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Endocrinology Vol. 147, No. 10 4688-4694
Copyright © 2006 by The Endocrine Society

Reproductive Experience Increases Prolactin Responsiveness in the Medial Preoptic Area and Arcuate Nucleus of Female Rats

Greg M. Anderson, David R. Grattan, Willemijn van den Ancker and Robert S. Bridges

Centre for Neuroendocrinology and Department of Anatomy and Structural Biology (G.M.A., D.R.G., W.v.d.A.), University of Otago School of Medical Sciences, Dunedin 9054, New Zealand; and Department of Biomedical Sciences (R.S.B.), Tufts Cummings School of Veterinary Medicine, North Grafton, Massachusetts 01536

Address all correspondence and requests for reprints to: Dr. Greg Anderson, Centre for Neuroendocrinology and Department of Anatomy and Structural Biology, University of Otago School of Medical Sciences, P.O. Box 913, Dunedin, New Zealand. E-mail: greg.anderson{at}anatomy.otago.ac.nz.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The experience of pregnancy plus lactation produces long-term enhancements in maternal behavior as well as reduced secretion of prolactin, a key hormone for the initial establishment of maternal care. Given that prolactin acts centrally to induce maternal care as well as regulate its own secretion, we tested whether prolactin receptors in brain regions known to regulate behavioral and neuroendocrine processes were up-regulated and more responsive to prolactin in reproductively experienced females. Diestrous primiparous (8 wk after weaning) and age-matched virgin rats were treated with 250 µg ovine prolactin sc or vehicle and the brains collected 2 h later for measurement of mRNA for genes involved in prolactin signaling. Reproductively experienced rats had lower serum prolactin concentrations, compared with virgin rats, suggesting enhanced prolactin feedback on the arcuate neurons regulating prolactin secretion. In the medial preoptic area and arcuate nucleus (regions involved in regulating maternal behavior and prolactin secretion, respectively), the level of long-form prolactin receptor mRNA was higher in primiparous rats, and prolactin treatment induced a further increase in receptor expression in these animals. In the same regions, suppressors of cytokine signaling-1 and -3 mRNA levels were also markedly increased after prolactin treatment in reproductively experienced but not virgin rats. These results support the idea that reproductive experience increases central prolactin responsiveness. The induction of prolactin receptors and enhanced prolactin responsiveness as a result of pregnancy and lactation may help account for the retention of maternal behavior and shifts in prolactin secretion in reproductively experienced females.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
THE STATES OF pregnancy and lactation bring about a range of physiological adaptations in the maternal brain that enable the mother to care for her offspring. For example, it is well established that whereas virgin females require a considerable period of exposure before showing maternal care to foster young, parous rats display this response relatively quickly (1, 2). Having demonstrated maternal care, either as virgin animals after prolonged exposure to pups or in the primiparous state, this maternal ability is remembered for a number of months after separation from the young (2, 3, 4). Although exposure to pups appears to be a crucial factor for the development of maternal memory in natural situations (1, 5), rats that have never previously been exposed to young will also rapidly establish maternal care if they are treated exogenously with steroids plus prolactin before testing with foster pups (6). The requirement for this hormone in regulation of maternal behavior is evident from prolactin receptor knockout studies because both homozygous and heterozygous prolactin receptor-deficient mice show greatly impaired maternal care toward their offspring or foster pups (7). Prolactin appears to stimulate maternal care by acting on cells within the medial preoptic area (mPOA); a region that has long been known to mediate maternal behavior in rats (8) and that expresses protein (9, 10) and mRNA (11, 12) for the long form of the prolactin receptor, which is the predominant form in the hypothalamus (13). Infusion of prolactin or the closely related rat placental lactogens into the mPOA facilitates the onset of maternal care in pup retrieval experiments (14, 15, 16), whereas infusion of the human recombinant prolactin receptor antagonist S179D-PRL into this area impede expression of maternal care (17).

