Cellular Mechanisms Underlying Intrathalamic Augmenting Responses of Reticular and Relay Neurons
Igor Timofeev and
Mircea Steriade
Laboratoire de Neurophysiologie, Faculté de Médecine, Université Laval, Quebec G1K 7P4, Canada
 |
ABSTRACT |
Timofeev, Igor and Mircea Steriade. Cellular mechanisms underlying intrathalamic augmenting responses of reticular and relay neurons. J. Neurophysiol. 79: 2716-2729, 1998. Augmenting (or incremental) responses are progressively growing potentials elicited by 5- to 15-Hz stimulation within the thalamus, cerebral cortex, or by setting into action reciprocal thalamocortical neuronal loops. These responses are associated with short-term plasticity processes in thalamic and cortical neurons. In the present study, in vivo intracellular recordings of thalamic reticular (RE) and thalamocortical (TC), as well as dual intracellular recordings, were used to explore the mechanisms of two types of intrathalamic augmenting responses elicited by thalamic stimuli at 10 Hz in decorticated cats. As recently described, after decortication, TC cells display incremental burst responses to thalamic stimuli that occur through either progressive depolarization (high threshold, HT) or progressive hyperpolarization leading to deinactivation of low-threshold (LT) spike bursts. Here, low-intensity stimuli (10 Hz) to dorsal thalamic nuclei elicited decremental responses in GABAergic RE cells, consisting of a progressive diminution in the number of action potentials in successive spike bursts, whereas higher stimulation (>50% of maximal strength) induced augmentation characterized by an increased number of spikes in repetitive responses. These opposing discharge patterns occurred in the absence of changes in the membrane potential of RE cells. In TC cells, augmentation depended on the thalamic site where testing volleys were applied. With stimuli applied closer to the site of impalement, augmenting resulted from a transformation from LT spike bursts into HT responses. Augmenting responses were followed by self-sustained oscillatory activity, within the frequency of spindles (7-14 Hz) or clock-like delta oscillation (1-4 Hz). As LT augmentation in TC cells results from their progressive hyperpolarization, we tested the effects exerted by the activating depolarizing system arising in the mesopontine cholinergic nuclei and found that such conditioning pulse-trains prevented the hyperpolarizing-rebound sequences as well as the LT augmenting in TC cells. We propose that the depolarization-dependent (HT) augmenting responses in TC cells result from decremental responses in RE neurons that are due to intra-RE inhibitory processes leading to disinhibition in target TC neurons, whereas LT-type augmenting in TC cells is produced mainly by incremental responses in GABAergic RE neurons.
 |
INTRODUCTION |
Augmenting (or incremental) responses initially have been described as progressively growing potentials elicited in the cerebral cortex by thalamic stimuli within the frequency range of sleep spindles, 5-15 Hz (Morison and Dempsey 1942
, 1943
). Because augmenting cortical responses also have been obtained by white matter or callosal stimulation after lesions of appropriate thalamic nuclei (Ferster and Lindström 1985
; Morin and Steriade 1981
) or extensive thalamectomy (Nuñez et al. 1993
; Steriade et al. 1993
), it generally was assumed that the cortex could sustain incremental potentials in the absence of the thalamus. The role played by intrinsic properties and synaptic interconnections of layer V pyramidal neurons in the initiation of augmenting responses was investigated in rat motor cortex (Castro-Alamancos and Connors 1996a
,b
). However, the thalamus can itself generate frequency-dependent augmenting potentials in decorticated animals (Steriade and Timofeev 1997
). In that study, two types of augmentation were described in thalamocortical (TC) neurons from the ventral lateral (VL) nucleus. One type was based on progressively increased low-threshold spikes (LTSs) deinactivated by the hyperpolarization produced by preceding stimuli in the pulse train at 10 Hz, whereas the other type of incremental responses was associated with decreased inhibitory postsynaptic potentials (IPSPs) during the rhythmic repetition of thalamic stimuli, occurred at relatively depolarized levels (more positive than
55 to
50 mV), and thus was termed high-threshold (HT) augmenting. We proposed that the intrathalamic augmenting responses may be considered a form of short-term plasticity as repeated sequences of stimuli ~10 Hz produced a persistent and prolonged decrease in IPSPs' amplitudes as well as a persistent increase in the depolarizing responses of TC cells during HT augmenting (Steriade and Timofeev 1997
).
The present study was undertaken to reveal the mechanisms of augmenting responses in different dorsal thalamic nuclei and, in particular, to explore the possible role played by GABAergic thalamic reticular (RE) neurons in the hyperpolarization- and depolarization-dependent incremental responses of TC neurons. We hypothesized that differential response patterns of RE neurons may determine the two types of augmenting responses in TC cells because augmenting potentials initially were investigated to mimic spontaneously occurring spindle oscillations (Morison andDempsey 1943) and RE neurons play a pacemaking role in this sleep rhythm (see Steriade et al. 1997
). To this end, we performed intracellular, single-unit extracellular, and field potential recordings from RE, VL, lateral posterior (LP), and intralaminar central lateral (CL) nuclei, sometimes using dual simultaneous impalements of TC cells or RE and TC cells, in ipsilaterally decorticated cats. We report differential responses in various thalamic nuclei, as a function of the stimulated site, and we propose that the hyperpolarization-dependent augmenting responses of TC cells depend on incremental responses in GABAergic RE cells, whereas the depolarization-dependent augmentation in TC cells is due to intra-RE inhibitory processes with the consequence of disinhibition in target TC cells. Some of these results have been presented in abstract form (Steriade and Timofeev 1996
).
 |
METHODS |
Preparation
Adult cats (n = 33) were anesthetized with ketamine and xylazine (10-15 mg/kg and 2-3 mg/kg im). Seven of those 33 cats also were used for a previous paper by Steriade and Timofeev 1997
). Tissues to be excised and pressure points were infiltrated with lidocaine (2%). When the electroencephalogram (EEG) showed low-frequency, high-amplitude oscillations as in resting sleep, the animals were paralyzed with gallamine thiethiodide and artificially ventilated by maintaining the end-tidal CO2 concentration at 3.5-3.8%. The EEG was monitored continuously and additional doses of anesthetics were administered at the slightest tendency toward an activated pattern. The heartbeat was monitored and body temperature was maintained at 37-39°. The cortex was removed by suction on the right side (Fig. 1A). Recordings began 30-60 min after decortication. At the end of experiments, animals were given a lethal dose of pentobarbital sodium and perfused intracardially for histological control.

