Projection from the prefrontal cortex to histaminergic cell groups in the posterior hypothalamic region of the rat. Anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with immunocytochemistry of histidine decarboxylase.

We investigated the projection from the infralimbic division of the prefrontal cortex (area 25) to histaminergic neurons in the posterior hypothalamic area. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in the prefrontal cortex of rats. Frozen brain sections were subjected to combined PHA-L and histidine decarboxylase (HDC)-peroxidase immunocytochemistry, using nickel-enhanced diaminobenzidine (blue reaction product) to visualize the transported PHA-L, and diaminobenzidine (brown reaction product) to visualize simultaneously the HDC-containing neurons. PHA-L-labeled fibers could be seen coursing in the capsula interna, leaving the telencephalon via the anterior thalamic radiation and the medial forebrain bundle. In the lateral and posterior hypothalamic areas, PHA-L-labeled fibers leave the medial forebrain bundle and traverse the nuclei containing HDC-immunoreactive neurons. Varicosities on the PHA-L-labeled fibers, the majority of which occur en passant, could be observed in close association with the HDC-immunoreactive neurons. The results suggest that the hypothalamic histaminergic neurons receive afferent synaptic input from neurons of the infralimbic division of the prefrontal cortex.