Circadian thermosensitive characteristics of suprachiasmatic neurons in vitro.

The circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) affects several regulatory systems, including body temperature. To study circadian changes in the firing rate and thermosensitivity of SCN neurons, single-unit activity was recorded from the dorsomedial and ventrolateral SCN in frontal slices of rat hypothalamus during changes in tissue temperature. When analyzed according to circadian time (CT), 305 neurons were characterized by firing rate and 208 neurons were characterized by thermosensitivity. Circadian firing-rate changes were evident only in the dorsomedial SCN, with peak firing rates occurring during the subjective day. Circadian changes in SCN neuronal thermosensitivity also were observed, but the greatest thermosensitivity occurred during the subjective night. Increased thermosensitivity was most dramatic in the ventrolateral SCN, where > 40% of the neurons were warm sensitive in the CT 16- to 20-h period. These changes in neuronal thermosensitivity may reflect interactions between body temperature and circadian rhythms.