Substance P, neurotensin and enkephalin injections into the ventral tegmental area: comparative study on dopamine turnover in several forebrain structures.

A comparison of dopaminergic (DAergic) turnover changes in several forebrain structures was investigated after local injection of substance P (SP), neurotensin and D-Ala-Met-enkephalin (DALA) into the ventral tegmental area (VTA). A dose-dependent increase in the DOPAC/DA ratio was elicited by all 3 peptides in the nucleus accumbens and the septum. DAergic turnover was enhanced in the anteromedial prefrontal cortex only after SP injection and in the amygdala only after neurotensin injection. In the anteromedial striatum as well as in the posterolateral striatum, a significant increased DOPAC/DA ratio was observed following SP and DALA injection into the VTA. No significant changes were noticed in the olfactory tubercles after injection of the 3 peptides in the VTA. From these results, it appears that each peptide induced a different profile of DAergic activation. Taking into account the facilitatory role of the DA neurons at the level of the forebrain integrative structures, the differential activation may explain the difference in behavioral response obtained after injection of the 3 peptides in the VTA.