Influence of dorsal hippocampus stimulation on the excitability of medial hypothalamic neurons in the rat.

Extracellular recordings from medial hypothalamic neurons in pentobarbital-anesthetized male Sprague-Dawley rats were evaluated during electrical stimulation in the ipsilateral and contralateral dorsal hippocampus/fimbria region. Similar short latency orthodromic (excitatory or inhibitory) responses were noted from both ipsilateral and contralateral stimulation sites indicating a bilateral projection. 14.5% of neurons responded to both stimuli, usually with a similar pattern of response. When neurons were also tested with ipsilateral basal or corticomedial amygdala stimulation, 21.5-22.8% of cells displayed orthodromic responses to both the hippocampus and the amygdala stimulus. 12.5% of medial hypothalamic tuberoinfundibular neurons also responded orthodromically to the hippocampal stimulus; a preceding stimulus to the amygdala could block this response in two of three tuberoinfundibular cells. These observations provide preliminary electrophysiological analysis of the functional connectivity of hippocampal and amygdala afferents to medial hypothalamic neurons that may underlie the influence of these extrahypothalamic regions on adenohypophyseal secretions.