An analysis of nicotinic and opioid processes in the medulla oblongata and nucleus ambiguus of the dog.

Effects of opioids and nicotine on blood pressure and heart rate were studied by microinjections in the region of nucleus ambiguus of acutely decerebrated dogs. Nicotine (5 micrograms/0.5 microliter) microinjected into the region of nucleus ambiguus produced hypotension accompanied by either bradycardia or tachycardia. Both types of response were antagonized by mecamylamine (10 micrograms/0.5 microliter) but not by naloxone (10 micrograms/0.5 microliter). Fentanyl (5 micrograms/0.5 microliter) and ethylketazocine (10 micrograms/0.5 microliter) decreased blood pressure and heart rate after microinjection into the nucleus ambiguus. The actions of fentanyl were antagonized by naloxone (10 micrograms/0.5 microliter) and N-allylnormetazocine (10 micrograms/0.5 microliter), but not by mecamylamine (10 micrograms/0.5 microliter). Naloxone (10 micrograms/0.5 microliter) administered after ethylketazocine elevated blood pressure and tended to increase heart rate. Naloxone, mecamylamine or N-allylnormetazocine did not produce significant effect when microinjected alone. It is concluded that, under the present experimental conditions, nicotine and opioid mu, kappa but not sigma receptors are distributed in the region of nucleus ambiguus. No evidence for an interaction between nicotine and opioids in this nucleus was obtained. Furthermore, there was no detectable endogenous opioid or cholinergic nicotinic tone in the nucleus ambiguus of the acutely decerebrated dog.