Bronchial, cardiovascular and secretory responses after central administration of capsaicin in the guinea-pig

Abstract

Capsaicin was injected intracisternally (i.c.), intrathecally (i.th.) or intravenously (i.v.) into guinea-pigs anaesthetized with urethane and ventilated artificially. The effects of 0.2-100 micrograms capsaicin on insufflation pressure, heart rate, arterial blood pressure and salivation were recorded. Low i.c. doses of 0.2 and 2 micrograms capsaicin induced bradycardia, hypertension and salivation but no change in insufflation pressure. An insufflation pressure increase, i.e. bronchoconstriction, was observed with 20 or 100 micrograms capsaicin i.c. and this was associated with tachycardia and hypertension. Bronchoconstriction after 20 micrograms capsaicin i.c. was augmented by propranolol (1 mg/kg i.v.). It was, however, unaffected by bilateral cervical vagotomy and could also be induced by i.th. capsaicin injections in the lumbar region. Capsaicin (3 micrograms/kg) injected i.v. induced bronchoconstriction and tachycardia. Propranolol enhanced bronchoconstriction but did not reduce the tachycardia indicating that capsaicin led to activation of sympathetic bronchial but not cardiac fibers. These results also indicate that i.c. capsaicin caused reflex responses consisting of salivation, bronchodilatation bradycardia and hypertension. High doses injected i.c. or i.th. also caused tachycardia and bronchoconstriction. This latter effect, however, was neither a vagal reflex nor did it seem to result from activation of central terminals of afferent fibers with subsequent release of mediators from the peripheral endings due to antidromic spread of nerve impulses. Instead, capsaicin seemed to be readily resorbed into the systemic circulation and thus acting at peripheral endings to cause bronchoconstriction and tachycardia.