Institut National de la Santé et de la Recherche Médicale U.161, École Pratique des Hautes Études, F-75014 Paris, France
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ABSTRACT |
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Bester, Hervé,
Victoria Chapman,
Jean-Marie Besson, and
Jean-François Bernard.
Physiological Properties of the Lamina I Spinoparabrachial
Neurons in the Rat.
J. Neurophysiol. 83: 2239-2259, 2000.
Single-unit extracellular recordings of
spino-parabrachial (spino-PB) neurons (n = 53)
antidromically driven from the contralateral parabrachial (PB) area
were performed in the lumbar cord in anesthetized rats. All the
spino-PB neurons were located in the lamina I of the dorsal horn. Their
axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the
thin myelinated fibers range. They had an extremely low spontaneous
activity (median = 0.064 Hz) and a small excitatory receptive
field (2 toes or pads). They were all activated by both peripheral A
(mainly A
) and C fibers after intense transcutaneous electrical
stimulation. Their discharge always increased in response to noxious
natural stimuli of increasing intensities. The great majority (75%) of
spino-PB neurons were nociceptive specific, i.e., they were excited
only by noxious stimuli. The remaining (25%) still were excited
primarily by noxious stimuli but also responded moderately to innocuous
stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by
both mechanical and heat noxious stimuli. Among them, 35% were in
addition moderately activated by noxious cold (thresholds between +20
and
10°C). Only (8%, 4/53) responded exclusively to noxious heat.
Spino-PB neurons clearly encoded the intensity of mechanical
(n = 39) and thermal (n = 38)
stimuli in the noxious range, and most of the individual
stimulus-response functions were monotonic and positive up to 40/60
N · cm
2 and 50°C, respectively. For the
mechanical modality, the mean threshold was 11.5 ± 1.25 N
· cm
2 (mean ± SE), the response increased
almost linearly with the logarithm of the pressure between 10 and 60 N · cm
2, the mean p50
(pressure evoking 50% of the maximum response) and the maximum
responsiveness were: 30 ± 2.4 N · cm
2 and
40.5 ± 5 Hz, respectively. For the thermal modality, the mean
threshold was 43.6 ± 0.5°C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48°C interval, the mean t50 and the maximum responsiveness
were: 47.4 ± 0.3°C and 40 ± 4.4 Hz, respectively. Most of
the spino-PB neurons tested (13/16) had their noxiously evoked
responses clearly inhibited by heterotopic noxious stimuli. The mean
response to noxious stimuli during heterotopic stimuli was 31.7 ± 6.1% of the control response. We conclude that the nociceptive
properties of the lamina I spino-PB neurons are reflected largely by
those of PB neurons that were suggested to be involved in autonomic and
emotional/aversive aspects of pain.
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INTRODUCTION |
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Numerous retrograde and anterograde anatomic data
have clearly demonstrated that the parabrachial (PB) areaa group of
neurons that surround the superior cerebellar peduncle at the
ponto-mesencephalic junction
is one of the major projection targets
for the neurons located in the most superficial laminae (I and II
outer, or to simplify: lamina I) of the dorsal horn (Bernard et
al. 1989
, 1995
; Blomqvist et al. 1989
;
Burton et al. 1979
; Cechetto et al. 1985
; Craig 1995
; Feil and Herbert 1995
;
Hylden et al. 1989
; Kitamura et al. 1993
;
Lima and Coimbra 1989
; Menétrey and De
Pommery 1991
; Menétrey et al. 1992
;
Panneton and Burton 1985
; Slugg and Light 1994
; Wiberg and Blomqvist 1984
; Wiberg
et al. 1987
; Yamada and Kitamura 1992
; see also
Menétrey et al. 1982
; Saper 1995
;
Swett et al. 1985
).
The superficial laminae of the spinal cord receive heavy and direct
input from the thin primary afferents A and C fibers and contain
primarily nociceptive neurons some of which being identified as
projecting to the thalamus (Besson and Chaouch 1987
; Brown and Culberson 1981
; Cervero and Connell
1984
; Culberson and Brown 1984
;
Fitzgerald 1984
; Light and Perl
1979
; Mizumura et al. 1993
; Sugiura et
al. 1986
, 1988
, 1989
, 1993
; Woolf and Fitzgerald 1983
,
1986
). More recently, several studies also have demonstrated
that a high number of nociceptive neurons in the superficial laminae
project to the PB area (Hylden et al. 1985
, 1986a
,b
;
Light et al. 1987
, 1993
; see also McMahon and
Wall 1983
).
These studies, as a whole, strongly support a substantial noxious input
from the spinal cord to the PB area. The effect of this noxious input
was observed directly at the PB level. Several electrophysiological
studies have recorded the unitary activity of PB neurons, a large
number of which respond specifically to somatosensory stimuli in the
noxious range irrespective of whether these neurons were antidromically
driven from the central nucleus (Ce) of the amygdala or from the
ventromedial (VMH) nucleus of the hypothalamus or whether their
projecting target was not determined (Bernard and Besson 1988,
1990
; Bernard et al. 1994
; Bester et al.
1995b
; Hayashi and Tabata 1990
; Huang et
al. 1993
; Matsumoto et al. 1996
; Menendez
et al. 1996
; Slugg and Light 1990
). Further indirect evidence for the involvement of the PB area (Bellavance and Beitz 1996
; Bullitt 1990
; Clement et
al. 1996
; Hamba et al. 1994
; Hermanson and
Blomqvist 1996
; Lantéri-Minet et al. 1993
, 1994
; Pertovaara et al. 1993
) and spino-PB
pathway (Bester et al. 1997b
; Buritova et al.
1998
) in nociception was obtained from c-Fos studies. Because
the nociceptive region of the PB area is involved in numerous autonomic
regulatory functions and densely projects to the Ce and the
hypothalamus, the spino-PB pathway is thought to be involved in
autonomic and emotional/aversive reactions to painful stimuli (see refs
and discussion in Bernard and Besson 1990
; Bester
et al. 1995b
).
Paradoxically, at least in the rat, the properties of PB neurons now
are known more precisely than those of their main input, the spino-PB
neurons. More specifically, their precise encoding properties to
mechanical and thermal stimuli and the effect of stimuli applied
outside the excitatory receptive field (heterotopic stimuli) of a
consistent subgroup of PB neurons have been investigated in some detail
(Bernard and Besson 1990; Bernard et al.
1994
; Bester et al. 1995b
). To date, these
properties have not been investigated for the spino-PB neurons.
Although a study in the rat investigated the main characteristics of
cells projecting through the dorsolateral funiculus (McMahon
and Wall 1983
), it did not give details concerning spino-PB
neurons. This issue has been considered in the cat but neither the
encoding properties nor the effect of heterotopic stimuli were
determined (Hylden et al. 1985
, 1986a
,b
; Light et
al. 1987
, 1993
). Indeed, the encoding properties of superficial
laminae neurons only have been investigated precisely for
spino-thalamic neurons or for neurons without identified projection in
the cat and monkey (Bushnell et al. 1984
; Chung et al. 1979
; Craig and Kniffki 1985
;
Craig and Serrano 1994
; Ferrington et al.