Given the well-established role of prolactin for the development of maternal behavior, it is somewhat surprising that maternally experienced rats tend to have lower circulating levels of prolactin than age-matched virgin rats. A significant decrease in the magnitude of proestrus prolactin surges is observed in experienced rats (18). Multiparous rats (19) and women (20) also exhibit lower suckling-induced prolactin levels, compared with primiparous females. These observations suggest an overall decrease in prolactin secretion after reproductive experience. One possible explanation for this is that reproductive experience might cause an increase in the responsiveness of the hypothalamus to prolactin by inducing prolactin receptor expression, facilitating mPOA responses to prolactin and also leading to increased prolactin-induced stimulation of dopaminergic activity within the arcuate region and thus reduced prolactin secretion. In this study we investigated this hypothesis by measuring levels of long-form prolactin receptor mRNA using real-time quantitative PCR in primiparous and virgin rats in three hypothalamic regions in which prolactin is known to act: the mPOA, the paraventricular nucleus (PVN, in which prolactin may modulate stress responses and oxytocin secretion) (21), and the arcuate nucleus (ARC, the location of the tuberoinfundibular dopaminergic neurons that control the negative feedback regulation of prolactin secretion) (22). To assess the effect of reproductive experience on the signaling responsiveness of cells in these regions to prolactin, we also measured mRNA for suppressors of cytokine signaling (SOCS)-1 and -3 after acute prolactin treatment. The SOCS family of proteins is a recently described class of cytokine-inducible inhibitors of cytokine signaling (23, 24, 25) that act as feedback inhibitors for a range of cytokines that use Janus kinase/signal transducer and activator of transcription (STAT) pathways, including prolactin. In vitro studies of the action of these proteins indicate that they act as part of an intracellular feedback loop to inhibit the phosphorylation of STATs (26), thereby moderating signal transduction. In vivo, prolactin has been shown to induce mRNA for SOCS-1, and -3 in ovary, adrenal, and mammary glands (27) as well as in the ARC (50), and their levels therefore reflect the degree of long-form prolactin receptor signaling (28).


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals
Female Sprague Dawley rats were obtained from the University of Otago animal breeding facility. Rats were group housed (except during late pregnancy and when with their litters) under conditions of controlled lighting (lights on from 0500 to 1900 h) and temperature (22 ± 1 C), and had free access to food and water. Half of the animals were paired with males at 17–21 wk of age; these females were allowed a full gestation and reared their litters (normalized to 12 pups) for 3 wk before weaning. Age-matched control females were never exposed to males. Estrous cycles were subsequently monitored by daily cytological examination of vaginal smears. On the day of diestrus, when the rats were 31–35 wk old (8 wk after weaning) and weighing 300–400 g, all animals were injected with either ovine prolactin (Sigma, St. Louis, MO; 250 µg sc in 250 µl saline) (50) or vehicle (n = 6–8 per group) between 1000 and 1200 h. Two hours after injection, animals were killed by decapitation and their brains and trunk blood collected. The brains were immediately removed, frozen on dry ice, and stored at –80 C until microdissection of the mPOA, PVN, and ARC for measurement of prolactin receptor and SOCS mRNAs. Blood samples were allowed to clot and the harvested serum stored at –20 C until prolactin RIA. All animal experimental protocols were approved by the University of Otago Committee on Ethics in the Care and Use of Laboratory Animals.

Brain microdissection
Thick coronal brain sections (300 µm) were cut in a cryostat at –9 C, thaw mounted onto glass slides, and refrozen. The mPOA was dissected from three consecutive sections (between ~0.0 and –0.9 mm relative to Bregma) with four punches per section of a sterile 21-G micropunch needle (500 µm internal diameter), so that a 1-mm square was removed immediately dorsal to the optic chiasm. The PVN was dissected from two consecutive sections (between ~–1.4 and –2.0 mm relative to Bregma) with a single punch on either side of the third ventricle at its most dorsal extent. The ARC was dissected from five consecutive sections (between ~–2.2 and –3.7 mm relative to Bregma) with a single midline punch centered around the ventral extent of the third ventricle (29). A schematic representation of the micropunches within these planes of dissection for each brain area is shown in Fig. 1Go. Micropunched tissue was placed in 50 µl of a commercial lysis buffer (Cells-to-cDNA II cell lysis buffer; Ambion, Austin, TX), sonicated in an ultrasonic cell disrupter, and then incubated at 75 C for 10 min to rupture the cells and inactivate endogenous RNases. The tissue lysate was then stored at –80 C until reverse transcription.


