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.