Kainic acid-induced changes in histamine-immunoreactive nerve fibers in the rat brain

Histamine is found in neurons and their long projections in the mammalian brain. The mammalian histaminergic system consists of nerve cell bodies in the tuberomammillary nucleus, and extensive, crossing projections to various brain areas. In addition to the tuberomammillary histaminergic system, histamine is found in rhombencephalic neurons during fetal development of rat. To investigate if histamine has a function in growth and regeneration of the nervous system, small injections of kainic acid were made into different parts of the rat brain. Histamine-immunoreactive nerve fibers were seen in and around the lesions 2 to 15 weeks after the injection. The density of these fibers was higher than that of the control side. The results suggest that histamine is either accumulated in pre-existing fibers or that sprouting of histamine-containing nerves is induced by the lesions. The newly established in situ hybridization method for the enzyme histidine decarboxylase may reveal possible dynamic changes in enzyme regulation associated with the lesions.