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Endomorphins inhibit high-threshold Ca²⁺ channel currents in rodent NG108-15 cells overexpressing µ-opioid receptors

Haruhiro Higashida, Naoto Hoshi, Rimma Knijnik, James E. Zadina * and Abba J. Kastin *

1. Extracellular application of the novel brain peptides endomorphin 1 (EM1) and endomorphin 2 (EM2) inhibited high-threshold Ca²⁺ channel currents in NGMO-251 cells, a daughter clone of NG108-15 mouse neuroblastoma × rat glioma hybrid cells, in which µ-opioid receptors are overexpressed.

2. In contrast, EM1 and EM2 did not induce this inhibition in the parental NG108-15 cells that predominantly express endogenous -receptors.

3. The IC₅₀ for EM1 and EM2 was 7·7 and 23·1 nM, respectively.

4. EM-induced Ca²⁺ channel current inhibition was blocked by treatment or pretreatment of the cells with 100 µM N-methylmaleimide or 100 ng ml^-1 pertussis toxin.

5. These results show that a decrease in conductance of Ca²⁺ channels results following interaction of EMs with cloned µ-receptors, which couple via G_i/G_o-type G proteins, and that EMs fulfill one of the necessary synaptic conditions for them to be identified as neurotransmitters.

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