Mu and delta opioid receptor immunoreactivity and mu receptor regulation in brainstem cells cultured from late fetal and early postnatal rats.

Cultured cells from the rat brainstem were used to study opioid receptor (OpR) expression during late fetal and early postnatal development. Mu and delta opioid receptor (MOR and DOR) expression was investigated from embryonic day 16 (E16) to 6 days postnatal (P6). Postnatal neurons showed more intense MOR immunoreactivity (IR) than neurons cultured from fetal brainstem (P < 0.006). DOR IR showed a similar pattern, but the differences between fetal and neonatal animals were not statistically significant. Using confocal microscopy, MOR and DOR IR were shown to be present on both the cell membrane and within the cytoplasm, in a similar pattern to the IR seen in SH-SY5Y neuroblastoma cells that endogenously express both MOR and DOR. Double-labeling experiments demonstrated colocalization of MOR and DOR in the same brainstem neurons; however, not all MOR IR regions of a single neuron were also positively stained for DOR, and not all DOR IR regions were also positive for MOR. MOR was down-regulated after a 1- or 2-h treatment with 1 microM DAMGO, a potent mu opioid agonist, in both non-transfected and MOR-transfected SH-SY5Y cells and in primary cell cultures. It was concluded that many brainstem neurons express functional MOR or DOR or coexpress both receptors, although intracellular distributions of the receptors are unique for each receptor type.