View larger version (56K):
[in this window]
[in a new window]
| FIG. 1.
Experimental design. A: right hemidecortication. Ca, caudate nucleus; Cbl, cerebellum; IC, inferior colliculus; SC, superior colliculus; Th, thalamus. B: diagram showing the array of stimulating electrodes in right ventral lateral (VL) nucleus, anterior and posterior parts of central lateral nucleus (CLa, CLp), and lateral posterior (LP) nucleus and the recording intracellular micropipettes in rostral part of right reticular (RE) nucleus as well as VL and LP nuclei. C: electrophysiological identification of rostral RE cell by response to VL stimulus: a, antidromic response; o, orthodromic response. D, dual intracellular recordings showing opposite polarities ( ) of RE and VL neurons during spindle oscillation (RE-cell's spikes truncated). Inhibitory postsynaptic potentials (IPSPs) in VL neuron are indicated ( ). Average (AVG, n = 10) illustrates IPSPs in VL cells during depolarizing components of spindles in RE cell. Onset of IPSPs in TC cell was taken as time zero for averages. Small deflections in VL recording are due to capacitive coupling from RE-cell's action potentials.
|
|
Recording and stimulation
Intracellular recordings of RE, VL, and LP neurons were performed with micropipettes filled with a solution of 3 M potassium acetate (DC resistances of 30-60 M
). Recordings of RE neurons have been performed from the rostrolateral part of this nuclear complex. The stability of intracellular recordings was ensured by cisternal drainage, hip suspension, and bilateral pneuomothorax and by covering the decorticated hemisphere with a warm agar solution (4% in 1% saline). A high-impedance amplifier with active bridge circuitry was used to record from and inject current into neurons. Single units also were recorded extracellularly. Thalamic field potentials were recorded through bipolar (coaxial) electrodes, also used for stimulation (Fig. 1B). The signals were recorded on an eight-channel tape with a band-pass of 0-9 kHz, digitized at 20 kHz for off-line computer analysis.
Stimuli (0.05-0.2 ms, 0.01-0.3 mA) were delivered by means of coaxial electrodes inserted in the VL, anterior and posterior CL (CLa and CLp) and LP nuclei (Fig. 1B) and the pedunculopontine tegmental (PPT) nucleus. The VL and CL nuclei were stimulated because these are the major nuclei that are interconnected with the rostrolateral district of RE nuclear complex (Steriade et al. 1984
) where intracellular and extracellular recordings were performed. Besides, the LP also was stimulated, and intracellular recordings were made from that nucleus, to determine the types of augmenting responses as a function of the closeness of the stimulation site (VL- as compared with LP-evoked responses in VL and LP cells; see Fig. 9).

View larger version (22K):
[in this window]
[in a new window]
| FIG. 9.
Dual, simultaneous intracellular recordings from VL and LP neurons show variable augmenting responses to different thalamic sites. Note most pronounced incremental depolarizing responses to proximal stimuli (VL stimulation in the case of VL cell, LP stimulation in the case of LP cell, and intermediate responses to CLa CLp stimulation).
|
|
 |
RESULTS |
Database and neuronal identification
The results are based on intracellular recordings from 92 RE cells and 224 TC cells (144 from VL nucleus and 80 from LP nucleus). Of those, we performed dual simultaneous intracellular recordings from RE and TC cells (n = 6) and from VL and LP cells (n = 8). Neurons retained for analyses displayed stable activities for
20 min and up to 120 min, with a resting membrane potential (Vm) more negative than
55 mV and overshooting action potentials. In addition, extracellular single-unit and field potential recordings were made from 87 foci within the RE, VL, CL, and LP nuclei.

View larger version (54K):
[in this window]
[in a new window]
| FIG. 2.
VL-evoked augmenting responses in intracellularly recorded rostral RE neuron. Augmenting responses to pulse train (5 stimuli) at 10 Hz. Part marked by horizontal bar is expanded below (arrow) and response to 5th stimulus in the pulse train is further expanded at right (oblique arrow). Note accelerando-decelerando pattern of spike burst, typical for RE cell.
|
|

View larger version (44K):
[in this window]
[in a new window]
| FIG. 3.
Intensity-dependency of decremental and incremental intracellular responses of rostral RE neuron to repetitive (10 Hz) VL stimuli. Left: lower speed; responses to all 5 stimuli in a pulse train at 10 Hz. Middle and right: higher speed; responses to 1st and 2nd and 4th and 5th stimuli, respectively. Decremental responses were observed with stimuli at 15-30% of maximal intensity. Incremental responses occurred at high intensities (70-100%). At half intensity (50%) there was virtually no change in the number of spikes evoked by the 5 stimuli in the pulse train.
|
|
Intracellularly recorded RE neurons were identified by 1) antidromic and monosynaptic responses to dorsal thalamic stimuli (Fig. 1C); 2) depolarizing spindle oscillations that were concomitant to spindle-related IPSPs in TC cells (Fig. 1D); and 3) prolonged spike bursts with accelerando-decelerando patterns (expanded insets in Figs. 2 and 3) that are typical for RE neurons during the natural state of resting sleep (Domich et al. 1986
; Steriade et al. 1986
). The average Vm in RE neurons was
64.3 ± 2.3 (SE) mV and the apparent input resistance (Rin) measured by short hyperpolarizing current pulses was 37.2 ± 2.1 M
. The TC neurons were recognized from rhythmic IPSPs during spindle oscillations, short rebound spike bursts, with progressively diminished interspike intervals, and clock-like delta oscillation (see Figs. 6 and 7), resulting from the interplay between It and Ih at hyperpolarized levels (McCormick and Pape 1990
; Soltesz et al. 1991
). The Vm of TC cells was
62.1 ± 1.3 mV, and their Rin was 22.2 ± 0.8 M
.

View larger version (27K):
[in this window]
[in a new window]
| FIG. 6.
