Prolactin secretion and its response to stress during the estrous cycle of the rats

Serum and pituitary prolactin (PRL) concentrations were measured during the estrous cycle of the rat with particular attention to the afternoons of the days of proestrus and estrus. Homogenizing machines, a Polytron and Sonifier, were used to extract PRL from the pituitary gland. The effects of ether anesthesia and restraint were also examined on the afternoons of both proestrus and estrus. The occurrence of a surge in PRL secretion during proestrus was confirmed with a peak at 1500 h, and this was accompanied by a decline in pituitary PRL content. A relatively high level of serum PRL was observed in the afternoon of estrus, during which time pituitary PRL content increased progressively. Ether anesthesia had no effect on the proestrus PRL surge, while restraint enhanced it. On the afternoon of estrus, restraint completely suppressed the rise in serum PRL, but ether anesthesia failed to suppress it completely. From these results, the following conclusions were drawn: 1) the PRL surge on the afternoon of proestrus occurs without synthesis of the hormone in the pituitary; 2) PRL secretion on the afternoon of estrus is accompanied by its synthesis in the gland; 3) the PRL response is distinct for each type of stress applied; and 4) PRL secretion is thus regulated by different mechanisms in proestrus and estrus.     

Reproductive experience alters prolactin receptor expression in mammary and hepatic tissues in female rats

Recent studies have reported that reproductive experience in female rats alters prolactin (PRL) receptor gene expression in the brain as well as neural sensitivity to PRL. Given PRL's actions in nonneural tissues, that is, mammary tissue and liver, it was asked whether reproductive experience may also alter prolactin receptor (Prlr) gene expression in these tissues. Groups of age-matched female rats were generated with varying reproductive histories. Separate groups of primiparous (first lactation) and multiparous (second lactation) had mammary tissue and liver samples collected on Day 3 or 10 of lactation. A fifth group raised one litter to weaning and then resumed estrous cyclicity. This group and a final group of age-matched, virgin controls were killed on diestrus. Tissue was processed by quantitative PCR for expression rates of the long and short forms of Prlr mRNA as well as casein beta mRNA (mammary tissue only). Western blots were performed to quantify receptor protein content. Multiple lactations as well as lactation itself resulted in alterations in Prlr expression. Prlr gene expression in mammary tissue was increased in primiparous mothers compared with that in multiparous dams, whereas in the liver, Prlr expression was reduced during an initial lactation. In contrast, PRLR protein levels declined during lactation in mammary, but not hepatic, tissues. Overall, the results demonstrate that the prolactin receptor system is altered in nonneural tissues as a result of the female's reproductive history. The findings are discussed in the context of milk and bile production and PRL's possible role in breast cancer.     

Reproductive experience and opioid regulation of luteinizing hormone release in female rats

The objective of the present study was to determine whether reproductive experience that produces shifts in opioid regulation of prolactin secretion and behavioural functions also alters opioid regulation of LH during the oestrous cycle or lactation. In Expt 1 the effect of naloxone administration (i.v.) on LH was compared between age-matched, nulliparous and primiparous, catheterized female rats on dioestrus II. In Expt 2, the effects of multiple reproductive experiences on opiate control of LH were investigated using cyclic, nulliparous and multiparous (three litters) rats. In both experiments, no differences in naloxone-stimulated LH release were found between groups even though multiple reproductive experiences resulted in the prolongation of oestrous cyclicity. In Expt 3, day 8 lactating primiparous rats were administered 2, 5, 10 or 25 mg naloxone kg-1 i.v. The three lowest naloxone doses, but not the 25 mg kg-1 dose, significantly increased LH concentrations. The possible effects of prior reproductive experience on opioid control of LH during lactation were then investigated. Naloxone at 0.5 mg kg-1, but not at 2 mg kg-1 or 10 mg kg-1, stimulated a significantly greater rise in LH in multiparous (two litters) than in primiparous females. Overall, these data indicate that while modest differences were found in naloxone-induced LH responses between multiparous and primiparous animals during lactation, reproductive experience did not significantly alter opioid regulation of LH during subsequent oestrous cycles at the naloxone doses examined. Hence, the effects of reproductive experience on opioid regulation of LH are less pronounced than those previously found for opioid regulation of prolactin and behaviour.     