1987
; Hayes et al. 1981
; Hoffman et
al.,1981
; Kenshalo et al. 1979
; Light et
al. 1993
; McHaffie et al. 1994
; Price et al. 1976
, 1978
). The aim of the present study was to provide a detailed electrophysiological characterization of the responses of the
lamina I spinal neurons backfired antidromically from the contralateral
PB area. Furthermore the present investigation should permit an
accurate comparison between spino-PB and PB neurons because the studies
were made under identical experimental conditions. Part of these
results have been presented in abstract form (Bester et al.
1995a
).
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METHODS |
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Animal preparation
Experiments were performed on 53 Sprague-Dawley male rats weighing 250-275 g. In an anesthetizing box, animals were deeply anesthetized with 2% halothane (Belamont, France) in a nitrous oxide-oxygen mixture (2/3-1/3). This anesthesia regimen was maintained throughout the surgery procedures. A tracheal cannula was inserted, the jugular vein was cannulated, and then animals were paralyzed with a continuous intravenous injection of gallamine triethiodide (Flaxedil) and artificially ventilated using a Palmer pump. The expiratory CO2 was monitored continuously using a Capnomac (Datex Instruments) throughout the experiment, and the end-tidal CO2 was maintained at ~4%. Neither blood pressure nor electreoencephalogram (EEG) were monitored because these same conditions have been found to provide reproducibly stable and reliable anesthesia. Use of these conditions enables comparisons across all prior studies in this laboratory. The level of halothane, O2, and N2O also was checked systematically during the experimental procedure. The heart rate was monitored continuously, and the core temperature maintained at 37 ± 0.5°C using a homeothermic blanket system.
The animals were mounted in a stereotaxic frame. The head was fixed in
a dorsiflexed position with the incisor bar elevated 10 mm above the
standard position (Paxinos and Watson 1986). A craniotomy was performed at the parieto-occipital level on the right
side for the passage of the stimulating electrodes targeting the PB
area. A second small craniotomy was performed rostrally in the left
frontal bone to implant the reference electrode for central
stimulation. Vertebras L1-3 were exposed and
fixed in "spinal hooks." A laminectomy then was performed to expose the L4-5 spinal segments. After surgery, the
halothane level was reduced to 0.5%, and the gas mixture of nitrous
oxide-oxygen was maintained at 2/3-1/3. This level of anesthesia did
not excessively depress neuronal responses to noxious stimuli and was
adequate in terms of ethical purposes (Benoist et al.
1984
; Le Bars et al. 1980
). These conditions
were in agreement with the ethical guidelines of the International
Association for the Study of Pain (IASP) for investigations of
experimental animals (Zimmermann 1983
) and received the
approval from the local ethical committee.
Recordings
Extracellular unit recordings made with metal microelectrodes
(9-12 M), tungsten or stainless steel (Frederic Haer). Single-unit activity was fed into a window discriminator, the output of which was
stored on a digital audiotape (Biologic DTR 1200). A multichannel analyzer was used to perform further data processing.
The electrode was inserted vertically in the left spinal cord close to the surface. From this zero point, the search of a neuron was started by driving the electrode through the dorsal horn. Each time only one spike or any change in the background activity was observed, the response to electrical stimuli delivered in the PB area was tested. If no response was detected, the effects of high-intensity current delivered on different distal sites of the hindpaw were tried rapidly. If the presence of a unit was confirmed, antidromic identification was tested by applying electrical stimuli in the PB area at different depths from each of the three stimulating electrodes (see following text). When antidromic activation was obtained, the cell responses to different cutaneous stimuli (electrical, mechanical, and thermal) were tested. Only one neuron was studied per animal to avoid sensitization.
Response to PB area stimulation
Stimulations of the PB area were applied with a linear array of three concentric monopolar electrodes (model No. SNE-300 modified; insulated contact protrudes from shaft, 120-µm diam, 5-mm length; exposed contact, 100-µm diam, 150-µm length; Rhodes Medical Instruments) separated by 600 µm from each another. The set of three electrodes was inserted into the right PB area with the following coordinates for the middle electrode: 1.9 mm rostral to the lambda and 1.8 mm lateral to the midline. The depth was 7,000 µm from the brain surface. The three center contacts could be independently stimulated (1-mA square wave negative current, 0.2-ms duration).
When, in the spinal cord, a unit was backfired from the PB area, the stimulation thresholds were measured for each electrode. The site of minimum threshold was determined by repeating this measure at different depths (above and below). Finally, the set of electrodes was positioned at the depth where the lowest threshold had been observed, and we used the electrode from which this lowest threshold was obtained. Then the intensity of the central stimulation was set at twice the threshold to test the criteria of antidromic activation: 1) the lack of variance (<100 µs) of the spike latency (t), 2) the ability of the evoked response to follow a train of high-frequency stimulation (>300 Hz, for 5 stimuli). This test was coupled with the measure of the refractory period (R), determined as the shortest interval between two stimuli required to produce two antidromic spikes. The refractory period always corresponded to a higher frequency (1/r > 500 Hz). And 3) the observation of a systematic collision between an orthodromic spike (spontaneous or electrically evoked from the receptive field) and the antidromically evoked response in the 2t + R period.
Response to transcutaneous electrical stimulation
Electrical square-wave stimuli (2-ms duration) were delivered
through pairs of stainless steel stimulating electrodes inserted subcutaneously in the toes. We usually observed that a high-intensity stimulation evoked two periods of activation. The threshold of each
peak of activation was determined by a progressive increase in the
intensity of stimulation from a 0 starting point. The effects of
repetitive electrical stimuli (16 trials at a frequency of 0.5 Hz) at
an intensity corresponding to threefold the highest threshold (i.e.,
that evoking the late peak of activation) were analyzed by building
poststimulus time histograms (PSTHs) off-line on a MacIntosh 7100, with
the Spike2 software and CED 1401 hardware (CED, Cambridge England). The
length of the sciatic nerve was measured at 15 cm from the tip of the
toes (III-V) to the dorsal horn entry zone of the
L4-5 lumbar segments. The conduction velocity of
the peripheral fibers then was calculated as follows: 150 mm/[observed
latency of the first spike in each individual peak 1 ms]. The
subtracted 1 ms corresponds to an estimate of the synaptic delay,
assuming that spino-PB neurons were monosynaptically activated. All in
all, our results happened to be in good agreement, a posteriori, with
data obtained directly from peripheral fibers in the rat foot
(Leem et al. 1993a
,b
), validating thus the assumptions and the methods.
Response to natural cutaneous stimulation
INNOCUOUS STIMULI. Innocuous stimuli were applied to the receptive field of the recorded neuron. They consisted of mechanical light touch and brush (paintbrush Gerand Séries SD 1, No. 6) or thermal stimuli applied with a thermostatically controlled water jet (20-40°C). To apply thermal stimuli, a 50-ml syringe was filled with water just above the desired temperature. When ejected through a 12-gauge needle, the stimulating water jet maintained a temperature (measured from a thermocouple placed next to the receptive field) at the desired level over the 20 s of stimulation. From 20 to 30°C, 5°C steps were used, then 2°C steps were used from 30 up to 50°C. In the absence of an evoked response, intervals between the discrete stimuli were of 2 min. All stimuli were applied during a 20-s period.
