Mapping of locomotor behavioral arousal induced by microinjections of dopamine within nucleus accumbens septi of rat forebrain.

Dopamine (DA) at ca. ED50 (16 microg) or saline was stereotaxically microinjected unilaterally 2 h after pretreatment with an MAO inhibitor into left or right nucleus accumbens septi of 697 freely moving rats (1394 injections) to define subregions involved in DA-induced behavioral arousal throughout the anatomical extent of the accumbens. Locomotion was quantified electronically and behavioral responses were assigned to histologically verified injection sites; postural or stereotyped behaviors characteristic of DA injections in caudate-putamen did not occur. Screening with 60 injections across mid-accumbens (2.2-3.2 mm rostral to bregma) indicated that locomotion was elicited non-homogeneously, and was particularly intense dorsomedially. Sites yielding intense arousal and their inactive surround were mapped along the rostrocaudal axis (1.4-4.2 mm anterior to bregma) in coronal sections. Responses to DA showed lateral symmetry and were similar across rostrocaudal levels, with intense responses in dorsomedial accumbens along its border with the caudate-putamen. This functional localization does not coincide closely with reported distributions of DA or its receptors, nor with histologically or histochemically defined core-shell regions of this limbic structure. Nucleus accumbens in rat brain thus appears to be organized functionally into distinct subregions differing markedly in ability to produce locomotor hyperactivity in response to exogenous DA.