Inhibition of serotonin release in the mouse brain via presynaptic cannabinoid CB1 receptors

We studied whether serotonin release in the CNS is inhibited via cannabinoi d receptors. In mouse brain cortex slices preincubated with [H-3]serotonin and superfused with medium containing indalpine and metitepine, tritium ove rflow was evoked either electrically (3 Hz) or by introduction of Ca2+ (1.3 mM) into Ca2+-free K+-rich (25 mM) medium containing tetrodotoxin. The eff ects of cannabinoid receptor ligands on the electrically evoked tritium ove rflow from mouse brain cortex slices preincubated with [H-3]choline and on the binding of [H-3]WIN 55,212-2 and [S-35]GTP gamma S to mouse brain corte x membranes were examined as well. In superfused mouse cortex membranes preincubated with [H-3]serotonin, the electrically evoked tritium overflow was inhibited by the cannabinoid recep tor agonist WIN 55,212-2(maximum effect of 20%, obtained at 1 mu M; pEC(50) =7.11) and this effect was counteracted by the CB1 receptor antagonist SR 1 41716 (apparent pA(2)=8.02), which did not affect the evoked tritium overfl ow by itself. The effect of WIN 55,212-2 was not shared by its enantiomer W IN 55,212-3 but was mimicked by another cannabinoid receptor agonist, CP-55 ,940. WIN 55,212-2 also inhibited the Ca2+-evoked tritium overflow and this effect was antagonized by SR 141716. Concentrations of histamine, prostagl andin E-2 and neuropeptide Y, causing the maximum effect at their respectiv e receptors, inhibited the electrically evoked tritium overflow by 33, 69 a nd 73%, respectively. WIN 55,212-2 (1 mu M) inhibited the electrically evok ed tritium overflow from mouse brain cortex slices preincubated with [H-3]c holine by 49%. [H-3]WIN 55,212-2 binding to mouse cortex membranes was inhi bited by CP-55,940, SR 141716 and WIN 55,212-2 (pK(i)=9.30, 8.70 and 8.19, respectively) but not by the auxiliary drugs indalpine, metitepine and tetr o-dotoxin (pK(i)<4.5). [35S]GTP gamma S binding was increased by WIN 55,212 -2 (maximum effect of 80%, pEC(50)=6.94) but not affected by WIN 55,212-3. In conclusion, serotonin release in the mouse brain cortex is inhibited via CB1 receptors, which may be located presynaptically and are not activated by endogenous cannabinoids. The extent of inhibition is smaller than that o btained (1) via another three presynaptic receptors on serotoninergic neuro nes and (2) via CB1 receptors on cholinergic neurones in the same tissue.