Histaminergic system in the cat hypothalamus with reference to type B monoamine oxidase

Abstract

It is known that histamine (HA) and type B monoamine oxidase (MAO-B), an enzyme involved in its metabolism, are present in the posterior hypothalamus, but the sites where MAO-B intervenes in HA metabolism remain uncertain. The present study examined and compared the detailed distribution and morphology of neurons immunoreactive to HA (HA-ir) and MAO-B (MAO-B-ir) in the cat hypothalamus. HA-ir neurons were localized almost exclusively in the posterior hypothalamus with the largest group in the tuberomammillary nucleus and adjacent areas. MAO-B-ir staining was detected in the vast majority of HA-ir neurons, suggesting that the degradation of tele-methylhistamine (t-MHA), the direct metabolite of HA, may occur within these cells. Nevertheless, a few HA-ir cells showed no detectable or very weak MAO-B-ir labeling; a small group of neurons containing MAO-B alone was detected in the area dorsolateral to the caudal part of the arcuate nucleus. Numerous HA-ir axons and terminal-like structures were distributed unevenly in virtually all hypothalamic regions. One of their principal trajectories ascended through the ventrolateral part of the hypothalamus and rostrally formed an axon column, which ascended into the preoptic area and contributed fibers to the diagonal band of Broca and bed nucleus of the stria terminalis. Other HA-ir axons passed laterally, dorsal to the zona incerta or ventrally through a narrow zone dorsal to the optic tract. Numerous long HA-ir axons coursed dorsomedially from the ventrolateral posterior hypothalamus to the dorsal hypothalamic area. Many are oriented vertically to the thalamus in the midline. MAO-B-ir axons and fibers were detectable throughout the hypothalamus and overlapped the areas distributing HA-ir fibers. They were, however, weaker in staining intensity and apparently fewer than the HA-ir fibers. MAO-B-ir glial cells were numerous in all hypothalamic structures rich in HA-ir fibers. These results suggest that the metabolism of t-MHA may also occur within HA terminals and glial cells.

Histaminergic neurons in the sheep diencephalon

“…In sheep, the rostral groups E4 and E5 contained a lower density of labeled perikarya than in rodents (rats and guinea pigs); only a few labeled neurons were found close to the third ventricle in the dorsomedial hypothalamus. This pattern of distribution was also observed in the cat (Lin et al, 1993).…”

“…In rodents and tree shrews (Airaksinen et al, 1989), the anterior hypothalamic nuclei, i.e., supraoptic, paraventricular, and infundibular nuclei, received a dense innervation with HA-IR fibers, whereas, in sheep, HA-IR fibers were quite rare in the supraoptic and paraventricular nuclei and moderate in the infundibular nucleus. Such a distribution was also found in the cat (Lin et al, 1993). In sheep and cats, the density of fibers was very high just dorsal to the supraoptic nucleus.…”

“…The distribution of HA-IR fibers in the sheep diencephalon follows a general pattern similar to that seen in rodents (Panula et al, 1984;Steinbusch and Mulder, 1984;Airaksinen and Panula, 1988;Inagaki et al, 1988;Smits et al, 1990), cats (Lin et al, 1993), tree shrews (Airaksinen et al, 1989), and humans (Airaksinen et al, 1991). However, some differences can be noted in several areas.…”

“…Similarly, HA-IR neurons contain monoamine oxydase in cats (Lin et al, 1993). In sheep, the presence of tyrosine hydroxylase (Tillet and Thibault, 1989), melanin-concentrating hormone (Tillet et al, 1996), somatostatin (Papadopoulos et al, 1986), and cyclooxygenase (Breder et al, 1992) has been described in the ventrocaudal hypothalamus, but the possible colocalization with HA neurons remains unknown.…”

“…At the same time, antibodies were produced against HA bound to a protein carrier. They allowed the detection of HA-containing neurons in different species, regardless of the interspecies divergence of HDC, and the detailed immunohistochemical distribution of histaminergic neurons was performed in the rat (Wilcox and Seybold, 1982;Panula et al, 1984;Steinbusch and Mulder, 1984), cat (Lin et al, 1986(Lin et al, , 1993, tree shrew (Airaksinen et al, 1989), guinea pig (Airaksinen and Panula, 1988), rabbit (Iwase et al, 1993), and human Airaksinen et al, 1991).…”