Ascending monoamine-containing fiber pathways related to intracranial self-stimulation: Histochemical fluorescence study

Lesions made via self-stimulation electrodes in the medial forebrain bundle (MFB) of rats always resulted in monoamine accumulation, as studied with the histochemical fluorescence method. Build-up was often in the monoamine systems of the MFB itself, those systems being the ventral norepinephrine bundle, the nigro-neostriate bundle, and the mesolimbic fibers. In other cases, build-up was restricted to a bundle of fibers dorsomedial to the MFB, probably originating in the locus coeruleus or the subcoeruleus area. In still other instances, build-up was seen in both this dorsal bundle of fibers and the catecholamine systems of the MFB. Electrodes in rats with sites of implantation in the region of the A1, A2 and A5 cell groups of origin of the ventral norepinephrine system, as identified with combined Nissl and histochemical examination, were never positive with respect to self-stimulation; nor were placements in this ventral system caudal to the origin of the dorsal bundle positive with respect to that behavior. These results argued against the association of the ventral system with self-stimulation. By implication, the catecholamine fibers of the MFB which are capable of supporting self-stimulation might be the two ascending dopamine systems of the ventral mesencephalic tegmentum. Lesions made via positive electrodes in the vicinity of the brachium conjuctivum always resulted in monoamine build-up in the dorsal norepinephrine fiber system. In sum, our results implicate the A9 and A10 dopamine ventral midbrain systems and the A6 norepinephrine dorsal bundle in intracranial self-stimulation.