Alteration in Prolactin Messenger Ribonucleic Acid Level during the Rat Estrous Cycle: Effect of Naloxone

The present study examines the physiological alterations in prolactin (PRL) messenger ribonucleic acid (mRNA) and serum PRL levels during the rat estrous cycle and the effed of naloxone, an endogenous opioid peptide receptor antagonist, on PRL gene expression during the rat estrous cycle. Adult female rats exhibiting at least two consecutive 4-day estrous cycles were used in this study. A single injection of naloxone (2mg/kg b.w.) or saline was given sc 30 mm prior to decapitation. Animals were sacrificed at 10:00 h of each stage of the estrous cycle, and at 2-h intervals from 10:00 h to 20:00 h during the proestrus. PRL mRNA and serum PRL levels were determined by an RNA-blot hybridization with the rat PRL cDNA probe and by a PRL radioimmunoassay, respectively. PRL mRNA and serum PRL levels were not dramatically altered in the morning of each stage of diestrus I, II, and proestrus, and naloxone failed to modify the two parameters. During estrus naloxone clearly suppressed serum PRL levels, but it was unable to modify PRL mRNA levels. A more detailed examination of the proestrus stage revealed that PRL mRNA and serum PRL levels fluctuated as a function of time: PRL mRNA levels reached a maximum level at 12:00 h and gradually decreased until 18:00 h. PRL mRNA levels then rose at 20:00 h. No difference in PRL mRNA levels between the control and naloxone-treated groups was observed. Changes in serum PRL level during proestrus were conversely related to changes in PRL mRNA: serum PRL levels were low from 10:00 h to 14:00 h, then increased and reached a maximum level at 16:00-18:00 h. Following then, serum PRL levels were decreased. Naloxone was effective in suppressing the characteristic afternoon surge of PRL from 16:00 h to 20:00 h. These data clearly showed that alterations in PRL mRNA levels were conversely correlated with changes in serum PRL levels on proestrus, indicating differential regulation of PRL gene expression and secretion.