Division of Cell Sciences, School of Biological Sciences, University of Southampton, Southampton SO16 7PX, United Kingdom
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ABSTRACT |
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Newland, Philip L.. Processing of Gustatory Information by Spiking Local Interneurons in the Locust. J. Neurophysiol. 82: 3149-3159, 1999. Despite the importance of gustation, little is known of the central pathways responsible for the processing and coding of different chemical stimuli. Here I have analyzed the responses of a population of spiking local interneurons, with somata at the ventral midline of the metathoracic ganglion, during stimulation of chemo- and mechanoreceptors on the legs of locusts. Volatile acidic stimuli were used to selectively activate the chemosensory neurons. Different members of the population of local interneurons received depolarizing or hyperpolarizing inputs during chemosensory stimulation. Many of the same interneurons that received chemosensory input also received mechanosensory inputs from tactile hairs on the leg, but others received exclusively mechanosensory inputs. Chemosensory inputs occurred with a short and constant latency, typical of monosynaptic connections. The chemosensory receptive fields of the spiking local interneurons mapped the surface of a hind leg so that spatial information relating to the location of a taste receptor was preserved. The amplitude of potentials in interneurons during chemosensory stimulation varied in a graded manner along the long axis of the leg, thus creating gradients in the chemosensory receptive fields of interneurons. Some interneurons were depolarized to a greater extent by chemical stimuli applied to basiconic sensilla on distal parts of the leg, whereas others were depolarized more by chemical stimulation of more proximal sensilla.
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INTRODUCTION |
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Sensory information from taste receptors is
vitally important for all animals and is essential if an animal is to
make appropriate selections of food. In insects, contact chemoreception
plays a major role in a number of behaviors, including avoidance of
noxious chemicals (White and Chapman 1990), the
selection of egg-laying sites (Ma and Schoonhoven 1973
;
Städler et al. 1995
), and the detection and
selection of food (Dethier 1976
). The gustatory receptors involved in these responses are called basiconic sensilla and
are found on the mouthparts, where they have been extensively studied,
on the body, wings, and legs (Blaney 1974
;
Chapman 1982
; Kendall 1970
; Thomas
1966
).
On the locust leg each basiconic sensillum is thought to contain a
single mechanosensory neuron and at least four chemosensory neurons
(White and Chapman 1990). We now know that the
mechanosensory afferents from basiconic sensilla converge onto the same
spiking local interneurons that process signals from tactile hairs
(Burrows and Newland 1994
), and together they form an
elaborate tactile detector system covering the surface of the legs
(Newland and Burrows 1994
). Although much is known of
how the sensory neurons within each receptor respond to chemical
stimuli (Blaney 1974
; White and Chapman
1990
) and touch (Newland 1991
; Newland
and Burrows 1994
), we know little of where and how the
chemosensory signals are subsequently processed and integrated in local
circuits in the CNS.
Only one study has described interneurons involved in processing
gustatory signals (Mitchell and Itagaki 1992), but that
did not analyze the physiological properties of particular populations of interneurons in any detail, nor was there any attempt made to
understand the role of those interneurons in any taste related behavior. Part of the problem in analyzing the networks of central neurons involved in taste processing lies in the difficulties inherent
in working with taste receptors in insects. Because each taste receptor
is innervated by a mechanosensory neuron, conventional methods of
stimulation using drops of chemical stimuli activate that
mechanosensory neuron in addition to the chemosensory neurons. Recently, however, I showed that gustatory receptors on the hind leg of
the desert locust respond to acidic odors, and that such stimulation
leads to a characteristic avoidance reflex of the leg (Newland
1998
). The great benefit of this method of stimulation for
analyzing central pathways involved in aversive taste processing is
that it is free of the very rapid sensory adaptation associated with
chemical stimuli placed directly over the sensillum tip, and more
importantly, specific to the chemosensory neurons themselves. This
means that there is no simultaneous stimulation of the mechanosensory sensory neuron within the receptor, and that any evoked responses are a
consequence of purely chemical cues. The avoidance reflex evoked by
acid-odor stimulation is somewhat different to that evoked by tactile
stimulation (Pflüger 1980
), but the movements and
activation of the underlying motor neurons imply that they share common
neuronal elements and pathways. This then points to important
contributions from the different types of local interneuron that are
part of the local circuits that control leg movements in the processing
of chemosensory signals. Spiking local interneurons are involved in the
initial processing of sensory signals in those local circuits (see
review by Burrows 1992
). One group of these interneurons
has its somata on the ventral midline of the thoracic ganglia and is
known to play a key role in the motor pattern formation (Siegler
and Burrows 1986
). This ventral midline population of interneurons uses the transmitter
-aminobutyric acid (Watson and Burrows 1987
), and hence has inhibitory outputs
(Burrows and Siegler 1982
). It is possible that the
inhibitory influences found in the responses of some of the tibial and
tarsal motor neurons during odor stimulation of the taste receptors
(Newland 1998
) may directly reflect the action of some
of these spiking local interneurons.
