Mu-selective opioid peptides stimulate prolactin release in lactating rats

Opioid system functional regulation in neurological disease management

There is increasing evidence to suggest a role for the opioid system in the control of pathophysiology of neurological disorders (Alzheimer's, Parkinson's, and Huntington's diseases, spinal cord injury, epilepsy, hypoxia, and autism). Resuscitation of the altered expression of the opioid system in various neurological disorders is of therapeutic importance. Such treatment may be beneficial in ameliorating the clinical symptoms of the disorder. This Mini-Review provides a brief update on opioid system regulation in neurological disorders and focuses on the opioids' pharmacological importance.

Participation of opioid peptides in sucking-induced oxytocin and prolactin secretions in lactating goats

The role of opioid peptides in the secretion of oxytocin (OT) and prolactin (PRL) induced by sucking was studied in goats. Seven goats were isolated with their kids (four singletons and three twins) in individual corrals 3-4 weeks after parturition. On day 1 of the experiment, the kids were separated from the does for 7 h and were weighed before and 15 min after being reunited with their mothers to assess the amount of milk obtained by sucking. The does were blood-sampled 10 min before and at the end of the sucking period. On day 2, a similar protocol was followed, but naloxone was given immediately after the first blood sample. On day 3, the protocol was repeated but saline vehicle was injected instead of naloxone. On day 5, the naloxone experiment was repeated as on day 2. Milk ejection was evaluated as the difference in the weight of the kids before and after sucking for 15 min, and the maternal serum levels of OT and PRL were measured by radioimmunoassay. A significant decrease in the weight gain of the kids was obtained when the mothers were treated with naloxone on day 2. Consistently, serum levels of OT and PRL induced by sucking were significantly reduced; indicating that sucking-induced OT secretion for milk ejection in lactating goats is facilitated by opioid peptides. In a second experiment performed in the same animals 10 days later, the administration of OT, immediately after naloxone administration, prevented the decrease in the weight gain induced by naloxone, suggesting that the effect of the opioid antagonist on milk ejection in goats is a result of a reduced OT secretion. The results of this study confirm the importance of sucking-induced OT secretion for milk ejection in lactating goats, and indicate that OT and PRL secretion are regulated by opioid peptides in this species.

Chronic morphine exposure during puberty decreases postpartum prolactin secretion in adult female rats

Opiate use in teenage populations has been increasing in recent years. The potential impact of exposure to high levels of opiates at a time when reproductive systems are maturing has not been well studied, especially in females. The present study used an animal model of adolescent opiate abuse in females to examine the potential impact of high levels of opiates during puberty on several reproductive parameters, including suckling-induced prolactin secretion. Two groups of juvenile female rats were administered increasing doses of morphine sulfate or saline (s.c.) from age 30-50 days, beginning with a dose of 2.5 mg/kg and achieving a maximal dose of 50 mg/kg. As adults, these females were mated and reared either their own or foster pups. On either postpartum day 5 or 10, following a 4 h separation, suckling-induced prolactin secretion was measured. In addition, on postpartum day 5 maternal behavior latencies were determined. The results demonstrate reduced suckling-induced prolactin secretion on postpartum day 5 in females previously exposed to morphine during pubertal development. These effects were observed in females rearing either their own or fostered pups. These effects were not due to any differences in maternal behavior latencies, as retrieval or crouching latencies were unaffected. In summary, chronic morphine exposure during puberty results in changes in the regulation of prolactin secretion during early lactation, which are observed several weeks after cessation of drug treatment. These data suggest that prior opiate use during puberty can continue to affect the regulation of prolactin secretion into adulthood.

