Effects of cannabinoids on adrenaline release from adrenal medullary cells

The objective of the present study was to analyse the peripheral effects of cannabinoids on adrenaline release from adrenal chromaffin cells. In pithed rabbits with electrically stimulated sympathetic outflow, intravenous injection of the cannabinoid receptor agonists WIN55212-2 and CP55940 (5, 50 and 500 μg kg−1) markedly lowered the plasma adrenaline concentration. The effect of WIN55212-2 was attenuated by the selective CB1 cannabinoid receptor antagonist SR141716A (500 μg kg−1). WIN55212-3 (same doses as WIN55212-2), the enantiomer of WIN55212-2 lacking affinity for cannabinoid receptors, had no effect on the plasma adrenaline concentration. In rabbit isolated adrenal glands, the release of adrenaline elicited by electrical stimulation was measured by fast cyclic voltammetry. Electrically-evoked adrenaline release was inhibited by WIN55212-2 (0.3, 1, 3 and 10 μM) and this effect was antagonized by SR141716A (1 μM). The non-cholinergic component of adrenaline release observed after blockade of nicotinic (by hexamethonium 100 μM) and muscarinic (by atropine 0.5 μM) acetylcholine receptors was not depressed by WIN55212-2. WIN55212-3 (10 μM) had no effect on adrenaline release. No detectable specific CB1 receptor binding and mRNA expression were found in rabbit adrenal glands with autoradiography and in situ hybridization. The results show that cannabinoids inhibit adrenaline secretion in rabbit isolated adrenal glands; the likely mechanism is a presynaptic CB1 receptor-mediated inhibition of acetylcholine release from preganglionic sympathetic neurons. The inhibition of adrenaline secretion in adrenal glands most probably accounts for the decrease in the plasma adrenaline concentration observed after cannabinoid administration in pithed rabbits. Keywords: Adrenaline release, autoradiography, cannabinoid receptor, fast cyclic voltammetry, in situ hybridization, rabbit isolated adrenal gland, plasma adrenaline, pithed rabbit, sympathetic regulation Introduction Systemic administration of natural and synthetic cannabinoid agonists elicits prominent changes in cardiovascular homeostasis both in humans and in experimental animals. However, little is known on the mechanisms involved in these effects (for review, see Dewey, 1986; Compton et al., 1996; Wagner et al., 1998). Especially, the interaction of cannabinoids with the peripheral components of cardiovascular regulation has seldom been investigated. Recent studies have shown that activation of CB1 cannabinoid receptors inhibits noradrenaline release from cardiac and vascular postganglionic sympathetic neurons (Ishac et al., 1996; Malinowska et al., 1997; Molderings et al., 1999; Niederhoffer & Szabo, 1999; Szabo et al., 2001), resulting in cardiovascular depression (Malinowska et al., 1997; Niederhoffer & Szabo, 1999). In contrast, the effects of cannabinoids on catecholamine secretion from the adrenal medulla are not known. In humans, smoking or oral administration of Δ9-tetrahydrocannabinol induces no change or an increase in urinary adrenaline excretion (Hollister et al., 1970; Weiss et al., 1972; Messiha & Soskin, 1973). Effects of cannabinoids on the adrenaline concentration in plasma were, to our knowledge, investigated only in rats. In this species, systemically administered Δ9-tetrahydrocannabinol produced either no change or a decrease in the plasma adrenaline concentration (Kumar et al., 1984; Patel et al., 1985a, 1985b). In all the studies cited above, the primary site of action of Δ9-tetrahydrocannabinol – central nervous system or adrenal medulla – could not be identified since the drug was administered systemically. Altered adrenal medullary function could be the consequence of Δ9-tetrahydrocannabinol-induced changes in plasma levels of pituitary hormones (Rodriguez de Fonseca et al., 1991; 1992). Direct effects of cannabinoids in the adrenal gland are, however, also possible. The objective of the present study was to analyse the peripheral effects of cannabinoids on adrenaline release. Three kinds of experiments were carried out. First, effects of cannabinoids on the plasma adrenaline concentration were evaluated in a whole animal preparation, the pithed rabbit with electrically stimulated sympathetic outflow. Second, effects on adrenaline release were studied in rabbit isolated adrenals; in these experiments, the release of adrenaline was evoked by electrical stimulation and measured by fast cyclic voltammetry. In both experimental models, the effects of the mixed CB1/CB2 cannabinoid receptor agonist WIN55212-2 were compared to those of WIN55212-3, an enantiomer of WIN55212-2 possessing very low affinity for cannabinoid binding sites (Kuster et al., 1993; Pertwee, 1999). The interaction between WIN55212-2 and the CB1 cannabinoid receptor antagonist SR141716A (Rinaldi-Carmona et al., 1994; Pertwee, 1999) was also studied. Finally, CB1 cannabinoid receptor protein and mRNA were sought for in rabbit adrenal glands using autoradiography and in situ hybridization, respectively.