Evidence that NPY-containing neurons in the brainstem project into selected hypothalamic nuclei: implication in feeding behavior.

Recent studies show that bilateral neural transections (NT) at the level of dorsal tegmentum in the mesencephalon significantly increase food intake and decrease latency to onset of feeding behavior in response to neuropeptide Y (NPY). The increased responsiveness to NPY may be due to denervation-induced hypersensitivity to NPY in hypothalamic sites that mediate feeding behavior in rats. To test this hypothesis we have studied the effect of NT on NPY concentrations in 7 neural sites of male rats. Two weeks after NT, NPY levels in 3 hypothalamic nuclei--suprachiasmatic nucleus, arcuate nucleus and ventromedial hypothalamus--were not altered by NT thereby suggesting that NPY innervations in these nuclei may be derived mainly from NPY perikarya in the ARC and elsewhere in the diencephalon. On the other hand, NPY concentrations were markedly decreased (50-60%) in the medial preoptic area, paraventricular nucleus, median eminence and dorsomedial nucleus indicating that a substantial number of neurons in the brainstem, which show coexistence of NPY and adrenergic transmitters, project into these 4 diencephalic nuclei. These findings indicate that NPY-containing neurons in the brainstem may project into selected hypothalamic sites and the reduction in the NT rats of NPY levels, especially in the paraventricular nucleus, may be responsible for the reported increase in sensitivity of the NPY-induced feeding response.