Visualization of mu opiate receptor downregulation following morphine treatment in neonatal rat brain.

In order to visualize neuroanatomical alterations in specific brain regions, light microscopy autoradiography was carried out on neonatal brain sections from controls and neonates chronically treated with morphine. In the case of brains exposed to morphine from postnatal day (PD) 1-4, mu receptor density on PD 5 was non-existent in the patches of the striatum. There were also decreases in mu opiate receptor density in the surrounding matrix area, and in the nucleus accumbens and amygdala. Longer durations of morphine treatment (PD 1-8) did not show these alterations in mu opiate receptor density. These data demonstrate the unique plasticity seen in the immature opioid system. It is suggested that the differences observed in neonatal vs adult central nervous system may be due to interactions with and the differential development of the G-protein/cyclic adenosine 3',5'-monophosphate (cAMP) system.