Involvement of Cannabinoid Receptors in the Regulation of Neurotransmitter Release in the Rodent Striatum: A Combined Immunochemical and Pharmacological Analysis

Despite the profound effect of cannabinoids on motor function, and their therapeutic potential in Parkinson's and Huntington's diseases, the cellular and subcellular distributions of striatal CB1 receptors are not well defined. Here, we show that CB1 receptors are primarily located on GABAergic (vesicular GABA transporter-positive) and glutamatergic [vesicular glutamate transporter-1 (VGLUT-1)- and VGLUT-2-positive] striatal nerve terminals and are present in the presynaptic active zone, in the postsynaptic density, as well as in the extrasynaptic membrane. Both the nonselective agonist WIN55212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl] pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate salt] (EC50, 32 nm) and the CB1-selective agonist ACEA [N-(2-chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide] inhibited [3H]GABA release from rat striatal slices. The effect of these agonists was prevented by the CB1-selective antagonists SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (1 μm) and AM251 [1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide trifluoroacetate salt] (1 μm), indicating that cannabinoids inhibit the release of GABA via activation of presynaptic CB1 receptors. Cannabinoids modulated glutamate release via both CB1 and non-CB1 mechanisms. Cannabinoid agonists and antagonists inhibited 25 mm K+-evoked [3H]glutamate release and sodium-dependent [3H]glutamate uptake. Partial involvement of CB1 receptors is suggested because low concentrations of SR141716A partly and AM251 fully prevented the effect of WIN55212-2 and CP55940 [5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol]. However, the effect of CB1 agonists and antagonists persisted in CB1 knock-out mice, indicating the involvement of non-CB1,CB1-like receptors. In contrast, cannabinoids did not modulate [3H]dopamine release or [3H]dopamine and [3H]GABA uptake. Our results indicate distinct modulation of striatal GABAergic and glutamatergic transmission by cannabinoids and will facilitate the understanding of the role and importance of the cannabinoid system in normal and pathological motor function.