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

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Mu and kappa opioid receptor expression in the mediobasal hypothalamus and effectiveness of selective antagonists on prolactin release during lactation

Endogenous opioid peptides are involved in prolactin release during lactation, in part by decreasing tuberoinfundibular dopaminergic (TIDA) neuronal activity. Both mu (mu) and kappa (kappa) opioid receptors have a role in the suckling-induced prolactin rise after 4-5 h up deprivation. The aim of this study was to investigate effects of mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), and kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI), on prolactin secretion and TIDA neuronal activity in lactating rats after 18 h pup deprivation. After 4 h separation from pups, the suckling-induced prolactin rise was abolished by 16 microg nor-BNI and 5 microg beta-FNA, coincident with increased dihydroxyphenylacetic acid (DOPAC):dopamine ratio in the stalk-median eminence (SME). However, after 18 h pups separation, these same doses of nor-BNI and beta-FNA did not alter the prolactin surge or DOPAC:dopamine ratios in the SME. Higher doses of nor-BNI (32 microg) and beta-FNA (10 microg) were required to inhibit suckling-induced prolactin secretion. beta-FNA (10 microg) increased the DOPAC:dopamine ratio in the SME, whereas nor-BNI (32 microg) treatment had no effect. The mu and kappa opioid receptor mRNA levels in the mediobasal hypothalamus were similar to suckled control rats after 4 h pup deprivation, but increased 1.4-fold after 18 h pup deprivation. These data support involvement of endogenous opioidergic systems in the suckling-induced prolactin rise after a prolonged (18 h) period of pup deprivation, as well as the shorter (4 h) pup deprivation period previously reported. Suppression of TIDA neuronal activity likely played a part in mu opioid receptor input to the suckling-induced prolactin rise after both 4 h and 18 h separation, whereas non-dopaminergic input was implicated with kappa opioid receptors after 18 h pup deprivation. Increased mu and kappa opioid receptors gene expression in the mediobasal hypothalamus may contribute to reduced effectiveness of opioid receptor antagonists to block suckling-induced prolactin release after 18 h pup deprivation.

Orphanin FQ/nociceptin is a physiological regulator of prolactin secretion in female rats

Orphanin FQ/nociceptin (OFQ/N), the most recently identified endogenous opioid peptide, stimulates prolactin secretion in both male and female rats. OFQ/N, however, did not elicit this stimulatory effect through the mu-, delta-, or kappa-opiate receptor subtype. The role OFQ/N plays in prolactin regulation under physiological conditions and its mechanism of action are not known. The purpose of these studies was to determine the physiological significance and pharmacological specificity of the prolactin secretory response to OFQ/N. In addition, the role of the tuberoinfundibular dopaminergic (TIDA) neurons in mediating this response was examined. Opioid receptor-like-1 (ORL-1) receptors were blocked by pretreatment with compound B (Comp B), a purported OFQ/N antagonist, or receptor synthesis was disrupted by pretreatment with ORL-1 receptor antisense oligonucleotides. The prolactin secretory response to OFQ/N administration in diestrous females was measured. Furthermore, the suckling-induced prolactin response was also determined after Comp B pretreatment. TIDA neuronal activity was quantified in diestrous female rats to determine whether OFQ/N stimulates prolactin release by inhibiting TIDA neurons. OFQ/N significantly inhibited the TIDA neurons by 1 min, preceding the prolactin secretory response. Both Comp B and antisense pretreatment blocked the stimulatory effects of OFQ/N on prolactin release, and Comp B abolished the suckling-induced prolactin response. These studies indicate that OFQ/N is a potent stimulus for prolactin secretion in female rats and that it mediates this effect by rapid and transient inhibition of TIDA neuronal activity. Furthermore, OFQ/N plays a physiologically significant role in the regulation of prolactin secretion during lactation, and it mediates its effects via actions at the ORL-1 receptor subtype.

Effect of steroids and nitric oxide on pituitary hormone release in ovariectomized, peripubertal rats

The purpose of this study was to determine the effects of the duration of steroid depletion on the steroid-induced luteinizing hormone and prolactin surges in ovariectomized, peripubertal female rats. Additionally, the role of nitric oxide (NO) in mediating the surge responses was determined. Peripubertal, 6-week-old, female Sprague-Dawley rats were ovariectomized. One or three weeks later, animals were injected with 17β-estradiol (50 μg, sc) followed 48 h later by progesterone (2.5 mg, sc). Effects of NO were examined by administeringl-arginine (300 mg/kg, ip). The response of ovariectomized, adult females to steroid treatment was also determined.