Transformation, in LP neuron, of augmenting from low-threshold (LT) responses deinactivated by hyperpolarization to high-threshold (HT) responses activated at a depolarized level. Stimulation at 10 Hz within the LP nucleus. Top: note diminution up to disappearance of self-sustained clock-like delta oscillation (2 Hz), that followed augmenting responses, by diminishing the stimulation intensity from maximum (100%) to 20% of maximal intensity. Stimuli at 80% intensity (bottom, arrow) evoked an LT augmented response to the 2nd shock and, thereafter, progressively enhanced HT augmented responses [oblique arrow points to fast ripples that will develop to bursts (200-250 Hz) of full-blown action potentials with further depolarization].
|
|

View larger version (25K):
[in this window]
[in a new window]
| FIG. 7.
Variations in amplitudes of LP-cell's augmenting responses by changing the thalamic site of stimulation (CLp, CLa and VL). Top: effects of 5 hyperpolarizing pulses (20 ms, 1 nA) at the same frequency as thalamic stimuli (10 Hz), followed by pulse trains consisting of 5 stimuli at 10 Hz applied to LP, CLp, CLa, and VL nuclei. Responses are expanded (bottom, spikes in CLp sequence are truncated). Note alternate LT responses to hyperpolarizing pulses (1 response to every 2 current pulses) and augmenting responses to stimulation of thalamic sites (LP and CLp) located in the vicinity of the recorded LP neuron but lack of overt augmentation in response to more distant (VL) stimuli.
|
|
The hemidecortication was total (see Fig. 1A) with the exception of ventrally located (prepiriform and periamygdaloid) areas that do not project to RE and dorsal thalamic nuclei investigated here (Witter et al. 1989).
Augmenting responses in RE neurons
At high intensities used in this study (generally 0.3 to 0.4 mA), repetitive stimuli (5 shocks at 10 Hz) applied to VL nucleus evoked augmenting responses in rostrolateral RE cells. Augmentation was characterized by an increased number of action potentials within the spike burst, from the response evoked by the first stimulus in the pulse train (range, 4-10 spikes; mean ± SE, 6.3 ± 3.3) to the response evoked by the fifth stimulus (6-13 spikes; 8.7 ± 4.4). This difference was significant (P < 0.05) and is illustrated in Figs. 2 and 3. However, progressive diminution of stimulation intensities led to transformation of augmenting into decremental responses. This change invariably was seen in all RE cells (n = 16) that were tested with 5-10 different intensities, from maximal ones down to 10-20% of the highest strengths. A typical example is illustrated in Fig. 3 showing augmentation at 70-100% intensity (after the invariant antidromic spike, 6-7 spikes evoked by the 1st stimulus and 9-11 spikes evoked by the 5th stimulus); no change in the number of spikes at midintensity (50%); and progressive diminution in spike number evoked by successive stimuli in the pulse train, by decreasing the stimulation intensity from 30 to 15%. These opposing response patterns occurred in the absence of changes in Vm, around
68 mV.
In a few number of RE neurons (n = 7), the responses of RE neurons to repetitive dorsal thalamic stimuli occurred in association with a selective increase in amplitude of a secondary depolarizing response. This was apparent even at low intensities that produced a decreased number of action potentials toward the end of pulse trains (Fig. 4).

View larger version (49K):
[in this window]
[in a new window]
| FIG. 4.
Two depolarizing components of rostral RE-cell's responses to rhythmic (10 Hz) VL stimuli. Left, top to bottom: pulse trains of 5 stimuli, from 20 to 50% of maximal intensity. Right: 6 superimposed responses (spikes truncated) to 1st, 2nd, and 5th stimuli in the pulse trains (with intensities shown at left, plus an additional intensity at 40%). Note that 1st stimulus in the train elicited a single depolarizing response, whereas from the 2nd stimulus to the end of the pulse train, the responses consisted of two depolarizing components (a and b) with latencies of 6-7 and 16 ms, respectively. Bottom traces in the superimpositions of responses to 2nd and 5th stimuli, without action potentials, belong to responses elicited by the pulse train at 20% intensity (see top trace on left).
|
|
Self-sustained oscillations within the frequency range of sleep spindles followed the responses of RE neurons to either a single testing stimulus or a pulse train at 10 Hz. The duration of the self-sustained oscillation decreased, and the frequency increased, with increasing number of stimuli in the pulse trains. In a sample of 12 RE neurons, the self-sustained oscillation lasted for 1.5-3.3 s (2.3 ± 0.6 s) after one stimulus, but it lasted for 0.5-1.5 s (0.9 ± 0.3) after five stimuli at 10 Hz. This difference was significant (P < 0.0001) and is illustrated in Fig. 5. In Fig. 5A, the self-sustained oscillatory activity of an RE cell lasted for ~2 s, whereas it lasted for ~1 s when it followed the augmenting response to a pulse train at 10 Hz. The other RE neuron (Fig. 5B) displayed a self-sustained spindle-like sequence after the response to one stimulus, but the oscillatory was strikingly diminished by progressively increasing the number of stimuli within repetitive pulse trains. The reduction in poststimulus oscillatory activity by increasing the number of stimuli in the testing pulse train also was reflected by a diminished frequency and/or reduced numbers of action potentials within spike bursts.

View larger version (40K):
[in this window]
[in a new window]
| FIG. 5.
Self-sustained spindle-like oscillation, which follows evoked responses in rostral RE neuron, is diminished by increasing the number of repetitive (10 Hz) thalamic VL stimuli. A and B: 2 different rostral RE cells. In both neurons, VL pulse trains elicited augmenting responses (compare, in the expanded inset, the response to 1st stimulus consisting of spike burst with 7 action potentials, with the spike burst to 5th stimulus with 13 spikes). However, in contrast with the progressively increased augmenting responses, the self-sustained oscillatory activity within the lower frequency range of spindles (5-8 Hz), that followed the VL-evoked augmenting responses, progressively diminished in duration.
|
|
Differential augmenting responses in TC cells as a function of stimulation sites
With repetitive stimuli (10 Hz) that were applied relatively close to the recording site, within the same nucleus or to adjacent dorsal thalamic nuclei, the intrathalamic augmentation generally developed from LT to HT incremental responses in TC cells. This means that the second (0.1-s delayed) stimulus in the train fell during the late part of the biphasic hyperpolarizing IPSP elicited by the first stimulus and, thus, produced a rebound LTS crowned by an action potential or a spike burst; and subsequent stimuli found the neuron more depolarized than
55 mV and triggered spike bursts with inactivation of action potentials (Fig. 6). As LTS is largely inactivated at Vms more positive than
55 mV (Deschênes et al. 1984
; Jahnsen and Llinás 1984
), the spike bursts evoked by the last stimuli in the pulse train, at relatively depolarized levels, are termed HT bursts.
