THE NEUROTRANSMITTER OF INHIBITORY SYNAPTIC INPUTS ON INHIBITORY BURST NEURONS (IBNS) CONTROLLING VESTIBULAR-NYSTAGMUS IN THE CAT

In the pontine neural networks that govern vestibular nystagmus in the cat, inhibitory burst neurons (IBNs) are known to fire in bursts during the quick phase, and to suppress firing of the abducens motoneurons on the contralateral side. The present experiments were designed to identify the neurotransmitter controlling the burst firing pattern of IBNs. Inhibitory burst neuron activity was recorded extracellularly, and various chemicals were applied to the IBNs iontophoretically through multibarrel micropipettes. GABA and muscimol (a GABA(A)-receptor agonist) strongly suppressed IBN activity and eliminated the burst pattern. Bicuculline (a GABA(A)-receptor antagonist) increased the firing of IBNs and suppressed the inhibitory effect of GABA when applied simultaneously. Although baclofen (a GABA(B)-receptor agonist) had no inhibitory effect, it slightly shortened the duration of burst firing. Neither glycine nor serotonin, other inhibitory transmitter candidates, nor their respective antagonists, strychnine and methysergide, had any effect. Systemically administered picrotoxin prevented the GABA-induced suppression of IBNs. These results suggest that IBNs possess GABA(A) receptors and are controlled by higher GABA-liberating neurons and Cl- channels.