S A Eaton & T E Salt
The ventrobasal thalamus (VB) is the principal somatosensory thalamic relay nucleus, and it
receives two major sources of excitatory input: firstly an input from ascending sensory afferents,
and secondly a descending projection from the primary somatosensory cortex 16. There is
considerable anatomical evidence to suggest that both of these projections utilise the excitatory
amino acid L-glutamate as their neurotransmitter 4,8,9,17. Previous work from this laboratory has
shown that the sensory input to the rat VB in vivo is mediated by ionotropic excitatory amino acid
receptors of both the N-methyl-D-aspartate (NMDA) and non-NMDA type 27,28. These findings
are consistent with data from other studies in various thalamic relay nuclei 15,33,36. In contrast,
there are considerably less data available concerning the synaptic pharmacology of the
cortico-thalamic projection although there have been both speculation and studies concerning the
functional significance of this pathway 18,34. There is some evidence to suggest an involvement of
NMDA receptors 10,30 and metabotropic glutamate receptors (mGluRs) 21. The aim of this study
was to determine which excitatory amino acid receptors might mediate cortically-elicited EPSPs
in the VB in vivo. Intracellular recordings were made, and neurotransmitter antagonists were
applied onto rat VB neurones by micro-iontophoresis. Cortically-elicited EPSPs were reduced by
the NMDA antagonist CPP, or the Group I metabotropic antagonist (S)-4CPG. These data
indicate that both NMDA receptors and Group I (possibly mGluR1) metabotropic receptors are
involved in the mediation of cortico-thalamic transmission. Such a transmitter mechanism would
allow a modulatory system that could selectively enhance other excitatory inputs. Some of these
data have been reported in abstract form 13.
Recordings were made from 13 neurones in nine adult Wistar rats (300-450g) anaesthetised with
urethane (1.2g/kg, i.p.) as detailed previously 11,27,29. These cells had resting membrane potentials
of -55 to -65 mV. Input resistance of the neurones was not routinely measured, as the high
impedance of the recording electrodes in the tissue (up to 700M-Ohm) made this impossible.
Nevertheless, it was possible to make stable recordings for periods of 45 minutes or more, thus
indicating that intracellular impalements were of sufficient quality for the purposes of this study.
Cortical stimulation typically evoked a response which consisted of one or more of the following
components: an initial antidromic action potential and/or EPSP (which could give rise to action
potentials in four cases), followed by a multi-component IPSP which appeared to curtail the EPSP
and which could display oscillatory activity for several hundreds of milliseconds 1. As it is now
well-established that there are virtually no intrinsic inhibitory interneurones within the VB of
rodents, IPSPs elicited in this study by cortical stimulation are mediated by GABAergic neurones
of the reticular thalamic nucleus (nrt), which is located adjacent to, but distinct from VB 2,7,8,24.
The EPSP is almost certainly due to activation of cortico-thalamic fibres as it is unlikely that
thalamic relay neurones make recurrent connections onto other relay neurones 16. In order to
reveal cortically-elicited EPSPs more prominently in neurones where such EPSPs were curtailed
by IPSPs, we applied the GABAA receptor antagonist SR95531 22 by iontophoresis (25-100nA):
this reduced the amplitude of the IPSPs, particularly in the earlier phases, and resulted in an
enhanced cortically-elicited EPSP, that could give rise to action potentials (Figure 1). The mean
amplitude of these enhanced cortically-elicited EPSPs was 6.1 + 2.9 mV. However, given that
the blockade of IPSPs was never complete with SR95531 [possibly reflecting a GABAB receptor
IPSP component 5], these values should be treated with caution. Similarly, it was thus not
possible to make reliable estimates of the EPSP time courses. The cortically-elicited EPSPs
which were evident before GABAA antagonism, or which were revealed during SR95531
application were challenged with iontophoretic applications of the NMDA receptor antagonist
CPP 6 or the mGluR antagonist (S)-4CPG 12. In either case, the iontophoretic currents and
durations of ejection of the antagonists were adjusted so that depolarising responses to NMDA or
ACPD respectively of the same neurones were reduced in a selective manner (Figure 1E).
Application of CPP (10-50nA) resulted in a reduction of the EPSP amplitude (to 60+12% of
control) that was accompanied by a reduction in the number of evoked action potentials, in four
of six neurones (Figure 1). Similarly, application of the mGluR antagonist (S)-4CPG reduced the
cortico-fugal EPSP (to 48+11% of control) on seven of nine neurones (Figure 2) when applied
with iontophoretic current of between 30-100nA. On the remaining two of these neurones,
ejection of (S)-4CPG with currents of 200nA and 300nA did not lead to a reduction of the
cortico-thalamic EPSP.
