We have determined the effect of naltrexone, naloxone, [D-Ala 2,D-Leu5]enkephalin (DADLE), and morphine on the μ-S196A opioid receptor knock-in and μ-opioid receptor knockout mouse vas deferens preparations. The antagonists, naltrexone and naloxone, exhibited agonist activity and possessed IC50 values that were 14- and 37-fold greater than morphine on the S196A preparation. Morphine was found to be threefold more potent at S196A relative to wild-type μ-opioid receptor. The mouse vas deferens data suggest that S196 in transmembrane helix 4 of the μ-opioid receptor modulates efficacy. It is proposed that this may be due to decreased dimerization of the receptor. Identical IC50 values of DADLE obtained on the wild-type, S196A knock-in, and μ-opioid receptor knockout preparations support the absence of μ-δ heterodimers in the mouse vas deferens.
Bibliographical noteFunding Information:
We thank Mary Lunzer for conducting the mouse vas deferens experiments. This research was supported by NIH research grants DA01533 (PSP), DA07339, DA0564, DA11806, KO5-DA70554 (HHL) and KO5-DA-00513 (PYL).
- Opioid receptor antagonist
- Vas deferens
- μ-Opioid receptor knockout