PHARMACOLOGICAL CHARACTERIZATION OF EXCITATORY SYNAPTIC TRANSMISSION IN THE CAT RED NUCLEUS IN-VIVO

Extracellular recordings were made from the magnocellular neurones of the red nucleus (mRN) in anaesthetised cats. A study was made of the e ffects of selective excitatory amino acid receptor antagonists on exci tatory monosynaptic responses evoked from the sensorimotor cortex (SMC ) and cerebellar interpositus nucleus (IPN). Iontophoretically applied CNQX and NBQX antagonised both SMC and IPN responses whereas, D-AP5 i nhibited the SMC response but was ineffective to the IPN. At currents that selectively antagonised NMDA responses, CPPene had no effect on e ither SMC or IPN responses. 7-chlorokynurenate inhibited both SMC and IPN responses but required currents that antagonised both AMPA and NMD A responses and was therefore acting in a non-selective manner. Iontop horetically applied glycine was inhibitory to both agonist and synapti c responses, whilst D-serine potentiated NMDA responses but did not en hance monosynaptic responses of the SMC. However in the presence of ei ther 7-chlorokynurenate or high currents of CNQX that reduced the SMC synaptic activation of the mRN neurones, D-serine attenuated the inhib itory action of these antagonists. It is concluded that monosynaptic r esponses from the SMC are mediated by both NMDA and non-NMDA receptors whereas the monosynaptic responses evoked from the IPN are mediated o nly by non-NMDA receptors. The lack of effect of CPPene is consistent with the postulate that two NMDA receptor subtypes are present on mRN neurones.