HISTAMINERGIC NEURONS IN THE SHEEP DIENCEPHALON

The distribution of histaminergic neurons in the sheep brain was studi ed by immunohistochemistry by using antibodies raised against histamin e. For the first time in this species, the presence of histamine-immun oreactive neurons was described in the caudal diencephalon, around the mammillary bodies, and in the tuberomammillary area. The general patt ern of distribution of these neurons was similar to that described pre viously in other species, i.e., rodents and humans. The distribution i n the five neuronal groups described in rodents was not easy to demons trate in sheep, because the boundaries between each group were not cle ar. The labeled neurons appeared to form a continuous cell system, as in humans. Numerous histamine-immunoreactive mast cells were found in the habenula and the thalamus. Histamine-immunoreactive fibers were fo und in almost all of the structures studied. The highest density of fi bers was seen in the tuberomammillary area, from which dense bundles o f fibers ran rostrally and dorsally along the third ventricle in a par asagittal plane. Numerous immunostained fibers were found close to the wall of the ventricles; some of them appeared to reach the cerebrospi nal fluid through the ependymal cell layer. Some fibers were also obse rved in the optic tract, and the lowest density was found in the supra optic and paraventricular nuclei. These results should be useful for d eveloping further physiological studies on the role of histaminergic n euronal systems in sheep. (C) 1998 Wiley-Liss, Inc.