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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.

We have developed a new technique for altering neural construction by implanting self-swelling, the hydrogel of water-absorbent polymer (WAPGEL) into restricted areas of the brain. Using this technique, we found that the VMH is responsible for expression of running activity. Implantation of 1.5 microliters of WAPGEL into the dorsomedial portion of the ventromedial nucleus of the hypothalamus (VMH) of the rat specifically induced bursts of running, whereas implantation into other sites of hypothalamus, e.g., the anterior, lateral and posterior areas, and the dorsomedial nucleus, caused no significant running activity. The WAPGEL-injected rat showed less aversive reactions than the sham-operated rat. Since deafferentation of the VMH after WAPGEL injection prevented the induction of running activity, the output from the VMH appeared to be responsible for the running activity.