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 int racellular recordings from substantia gelatinosa (SG) neurones in tran sverse slices of the adult rat spinal cord which retained an attached dorsal root. 2. Stimulation of primary afferent BS fibres evoked an in itial excitatory postsynaptic potential (fast EPSP) followed by a shor t and/or long inhibitory postsynaptic potential (short and long IPSP). The short IPSP, observed in twenty-nine SG neurones (37%) which recei ved inhibitory inputs, had a mean latency of 3.6 ms and a half-decay t ime of 11 ms, while the long IPSP had a mean latency of 3.7 ms and a h alf-decay time of 42 ms and was observed in thirty-seven SG neurones ( 47%). The remaining twelve neurones (16%) exhibited both short and lon g 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 reversibl y 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 aci d (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-10 mu M). Both IPSPs were less sensitive (depressed by less t han 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 gl utamate (0.5-2 mM) increased the amplitude and frequency of IPSPs, whi ch had a similar time course to that of the short IPSP evoked by affer ent BS fibres. The glutamate-induced short IPSPs were blocked by tetro dotoxin (0.5 mu M) or strychnine (0.5-1 mu M). In twelve neurones (16% ), glutamate hyperpolarized the membrane or increased the amplitude an d frequency of IPSPs that had a similar time course to that of the AS fibre-evoked long IPSPs. The glutamate-induced membrane hyperpolarizat ion and long IPSPs decreased in amplitude with membrane hyperpolarizat ion and reversed polarity at -70 +/- 6 mV. These hyperpolarizing respo nses were blocked by tetrodotoxin (0.5 mu M) or bicuculline (10 mu M). 5. These observations suggest that primary afferent A delta fibres ac tivate glycinergic and/or GABAergic interneurones primarily through th e non-NMDA receptor subclass and result in inhibition of nearby SG neu rones 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.