Junctional versus extrajunctional glycine and GABAA receptor-mediated IPSCs in identified lamina I neurons of the adult rat spinal cord

Colocalization of GABA and glycine in synaptic terminals of the superficial dorsal horn raises the question of their relative contribution to inhibition of different classes of neurons in this area. To address this issue, miniature IPSCs (mIPSCs) mediated via GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs) were recorded from identified laminae I-II neurons in adult rat spinal cord slices. GABA(A)R-mediated mIPSCs had similar amplitude and rise times, but significantly slower decay kinetics than GlyR-mediated mIPSCs. Lamina I neurons appeared to receive almost exclusively GlyR-mediated mIPSCs, even after application of hypertonic solutions. Yet, all neurons responded to exogenous applications of both GABA and glycine, indicating that they expressed both GABA(A)Rs and GlyRs. Given that virtually all glycinergic interneurons also contain GABA, the possibility was examined that GABA(A)Rs may be located extra-synaptically in lamina I neurons. A slow GABA(A)R-mediated component was revealed in large, but not minimally evoked monosynaptic IPSCs. Administration of the benzodiazepine flunitrazepam unmasked a GABA(A)R component to most mIPSCs, suggesting that both transmitters were released from the same vesicle. The isolated GABA(A)R component of these mIPSCs had rising kinetics 10 times slower than that of the GlyR component (or of GABA(A)R mIPSCs in lamina II). The slow GABA(A)R components were prolonged by GABA uptake blockers. It is concluded that, whereas GABA and glycine are likely released from the same vesicle of transmitter in lamina I, GABA(A)Rs appear to be located extrasynaptically. Thus, glycine mediates most of the tonic inhibition at these synapses. This differential distribution of GABA(A)Rs and GlyRs confers distinct functional properties to inhibition mediated by these two transmitters in lamina I.