Development of the histaminergic neurons and expression of histidine decarboxylase mRNA in the zebrafish brain in the absence of all peripheral histaminergic systems

The histamine-storing neural system in adult and developing zebrafish (Dani o rerio) was studied with immunocytochemical and chromatographical methods. Furthermore, the gene for histidine decarboxylase was partially cloned and its expression mapped with in situ hybridization. The histamine-storing ne urons were only seen in the caudal hypothalamus, around the posterior reces s of the diencephalic ventricle. Almost all parts of the brain, except the cerebellum, contained at least some histamine-immunoreactive fibres. The as cending projections had the rostral part of the dorsal telencephalon as a m ajor target. Descending projections terminated in the torus semicircularis, central grey and inferior olive. A prominent innervation of the optic tect um, which has not been reported in other fish, was seen. The in situ hybrid ization gave a strong signal in cells with the same anatomical position as the histamine-immunoreactive neurons. The first histamine-immunoreactive ne urons appeared in the ventral hypothalamus at about 85 h post-fertilization , and at 90 h, immunoreactive fibres terminated in the dorsal telencephalon . The embryonic histamine production described in mammals was lacking in th is species. Both immunocytochemical and chromatographical studies indicated that histamine is absent in all other parts of the zebrafish body, and no specific hybridization was seen in any other part of the fish than the hypo thalamus. The zebrafish could therefore be a very useful model for pharmaco logical in vivo studies of the histaminergic system of the brain, since the powerful peripheral actions of histamine should be lacking in this species .