Figure 1
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  		FIG. 1. Schematic coronal rat brain sections, taken from the Paxinos and Watson brain atlas, showing the locations of micropunches of hypothalamic nuclei (circles). The section coordinates represent the distance (relative to Bregma) of the rostral face of each section. Diameter of the micropunch needle is 500 µm.

 
Preparation of cDNA
The crude tissue lysate was treated with 4 U DNase 1 (Ambion) according to the manufacturer’s instructions to degrade genomic DNA. After DNase 1 inactivation, reverse transcription of RNA into cDNA was performed in the Cells-to-cDNA II cell lysis buffer (RNA isolation is not required with this procedure). A 20-µl volume of lysate (containing 56 ± 2 ng total RNA) was reverse transcribed into cDNA in a 33-µl reaction with 30 U MultiScribe reverse transcriptase (Applied Biosystems, Foster City, CA) using random hexamer primers (Applied Biosystems). Each run, which was conducted in a 96-well PCR plate, included a control tube with reverse transcriptase enzyme omitted to demonstrate that the template for the PCR product was not genomic DNA.

Real-time quantitative PCR measurement of long-form prolactin receptor and SOCS-1 and -3 mRNA
Taqman probes and forward (sense) and reverse (antisense) primers to the rat genes encoding long form prolactin receptor and SOCS-1 and -3 (see Table 1Go) were designed using Primer Express software (Applied Biosystems). Real-time PCR was conducted as described previously (30) with minor modifications. A 20-µl reaction mix was prepared containing primers and probes at optimized final concentrations (Table 1Go), ABsolute qPCR Rox master mix (Abgene; Epsom, Surrey, UK) and RNase-free water. Template cDNA (2.5 µl) was added. A stock of cDNA containing relatively high levels of long-form prolactin receptor and SOCS mRNA was created by pooling many punches of liver (23), and 4-fold dilution series of this were run on each plate as external standards. An ABI PRISM 7000 sequence detection system (Applied Biosystems) was used to detect fluorescence during each PCR cycle. Standard curves of dilution factor vs. the cycle number at which fluorescence first exceeded a given threshold were linear (r2 = < 0.98); these curves were used to calculate relative levels of long-form prolactin receptor and SOCS mRNA from the sample threshold values. These data were then normalized to total RNA content [which is advocated as a less error-prone method than normalization to housekeeping genes (31)]. Total RNA in the remaining tissue lysate was measured in 96-well plates using a Quant-iT RNA assay kit (Molecular Probes, Eugene, OR), which has a lower detection limit of around 2 ng and tolerates a range of contaminating substances in the solution. The fluorescent signal was read on a POLARstar Optima fluorescence microplate reader (BMG Labtechnologies, Offenburg, Germany) at excitation and emission wavelengths of 640 and 680 nm, respectively. The prolactin receptor and SOCS mRNA levels were then expressed as arbitrary (relative) units per nanogram total RNA. Coefficients of variation (CV) for the sample duplicates were almost always less than 1%.


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  		TABLE 1. Details of oligonucleotides used for real-time quantitative PCR

 
Prolactin and estradiol RIA
Serum prolactin concentration was measured in 10-µl sample volumes by RIA. Values are expressed in terms of the rat standard NIDDK-rat PRL-RP-3. Iodinated rat prolactin (NIDDK-rat PRL-I-6) was used as tracer and primary antisera was NIDDK rabbit antirat PRL-RIA-9 (final dilution 1:150,000). The sensitivity of the prolactin assay (95% confidence interval at 0 ng/ml on the standard curve) was 2 ng/ml. The intraassay CV was 15% for a serum pool falling in the middle of the standard curve. All samples were measured in a single assay.

