Synergistic interaction between baclofen administration into the median raphe nucleus and inconsequential visual stimuli on investigatory behavior of rats.

RATIONALE:Noncontingent administration of amphetamine into the ventral striatum or systemic nicotine increases responses rewarded by inconsequential visual stimuli. When these drugs are contingently administered, rats learn to self-administer them. We recently found that rats self-administer the GABA(B) receptor agonist baclofen into the median (MR) or dorsal (DR) raphe nuclei. OBJECTIVES:We examined whether noncontingent administration of baclofen into the MR or DR increases rats' investigatory behavior rewarded by a flash of light. RESULTS:Contingent presentations of a flash of light slightly increased lever presses. Whereas noncontingent administration of baclofen into the MR or DR did not reliably increase lever presses in the absence of visual stimulus reward, the same manipulation markedly increased lever presses rewarded by the visual stimulus. Heightened locomotor activity induced by intraperitoneal injections of amphetamine (3 mg/kg) failed to concur with increased lever pressing for the visual stimulus. These results indicate that the observed enhancement of visual stimulus seeking is distinct from an enhancement of general locomotor activity. Visual stimulus seeking decreased when baclofen was co-administered with the GABA(B) receptor antagonist, SCH 50911, confirming the involvement of local GABA(B) receptors. Seeking for visual stimulus also abated when baclofen administration was preceded by intraperitoneal injections of the dopamine antagonist, SCH 23390 (0.025 mg/kg), suggesting enhanced visual stimulus seeking depends on intact dopamine signals. CONCLUSIONS:Baclofen administration into the MR or DR increased investigatory behavior induced by visual stimuli. Stimulation of GABA(B) receptors in the MR and DR appears to disinhibit the motivational process involving stimulus-approach responses.