Given this possibility, the aim of this study was therefore to analyze the responses of this population of spiking local interneurons in the locust during chemosensory and mechanosensory stimulation, to start to describe the pathways involved in the processing of gustatory signals.
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METHODS |
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Experiments were performed on adult male and female desert locusts, Schistocerca gregaria (Forskål), taken from our crowded laboratory colony at Southampton University. Each locust was restrained ventral side uppermost in modelling clay with its left hind leg fixed so that the anterior surface was accessible. All other legs were firmly fixed at the femur, but with the tibiae and tarsi free to move.
The meso- and metathoracic ganglia were exposed by removing a small window of cuticle from the ventral thorax and then supported firmly on a wax-coated silver platform. The abdominal connectives were cut posterior to the metathoracic ganglion, and the connectives between the meso- and metathoracic ganglia crushed. To aid electrode penetration, the sheath of the metathoracic ganglion was treated with protease (Sigma type XIV) for 45-60 s before recording. The thorax was continuously superperfused with locust saline at 22-25°C throughout an experiment.
Intracellular recordings were made from the somata of spiking local
interneurons of a ventral midline group (Burrows and Siegler 1984; Siegler and Burrows 1984
) using
glass microelectrodes filled with 2 M potassium acetate and with DC
resistances of 50-80 M
. Interneurons of this population were
identified by their responses to stimulation of tactile hairs on the
leg that define their receptive fields (Burrows 1985
;
Burrows and Siegler 1984
; Siegler and Burrows 1983
). Intracellular recordings were made using an Axoclamp 2A amplifier (Axon Instruments).
The activity of chemosensory neurons was recorded in one of two ways
depending on whether odor or aqueous solutions of chemicals were used
as the method of stimulation. For odor stimulation, a glass
microelectrode filled with standard locust saline was driven through
the soft cuticle at the base of a receptor close to the somata of its
sensory neurons. Signals were fed to a standard high-impedance DC
amplifier and then AC coupled. This method of odor stimulation has
already been shown to be specific to chemosensory neurons; it does not
cause injury damage to sensory afferents and does not activate the
mechanosensory afferents (Newland 1998). For chemical
stimulation with aqueous solutions, the tip-recording technique was
used (Hodgson et al. 1955
) in which a blunt recording microelectrode containing 50 mM sodium chloride was placed directly over the tip of a receptor. Movements of the electrode deflected the
sensillum and elicited spikes in the mechanosensory afferent that were
clearly distinguishable from spikes in chemosensory afferents
(Newland and Burrows 1994
). The same electrode was
therefore used to simultaneously evoke and record the spikes of both
the mechano- and chemosensory afferents. Tactile hairs (trichoid
sensilla) were first cut to approximately half their lengths and
stimulated and recorded using the same tip-recording technique.
