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The aim of the present study was to analyse the cardiovascular actions of the synthetic CB1/CB2 cannabinoid receptor agonist WIN55212-2, and specifically to determine its sites of action on sympathetic cardiovascular regulation.
Pithed rabbits in which the sympathetic outflow was continuously stimulated electrically or which received a pressor infusion of noradrenaline were used to study peripheral prejunctional and direct vascular effects, respectively. For studying effects on brain stem cardiovascular regulatory centres, drugs were administered into the cisterna cerebellomedullaris in conscious rabbits. Overall cardiovascular effects of the cannabinoid were studied in conscious rabbits with intravenous drug administration.
In pithed rabbits in which the sympathetic outflow was continuously electrically stimulated, intravenous injection of WIN55212-2 (5, 50 and 500 μg kg−1) markedly reduced blood pressure, the spillover of noradrenaline into plasma and the plasma noradrenaline concentration, and these effects were antagonized by the CB1 cannabinoid receptor-selective antagonist SR141716A. The hypotensive and the sympathoinhibitory effect of WIN55212-2 was shared by CP55940, another mixed CB1/CB2 cannabinoid receptor agonist, but not by WIN55212-3, the enantiomer of WIN55212-2, which lacks affinity for cannabinoid binding sites. WIN55212-2 had no effect on vascular tone established by infusion of noradrenaline in pithed rabbits.
Intracisternal application of WIN55212-2 (0.1, 1 and 10 μg kg−1) in conscious rabbits increased blood pressure and the plasma noradrenaline concentration and elicited bradycardia; this latter effect was antagonized by atropine.
In conscious animals, intravenous injection of WIN55212-2 (5 and 50 μg kg−1) caused bradycardia, slight hypotension, no change in the plasma noradrenaline concentration, and an increase in renal sympathetic nerve firing. The highest dose of WIN55212-2 (500 μg kg−1) elicited hypotension and tachycardia, and sympathetic nerve activity and the plasma noradrenaline concentration declined.
The results obtained in pithed rabbits indicate that activation of CB1 cannabinoid receptors leads to marked peripheral prejunctional inhibition of noradrenaline release from postganglionic sympathetic axons. Intracisternal application of WIN55212-2 uncovered two effects on brain stem cardiovascular centres: sympathoexcitation and activation of cardiac vagal fibres. The highest dose of systemically administered WIN55212-2 produced central sympathoinhibition; the primary site of this action is not known.
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Pharmacological effects of cannabinoids in animals include hypokinesia, analgesia, catalepsy and hypothermia (for review see Howlett, 1995; Compton et al., 1996; Pertwee, 1997). Cannabinoids also elicit cardiovascular changes (for review see Dewey, 1986; Compton et al., 1996). In most experiments in anaesthetized animals, cannabinoids lowered blood pressure and heart rate, and this was generally attributed to depression of sympathetic tone and enhancement of cardiac vagal activity (rat: Graham & Li, 1973; Adams et al., 1976; Estrada et al., 1987; Varga et al., 1995, 1996; Vidrio et al., 1996; Lake et al., 1997a,1997b; dog: Cavero et al., 1973a,1973b, 1974; Jandhyala & Hamed, 1978; cat: Vollmer et al., 1974). In conscious animals, cannabinoids either caused moderate cardiovascular depression (rat: Birmingham, 1973; Vidrio et al., 1996; rabbit: Stark & Dews, 1980; monkey: Fredericks et al., 1981), or they had no effect or elicited hypertension or tachycardia (rat: Osgood & Howes, 1977; Kawasaki et al., 1980; Stein et al., 1996; Lake et al., 1997b; dog: Jandhyala & Hamed, 1978). In conscious humans, acute administration of cannabinoids elicited marked tachycardia accompanied by a small increase in blood pressure (Benowitz et al., 1979; Huestis et al., 1992), whereas long-term cannabinoid application produced hypotension and bradycardia (Benowitz & Jones, 1975). In the majority of cardiovascular studies, Δ9-tetrahydrocannabinol (the main active component from Cannabis sativa) or anandamide (a putative endogenous cannabinoid) was used as the agonist. Both compounds are agonists at CB1 and CB2 receptors (Felder et al., 1995; Showalter et al., 1996; see Pertwee, 1997) but also elicit effects independent of cannabinoid receptors (see Martin, 1986; Lake et al., 1997a,1997b).
In the majority of the above mentioned cardiovascular studies only blood pressure and heart rate were measured, and only few experiments were carried out which permitted determination of the site of interaction of cannabinoids with the cardiovascular system. To our knowledge, effects on sympathetic nerve activity have been determined only by Vollmer et al. (1974) and effects on the plasma concentration of catecholamines have not been examined. The information on cardiovascular effects of centrally administered cannabinoids is also limited (Cavero et al., 1973a,1973b; Vollmer et al., 1974). The aim of the present study was to determine the sites of interaction of cannabinoids with the sympathetic nervous system. To reach this goal, four kinds of experiment were carried out. (i) Peripheral prejunctional effects on noradrenaline release from sympathetic neurons were studied in pithed rabbits with electrically stimulated sympathetic outflow. (ii) Peripheral postjunctional vascular effects were studied in pithed rabbits which received a pressor infusion of noradrenaline. (iii) Effects on cardiovascular centres in the brain stem were examined by administration of cannabinoids into the cisterna cerebellomedullaris of conscious rabbits. (iv) Finally, the overall effect of systemically administered cannabinoids on cardiovascular regulation was studied in conscious rabbits; in these experiments, the electrical activity of renal postganglionic sympathetic axons was recorded by means of a chronically implanted electrode.
In most experiments, we used the synthetic aminoalkylindole compound WIN55212-2 as a cannabinoid agonist. This compound possesses affinity for both CB1 and CB2 cannabinoid receptors (in this respect it is similar to Δ9-tetrahydrocannabinol and anandamide), its affinity for these receptors is high, and more importantly, its lack of affinity for a great number of neurotransmitter receptors and ion channels has been documented (Felder et al., 1995; Showalter et al., 1996; Kuster et al., 1993; for review see Pertwee, 1997). In a few experiments, the effects of WIN55212-2 were compared with the effects of WIN55212-3, the enantiomer of WIN55212-2, which in binding studies has very low affinity for cannabinoid receptors, and with the effects of CP55940, a mixed CB1/CB2 cannabinoid receptor agonist with a chemical structure markedly different from WIN55212-2.