At relatively high intensities (>50% of maximum strength), the evoked augmenting responses were followed by self-sustained, clock-like potentials within the frequency range of 2-4 Hz (Fig. 6), representing the delta sleep oscillation of TC cells (Steriade et al. 1991
) that results from an interplay between two intrinsic currents (see earlier text). To shed light on the participation of intrinsic versus synaptic actions implicated in the generation of intrathalamic augmenting responses, we compared the effects of short (20 ms) intracellular current pulses at 10 Hz with those of thalamic stimuli at the same frequency (n = 7). Figure 7 shows that hyperpolarizing current pulses deinactivated rebound LTSs, crowned by full-blown spike bursts, that recurred after the first and fourth pulses, whereas stimulation of synaptic pathways elicited typical augmenting responses. Comparison between the effects of different stimulation sites on responses of an LP neuron revealed typical LT-to-HT augmentation when stimulating close to the impaled cell within the LP nucleus, purely LT responses when stimuli were applied to the adjacent CLp site, and barely visible augmenting with stimuli applied 3 mm apart, to the VL nucleus (Fig. 7). Similar differences in the amplitude of augmenting responses were observed when impaling VL cells and stimulating closely located or more distant thalamic nuclei (Fig. 8). Indeed, clear-cut augmenting, leading to rebound spike bursts, resulted by stimulating a rostral site in intralaminar nuclei (CLa), followed by augmenting without spike bursts by stimulating intralaminar nuclei more posteriorly (CLp), and absence of augmentation, despite a hyperpolarizing shift, by LP stimulation. The area of depolarization during augmenting responses was measured by mV × ms from the resting Vm, thus including all depolarizing events above the baseline (see Fig. 8, bottom, dotted line). This area increased by >300-400% from the first to the fifth stimulus in the train when stimulating intranuclearly or the adjacent CLp site, whereas there was no visible increase or even a slight decrease in area of depolarization during repetitive responses elicited by stimulating the more distant VL site (graph in Fig. 8).

View larger version (25K):
[in this window]
[in a new window]
| FIG. 8.
Increasing amplitudes of augmenting responses in VL neuron depending on the proximity of various stimulated thalamic sites (CLa, CLp, and LP). Simultaneous recordings of intracellular and field potential activities in VL nucleus. The intracellular responses to 5 stimuli at 10 Hz are expanded (bottom). Graph depicts the depolarization area of the augmented responses (ordinate) to each stimulus in the 5-shock pulse train (abscissa) applied to CLa, CLp, and LP.
|
|
Dual simultaneous intracellular recordings from VL and LP neurons (n = 8) confirmed the above findings, namely, that stimuli applied to the same nucleus or adjacent nuclei induced more marked augmentation than that elicited by stimulating distant sites. Thus, with recordings from LP neurons, the LT-to-HT augmentation was obtained by stimulating within the LP nucleus, whereas only LT augmenting was elicited from CLa or CLp stimuli, and no augmenting was elicited from VL nucleus; with simultaneous intracellular recordings from VL neurons, the most marked augmentation was elicited by stimulating within the VL nucleus and much less obvious augmentation was elicited from more distant stimulation sites (Fig. 9).
Augmenting responses were followed by self-sustained oscillatory activity, consisting of either clock-like LTSs within the delta frequency range, i.e., 1-4 Hz (see earlier text, Figs. 6-7) or spindles at 7-14 Hz (Fig. 10). Simultaneous intra- and extracellular recordings from different thalamic nuclei revealed propagation patterns of spontaneously occurring or stimulus-induced spindles that outlasted augmenting responses (n = 9). Although the temporal patterns of spindle propagation varied in different animals, as a function of recorded sites, they were consistent in the same animal with multisite recordings from the same RE and/or dorsal thalamic sites. Figure 10 is an example of such propagation patterns of both spontaneously occurring and stimulus-induced spindles after augmenting responses. In this case, VL stimulation evoked LT augmenting responses in an intracellularly recorded LP neuron, which gave rise to a sequence of spindle oscillation in the same neuron, thereafter followed by a spindle sequence in field potentials recorded from CLp and, finally, in an extracellularly recorded RE unit (Fig. 10A). These complex temporal events, outlasting evoked augmenting responses, were verified in spontaneously occurring spindles showing the same succession in spindle propagation (Fig. 10B). Although the intracellularly recorded LP neuron sometime failed to display spindles, the spike-triggered average (using RE-cell's discharges and local field potentials from CLp) clearly demonstrated the precession of RE firing by CLp oscillatory activity.

View larger version (28K):
[in this window]
[in a new window]
| FIG. 10.
Intrathalamic propagation of rhythmic responses leading to spindle oscillation. Simultaneous extracellular unit recording from lateral part of the RE nucleus (~2 mm rostral to LP nucleus), local field potential (LFP) activity from rostral part of CL nucleus, and intracellular activity of LP cell. A: stimulation (5 shocks at 10 Hz) of VL nucleus (horizontal bar) elicited augmenting responses in LP neuron leading to self-sustained spindles, followed by CL spindles, and finally by spindle-related spike bursts in RE cell. LP-evoked augmenting response of LP cell is expanded (right, , spikes truncated). B: spontaneous activity; same arrangement of neurons and field potentials as in the top panel. Intrathalamic propagation path suggested by the LP-evoked potentials in A is supported by analysis of spontaneously occurring spindles. Two spindle sequences are illustrated showing the LP-CL-RE temporal evolution of oscillatory activity. Whereas the LP-cell's spindles were not always seen in intracellular recording, the CL-to-RE relation was constant and is shown by spike-triggered average in which time 0 is the first spike in RE-cell's spindle-related spike bursts.
