Maintained discharge of rat suprachiasmatic neurons at different adaptation levels.

In the rat the response of suprachiasmatic nucleus neurons to steady retinal illumination at different adaptation luminances was studied. The results confirm that in this nucleus the discharge rate is more commonly increased than suppressed by light. Interval histograms indicate that a complex process underlies the discharge at any adaptation level. The suppressive or excitatory effect of light has a high threshold. Phasic stimuli appear to be relatively ineffective. These findings are consistent with the retino-suprachiasmatic function in circadian rhythm synchronization. An attempt was made to separate the effects of lateral geniculate from direct retinal inputs to the hypothalamus.