Subicular efferents to histaminergic neurons in the posterior hypothalamic region of the rat studied with PHA-L tracing combined with histidine decarboxylase immunocytochemistry.

The projections from the subiculum to histaminergic cells in the posterior hypothalamic region of the rat were studied by means of anterograde neuroanatomical tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L) combined with histidine decarboxylase (HDC)-immunohistochemistry. PHA-L was injected at various loci along the dorsoventral and proximodistal axes of the subiculum. This resulted in labeling of the fornix and of terminal plexuses at various locations in the diencephalon and the mammillary body. Following deposition of PHA-L in the proximal part of the dorsal subiculum, labeled fibers in the posterior hypothalamus were confined to the mammillary nuclei, whereas after injections of PHA-L in the distal part of the dorsal subiculum and the entire ventral subiculum, labeled fibers were also present in clusters of histaminergic cells located around the mammillary nuclei. The density of the PHA-L labeled fibers within these clusters increased from low to moderate in association with a shift of the injection sites from dorsal to ventral and from proximal to distal parts of the subiculum, i.e., the highest fiber labeling was seen after injections of PHA-L in the distal part of the ventral subiculum. In the latter experiments, PHA-L labeled fibers reached HDC-immunoreactive neurons in the tuberal magnocellular nucleus, the deepest layer of the caudal magnocellular nucleus, the two bridges of histaminergic cells in the posterior hypothalamus, and the histaminergic neurons scattered in the supramammillary region. A few labeled fibers invaded the postmammillary caudal magnocellular nucleus. The presence of varicosities on the PHA-L labeled fibers in close proximity to the cell bodies and dendrites of the histaminergic neurons suggest the existence of synaptic contacts.