Expression of mu-opioid receptors in developing rat spinal cord: an autoradiographic study

The expression of mu-opioid receptors in the developing rat spinal cord (Postnatal days 7, 14, 30) was studied by autoradiography using [3H]DAMGO. When compared to camera lucida drawings, the receptor was noted over the entire gray matter and dorsal root ganglia at postnatal days 7 and 14. At postnatal day 30, the receptor expression decreased over the gray matter except the superficial laminae (laminae I and II). At all age groups studied, a higher expression of the receptor was noted over the superficial laminae. The study shows that micro-opioid receptors appears early in postnatal development and attains mature receptor distribution relatively late in ontogeny, suggesting a possible role in the normal development of nervous system. This is affirmed by an impairment of psychomotor development in babies born to mothers, addicted to opiates.