Search for an endogenous cannabinoid-mediated effect in the sympathetic nervous system

Activation of CB1 cannabinoid receptors by exogenous agonists causes presynaptic inhibition of neurotransmitter release from axon terminals. In the central nervous system, presynaptic CB1 receptors can also be activated by endogenous cannabinoids (endocannabinoids) released from postsynaptic neurons. Except in the vas deferens, there is no indication of endocannabinoid-mediated presynaptic inhibition in the sympathetic nervous system. The aim of the present study was to search for such inhibition in pithed rats. Artificial sympathetic tone was established by continuous electrical stimulation of preganglionic sympathetic axons. The CB1 cannabinoid receptor antagonist rimonabant (0.5 and 2 mg kg−1 i.v.) did not change blood pressure, heart rate or plasma noradrenaline concentration. Since activation of Gαq/11 protein-coupled receptors enhances endocannabinoid synthesis in the central nervous system, we attempted to stimulate endocannabinoid production by infusion of arginine vasopressin and phenylephrine (both activate Gαq/11 protein-coupled receptors). Rimonabant (2 mg kg−1 i.v.) did not change blood pressure, heart rate or plasma noradrenaline concentration during infusion of phenylephrine or vasopressin. In the final series of experiments we verified that an exogenous cannabinoid agonist produces sympathoinhibition. The synthetic CB1/CB2 receptor agonist WIN55212-2 (0.1 and 1 mg kg−1 i.v.) markedly lowered blood pressure and plasma noradrenaline concentration in pithed rats with electrically stimulated sympathetic outflow. In contrast, in pithed rats with a pressor infusion of noradrenaline, WIN55212-2 did not change blood pressure or heart rate. The results verify that activation of peripheral presynaptic CB1 receptors inhibits noradrenaline release from sympathetic nerve terminals. The lack of effect of the CB1 receptor antagonist rimonabant indicates that, even under conditions favouring endocannabinoid synthesis, endocannabinoid-mediated presynaptic inhibition is not operating in the sympathetic nervous system of the pithed rat.