Circadian rhythms of tonic immobility in the rat: Evidence of an endogenous mechanism

Circadian rhythms of tonic immobility were found in male albino rats raised on a 12-h diurnal light cycle and tested at 6-h intervals. Durations of immobility were twice as long at 2000 h as at 1400 h. These differences persisted when rats were exposed to constant darkness for 10 days, but disappeared when rats were maintained in constant light for the same period. Since endogenous circadian rhythms of certain monoamine levels persist in constant darkness yet disappear under constant light, it is suggested that cycles of tonic immobility in rats are also endogenous. When the diurnal pattern of tonic immobility duration is compared to that of various neurohumors, immobility duration appears to parallel melatonin production and to be opposite in phase to the cycle of serotonin levels. Duration of immobility increased over trials, although the number of inductions required to produce immobility decreased. This suggests that instrumental conditioning may modify the immobility response to some extent.