CENTRAL HISTAMINERGIC NEURONS REGULATE RABBIT TRACHEAL TENSION THROUGH THE CERVICAL SYMPATHETIC-NERVE

We previously showed that stimulation of the posterior hypothalamus de creases tracheal tension and involves central histaminergic neurons. I n the present study, we reveal that central histaminergic neurons proj ect to the rostral ventrolateral medulla and affect cervical sympathet ic nervous activity in rabbits. Administration of histamine into the f ourth ventricle increased cervical sympathetic nervous activity and de creased tracheal tension. These effects were inhibited by administrati on of a histamine H-1, receptor antagonist, pyrilamine, into the fourt h ventricle. Unilateral injection of DL-homocysteic acid into the tube romammillary nucleus increased cervical sympathetic nervous activity, an effect was antagonized by bilateral injection of pyrilamine into th e rostral ventrolateral medulla. The pulse correlogram between the sti mulation pulse applied to the tuberomammillary nucleus and the cervica l sympathetic nerve activity showed a mode at 150 to 200 ms, which was reduced by pyrilamine administration into the fourth ventricle. Fiber s anterogradely labeled by Phaseolus vulgaris leucoagglutinin (PHA-L) injected into the tuberomammillary nucleus were distributed in the A1, A2, C1, and C2 areas which are determined by tyrosine hydroxylase-imm unohistochemistry. PHA-L positive neurons were in close contact with t yrosine hydroxylase-immunoreactive neurons in these four areas. Cell b odies in the tuberomammillary nucleus retrogradely labeled with fluoro gold from the rostral ventrolateral medulla were immunoreactive with h istamine. These results suggest that an excitatory efferent pathway pr ojects from the tuberomammillary nucleus to the cervical sympathetic n erve and that the histaminergic neurons of this pathway influence trac heal tension through the rostral ventrolateral medulla. (C) 1998 Elsev ier Science B.V. All rights reserved.