Endogenous opioid peptides contribute to suckling-induced prolactin release by suppressing tyrosine hydroxylase activity and messenger ribonucleic acid levels in tuberoinfundibular dopaminergic neurons

The endogenous opioid peptides have been implicated in the control of the suckling-induced PRL rise during lactation. This study examined the role of the endogenous opioid peptides in suppressing tuberoinfundibular dopaminergic neuronal activity during lactation. In the first experiment, lactating rats were constantly exposed to pups. Naloxone (NAL; 60 mg/kg x h; i.v.), an opioid receptor antagonist, or saline was infused for 12 h. Blood was collected before and at 2-h intervals during the infusion. NAL suppressed circulating PRL levels to less than 36% of control values at 4, 6, 8, and 12 h after the onset of the infusion. Tyrosine hydroxylase (TH) activity in the stalk-median eminence and TH messenger RNA signal levels in the arcuate nucleus were determined at the end of the NAL infusion. TH activity and TH messenger RNA signal levels were increased 2.5- and 2.7-fold, respectively, after the 12-h NAL infusion. Even though the time spent with their pups was similar between the two groups, the pups in the NAL-treated group failed to gain weight during the 12-h NAL infusion period, whereas the control litters (8 pups) gained 5 g. In a second experiment, pups were removed from the dams before the 12-h NAL infusion and were returned after 11 h. Blood was collected before the infusion, at 3-h intervals during the pup separation period, and at 15-min intervals after reunion with the pups. Plasma PRL in control and NAL-treated rats was low (1-15 ng/ml) and similar during the separation period. The suckling-induced PRL surge in NAL-treated rats was markedly attenuated to 9-25% of control levels (350-650 ng/ml). After a 1-h suckling episode, TH activity in the stalk-median eminence of NAL-treated rats was 4.5-fold greater than controls. Litter weight gains were significantly less in NAL-treated rats during the 1-h suckling episode. These data indicate that the endogenous opioid peptides are an integral component for increasing PRL release in response to suckling and they act to decrease tuberoinfundibular dopaminergic neuronal activity during lactation, in part, by suppressing TH gene expression.

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.

Regulation of prolactin secretion by adrenal steroids in oestrogen-treated ovariectomized rats: participation of endogenous opioid peptides

The purpose of the present study was to determine whether glucocorticoid inhibition of prolactin (PRL) release in oestrogen-treated ovariectomized (OVX) rats is mediated by endogenous opioid peptides (EOPs). All the animals were OVX and given oestradiol benzoate (OB, 20 microg/rat, s.c.) 2 weeks later (day 0). On day 3 they received vehicle, mifepristone (MIF, 10 mg/kg, s.c.) or hydrocortisone (HYD, 2 mg/rat, s.c.), in combination with the opioid antagonist naloxone (NAL, 2 mg/kg, i.p.) or vehicle. Serum PRL concentration was then measured by RIA at 13.00 and 18.00 hr, to include assessment of diurnal variation of PRL secretion. At 13.00 hr either MIF or NAL alone increased PRL secretion with no additional effect when NAL was combined with MIF. HYD had no significant inhibitory effect, but NAL with HYD increased PRL secretion. At 18.00 hr serum PRL concentration was higher than at 13.00 hr, and not affected significantly by MIF or NAL alone, although PRL secretion was increased by treatment with both. HYD inhibited PRL secretion and this inhibition was prevented by NAL. In a second experiment to distinguish antiglucocorticoid and antiprogesterone effects of MIF, we administered progesterone (2 mg/rat, s.c.) or a specific progesterone antiserum. In contrast with MIF, the progesterone antibody had no effect on PRL secretion at 13.00 hr, nor on the stimulation by NAL, while progesterone (unlike HYD) increased PRL secretion and NAL attenuated this response; this was opposite to the effect of NAL with HYD. Similarly, at 18.00 hr the interaction of MIF and NAL was not explained by antagonism of progesterone. Together, these results indicate inhibition of PRL by glucocorticoids but not progesterone, mediated in part by EOPs. At 18.00 hr endogenous glucocorticoids do not regulate oestrogen-stimulated PRL release, although HYD is inhibitory through EOPs.