NOXIOUS MECHANICAL STIMULI.
With a calibrated forceps (strain gauge on the arm connected to an
amplifier: Hottinger, Baldwin Messtechnick, Darmstadt, Germany), we
manually applied serially increasing pressures of 20-s duration (5, 10, 20, 40, 60, and 80 N · cm2; 1 N · cm
2 = 10 kPa
100 g · cm
2) on the receptive field of the spino-PB
neurons. The stimulating area was 10 mm2. The
level (intensity) of the applied pressure was read, on-line, on the
digital screen (4 digits) of the apparatus, allowing the trained
experimenter to maintain, with a feedback reaction, an almost constant
pressure (variation <10%) over the stimulating period. Between two
stimulations, an interval of 5 min was observed, to avoid fatigue
and/or sensitization of peripheral receptors.
NOXIOUS THERMAL STIMULI.
Both noxious heat and noxious cold stimuli were applied using a
thermostatically controlled water jet, directly on the receptive field,
for 20 s periods. With the approach used for innocuous thermal
stimuli, scanning the effect of heat stimuli toward the noxious range
consisted of applications of discrete water jets of increasing
temperatures from innocuous 30°C up to highly noxious 52°C in 2°C
steps. Similarly, cold stimuli consisted of applications of discrete
water jets of decreasing temperatures from innocuous 30°C down to
highly noxious 15°C, with a 5°C step. In this case, the
stimulating fluid was an alcohol-water mixture that was set just below
the desired temperature. This procedure allowed the desired temperature
to be achieved at the level of the receptive field (measured with a
thermocouple) for the 20-s stimulation period. For both heat and cold
stimuli, a minimum interval of 5 min (and 10 min for higher intensity
stimuli) was observed, to avoid fatigue and/or sensitization of
peripheral receptors.
Effects of heterotopic noxious stimulation on a noxiously evoked response
On 16 spino-PB neurons, we have tested the effects of a noxious
stimulus applied far outside the receptive field (heterotopic stimuli)
on the noxiously evoked responses of the spino-PB neurons. Briefly, on
a standard response to noxious stimuli (50°C, or 60 N · cm2) applied on the receptive field of a
spino-PB neuron (toe or pad), we have tested the effects of a noxious
stimuli (50°C, or 60 N · cm
2) applied
far outside the receptive field, basically, a forepaw (and in some
cases the tongue). The heterotopic stimulation started 10 s before
the noxious stimulation of the receptive field, continued throughout
this stimulation (20 s), and was prolonged 10 s after the end of
the noxious stimulation of the receptive field (total duration of the
heterotopic stimulus: 40 s).
Histological controls
For all the experiments, the location of the PB electrode tip that provided the lowest threshold to obtain an antidromic spike was marked by passing a 10 µA DC for 30 s (positivity to the electrode). This procedure allowed a small electrophoretic iron deposit at the tip of the stimulating electrode.
In a first series of 28 experiments, we used stainless steel recording electrodes. At the end of each experiment, the depth of the electrode was noted, and a small deposit of iron was made at the tip of the electrode by passing a 10 µA DC current (positivity to the electrode) during 30 s.
After the iron deposits, animals were killed by an overdose of intravenous sodium pentobarbitone. The brain and lumbar spinal cord were removed and fixed for 5 days in a mixture that would label the iron with a Prussian blue color (1 volume of 1% potassium ferrocyanide in 10% formalin solution added to 2 volumes of 2% acetic acid in 95% alcohol solution). The tissues then were cut into 100-µm thick sections, mounted on slides, Nissl stained with safranin, and coverslipped. Because the depth of electrode was related very closely to a Prussian blue point in the most superficial laminae of the dorsal horn (lamina I), it appeared reasonable to use only the depth measurement to locate the recordings when we used tungsten microelectrodes, given that all the experiments were performed with identical paradigms.
Both the recording and the stimulating sites were drawn individually through an optical microscope, using an attached camera lucida. The stimulation sites were plotted on a series of 12 camera lucida drawings of the PB area (150 µm apart). The spinal recording sites were plotted on camera lucida drawings of the L4-5 levels because no recordings were made outside these lumbar segments. For illustration purposes, the stimulation sites in the PB area were collated on four representative sections (2 pontine, 2 mesencephalic), and the recording sites on a representative section passing through the L4 segment.
Data analysis
The magnitude (or intensity) of the responses was defined as the mean spike frequency increase during the whole period of stimulation (20 s), i.e., mean spike frequency during the stimulation minus mean spike frequency during the 20 s preceding the stimulation. The responsiveness of a neuron was defined as the magnitude of the highest response observed for the considered modality.
In the case of heterotopic stimulation, the effect (inhibition) was defined as the percentage of the residual response as compared with the control test value. The results therefore were expressed as percentages of control responses.
Unless noted otherwise, the results were expressed as means ± SE. Statistical ANOVA were made using the Fisher's PLSD (protected least significant difference) tests. However, in some cases, we used the 10th, median, and 90th percentile because the distribution of the data were obviously not normal.
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RESULTS |
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Fifty-three dorsal horn neurons were recorded extracellularly and unitarily at the lumbar segment level of 53 rats. These neurons were selected by the following criteria: antidromically backfired from the contralateral PB area and located in the most superficial laminae of the dorsal horn.
General findings
The spontaneous activity of the spino-PB neurons was extremely low (10th percentile < median <90th percentile): 0.001 < 0.064 < 1.88 Hz (n = 53), being often very close to silent. All of these neurons were clearly nociceptive, i.e., responses were markedly increased to stimuli in the noxious range, irrespective of whether or not they responded to innocuous stimuli. Most of them (75%, see following text) were nociceptive specific (NS), responding only to stimuli in the noxious range. Almost all of them (92%, 49/53) responded to two modalities of noxious stimuli (mechanical and thermal). Generally, they had small receptive fields including a restricted portion of the paw. All of the spino-PB neurons responded to transcutaneous electrical stimulation applied to the receptive field with, at least, two distinct early and late periods of activity.
Antidromic activation of the spino-PB neurons
All of the spino-PB neurons were driven antidromically from the contralateral PB area and satisfied the antidromic criteria (Fig. 1): the latency of the antidromic spike was stable (Fig. 1B), i.e., with a variation <100 µs, or <1% of the latency; it was always possible to obtain a collision between the antidromic spike and a spontaneous or electrically evoked orthodromic spike (Fig. 1, C and C'); the antidromic spike could follow a high-frequency stimulation (>500 Hz; Fig. 1, D and D'). The refractory period of the spino-PB neurons was short with a mean of 0.7 ± 0.2 ms (mean ± SD, n = 53).
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The mean latency of the antidromic spikes was 9 ± 6.1 ms (mean ± SD, n = 53). The mean of the calculated conduction velocities for the spino-PB neurons (~100 mm distance between the L4 segment and the contralateral PB area divided by the spike latency) was 15.3 ± 7.3 m/s (mean ± SD, n = 53). The lowest conduction velocity was 2.8 m/s (Fig. 2), indicating that all of the spino-PB neurons recorded here had a thin myelinated axon.