For odor delivery, controlled pulses of compressed air were delivered
through a World Precision Instruments Picopump and passed through a
small bottle containing filter paper soaked in glacial acetic acid
(Newland 1998). The output from this bottle was then connected to a metal pipette (2.0 mm OD) and odors of acetic acid delivered in an airstream at a rate of 1.75 cm3 · s
1. The odor delivery pipette was positioned 2-3 mm from
the receptor being analyzed. All recordings from the sensory neurons
and interneurons were stored on a Biologic DAT recorder for subsequent
analysis and display on either a digital oscilloscope (Tektronix), or
on a computer following digitization using a Cambridge Electronics Design interface (CED1401) and Spike 2 software. The results are based
on recordings from 65 interneurons in 41 locusts. Data were collected
only for interneurons in which the resting potential remained constant
throughout the entire experiment. Moreover, any interneuron that showed
adaptation to a stimulus presented to a specific location on the leg at
the start and at the end of an experiment was excluded from this analysis.
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RESULTS |
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Spiking local interneurons respond to chemosensory stimulation
Many spiking local interneurons of the ventral midline population
(Burrows and Siegler 1984; Siegler and Burrows
1983
) that responded to mechanosensory stimulation also
responded to gustatory stimuli. For example, the interneuron shown in
Fig. 1 had a tactile receptive field
restricted to the tibia and tarsus of the hind leg. Thus mechanical
stimulation of tactile hairs and basiconic sensilla on the tibia and
tarsus, but not on the femur, evoked depolarizations and spikes in the
interneuron (Fig. 1A). Stimulation with an odor of acetic
acid (Newland 1998
) evoked a long-lasting depolarization
and spikes in the same interneuron (Fig. 1B) when directed
toward basiconic sensilla on the ventral tarsus, but had little effect
when directed toward the femur. Moreover, drops of water or 250 mM
sodium chloride applied to groups of basiconic sensilla on the tibia
also evoked depolarizations and spikes in the same interneuron (Fig.
1C). Although these drops may also have activated the
mechanosensory afferents of both tactile hairs and basiconic sensilla,
interneurons responded differently (i.e., with different durations) to
different chemical solutions, with 250 mM NaCl evoking a response with
twice the duration of that evoked by water. These differing responses
of the spiking local interneurons to NaCl and water were consistent
from animal to animal (n = 19 interneurons).
Stimulation using an acid odor, however, avoided the problem of
simultaneously activating mechanosensory and chemosensory neurons and
was therefore used as the preferred method for analyzing the
physiological properties of the spiking local interneurons. Not all
members of this midline population of local interneuron received inputs
during chemosensory stimulation (Fig. 2),
but all that did also received exteroceptive inputs from mechanosensory
neurons innervating both tactile hairs and basiconic sensilla.
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Chemosensory stimulation of basiconic sensilla on the locust leg with a
number of volatiles showed that the spiking local interneurons only
responded to odors of acids (Fig. 3),
such as formic and acetic acid [in a similar manner to the responses
of leg motor neurons to odorants (Newland 1998)], and
not to any other volatile, including amyl acetate, xylene, and clove
oil, which are known to have powerful olfactory effects (Laurent
and Naraghi 1994
; Slifer 1954
,
1956
). This contrasts with the responses of interneurons
that receive olfactory inputs from receptors on the antennae of insects
(Laurent and Davidowitz 1994
; Laurent et al.
1996
).
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Mechano- and chemosensory signals converge on spiking local interneurons
With the use of the "tip recording" technique, it is possible
to simultaneously record and stimulate mechanosensory neurons of the
basiconic sensilla. Small movements of the recording electrode caused
deflections of the sensilla, and each deflection evoked a spike in a
mechanosensory neuron that was followed by a depolarizing potential in
an interneuron (Fig. 4A).
