Regulation of G protein-coupled muscarinic acetylcholine receptors (mAChRs) and delta-opioid receptors has been studied in neuroblastoma hybrids NCB-20 and NG108-15 cells. Exposure of NCB-20 cells to some mAChR agonists (carbachol, oxotremorine-M, and McN-A343) induce mAChR down-regulation, but other agonists such as pilocarpine and oxotremorine induce up-regulation, while another agonist, bethanecol, causes no change in the receptor number. These up- and down-regulations are blocked by atropine and involve corresponding changes in the density, but not affinity, of the receptor binding sites. A change in the m4-mAChR mRNA level is not involved in this modulation. In conjunction with Dr. Carmine Coscia's laboratory, adaptation of delta-opioid receptors has been studies in NG108-15 cells. Up-regulation of delta-receptors induced by two-day treatment with delta-receptor antagonists is preceded by a transient down-regulation of the receptor sites that occurs about 5 min after treatment. This down-regulation is accompanied by a decrease in the heavy membrane population of receptors and an increase in the binding sites in the light membrane fraction. Moreover, treatment with several delta-receptor antagonists induces a decrease in the specific activities of the lysosomal enzymes, suggesting that a retardation of receptor degradation is involved in the up-regulation. Novel nuclear delta- receptor binding sites are detected in intact purified nuclei and nuclear subfractions. Nuclear membranes contain newly synthesized G protein- coupled delta receptors, while chromatins contain internalized, uncoupled receptor binding sites. These nuclear delta-opioid receptors may partic- ipate in opioid functions that require gene transcription, such as opioid tolerance and dependence and modulation of brain development.
