The κ-opioid receptor agonist U-50488 blocks Ca²⁺ channels in a voltage- and G protein-independent manner in sensory neurons

Background and Objectives: κ-Opioid receptor (κ-OR) activation is known to play a role in analgesia and central sedation. The purpose of the present study was to examine the effect of the κ-OR agonist, U-50488 (an arylacetamide), on Ca²⁺ channel currents and the signaling proteins involved in acutely isolated rat dorsal root ganglion (DRG) neurons expressing the putative promoter region of the tetrodotoxin-resistant Na⁺ channel (NaV 1.8) that is known to be involved in pain transmission. Methods: Acutely isolated rat DRG neurons were transfected with cDNA coding for enhanced green fluorescent protein (EGFP), whose expression is driven by the NaV 1.8 promoter region. Thereafter, the whole-cell variant of the patch-clamp technique was used to record Ca²⁺ channel currents in neurons expressing EGFP. Results: Exposure of EGFP-expressing DRG neurons to U-50488 (0.3-40 μM) led to voltage-independent inhibition of the Ca²⁺ channel currents. The modulation of the Ca²⁺ currents did not appear to be mediated by the Gα protein subfamilies: Gα_i/o, Gα_s, Gα_q/11, Gα₁₄, and Gα_z. Furthermore, dialysis of the hydrolysis-resistant GDP analog, GDP-β-S (1 mM), did not affect the U-50488-mediated blocking effect, ruling out involvement of other G protein subunits. Finally, U-50488 (20 μM) blocked Ca ²⁺ channels heterologously expressed in HeLa cells that do not express κ-OR. Conclusion: These results suggest that the antinociceptive actions mediated by U-50488 are likely due to both a direct block of Ca ²⁺ channels in sensory neurons as well as G protein modulation of Ca²⁺ currents via κ-OR-expressing neurons.