En bloc immunohistochemistry reveals extensive distribution of histidine decarboxylase-immunoreactive neurons on the ventral surface of the rat hypothalamus

En bloc immunohistochemistry was used to examine the distribution of l-histidine decarboxylase (HDC)-immunoreactive neurons on the ventral surface of the rat hypothalamus. Following standard para-formaldehyde fixation, the ventral hypothalamus containing the tuber cinereum was carefully dissected free and incubated en bloc with antisera to HDC followed by standard avidin-biotin complex immunohistochemistry. Microscopic examination of these en block preparations revealed the existence of an extensive plexus of HDC-immunoreactive neurons located directly upon the ventral hypothalamic surface. HDC-immunoreactive neurons were largely restricted to the caudal half of the hypothalamic surface, and were multipolar, with 2-5 dendrites radiating in all directions from the soma. The proximity of these neurons to the ventral surface of the brain suggests that histaminergic neurons might be capable of responding to cerebrospinal fluid borne substances. Thus, one form of input to hypothalamic histaminergic neurons may be humoral rather than synaptic.