Central histaminergic neurons regulate rabbit tracheal tension through the cervical sympathetic nerve.

We previously showed that stimulation of the posterior hypothalamus decreases tracheal tension and involves central histaminergic neurons. In the present study, we reveal that central histaminergic neurons project to the rostral ventrolateral medulla and affect cervical sympathetic nervous activity in rabbits. Administration of histamine into the fourth ventricle increased cervical sympathetic nervous activity and decreased tracheal tension. These effects were inhibited by administration of a histamine H receptor antagonist, pyrilamine, into the fourth ventricle. Unilateral injection of DL-homocysteic acid into the tuberomammillary nucleus increased cervical sympathetic nervous activity, an effect was antagonized by bilateral injection of pyrilamine into the rostral ventrolateral medulla. The pulse correlogram between the stimulation pulse applied to the tuberomammillary nucleus and the cervical sympathetic nerve activity showed a mode at 150 to 200 ms, which was reduced by pyrilamine administration into the fourth ventricle. Fibers 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-immunohistochemistry. PHA-L positive neurons were in close contact with tyrosine hydroxylase-immunoreactive neurons in these four areas. Cell bodies in the tuberomammillary nucleus retrogradely labeled with fluorogold from the rostral ventrolateral medulla were immunoreactive with histamine. These results suggest that an excitatory efferent pathway projects from the tuberomammillary nucleus to the cervical sympathetic nerve and that the histaminergic neurons of this pathway influence tracheal tension through the rostral ventrolateral medulla.