Presynaptic inhibition in the cercal-afferent giant-interneurone synapses of the cockroach, Periplaneta americana L.

The occurrence of presynaptic control of synaptic transmission in the cercal-afferent giant-interneurone system of the cockroach was investigated. Reduction in amplitude (up to 50%, lasting about 200–250 ms) of the compound evoked EPSP followed repetitive (300 to 500 Hz) supra-threshold stimulation of cercal nerves XI. A similar but weaker depressive effect was detected on the unitary EPSP resulting from stimulation of an ipsilateral cercal mechanoreceptor.This inhibition is attributed to multisynaptic inhibitory pathways impinging upon presynaptic excitatory neurones. The involvement of chloride ions is suggested by the observation that both picrotoxin and chloride-deficient salines abolished the inhibitory phenomenon. Presynaptic mannitolgap recording from cercal nerve XI revealed a chloride-dependent hyperpolarization in response to repetitive conditioning stimulation. The time course of this response was similar to that of presynaptic inhibition. Bath-application of GABA (20 mM) produced a chloride-dependent hyperpolarization followed by a depolarization of the intraganglionic part of the cerca-afferents. GABA-induced hyperpolarization and electrically-induced presynaptic hyperpolarization were both reversed in low chloride saline (166 mM chloride). It is proposed that presynaptic modulation of acetyl-choline release occurs at the cercal-afferent giant-interneurone synapses. The role played by GABA is duscussed.