SUPRACHIASMATIC NUCLEI AND CIRCADIAN-RHYTHMS - ROLE OF SUPRACHIASMATIC NUCLEI ON RHYTHMIC ACTIVITY OF NEURONS IN LATERAL HYPOTHALAMIC AREA, VENTROMEDIAN NUCLEI AND PINEAL-GLAND

Unit activity of lateral hypothalamic area (LHA) and ventromedian nuclei (VMN) was recorded in urethane anesthetized male rats. At 5-10 s, a 3-5 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. Stimulation of the optic nerve at a rate of 5-10/s increased firing rates in 1/3 of the neurons of SCN. Iontophoretically applied acetylcholine increased the firing rate in 80% of the tested neurons of SCN, whereas norepinephrine, dopamine and 5HT [5-hydroxytryptamine] inhibited 64, 60 and 75% of the SCN neurons, respectively. These inhibitions were much stronger in neurons whose activity was increased by optic nerve stimulation. Stimulation of the SCN inhibited the tonic activity in cervical sympathetic nerves. Apparently, 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.