The μ-opioid receptor gene-dose dependent reductions in G-protein activation in the pons/medulla and antinociception induced by endomorphins in μ-opioid receptor knockout mice

There appear to be different relationships between μ-opioid receptor densities and the acute and neuroadaptive μ-opioid agonist-induced responses of the multiple opioid neuronal systems, including important pons/medulla circuits. The recent success in creating μ-opioid receptor knockout mice allows studies of μ-opioid agonist-induced pharmacological and physiological effects in animals that express no, one or two copies of the μ-opioid receptor gene. We now report that the binding of μ-opioid receptor ligand, [³H][D-Ala²,NHPhe⁴,Gly-ol]enkephalin to membrane preparations of the pons/medulla was reduced by half in heterozygous μ-opioid receptor knockout mice and eliminated in homozygous μ-opioid receptor knockout mice. The endogenous μ-opioid agonist peptides endomorphin-1 and -2 activate G-proteins in the pons/medulla from wild-type mice in a concentration-dependent fashion, as assessed using [³⁵S]guanosine-5'-o-(3-thio)triphosphate binding. This stimulation was reduced to half of the wild-type levels in heterozygous mice and eliminated in homozygous knockout mice. The intracerebroventricular injection of either endomorphin-1 or endomorphin-2 produced marked antinociception in the hot-plate and tail-flick tests in wild-type mice. These antinociceptive actions were significantly reduced in heterozygous μ-opioid receptor knockout mice, and virtually abolished in homozygous knockout mice. The μ-opioid receptors are the principal molecular targets for endomorphin-induced G-protein activation in the pons/medulla and the antinociception caused by the intracerebroventricular administration of μ-opioid agonists. These data support the notion that there are limited physiological μ-opioid receptor reserves for inducing G-protein activation in the pons/medulla and for the nociceptive modulation induced by the central administration of endomorphin-1 and -2.