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The stimulation site was always located in or in the close vicinity of the PB area (Fig. 3, A-E). The thresholds needed to evoke an antidromic spike were often low (150-300 µA) or very low (5-150 µA): 26% (14/53) and 57% (30/53), respectively (Fig. 3, A-E). A few thresholds (17%, 9/53) were higher (300-500 µA) when a systematic search for the lowest threshold was not achieved. In numerous experiments, a low threshold could be obtained from only one of the three stimulating electrodes, whereas from the other two, even a current >1 mA was not always sufficient.
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Location of the spino-PB neurons
The depths of the recording sites of the 53 spino-PB neurons were in the 118- to 363-µm range with a mean of 240 ± 12 µm (n = 53), i.e., corresponding to the superficial dorsal horn. The relevance of this measurement was confirmed further in a subgroup of 28 spino-PB neurons for which the recording sites were labeled with a Prussian blue point (Fig. 4A) in the superficial laminae (Fig. 4B). The mean depth of the recordings corresponding to these labeled sites was 260 ± 23 µm (n = 28). The mean depth of the 25 neurons recorded with tungsten microelectrodes was similar 233 ± 14 µm (n = 25), indicating that sites without histological confirmation were similarly located close to the lamina I.
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Receptive field
Overall the spino-PB neurons had small receptive fields. In most cases (72%, 38/53), the receptive field was extremely small (Fig. 5), including only one toe (n = 33) or one pad (n = 5). In some cases (28%, 15/53), the receptive field extended to two toes (n = 10), two pads (n = 2), or one toe and one pad (n = 3).
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Characterization of spino-PB neurons by electrical stimuli
All the spino-PB neurons responded to suprathreshold transcutaneous electrical stimulation of the receptive field with at least two periods of activation (Fig. 6). The mean intensity threshold of these early and late periods of activation were 0.4 ± 0.1 and 3.1 ± 0.5 mA, respectively (n = 53). The mean latency (±SD) of the early and late periods were 7.7 ± 3 ms (range 3.9-20 ms, n = 53) and 162 ± 54 ms (range 51-290 ms, n = 53), respectively. These latencies correspond to calculated mean conduction velocities (±SD) of 26 ± 9 m/s (range 7.9-52 m/s, n = 53) and 1.1 ± 0.6 m/s (range 0.5-2.9 m/s, n = 53), respectively. These ranges of conduction velocities correspond with those of A and C fibers, respectively. Intense electrical stimulation (three times the C-fibers threshold) evoked 3.7 ± 0.4 and 14 ± 1.4 spikes (n = 53), in the early (A fibers) and late (C fibers) periods of activation, respectively.
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The distribution of the A-fiber conduction velocities, calculated from
the latency of the earliest spike in the early peak of activation for
each single spino-PB neuron appeared normal (Fig.
7A). The majority of the
conduction velocities (58%, 31/53) were in the 8- to 26-m/s range,
i.e., clearly in the range of conduction velocities of A fibers. The
remaining 42% (22/53) corresponded to conduction velocities between
either 26-38 or 38-60 m/s (Fig. 7A), i.e., to conduction
velocities of peripheral fibers that were either at the border between
A
and A
fibers (n = 18) or within A
-fiber
(n = 4) ranges, respectively. Even in neurons with a
clear A
-fiber activation, the duration of the first period of
activation was such that most of the evoked spikes, and more
specifically the relatively late ones, were likely to be triggered by
A
fibers. Furthermore in 12 cases, two peaks of activation, with
mean conduction velocities of 28 ± 6 and 13.6 ± 2 m/s,
respectively (mean ± SD, n = 12), were
identified. These clearly correspond to A
/A
-fiber and A
-fiber
conduction velocity ranges, respectively.
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The distribution of the C-fiber conduction velocities, calculated from the latency of the earliest spike in the late peak of activation for each single spino-PB neuron (Fig. 7B) was also normal and clearly separated from the A-fiber group (Fig. 7A). The great majority of the C fibers (87%, 47/53) activating the spino-PB neurons had very slow conduction velocities, between 0.3 and 1.4 m/s, the few remaining cases corresponded to intermediate weaker peaks (Fig. 7A). Furthermore, as clearly illustrated in the single sweep of Fig. 6, numerous spino-PB neurons responded to suprathreshold electrical stimuli with two peaks in the late period of activation. In 26 cases where the two late peaks were clearly separated, their mean conduction velocities were 1.2 ± 0.6 and 0.6 ± 1 m/s, respectively (mean ± SD, n = 26).
Figure 8 shows that in the late period corresponding to C-fiber activation, for 36 representative spino-PB neurons stimulated over 16 trials at 0.5 Hz with the same intensity (3 times the threshold), the number of spikes evoked by the first stimulus increased slightly after a few stimulations and remained stable thereafter. Thus the wind-up was relatively weak in the present population of superficial spino-PB neurons.
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Characterization of the spino-PB neurons by natural stimuli
RESPONSIVENESS. All the spino-PB neurons responded with a strong and sustained discharge to thermal and/or mechanical noxious stimuli. From a very low or absent spontaneous activity (see preceding text), the responses of the spino-PB neurons to noxious stimuli had a rapid onset that could reach values ~100 Hz (Figs. 5, 10, and 13), giving to the initial part of the response a phasic aspect. The response then decreased but remained strong and sustained throughout the period of stimulation, giving to the second part a tonic pattern. The majority of neurons had an afterdischarge, especially for the highest noxious intensities of stimulation (Figs. 5, 10, and 13). Overall, the spino-PB neurons had a high responsiveness, the mean of the maximum response (whatever the modality) was 47 ± 5 Hz (n = 53). Furthermore the response to noxious stimuli was clearly reproducible, the variation of responsiveness to two noxious heat stimulation of the skin (5-20 min apart) being in the 0.5-25% range (n = 29). Figure 5, B and C, shows two individual examples of the reproducibility of responsiveness to heat stimuli.
Most of the spino-PB neurons (75%, 40/53) were clearly NS, i.e., they were excited only by stimuli in the noxious range. The remaining spino-PB neurons were not so easy to classify. They could be activated by innocuous stimuli with a low responsiveness (mean response to brush: 4.6 ± 1.7 Hz, n = 13). When noxious stimuli were applied, these neurons had a much higher responsiveness, which, in fact, did not differ from that of the main group (see, however, encoding properties to thermal stimuli). They were then considered as nociceptive nonspecific neurons or wide dynamic range (WDR) to match the nomenclature in the current literature. The spontaneous activity of this population of WDR neurons is 0.08 < 0.36 < 3.25 Hz (10th percentile < median <90th percentile), i.e., there was a tendency to be moderately greater than that of the general population. Interestingly, consistent with the lack of substantial "wind-up" in response to electrical stimuli, we observed that repeated stimuli of same intensity after 5-min intervals did not lead to increased responsiveness (Fig. 5, B and C). Almost all the spino-PB neurons (92%, 49/53) were activated by both intense noxious thermal (50°C) and noxious mechanical (60 N · cm
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ENCODING PROPERTIES TO MECHANICAL STIMULI.
