The role of protein kinase C (PKC) on dopamine inhibition of PRL messenger RNA (mRNA) levels was studied in anterior pituitary cells kept in primary culture. PKC was desensitized by long-term exposure to 12-O-tetradecanoylphorbol 13-acetate (TPA). Effectiveness of PKC desensitization was confirmed by the fact that after TPA pretreatment, short-term (1-h) exposure to TPA was no longer able to trigger PRL release. In contrast, the capacity of nonreceptor-mediated secretagogues as depolarization with 48 mM K+ to release the hormone was preserved. Pretreatment with TPA did not affect basal PRL mRNA levels. In contrast, it significantly reduced the dose-dependent inhibition of PRL mRNA induced by 1 nM bromocriptine after a 4-day incubation period. Since dopamine inhibition of PRL release is mediated by several second messager pathways, including cAMP, inositol phosphates, and Ca2+, we investigated whether PKC depletion was able to interact with direct stimulation of these pathways. Pretreatment with PKC suppressed stimulation of PRL mRNA levels induced by either Forskolin (FK) or 8Br-cAMP. In parallel, it reduced basal as well as FK stimulated intracellular cAMP levels. In addition, chronic exposure to TPA completely suppressed PRL mRNA inhibition induced by nifedipine, a dihydropyridine antagonist which blocks voltage-dependent Ca2+ channels. TPA desensitization also affected the action of bromocriptine, FK or nifedipine on PRL release measured under the same conditions. The data indicate that endogenous PKC can interfere with the regulation of PRL gene expression induced by both cAMP and Ca2+ pathways, two second messengers associated with the action of dopamine in lactotroph cells.