SUBSTANCE-P IN THE VENTRAL PALLIDUM - PROJECTION FROM THE VENTRAL STRIATUM, AND ELECTROPHYSIOLOGICAL AND BEHAVIORAL CONSEQUENCES OF PALLIDAL SUBSTANCE-P

The ventral pallidum of the basal forebrain contains a high concentrat ion of substance P and receives a massive projection from the nucleus accumbens. The present study was designed to determine whether the acc umbens serves as a source for substance P-containing fibers in the Ven tral pallidum and characterize the function of this tachykinin peptide within the ventral pallidum. By combining in situ hybridization for m essengerRNA of the substance P prohormone, beta-preprotachykinin, with Fluoro-Gold retrograde labeling from iontophoretic deposits in the ve ntral pallidum, a population of substance P-containing neurons was dem onstrated in the shell and core components of the nucleus accumbens an d the ventromedial striatum. The function of substance P within the ve ntral pallidum was characterized at the level of the single neuron, an d the behaving animal. Electrophysiological assessment revealed that a pproximately 40% of the 97 Ventral pallidal neurons tested were readil y excited by microiontophoretic applications of substance P or a metab olically stable agonist analog, DiMeC7 [(pGlu5, MePhe8, MeGly9)-substa nce P5-11]. Response characteristics were distinguished from glutamate -induced excitations by a slower onset and longer duration of action. Recording sites of tachykinin-sensitive neurons were demonstrated to b e located throughout the ventral pallidum and within high densities of fibers exhibiting substance P-like immunoreactivity. When behaving ra ts received microinjections of DiMeC7 into this same region, the anima ls displayed an increase in motor activity, with a response threshold of 0.1 nmol per hemisphere. These results verify the existence of a su bstantial substance P-containing projection from the nucleus accumbens to the ventral pallidum. The projection likely serves to excite ventr al pallidal neurons for these neurons readily increased firing followi ng local exposure to tachykinins. Furthermore, an increase in motor be havior appears to be a consequence of this neuronal response.