The spino-PB neurons increased their firing rate when the pressure
stimulus increased in the noxious range as shown in Fig. 10, A-C, for three neurons
having different thresholds (5, 10, and 20 N · cm2) close to and in the noxious range. The
progressive increase in neuronal firing paralleled the increase in
intensity of the noxious pressure, this was true for the whole tonic
excitation throughout the stimulation period and the afterdischarge
where present (Fig. 10, B and C).
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ENCODING PROPERTIES TO HEAT STIMULI. The firing rate of spino-PB neurons increased with increasing temperature of stimulation in the noxious range as shown for three individual examples (Fig. 13, A-C) of neurons with different noxious thresholds (44, 46, and 48°C, respectively). The response increase, primarily in the 46-50°C range, was maintained over the whole period of stimulation in addition to the afterdischarge in some cases (Fig. 13B).
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RELATIONSHIPS BETWEEN ENCODING PROPERTIES OF NEURONS TO NOXIOUS
HEAT AND MECHANICAL STIMULI.
Neurons were divided into two groups with respect to mechanical
thresholds, low (5 N · cm
2,
n = 8) and high (
20 N · cm
2, n = 9), thus corresponding
to the commonly adopted classification of WDR and NS cells. The
encoding properties of these two neuronal groups to heat stimuli were
compared. Low mechanical threshold neurons had a tendency to lower heat
thresholds (42 ± 1.3°C; n = 8), whereas high
mechanical heat threshold neurons had higher heat thresholds (45 ± 0.8°C; n = 9). In addition, the mean
stimulus-response curves of the two groups to heat stimuli were
significantly different (Fig. 16;
ANOVA, F1,151 = 5, P = 0.027). Thus the low mechanical threshold neurons (WDR) had a higher
responsiveness than the high mechanical threshold neurons (NS) for all
the graded temperatures in the noxious range (
44°C).
|
Inhibition of spino-PB neurons by concomitant heterotopic noxious stimuli
Noxious heat stimulation (50°C) applied to the receptive field
of the recorded lumbar neuron could be considerably inhibited by a
noxious mechanical stimuli (60 N · cm2)
of the forepaw (Fig. 17A,
1 and 2). Heterotopic noxious stimuli (stimulation applied outside the receptive field of the recorded lumbar
neuron) clearly inhibited the response to the control noxious stimulation of the great majority of the spino-PB neurons (13/16) studied. The percentage of inhibition was clearly different between neurons (Fig. 17B), but in most cases (9/13) the neuronal
response was reduced to <50% of the control value. The mean intensity
of the responses during heterotopic noxious stimulation was 31.7 ± 6.1% of control responses (n = 13).
|
This phenomenon of heterotopic inhibition was maximum during concomitant stimulation, but decreased rapidly thereafter, rarely lasting more than a couple of minutes after cessation of the heterotopic stimulation. The level of inhibition did not seem to be dependent on the intensity of the control response, which indicates that whatever the amplitude of the initial responsiveness, the neurons seemed to have the same susceptibility to inhibition by heterotopic noxious stimuli.
We did not find any evidence for a difference between the different categories of spino-PB neurons (low or high mechanical threshold) when considering their susceptibility to inhibition by heterotopic stimuli.
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DISCUSSION |
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Results were obtained from 53 neurons, all recorded in or close to the lamina I at the lumbar level of the spinal cord, and backfired antidromically from the contralateral PB area. All spino-PB neurons were excited by and encoded the intensity of noxious mechanical and/or thermal stimuli applied to their small receptive field. None of the spino-PB neurons were activated by innocuous stimuli only. However, some neurons were lightly activated by innocuous mechanical stimuli, as well as responding to noxious stimuli. A distinction between NS (75%) and WDR (25%) spino-PB neurons was made. All neurons had a very low spontaneous activity, close to silent, and exhibited two distinct periods of activation, corresponding to A- and C-fiber-evoked activity, respectively, in response to suprathreshold transcutaneous electrical stimuli.
Methodological considerations
ANESTHETIC REGIMEN AND IMMOBILIZING DRUGS.
The anesthetic conditions used in the present study were the same as
those used in all of the previous electrophysiological studies of the
PB area in our laboratory (e.g., Bernard and Besson 1990; Bernard et al. 1994
; Bester et al.
1995b
). This point is of critical importance to ensure
accurate comparison between responses of spino-PB and PB neurons. It
must be emphasized that the precise measurement of the halothane
concentration together with the endtidal PCO2
allowed good control of the stability and reproducibility of the
anesthesia depth. Furthermore in our hands, this anesthetic regimen did
not seem too depressive in terms of neuronal responses. This indeed
allowed us to observe clear, and often strong, neuronal responses to
light and/or noxious stimuli in different spinal laminae and several
supraspinal nuclei (present results; see refs in Bernard et al.
1996
; Besson and Chaouch 1987
). Despite numerous advantages, our anesthesia regimen, like any other, inevitably induces
some changes in comparison to the awake state. This issue is probably
even more critical with other types of anesthesia regimen where the
physiological assessments of the anesthesia quality and levels of drug
delivery cannot be constantly monitored and accurately adapted.
ANTIDROMIC ACTIVATION AND LOCATION OF SPINO-PB NEURONS.
The PB area has been shown to be the densest field of projection of
contralateral spinal lamina I neurons in rats, as demonstrated recently
with the Phaseolus vulgaris leucoagglutinin (Bernard et al. 1995; Feil and Herbert 1995
; Slugg
and Light 1994
). The selectivity of the projection origin from
lamina I is supported further by studies using retrograde tracers
(Bernard et al. 1989
; Cechetto et al.
1985
; Hylden et al. 1989
; Kitamura et al.
1993
; Lima and Coimbra 1989
;
Menétrey and De Pommery 1991
;
Menétrey et al. 1982
, 1992
; Swett et al.
1985
; Yamada and Kitamura 1992
). In the present
study, most of the thresholds (83%) needed to evoke an antidromic
spike were low (
300 µA) or very low (<150 µA), and all the
stimulating sites were clearly located in the PB area, it is therefore
highly probable that spino-PB neurons terminals were primarily
stimulated. This confidence in the specificity of the antidromic
stimulation for spino-PB neurons is reinforced by the personal
observation that in some cases a current
1,000 µA did not spread
up to 600 µm (the distance between 2 monopolar electrodes). This
short distance of current spread is a little lower than indicated by
Ranck (1975)
. Furthermore identifying well-isolated
spikes that lasted long enough (
1 ms) and had a high enough amplitude
(
2 mV) supports the somatic origin of the recordings (Lipski
1981
). Consequently, as validated by the location of the
Prussian blue points in the lamina I and sometimes in the Lissauer's
tract, we can state that our recordings of neurons driven
antidromically from the contralateral PB area were those of lamina I
spino-PB neurons. However, because the axonal trajectory of
spinothalamic neurons could be in the vicinity of the PB area (unpublished anatomic data from PHA-L injection centered in lamina I)
(see also Mehler et al. 1960
), one cannot exclude the
possibility that the higher currents have stimulated spinothalamic neurons.