Does corpus luteum function autonomously during estrous cycle in the rat? A possible involvement of LH and prolactin

This study examined the of LH and prolactin in the control of corpus luteum function during 4-day cycles in the rat. Bromocriptine (BRC) treatment was performed on proestrus or/and estrus morning that means before or after the preovulatory release of LH. This caused complete blood prolactin depression from the time of injection until diestrus 1 afternoon. This decrease in blood prolactin concentration was associated with a rise in the tonic level of LH secretion in those females which received BRC as soon as on proestrus. We first observed that injection on the morning of proestrus of doses of BRC capable of blunting prolactin secretion on proestrus afternoon did not significantly impair the preovulatory release of LH and did not prevent ovulation occurring during the following night. The life span of the corpora lutea edified from ovarian follicles rupturing before or under BRC administration did not exceed that of those formed under physiological circumstances since 4-day cycles culminating in ovulation constantly took place in all the treated animals whatever the time of BRC injection. To determine the pattern of luteal activity in the absence of prolactin secretion, we measured blood progesterone concentration from estrus until late diestrus in female rats injected with BRC on proestrus and/or estrus at 1100 h. The initiation of the function of corpus luteum on estrus and the achievement of its full activity on diestrus 1 did not appear to be affected by BRC. By contrast the level of blood progesterone declined more rapidly on the morning of diestrus 2 in BRC-treated than in control females. The capacity for autonomous progesterone secretion by corpus luteum of the cycle was discussed in the light of previous and present observations.     

The female reproductive cycle is an important variable in the response to trauma-hemorrhage

Although immune functions in proestrus females are maintained after hemorrhage as opposed to decreased responses in males, it remains unknown whether such a sexual dimorphism also exists with regard to cardiovascular and hepatocellular functions under those conditions. To study this, male and female (estrus and proestrus) rats underwent a 5-cm midline laparotomy and were bled to and maintained at a mean blood pressure of 40 mmHg until 40% of the maximal bleed-out volume was returned in the form of Ringer lactate (RL). Rats were then resuscitated with four times the shed blood volume with RL. At 24 h thereafter, cardiac index; heart performance; hepatocellular function; and plasma estradiol, testosterone, and prolactin levels were measured. Cardiovascular and hepatocellular functions were depressed in males and estrus females (P < 0.05) but were not depressed in proestrus females after resuscitation. Plasma estradiol and prolactin levels were highest in proestrus females (P < 0.05), whereas males had high testosterone and the lowest estradiol levels (P < 0.05). Thus the female reproductive cycle is an important variable in the response to hemorrhage. Because low testosterone and high estradiol and prolactin levels appear to be beneficial for organ functions after trauma-hemorrhage, antagonism of testosterone receptors and/or increases in estradiol and prolactin levels in males and estrus females, respectively, may be novel approaches for improving organ functions under such conditions.     

A secondary surge of prolactin on the estrus afternoon

It has been described that throughout the estrous cycle of the rat, plasma prolactin (PRL) is basal except on proestrus afternoon when a preovulatory surge occurs. However, there have been controversies about PRL levels on the estrus day. Thus, the aim of this study was to evaluate the existence of a secondary surge of PRL on estrus afternoon and correlate it with plasma estradiol levels. The jugular vein of cycling rats was cannulated at 14:00 h on proestrus and a blood sample was withdrawn at 17:00 h for plasma LH measurement and determination of the preovulatory LH surge occurrence. In order to exclude the regular cycling rats that do not present the gonadotropins preovulatory surge and do not ovulate, only rats showing the LH surge on proestrus were considered in this study. Blood samples were collected hourly during estrus from midnight to 9:00 h (group 1) and from 10:00 to 18:00 h (group 2). In group 1, PRL showed a descending profile from midnight to 9:00 h, whereas the estradiol concentrations were constant. In group 2, a secondary surge of PRL was observed in 20 of 25 (80%) rats and plasma estradiol remained constant, but was higher in animals with the PRL surge. Thus the present data suggest the occurrence of a secondary surge of PRL in the afternoon of estrus that seems to be related to plasma estradiol levels of estrus day, which might exert only a permissive role in this surge generation.     

Effects of intraventricular injection of delta receptor antagonist ICI 154, 129 on the secretion of luteinizing hormone and prolactin in the proestrous rat.

In order to ascertain the role of delta receptors in the control of gonadotropin secretion, a preferential delta receptor antagonist ICI 143,129 was microinjected into the third ventricle through chronically implanted cannulae and the effects on the serum concentration of luteinizing hormone (LH) and prolactin (PRL) were determined in female rats in proestrus. When the injection was given at 1030 h, ICI 154,129 (50 micrograms) exerted no significant effects on either LH or PRL. However, in the rat given a microinjection of ICI 154,129 at 1300 h, an afternoon rise in LH occurred in advance and was of greater magnitude, with the peak time more than 1 h earlier and the peak amplitude approximately 100% greater than that in the control rat, respectively. The injection also suppressed the PRL rise during the plateau phase. The results indicate that delta receptors are involved in the mediation of the inhibitory influence of endogenous opioids on the surge of LH in proestrus, and that delta receptors mediate the facilitatory influence of opioids on the PRL surge during the plateau phase.     

