Opioid receptors localize to the external granular cell layer of the developing human cerebellum.

The distribution of opioid receptors in the developing human cerebellum was determined by tissue autoradiography using [3H]naloxone. In infants, opioid receptors were heavily concentrated in the external granular layer, a matrix of germinal cells, and were substantially less concentrated in the internal granular layer, differentiating progeny of external granular cells. In the mature internal granular layer of the child and adult, opioid receptors were negligible. Thus, in the human cerebellum, opioid receptors localize to a population of germinal cells and are negligible in their mature progeny. These data support the idea that endogenous opioids play a role in human brain development and may function as receptor-mediated growth factors. The cerebellum provides a model site to examine abnormal opioid effects upon human brain development, particularly in infants exposed to narcotics in utero.