Role of the suprachiasmatic nuclei in the circadian timing system of the squirrel monkey. II. Light-dark cycle entrainment

Abstract

The role of the suprachiasmatic nuclei (SCN) in light-dark (LD) cycle entrainment in the squirrel monkey (itSaimiri sciureus) was evaluated by continuously monitoring core body temperature and drinking behavior in both intact and SCN-lesioned animals exposed to different LD conditions. In a 24-h LD cycle with 12 h of light per day (LD 12:12), both intact and SCN-lesioned monkeys had a prominent rhythm in body temperature with an acrophase 6.51 ± 0.36h (SCN-lesioned) after light onset. In a 23-h LD cycle composed of 1 hr of light and 22 h of dark (LD 1:22), the temperature rhythm in both intact and SCN-lesioned monkeys became synchronized with the 23-h period of the LD cycle. Studies examining the effects of LD cycle exposure on the temporal pattern of drinking behavior indicated that both intact and SCN-lesioned monkeys confined their drinking to the daily period of light. Each monkey was subsequently exposed to an LD cycle with 6 h of bright light and 18 h of darkness or dim background illumination (i.e. 60:0, 66:6 and 76:16 lux cycles). In intact monkeys systematic increases in the average intensity of the daily LD cycle produced corresponding delays in the phase of the daily offset of drinking behavior but did not alter the basic 24-h rhythmicity in the behavior. In contrast, increasing the intensity of the 24-h LD cycle in SCN-lesioned monkeys produced no delay in the daily termination of drinking, but instead produced a disintegration in the 24-h rhythm of drinking behavior. These data indicate that 24-h drinking rhythms in SCN-lesioned monkeys housed under LD cycles are not generated by the circadian system but are the result of a ‘masking’ effect of the LD cycle on drinking behavior. These studies demonstrate that the SCN play a fundamental role in LD cycle entrainment of circadian drinking behavior but are not required for LD cycle entrainment of the circadian rhythm of core body temperature, or the mediation of the masking effects of LD cycles on drinking behavior.