The sympathetic axons innervating the sinus node of the rabbit possess presynaptic opioid kappa- but not mu- or delta-receptors.
The postganglionic sympathetic nerves of rabbit isolated hearts were stimulated with pulses delivered at 5 Hz and train durations of 1-5 s. Ethylketocyclazocine 0.01-1 mumol/l and fentanyl 1 and 10 mumol/l but not morphine 1 and 10 mumol/l, Met-enkephalin 1 and 4 mumol/l or D-Ala2, D-Leu5-enkephalin 0.5 and 5 mumol/l diminished the stimulation-evoked increase in heart rate. The effect of ethylketocyclazocine 0.1 mumol/l was antagonized by naloxone 1 and 10 mumol/l. In contrast, the effect of fentanyl was not changed by naloxone 10 mumol/l. Ethylketocyclazocine 0.03 and 1 mumol/l did not reduce the tachycardia elicited by exogenous noradrenaline. The results suggest that, under in vitro conditions, only presynaptic opioid kappa- but not mu- or delta-receptors inhibit the release of noradrenaline from the sympathetic neurones innervating the sinus node.
Starke
K
K
Schöffel
E
E
Illes
P
P
D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D004558:Electric Stimulation; D006321:Heart / Q000502:physiology; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology; D011817:Rabbits; D011957:Receptors, Opioid / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D012849:Sinoatrial Node / Q000187:drug effects / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000187:drug effects / Q000502:physiology*
D009294:Narcotics; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D009270:Naloxone; D009638:Norepinephrine