Ventral striatopallidal parts of the basal ganglia in the rat: I. Neurochemical compartmentation as reflected by the distributions of neurotensin and substance P immunoreactivity.

The distribution of neurotensin immunoreactivity in the basal ganglia of the adult rat was evaluated by studying alternate serial vibratome sections that were exposed to antiserum against neurotensin, substance P, or cholecystokinin. It was observed that a heterogeneous distribution of neurotensin-immunoreactive fibers and terminals contributes to the neurochemical compartmentation of the ventral pallidum and ventral striatum, and that significant numbers of neurotensin-immunoreactive neurons occupy striatal districts of the olfactory tubercle, nucleus accumbens, and ventromedial caudate-putamen. An intense band of pallidal neurotensin immunoreactivity characterizes the medial part of the ventral pallidum adjacent to the nucleus accumbens, whose medial boundary is conveniently defined in sections incubated with cholecystokinin antiserum. Electron microscopic studies showed that the pallidal plexus of neurotensin-immunoreactive elements consists primarily of boutons, which contact large dendrites in arrangements that in all respects appear to be of the classical striatopallidal variety. A gradual decrease in immunolabel was observed approaching the lateral parts of the ventral pallidum, which display sparse neurotensin immunoreactivity. The results thus indicate the existence of a significant neurotensinergic striatopallidal pathway confined primarily, if not exclusively, to the medial part of the ventral striatopallidal system. The contribution of neurotensin-immunoreactive fibers and terminals to the compartmentation of ventral striatum is expressed most vividly in their exclusion from clusters of tightly packed medium-sized neurons, many of which are intensely substance P immunoreactive. Such clusters appear identical with those previously described as rich in opiate receptors and poor in acetylcholinesterase activity. In the ventral striatal region where the nucleus accumbens and ventromedial caudate-putamen merge, neurotensin-immunoreactive neurons are organized in clusters. Further rostral in the nucleus accumbens, they are more evenly distributed. Few were found in the dorsolateral quadrant of the neostriatum.