Excitation of hypothalamic paraventricular neurons by stimulation of the raphe nuclei

Abstract

Extracellular recordings were made from 467 anti dromically identified neurosecretory neurons and 148 non-neurosecretory neurons in the paraventricular nucleus of the hypohalamus of hemispherectomized cats under pentobarbital anesthesia. Stimulation of the dorsal, median, and pontine raphe nuclei excited 31%, 26%, and 12% of neurosecretory neurons tested, respectively, and inhibited 9%, 7%, and 8%. The excitatory responses in 13 of 14 neurons tested were blocked by either of two intravenously administered 5-HT 2 antagonists, cyproheptadine or methysergide. The 5-HT 1A antagonist, (−)pindolol, partially blocked the excitatory responses elicited by raphe stimulation in three of live neurons tested. The inhibitory responses to raphe stimulation were not affected by application of these antagonists. More non-neurosecretory neurons than neurosecretory neurons were excited in response to raphe stimulation and these excitatory responses were also blocked by these antagonists. We conclude that most electrically stimulated synaptic inputs from the midbrain raphe nuclei to the hypothalamic paraventricular nucleus are excitatory and are mainly mediated by 5-HT 2 receptors.