|
|
Diminished LT augmenting responses in TC cells by brain stem-thalamic depolarizing impulses
As LT augmenting develops from progressive hyperpolarization in TC cells, which deinactivates rebound spike bursts, we attempted to modulate these incremental responses by activating thalamic networks through ascending brain stem volleys. It is known that stimulation of mesopontine cholinergic nuclei induces prolonged depolarization of TC cells recorded from different dorsal thalamic nuclei and transforms their bursting firing into tonic discharges, thus leading to activation of cortical electrical activity (Curró Dossi et al. 1991
; Hu et al. 1989
; Steriade and Deschênes 1984
, 1988
). Therefore, it was expected that LT augmenting responses evolving with hyperpolarization would be diminished as consequence of PPT stimulation. Dual intracellular recordings of TC cells demonstrated that this is indeed the case (n = 6). Figure 11 shows the CLp-evoked augmenting responses in simultaneously recorded LP and VL neurons. Augmentation was of LT type, with increasing hyperpolarizations and alternating postinhibitory rebound spike bursts in LP cell; and similar responses, though with lower amplitudes, in VL cell. A conditioning pulse train (0.15 s, 300 Hz) to the PPT prevented the CLp-induced hyperpolarizing-rebound sequences, thus blocking augmentation. The LT-type augmentation recovered in control conditions, a few seconds after PPT stimulation.

View larger version (25K):
[in this window]
[in a new window]
| FIG. 11.
Stimulation of pedunculopontine tegmental (PPT) nucleus blocks augmenting responses in TC cells. Dual intracellular recording from LP and VL neurons. In control conditions (before and after PPT stimulation), stimulation of CLp (5 shocks at 10 Hz) elicited clear-cut augmenting responses in LP cell, developing from LT rebounds and followed by a self-sustained rebound cycle, whereas immediately after a pulse train to PPT (300 Hz, lasting for 150 ms), incremental LTSs de-inactivated by hyperpolarization were blocked because of the PPT-elicited reduction in hyperpolarizing responses. Three epochs depicted in the top traces (control before the PPT pulse train, effects of PPT stimulation on augmentation, and recovery to control values after the PPT pulse train) are superimposed and expanded (bottom). Arrow head, LT responses (developing from excitatory postsynaptic potentials) evoked in LP cell by the 3rd and 5th stimuli in the train. Full-blown spike-bursts crowning LTSs are expanded at right. CLp stimulation evoked less obvious augmentation in VL neuron.
|
|
 |
DISCUSSION |
The main results are that on repetitive (10 Hz) stimulation of the dorsal thalamus, RE neurons display two types of responses, decremental and incremental, which depended on stimulation intensity; although the incremental responses of TC cells are more pronounced when testing stimuli are applied within the nucleus where recordings are made, clear-cut augmentation in TC cells also occurs with stimuli delivered to other dorsal thalamic nuclei; and LT augmenting, developing from postinhibitory rebound bursts, is reduced when conditioning pulse trains are applied to brain stem-thalamic activating systems.
Decremental and incremental responses of RE cells determine two types of augmentation in TC cells
Low-intensity (generally <50% of maximal intensity) repetitive volleys to the dorsal thalamus elicited decremental responses in RE cells, i.e., a progressive decrease in the number of action potentials within spike bursts evoked by successive stimuli (Fig. 3). This is ascribed to intra-RE inhibitory processes within cat and ferret RE nucleus, through both dendrodendritic contacts and recurrent axonal collaterals (Deschênes et al. 1985
; Liu et al. 1995
; Sanchez-Vives and McCormick 1997
; Yen et al. 1985
).
-aminobutyric acid-A (GABAA)- and GABAB-mediated IPSPs are produced by intra-RE interactions (Bal et al. 1995
; Destexhe et al. 1994
; Huguenard and Prince 1994a
; Ulrich and Huguenard 1996
). Glutamate-induced excitation of local regions in the perigeniculate part of the RE nuclear complex in ferret slices produces GABAA as well as, in a subset of neurons, GABAB-mediated IPSPs that may reduce or abolish LT spike bursts and tonic discharges (Sanchez-Vives et al. 1997
; Ulrich and Huguenard 1997
). In view of these data, Sanchez-Vives et al. (1997)
suggested that activation of a neuronal pool within the interactive GABAergic neurons would result in the inhibition of neurons surrounding the activated focus, with the consequence that a pattern of inhibition surrounded by disinhibition would appear in the related relay nucleus of the dorsal thalamus. This is in line with a model of intra-RE interactions influencing operations in relay nuclei of cat (see Fig. 14 in Steriade 1991
), a species in which local-circuit GABAergic neurons intrinsic to dorsal thalamic nuclei may, however, change the sign of RE-to-TC synaptic actions (see further text).
Thus, especially when IPSPs in RE neurons are associated with large conductance increases, these IPSPs may shunt excitatory responses evoked by successive stimuli in the pulse train at 10 Hz, especially if inputs are relatively weak. Another, nonexclusive factor underlying the depression phenomenon calls on a presynaptic mechanism. In neocortical neurons, repetitive activation of excitatory synapses may evoke not only facilitation but also depression (Deisz and Prince 1989
; Fleidervish and Gutnick 1995
; Thomson 1997
; Thomson et al. 1993
), especially at interstimulus intervals >50 ms.
For higher intensity of repetitive stimuli to the dorsal thalamus, target RE cells displayed incremental, instead of decremental, responses (see Fig. 3), as if an enhanced afferent excitatory drive was capable of overwhelming the intra-RE inhibitory processes observed at lower intensities. The GABAB-mediated presynaptic inhibition of GABA release in the RE nucleus (Ulrich and Huguenard 1996
) also may have the consequence of favoring the prevalence of excitatory signals from the dorsal thalamus at the expense of intra-RE inhibition.
We propose that these two response types (decremental and incremental) in RE cells are a major factor in determining the two modalities (HT and LT) of augmenting responses in target TC cells. The decremental responses in RE cells presumably produce a progressive release from inhibition in TC cells, with the consequence of developing HT responses occurring at a relatively depolarized level (Jahnsen and Llinás 1984
; Roy et al. 1984
; Steriade and Timofeev 1997
). The Ca2+-dependent high-voltage current (Hernández-Peon and Pape 1989) has at least four different components that have been dissociated by channel blocking agents (Kammermeier and Jones 1997
). On the contrary, the incremental responses in RE cells produce a progressive hyperpolarization in target TC cells, with the consequence of progressively deinactivating the LT conductance and increasing the postinhibitory rebound bursts. It is known that activation of RE neurons produces both GABAA- andGABAB-mediated IPSPs in TC cells (Huguenard and Prince 1994b
; Sanchez-Vives and McCormick 1997
; Thomson 1988
; Warren et al. 1994
). Of course, these relations in the simple recurrent inhibitory RE-TC circuit are simplified when GABAergic local-circuit neurons, intrinsic to cat and primate dorsal thalamic nuclei, are neglected. Although only 10% of RE axon terminals contact local interneurons (Liu et al. 1995
), their synaptic weight is not known and their actions ultimately may lead to disinhibition of TC cells. Indeed, after disconnection from inputs arising in the RE nucleus, TC cells display an increased number of Cl
-dependent IPSPs (Steriade et al. 1985
) as if local-circuit inhibitory neurons were released from RE-induced inhibition. The complication introduced by adding local interneurons in this circuit is generally not taken into consideration and should be worked out in further experimental and modeling studies.
