Circadian differences in neuronal activity of the suprachiasmatic nucleus in brain slices prepared from photo-responsive and photo-non-responsive Djungarian hamsters.

The suprachiasmatic nucleus (SCN) of the hypothalamus, as the putative generator of circadian rhythmicity, plays an important role in mammalian photoinduction. To determine if SCN function differs in photo-non-responsive Djungarian hamsters, we defined the pattern of spontaneous neuronal discharge of single cells from SCN slices in vitro of photo-responsive and photo-non-responsive phenotypes. Responsive hamsters exhibited a peak neuronal discharge rate (4.8 +/- 0.5 Hz) during the mid day which gradually attenuated through the late day and early night. In non-responsive hamsters, a similar discharge rate (5.1 +/- 0.5) was maintained through the late day and early night. The delayed decline in spontaneous firing rate of non-responders correlates with their delayed activity onset and delayed nocturnal pineal melatonin pulse. These data support the argument that the absence of photoperiodic adjustments in Phodopus sungorus rests with differences in SCN function.