Superimposing sweeps of the oscilloscope triggered from the
mechanosensory afferent spike showed that these potentials occurred
with a constant latency (Fig. 4B), typical of known
monosynaptic mechanosensory inputs from basiconic sensilla onto these
interneurons (Burrows 1992; Newland and Burrows
1994
). A recording from a neighboring tactile hair (trichoid
sensillum) showed that mechanosensory input from this class of receptor
converged onto the same interneurons as those receiving mechanosensory
input from basiconic sensilla. Superimposed sweeps of the oscilloscope
triggered from the afferent spike of the tactile hair show excitatory
postsynaptic potentials in the interneuron that again followed with a
short and constant latency (Fig. 4C). An odor of acetic acid
directed to a similar location on the leg also evoked a depolarization
and spikes in the interneuron (Fig. 4D), revealing a
convergence of mechano- and chemosensory signals onto members of the
same population of interneurons.
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The connections made by mechanosensory neurons of tactile hairs and
basiconic sensilla with these interneurons are mediated through
monosynaptic pathways (Burrows and Newland 1994). The connections made by the chemosensory neurons with the same interneurons were more difficult to analyze because signal averaging failed to
reveal chemosensory afferent spikes in nerve 5, which could be used to
analyze synaptic latency. In addition, although chemosensory spikes
could be readily elicited in an individual receptor by application of
different chemicals in the recording electrode, these spikes adapted so
rapidly that signal averaging failed to reveal a potential in an
interneuron. To analyze the patterns of connectivity of the
chemosensory neurons, it was therefore necessary to stimulate the
basiconic and trichoid sensilla electrically and analyze the evoked
potentials in an interneuron (n = 7). Superimposed sweeps of the oscilloscope triggered from the stimulus pulse showed that the potentials evoked by stimulation of a trichoid sensillum with
a stimulus intensity just above threshold (Fig.
5A,
) and another
suprathreshold (· · ·) were of a similar amplitude. Similar results were obtained for two other neighboring tactile hairs on the
dorsal distal femur (numbered 2 and 3). The amplitude of the potential
evoked by electrical stimulation of a basiconic sensillum, however,
depended on the amplitude of electrical stimulation. At threshold, the
depolarizing potential in an interneuron was of a relatively small
unitary type, with an amplitude and delay characteristic of input from
the mechanosensory neurons of the basiconic sensilla. Suprathreshold
electrical stimulation recruited a later delayed component that summed
with the initial depolarization (Fig. 5B), that is assumed
to be from one or more chemosensory neurons from the same sensillum.
Neighboring basiconic sensilla on the distal dorsal femur again also
produced similar responses in the interneuron, and similar results were
obtained with stimulus durations of 0.5, 1.0, and 2.0 ms. Of the
interneurons tested, the example in Fig. 5B showed the
clearest increase in amplitude during stimulation of a basiconic
sensillum. More commonly, the increased amplitude of stimulation of a
basiconic sensilla led to a 10-20% increase in the amplitude of the
potential in an interneuron, which often gave rise to spikes. It is
unlikely that current spread to neighboring basiconic sensilla is
causing the delayed component because similar electrical stimulation to
the tactile hairs results in a single amplitude of evoked potential
even though the tactile hairs have a similar spatial distribution on
the leg.
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Receptive field properties of local interneurons
Spiking local interneurons were not always depolarized by odor stimulation. Instead the polarity of input during chemosensory stimulation always matched that during mechanosensory stimulation of basiconic and trichoid sensilla. Thus interneurons that were inhibited during mechanosensory stimulation were also inhibited by chemosensory stimulation with acetic acid (Fig. 6, A and B). Other interneurons were depolarized when sensilla on the leg were displaced and also when an odor of acetic acid was directed toward basiconic sensilla (Fig. 6, C and D). Interneurons that were inhibited were encountered less often than those that were depolarized and represented only 6 of the total of 65 interneurons that were encountered in this study.