The influence of reproductive experience on milk energy output and lactation performance in the grey seal (Halichoerus grypus)

Although evidence from domestic and laboratory species suggests that reproductive experience plays a critical role in the development of aspects of lactation performance, whether reproductive experience may have a significant influence on milk energy transfer to neonates in wild populations has not been directly investigated. We compared maternal energy expenditures and pup growth and energy deposition over the course of lactation between primiparous and fully-grown, multiparous grey seal (Halichoerus grypus) females to test whether reproductive experience has a significant influence on lactation performance. Although there was no difference between primiparous females in milk composition and, thus, milk energy content at either early or peak lactation primiparous females had a significantly lower daily milk energy output than multiparous females indicating a reduced physiological capacity for milk secretion. Primiparous females appeared to effectively compensate for lower rates of milk production through an increased nursing effort and, thus, achieved the same relative rate of milk energy transfer to pups as multiparous females. There was no difference between primiparous and multiparous females in the proportion of initial body energy stores mobilised to support the costs of lactation. Although primiparous females allocated a greater proportion of energy stores to maternal maintenance versus milk production than multiparous females, the difference was not sufficient to result in significant differences in the efficiency of energy transfer to pups. Thus, despite a lower physiological capacity for milk production, primiparous females weaned pups of the same relative size and condition as multiparous females without expending proportionally more energy. Although reproductive experience does not significantly affect the overall lactation performance of grey seals, our results suggest that increases in mammary gland capacity with reproductive experience may play a significant role in the age-related increases in neonatal growth rates and weaning masses observed in other free-ranging mammals.     

Differential changes in the mechanisms controlling luteinizing hormone and prolactin secretion in middle-aged female rats

Serum luteinizing hormone (LH) and prolactin (PRL) concentrations were measured in young (3-4 month old) and middle-aged (10-12 month old) intact female rats on proestrus, in ovariectomized rats after two estrogen injections (estradiol benzoate; EB, 10 micrograms/100 g body weight, s.c.) or after preoptic stimulation in EB-primed ovariectomized rats. Only animals showing regular 4-day estrous cycles were selected for the experiment. The magnitude of proestrous LH surge was significantly smaller in middle-aged than in young rats. Two BE injections, at noon on Days 0 and 3, in ovariectomized middle-aged rats failed to induce surges in LH secretion on Day 4 whereas the same treatment produced LH surges in ovariectomized young rats. The preoptic electrochemical stimulation (50 microA for 60 sec) produced a prompt rise in serum LH levels in ovariectomized EB-primed young but not in middle aged rats. The preoptic stimulation with a larger current (200 microA) induced LH secretin in middle-aged rats. In none of these situations serum PRL concentrations were different between young and middle-age rats. These results suggest differential aging rates in the preoptic mechanisms governing LH and PRL secretion in the rat. The function of the preoptic ovulatory center in responding to the estrogen positive feedback action and inducing LH secretion may become impaired and independent of the PRL control mechanism, even before the regular estrous cycle terminates.     

Patterns of gonadotrophin secretion associated with ovulation

A detailed analysis was made of circulating luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin throughout the estrous cycle in rats. Vaginal cytology was recorded daily for 18 or 23 days in 2 groups of adult female rats, 300 and 400 per group. The presence of distended uteri or tubal ova was noted at the time of exsanguination under ether anesthesia. Serum concentrations of LH, FSH, and prolactin were determined by radioimmunoassay. In 404 rats with regular 5-day cycles, all 3 hormones reached maximum concentrations on the afternoon of functional proestrus. During the next 3 days FSH decreased progressively, and LH levels indicated a diurnal fluctuation with minimum concentrations during darkness. Serum prolactin may have b een elevated due to anesthesia and exsanguination and showed a rapid increase which occurred simultaneously with elevated LH and FSH on the afternoon of proestrus. Radioimmunoassay of all 3 hormones indicated that the increases in serum LH and prolactin did not necessarily occur simultaneously. Hypophysectomy and sequential heart puncture of proestrus rats during maximum hormone release showed disappearance rates (half-lives) for LH, FSH, and prolactin of 20, 110, and 13 minutes, respectively.     

