We have recently demonstrated that acute and chronic treatments with estradiol and progesterone induce changes in the responsiveness of endogenous opioid systems to painful stimulation. In the present study the neuroblastoma SH-SY5Y subclone known to contain predominantly mu opioid receptors was used as a model to characterize the gonadal steroid effect on this opioid receptor system. The function of opioid receptors was assessed by measuring prostaglandin E1 (PGE1)-induced cyclic AMP accumulation after various treatments with estradiol and progesterone. Differentiated SH-SY5Y cells respond to PGE1 with a dramatic increase in cAMP level. Morphine (MOR) inhibits by about 75% the stimulatory effect of PGE1 on cAMP. Pretreatment with 5 nM of estradiol for 6 days resulted in a significant increase of PGE1-stimulated cAMP accumulation. Exposure of cells for 48 h to estradiol in doses of 5 nM or 50 nM did not affect cell sensitivity to the PGE1 effect on cAMP. Moreover, neither dose of estradiol changed the inhibitory effect of morphine on PGE1-induced cAMP response. There was a significant increase in PGE1-stimulated cAMP accumulation after treatment with 100 nM progesterone for 1 h or 15 min and a marked elevation of cAMP levels was also measured after 15 min treatment with 10 nM progesterone. Exposure to either dose of progesterone for 8 h, 48 h or 6 days did not affect basal or PGE1-induced cAMP in neuroblastoma cells. Progesterone-treated groups responded to MOR with 56-67% inhibition of PGE1-stimulated cAMP accumulation. The potency of MOR-induced inhibition was comparable to the MOR effect in cells not treated with the steroid.(ABSTRACT TRUNCATED AT 250 WORDS)