Activity of suprachiasmatic and hypothalamic neurons during sleep and wakefulness in the rat.

Single unit activity was recorded from the suprachiasmatic nucleus (SCN) and preoptic/anterior hypothalamus (POAH) of unrestrained Wistar rats during sleep and wakefulness. Regularly firing cells, which are abundant in in vitro SCN preparations and have been considered the basis of a central neuronal oscillator, were conspicuously absent in this preparation and in other in vivo studies. Most of the 55 cells recorded in the SCN and POAH were characterized by spontaneous firing rates below 12 Hz and with heterogeneous patterns of changes in frequency with arousal states. In vivo neurophysiological studies of the SCN in which the anesthetic agent urethane is used should consider the effect of different levels of arousal, as indicated by the cortical EEG, in evaluating the relationship between sensory stimulation and single unit activity.