Mechanisms for the stimulatory effects of opioidergic and serotonergic input signals on prolactin in pregnant rats

Opioid modulation of prolactin secretion induced by stress during late pregnancy. Role of ovarian steroids

BACKGROUND

The opioid system modulates prolactin release during late pregnancy. Its role and the participation of ovarian hormones in this modulation are explored in ether stress-induced prolactin release.

METHODS/RESULTS

Estrous, 3-day and 19-day pregnant rats were used. We administered the antagonist mifepristone (Mp) and tamoxifen to evaluate progesterone and estradiol action in naloxone (NAL, opioid antagonist) or saline treated rats. Ether stress had no effect on serum prolactin levels in controls but increased prolactin release in NAL-treated rats. Prolactin response to stress in NAL-treated rats was blocked by l-DOPA administration. Mp treatment on day 18 of pregnancy increased prolactin levels after stress without alterations by NAL. Tamoxifen on days 14 and 15 of pregnancy completely blocked Mp and NAL effects on prolactin release at late pregnancy. In contrast, stress significantly increased prolactin levels in estrous rats and pretreatment with NAL prevented this. On day 3 of pregnancy, at 6.00 p.m., stress and NAL treatment inhibited prolactin levels in saline-treated rat. No effect of stress or NAL administration was detected on day 3 of pregnancy at 9.00 a.m. icv administration of specific opioids antagonist, B-Funaltrexamine but not Nor-Binaltorphimine or Naltrindole, caused a significant increase in stress-induced prolactin release.

CONCLUSIONS

Opioid system suppression of prolactin stress response during late pregnancy was observed only after progesterone withdrawal, involving a different opioid mechanism from its well-established stimulatory role. This mechanism acts through a mu opioid receptor and requires estrogen participation. The opioid system and progesterone may modulate stress-induced prolactin release, probably involving a putative prolactin-releasing factor.

Co-regulation of female sexual behavior and pregnancy induction: an exploratory synthesis

This paper will review both new and old data that address the question of whether brain mechanisms involved in reproductive function act in a coordinated way to control female sexual behavior and the induction of pregnancy/pseudopregnancy (P/PSP) by vaginocervical stimulation. Although it is clear that female sexual behavior, including pacing behavior, is important for induction of P/PSP, there has been no concerted effort to examine whether or how common mechanisms may control both functions. Because initiation of P/PSP requires that the female receive vaginocervical stimulation, central mechanisms controlling P/PSP may be modulated by or interactive with those that control female sexual behavior. This paper presents a synthesis of the literature and recent data from our lab for the purpose of examining whether there are interactions between behavioral and neuroendocrine mechanisms which reciprocally influence both reproductive functions.

Neurotransmitters involved in the opioid regulation of prolactin secretion at the end of pregnancy in rats

Using a pharmacological approach, we explored potential mechanisms for the regulation of prolactin secretion by opioid peptides at the end of pregnancy in rats. On day 19 of pregnancy, intracereboventricular administration of the mu-opioid receptor agonist (D-Ala2, NMe-Phe4, Gly-ol5)-enkephalin (DAMGO) or beta-endorphin (beta-END) induced a dose-related increase in serum prolactin levels 30 min later. Pretreatment with the opioid antagonist naloxone abolished the increase induced by DAMGO injection. At lower doses, DAMGO and beta-END did not modify the 3,4-dihydroxyphenylacetic acid/dopamine ratio, but at higher doses, the mu-agonists evoked a significant increase of the dopaminergic activity as compared with saline control. The time course of the effects of beta-END (2.5 microg/rat) showed a higher increase in serum prolactin levels at 15 min than at 30 min after treatment. The 3,4-dihydroxyphenylacetic acid/dopamine ratio increased 15 min after beta-END administration and was even higher 30 min later. Neither the selective kappa-agonist U50,488H nor the selective delta-agonist (D-Pen2, D-Pen5)- enkephalin were able to modify the serum prolactin levels at the doses studied. To evaluate potential neurotransmitters involved in the regulation of prolactin secretion at the end of pregnancy, we combined the administration of serotoninergic or GABAergic antagonists with the opioid agonist DAMGO. The serotonin 5-HT2 receptor antagonist ketanserin increased the serum prolactin levels and potentiated the effect of DAMGO. The intracerebroventricular administration of SR-95531 did not modify the serum prolactin concentration under basal conditions, but partially prevented the increase induced by DAMGO injection. The intracerebroventricular administration of the GABA(B) receptor antagonist phaclofen had no effect on the serum prolactin levels either in naive or DAMGO-treated rats. The present results support the proposal that activation of mu-opioid receptors stimulates prolactin secretion at the end of pregnancy. Although the exact mechanisms by which the opioid system modulates prolactin secretion at the end of pregnancy are unclear, these results suggest an interaction of the opioidergic system with serotoninergic and GABAergic systems, without ruling out a direct or indirect action on dopaminergic neurons. In conclusion, the opioid system may regulate prolactin secretion at the end of pregnancy through either stimulatory (present results) or inhibitory actions previously described.

