Alpha-bungarotoxin binding in relation to functional organization of the rat suprachiasmatic nucleus.

[125I]alpha-Bungarotoxin (alpha-Btx) binding was quantified in autoradiographs of in vitro labeled sections through the rat suprachiasmatic nucleus (SCN). Recent evidence suggests that acetylcholine and alpha-Btx binding sites in the SCN region may mediate effects of light on circadian functions. In contrast to the ventrolateral termination of retinal axons, there was a consistent pattern of relatively high alpha-Btx binding levels in the dorsolateral SCN and low levels in the ventromedial SCN. In addition, enucleation had little or no effect. We also investigated the possibility that phase-dependent influences of light on circadian functions could reflect the presence of light-induced or daily rhythms in alpha-Btx binding. Animals were tested in a light-dark cycle or after 1-2 days in constant light. The dorsolateral SCN showed a small but significant decline of alpha-Btx binding in animals in light at night compared to darkness at night. alpha-Btx binding in the SCN showed no statistically significant daily variation in animals in the light-dark cycle.