Inhibition of human recombinant T-type calcium channels by the endocannabinoid N-arachidonoyl dopamine

Background and purpose: N-arachidonoyl dopamine (NADA) has complex effects on nociception mediated via cannabinoid CB ₁ receptors and the transient receptor potential vanilloid receptor 1 (TRPV1). Anandamide, the prototypic CB ₁/TRPV1 agonist, also inhibits T-type voltage-gated calcium channel currents (I _Ca). These channels are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NADA and the prototypic arachidonoyl amino acid, N-arachidonoyl glycine (NAGly) modulate T-type I _Ca Experimental approach: Human recombinant T-type I _Ca (Ca _V3 channels) expressed in HEK 293 cells and native mouse T-type I _Ca were examined using standard whole-cell voltage clamp electrophysiology techniques. Key results: N-arachidonoyl dopamine completely inhibited Ca _V3 channels with a rank order of potency (pEC ₅₀) of Ca _V3.3 (6.45) ≥ Ca _V3.1 (6.29) > Ca _V3.2 (5.95). NAGly (10 μmol·L ^-1) inhibited Ca _V3 I _Ca by approximately 50% or less. The effects of NADA and NAGly were voltage- but not use-dependent, and both compounds produced significant hyperpolarizing shifts in Ca _V3 channel steady-state inactivation relationships. By contrast with anandamide, NADA and NAGly had modest effects on Ca _V3 channel kinetics. Both NAGly and NADA inhibited native T-type I _Ca in mouse sensory neurons. Conclusions and implications: N-arachidonoyl dopamine and NAGly increase the steady-state inactivation of Ca _V3 channels, reducing the number of channels available to open during depolarization. These effects occur at NADA concentrations at or below to those affecting CB ₁ and TRPV1 receptors. Together with anandamide, the arachidonoyl neurotransmitter amides, NADA and NAGly, represent a new family of endogenous T-type I _Ca modulators.