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 CB₁ receptors and the transient receptor potential vanilloid receptor 1 (TRPV1). Anandamide, the prototypic CB₁/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 (pEC₅₀) 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 CB₁ and TRPV1 receptors. Together with anandamide, the arachidonoyl neurotransmitter amides, NADA and NAGly, represent a new family of endogenous T-type I_Ca modulators.