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  • Title: Inhibition of [3H][3-MeHis2]thyrotropin releasing hormone recognition sites in the rat brain by tifluadom, a kappa opiate receptor agonist.
    Author: Das S, Bhargava HN.
    Journal: Neuropharmacology; 1987 Aug; 26(8):1141-6. PubMed ID: 2889163.
    Abstract:
    The effect of tifluadom (TIF), a postulated kappa-opiate agonist with a benzodiazepine (Bz) structure, on the binding of [3H][3-MeHis2]thyrotropin releasing hormone [( 3H]MeTRH) to receptors for thyrotropin releasing hormone (TRH) in membranes from rat brain was determined. Tifluadom inhibited the binding of [3H]MeTRH with an IC50 value of 1.88 microM. When the binding was carried out in the presence of an IC20 concentration of tifluadom, the Bmax value of [3H]MeTRH was decreased by 20% but no change in the Kd value was noted, indicating that the inhibition was apparently noncompetitive. Ro 15-1788 a benzodiazepine antagonist, as well as bicuculline, a gamma-aminobutyric acid (GABA) antagonist did not antagonize the effect of tifluadom on the binding of [3H]MeTRH suggesting that the benzodiazepine receptors are not involved in the action of tifluadom. Since tifluadom is suggested to be a kappa-opioid agonist, the effect of other kappa-opiate ligands were also tested for their ability to affect TRH receptors. The drugs used were ethylketocyclazocine, dynorphin(1-13) and 5-bromo-6-(2-imidazoline-2-ylamino)quinoxaline (U-50,488H). The order of potency of these compounds to inhibit the binding of [3H]MeTRH to membrane from the rat brain was in the following order: tifluadom greater than U-50,488H greater than dynorphin-(1-13) greater than ethylketocyclazocine. It is concluded that tifluadom inhibits the binding of [3H]MeTRH to receptors in brain in a noncompetitive manner. The effect does not appear to be mediated through benzodiazepine receptors but possibly through kappa-opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)
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