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Each interneuron was excited or inhibited by stimulation of particular
arrays of basiconic sensilla and tactile hairs on the leg, so that its
receptive field could be described according to the distribution of
those receptors on different parts of the leg. The mechanosensory
receptive fields of spiking local interneurons have been described in
detail for tactile hairs alone (Burrows 1992) and for
tactile hairs and the mechanosensory afferents of basiconic sensilla
(Burrows and Newland 1994
). I analyzed in detail the
receptive fields of 39 interneurons, and these could be divided into 16 types. The receptive fields were plotted by mechanically stimulating
arrays of receptors with a fine brush and also by directing acidic
odors to those same arrays of receptors. Given the problems associated
with assessing the contributions of individual chemosensory receptors
to the receptive fields of interneurons (rapid adaptation, small
amplitude of extracellular spikes, and lack of signal averaging),
stimulation with an odor of acetic acid was used to analyze the extent
of the chemosensory receptive fields over the three distal segments
(femur, tibia, and tarsus) of the hind leg. The interneuron shown in
Fig. 7A was depolarized and
produced spikes when tactile hairs and basiconic sensilla on the dorsal
tarsus were deflected using a fine paintbrush. No mechanoreceptors on
other surfaces or other leg segments provided an input to this
interneuron, and therefore its mechanosensory receptive field was
restricted to the dorsal surface of the tarsus. An odor of acetic acid
directed toward various sites on the leg showed that the chemosensory
receptive field overlapped, or matched, its mechanosensory field. Thus
an odor directed toward the dorsal distal femur evoked no responses in
the interneuron (Fig. 7B, top trace), whereas an odor
directed to the dorsal tarsus evoked a large depolarization and spikes
in the interneuron (Fig. 7B, middle trace). A different
interneuron was depolarized during mechanosensory stimulation of
receptors on the dorsal surface of the femur only (Fig. 7C, top
trace). Similarly, only odors directed to the dorsal femur evoked
depolarization and spikes in the interneuron (Fig. 7D), but
not when directed elsewhere.
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Of the 16 types of interneuron sampled, the receptive fields evoked by mechano- and chemosensory stimulation were, with one exception, contiguous and overlapping (Fig. 8). Three basic classes were found: those that received purely excitatory input, those that received inhibitory input, and those that received both excitatory and inhibitory inputs. The extent of the receptive fields varied considerably so that for some interneurons the mechanosensory field covered the entire dorsal surface, but the chemosensory field was restricted to the tarsus. Some interneurons received both excitatory and inhibitory inputs, but they did not contribute equally to the mechano- and chemosensory receptive fields. Thus for example, an interneuron that was excited by mechanosensory stimuli to receptors on the dorsal tibia and tarsus was inhibited by mechanosensory stimulation of the ventral tarsus. Chemosensory stimulation resulted in an inhibition of the same interneuron, but the excitatory inputs were absent.
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Interneurons express gradients of excitability in response to chemosensory stimulation
A key feature of the mechanosensory receptive fields of spiking
local interneurons is their gradients of excitability from receptors on
different locations on the leg (Burrows 1992;
Burrows and Newland 1994
). This means that for
particular interneurons receptors on one part of the leg have a greater
effect on the interneuron than receptors on another area of the leg.
Similar gradients in chemosensory receptive fields were also commonly found in this study. For example, the interneuron shown in Fig. 9 had a chemosensory receptive field
covering most of the hind leg; however, odor pulses of acetic acid
directed to the distal region of the femur had a far greater excitatory
effect on the interneuron than similar odor pulses directed toward
surrounding areas. The latency to the onset of the depolarization was
similar for all areas of the leg that evoked a response in the
interneuron, suggesting that this gradient in response amplitude was
not the result of spread of the odor from areas that had no effect to areas that did.
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DISCUSSION |
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In this study I have demonstrated that a population of identified
spiking local interneurons processes chemosensory signals from
basiconic sensilla on the leg of the desert locust. All of the
interneurons of the ventral midline population that received chemosensory input from the basiconic sensilla during aversive odor
stimulation also received convergent exteroceptive input from
mechanosensory neurons of both tactile hairs and basiconic sensilla.
