Time course of the insensitivity of prolactin release to morphine administration in the lactating female rat

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.

Immunoneutralization of endogenous opioid peptides prevents the suckling-induced prolactin increase and the inhibition of tuberoinfundibular dopaminergic neurons

Previous studies have shown that the endogenous opioid peptides, acting at specific opiate receptor subtypes, are involved in the suckling-induced prolactin secretory response. The prolactin increase elicited by suckling is due, at least in part, to an inhibition of tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We investigated the effects of immunoneutralization of dynorphin, leu-enkephalin and met-enkephalin on the suckling-induced prolactin increase and on the activity of the TIDA neurons in lactating female rats between days 7 and 12 postpartum. Rats were injected into the right lateral ventricle with antiserum specific for one of these three peptides. Control rats were administered equal amounts of immunoglobulin proteins. Suckling produced a profound and significant increase in prolactin levels, as well as a decrease in DOPA accumulation in the median eminence of lactating rats. Administration of immunoglobulin concentrations of up to 3.6 microg did not inhibit the prolactin secretory response to the suckling stimulus and did not prevent the suckling-induced inhibition of TIDA neurons. Antisera to all three endogenous opioid peptides abolished the suckling-induced prolactin increase and prevented the inhibition in DOPA accumulation in the median eminence. Thus, the endogenous opioid peptides, dynorphin, leu-enkephalin and met-enkephalin, are essential for the prolactin secretory response to suckling and inhibition of TIDA neuronal activity is at least one of the mechanisms of action utilized by these peptides.

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.

Suckling-induced changes in responsivity to the hypoalgesic effect of morphine

The hypoalgesic effect of morphine in lactating rats was assessed using the hot-plate test. At midlactation (days 12 and 18 postpartum) females nursing litters of 8 pups were less responsive to the hypoalgesic effect of morphine than ovariectomised or cycling females and females on days 6 or 24 of lactation. Subsequent studies showed that the hypoalgesic response to morphine was inhibited in lactating rats at a number of time points after drug administration and across a variety of doses. This effect was not dependent on milk delivery but was dependent on the hormonal state of the female since separation of dams and their litters for 96 h was sufficient to reinstate the response to morphine if it resulted in a reappearance of vaginal estrus.

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.

Estrogen fails to reduce tuberoinfundibular dopaminergic neuronal activity and to cause a prolactin surge in lactating, ovariectomized rats

The activity of tuberoinfundibular dopaminergic (TIDA) neurons was estimated by measuring the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence of ovariectomized lactating rats, ovariectomized non-lactating female rats and castrated male rats in correlation with the estrogen-induced prolactin (PRL) surge. PRL surge with a peak at 17.00 h was observed in ovariectomized non-lactating rats but not in the other groups. DOPAC levels were significantly lower at 17.00 h than at 11.00 h in non-lactating female rats, but there was no change of DOPAC levels in lactating rats and male rats. These results indicate that TIDA neuronal activity is inhibited at the time of estrogen-induced PRL surge and that the TIDA neurons may alter their function to result in refractoriness of PRL response to estrogen in the lactating rat.

Effects of immobilization stress on tuberoinfundibular dopaminergic (TIDA) neuronal activity and prolactin levels in lactating and non-lactating female rats

The effects of immobilization stress on the prolactin secretory response and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons were determined in intact, virgin female rats on the morning of diestrus or proestrus and in post-partum, lactating female rats. The virgin females exhibited a significant increase in circulating levels of prolactin which was evident by 1 minute and persisted during the immobilization (5 minutes). In contrast, the prolactin secretory response in lactating females was significantly attenuated compared to non-lactating animals. The activity of the TIDA neurons was not altered by the 5 minutes of stress. Even after 30 minutes of immobilization, TIDA neuronal activity was not affected in either the lactating or cycling females. These data suggest that the cycling female rat is capable of a prolactin secretory response to the stressor without inhibition of TIDA neuronal activity. It seems likely that prolactin releasing factors mediate this response. In contrast, stress did not produce a similar prolactin increase during lactation. It seems likely that, during lactation, the pituitary is not sensitive to releasing factors unless the TIDA neurons are inhibited. There appear to be differences in the sensitivity of the pituitary depending on the physiological state of the model employed.

Mu-selective opioid peptides stimulate prolactin release in lactating rats

Abstract The fact that opiates elicit prolactin secretion is well known. However, we have recently discovered that morphine does not stimulate prolactin release in lactating rats. The physiological basis for this alteration in opiate sensitivity during lactation is not known. Since morphine-induced prolactin secretion in male rats is mediated via the mu opioid receptor subtype, one possible explanation is that mu receptors are down-regulated during lactation. To address this possibility, the effects of mu opioid peptides on prolactin secretion were examined in lactating rats. The presumed mu-selective peptides DAGO ([D-Ala(2), Me-Phe(4), Gly-ol(5)]-enkephalin) and PLO-17 ([NMe-Phe(3), D-Pro(4)]-morphiceptin) were administered to primiparous lactating rats and the resulting hormone responses measured. Both DAGO and PLO-17 caused a rapid and significant rise in plasma prolactin during lactation. The prolactin-releasing effects of both peptides were naloxone reversible, suggesting involvement of opioid receptors. Moreover, the DAGO-induced secretion of prolactin could be completely abolished by pretreatment with the irreversible mu antagonist beta-funaltrexamine. In lactating rats, DAGO and PLO-17 were poor growth hormone-releasing agents, providing further evidence for the mu specificity of these peptides. These results imply that during lactation, as in other reproductive states, mu opioid receptor sites are positively coupled to the prolactin secretory mechanism. Thus, the previously observed inability of morphine to elicit prolactin release in lactating rats cannot be explained on the basis of down-regulation of mu opioid receptors.