[Effect of thyrotropin releasing hormone (TRH) on GABA (gamma aminobutyric acid) metabolism in mouse and rat brains: as to the activities of GAD (glutamic acid decarboxylase), GABA-T (GABA-transaminase) and GABA re-uptake].

It has been reported that thyrotropin-releasing hormone (TRH) improves the ataxia of cerebellar type. The mechanism of action is unclear. As well recognized, GABA (gamma aminobutyric acid) is an important neurotransmitter in cerebellar system. So, if TRH acts on cerebellum, it is expected that the GABA metabolism will be modified by in vivo or in vitro TRH application. The purpose of this experiment is to clarify whether or not TRH affects on GABA system in cerebellar system. The first experiment was to determine the effect of TRH on the two GABA related enzyme activities, that is, GAD (glutamic acid decarboxylase) and GABA-T (GABA-transaminase). TRH was intraperitoneally injected at a dose of 5 mg/kg. In mouse brains, the two enzyme activities of hindbrains increased after 60 minutes. Next experiment assaying GAD activities at two parts of hindbrain revealed that the increase in hindbrain observed above was due to marked increase in brain-stem (p less than 0.001), but not in cerebellum itself in which the GAD activities decreased (p less than 0.05). On the other hand, in the forebrains, the same dose of TRH failed to change both GAD and GABA-T activities. In order to ascertain the effect more precisely, we assayed GAD activities at seven parts of the brain of Wistar male rats. By this experiment, it was found that GAD activities increase at two portions, namely, at thalamo-midbrain after 30 minutes and at pons-medulla after 180 minutes of TRH injection (p less than 0.05, in both). Other five portions, including cerebellum, showed no significant change of GAD activities.(ABSTRACT TRUNCATED AT 250 WORDS)