Other members of the midline group, however, did not receive
chemosensory inputs, although it is possible that these particular
interneurons could receive input during stimulation with other
chemicals. The input from the mechanosensory afferents of basiconic
sensilla was mediated through monosynaptic connections (see also
Burrows and Newland 1994), but it is not yet firmly established whether the inputs from the chemosensory neurons were made
through mono- or polysynaptic pathways. The results obtained indicating
a recruitment of synaptic inputs with short and constant latency in
response to an increased amplitude of electrical stimulation were
indicative of a monosynaptic input from at least one chemosensory neuron. In addition, we already know that, although chemosensitive afferents are thought to project to a slightly more dorsal area of the
ganglion than exteroceptive afferents (Burrows and Newland 1994
), there is considerable overlap of the branches of spiking local interneurons with afferents of both modalities (Burrows and Newland 1993
). Further tests of connectivity have not been possible because electrical stimulation, which necessarily activates the mechanosensory neuron innervating the same receptor, is the only
reliable method of stimulating the basiconic sensilla. Direct contact
with aqueous solutions of chemicals results in a sensory response that
adapts rapidly (Newland 1998
) precluding this method during tests of synaptic connectivity (Burrows and Newland
1994
). The consistency and constant latency of the responses in
the spiking local interneurons provide compelling evidence that the
connections are monosynaptic.
Organization of receptive fields of interneurons
Spiking local interneurons of the midline group, including those
analyzed in this study, are known to play a major role in forming the
pattern of movements of the hind leg (Burrows 1985). Through their characteristic receptive fields these interneurons provide a map of the mechanoreceptors on the surface of the hind leg
(Burrows 1992
; Burrows and Newland 1994
)
in which spatial information is preserved (Burrows and Siegler
1985
). Information about the spatial location of a
stimulus on a leg is essential for the production of appropriate local
reflex movements of that leg that move it away from the source of
stimulation. The precise movement depends on the location of the
stimulus on the leg so that the movements are different when ventral
receptors are stimulated compared with when dorsal receptors are
stimulated (Pflüger 1980
).
The chemosensory receptive fields of these same interneurons are also
very specific so that spatial information relating to the location of
the chemoreceptors is again preserved across the population.
Chemosensory stimulation, however, appears to evoke only a single
reflex pattern of movements of the leg (Newland 1998),
and the need for information about stimulus location appears unclear.
Stimulation of chemoreceptors on different surfaces or segments of the
leg results simply in a change in amplitude of responses of the leg
motor neurons (Newland 1998
) and not a reversal in the
sign of input that occurs in tactile reflexes (Siegler and
Burrows 1986
). Although it remains unclear why it is necessary to preserve the spatial information related to the location of the
chemoreceptors, it is more than likely that the input the local
interneurons receive from the chemoreceptors will modify their
contribution to reflex effects in local circuits and controlling leg
movements (Burrows 1996
).
Role of local interneurons in processing taste signals
Because the spiking local interneurons described here play a major
role in shaping movements of the legs, this implies that chemosensory
inputs will also contribute greatly to their output effects on
nonspiking local interneurons (Burrows 1987) and motor neurons (Burrows and Siegler 1982
). The only other study
to examine the central processing of taste inputs also describes local
interneurons receiving chemosensory inputs. In the blowfly,
Mitchel and Itagaki (1992)
showed that many of the
interneurons responding to taste stimuli applied to the labelum were
spiking local interneurons with extensive branching patterns in the
suboesophageal ganglion. The detailed analyses of interneurons in the
local circuits of locusts (see Burrows 1996
) have
implicated spiking local interneurons in the initial processing of
sensory signals from many types of receptor on the leg, and of
different sensory modalities. For example, in addition to processing
exteroceptive signals from tactile hairs on the legs (Burrows
1992
; Siegler and Burrows 1983
) spiking local
interneurons also process inputs from the mechanosensory afferents of
basiconic sensilla and from campaniform sensilla that monitor cuticular
stress (Siegler and Burrows 1983
). They may also receive
convergent proprioceptive input from receptors in a leg (Burrows
1988
). Because these interneurons are responsible for the
integration of so many sensory inputs, it is perhaps not surprising
that the same interneurons are also involved in processing chemosensory
signals from taste receptors on the leg, especially because the
stimulation of the gustatory receptors evokes an avoidance reflex
(Newland 1998
) similar in form to that evoked by tactile hairs on the leg (Pflüger 1980
).
