Brain Research, 398 (1986) 403-406 403 Elsevier BRE 21900 Melatonin increases in vivo GABA accumulation in rat hypothalamus, cerebellum, cerebral cortex and pineal gland Ruth E. Rosenstein and Daniel P. Cardinali Centro de Estudios Farmacologicos y de Principios Naturales, CEFAPRIN. Buenos Aires (Ar~entitmJ (Accepted 29 July 1986) Key words: 7-Aminobutyric acid (GABA): Melatonin: Pineal gland: Cerebellum; Hypothalamus; Cerebral cortex The effect of melatonin on in vivo y-aminobutyric acid (GABA) accumulation in several brain regions was determined by measuring the increase of GABA levels following inhibition of GABA transaminase. A single melatonin injection (25 300 ug/kg) augmented sig- nificantly, by 17-20%, GABA accumulation in the hypothalamus and caused a dose-dependent increase of this parameter in the pinc- al gland. Significant rises of GABA accumulation were found in the cerebellum and cerebral cortex after administering 100 300 and 300 ug/kg of melatonin, respectively. The critical role of the pineal gland in the photope- riodic circannual control of endocrine activity of cer- tain mammals has been well established 9"u22. This function is mediated by the secretion of melatonin, the discrete, nocturnal elevation of this hormone en- coding daylength information to the neuroendocrine apparatus. Melatonin may also participate as a me- diator of circadian variations of sleep and sleepiness in man eS. Melatonin acts on receptor sites in the brain, particularly the hypothalamus, by affecting several presumptive second messengers, including cyclic nucleotides, prostaglandins and calcium in neuronal and glial cells ~'. However its primary mech- anism of action in the brain remains elusive. Recent data revealed 7-aminobutyric acid (GABA)-containing neurons as one of the possible CNS targets for pineal melatonin. Pineal removal, a procedure that causes a convulsive-prone state ~ 2<>, disrupts the diurnal rhythmicity of GABA 2 and benzodiazepine I binding in rat brain, by increasing site number of the former and decreasing that of the latter. On the other hand, melatonin, a compound causing mild sedation and torpor 23'25, restored nor- mal brain GABA and benzodiazepine binding of pinealectomized rats 121~. Although pinealectomy decreased brain GABA content ~, conflictive reports have been published about melatonin activity which was found either to increase -~ or decrease Is brain GABA content. Since this apparent contradiction could be due to a complex effect of melatonin on the synthesis and/or degradation of brain GABA, we considered it worthwhile to examine the effect of the pineal hormone on the in vivo turnover of GABA in several regions of the rat brain. GABA accumula- tion, an estimate of in vivo GABA turnover 5'13'15, was determined by measuring the increase of GABA levels following inhibition of the GABA-},-ketoglu- taric acid aminotransferase (GABA transaminase) by },-acetylenic GABA (4-amino-hex-5-enoic acid, GAG). GABA accumulation assessed in this way correlates with traffic neural signals in GABAergic neurons of rat substantia nigra ~. Adult male Wistar rats were kept under light from 07.1)0 to 19.00 h daily and were given free access to water and food. Animals received a single s.c. injec- tion of 25-300,ug/kg of melatonin (Sigma Chemicals, St. Louis, MO, U.S.A.) or vehicle (saline-ethanol, 9:1, v/v) at 09.00 h and were killed 3 h later. The GABA transaminase inhibitor GAG (a gift from Merrel Dow Research Institute, Strasbourg, France) ('orre.v~ondence: D.P. Cardinali. Ccntro de Estudios Farmacol6gicos y de Principios Naturales. C'EFAPRIN Serrano 665, 1414 Bue- nos Aires, Argentina.