Action of cholecystokinin and serotonin on lateral hypothalamic neurons of rats.

Discharges of spontaneously active lateral hypothalamic neurons were extracellularly recorded during iontophoretic administration of cholecystokinin (CCK-8S) or/and serotonin (5-HT) in anesthetized rats. The main results are the following. (1) The proportion of neurons responsive to CCK-8S was 62% (61/99) and that responsive to 5-HT 42% (33/78). (2) Out of the neuronal sample, 36% were influenced by both transmitters, allowing an interaction between the two systems. (3) Co-ejection of CCK and 5-HT elicited a response in 40% of the tested neurons, which was a significantly smaller responsiveness than with separate ejection of CCK-8S. The effect resulted from a reduced number of excited neurons whereas the number of inhibitions did not change. The results show that effects of 5-HT and CCK can converge on the same neuron within the lateral hypothalamus. This might be of relevance in the regulation of feeding behavior.