Delta opiate receptors account for the castration-induced unmasking of gonadotropin-releasing hormone binding sites in the rat pituitary

Under control incubation conditions, gonadotropin-releasing hormone (GnRH) binds only a fraction of its receptors in rat-cultivated pituitary cells. U nmasking of the remaining receptors, which have been termed 'cryptic', requ ires drug- or peptide-induced protein kinase activation. Spontaneous maskin g however is not observed on pituitary cells sampled from castrated male ra ts, suggesting the presence of an intrinsic unmasking factor. Many endogeno us factors could theoretically account for the effect. Here we attempted to identify the factor involved by taking advantage of their differential dep endency upon second messengers and transduction cascades. Spontaneous unmas king of GnRH binding was found reversed by pertussis toxin (PTX), an inhibi tor of alpha(i) and alpha(o) subunits of heterotrimeric G proteins, and by U73122, a phospholipase C (PLC) inhibitor. In contrast, desensitization of protein kinase C (PKC) or inhibition of tyrosine kinase by herbimycin were ineffective. Among endogenous pituitary factors able to unmask GnRH recepto rs in pituitary cells from normal male rats, as EGF, NPY or opiate peptides , only the latter were found to correspond to this transduction profile. In an attempt to characterize the pharmacology of opiate effects, naloxone (1 0 mu M), a poorly selective opiate antagonist, restored masking of GnRH bin ding in cells from castrates. Only the delta antagonist naltrindole (1 mu M ) was able to mimick the action of naloxone. Conversely, when tested on cel ls from intact animals, morphine (10 mu M), as well as dslet (1 mu M) and m et-ENK (10 nM), preferential delta agonists, but not dago and beta-endorphi n or U50488 H and dynorphin, respectively mu and kappa agonists, were able to suppress masking. Among opioid peptides endogenous to the pituitary, onl y met-ENK was able to unmask cryptic receptors, an effect antagonized by na ltrindole. We conclude that an opiate delta receptor subtype is endogenousl y activated in the pituitary of castrated male rats to prevent masking of G nRH binding.