PubMed · 4495985
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A Johansson. 1974-05-31. [Pain--a warning!].. https://pubmed.ncbi.nlm.nih.gov/4495985/
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Inflammatory hyperalgesia was induced in wild-type, heterozygous and mu-opioid receptor knockout mice after an intraplantar injection of complete Freund's adjuvant. micro-Opioid receptor knockout mice exhibited faster recovery from hyperalgesia as compared to heterozygous (P<0.05) and wild-type (P<0.01) mice. Naloxone restored hyperalgesia in all genotypes. Naltrindole (delta-opioid receptor-selective antagonist) partially restored the hyperalgesia only in mu-opioid receptor knockout mice (P<0.001). Nor-binaltorphimine (kappa-opioid receptor-selective antagonist) had no effect. The mu-opioid receptor-selective agonist, [D-Ala(2), MePhe(4),Gly-ol(5)]enkephalin (DAMGO), reduced the hyperalgesia in heterozygous and wild-type but not in mu-opioid receptor knockout mice while U69,593 ¿(+)-(5alpha,7alpha, 8beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4. 5]dec-8-yl]-benzeneacetamide, kappa-opioid receptor-selective¿ produced similar effects in all mice. The delta-opioid receptor-selective agonists, [D-Pen(2), D-Pen(5)]enkephalin (DPDPE) and deltorphin ([D-Ala(2)]deltrophin-II), produced significantly greater antihyperalgesia in knockout mice (P<0.05). The findings suggest that mu-opioid receptors may be involved in the persistence of inflammatory hyperalgesia and that a delta-opioid receptor-mediated compensatory mechanism in the absence of the mu-opioid receptor is activated by persistent hyperalgesia.
Previous research has shown that rats consuming a sucrose solution and chow are more sensitive to the analgesic actions of morphine, a selective mu opioid agonist, and the anorectic actions of opioid antagonists, than rats eating only chow. However, from these data, it cannot be determined if sucrose intake only modifies the behavioral consequences of drugs that act at the mu opioid receptor, or if the sugar also alters the actions of opioid drugs that act at other opioid receptor subtypes. Thus, the present experiments examined the effects of sucrose intake on the actions of spiradoline, a selective kappa opioid agonist, on analgesia and food intake in male and female Long-Evans rats. In Experiment 1, male and female rats consumed either chow, a 32% sucrose solution and water, or only chow and water. After 3 weeks, antinociceptive responses on the tail-flick test were determined after spiradoline injections (0.0, 0.3, 1.0, and 3.0 mg/ kg, s.c.). Rats fed sucrose were more sensitive to the analgesic actions of spiradoline than rats fed only chow. In Experiment 2, drug-naive male and female rats were maintained under the same dietary conditions as in Experiment 1. Food intake was measured 1, 2, 4, and 6 h after spiradoline injections (0.0, 0.3, 1.0, and 3.0 mg/kg, s.c.). Spiradoline led to significant dose-related decreases in food intake for males and females in both dietary conditions. However, the anorectic effects of the drug were more pronounced in rats fed sucrose than in those eating only chow. These results support the hypothesis that intake of palatable foods and fluids alters the activity of the endogenous opioid system.
The work in our laboratory has been designed to characterize the transducer mechanisms coupled to neurotransmitter receptors in the plasma membrane. Particular attention has been paid to the physiological/pharmacological effects mediated by the opioid system. Antisense oligodeoxynucleotides have proved useful in correlating opioid receptor clones with those defined pharmacologically. The involvement of the cloned opioid receptors mu, delta, and kappa in analgesia has been determined by means of in vivo injection of ODNs directed to the receptor mRNAs. Using this strategy the classes of G-transducer proteins regulated by each type/subtype of opioid receptor in the promotion of antinociception have also been characterized. After displaying different patterns of binding to their receptors, opioids trigger a variety of intracellular signals. The physiological implications and therapeutic potential of these findings merit consideration.