DESENSITIZATION OF DELTA-OPIOID-INDUCED MOBILIZATION OF CA2-15 CELLS(STORES IN NG108)

Activation of delta-opioid receptors in NG108-15 cells induces the rel ease of calcium from an inositol 1,4,5-trisphosphate-sensitive intrace llular store. We used fura-2-based digital imaging to study the effect s of prolonged exposure to agonist on opioid-induced increases in [Ca2 +](i). Exposure to D-Ala(2)-D-Leu(5) enkephalin (DADLE) (1 mu M) for 3 0 min completely desensitized NG108-15 cells to a second DADLE-induced response. The cells recovered gradually over 25 min following washout of DADLE. The desensitization was not due to depletion of intracellul ar calcium stores and bradykinin failed to cross-desensitize the DADLE -evoked response, although both agonists mobilized the same Ca2+ store . Desensitization induced by 100 nM DADLE was overcome by a higher con centration of DADLE (100 mu M). Treatment with 8-cpt-cAMP (0.1 mM) for 30 min did not influence the DADLE-induced increases in [Ca2+](i). Ph orbol dibutyrate (PdBu) (1 mu M) blocked the response completely. Trea tment with the inhibitor of cyclic nucleotide-dependent kinases H8 (1 mu M) for 45 min did not prevent DADLE-induced desensitization. Treatm ent with the protein kinase C (PKC) inhibitors staurosporin (10 nM) an d GF-109203X (200 nM) for 45 min reduced desensitization. However, dow n-regulation of PKC by 24 h exposure to PdBu (1 mu M) failed to preven t the DADLE-induced desensitization in NG108-15 cells. Thus, we conclu de that multiple pathways participated in desensitization of delta-rec eptor-mediated Ca2+ mobilization, one of which includes PKC. (C) 1998 Elsevier Science B.V. All rights reserved.