In vitro electrical activity in the suprachiasmatic nucleus following splitting and masking of wheel-running behavior.

Djungarian hamsters (Phodopus sungorus sungorus) were chronically exposed to constant light (60 lux) in order to generate a split rhythm of wheel-running behavior. The animals were killed and coronal hypothalamic slices prepared for extracellular recording from the left and right suprachiasmatic nuclei (SCN). Nine hamsters which exhibited a split in overt behavior also had bimodal peaks (280 cells) of SCN firing frequency (5.4 +/- 1.1 and 5.8 +/- 0.7 Hz); these peaks were about 180 degrees antiphase. Two troughs in firing frequency were also apparent (1.8 +/- 0.4 and 2.4 +/- 0.5 Hz) and coincided with the projected time of wheel-running activity. Differences in circadian pattern of electrical activity between the right and left SCN were not observed. When wheel-running activity in 5 hamsters was suppressed with high intensity (500 lux) constant light, the SCN firing profile (154 cells) failed to exhibit a daily rhythm. Firing frequency was consistently high (5.8 +/- 1.4 Hz) throughout the 24-h period. These results indicate that in vitro electrical activity of the SCN is related to overt behavior in Djungarian hamsters, in that high electrical activity occurs when locomotor activity is minimal.