DIURNAL CHANGES OF GABA TURNOVER RATE IN BRAIN AND PINEAL-GLAND OF SYRIAN-HAMSTERS

Circadian rhythms of GABA turnover rate in cerebral cortex, preoptic a rea-medial basal hypothalamus (PMBH), cerebellum, and pineal gland wer e 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 turnov er rate was measured by the increase of GABA levels following inhibiti on of GABA-transaminase by gamma-acetylenic GABA. Under long photoperi ods, a significant rhythm of GABA turnover was detected in the four ar eas studied (cerebral cortex, PMBH, cerebellum, and pineal gland), wit h maxima at night. A Cosinor analysis indicated acrophases which varie d from 2300 to 0400 h (3rd to 8th h of darkness). Under short photoper iods, 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 eith er during the light phase of daily photoperiod (PMBH) or at night (cer ebellum, pineal gland). Steady state levels of GABA also changed perio dically in the same brain regions under both lighting environments, al though phase relationships of circadian rhythms in GABA content and tu rnover rate varied significantly among tissues, as well as on photoper iodic conditions.