It appears that the RE-TC interactions are essential for the production of intrathalamic augmenting responses in decorticated animals because repetitive stimuli applied to prethalamic (cerebellar or brain stem) pathways, that do not directly activate RE cells, fail to induce spindle oscillations (Steriade 1984
) as well as their experimental counterpart, incremental responses (see Fig. 3 in Timofeev et al. 1996
; also unpublished data). In modeling studies, stimulation of only TC cells, without concomitantly driving afferent axons to the RE nucleus, results in only weak or no intrathalamic augmenting, whereas clear-cut intrathalamic augmentation only results when both TC and RE neurons are stimulated (Bazhenov et al. 1997
, 1998
).
Two types of self-sustained oscillatory activities followed augmenting responses. In either RE or TC cells, spindle-like rhythmic potentials (7-14 Hz), lasting for 1.5-3 s, occurred after repetitive responses elicited by volleys within the spindle frequency range. The self-sustained activity diminished in RE neurons by increasing the number of rhythmic volleys (Fig. 5), again suggesting a summation of intra-RE inhibitory processes (see earlier text). As for the self-sustained spindle oscillations in TC cells, multisite recordings showed their successive appearance in various dorsal thalamic nuclei (Fig. 10), probably from activities in reciprocally related relay nuclei and appropriate sectors of the RE nuclear complex. The other type of self-sustained oscillation in TC cells was the clock-like delta rhythm. Although this activity can be generated intrinsically by the interplay of two hyperpolarization-activated currents (McCormick and Pape 1990
; Soltesz et al. 1991
), the occurrence of this oscillation after augmenting responses implicates the progressive activation of related RE neurons, with consequent hyperpolarization of TC cells, because synaptic volleys induced more powerful delta potentials outlasting the stimulation period than current pulses injected into TC cells (see pulses and LP stimuli in Fig. 7).
Augmenting responses in TC cells and their blockage by brain stem-thalamic activating systems
The fact that the closer the stimulation site, the more marked the augmenting response and the more obvious the depolarization leading to HT spike bursts (see Figs. 6-8) indicates that one of the main factors in producing the depolarization of TC cells is the afferent thalamic volley eliciting EPSPs in the target neuron. The fact that augmentation also appeared in distant TC cells, by stimulating thalamic nuclei other than those in which impalements were made (see Figs. 7-9), suggests that the thalamic inhibitory neurons playing the crucial role in the generation of hyperpolarization-rebound sequences of augmentation are RE neurons and much less or not at all local GABAergic cells in various thalamic nuclei. Modeling studies also point to the role played by RE neurons in this phenomenon even in the absence of TC cells, as isolated neuronal pools are capable of displaying augmentation if slightly depolarized, to enhance GABAB IPSPs (Bazhenov et al. 1998
).
The LT-type augmenting in TC cells is modulated by the brain stem-thalamic cholinergic system. Stimulation of mesopontine cholinergic nuclei elicits a muscarinic-mediated depolarization of TC cells, associated with increased input resistance (Curró Dossi et al. 1991
; Hu et al. 1989
), similar to the muscarinic action observed in thalamic slices (McCormick and Prince 1987
). The depolarization elicited by brain stem-thalamic activating systems reduces augmenting responses in TC cells or even prevents them from developing the LT increment (see Fig. 11). A similar suppressive action on augmenting responses was reported previously for more rostral stimulation at the level of the mesencephalic reticular formation (Steriade and Morin 1981
). The behavioral counterpart of brain stem core influences on augmenting responses is seen on awakening in chronically implanted cats (Steriade et al. 1969
) and during strong arousal in unrestrained rats (Castro-Alamancos and Connors 1996c
). All these data demonstrate that augmenting responses are dynamically modulated by behavioral states of vigilance.
 |
ACKNOWLEDGEMENTS |
We thank P. Giguère and D. Drolet for technical assistance.
This study was supported by the Medical Research Council of Canada and Human Frontier Science Program. I. Timofeev is a postdoctoral fellow partially supported by the Savoy Foundation.
 |
FOOTNOTES |
Address reprint requests to M. Steriade.
Received 8 October 1997; accepted in final form 23 December 1997.
 |
REFERENCES |
-
BAL, T.,
VON KROSIGK, M.,
MCCORMICK, D. A.
Role of the ferret perigeniculate nucleus in the generation of synchronized oscillations in vitro.
J. Physiol. (Lond.)
483: 665-685, 1995.[Abstract]
-
BAZHENOV, M.,
TIMOFEEV, I.,
STERIADE, M.,
SEJNOWSKI, T. J. A
computational model of intrathalamic augmenting responses.
Soc. Neurosci. Abstr.
23: 1306, 1997.
-
BAZHENOV, M.,
TIMOFEEV, I.,
STERIADE, M.,
SEJNOWSKI, T. J.
Cellular and network models for intrathalamic augmenting responses during 10-Hz stimulation.
J. Neurophysiol.
79: 2730-2748, 1998.[Abstract/Free Full Text]
-
CASTRO-ALAMANCOS, M. A.,
CONNORS, B. W.
Spatiotemporal properties of short-term plasticity in sensorimotor thalamocortical pathways of the rat.
J. Neurosci.
16: 2767-2779, 1996a.[Abstract]
-
CASTRO-ALAMANCOS, M. A.,
CONNORS, B. W.
Cellular mechanisms of the augmenting response: short-term plasticity in a thalamocortical pathway.