SELECTION CRITERIA. Neurons were selected primarily on the basis of the fulfillment of antidromic activation from the PB area. Consequently we have investigated the physiological characteristics of all of the recorded spino-PB neurons, and we were able to determine, with minimal bias, that all lamina I spino-PB neurons studied were excited by noxious stimuli. With the aim of avoiding central and peripheral sensitization, we have described only one neuron per animal, thus allowing us to report a homogenous neuronal population with homogenous properties.
Physiological properties of spino-PB neurons
CENTRAL CONDUCTION VELOCITIES.
A wide range of central conduction velocities was observed. Because the
slowest fiber we recorded had a conduction velocity of 2.8 m/s, we can
assume that all the axons of the spino-PB neuronal population were
myelinated. Previous studies have also described a wide range of
central conduction velocities of spino-PB axons; however, a higher
representation of low to very low conduction velocities were then
reported (Hayashi and Tabata 1989; Hylden et al.
1985
, 1986a
,b
; Light et al. 1993
; McMahon
and Wall 1985
). Furthermore one study (Light et al.
1993
), suggested that a substantial proportion (~40%) of
axons of NS spino-PB neurons had very low conduction velocities in the
unmyelinated range. The basis for these discrepancies may be due to
species differences (Hayashi and Tabata 1989
;
Hylden et al. 1985
, 1986a
,b
; Light et al.
1993
). Furthermore in our experimental condition the few spikes
evoked with very long latencies were not sufficiently stable to be
recognized as antidromic. Consequently it is not possible to exclude
the possibility that we missed a few antidromic spikes carried by very
thin unmyelinated fibers. Interestingly, others have reported antidromic activation of unmyelinated lamina I axons with the use of
long (2 ms) stimulus pulses (Craig and Kniffki 1985
).
Although, Light et al. (1993)
have suggested that
multireceptive (WDR) neurons have the highest conduction velocities, we
did not find any evidence for a relationship between the central
conduction velocity and the physiological properties of the spino-PB neurons.
RECEPTIVE FIELD.
Spino-PB neurons responding from the distal territory of the sciatic or
saphenous nerves had small receptive fields. This observation is
consistent with earlier studies of spinal neurons of the superficial
and deep dorsal horn, projecting either to the PB area, the thalamus or
other undefined projecting targets in the rat, cat, and monkey
(Christensen and Perl 1970; Hylden et al. 1985
,
1986a
,b
; Kumazawa et al. 1975
; Light et
al. 1993
; McMahon and Wall 1983
; Woolf
and Fitzgerald 1983
).
SPONTANEOUS ACTIVITY.
In our hands, spino-PB neurons had very low spontaneous activity, being
close to silent. This physiological parameter of the lamina I neurons
has rarely been investigated systematically but has been reported to be
low (<2 Hz) in several species (rat, cat, and monkey), irrespective of
the projection site of the neurons (Christensen and Perl
1970; Ferrington et al. 1987
; Hayashi and Tabata 1989
; Hylden et al. 1986a
,b
; Light
et al. 1993
; McHaffie et al. 1994
;
McMahon and Wall 1983
; Meng et al.
1997
; Woolf and Fitzgerald 1983
).
PERIPHERAL ELECTRICAL STIMULATION.
After intense electrical stimulation (3.1 ± 0.5 mA;
n = 53) of the receptive field of all of the spino-PB
neurons, at least two periods of activation, corresponding to
peripheral A and C fibers, were observed. This is in agreement with
earlier anatomic and electrophysiological work, showing that
superficial laminae neurons are contacted monosynaptically by A and C
fibers (Craig and Kniffki 1985; Fitzgerald and
Wall 1980
; Gobel et al. 1981
). The constant
coinnervation by A- and C-fiber inputs found in the present study was
not consistently observed in earlier studies of spino-PB neurons that
reported a dissociated innervation by either A- or C-fibers inputs
(Hylden et al. 1986a
,b
; Light et al.
1993
; McMahon and Wall 1983
; Meng et al.
1997
).
RESPONSIVENESS TO NATURAL STIMULI.
We have shown that all of the spino-PB neurons were nociceptive, the
large majority of them (75%) being NS and the remainder WDR. None of
the lamina I spino-PB neurons responded exclusively to innocuous
stimuli; this is in agreement with some previous studies
(Hayashi and Tabata 1989; Hylden et al.
1986a
,b
; Meng et al. 1997
). However, the
literature describes spino-PB neurons responding to innocuous stimuli
only (Light et al. 1993
; McMahon and Wall
1983
). In the latter studies, the percentage of innocuous spino-PB neurons was low (3 and 12%). The 12% of innocuous spino-PB neurons found in the cat corresponded to neurons responding to innocuous cooling (Light et al. 1993
).
ENCODING PROPERTIES TO THERMAL STIMULI.
This is the first study to systematically analyze the encoding
properties of a well identified population of lamina I spino-PB neurons. The capacity of lamina I neurons of the dorsal horn to increase firing rates with noxious temperature was demonstrated in
early studies, in the cat (Christensen and Perl 1970),
monkey (Kumazawa and Perl 1978
; Price et al.
1976
, 1978
), and rat (Menétrey et al. 1977
,
1979
). However, systematic studies of the thermal encoding
properties of neurons were concentrated on lamina I neurons projecting
to the thalamus in the monkey (Bushnell et al. 1984
; Ferrington et al. 1987
; Hayes et al.
1981
; Hoffman et al. 1981
; Kenshalo et
al. 1979
; Price et al. 1976
, 1978
) and the cat
(Craig and Kniffki 1985
; Craig and Serrano
1994
) and neurons with unidentified projection in the rat
(McHaffie et al. 1994
). These previous studies classified nociceptive neurons into two main groups (NS and WDR) according to responses to mechanical stimuli. Therefore we first used
the same classification (Fig. 16) to make an accurate comparison. In
our study, the mean firing rate of WDR neurons was higher than that of
NS neurons, which is in agreement with previous systematic studies of
lamina I spino/trigeminothalamic neurons (Bushnell et al. 1984
; Ferrington et al. 1987
;
Hayes et al. 1981
; Hoffman et al. 1981
).
Furthermore in both WDR and NS neurons, the mean discharge increased
progressively with temperatures from ~40 to 52°C.
ENCODING PROPERTIES TO MECHANICAL STIMULI.
The present study is the first to give a detailed analysis of the
mechanical encoding properties of lamina I spino-PB neurons. We have
demonstrated using a semilogarithmic scale that spino-PB neurons
increase their firing rate linearly in response to increasing mechanical stimuli over a very large noxious range (10-80 N · cm2). Even though most earlier studies used
brush, pinch, and squeeze as mechanical stimuli without building
stimulus-response curves, they showed, as does the present study, that
spino-PB, and spino(trigemino)-thalamic neurons increased their firing
rate with increasing intensities of mechanical stimuli in the noxious
range (Chung et al. 1979
; Craig and Serrano
1994
; Light et al. 1993
; McHaffie et al.
