Action of nicotine on the respiratory activity of the diaphragm and genioglossus muscles and the nerves that innervate them.

Nicotine is known to alter respiration by stimulating peripheral chemoreceptors and receptors within the brain. In this study the sites of action and the effects of nicotine on hypoglossal nerve activity were compared to its effects on phrenic activity in paralyzed, vagotomized and chloralose-anesthetized cats. Since anesthesia is known to affect respiratory responses, we also compared the effects of intravenous nicotine given to conscious unsedated cats on genioglossus and diaphragm electrical activity. In eight conscious animals intravenous doses of nicotine ranging between 10 ng and 200 micrograms increased genioglossus activity significantly more than diaphragm activity. Studies in 26 anesthetized animals included injection of nicotine, intravenously, in the lateral ventricles, and application of nicotine to the ventrolateral surface of the medulla (the putative site of the central chemoreceptors) before and after section of the carotid sinus nerves. With all these interventions, changes in hypoglossal nerve activity were significantly greater than changes in phrenic nerve activity. The responses to nicotine could be blocked by application of hexamethonium to the ventrolateral medullary surface or by cooling the same area. The results indicate that: nicotine increases hypoglossal nerve activity by both its peripheral and central effects; nicotine has differential effects on different respiratory muscles and nerves; and the central action of nicotine may be mediated largely through receptors located near the ventral medullary surface.