THE EFFECTS OF CONTINUOUS LIGHT EXPOSURE ON SYRIAN-HAMSTER SUPRACHIASMATIC (SCN) NEURONAL DISCHARGE ACTIVITY INVITRO

The circadian variation in neuronal discharge activity recorded in vitro from the Syrian hamster suprachiasmatic nucleus (SCN), identified as a circadian clock, was used as an index of SCN circadian function. SCN neurones (n = 89) following in vitro commissural section of the paired SCN showed a similar peak in spontaneous discharge activity during the projected light phase (between CT 06.00 and 08.00 h) as the commissural intact LD 12:12 preparations (n = 230 neurones). Long-term exposure to continuous lighting (LL) may induce either arrhythmicity or splitting of locomotor behaviour. Circadian variation in SCN discharge activity was absent in hamsters (44 cells, 6 animals) showing LL-induced arrhythmicity. In 4 hamsters exhibiting split free-running behaviour two peaks in SCN discharge activity (n = 32 cells) were observed. Fifteen LL free-running animals showed no evidence of splitting or arrhythmicity and subsequent SCN recordings revealed only a single peak in SCN discharge activity. These LL-induced changes in overt circadian behaviour appeared paralleled by changes in neuronal discharge activity of the SCN circadian clock and support the view that the paired SCN act as a mutually coupled circadian oscillator.