1994
; Price et al. 1976
). The mechanical
encoding properties of NS neurons located in the sacral dorsal horn was
systematically studied in the rat by Cervero et al.
(1988)
. The encoding properties of the spino-PB neurons we
describe here resemble those of NS neurons previously described
(Cervero et al. 1988
). The mean mechanical threshold
(11.5 N · cm
2) we observed in the
present study using a stimulation area of 10 mm2
is comparable with the mean threshold (16.7 N · cm
2) observed in spino-thalamic neurons
recorded in cats with a 3 mm2 probe (Craig
and Serrano 1994
).
COLD INFORMATION PROCESSED BY SPINO-PB NEURONS.
In our hands, some spino-PB neurons responded moderately to cold
stimuli, with thresholds on the border (20-15°C) or frankly in the
noxious ranges (<0°C). None of the spino-PB neurons responded only
to cooling in the innocuous range, whereas they increased firing rates
when the temperature was lowered within the noxious range (down to 5
and
10°C). All of the cold activated neurons described in the
present study also responded to mechanical noxious stimuli. These
spino-PB neurons sensitive to noxious cold resembled the HPC neurons
described by Craig and Kniffki (1985
; see also Craig and Serrano 1994
). These results contrast a
previous study describing cooling specific spino-PB neurons in cat
(Light et al. 1993
) and spino-thalamic neurons in cat
and monkey (Craig and Hunsley 1991
; Craig and
Kniffki 1985
; Craig and Serrano 1994
; Dostrovsky and Craig 1996
). In these latter species, the
innocuous cooling neurons are mostly of pyramidal shape, whereas the
HPC and NS neurons are mostly of multipolar and fusiform shape,
respectively (Han et al. 1998
; Zhang and Craig
1997
; Zhang et al. 1996
). If we admit similar
morpho-functional relationships in the rat, it is tempting to explain
this discrepancy by the fact that pyramidal cells (potentially
innocuous cooling neurons) project only weakly to the PB area in the
rat (Lima and Coimbra 1989
). Interestingly, the
stimulus-response curves for cold stimuli of these spino-PB neurons are
relatively close to those of PB neurons. However, it cannot be excluded
that our sample missed these innocuous cold responsive neurons because
of a relatively restricted searching window in the dorsal horn. Indeed
although a particular distribution of innocuous cooling neurons was not
demonstrated in the rat, some data in the medullary dorsal horn of the
monkey were in favor of special location of this type of neurons
(Craig et al. 1999
). Finally, one cannot exclude a
specific depressant effect of our anesthetic regimen on responses to
innocuous cold, thus explaining the failure to observe neurons
responding to that type of stimuli in our study. Indeed,
N2O, used as a sole anesthetic drug, has been
shown to change responses to cold in humans (Cheung and Mekjavik 1995
). Nevertheless, this question of responses to cold
stimulation would need further investigation devoted specifically to
this particular but important point.
INHIBITION OF SPINO-PB NEURONS BY A CONCURRENT HETEROTOPIC
NOXIOUS STIMULI.
Our results showed that a large proportion of spino-PB lamina I neurons
were subject to the effects of inhibitory controls generated from
noxious stimuli applied far outside their receptive field. This
phenomenon was very strong, virtually abolishing responses in some
cases. Inhibition was obtained with different types of heterotopic
noxious stimuli (thermal or mechanical) and was effective on the
different modalities that activated the spino-PB neurons. Because it
was possible to obtain inhibition with heterotopic stimuli delivered to
all sites of the rat's body, it is probable that this inhibition
corresponds to the diffuse noxious inhibitory controls (DNIC)
(Dickenson et al. 1980; Le Bars et al. 1979a
,b
, 1981
; Villanueva and Le Bars 1986
;
Villanueva et al. 1984
). Previously, DNIC was described
predominantly for WDR neurons located around the lamina V, whereas DNIC
exerted considerably less effect on lamina I NS neurons. Our findings
seem to extend the concept of DNIC, showing that lamina I neurons
projecting to the PB area are inhibited greatly by heterotopic stimuli.
These results are in keeping with a recent study showing WDR, as well
as NS, neurons (in the spinal trigeminal nucleus) are sensitive to DNIC
(Meng et al. 1997
). These studies are supported
indirectly by a study of c-Fos expression in the lumbar spinal cord
level that showed a significant decrease in the number of noxious-pinch
evoked Fos-ir neurons in lamina I by immersion of the tail in 50°C
(Morgan et al. 1994
). Thus the present results and
previous literature suggest that the concept of DNIC preferentially
acting on WDR neurons in the deep dorsal horn should be questioned, and
this phenomenon extended to NS neurons of the superficial dorsal horn.
Relationship between spino-PB and primary afferent fibers
The electrophysiological data presented in the preceding text
taken together with earlier studies of spino-PB neurons (Hylden et al. 1986a,b
; Light et al. 1993
;
McMahon and Wall 1983
; Meng et al. 1997
)
and data obtained from the rat foot (Leem et al. 1993a
)
suggest that lamina I spino-PB neurons primarily were innervated by
A
and C fibers. This is in keeping with anatomic data showing that
these fibers densely arborize in the superficial spinal laminae (Brown and Culberson 1981
; Cervero and Connell
1984
; Cervero et al. 1988
; Culberson and
Brown 1984
; Gobel et al. 1981
; Mizumura et al. 1993
; Sugiura et al. 1986
, 1988
, 1989
,
1993
; Swett and Woolf 1985
). In fact, A
and C
fibers are involved in detection of noxious heat and mechanical and
cold stimuli and also encode the intensity of the stimulation.
Responses of spino-PB neurons to noxious heat may essentially be driven
by C-polymodal receptors the properties of which were shown to be
similar to those of spino-PB neurons. Indeed, in the rat hindpaw,
Leem et al. (1993a,b
) have shown that the C-polymodal receptors are numerous, that their heat thresholds were in the 37-47°C range (many being in 43-47°C range), and that they
encoded thermal stimuli in the noxious range (see also Long
1977
).
Responses of spino-PB neurons to noxious mechanical stimuli chiefly
would be driven by A-mechanical and C-polymodal receptors. Indeed,
Leem et al. (1993b)
, using a 20 mm2 probe (see their Fig. 5) have shown that
A
- and C-mechanical cutaneous nociceptors are numerous and have
clear encoding properties at least in the 1-6 N range (5-30 N
· cm
2 range). These data support the mean
stimulus-response curve obtained from spino-PB neurons, even if
spino-PB neurons increased their firing rate over a larger range (5-60
N · cm
2). However, considering the
difficulty of applying adequately calibrated stimuli to the paw, or toe
of the rat, it cannot be excluded that the moderate discrepancies
observed may mainly be due to methodological differences. The weak
response of some spino-PB neurons to innocuous stimuli may result from
activation of C mechanoreceptors (which are numerous and respond to
innocuous stimuli) rather than from activation of A
fibers, which
did not seem to substantially innervate these neurons.
Finally in our study, responses of spino-PB neurons to noxious cold may
be driven by A-mechano-cold, A
-cold, C-mechano-heatcold, and
C-mechano-cold fibers as observed in the rat foot (Leem et al.
