Localization and mechanisms of action of cannabinoid receptors at the glutamatergic synapses of the mouse nucleus accumbens

Despite the role of excitatory transmission to the nucleus accumbens (NAc) in the actions of most drugs of abuse, the presence and functions of cannab inoid receptors (CB1) on the glutamatergic cortical afferents to the NAc ha ve never been explored. Here, immunohistochemistry has been used to show th e localization of CB1 receptors on axonal terminals making contacts with th e NAc GABAergic neurons. Electrophysiological techniques in the NAc slice p reparation revealed that cannabimimetics [WIN 55,212,2 (WIN-2) and CP55940] strongly inhibit stimulus-evoked glutamate-mediated transmission. The inhi bitory actions of WIN-2 were dose-dependent (EC50 of 293 +/- 13 nM) and rev ersed by the selective CB1 antagonist SR 141716A. In agreement with a presy naptic localization of CB1 receptors, WIN-2 increased paired-pulse facilita tion, decreased miniature EPSC (mEPSC) frequency, and had no effect on the mEPSCs amplitude. Perfusion with the adenylate cyclase activator forskolin enhanced glutamatergic transmission but did not alter presynaptic CB1 actio ns, suggesting that cannabinoids inhibit glutamate release independently fr om the cAMP-PKA cascade. CB1 did not reduce evoked transmitter release by i nhibiting presynaptic voltage-dependent Ca2+ currents through N-, L-, or P/ Q-type Ca2+ channels, because CB1 inhibition persisted in the presence of o mega -Conotoxin-GVIA, nimodipine, or omega -Agatoxin-IVA. The K+ channel bl ockers 4-aminopyridine (100 muM) and BaCl2 (300 muM) each reduced by 40-50% the inhibitory actions of WIN-2, and their effects were additive. These da ta suggest that CB1 receptors are located on the cortical afferents to the nucleus and can reduce glutamate synaptic transmission within the NAc by mo dulating K+ channels activity.