Serum estradiol concentration was measured in 200-µl sample volumes using a commercially available RIA kit (DSL-39100; Diagnostic Systems Laboratories, Inc., Webster, TX). The sensitivity of the estradiol assay was 0.5 pg/ml. The intraassay CV was 23% for a serum pool falling in the middle of the standard curve. All samples were measured in a single assay.

Statistical analysis
All significant treatment effects were identified using two-way ANOVA. Where data failed equal variance or normality tests, they were log transformed (base 10). This was followed by the Bonferroni t test for post hoc analysis to determine where significant effects occurred. Results are presented as mean ± SEM, and differences were considered significant at P < 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The concentration of prolactin in serum from control rats was generally low (20 ± 10 ng/ml); however, a 5-fold reduction (P < 0.05) in the level of this hormone was evident in primiparous rats, with most animals having values below the assay detection level (Fig. 2Go). Serum estradiol concentration tended to be high (40–70 pg/ml) in control rats, possibly reflecting their middle age. As was the case with prolactin, the level of estradiol was significantly lower in primiparous rats (P < 0.05; Fig. 2Go). Ovine prolactin treatment did not effect the serum concentration of endogenous rat prolactin or estradiol in either group, as was expected because such an effect usually takes longer than the 2-h time frame after treatment in the present experiment (32).


Figure 2
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  		FIG. 2. Serum prolactin and estradiol concentration in age-matched cycling virgin and cycling primiparous rats treated with ovine prolactin (250 µg, sc) or vehicle 2 h previously on the day of diestrus. *, Main effect of reproductive experience, P < 0.05.

 
Basal levels of prolactin receptor were 2–3 times higher in the ARC than in the mPOA and PVN (Fig. 3Go). In the mPOA and ARC, primiparity caused an approximately 3-fold increase in the levels of prolactin receptor mRNA (P < 0.001). Prolactin treatment induced a further increase in these animals (P < 0.05) but did not affect prolactin receptor mRNA levels in virgin control rats. In the PVN there was a trend toward an effect of primiparity on this variable (3-fold increase; P = 0.06), but no effect of prolactin in either group.


Figure 3
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  		FIG. 3. Levels of mRNA for prolactin receptor (long form) in the mPOA, PVN, and ARC, microdissected from cycling virgin and cycling primiparous rats treated with ovine prolactin (PRL; 250 µg, sc) or vehicle 2 h previously on the day of diestrus. *, P < 0.05 vs. the appropriate vehicle-treated controls; **, main effect of reproductive experience, P < 0.001.

 
Basal levels of SOCS-1 (Fig. 4Go) and -3 (Fig. 5Go) were similar across all three brain areas studied. In primiparous rats, SOCS-1 and -3 mRNA levels were markedly increased after prolactin treatment in the mPOA (4-fold increase) and ARC (2- to 3-fold increase) (P < 0.001). In contrast, no effects of prolactin were seen in virgin rats or in the PVN for either group.


Figure 4
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  		FIG. 4. Levels of mRNA for SOCS-1 in the mPOA, PVN, and ARC, microdissected from cycling virgin and cycling primiparous rats treated with ovine prolactin (PRL; 250 µg, sc) or vehicle 2 h previously on the day of diestrus. **, P < 0.001 vs. the appropriate vehicle-treated controls.

