Circadian oscillation of the multiple unit activity in the guinea pig suprachiasmatic nucleus

In the guinea pig with chronically implanted electrodes, neuronal multiple unit activity (MUA) was recorded inside and outside the suprachiasmatic nucleus (SCN). Long-term recording of the SCN indicated distinct daily rhythms with a daytime peak in MUA during a 24-h light-dark (LD 12:12) cycle. On the other hand, MUA recorded from adjacent hypothalamic regions outside the SCN showed a phase reversal with a nighttime peak, similarly to the rat. The amplitude of the rhythms recorded outside the SCN was much smaller (one-half to one-quarter) than that inside the SCN. These rhythms persisted during constant darkness indicating characteristics of endogenous circadian rhythmicity. When the external light-dark cycle was delayed abruptly for 12 h, MUA rhythms showed a gradual phase shift taking 7-10 days for complete reentrainment. Overt behavior including sleep-wakefulness did not show significant and consistent daily or circadian rhythms in spite of the distinct oscillation in neuronal activity inside the SCN.