Functional role of basiconic sensilla on the leg of the locust
It is intriguing as to why the surface of the body and limbs of
insects are covered with taste receptors (Stocker
1994). The role of some of these receptors has been
demonstrated in some species. For example, the work of Dethier
(1976)
and Getting (1971)
showed that
stimulation of tarsal chemoreceptors on the forelegs of flies elicits
an extension of the proboscis. If the proboscis makes contact with the
same stimulant, then feeding is likely to occur. Those on the palps of
Manduca sexta are thought to be involved in food
rejection (Glendenning et al. 1998
), whereas those on
the antennae of cockroaches are involved in food selection and
conspecific recognition (Hansen-Delkeskamp 1972
). This,
however, is only a small subset of taste receptors on the body, and the role played by the majority has yet to be described.
The receptive field properties of the spiking local interneurons
and their role in local circuits controlling leg movements provide some
clues to the role of the basiconic sensilla on the legs. I have shown
that the areas on the leg providing inputs to an interneuron, its
receptive field, were overlapping for both mechano- and chemosensory
inputs. Basiconic sensilla within the receptive fields of an
interneuron did not appear to contribute equally to its receptive
field; one area evoked a greater response, whereas surrounding areas
have a lesser effect. This pattern of input had been analyzed in detail
by Burrows (1992) and Burrows and Newland
(1994)
for the mechanosensory afferents innervating the tactile
hairs and basiconic sensilla, respectively, and points to a general
organization feature of these interneurons and their presynaptic
neurons. The convergent input from both neurons of mechanosensory and
chemosensory modalities indicates a summation of inputs. The
implication is therefore that the responses to mechanosensory signals
will be enhanced by the appropriate chemical stimulation of the same
receptors, in addition to the tactile hairs. This is only one possible
role of the basiconic sensilla, and it is likely that information about
different chemicals is also transmitted to other centers. Furthermore,
it is also possible that different chemicals activate spiking local
interneurons in other populations and that processing of different
tastes could occur at separate sites. The role of local spiking
interneurons is likely to be one that would enhance an avoidance
movement of the leg (Newland 1998
; Pflüger
1980
; Siegler and Burrows 1986
) and
could possibly play a role in food rejection.
Although the coding of different odors has been analyzed in detail and
is now known to involve oscillatory coding and temporal shifts in the
oscillations and neurons involved in a network (Laurent and
Davidowitz 1994), nothing is yet known about the coding of different chemicals in the local circuits, and it remains the next step
in this analysis. Initial experiments suggest that the interneurons
described here respond differently to different chemicals. Now I have
identified a class of interneuron that is known to receive chemosensory
input; the task now is to analyze how these interneurons encode
different tastes and how that information is integrated in the networks
that control movements of the locust leg, and how that information may
be used by an animal in the selection or rejection of food.
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ACKNOWLEDGMENTS |
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I am grateful to Drs. Hitoshi Aonuma, Stephen Rogers, Hans Schuppe, and David Shepherd for valuable comments on earlier drafts of this manuscript.
This work was supported by an Advanced Fellowship from the Biotechnology and Biological Sciences Research Council (U.K.) and a research grant from the Royal Society (U.K.).
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FOOTNOTES |
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Address for reprint requests: P. L. Newland, School of Biological Sciences, University of Southampton, Biomedical Sciences Building, Bassett Crescent East, Southampton SO16 7PX, United Kingdom.
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 21 June 1999; accepted in final form 13 August 1999.
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REFERENCES |
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