Changes in multiple opioid receptors in regions of the brain in rats treated chronically with thyroxine

The effect of the administration of thyroxine (T4) (1 mg/kg, s.c.), to male Sprague-Dawley rats on alternate days for 18 days, on the binding of opioid ligands, [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO, mu receptors), [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr (DSTLE, delta receptors) and [3H]ethylketocyclazocine (EKC, kappa receptors) to membranes of regions of the brain was determined. The chronic administration of thyroxine to rats decreased their rate of gain of body weight, increased colonic temperature and increased systolic blood pressure and heart rate, in comparison to vehicle-injected rats. The administration of thyroxine also increased the serum concentration of triiodothyronine (total T3) and thyroxine, when compared to vehicle-injected rats. The binding of [3H]DAGO to membranes of the striatum of rats treated with thyroxine was greater than in vehicle-treated rats; however, the binding to membranes of pons and medulla, amygdala, hypothalamus, midbrain and cortex in the two treatment groups did not differ. The increased binding of [3H]DAGO in rats treated with thyroxine was due to an increase in the Bmax value. The binding of [3H]DSTLE in the midbrain, hypothalamus, pons and medulla and striatum of rat treated with thyroxine and vehicle-injected rats did not differ but it was significantly less in the amygdala of rats injected with thyroxine than in vehicle-injected rats. The decreased binding in the amygdala was due to changes in the Kd value of [3H]DSTLE.(ABSTRACT TRUNCATED AT 250 WORDS)