Prolactin Induces Apoptosis of Lactotropes in Female Rodents

Anterior pituitary cell turnover occurring during female sexual cycle is a poorly understood process that involves complex regulation of cell proliferation and apoptosis by multiple hormones. In rats, the prolactin (PRL) surge that occurs at proestrus coincides with the highest apoptotic rate. Since anterior pituitary cells express the prolactin receptor (PRLR), we aimed to address the actual role of PRL in the regulation of pituitary cell turnover in cycling females. We showed that acute hyperprolactinemia induced in ovariectomized rats using PRL injection or dopamine antagonist treatment rapidly increased apoptosis and decreased proliferation specifically of PRL producing cells (lactotropes), suggesting a direct regulation of these cell responses by PRL. To demonstrate that apoptosis naturally occurring at proestrus was regulated by transient elevation of endogenous PRL levels, we used PRLR-deficient female mice (PRLRKO) in which PRL signaling is totally abolished. According to our hypothesis, no increase in lactotrope apoptotic rate was observed at proestrus, which likely contributes to pituitary tumorigenesis observed in these animals. To decipher the molecular mechanisms underlying PRL effects, we explored the isoform-specific pattern of PRLR expression in cycling wild type females. This analysis revealed dramatic changes of long versus short PRLR ratio during the estrous cycle, which is particularly relevant since these isoforms exhibit distinct signaling properties. This pattern was markedly altered in a model of chronic PRLR signaling blockade involving transgenic mice expressing a pure PRLR antagonist (TG^{Δ1–9-G129R-hPRL}), providing evidence that PRL regulates the expression of its own receptor in an isoform-specific manner. Taken together, these results demonstrate that i) the PRL surge occurring during proestrus is a major proapoptotic signal for lactotropes, and ii) partial or total deficiencies in PRLR signaling in the anterior pituitary may result in pituitary hyperplasia and eventual prolactinoma development, as observed in TG^{Δ1–9-G129R-hPRL} and PRLRKO mice, respectively.     

Suppression of the proestrus prolactin surge in the rat estrous cycle by urethane anesthesia

Urethane-anesthetized male rats have been used for the analysis of prolactin (PRL)-releasing substances on PRL secretion. However, there are only a few reports investigating the effect of urethane anesthesia on PRL secretion in female rats. In this study, we intended to examine the effects of urethane anesthesia on PRL secretion during proestrus in the rat. Proestrus PRL surge was completely blocked when urethane was administered to rats prior to the critical period of proestrus both at doses of 1.0 g/kg and 1.5 g/kg. Additionally, urethane, at a dose of 1.5 g/kg, was also effective in blocking spontaneous ovulation. An experiment examining pituitary PRL concentration at 1800 h confirmed that urethane (1.0 g/kg) anesthesia prevents the PRL surge from the pituitary. Similarly, urethane anesthesia blocked the LH surge from the pituitary, but LH levels in the urethane-treated group were higher than those in the pentobarbital-treated group.     

Effects of estrogen and testosterone on specific prolactin binding in the kidneys and adrenal of rats.

The effects of estradiol benzoate in the female rat, testosterone propionate in the male rat, and castration in both sexes on specific prolactin binding sites in the particulate membranes of the kidneys and adrenals were studied. Castration resulted in a significant increase in PRL binding activity in the kidneys of both males and females, and in a significant increase in PRL binding activity in the adrenals of the females. The increase in PRL binding with castration and the decrease seen with testosterone treatment were similar in both immature and mature rats. Progesterone administration to castrate females failed to alter PRL binding in both tissues. The present results suggest that estrogen and testosterone participate in the PRL osmoregulatory system in rat.     

Steroid regulation of gonadotropins in genetically hypoprolactinemic females (IPL nude rats)

IPL nude females present an absence of lactation with hypoprolactinemia. While males present a slight but significant decrease in serum testosterone and gonadotropins, females show normal values of estradiol, progesterone, LH and FSH during all estrus cycle stages. In this work, we observed that the postovariectomy rise of LH and FSH was significantly lower in the IPL nude females. We studied also the effect of acute (1 injection of 25 micrograms/rat E2Bz) or long-term (E2Bz capsule for 8 days) estradiol benzoate (E2Bz) treatment, with or without progesterone injection (5 mg/rat) in ovariectomized (OVX) IPL and normal females. The sensitivity of gonadotropins to E2 negative feedback is decreased in the IPL nude rats, result in agreement with previous reports and which could be linked to both hypoprolactinemia and decreased beta-endorphin observed in the IPL nude rat. The responsiveness of LH to LHRH was also tested in OVX and OVX + E2Bz or OVX + E2B + P treated. In OVX females responsiveness of LH to LHRH was similar in IPL nude to that of normal females. However, LH responsiveness in acute and long-term steroid-treated OVX IPL nude was significantly depressed. Since the mechanism whereby PRL interacts with steroids to modify gonadotropin secretion is still unexplained, IPL nude rat could be a good model to study it.     

