Glycinergic inhibition in thalamus revealed by synaptic receptor blockade

Using juvenile rat brain slices, we examined the possibility that strychnine-sensitive receptors for glycine-like amino acids contributed to synaptic inhibition in ventrobasal thalamus, where γ-aminobutyrate (GABA) is the prevalent inhibitory transmitter. Ventrobasal nuclei showed staining for antibodies against α1 and α2 subunits of the glycine receptor. Exogenously applied glycine, taurine and β-alanine increased membrane conductance, effects antagonized by strychnine, indicative of functional glycine receptors. Using glutamate receptor antagonists, we isolated inhibitory postsynaptic potentials and currents (IPSPs and IPSCs) evoked by high-threshold stimulation of medial lemniscus. Like the responses to glycine agonists, these synaptic responses reversed near ECl. In comparative tests with GABA receptor antagonists, strychnine attenuated inhibition in a majority of neurons, but did not alter slow, GABAB inhibition. For complete blockade, the majority of fast IPSPs required co-application of strychnine with bicuculline or gabazine, GABAA receptor antagonists. Strychnine acting with an IC50≅33 nM, eliminated residual fast inhibition during selective GABAA receptor blockade with gabazine. The latency of onset for IPSPs was compatible with polysynaptic pathways or prolonged axonal propagation time. Strychnine lacked effects on monosynaptic, GABAergic IPSPs from zona incerta. The specific actions of strychnine implicated a glycine receptor contribution to fast inhibition in somatosensory thalamus.