Endomorphins Inhibit N-Type Ca²⁺ Channel Currents Through µ-Opioid Receptors in NG108-15 Cells Expressing Cloned µ-Receptors

Abstract

Endomorphin 1 (EM1) and endomorphin 2 (EM2) recently isolated from rat brain are tetrapeptides with C-terminal amidation (Zadina et al. 1997). EMs have been shown to have the highest affinity and specificity for µ-opioid receptors. The question arises whether EMs function as neurotransmitters to mediate analgesia, opioid dependence, and neuroendocrine effects. To answer this question, we used cultured NGMO-251 cells overexpressing cloned rat µ-opioid receptors (Morikawa et al. 1995 ). The high-threshold Ca²⁺ channel currents were evoked by a single-pulse protocol in which a 200-ms depolarizing pulse to +20 mV and +40 mV from a holding potential of -40 mV was applied. Ba²⁺ was used as a charge carrier for Ca²⁺ channel currents.