BACKGROUND AND PURPOSE The belief of pain and its inhibition varies considerably between individuals and this variability is still unexplained. antagonist naltrindole. Inhibition of the binding of [3H] naltrindole by μ-opioid receptor agonists was different in brain membranes from SDU and Wistar rats. Differences were also obvious in the effect of δ-opioid receptor ligands around the binding of [35S]GTP-γ-S stimulated by μ-opioid receptors agonists. No strain-related differences were detected in spinal cord membranes. The potency of morphine Rabbit polyclonal to AFF3. to inhibit cAMP production in brain membranes varied between the strains in the presence of deltorphin II and naltrindole. Co-immunoprecipitation experiments exhibited that δ-opioid receptors were associated with μ-opioid receptors to a higher extent in brain synaptosomal fractions from SDU than in those from Wistar rats. CONCLUSIONS AND IMPLICATIONS There was increased supraspinal cross-talk between μ and δ-opioid receptors in SDU as compared with Wistar rats. This was related to an enhanced sensitivity to anti-nociception induced by μ-opioid receptor agonists. 2011 Opioid receptors belong to the family of GPCRs and their multiplicity provides a basis for explaining the complex pharmacology of opioids. At present the presence of interactions between opioid receptors is usually widely assumed. There is indirect evidence that opioid receptors do not necessarily take action independently from each other. The presence of opioid receptor complexes was reported more than 30 years ago from radioligand binding and anti-nociception experiments (Vaught and Takemori 1979 The cross-talk between μ-opioid receptors and δ-opioid receptors is usually documented mainly from your observation that δ receptor agonists modulate μ receptor-mediated analgesia (Vaught and methods in two strains of rats – Sprague-Dawley bred at our university or college (SDU) and Wistar – that differ in their sensitivity to morphine. Our findings demonstrate that this sensitivity to the anti-nociceptive effect of μ-opioid receptor agonists was related to the extent of the conversation between μ- and δ-opioid receptors at a supraspinal level. Methods Animals All animal care and experimental procedures were approved by the Institutional Animal Care and Use Committee and followed guidelines regarding ethical requirements for the experimental investigation of pain in animals (Zimmermann 1983 All experiments Isolinderalactone were carried out on adult male rats that were 12-15 weeks aged. The strains used were SDU rats derived from a collection bred at our University or college and Wistar rats that were purchased commercially (Harlam Barcelona Spain). The animals were housed in obvious plastic cages three to four rats per cage and managed on a 12 h light/dark cycle with sawdust bed linens. Food and water were provided < 0.05. Receptor binding For each batch pooled membranes from the whole brain (minus cerebellum) or spinal cord from six rats were prepared (Fang < 0.05. Co-immunoprecipitation experiments Co-immunoprecipitation experiments were performed as explained by Garzón Isolinderalactone < 0.05). This resulted in suppression of the significant differences in the estimates of Isolinderalactone ED50 of morphine and in the dose-response curves (Table 1). Naltrindole (10 mg·kg-1 s.c.) also reversed the increased effect of 10 and 20 μg·kg-1 of the highly selective μ receptor agonist fentanyl in SDU rats compared with Wistar rats (< 0.01) (Physique 1D). Binding of [3H]naltrindole and [3H]DAMGO to the brain and spinal cord microsomal fraction To determine the density of brain μ- and δ-opioid receptors in SDU and Wistar Isolinderalactone rats saturation binding of [3H]naltrindole and [3H] DAMGO to the brain and spinal cord microsomal portion was decided The apparent < 0.01 in both cases). The percentage of high-affinity binding sites for morphine and DAMGO showed affordable agreement. In contrast for the brain membranes of Wistar rats the results fitted significantly to a one binding-site model. The < 0.01). In the brain membranes of SDU rats deltorphin II and naltrindole shifted the concentration-response curves of morphine significantly to the left and right respectively (Physique 8D). In the presence of 0.5 μM deltorphin II (a concentration that by itself did not inhibit significantly the adenylyl cyclase activity stimulated by forskolin) the.