Effects of serotonin, GABA and glycine on the activity of pause neurons during vestibular nystagmus in the cat.

Pause neurons (PNs), found in the pontine raphe nuclei, are important in the regulation of fast eye movements. The present study focuses on transmitters regulating the activity of the PNs. Extracellular spikes of single PNs and ocular motor nerve discharges were simultaneously recorded during horizontal vestibular nystagmus in the alert cat. Several transmitter candidates were iontophoretically applied, while methysergide, a serotonin antagonist, was administered both systemically and topically. The following data supported the conclusion that PNs are probably under the control of two different types of neurons, and the firing of PNs may be caused partly by GABAergic, but not by glycinergic neurons: 1) serotonin markedly decreased the spontaneous firing of the PNs; 2) methysergide abolished the inhibitory effect of serotonin on the PN firing; however, a pause pattern was maintained under this condition; 3) GABA moderately suppressed the firing but only in 25% of PNs; and 4) glycine hardly changed the PN firing in any case examined.