Is dopamine involved in the hyperactivity produced by injections of muscimol into the median raphe nucleus?

Many studies have shown that experimental manipulations of the median raphe nucleus are able to produce profound effects on locomotor activity. Other data indicate that the raphe nuclei may exert an inhibitory influence over dopamine systems projecting to the forebrain. These results raise the possibility that the median raphe's influence over locomotion may be mediated through alterations in forebrain dopamine release. We examined this possibility in the current report by studying the role of dopamine in the hyperactivity produced by microinjections of the GABA agonist muscimol into the median raphe. Muscimol injections resulted in pronounced hyperactivity which was accompanied by a decrease in serotonin metabolism within the hippocampus and an increase in dopamine metabolism within the nucleus accumbens. Systemic injections of high doses of the dopamine antagonist haloperidol, however, were not able to attenuate muscimol's effect on activity. These results suggest that dopaminergic mechanisms do not play an essential role in mediating the effects of intraraphe muscimol on locomotor activity.