Effect of photoperiod reversal on twenty-four hour patterns for GABA levels in rat brain.

GABA levels were measured in various areas of rat brain at 3 or 6 hour intervals for a period of 24 hours in animals adapted for 3 weeks to a programmed-lighting schedule. On the normal cycle rats were illuminated from 08 to 20 hours; on the reverse cycle, from 20 to 08 hours. The cerebellar pattern is unimodal with a peak in the dark at 06 hours and a trough in the light at 18 hours which coincides with maxima and minima of motor activity in the nocturnal rat. The cortical pattern also peaks in the dark at 06 hours but is less well-defined. On the reverse illumination cycle both patterns peak significantly in the dark at 18 hours. Only the cerebellum displays significant differences when averaged GABA levels for light and dark phases are compared. A circadian pattern peaking in the dark described for GABA levels of the corpora quadrigemina which is partly-reversed by reversing the photoperiod. The rhythm in thalamus-hypothalamus which peaks in the dark is complex, consisting of an ultradian rhythm super-imposed on a circadian one. The former rhythm appears to be endogenous and unaltered by a change in the illumination cycle; the latter exogenous since it varies with the illumination cycle. Lower GABA levels in rat brain were found after photoperiod reversal.