Diurnal changes of GABA turnover rate in brain and pineal gland of Syrian hamsters.

Circadian rhythms of GABA turnover rate in cerebral cortex, preoptic area-medial basal hypothalamus (PMBH), cerebellum, and pineal gland were examined in Syrian hamsters kept for 3 months under either long (14 h of light/day) or short days (10 h of light/day). In vivo GABA turnover rate was measured by the increase of GABA levels following inhibition of GABA-transaminase by gamma-acetylenic GABA. Under long photoperiods, a significant rhythm of GABA turnover was detected in the four areas studied (cerebral cortex, PMBH, cerebellum, and pineal gland), with maxima at night. A Cosinor analysis indicated acrophases which varied from 2300 to 0400 h (3rd to 8th h of darkness). Under short photoperiods, there were no significant circadian variations in GABA turnover in the cerebral cortex, and the synchronization in turnover rate among the remaining regions was lost, with acrophases being detectable either during the light phase of daily photoperiod (PMBH) or at night (cerebellum, pineal gland). Steady state levels of GABA also changed periodically in the same brain regions under both lighting environments, although phase relationships of circadian rhythms in GABA content and turnover rate varied significantly among tissues, as well as on photoperiodic conditions.