Study of the efferent connections of the enkephalinergic magnocellular dorsal nucleus in the guinea-pig hypothalamus using lesions, retrograde tracing and immunohistochemistry: evidence for a projection to the lateral septum.

The efferents of enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus of the guinea-pig were studied using different neuroanatomical methods and indirect immunocytochemical technique. Following unilateral implantation of the fluorescent dye 4',6-diamidino-2-phenylindole in the lateral septal nucleus, retrogradely-labeled perikarya were found in the magnocellular dorsal nucleus. These labeled perikarya reacted with antiserum against enkephalin, demonstrating that enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus project to the lateral septal nucleus. In other experiments, complete bilateral lesions were produced in the magnocellular dorsal nucleus by electrocoagulation. Enkephalin-immunoreactive nerve fibers and terminals were totally depleted in the lateral septal nucleus. This confirms that septal enkephalin-immunoreactive terminals originate in the magnocellular dorsal nucleus and further suggests that this nucleus is the source of all the enkephalin-immunoreactive material found in the septum. Experiments utilizing two different fluorescent dyes, 4',6-diamidino-2-phenylindole and propidium iodide, injected in each side of the lateral septal nucleus, respectively, demonstrated that the magnocellular dorsal nucleus gives off axon collaterals to both sides of the septum, since double-labeling of individual cell bodies was detected in the nucleus. By relating this finding to the results obtained after unilateral destruction of the nucleus, which caused an incomplete loss of enkephalin- immunoreactive material in the lateral septal nucleus ipsilaterally, it is suggested that the enkephalinergic hypothalamo-septal pathway contains unbranching neurons projecting ipsilaterally and branching neurons distributing fibers ipsilaterally and contralaterally. Lesion experiments, and experiments based on the retrograde axonal transport of horseradish peroxidase after intravenous injections, demonstrated that the magnocellular dorsal nucleus contributes neither to the tubero-infundibular nor to the hypothalamo-neurohypophyseal tracts. The lateral septal nucleus receives numerous aminergic and peptidergic projections, indicating the potential importance of this region in physiological and behavioral events. In the guinea-pig, the well-demarcated enkephalinergic pathway demonstrated in this study provides a convenient model for the experimental study of the enkephalinergic innervation of the lateral septal nucleus.