EFFECTS OF MORPHINE AND ITS METABOLITES ON OPIATE RECEPTOR-BINDING, CAMP FORMATION AND [H-3] NORADRENALINE RELEASE FROM SH-SY5Y CELLS

Opiate receptor occupation leads to a variety of intracellular events including inhibition of adenylyl cyclase and cAMP formation. We have examined the opiate binding characteristics, effects on cAMP formation and [H-3]noradrenaline release of morphine, morphine-6 (M6G) and -3 (M3G)-glucuronides, and fentanyl in SH-SY5Y human neuroblastoma cells. M6G and M3G are the major metabolites of morphine formed in vivo whose cellular action remains to be fully elucidated. In binding experiments morphine (affinity, K50 = 96 nM) and fentanyl (K50 = 99 nM) were more potent than M6G (K50 = 393 nM), while M3G was inactive. However, for cAMP inhibition morphine (half maximum inhibition, IC50 = 193 nM) and M6G (IC50 = 113 nM) were roughly equipotent, with fentanyl (IC50 = 27 nM) being more potent and producing a greater maximum inhibition (56%). M3G was inactive. These in vitro data are in general agreement with the in vivo effects of these glucuronides. Moreover, all of the opiates tested failed to inhibit K+-evoked release of [H-3]noradrenaline. Whilst these data do not support a role for cAMP in neurotransmitter release, alterations in cAMP formation may still have a role to play in the mechanism of analgesia.