Downregulation of mu-opioid receptor mRNA in the mediobasal hypothalamus of the female guinea pig following morphine treatment.

Ribonuclease protection assays were performed on the basal hypothalamus (BH), the preoptic area (POA) and the thalamus (THAL) from female guinea pigs that were ovariectomized and implanted with morphine (n = 8) or placebo (n = 8) pellets for 1 week. An antisense [32P]rUTP labeled riboprobe, representing a 280 bp fragment of the guinea pig mu-opioid receptor gene (spanning putative TM II through eight residues of TM IV), protected a single RNA band of 280 bp. In contrast to the rat, the guinea pig THAL expressed less mu-opioid receptor mRNA than both POA and BH. Morphine treatment caused a significant decrease (15.6 +/- 5.8%) in mu-opioid receptor mRNA expression in the BH, while POA and THAL were not different from placebo controls. Therefore, in conjunction with our previous findings of a downregulation of mu-opioid receptors, mu-opioid receptor mRNA is downregulated in the mediobasal hypothalamus of female guinea pigs following chronic morphine treatment.