Influence of lateral septum and amygdala stimulation on the excitability of hypothalamic supraoptic neurons. An electrophysiological study in the rat.

Extracellular recordings were obtained from 116 phasically-active (putative vasopressinergic) and 113 continuously-active (putative oxytocinergic) neurosecretory neurons in the hypothalamic supraoptic nucleus of urethane or pentobarbital anesthetized male Sprague-Dawley rats. Single 1 Hz pulse stimulation in most regions of the amygdala and the ipsilateral lateral septum was followed by a transient (20-140 ms) reduction in the excitability of more than 90% of responsive cells; one third displayed a reduction in excitability to both amygdala and lateral septum stimulation. Amygdala or lateral septum stimuli delivered in brief trains of 20-100 pulses at 5-20 Hz during ongoing phasic discharges could induce silent periods lasting up to 30 or more seconds beyond the time of application. The same stimuli also reduced ongoing activity among continuously-firing SON cells but their response lasted only as long as the duration of the applied stimulus. These data suggest that neurons in both the ipsilateral septum and the various amygdaloid nuclei exert a predominantly inhibitory influence on the excitability of both vasopressinergic and oxytocinergic SON neurons in the rat.