Diverging respiratory effects of serotonin and nicotine in vagotomised cats prior to and after section of carotid sinus nerves

Respiratory effects of intravenous serotonin and nicotine were investigated prior to and after bilateral neurotomy of the carotid sinus nerves (CSNs) in eight pentobarbitone/chloralose-anaesthetised, bilaterally vagotomised a nd superior laryngeal nerves-sectioned cats. Injection of 188 nmol . kg(-1) serotonin (hydrogen oxalate salt, 50 mug . kg(-1)) prior to and after CSNs section induced an expiratory apnoea of, respectively, 7.9 +/-1.25 a and 8 .3 +/-1.6 s duration (mean +/-S.E.M.) in, respectively, five and three of t hose cats. In all cats, the serotonin challenge produced a period of accele rated breathing (P < 0.05) both prior to and after section of CSNs. Injecti on of a 433 nmol nicotine bolus (hydrogen tartrate salt, 200 <mu>g) increas ed tidal volume by 25 +/-8% in cats with intact CSNs (P < 0.01), but decrea sed it by 13<plus/minus>10% (P < 0.05) after CSNs section. Nicotine, but no t serotonin, transiently increased mean arterial blood pressure in our cats , which rise was delayed by CSNs cut. Results of this study indicate that t he respiratory response to serotonin occurs beyond carotid body chemorecept ors in vagotomised cats, and suggest that the volume response to intravenou s nicotine depends qualitatively on carotid body chemoreceptor input in thi s experimental model.