The cannabinoid agonist Win55,212-2 inhibits calcium channels by receptor-mediated and direct pathways in cultured rat hippocampal neurons

The effects of the cannabinoid receptor agonist Win55,212 on Ca²⁺ channels were studied in rat hippocampal neurons grown in primary culture. Win55,212-2 inhibited whole-cell Ba²⁺ currents through Ca²⁺ channels by both CB1 receptor-mediated and direct mechanisms. The concentration dependent inhibition of the current showed two clear phases, a high-affinity receptor- mediated phase (IC₅₀ = 14 ± 2 nM) that was stereoselective and sensitive to a CB1 receptor antagonist, 300 nM SR141716, and a non-saturating phase that was neither stereoselective nor inhibited by SR141716. These concentration-dependent effects were paralleled by Win55212-induced inhibition of glutamatergic synaptic transmission. Win55,212-2 (100 nM) inhibited both ω-agatoxin IVA- and ω-conotoxin GVIA-sensitive currents. Thus, activation of cannabinoid receptors inhibits N- and P/Q-type Ca²⁺ channels. Activation of cannabinoid receptors inhibited only a fraction of the whole-cell Ca²⁺ channel current (17 ± 2%) even though more than half of the whole-cell Ba²⁺ current was carried by N- and P/Q-type Ca²⁺ channels. Concentrations of agonist greater than 1 μM inhibited Ca²⁺ channels directly.