Activity of neurons of the hypothalamic suprachiasmatic nuclei and circadian rhythms: the role of the suprachiasmatic nuclei in rhythmic activities of neurons in the lateral hypothalamic area and ventromedian nuclei and the control of the pineal gland

Abstract

Unit activity of lateral hypothalamic area (LHA) and ventromedian nuclei (VMN) was recorded in urethane anesthetized male rats. A 5 to 10 sec, a 3 to 5 min and a circadian rhythmicity were observed. In about 15% of all neurons, spontaneous activity of LHA and VMN showed reciprocal relationships. Subthreshold stimuli applied at a slow rate in the septum and the suprachiasmatic nuclei (SCN) suppressed the rhythms without changing firing rates. On the other hand stimulation of the optic nerve at a rate of 5 to 10/sec increased firing rates in one third of neurons of SCN. Iontophoretically applied acetycholine increased 80% of tested neurons of SCN, whereas norepinephrine, dopamine and 5HT inhibited 64%, 60% and 75% of SCN neurons, respectively. These inhibitions were much stronger in neurons, the activity of which was increased by optic nerve stimulation. Stimulation of the SCN inhibited the tonic activity in cervical sympathetic nerves. These data suggest that SCN plays a significant role in controlling the rhythmic activity of LHA, VMN and the pineal gland. Dopamine and 5HT may be involved in regulation of the firings of SCN neurons.