These data indicate that components of the cortically-elicited EPSP in VB neurones are mediated
via NMDA receptors and metabotropic glutamate receptors. It is evident that there are
components of the EPSPs that are not blocked by these two antagonists. It is possible that this
represents an inability of the antagonists to penetrate the synaptic clefts to a sufficient extent, a
well-known problem with the iontophoretic technique. This may be particularly relevant given the
location of the cortico-thalamic terminals on the distal dendrites of VB neurones 20,24,25. An
alternative possibility is that there are other receptor types that contribute to this EPSP, and
indeed it would appear likely that AMPA receptors may also be involved. However, this remains
to be resolved. Our current findings are consistent with previous micro-injection studies where
administration of NMDA antagonists into the thalamus of nrt-lesioned cats was found to reduce
corticofugal EPSPs 10.
Eight mGluR types (and several splice variants) have been cloned, and these can be placed into
three groups (Group I comprises mGluR1 and mGluR5, Group II comprises mGluR2 and
mGluR3, Group III comprises mGluR4, mGluR6, mGluR7 mGluR8) on the basis of sequence
homology, transduction mechanism and agonist pharmacology 26,37. It is known that (S)-4CPG is
an effective competitive antagonist at Group I mGluRs (showing a preference for mGluR1
compared with mGluR5 17a ), with less action at the other receptors 12,14,35,37. A complicating
factor is that (S)-4CPG has been shown to have weak agonist activity at a Group II receptor,
mGluR2 14. However, we have shown that activation of such receptors in VB does not reduce
excitatory transmission 29. Furthermore, there is abundant mGluR1 and little mGluR5 mRNA in
the thalamus 31,32, and mGluR1 is located postsynaptically on dendrites of rat VB neurones 19. It
thus seems likely that (S)-4CPG is acting at mGluR1 in the present study, and therefore that the
EPSP portion that was sensitive to (S)-4CPG is mediated via mGluR1, although it is not possible
to exclude a contribution of mGluR5. It is possible that these receptor types make an even more
substantial contribution to cortico-thalamic synaptic transmission than that which has been
revealed in the present study and it may be that this would be revealed using repetitive
stimulation21. In the present experiments, cortical stimulation elicited a powerful inhibitory
response which was only reduced in part by the GABAA antagonist, SR95531. It is therefore
probable that longer latency cortical EPSP components are present, but that these were occluded
by the residual IPSP sequence(s), as has been described in the cerebellum 3.
NMDA-receptor mediated responses are voltage-dependent by virtue of the blockade of the
NMDA-receptor-channel by Mg++, and it has been shown that mGluR activation on thalamic
neurones reduces a K+-conductance that causes a depolarisation with decreased membrane
conductance 21. Both of these mechanisms are ideally suited to provide a modulatory input onto
thalamic cells that could enhance other depolarising inputs. Thus, such a cortico-thalamic input
could be the basis of selective enhancement or gating of sensory transmission through the
thalamus, a concept which has been suggested by several groups of workers18,34. An interesting
question is why the cortico-thalamic projection would impinge upon both NMDA receptors and
mGluRs. It could be that these receptors are activated under different conditions of input, for
example different frequencies and/or durations of activation. This is clearly a matter for further
study, perhaps using different and more complex stimulus protocols than those used at present. It
is certainly evident from other studies that mGluR-mediated EPSPs are of longer duration than
EPSPs mediated by ionotropic receptors, and thus the different receptors could contribute to
different time courses of activation.
It has recently been shown on cerebellar neurones that ionotropic glutamate receptors and
mGluRs are located adjacent to each other in such a way that suggests that ionotropic glutamate
receptors are activated in the synaptic cleft, and that mGluRs are activated by glutamate if
sufficient glutamate is released from the pre-synaptic terminal 23. Preliminary results suggest that
this is the case in the thalamus for NMDA receptors and mGluR1 17: this would offer a
mechanism for the differential activation of ionotropic glutamate receptors and mGluRs in the
cortico-thalamic system, and open up the possibility of pattern-dependent modulation of thalamic
transmission by the sensory cortex. Clearly, it will be important to determine the relative
distributions of glutamate receptor types on different parts of the thalamic neurones in order to
progress in this area.
Acknowledgements
We are indebted to Prof J C Watkins for gifts of (S)-4CPG. This work was supported by the
Wellcome Trust. Technical assistance was provided by D J Rhodes.
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Abbreviations
ACPD (1S,3R)-1-amino-1,3-cyclopentane dicarboxylate
AMPA D,L-a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate
CPP 3-((+)-2-carboxy-piperazin-4-yl)-propyl-1-phosphonate
EPSP excitatory postsynaptic potential
IPSP inhibitory postsynaptic potential
mGluR metabotropic glutamate receptor
NMDA N-methyl-D-aspartate
(S)-4CPG (S)-4-carboxyphenylglycine
VB ventrobasal thalamus
nrt thalamic reticular nucleus
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Last Updated 22 January 1997.