Our previous study showed that injection of the hydrogel of a water-absorbent polymer (WAPGEL) into the ventromedial nucleus of the hypothalamus (VMH) induced stereotyped long-lasting running activity without association of any aggressive reactions, and suggested that swelling of the polymer at the site of injection affected the inhibitory neuron system, resulting in the expression of running activity. In the present study, we injected WAPGEL impregnated with different doses of GABA. By this treatment, the expression of running activity was inhibited in a dose-dependent manner. Contrarily, microinjection of a GABA-A receptor blocker, bicuculline methiodide (BM), released the running reaction in a dose-dependent manner with similar features to that induced by WAPGEL. It is suggested that the running-related neurons in the VMH are inhibited by GABA when the animals are at rest.
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