J. Neurosci.
16: 7742-7756, 1996b.[Abstract/Free Full Text]
-
CASTRO-ALAMANCOS, M. A.,
CONNORS, B. W.
Short-term plasticity of a thalamocortical pathway dynamically modulated by behavioral state.
Science
272: 274-277, 1996c.[Abstract]
-
CURRÓ DOSSI, R.,
PARÉ, D.,
STERIADE, M.
Short-lasting nicotinic and long-lasting muscarinic depolarizing responses of thalamocortical neurons to stimulation of mesopontine cholinergic nuclei.
J. Neurophysiol.
65: 393-406, 1991.[Abstract/Free Full Text]
-
DEISZ, R. A.,
PRINCE, D. A.
Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABAB receptor feed-back on GABA release.
J. Physiol. (Lond.)
412: 513-542, 1989.[Abstract]
-
DESCHÊNES, M.,
MADARIAGA, A.,
STERIADE, M.
Dendrodendritic synapses in the cat reticularis thalami nucleus: a structural basis of thalamic spindle synchronization.
Brain Res.
334: 165-168, 1985.[Medline]
-
DESCHÊNES, M.,
PARADIS, M.,
ROY, J. P.,
STERIADE, M.
Electrophysiology of neurons of lateral thalamic nuclei in cat: resting properties and burst discharges.
J. Neurophysiol.
51: 1196-1219, 1984.[Abstract/Free Full Text]
-
DESTEXHE, A.,
CONTRERAS, D.,
SEJNOWSKI, T. J.,
STERIADE, M. A
model of spindle rhythmicy in the isolated thalamic reticular nucleus.
J. Neurophysiol.
72: 803-818, 1994.[Abstract/Free Full Text]
-
DOMICH, L.,
OAKSON, G.,
STERIADE, M.
Thalamic burst patterns in the naturally sleeping cat: a comparison between cortically-projecting and reticularis neurones.
J. Physiol. (Lond.)
379: 429-450, 1986.[Abstract]
-
FERSTER, D.,
LINDSTRÖM, S.
Augmenting responses evoked in area 17 of the cat by intracortical axonal collaterals of cortico-geniculate cells.
J. Physiol. (Lond.)
367: 217-232, 1985.[Abstract]
-
FLEIDERVISH, I. A.,
GUTNICK, M. J.
Paired-pulse facilitation of IPSCs in slices of immature and mature mouse somatosensory cortex.
J. Neurophysiol.
73: 2591-2595, 1995.[Abstract/Free Full Text]
-
HERNANDÉZ-CRUZ, A.,
PAPE, H.-P.
Identification of two calcium currents in acutely dissociated neurons from the rat lateral geniculate nucleus.
J. Neurophysiol.
61: 1270-1283, 1989.[Abstract/Free Full Text]
-
HU, B.,
STERIADE, M.,
DESCHÊNES, M.
The effects of brainstem peribrachial stimulation on neurons of the lateral geniculate nucleus.
Neuroscience
31: 13-24, 1989.[Medline]
-
HUGUENARD, J. R.,
PRINCE, D. A.
Clonazepam suppresses GABAB-mediated inhibition in thalamic relay neurons through effects in nucleus reticularis.
J. Neurophysiol.
71: 2576-2581, 1994a.[Abstract/Free Full Text]
-
HUGUENARD, J. R.,
PRINCE, D. A.
Intrathalamic rhythmicity studied in vitro: nominal T current modulation causes robust anti-oscillatory effects.
J. Neurosci.
14: 5845-5502, 1994b.
-
JAHNSEN, H.,
LLINÁS, R.
Ionic basis for the electroresponsiveness and oscillatory properties of guinea-pig thalamic neurones in vitro.
J. Physiol. (Lond.)
349: 227-247, 1984.[Abstract]
-
KAMMERMEIER, P. J.,
JONES, S. W.
High-voltage-activated calcium currents in neurons acutely isolated from the ventrobasal nucleus of the rat thalamus.
J. Neurophysiol.
77: 465-475, 1997.[Abstract/Free Full Text]
-
LIU, X. B.,
WARREN, R. A.,
JONES, E. G.
Synaptic distribution of afferents from reticular nucleus to ventroposterior nucleus of cat thalamus.
J. Comp. Neurol.
352: 187-202, 1995.[Medline]
-
MCCORMICK, D. A.,
PAPE, H. C.
Properties of a hyperpolarization-activated cation current and its role in rhythmic oscillation in thalamic relay neurones.
J. Physiol. (Lond.)
431: 291-318, 1990.[Abstract]
-
MCCORMICK, D. A.,
PRINCE, D. A.
Actions of acetylcholine in the guinea pig and cat medial and lateral geniculate nuclei, in vitro.
J. Physiol. (Lond.)
392: 147-165, 1987.[Abstract]
-
MORIN, D.,
STERIADE, M.
Development from primay to augmenting responses in primary somatosensory cortex.
Brain Res.
205: 49-66, 1981.[Medline]
-
MORISON, R. S.,
DEMPSEY, E. W. A
study of thalamocortical relations.
Am. J. Physiol.
135: 281-292, 1942.
-
MORISON, R. S.,
DEMPSEY, E. W.
Mechanism of thalamocortical augmentation and repetition.
Am. J. Physiol.
138: 297-308, 1943.[Free Full Text]
-
NUÑEZ, A.,
AMZICA, F.,
STERIADE, M.
Electrophysiology of cat association cortical cells in vivo: intrinsic properties and synaptic responses.
J. Neurophysiol.
70: 418-430, 1993.[Abstract/Free Full Text]
-
ROY, J. P.,
CLERCQ, M.,
STERIADE, M.,
DESCHÊNES, M.
Electrophysiology of neurons of the lateral thalamic nuclei in cat: mechanisms of long-lasting hyperpolarizations.
J. Neurophysiol.
51: 1220-1235, 1984.[Abstract/Free Full Text]
-
SANCHEZ-VIVES, M. V.,
BAL, T.,
MCCORMICK, D. A.
Inhibitory interactions between perigeniculate GABAergic neurons.
J. Neurosci.
17: 8894-8908, 1997.[Abstract/Free Full Text]
-
SANCHEZ-VIVES, M. V.,
MCCORMICK, D. A.
Functional properteis of perigeniculate inhibition of dorsal lateral geniculate nucleus thalamocortical neurons in vitro.
