SEROTONIN-EVOKED MODIFICATIONS OF THE NEURONAL FIRING RATE IN THE SUPERIOR VESTIBULAR NUCLEUS - A MICROIONTOPHORETIC STUDY IN THE RAT

Microiontophoretic ejection (10-100 nA) of serotonin (5-hydroxytryptamine) into the superior vestibular nucleus induced modifications of the mean firing rate in 87% of the neurons examined. The responses to 5-hydroxytryptamine application were excitatory in 48% of the cells, inhibitory in 29%, and biphasic (inhibitory/excitatory) in the remaining 10%. The excited neurons were scattered throughout the nucleus; the units inhibited or characterized by biphasic responses were distinctly more numerous in the ventrolateral sector of the nucleus. The magnitude of both excitatory and inhibitory effects was doge-dependent. The excitatory responses to 5-hydroxytryptamine were blocked or greatly reduced by two 5-hydroxytryptamine antagonists, methysergide and ketanserin, or even reversed in many cases. Inhibitory responses were enhanced by simultaneous application of 5-hydroxytryptamine antagonists in half of the units studied. In the remaining units, ketanserin left the response unmodified, whereas methysergide reduced but never quite blocked it. The application of 5-methoxy-NN-dimethyltryptamine, a 5-hydroxytryptamine agonist more effective on 5-hydroxytryptamine, than on 5-hydroxytryptamine2 receptors, and of 8-hydroxy-2(di-n-propyl-amino) tetralin, a 5-hydroxytryptamine1A-specific agonist, induced a decrease in the firing rate which was unaffected by methysergide. These results support the hypothesis that 5-hydroxytryptamine exerts various functions throughout the superior vestibular nucleus by various receptors and that the inhibitory action is limited to an area of it. A comparison between the responses to 5-hydroxytryptamine of lateral vestibular neurons, mostly projecting to the spinal cord, and the present data, which were detected in units projecting to the oculomotor system, demonstrate that the responses to this indoleamine are more various in pattern, even if less strong in magnitude, in superior than in the lateral vestibular nucleus.