 

Figure 5
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  		FIG. 5. Levels of mRNA for SOCS-3 in the mPOA, PVN, and ARC, microdissected from cycling virgin and cycling primiparous rats treated with ovine prolactin (PRL; 250 µg, sc) or vehicle 2 h previously on the day of diestrus. **, P < 0.001 vs. the appropriate vehicle-treated controls.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The results of this study support the concept that reproductive experience increases central responsiveness to prolactin. In the mPOA and ARC, areas crucial for maternal behavior and regulation of prolactin secretion, respectively, basal levels of expression of the long form of the prolactin receptor were increased in reproductively experienced females. Moreover, prolactin treatment caused an up-regulation of components of the Janus kinase/STAT signaling pathway, i.e. SOCS mRNAs, in primiparous but not virgin rats. Our results provide evidence that an increase in levels of the long-form prolactin receptor in the mPOA and ARC and a consequent increase in responsiveness to prolactin may be a mechanism by which parity causes this long-term change in maternal care.

In the present study, reproductive experience induced an increase in the basal levels of prolactin receptor mRNA in the mPOA and ARC. This finding extends an earlier report showing that high prolactin receptor mRNA levels are maintained in these areas 1 wk after weaning (30). A similar trend was observed in the PVN in our experiment. Both prolactin (33) and estradiol (9, 34) have been shown to up-regulate prolactin receptor mRNA in the hypothalamus, although the evidence for this is less definitive for prolactin. It is noted, however, that in the current experiment, reproductive experience resulted in a reduction of the serum concentrations of both hormones. Hence, it is unlikely that these hormones are directly responsible for the induction of prolactin receptor mRNA in reproductively experienced rats. However, there are two possible caveats to this situation. First, the reproductively experienced rats would have been exposed to high circulating levels of estradiol, prolactin, and placental lactogens at various times throughout pregnancy and lactation, processes that are known to induce prolactin receptors (10). Because the animals in the present study were weaned more than 8 wk before sampling, it seems unlikely that prolactin receptor levels could still be elevated simply as a carry-over from the time of parity (30). Second, there is a small body of evidence that suggests the brain itself can produce prolactin. Prolactin gene and protein expression have been reported in the mPOA, PVN, and supraoptic nuclei as well as in the stria terminalis, amygdala, and locus caeruleus, suggesting these brain regions are capable of producing prolactin (35, 36, 37). Because it has been shown that pregnancy and lactation are associated with an increase in hypothalamic prolactin gene expression (36), it is conceivable that the induction of prolactin receptors in reproductively experienced animals in the current study could have been caused by an increase in brain-derived prolactin production that is induced by the female having experienced a prior pregnancy and lactation.

It is possible that if the prolactin receptor up-regulation is maintained after parity that such an up-regulation in the ARC may contribute to the observed suppression prolactin secretion found in the reproductively experienced diestrous rats. An increase in prolactin receptor activity would be expected to result in a concomitant increase in tuberoinfundibular dopamine (TIDA) activity, thereby increasing dopamine release and a suppression or attenuation of prolactin synthesis and secretion from lactotrophs (22). The suppression of prolactin secretion in reproductively experienced females would likewise be greatly favored by enhanced neural prolactin production because a central source of prolactin would be outside of the negative feedback loop of circulating prolactin on the TIDA system (38).

The results of the present study demonstrate that reproductive experience alters the responses of prolactin-induced intracellular signaling pathways on SOCS mRNA expression. Because changes in SOCS mRNA levels were observed only after prolactin injection (i.e. not seen in the vehicle-treated controls), the up-regulation of this signaling mediator (which is known to be induced by a range of cytokines and hormones) can be directly attributed to prolactin stimulation in this experiment. It has been reported that SOCS proteins act as part of an intracellular feedback loop to restrain the level of STAT phosphorylation and signal transduction (26). We have recently shown that STAT5b in TIDA neurons is translocated to the cell nucleus between 0.5 and 2 h after prolactin treatment in rats (39), and induction of SOCS mRNA is maximal about 2 h after treatment (50). Thus, the high SOCS mRNA levels we observed in the present study in reproductively experienced rats after exogenous prolactin treatment are indicative of increased prolactin receptor signaling (28). Although estradiol has been recently implicated as an inducer of SOCS proteins (40), this does not appear to have been a factor behind the marked SOCS mRNA responses to prolactin in the current experiment, especially because estradiol levels were in fact reduced in reproductively experienced animals. The cells that respond to prolactin with an induction of SOCS mRNA after maternal experience have not been specifically identified. Likely phenotypic candidates, based on results of pharmacological experiments that have shown facilitation or inhibition of maternal learning and memory in response to agonist or antagonist treatments, respectively, include dopaminergic (41), opioidergic (42), and noradrenergic (43) cell types.

