Biochemical evidence of functional interaction between mu- and delta-opioid receptors in SK-N-BE neuroblastoma cell...

Radioligand binding assays and functional experiments revealed that the SK-N-BE neuroblastoma cell line expresses a similar ratio of mu- and delta-opioid receptors, both negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. Our findings also indicate that some functional interaction occurred between the two opioid subtypes; in fact, long-term exposure to [D-Ala2-N-methyl-Phe4-Gly-ol5]enkephalin (DAMGO), a mu-selective agonist, sensitized the functional response of the delta-selective agonist but not vice versa. It is interesting that in acute interaction experiments, we observed a shift to the right of the concentration-effect curve of either DAMGO or [D-Pen2,5]enkephalin (DPDPE), a delta-selective agonist, as a result of DPDPE or DAMGO administration, respectively. In addition, low doses of naloxone, an antagonist selective for mu receptors, increased the inhibitory effect [D-Ala2-D-Met5]enkephalinamide (DAME), a mixed mu/delta agonist, on adenylyl cyclase activity. Taken overall, these data support the hypothesis of the existence of a cross talk between mu and delta receptors in the SK-N-BE cell line.