Effects of capsaicin applied perineurally to the vagus nerve on cardiovascular and respiratory functions in the cat.

The effects of capsaicin applied perineurally to the cervical vagus nerves have been studied on cardiovascular and respiratory functions in urethane anaesthetized cats. Application of capsaicin resulted in a moderate but significant decrease in the mean arterial blood pressure and in changes of the heart rate whose direction and magnitude depended on the initial cardiac frequency. Subsequent to these alterations, which may be attributed to a direct stimulation by capsaicin of vagal afferents, a transient block of impulse propagation was observed. Three to five days after pre-treatment of the cervical vagus nerves with capsaicin, phenyldiguanidine and veratrine given intravenously invariably evoked bradycardia, hypotension and apnoea, while the reflex responses to intravenous injection of capsaicin and some of its pungent congeners were greatly reduced or even abolished. It is suggested that vagal afferent fibres mediating cardiovascular and respiratory chemo-reflexes are separated into chemo-specifically different populations. Perineural application of capsaicin may be a useful tool for elucidating the role of different populations of peptide-containing vagal afferent fibres in the regulation of cardiovascular and respiratory functions.