The role of suprachiasmatic nuclei of the hypothalamus in the production of circadian rhythm.

(1) Action potentials were recorded from single neurons in suprachiasmatic nuclei of hypothalamus in urethane anesthetized male rat. The rate of firing ranged from less than 1 to over 10/sec but was generally 4-8/sec, and it varied from one cell to another in the same animal. (2) Repetitive stimulation of optic nerve or light acting on the eye augmented the activity of approximately half of the the suprachiasmatic neurons examined (67 out of 159 cells) while 37 neurons (23%) showed clear inhibition by the same stimuli. (3) Stimulation of the suprachiasmatic nuclei strongly inhibited the electrical activity of cervical sympathetic nerves. Light or optic nerve stimulation also inhibited activity of cervical sympathetic nerves. (4) The rate of discharge of suprachiasmatic nuclei showed, at times, some short duration oscillations occurring every 3-5 min, but at other times the same neuron showed a steady low frequency of firing. (5) Projection of optic nerves to the suprachiasmatic nuclei was demonstrated by anterograde migration of horseradish peroxidase placed in the vitreous body. (6) It was suggested that light excites certain groups of neurons in the suprachiasmatic nuclei which exert inhibitory action on cervical sympathetic nerve. This, in turn, caused a reduction in norepinephrine release by nerve fibers innervating the pineal and a reduction in pineal enzyme production. Thus, neurons in these nuclei contribute to the suppression of pineal enzyme production produced by light.