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.

Rhythmicity of beta-endorphinergic neuronal activity in the mediobasal hypothalamus during pregnancy in the rat

During the first half of gestation in the rat, prolactin (PRL) from the anterior pituitary gland exerts its luteotropic function on the ovary to stimulate progesterone secretion. During this period, beta-endorphin stimulates PRL secretion by regulation of dopaminergic neurons in the hypothalamus. During the second half, placental lactogens (PLs) take the place of PRL in maintenance of pregnancy, and initiate a negative feedback to suppress PRL secretion. However, the effect of PLs on beta-endorphinergic neurons is not known. The aim of this study was to examine the possibility that PLs suppress PRL secretion by inhibiting beta-endorphinergic neuronal activity. To accomplish this aim, we examined the changes in the neuronal activity of beta-endorphinergic neurons in the mediobasal hypothalamus, as measured by Fos immunoreactivity, after manipulating the levels of PRL and PLs during pregnancy. On day 4 of pregnancy, animals received either Rcho-1 cells in the lateral ventricle that secrete PLs or HRP-1 cells as controls. In a separate experiment on day 12, hysterectomy was performed to remove the intrinsic source of PLs. These rats received Rcho-1 cells, HRP-1 cells, or nothing. Intracerebroventricular (i.c.v.) injection of Rcho-1 into hysterectomized rats was done to examine the effect of PL replacement. Sham-hysterectomy was also performed as a control. Animals were sacrificed 2 days after each treatment at 0200 h, 1400 h, and 1800 h. Brains were used for dual immunocytochemistry of Fos/beta-endorphin. The neuronal activity of beta-endorphinergic neurons of HRP-1 i.c.v. injected animals showed a daily rhythm, with high levels at 0200 h and 1800 h, and a low level at 1400 h. These animals also exhibited two surges of PRL secretion on day 6 of pregnancy. This rhythmicity of beta-endorphinergic neurons was also observed in Rcho-1 i.c.v. injected animals, which showed very low and unchanging PRL levels. However, the magnitude of neuronal activity was reduced. On day 14 of pregnancy, all four experimental groups showed diurnal rhythms of beta-endorphinergic neurons. This rhythmicity occurred even though PRL was elevated at all three time points in the hysterectomized rats and very low in the Rcho-1 i.c.v. injected hysterectomized and sham-hysterectomized rats. Our results demonstrate that there is a diurnal rhythm of beta-endorphinergic neuronal activity in the mediobasal hypothalamus during pregnancy in the rat. PLs might reduce the neuronal activity of beta-endorphinergic neurons, but only during the first half of pregnancy, partially explaining the suppression of PRL surges.

Lactotroph hyperplasia in the pituitaries of female mice expressing high levels of bovine growth hormone

PEPCK/bGH transgenic mice have very high blood levels of foreign GH, and prominent reproductive disturbances, especially in females. To obtain a deeper insight into the causes of these abnormalities, pituitaries of PEPCK/bGH transgenics were studied by immunocytochemistry, electron microscopy and in situ hybridization (ISH) techniques. Pituitary weights were significantly reduced (P < 0.05) in transgenic males, while in transgenic females they were increased without reaching significance compared to nontransgenic controls. In both sexes, GH cells were inhibited, as previously described in other lines of GH transgenic mice. In females, PRL cells were increased by 37% compared to controls. Ultrastructurally, the lactotrophs had characteristics of stimulation and PRL mRNA was increased by 35%. In males the increase in the number of PRL immunoreactive cells was not significant, the PRL mRNA signal did not differ from controls, and there were no changes in their ultrastructure. Only in females ACTH cells were significantly reduced (P < 0.05) in number and unchanged in males; however, POMC mRNA signal was increased in both genders and reached significance (P < 0.05) in males. In females, but not in males, the percentage of LH cells was lower than in control mice. In conclusion, the high blood bGH levels induced sex related changes in transgenic mice from the present line. The infertility of PEPCK/bGH transgenic females may be attributed to lactotroph hyperplasia and marked reduction in number of gonadotrophs.

Immunoneutralization of beta-endorphin blocks prolactin release during suckling without affecting tuberoinfundibular dopaminergic neural activity

The effect of immunoneutralization of beta-endorphin on the suckling-induced prolactin increase and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons was determined in lactating female rats between days 8 - 12 post-partum. Two antisera were used in the immunoneutralization studies. Both were specific for beta-endorphin, exhibiting little cross reactivity with met- or leu-enkephalin or dynorphin. Antisera to beta-endorphin completely abolished the suckling-induced prolactin increase indicating that this endogenous opioid peptide is involved in this response. Suckling significantly inhibited DOPA accumulation in the median eminence and antiserum to beta-endorphin did not prevent this inhibition. Additionally, 5-endorphin antiserum significantly reduced TIDA neural activity even in pup-deprived dams. These results indicate that beta-endorphin is involved in the prolactin secretory response to suckling but that inhibition of TIDA neuronal activity is not its mechanism of action. Other possible mechanisms are discussed.