Neurotensin in the ventromedial mesencephalon of the rat: anatomical and functional considerations.

Neurotensin is an endogenous neuropeptide that fulfills some of the criteria for a neurotransmitter in the mammalian central nervous system. It exists in high concentrations in the ventral tegmental area and adjacent midline nuclei of the ventromedial mesencephalon, and recent microinjection studies have demonstrated that neurotensin can act in this brain region to produce both a decrease in colonic temperature, and an increase in spontaneous motor activity. In this study it was found that hypothermia was most successfully evoked following neurotensin injection along the midline of the ventral mesencephalon, corresponding to the nucleus linearis centralis. In contrast, behavioral hyperactivity was produced with greatest consistency in the ventral tegmental area, corresponding to the nucleus paranigralis and nucleus parabrachialis pigmentosus. However, in its caudal aspect, the nucleus paranigralis was found unresponsive to neurotensin. Behavioral hyperactivity was also observed after neurotensin injection along the midline into the nucleus interfascicularis. Only injections made into the nucleus linearis rostralis produced hypothermia and hyperactivity in the same rat. This distribution of neurotensin-responsive nuclei corresponded to the distribution of neurotensin containing perikarya and fibers. With the exception of the nucleus interfascicularis, neurotensin-containing neurons were distributed throughout the rostral portion of the ventromedial mesencephalon, the nucleus parabrachialis pigmentosus containing the greatest density. However, in the caudal portion, neurotensin neurons were found almost exclusively in the nucleus linearis centralis. Neurotensin-containing fibers were of greatest density in the nucleus interfascicularis and the nucleus linearis centralis. Considering the known capacity of neurotensin to activate dopamine neurons in the ventromedial mesencephalon, and the partial mediolateral topographical distribution of dopaminergic projections from this region to the limbic forebrain, it is possible that neurotensin may be activating two distinct populations of dopamine neurons to produce hypothermia and behavioral hyperactivity.