Amino acid‐mediated EPSPs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord.

Abstract

1. Fast excitatory postsynaptic potentials (EPSPs) evoked by stimulation of A delta and C fibres were examined by intracellular recording from substantia gelatinosa (SG) neurones in a transverse slice preparation of adult rat spinal cord. 2. Single low‐intensity stimuli applied to the dorsal root activated A delta fibres and evoked monosynaptic EPSPs in 70% of SG neurones. In 5% of SG neurones, increasing the intensity and duration of stimulation evoked solely C fibre‐mediated EPSPs. About 20% of neurones received both A delta and C fibre input from primary afferents. 3. Low concentrations of tetrodotoxin (TTX, approximately 50 nM) blocked EPSPs evoked by stimulation of A delta fibres without affecting those evoked by C fibre stimulation. Higher concentrations of TTX (500 nM) also blocked C fibre‐evoked responses. 4. EPSPs evoked by A delta and C fibre stimulation reversed in polarity at membrane potentials near 0 mV, similar to the reversal potential of spontaneous EPSPs and of the potential change evoked by exogenous glutamate. 5. A delta and C fibre‐evoked EPSPs were depressed by kynurenate and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX); C fibre‐evoked EPSPs appeared to be less sensitive. 6. In the presence of TTX, only 50% of SG neurones were depolarized by L‐glutamate. However, neurones which exhibited no direct response to L‐glutamate received afferent‐evoked EPSPs which were sensitive to CNQX. In sensitive neurones, the depolarization evoked by L‐glutamate was depressed by only approximately 15% in the presence of CNQX, whereas afferent‐evoked EPSPs recorded from the same neurone were almost completely suppressed. Combined application of DL‐2‐amino‐5‐phosphonovaleric acid (APV) and CNQX depressed the response to L‐glutamate by only approximately 25%. 7. These findings suggest that A delta and C fibres use L‐glutamate or a related amino acid as a transmitter at synapses with substantia gelatinosa neurones. The postsynaptic actions of this transmitter are mediated predominantly by non N‐methyl‐D‐aspartic acid (NMDA) receptors. The failure of CNQX and APV to completely block the L‐glutamate‐evoked depolarization of substantia gelatinosa neurones raises the possibility that exogenously applied L‐glutamate activates a non‐NMDA receptor distinct from that which mediates the actions of the synaptically released afferent transmitter.