Morphine induced analgesia is attenuated in post-partum lactating rats

The analgesic effects of morphine administration were determined in post-partum, lactating female rats, as well as in intact, cycling females during the diestrous stage of the estrous cycle. All doses of morphine (2.5, 5 and 10 mg/kg, iv) produced a significant analgesic response in both post-partum and diestrous females using the hot water tail immersion latency test. However, the analgesic response in the post-partum females was significantly less than during diestrus at all doses tested. In addition, pretreatment with the mu 1 specific antagonist, Naloxonazine, significantly blunted the analgesic response in diestrous females, but did not significantly affect analgesia in post-partum females. These results indicate that morphine is less effective in producing analgesia in post-partum females. The mu 1 opiate receptor site does not appear to be involved in the analgesia produced during the post-partum period.

The role of the mu(1) opioid receptor subtype in the regulation of prolactin and growth hormone secretion by Beta-endorphin in female rats: studies with naloxonazine

The μ opioid receptor subtype has been reported to mediate the prolactin secretory response to opioids. This receptor subtype has been implicated in the morphine-induced prolactin increase, as well as the prolactin response to μ-specific opioid peptides. Subtypes of the μ receptor have been proposed and the μ(1) , site has been postulated as the receptor subtype involved in the morphine-induced prolactin secretory response. However, the role of this receptor subtype in mediating the endocrine effects of the endogenous opioid peptides has not been characterized. In order to determine the physiological significance of this receptor subtype, animals were pretreated with saline, WIN 44,441-3 (a μ, δ and κ antagonist) or naloxonazine (a μ(1) antagonist) followed by a stimulatory dose of morphine or β-endorphin. A dose response study for β-endorphin was conducted to determine the minimal stimulatory dose of β-endorphin on the prolactin and growth hormone (GH) secretory response. The dose response study indicated that β-endorphin is a more potent stimulus for prolactin release than for GH. A dose as low as 25 ng increased prolactin levels as much as 100-fold in both lactating and diestrous female rats. In contrast, 2.5 μg β-endorphin was required in order to consistently and significantly increase circulating levels of GH by 2- to 3-fold. WIN 44,441-3 antagonized the stimulatory effects of β-endorphin on both prolactin and GH secretion. Naloxonazine pretreatment abolished the morphine-induced prolactin secretory response, without affecting the GH increase in diestrous females. Naloxonazine also antagonized the prolactin response to β-endorphin in both lactating and diestrous females. In addition, it attenuated the GH secretory response but did not totally abolish it. These data indicate that β-endorphin elicits an increase in prolactin release through an opioid specific receptor which appears to be the μ(1) opioid receptor subtype. They further suggest that β-endorphin may increase GH levels, at least partially, via its action at this μ(1) site.

Inhibition of suckling-induced prolactin release by mu- and kappa-opioid antagonists

Evidence suggests that endogenous opioid peptides (EOP) are involved in the hyperprolactinemia and suppression of luteinizing hormone (LH) release associated with lactation. To address this hypothesis, we investigated the effects of various opioid receptor antagonists on suckling-induced prolactin (PRL) and LH responses in primiparous, lactating rats. All animals were fitted with indwelling jugular catheters to allow serial blood sampling, and some rats received intracerebroventricular (i.c.v.) cannulae for central drug injection. Naloxone (2.0 mg/kg, i.v.) was employed as a broad spectrum opioid antagonist, whereas beta-funaltrexamine (beta-FNA, 1.0-5.0 micrograms, i.c.v.), naloxonazine (NAZ, 20 mg/kg, i.v.) and nor-binaltorphimine (nor-BNI, 4.0-16.0 micrograms, i.c.v.) were used to block mu, mu 1 and kappa receptor sites, respectively. In vehicle-treated rats, pup suckling evoked a dramatic increase in plasma PRL and a concurrent decrease in circulating LH. Naloxone caused a modest, though significant, attenuation of the PRL surge during nursing. beta-FNA and nor-BNI inhibited suckling-induced PRL release in a dose-related fashion, and at sufficient doses, both antagonists abolished the PRL response. Conversely, the suckling-induced rise in plasma PRL was not affected by NAZ. Naloxone, beta-FNA, and NAZ did not alter the profile of circulating LH in suckled rats, but the highest dose nor-BNI (16 micrograms, i.c.v.) produced a significant elevation in plasma LH. However, even in rats treated with 16.0 micrograms of nor-BNI, plasma LH levels declined in response to the nursing stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)