Role of the suprachiasmatic nuclei in the circadian timing system of the squirrel monkey. I. The generation of rhythmicity.

The circadian organization of squirrel monkey (Saimiri sciureus) drinking behavior was evaluated before and after the placement of radiofrequency lesions which completely destroyed the suprachiasmatic nuclei (SCN) in 4 monkeys and partially ablated the SCN in another 4 animals. In continuous illumination (LL: 600 lux) prior to surgery, each monkey had a precise free-running circadian rhythm of drinking behavior with a period of 25.31 +/- 0.21 h (means +/- S.E.M.). By 4-6 weeks following the lesions, the temporal organization of drinking behavior had become disrupted, but a statistically significant free-running circadian rhythm was still detectable by time series analyses. Subsequently, the circadian organization of drinking behavior in 7 out of 8 monkeys gradually decayed with either no statistically significant rhythmicity or only weak circadian and/or ultradian rhythmicity detected by time series analyses by 10-92 weeks post-lesion. The remaining animal which maintained a statistically reliable free-running rhythm in drinking behavior received the least damage (less than 50%) to the SCN. Despite the major alterations in the temporal patterning of behavior, the overall amount of drinking behavior per 24 h was unchanged. The SCN are thus essential for maintaining the circadian organization of squirrel monkey drinking behavior. However, the existence of residual circadian rhythmicity following SCN lesions and the gradual decay of circadian organization thereafter suggest that the SCN may coordinate the activity of circadian oscillators which lie outside its borders.