One and three weeks after ovariectomy, steroid replacement produced an LH and prolactin surge in peripubertal animals. However, both the magnitude and duration of the LH surge was greater 3 weeks after ovariectomy. Whilel-arginine significantly enhanced the magnitude of the LH surge 1 week after ovariectomy, by 3 weeksl-arginine caused a decrease in the duration, but not the magnitude of the surge. In contrast,l-arginine did not affect either the magnitude or duration of the prolactin surge one week after ovariectomy, but diminished the magnitude after 3 weeks of steroid depletion. In adults, steroids induced significant increases in both LH and prolactin. These results demonstrate that sensitivity to NO stimulation of LH, but not prolactin secretion, is modulated by the duration of gonadal steroid hormone depletion. The differences in the responsiveness of LH and prolactin to steroid-induced stimulation in peripubertal animals demonstrate that these hormones are regulated by NO through different mechanisms.

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.

Acetyl-RYYRIK-NH2, a purported orphanin FQ antagonist, does not block the orphanin FQ-induced prolactin secretory response in female rats

Acetyl-RYYRIK-NH2, a drug reported to act as an orphanin FQ/nociceptin (OFQ/N) receptor antagonist, did not block the OFQ/N-induced prolactin increase in female rats. Indeed, Acetyl-RYYRIK-NH2 pretreatment increased the magnitude and duration of the prolactin secretory response. Furthermore, by 15 min after the Acetyl-RYYRIK-NH2 pretreatment, and prior to any OFQ/N administration, circulating levels of prolactin were significantly increased in a dose-related manner, providing further evidence that this drug exhibits agonist activity. In contrast, neither drug affected basal levels of luteinizing hormone nor had any effect on tail flick latency in rats. These results indicate that Acetyl-RYYRIK-NH2 does not antagonize the prolactin secretory activity of OFQ/N.

Pivotal roles of alpha-melanocyte-stimulating hormone and the melanocortin 4 receptor in leptin stimulation of prolactin secretion in rats

Leptin, the obese gene product, was reported to stimulate prolactin (PRL) secretion, but the neuroendocrine mechanism underlying this hormonal response is largely unknown. Thus, in this study we examined the involvement of several important PRL regulators in the leptin-induced PRL secretion in male rats. Compared with the values in normally fed rats, food deprivation for 3 days significantly decreased both PRL and leptin levels in the plasma. These changes were reverted to normal by a 3-day constant infusion of 75 microg/kg/day of leptin to the fasted rats, while 225 microg/kg/day of leptin further elevated both PRL and leptin levels. These four groups of animals were used for the following experiments. Results of dopamine and serotonin turnover studies in the brain and the pituitary indicated that neither of these biogenic amines plays a primary role in mediating leptin's effects on PRL. Repeated intracerebroventricular injections over 72 h of neutralizing antibodies against vasoactive intestinal peptide, PRL-releasing peptide, or beta-endorphin, did not significantly suppress the leptin actions. However, both the blockade of the melanocortin (MC) 4 receptor (R) and the immunoquenching of brain alpha-melanocyte-stimulating hormone (alpha-MSH) completely abolished the leptin-induced PRL release, and the stimulation of the MC4-R, but not the MC3-R, significantly elevated PRL levels in the fasted rats. These results suggest that alpha-MSH, a cleaved peptide from pro-opiomelanocortin of which synthesis is stimulated by leptin, may be the pivotal neuropeptide in the brain mediating the leptin's stimulatory influence on PRL secretion. It was also suggested that the MC4-R may be the primary subtype of the MC-Rs mediating this action of alpha-MSH.

[Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) does not antagonize orphaninFQ/nociceptin-induced prolactin release

The specificity of the orphaninFQ (OFQ)/nociceptin (N)-induced prolactin increase was determined in male and female rats by pretreating animals with different doses of [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2), a compound originally reported to be a specific OFQ/N antagonist. In addition, the effect of naloxone pretreatment on OFQ/N-induced prolactin release was examined to determine if OFQ/N's effects were mediated by opiate receptors. Furthermore, dose response studies using [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) only were performed to determine potential agonist activity of this drug. Finally, growth hormone (GH) levels were determined as an index of specificity of the prolactin response. Our results confirm previous findings that OFQ/N potently stimulates prolactin release and that a gender difference exists in the magnitude of the response, with females showing a much greater response than male rats. The endocrine response is specific because OFQ/N potently stimulated prolactin, but not GH secretion. The prolactin response is not mediated by actions at opiate receptors because naloxone did not inhibit OFQ/N's effects on prolactin release. However, [Phe(1)Psi(CH(2)-NH) Gly2]NC(1-13) NH(2) did not antagonize OFQ/N's effects on prolactin release. Indeed, this drug acted as a potent agonist. Demonstrating pharmacological specificity of OFQ/N's effects on prolactin release awaits the development of more selective, specific antagonists.