The effects of continuous light exposure on Syrian hamster suprachiasmatic (SCN) neuronal discharge activity in vitro.

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.