Circadian and other rhythmic activity of neurones in the ventromedial nuclei and lateral hypothalamic area.

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Abstract
The frequency of firing was simultaneously recorded from single neurons of the ventromedial nuclei (VMN) and the lateral hypothalamic area (LHA) in urethane anesthetized rats for many hours. There were circadian changes of VMN and LHA neuron activity. The pattern of this circadian rhythm was as follows: throughout the day LHA neurons showed higher activity than that of VMN, as indicated by higher frequency and more fluctuations in their rates of firing. In late afternoon the discharge rate of LHA neurons increased further, showing oscillations of short duration. In the early evening hours LHA neuron activity gradually went down, as the VMN neurons became active. Throughout the night, VMN neurons were more active than those of LHA, just the opposite of the day period. In early morning hours VMN neurons gradually became quiet, while LHA neurons began to show activity. Superimposed on the circadian rhythm, at certain periods of the day, VMN and LHA neurons showed short duration oscillations in rate of firing, roughly every 7-15 s and every 3-5 min. Activities in neurons of the VMN and LHA were reciprocally related; a decrease in firing rate of one was associated with an increase in the other. This phenomenon was shown clearly by analysis of auto- and cross-correlation functions of firing patterns of VMN and LHA neurons. The effects of stimulations of the prefrontal cortex and splanchnic afferents on VMN and LHA neurons depended on the basic firing frequency, thus they varied with the time of day. Definite relationships existed between basic firing frequency of a cell and the magnitude of changes evoked by these stimuli. Reactions of VMN and LHA neurons were the opposite in most instances. Septal stimulations (at more than 10/s) always produced inhibition of LHA neuron activity. I.v. injection of glucose inhibited LHA neurons and accelerated firing of VMN cells. This was true during the day and at night when background activities of VMN and LHA neurons were different from that of the day. Stimulation of the septal area with subthreshold pulses at a low rate (1-0.3/s) suppressed or altered oscillations in firing frequency of LHA neurons. Severance of connection between LHA and structures caudal thereto had no effect on LHA neuron firing rates or rhythms. Sections between the septal area and LHA abolished or greatly altered the oscillatory rhythms of LHA cell activity, although spontaneous discharges continued at a somewhat lower rate for periods of hours. Stimulation of suprachiasmatic nuclei with weak intensity and low frequency also changed oscillatory fluctuations in firing of LHA neurons. Possible origins of circadian rhythm and oscillations of short duration in firing pattern of VMN and LHA neurons were discussed.