Modulation of the cardiac autonomic transmission of pithed rats by presynaptic opioid OP4 and cannabinoid CB1 receptors.

We studied the effects of nociceptin, the endogenous ligand of the opioid OP4 receptor, and of two cannabinoid receptor agonists WIN 55,212-2 and CP-55,940 (0.001-1 micromol/kg each) on the neurogenic tachycardia and bradycardia in pithed rats. Electrical stimulation (1 Hz, 1 ms, 50 V for 10 s) of the preganglionic sympathetic nerve fibres and injection of nicotine 2 micromol/kg or isoprenaline 0.5 nmol/kg increased heart rate by about 70 beats/min (bpm) in pithed rats pretreated with atropine 1.5-2 micromol/kg. The electrically induced tachycardia was reduced dose dependently by nociceptin, WIN 55,212-2 and CP-55,940 (by 60, 30 and 20% at the highest dose, respectively). The OP4 and cannabinoid receptor agonists diminished the nicotine- but not the isoprenaline-stimulated increase in heart rate. In pithed rats pretreated with propranolol 3 micromol/kg, vagal stimulation (5 Hz, 1 ms, 15 V for 10 s) or injection of methacholine (5-10 nmol/kg) decreased heart rate by about 30 bpm. Nociceptin, but not WIN 55,212-2 or CP-55,940 decreased the vagal bradycardia dose dependently (the inhibitory effect of 1 micromol/kg was about 40%). Nociceptin failed to modify the methacholine-induced decrease in heart rate. The OP4 receptor antagonists naloxone benzoylhydrazone 5 micromol/kg and/or [Phe1Psi(CH2-NH)Gly2]-nociceptin(1-13)NH2 0.7 micromol/kg, but not the OP(1-3) receptor antagonist naloxone 10 micromol/kg, diminished the inhibitory action of nociceptin on the neurogenic tachycardia and bradycardia. The inhibitory effect of both cannabinoid receptor agonists on the neurogenic tachycardia was abolished by the CB1 receptor antagonist SR 141716 0.1 micromol/kg. The present data suggest that the postganglionic sympathetic nerve fibres innervating the rat heart are endowed with presynaptic opioid OP4 and cannabinoid CB1 receptors, activation of which inhibits the neurogenic tachycardia. The parasympathetic nerve fibres innervating the heart and causing bradycardia are endowed with presynaptic opioid OP4 but not cannabinoid receptors.