Guinea pig mu opioid receptor: brainstem expression in the morphine-exposed neonate

The mu opioid receptor (MOR) has been characterized in the mouse and rat. The guinea pig has several advantages as a model for in utero exposure to morphine; the placental structure is similar to the human; and the guinea pig metabolizes morphine to its active metabolite, morphine-6-glucuronide, at similar rates as humans. Guinea pig MOR cDNA was sequenced. To evaluate expression in respiratory control areas, a guinea pig specific cRNA probe was prepared and utilized for in situ hybridization in brainstem slices. In situ results were compared with immunohistochemistry to determine a correlation between mRNA expression and MOR protein expression. Neonatal animals exposed in utero to chronic intermittent morphine were compared with saline-exposed controls. Quantitative RT-PCR comparison of mRNA expression levels in brainstem regions was also performed. No differences were found in brainstem areas expressing mRNA or amount of mRNA expression between treatment and neonatal age groups. Immunohistochemistry showed that protein expression correlates anatomically with mRNA expression and no difference in protein levels could be detected between treatment groups. Lack of difference in mRNA or protein levels of MOR following in utero exposure to morphine suggests that differences seen between these groups with DAMGO-stimulated 35S-GTPgammaS binding involve a change in efficiency of coupling to G-proteins when MOR is activated.