1993a
). These receptors were shown to respond and encode the intensity of cold stimuli in the noxious range, with a wide range of
thresholds (22 to
14°C). Furthermore, none of these peripheral fibers responded exclusively to noxious cold stimuli in the rat (Fleischer et al. 1983
; Handwerker et al.
1991
; Kajander et al. 1994
; Kress et al.
1992
; Leem et al. 1993a
,b
; Lynn and
Carpenter 1982
). These properties are reminiscent of those of
cold spino-PB neurons, which all responded also to mechanical and/or
heat noxious stimuli. On the other hand, as discussed in the preceding
text, we have probably not encountered the specific cooling neurons which are probably driven by "cold responsive C fibers" with a maximum sensitivity in the innocuous range (27-17°C).
Spino-PB and PB neurons: functional considerations
Comparison of the physiological properties of spino-PB and PB
neurons is a relevant question to our further understanding of the
manner in which noxious information is processed along the dense
pathways relaying in the PB area and terminating in the Ce, the VMH,
and the bed nucleus of the stria terminalis (Aldén et al.
1994; Bernard et al. 1993
, 1995
; Bester
et al. 1997a
; Feil and Herbert 1995
;
Slugg and Light 1994
). Anatomic studies emphasized that
spinal lamina I neurons send a dense projection to the PB area
(Bernard et al. 1995
; Craig 1995
;
Feil and Herbert 1995
; Slugg and Light
1994
). Consequently, it is not surprising that the properties
of spino-PB neurons were in many respects very similar to those already
observed in the target PB area (Bernard and Besson 1990
;
Bernard et al. 1994
; Bester et al. 1995b
;
Huang et al. 1993
; Matsumoto et al. 1996
;
Menendez et al. 1996
). Thus at both levels of the
spino-PB pathway, neurons had a very low spontaneous activity,
responded only, or preferentially to noxious stimuli, encoded the
stimuli intensities in the noxious range, and were activated by A
and C fibers at the periphery. At both levels, neurons had similar
thermal thresholds: 43.6°C for the spino-PB neurons, and 44-44.3°C
for PB neurons (Bernard and Besson 1990
; Bester
et al. 1995b
; Matsumoto et al. 1996
). Similarly, the mean stimulus/response curve has a maximal slope between 46 and
50°C for both the spino-PB and the PB neurons (Bernard and Besson 1990
; Bernard et al. 1994
; Bester
et al. 1995b
; Matsumoto et al. 1996
). In
addition, the t50 were similar:
47.4°C for the spino-PB neurons, and 47.5-47.7°C for PB neurons
(Bester et al. 1995b
; Matsumoto et al.
1996
). The similar encoding properties to heat stimuli at both
levels of the spino-PB pathway also has been demonstrated by studies of
the expression of the c-Fos protein (Bester et al.
1997b
). Similarly, the mean mechanical thresholds at the spinal
and PB levels were close, 11.5 and 15.8 N · cm
2, respectively (Matsumoto et al.
1996
), with a comparable encoding of mechanical noxious stimuli
over the 10-60 N · cm
2 range.
In contrast to this similarity in neuronal response profiles, the
receptive fields of spino-PB neurons were very small cutaneous areas,
whereas PB neurons had rather large receptive fields, often including
some parts of the body, and in some cases the body in its whole
(Bernard and Besson 1990; Bernard et al.
1994
; Bester et al. 1995b
; Huang et al.
1993
; Matsumoto et al. 1996
). This observation
suggests a huge convergence of noxious information arising from lamina
I neurons all along the neuro-axis toward the PB neurons. Considering
that some PB neurons have a complex receptive field
the simultaneous
stimulation of two different parts of the body can diminish the
response evoked from only one these two parts (Bernard et al.
1994
; Bester et al. 1995b
; Huang et al.
1993
)
one can assume that this characteristic of these PB
neurons could be linked, at least in part, to the inhibition of
spino-PB neurons by heterotopic noxious stimuli (DNIC phenomenon).
The latter considerations suggest that the PB area is a region for
integration of noxious information. Interestingly, a substantial number
of PB neurons are modality specific. Thus for example 78% of the
PB-VMH neurons responded exclusively or very preferentially either to
mechanical or to heat noxious stimuli (Bester et al. 1995b) and about half of the PB cold neurons were excited only by cold (Menendez et al. 1996
). In contrast, a huge
majority of spino-PB neurons responded to at least two modalities of
noxious stimulation (mechanical and/or heat and/or cold) with a less
marked preference for one modality and only very few (8%) of them
responded specifically to noxious heat. It appears clearly that the
nociceptive region of the PB area can be considered as a center where
noxious information originating in lamina I is extracted,
modality-wise, and where further links with autonomic controls are engaged.
In conclusion, considering all the similarities between spino-PB
and PB neurons, it is suggested that the nociceptive neurons in the PB
area behave more or less like an extension of lamina I neurons of the
spinal cord. However, the remaining discrepancies can reflect the fact
that noxious information is not only transmitted along that pathway,
but also processed, integrated, and modified before reaching higher
centers. In the rat, lamina I neurons send their densest projections to
the contralateral PB area (Bernard et al. 1995;
Craig 1995
; Feil and Herbert 1995
;
Slugg and Light 1994
) and in turn the PB area further
projects very densely to the amygdala (Bernard et al.
1993
), hypothalamus (Bester et al. 1997a
), and
bed nucleus of the stria terminalis (Aldén et al. 1994
). It has been shown that PB-Ce and PB-VMH neurons are NS with large receptive fields (Bernard and Besson 1990
;
Bester et al. 1995b
), and their general physiological
properties resemble those of the lamina I spino-PB neurons. Taking into
account both the roles of the brain targets (amygdala and hypothalamus)
and the sizes of the receptive fields of the PB neurons, these pathways relaying in the PB area are unlikely to be involved in the sensory discriminative aspects of pain that usually are thought to be dependent
on the classically studied spino-thalamic tract (see references in
Besson and Chaouch 1987
). In contrast, according to
earlier studies (Bernard and Besson 1990
; Bernard
et al. 1994
; Bester et al. 1995b
, 1997a
,b
;
Huang et al. 1993
; Matsumoto et al. 1996
;
Menendez et al. 1996
), it is suggested that the
nociceptive spino-PB pathway, which further relays noxious inputs to
the amygdala, hypothalamus and bed nucleus of the stria terminalis,
would be a major entry point for the integration of the autonomic and
emotional/aversive aspects of pain.
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ACKNOWLEDGMENTS |
---|
We thank J. Martin for technical support and R. Rambur for photographs.
This work was supported by the Institut National de la Santé et de la Recherche Médicale and by the Institut UPSA de la Douleur.
Present address of V. Chapman: School of Biomedical Sciences, E Floor Medical School, Queen's Medical Centre, University of Nottingham, Nottingham NG7 2UH, UK.
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FOOTNOTES |
---|
Address for reprint requests: H. Bester, INSERM U-161, 2 rue d'Alésia, F-75014 Paris, France.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 29 January 1999; accepted in final form 21 December 1999.
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REFERENCES |
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