J. Neurosci.
17: 8880-8893, 1997.[Abstract/Free Full Text]
-
SOLTESZ, I.,
LIGHTOWLER, S.,
LERESCHE, N.,
JASSIK-GERSCHENFELD, D.,
POLLARD, C. E.,
CRUNELLI, V.
Two inward currents and the transformation of low-frequency oscillations of rat and cat thalamocortical cells.
J. Physiol. (Lond.)
441: 175-197, 1991.[Abstract]
-
STERIADE, M.
The excitatory-inhibitory response sequence in thalamic and neocortical cells: state-related changes and regulatory systems.
In: Dynamic Aspects of Neocortical Function, edited by
G. M. Edelman,
W. E. Gall,
and W. M. Cowan
New York: Wiley, 1984, p. 107-157.
-
STERIADE, M.
Alertness, quiet sleep, dreaming.
In: Cerebral Cortex, edited by
A. Peters,
and E. G. Jones
New York: Plenum, 1991, vol. 9, p. 279-357.
-
STERIADE, M.,
CURRÓ DOSSI, R.,
NUÑEZ, A.
Network modulation of a slow intrinsic oscillation of cat thalamocortical neurons implicated in sleep delta waves: cortically induced synchronization and brainstem cholinergic suppression.
J. Neurosci.
11: 3200-3217, 1991.[Abstract]
-
STERIADE, M.,
DESCHÊNES, M.
The thalamus as a neuronal oscillator.
Brain Res. Rev.
8: 1063, 1984.
-
STERIADE, M. AND DESCHÊNES, M. Intrathalamic and brainstem-thalamic networks involved in resting and alert states. In: Cellular Thalamic Mechanisms, edited by M. Bentivoglio and R. Spreafico. Elsevier: Amsterdam, 1988, p. 37-62.
-
STERIADE, M.,
DESCHÊNES, M.,
DOMICH, L.,
MULLE, C.
Abolition of spindle oscillations in thalamic neurons disconnected from nucleus reticularis thalami.
J. Neurophysiol.
54: 1473-1497, 1985.[Abstract/Free Full Text]
-
STERIADE, M.,
DOMICH, L.,
OAKSON, G.
Reticularis thalami neurons revisited: activity changes during shifts in states of vigilance.
J. Neurosci.
6: 68-81, 1986.[Abstract]
-
STERIADE, M.,
IOSIF, G.,
APOSTOL, V.
Responsiveness of thalamic and cortical motor relays during arousal and various stages of sleep.
J. Neurophysiol.
32: 251-265, 1969.[Free Full Text]
-
STERIADE, M.,
JONES, E. G.,
MCCORMICK, D. A.
In: Thalamus. Organisation and Function. Oxford: Elsevier, 1997, vol. 1
-
STERIADE, M.,
MORIN, D.
Reticular influence on primary and augmenting responses in somatosensory cortex.
Brain Res.
205: 67-80, 1981.[Medline]
-
STERIADE, M.,
NUÑEZ, A.,
AMZICA, F.
Intracellular analysis of relations between the slow (<1 Hz) neocortical oscillation and other sleep rhythms of the electroencephalogram.
J. Neurosci.
13: 3266-3283, 1993.[Abstract]
-
STERIADE, M.,
PARENT, A.,
HADA, J.
Thalamic projections of nucleus reticularis thalami: a study using retrograde transport of horseradish peroxidase and double fluorescent tracers.
J. Comp. Neurol.
229: 531-547, 1984.[Medline]
-
STERIADE, M.,
TIMOFEEV, I.
Intrathalamic mechanisms of short-term plasticity processes during incremental responses.
Soc. Neurosci. Abstr.
22: 2030, 1996.
-
STERIADE, M.,
TIMOFEEV, I.
Short-term plasticity during intrathalamic augmenting responses in decorticated cats.
J. Neurosci.
17: 3778-3795, 1997.[Abstract/Free Full Text]
-
THOMSON, A. M.
Inhibitory postsynaptic potentials evoked in thalamic neurons by stimulation of the reticularis nucleus evoke slow spikes in isolated brain slices.
Neuroscience
25: 491-502, 1988.[Medline]
-
THOMSON, A. M.
Activity-dependent properties of synaptic transmission at two classes of connections made by rat neocortical pyramidal axons in vitro.
J. Physiol. (Lond.)
502: 131-147, 1997.[Abstract]
-
THOMSON, A. M.,
DEUCHARS, J.,
WEST, D. C.
Large, deep layer pyramid-pyramid single axon EPSPs in slices of rat motor cortex display paired pulse and frequency-dependent depression, mediated presynaptically and self-facilitation, mediated postsynaptically.
J. Neurophysiol.
70: 2354-2369, 1993.[Abstract/Free Full Text]
-
TIMOFEEV, I.,
CONTRERAS, D.,
STERIADE, M.
Synaptic responsiveness of cortical and thalamic neurones during various phases of slow oscillation in cat.
J. Physiol. (Lond.)
494: 265-278, 1996.[Abstract]
-
ULRICH, D.,
HUGUENARD, J. R.
GABAB receptor-mediated responses in GABAergic projection neurones of rat nucleus reticularis thalami in vitro.
J. Physiol. (Lond.)
493: 845-854, 1996.[Abstract]
-
ULRICH, D.,
HUGUENARD, J. R.
GABAa-receptor-mediated rebound burst firing and burst shunting in thalamus.
J. Neurophysiol.
78: 1748-1751, 1997.[Abstract/Free Full Text]
-
WARREN, R. A.,
AGMON, A.,
JONES, E. G.
Oscillatory synaptic interactions between ventroposterior and reticular neurons in mouse thalamus in vitro.
J. Neurophysiol.
72: 1993-2003, 1994.[Abstract/Free Full Text]
-
WITTER, M. P., GROENEWEGEN, H. J., LOPES DA SILVA, F., AND LOHMAN, A.H.M. Functional organization of the extrinsic and intrinsic circuitry of the parahippocampal circuitry. Prog. Neurobiol. 33: 161-253.
-
YEN, C. T.,
CONLEY, M.,
HENDRY, S.H.C.,
JONES, E. G.
The morphology of physiologically identified GABAergic neurons in the somatic part of the thalamic reticular nucleus in the cat.
J. Neurosci.
5: 2254-2268, 1985.[Abstract]