Expression of μ-, κ- and δ-opioid receptors in P19 mouse embryonal carcinoma cells

P19 embryonal carcinoma cells are pluripotential and able to differentiate into a variety of cell types, including neurons, glia and fibroblast-like cells, upon retinoic acid treatment and cellular aggregation. Using reverse transcription-polymerase chain reaction, ligand binding and immunocytochemical methods, κ- and δ-opioid receptors were detected in undifferentiated P19 cells. The μ-opioid receptor was not observed until one day after plating, following one essential step of differentiation, but increased in number in the four days after plating. Several different expression patterns were detected in these differentiated cells. Some cells exhibited μ- and δ-opioid receptors co-expressed, with or without κ- opioid receptor; whereas some of the cells expressed only κ-opioid receptor. All three opioid receptors are detected on aggregated cells which are postmitotic and also expressing neurofilaments, indicating neuronal characteristics. Furthermore, those cells expressing μ- and δ-opioid receptors also expressed glutamate decarboxylase, characteristic of the GABAergic phenotype. Based on these findings, we propose that P19 cells may serve as a model system to study the developmental regulation of opioid receptors, and in particular their relationship with GABA.