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.
REFERENCES
1. Andersen P, Eccles JC, and Sears TA, (1964) The ventrobasal complex of the thalamus: types
of cells, their responses and their functional organization. J. Physiol. 174: 370-399.
2. Barbaresi P, Spreafico R, Frassoni C, and Rustioni A, (1986) GABAergic neurons are present
in the dorsal column nuclei but not in the ventroposterior complex of rats. Brain Res.
382:305-326.
3. Batchelor AM, Madge DJ, and Garthwaite J, (1994) Synaptic activation of metabotropic glutamate receptors in the parallel fibre-Purkinje cell pathway in rat cerebellar slices.
Neuroscience 63:911-915.
4. Bromberg MB, Penney JB, Stephenson BS, and Young AB, (1981) Evidence for glutamate as
the neurotransmitter of corticothalamic and corticorubral pathways. Brain Res. 215: 369-374.
4a. Chapin JK, and Lin CS, (1984) Mapping the body representation in the SI cortex of
anesthetized and awake rats. Journal of Comparative Neurology 229: 199-213.
5. Crunelli V, Haby M, Jassik-Gershenfeld D, Leresche N, and Pirchio M, (1988) Cl- and
K+dependent inhibitory postsynaptic potentials evoked by interneurones of the rat lateral
geniculate nucleus. J. Physiol. 399: 153-176.
6. Davies J, Evans RH, Herrling PL, Jones AW, Olverman HJ, Pook P, and Watkins JC, (1986)
CPP, a new potent and selective NMDA antagonist. Depression of central neuron responses,
affinity for (3H) D-AP5 binding studies on brain membranes and anticonvulsant activity. Brain
Res. 382: 169-173.
7. De Biasi S, Frassoni C, and Spreafico R, (1988) The intrinsic organisation of the
ventropostero-lateral nucleus of the rat: a double-labeling ultra-structural investigation with
g-aminobutyric acid immunogold staining and lectin-conjugated horseradish peroxidase.
Somatosensory Res. 5: 187-203.
8. De Biasi S, and Rustioni A, (1990) Ultrastructural immunocytochemical localization of
excitatory amino acids in the somatosensory system. J. Histochem. Cytochem. 38: 1745-1754.
9. DeBiasi S, Amadeo A, Spreafico R, and Rustioni A, (1994) Enrichment of glutamate
immunoreactivity in lemniscal terminals in the ventropostero lateral thalamic nucleus of the rat:
An immunogold and WGA-HRP study. Anat. Record 240:131-140.
10. Deschênes M, and Hu B, (1990) Electrophysiology and pharmacology of the corticothalamic
input to lateral thalamic nuclei: an intracellular study in the cat. Eur. J. Neuroscience 2:140-152.
11. Eaton SA, and Salt TE, (1991) Membrane and action potential responses evoked by
excitatory amino acids acting at N-methyl-D-aspartate receptors and non-N-methyl-D-aspartate
receptors in the rat thalamus in vivo. Neuroscience 44: 277-286.
12. Eaton SA, Jane DE, Jones PLSJ, Porter RHP, Pook PCK, Sunter DC, Udvarhelyi PM,
Roberts PJ, Salt TE, and Watkins JC, (1993) Competitive antagonism at metabotropic glutamate
receptors by (S)-4-carboxyphenylglycine (CPG) and (RS)-a-methyl-4-carboxyphenylglycine. Eur.
J. Pharmacology 244:195-197.
13. Eaton SA, and Salt TE, (1995) Reduction of cortico-thalamic EPSPs in vivo by antagonists
acting at metabotropic glutamate receptors (mGluRs) and NMDA receptors. Society for
Neuroscience Abstracts 21:250.2.
14. Hayashi Y, Sekiyama N, Nakanishi S, Jane DE, Sunter DC, Birse EF, Udvarhelyi PM, and
Watkins JC, (1994) Analysis of agonist and antagonist activities of phenylglycine derivatives for
different cloned metabotropic glutamate receptor subtypes. J. Neuroscience 14: 3370-3377.
15. Hu B, Senatorov V, and Mooney D, (1994) Lemniscal and non-lemniscal synaptic
transmission in rat auditory thalamus. J. Physiol. 479:217-232.
16. Jones EG, (1985) The Thalamus. Plenum Press, New York.
17. Kharazia VN, Phend KD, Weinberg RJ, and Rustioni A, (1995) Excitatory amino acids in
cortico-fugal projections: microscopic evidence. In "Excitatory Amino Acids and the Cerebral
Cortex" eds Conti F, Hicks TP, MIT PRESS Cambridge MA, IN PRESS.
