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

Abstract.

We studied whether serotonin release in the CNS is inhibited via cannabinoid receptors. In mouse brain cortex slices preincubated with [³H]serotonin and superfused with medium containing indalpine and metitepine, tritium overflow 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 effects of cannabinoid receptor ligands on the electrically evoked tritium overflow from mouse brain cortex slices preincubated with [³H]choline and on the binding of [³H]WIN 55,212–2 and [³⁵S]GTPγS to mouse brain cortex membranes were examined as well.

In superfused mouse cortex membranes preincubated with [³H]serotonin, the electrically evoked tritium overflow was inhibited by the cannabinoid receptor agonist WIN 55,212–2 (maximum effect of 20%, obtained at 1 µM; pEC₅₀=7.11) and this effect was counteracted by the CB₁ receptor antagonist SR 141716 (apparent pA₂=8.02), which did not affect the evoked tritium overflow by itself. The effect of WIN 55,212–2 was not shared by its enantiomer WIN 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, prostaglandin E₂ and neuropeptide Y, causing the maximum effect at their respective receptors, inhibited the electrically evoked tritium overflow by 33, 69 and 73%, respectively. WIN 55,212–2 (1 µM) inhibited the electrically evoked tritium overflow from mouse brain cortex slices preincubated with [³H]choline by 49%. [³H]WIN 55,212–2 binding to mouse cortex membranes was inhibited 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 tetrodotoxin (pK_i<4.5). [³⁵S]GTPγS binding was increased by WIN 55,212–2 (maximum effect of 80%, pEC₅₀=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 obtained (1) via another three presynaptic receptors on serotoninergic neurones and (2) via CB₁ receptors on cholinergic neurones in the same tissue.