Stimulation of prolactin release by prolactin-releasing peptide in rats.

We have previously reported a hypothalamic peptide that shows specific prolactin (PRL)-releasing activity in vitro, named prolactin-releasing peptide (PrRP). However, its activity in vivo has not yet been shown. In this study, we examined whether PrRP could induce specific PRL release in vivo using normal cycling female and male rats. Intravenous injection of PrRP31 increased plasma PRL levels in rats in a dose-dependent manner. PrRP31 (50 nmol/kg i.v.) significantly (P < 0.05) stimulated plasma PRL levels within 25 min after injection in rats in proestrus, estrus, and metestrus. A higher dose of PrRP31 (500 nmol/kg i.v.) was necessary for a significant increase in plasma PRL levels in male rats. These results clearly indicate that female rats, especially at proestrus, are more sensitive to PrRP-induced PRL secretion than male rats. The effect of PrRP on PRL release is affected considerably by the estrous cycle and sex, which suggests that PrRP sensitivity is controlled by the endogenous hormonal milieu, such as estrogen levels. PrRP31 did not affect other pituitary hormone secretions. The results indicate that PrRP shows specific PRL-releasing activity in vivo as well as in vitro and suggest that it plays an important role in the regulation of PRL release under certain physiological conditions.     

Reproductive experience alters anxiety-like behavior in the female rat

Reproductive experience (i.e. pregnancy and lactation) results in significant alterations in subsequent hormone levels in female rats. Several studies have demonstrated that circulating hormones can significantly affect anxiety-like behavior. Thus, the purpose of the present study was to determine whether reproductive experience induces alterations in anxiety-like behaviors in cycling female rats and in older, reproductively senescent rats. In Experiment 1, the elevated plus maze (EPM) was used to test young cycling (6-8 weeks post-weaning) and middle-aged (32-36 weeks post-weaning) primiparous rats and their age-matched nulliparous counterparts for anxiety-like responses. In Experiment 2, activity in the open field was used as an additional measure of anxiety-like behavior in young (proestrus) and middle-aged (constant estrus) primiparous and nulliparous rats. For Experiment 3, EPM testing was conducted in separate groups of young and middle-aged animals tested two weeks after ovariectomy. The results revealed that during proestrus, primiparous animals exhibited fewer anxiety-like behaviors on the EPM compared to nulliparous controls. In middle-aged animals, however, parity was associated with increased anxiety-like behavior. In the open field, young, non-lactating primiparous animals again exhibited fewer anxiety-like behaviors compared to nulliparous controls, an effect that was reversed in middle-aged animals. Effects of reproductive experience on the EPM in both age groups were eliminated by ovariectomy. Overall, the findings indicate that reproductive experience significantly alters anxiety-like behavior, effects that are influenced by the endocrine status and/or age of the female.     

Prolactin receptors in the rat mammary gland. Change during the estrous cycle

Ovine prolactin (o-PRL) binding to mammary gland membranes was studied during the estrous cycle in the rat. Groups of rats were decapitated throughout the 4-day estrous cycle at 10 h00 on the days of diestrus I, diestrus II and estrus and at 10 h00, 12 h00, 16 h00 during the day of proestrus. Daily vaginal smears were taken to determine the stage of the estrous cycle which was also controlled by PRL and LH serum levels. Prolactin receptors were quantified in the 100 000 g pellet. For one Scatchard analysis, mammary gland membranes from 5 animals were pooled. Results given are the mean of 4 or 5 pools. Results obtained showed that the apparent affinity constant (KA) remained unchanged during the days of diestrus II and at all the times studied of proestrus and showed a slight but significant decrease on the days of estrus and diestrus I (or metestrus). The binding capacity did not vary from the day of diestrus II to the proestrus 16h00 (11.3 +/- 2.8 fmoles/mg protein) but sharply increased on the day of estrus (190.4 +/- 35.9 fmoles/mg protein). Binding capacity remained elevated on the day of diestrus I. This increase of PRL receptors on the day of estrous would appear to be an important step in preparing mammary gland for pregnancy and lactation.     

