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  • Title: Opioid receptors mediating antinociception from beta-endorphin and morphine in the periaqueductal gray.
    Author: Smith DJ, Robertson B, Monroe PJ, Taylor DA, Leedham JA, Cabral JD.
    Journal: Neuropharmacology; 1992 Nov; 31(11):1137-50. PubMed ID: 1335557.
    Abstract:
    beta-Endorphin and morphine produce an increase in the latency of the tail-flick reflex when administered into the PAG of awake rats. The antinociceptive effect of both opioid agonists was blocked by the sequential local injection of either CTP (D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2), a selective mu opioid receptor antagonist, naltrexone, or beta-endorphin (1-27), a putative epsilon opioid receptor antagonist, with minimal selectivity. When either CTP or naltrexone was used as the antagonist, the dose-inhibition curves generated for beta-endorphin and morphine were not parallel, suggesting the involvement of separate and distinct receptors. Also, synergism occurred when a dose of morphine producing submaximum antinociception was administered simultaneously with either a submaximal or ineffective dose of beta-endorphin. Inhibition of the antinociceptive response to beta-endorphin by mu antagonists and the non-selective antagonism of both beta-endorphin and morphine by beta-endorphin (1-27) suggested that epsilon opioid receptors were not involved. Additionally, a mu/delta opioid receptor complex was not involved, since ICI 174,864 (Allyl2-Tyr-Aib-Aib-Phe-Leu-OH), a selective delta opioid receptor antagonist, did not alter the response to beta-endorphin. Thus, although additional characterization is required, beta-endorphin and morphine appear to act (at least in part) through different opioid receptors, demonstrable using selected mu opioid receptor antagonists.
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