PRIMARY AFFERENT-EVOKED GLYCINE-MEDIATED AND GABA-MEDIATED IPSPS IN SUBSTANTIA-GELATINOSA NEURONS IN THE RAT SPINAL-CORD IN-VITRO

1. The possible roles of glycine and gamma-aminobutyric acid (GABA) as inhibitory transmitters in the spinal dorsal horn were studied by intracellular recordings from substantia gelatinosa (SG) neurones in transverse slices of the adult rat spinal cord which retained an attached dorsal root. 2. Stimulation of primary afferent BS fibres evoked an initial excitatory postsynaptic potential (fast EPSP) followed by a short and/or long inhibitory postsynaptic potential (short and long IPSP). The short IPSP, observed in twenty-nine SG neurones (37%) which received inhibitory inputs, had a mean latency of 3.6 ms and a half-decay time of 11 ms, while the long IPSP had a mean latency of 3.7 ms and a half-decay time of 42 ms and was observed in thirty-seven SG neurones (47%). The remaining twelve neurones (16%) exhibited both short and long IPSPs. Both IPSPs reversed polarity at a membrane potential of -70 +/- 4 mV. The short IPSP was reversibly blocked by the glycine receptor antagonist strychnine (0.5-2 mu M), while the long IPSP was reversibly blocked Toy the GABA(A) receptor antagonist bicuculline (10-20 mu M). 3. In the majority of SG neurones, the short and long IPSPs appeared to be disynaptic and were blocked;by the non-lv-methyl-D-aspartic acid (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-10 mu M). Both IPSPs were less sensitive (depressed by less than 30%) to the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 50-100 mu M). 4. In ten SG neurones (13%), bath-applied glutamate (0.5-2 mM) increased the amplitude and frequency of IPSPs, which had a similar time course to that of the short IPSP evoked by afferent BS fibres. The glutamate-induced short IPSPs were blocked by tetrodotoxin (0.5 mu M) or strychnine (0.5-1 mu M). In twelve neurones (16%), glutamate hyperpolarized the membrane or increased the amplitude and frequency of IPSPs that had a similar time course to that of the AS fibre-evoked long IPSPs. The glutamate-induced membrane hyperpolarization and long IPSPs decreased in amplitude with membrane hyperpolarization and reversed polarity at -70 +/- 6 mV. These hyperpolarizing responses were blocked by tetrodotoxin (0.5 mu M) or bicuculline (10 mu M). 5. These observations suggest that primary afferent A delta fibres activate glycinergic and/or GABAergic interneurones primarily through the non-NMDA receptor subclass and result in inhibition of nearby SG neurones in the dorsal horn of the spinal cord. This inhibitory circuitry may play an important role in modulation of nociceptive transmission in the spinal dorsal horn.