POSTERIOR HYPOTHALAMIC CONTROL OF RABBIT TRACHEAL TENSION AND INVOLVEMENT OF CENTRAL HISTAMINERGIC NEURONS

The hypothalamus is involved in the control of the cardiovascular syst em, but airway tone is less well defined. In the posterior hypothalamu s, histaminergic neuronal cell bodies are located. Effects of electric al stimulation in the posterior hypothalamus on tracheal tension and t he cardiovascular system were examined in anesthetized, paralyzed and artificially ventilated rabbits. Tracheal tension was determined from pressure exerted on a balloon inserted in the trachea and measured by a pressure transducer. Electrical stimulation of the posterior hypotha lamus caused tracheal tension to decrease, arterial blood pressure to increase, and mild tachycardia followed by bradycardia. The tracheal t ension decrease induced by posterior hypothalamic stimulation was not affected by atropine nor by transection of either the superior larynge al nerve or the vagus nerve, but was depressed by adrenoceptor blockad e. Tracheal tension decrease was also reduced by pyrilamine, a histami ne H-1-receptor antagonist, administered into the fourth ventricle, bu t was not affected by cimetidine, a histamine H-2-receptor antagonist. The stimulation sites where these effects were evoked were interspers ed among the loci of histamine immunoreactive cell bodies previously r eported. Results suggest that posterior hypothalamic neurons decrease tracheal tension through the sympathetic nervous system, and involve t he histaminergic neurons in this route.