The binding of 3H-(3-MeHis2) thyrotropin releasing hormone to brain and pituitary membranes of morphine tolerant-dependent and abstinent rats

Abstract

The effect of chronic administration of morphine and subsequent withdrawal on brain and pituitary receptors for thyrotropin releasing hormone (TRH) was investigated in Sprague-Dawley rats. The rats were implanted subcutaneously with four morphine pellets (each containing 75 mg of morphine free base) during a 3-day period. Placebo pellets, which contained all the excipients of morphine pellets except the morphine, were implanted in rats which served as controls. Both tolerance and physical dependence on morphine have been shown to develop as a result of this procedure. TRH receptors were labeled with 3H-(3-MeHis2) TRH (MeTRH). 3H-MeTRH bound to brain membranes at a single high affinity site with Bmax (receptor density) value of 24.6 +/- 2.2 fmol/mg protein and Kd (apparent dissociation constant) value of 3.7 +/- 0.4 nM. The binding of 3H-MeTRH to five regions of the brain namely, hypothalamus, cortex, striatum, midbrain and pons + medulla, as well as pituitary was also investigated. The binding of 3H-MeTRH to pituitary membranes was increased during the development of tolerance, whereas the binding to membranes prepared from different brain regions was unaffected. Serum concentration of triiodothyronine (T3) and thyroxine (T4) were found to be lower in chronic morphine-treated rats when compared to placebo-treated rats, however, serum TSH level remained unaltered. Twenty-four hours after the removal of morphine pellets (natural withdrawal), the binding of 3H-MeTRH to pons + medulla membranes was greater than in placebo control group. Naloxone-precipitated withdrawal produced results which were qualitatively similar to those obtained in rats from which pellets had been removed. The results suggest that the development of tolerance to morphine may be associated with changes in the pituitary-thyroid axis.