17a. Kingston AE, Burnett JP, Mayne NG, and Lodge D, (1995) Pharmacological analysis of
4-carboxyphenylglycine derivatives: comparison of effects on mGluR1a and mGluR5a subtypes.
Neuropharmacology 34: 887-89.
18. Koch C, (1987) The action of the corticofugal pathway on sensory thalamic nuclei: a
hypothesis. Neuroscience 23: 399-406.
19. Martin LJ, Blackstone CD, Huganir RL, and Price DL, (1992) Cellular localization of a
metabotropic glutamate receptor in rat brain. Neuron 9:259-270.
20. McAllister JP, and Wells J, (1981) The structural organization of the ventral posterolateral
nucleus of the rat. J. Comp. Neurol. 197:271-301.
21. McCormick DA, and Von Krosigk M, (1992) Corticothalamic activation modulates thalamic
firing through glutamate "metabotropic" receptors. Proc. Nat. Acad. Sci. (USA) 89:2774-2778.
22. Michaud JC, Mienville JM, Chambon JP, and Biziere K, (1986) Interactions between three
pyridazinyl-GABA derivatives and central GABA and glycine receptors in the rat, an in vivo
microiontophoretic study. Neuropharmacology 25: 1197-1203.
23. Nusser Z, Mulvihill E, Streit P, and Somogyi P, (1994) Subsynaptic segregation of
metabotropic and ionotropic glutamate receptors as revealed by immunogold localization.
Neuroscience 61:421-427.
24. Ohara PT, and Lieberman AR, (1993) Some aspects of the synaptic circuitry underlying
inhibition in the ventrobasal thalamus. J. Neurocytol. 22:815-825.
25. Peschanski M, Lee CL, and Ralston HJ, (1984) The structural organization of the ventrobasal
complex of the rat as revealed by the analysis of physiologically characterised neurons injected
intracellularly with horseradish peroxidase. Brain Res. 297:63-74.
26. Pin J-P, and Duvoisin R, (1995) The metabotropic glutamate receptors: Structure and
functions. Neuropharmacology 34:1-26.
27. Salt TE, (1987) Excitatory amino acid receptors and synaptic transmission in the rat
ventrobasal thalamus. J. Physiol. 391: 499-510.
28. Salt TE, and Eaton SA, (1991) Sensory excitatory postsynaptic potentials mediated by
NMDA and non-NMDA receptors in the thalamus in vivo. Eur. J. Neuroscience 3: 296-300.
29. Salt TE, and Eaton SA, (1995) Distinct presynaptic metabotropic receptors for L-AP4 and
CCG1 on GABAergic terminals: pharmacological evidence using novel a-methyl derivative
mGluR antagonists, MAP4 and MCCG, in the rat thalamus in vivo. Neuroscience 65:5-13.
30. Scharfman HE, Lu S-M, Guido W, Adams PR, and Sherman SM, (1990)
N-Methyl-D-aspartate receptors contribute to excitatory postsynaptic potentials of cat lateral
geniculate neurons recorded in thalamic slices. Proc. Nat. Acad. Sci. (USA) 87: 4548-4552.
31. Shigemoto R, Nakanishi S, and Mizuno N, (1992) Distribution of the mRNA for a
metabotropic glutamate receptor (mGluR1) in the central nervous system: An in situ
hybridization study in adult and developing rat. J. Comp. Neurol. 322:121-135.
32. Shigemoto R, Nomura S, Ohishi H, Sugihara H, Nakanishi S, and Mizuno N, (1993)
Immunohistochemical localization of a metabotropic glutamate receptor, mGluR5, in the rat brain.
Neuroscience Letters 163:53-57.
33. Sillito AM, Murphy PC, and Salt TE, (1990) The contribution of the
non-N-methyl-D-aspartate group of excitatory amino acid receptors to retinogeniculate
transmission in the cat. Neuroscience 34: 273-280.
34. Sillito AM, Jones HE, Gerstein GL, and West DC, (1994) Feature-linked synchronization of
thalamic relay cell firing induced by feedback from the visual cortex. Nature 369:479-482.
35. Thomsen C, Boel E, and Suzdak PD, (1994) Actions of phenylglycine analogs at subtypes of
the metabotropic glutamate receptor family. Eur. J. Pharmacology 267:77-84.
36. Turner JP, Leresche N, Guyon A, Soltesz I, and Crunelli V, (1994) Sensory input and burst
firing output of rat and cat thalamocortical cells: the role of NMDA and non-NMDA receptors. J.
Physiol. 480:281-295.
37. Watkins JC, and Collingridge G, (1994) Phenylglycine derivatives as antagonists of
metabotropic glutamate receptors. Trends In Pharmacological Sciences 15:333-342.
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.