Maintenance of lung myeloperoxidase activity in proestrus females after trauma-hemorrhage: upregulation of heme oxygenase-1

Previous studies showed that females in the proestrus stage of the reproductive cycle maintain organ functions after trauma-hemorrhage. However, it remains unknown whether the female reproductive cycle is an important variable in the regulation of lung injury after trauma-hemorrhage and, if so, whether the effect is mediated via upregulation of heme oxygenase (HO)-1. To examine this, female Sprague-Dawley rats during diestrus, proestrus, estrus, and metestrus phases of the reproductive cycle or 14 days after ovariectomy underwent soft tissue trauma and then hemorrhage (mean blood pressure 40 mmHg for 90 min followed by fluid resuscitation). At 2 h after trauma-hemorrhage or sham operation, lung myeloperoxidase (MPO) activity and intercellular adhesion molecule (ICAM)-1, cytokine-induced neutrophil chemoattractant (CINC)-1, CINC-3, and HO-1 protein levels were measured. Plasma 17beta-estradiol concentration was also determined. The results indicated that trauma-hemorrhage increased lung MPO activity and ICAM-1, CINC-1, and CINC-3 levels in ovariectomized females. These parameters were found to be similar to sham-operated animals in proestrus female rats subjected to trauma-hemorrhage. Lung HO-1 protein level in proestrus females was increased significantly compared with female rats subjected to trauma-hemorrhage during diestrus, estrus, and metestrus phases of the reproductive cycle and ovariectomized rats. Furthermore, plasma 17beta-estradiol level was highest in proestrus females. Administration of the HO inhibitor chromium mesoporphyrin prevented the attenuation of shock-induced lung damage in proestrus females. Thus these findings suggest that the female reproductive cycle is an important variable in the regulation of lung injury following trauma-hemorrhage and that the protective effect in proestrus females is likely mediated via upregulation of HO-1.     

Patterns of progesterone secretion in free-living California ground squirrels (Spermophilus beecheyi)

During a long-term field study of a free-living population of California ground squirrels (Spermophilus beecheyi), blood samples were drawn at regular intervals from marked females via femoral venipuncture, and plasma progesterone (P) and prolactin (PRL) were measured by radioimmunoassay. Marked fluctuations with season and reproductive condition occurred in circulating levels of both hormones, with peak levels occurring during the spring breeding season. Two peaks in P concentrations were observed each spring, the first occurring during pregnancy, and the second during lactation. Peak PRL levels in females were also reached during the lactation interval, midway between the two P peaks. Analysis of repeated measures from individual females showed a marked decline in circulating P around the time of parturition. Juveniles had lower mean P levels than adults, and yearlings had lower peak levels during their initial reproductive episodes than older females did. The observed pattern of P secretion in S. beecheyi differs from that known for most mammals, but resembles those reported for other ground-dwelling sciurid rodents.     

Lactational [correction of Lacational] anovulation in rats and its dependency on progesterone

The relationship between prolactin (PRL) secretion and anovulation in lactating rats was studied. Normal lactating rats and lactating rats treated with antiserum against luteinizing hormone-releasing hormone at the time of postpartal ovulation were used. Normal lactating rats were treated with either a dopamine agonist (CB-154, 150 micrograms/rat) on Day 10 or 13, or pups removal on Day 7 or 10, and thereafter luteolysis and inhibition on PRL secretion were assessed. With the CB-154 treatment, the incidence of luteolysis increased as the lactational period advanced (42% vs 72%), whereas it decreased (73% vs 14%) with the pups removal. Thus, dopamine effectively inhibited PRL secretion during the later lactational stage, but could not do so during the earlier stage when there were mechanisms other than dopamine stimulating PRL secretion. Following luteal regression induced by CB-154, ovulation did not occur if the rats were treated with CB-154 on Day 10, whereas 50% of the rats ovulated within 4 days if treated on Day 13. Furthermore, in the lactating rats treated with anti-luteinizing hormone-releasing hormone serum during late pregnancy, ovulation was not observed until Day 10 of lactation. Since the serum progesterone levels were low in these rats due to the absence of ovulation and lactational corpora lutea, the blockade of ovulation was not due to elevated circulating progesterone during the early lactational period. The mechanism of ovulation blockade during lactation thus seems to shift from being progesterone independent to progesterone dependent at a similar period when the neuroendocrine control of PRL secretion shifts from dopamine independent to dependent.