Interestingly, given the increased prolactin receptor mRNA levels and SOCS response to prolactin treatment in the mPOA and ARC of primiparous females, there was no effect of reproductive experience on basal SOCS mRNA levels. SOCS mRNA up-regulation may not be fully reflective of the neural responsiveness to prolactin when levels are low, such as during diestrus. Our data imply that reproductively experienced females may be more responsive to a given stimulus, such as exposure to pups (or exogenous prolactin as used in the current experiment). Such an increased responsiveness does not appear to be associated with increased basal SOCS mRNA; indeed, an increase in basal SOCS might well reduce the responsiveness to external stimuli. The complete lack of SOCS mRNA response in virgin rats may also indicate that this endpoint does not always accurately reflect more subtle changes in prolactin receptor signaling. In contrast to this lack of responsiveness in virgin rats, we previously observed a robust SOCS mRNA response in the ARC of virgin ovariectomized rats treated with a similar dose of prolactin (50). This may indicate that the hormonal environment has a large bearing on SOCS mRNA responses to prolactin treatment. For example, the very low endogenous circulating concentrations of prolactin and/or progesterone in ovariectomized rats might render them more responsive to treatment with prolactin than diestrous rats. We have shown that diestrous rats can respond to prolactin with an increase in STAT5 phosphorylation in the ARC (50), but the present data show that this does not lead to significantly induced SOCS mRNA. Hence, whereas SOCS can be used as a marker of prolactin action, they are not activated under all conditions of prolactin signaling. Nevertheless, because both the virgin and primiparous groups were killed in diestrous, there were clearly differences in the level of signaling between the two groups.

Both circulating prolactin and estradiol levels were reduced in the reproductively experienced females in the present report. These findings are in agreement with our recent study in reproductively experienced rats sampled on proestrus during a subsequent estrous cycle (44). Whereas a primary cause of the reduced prolactin levels likely is associated with a shift in TIDA activity as noted earlier, a slight reduction in circulating estradiol might also be expected to contribute to lowered prolactin secretion because estradiol potently stimulates prolactin release from the lactotrophs (45). At this time the events responsible for the reduction in serum estradiol in reproductively experienced rats are unclear.

The precise contributions that pregnancy, parturition, and lactation play in the subsequent alteration of the neural prolactin receptor system are unknown. Our earlier studies in rats as to the effects of pregnancy and lactation on plasma prolactin levels in later adulthood indicate that 21 d of lactation is required to produce reductions in circulating prolactin (18). Likewise, when nonlactating ovariectomized parous rats were challenged with haloperidol, an attenuated prolactin response only occurred in females that had experienced pregnancy plus lactation; females that had given birth, but not raised young, failed to show diminished prolactin release after treatment with this dopamine antagonist (46). This dampened prolactin response to a dopamine antagonist has also been reported in parous women. Mothers who nursed their babies for a minimum of 3–4 months released less prolactin after an oral challenge with metoclopramide than did nulliparous women (47) Therefore, based on these studies, it appears that events associated with lactation after pregnancy, perhaps hyperprolactinemia, are crucial to inducing these endocrine and neuroendocrine changes. At this point it is unknown whether similar reproductive experiences are required for producing the alterations in neural prolactin receptor responsiveness found in the present study. This is a subject of further study.

Central prolactin actions appear to be vital for regulating the physiological and behavioral adaptations in the adult mammal that enable the mother to care for her offspring (48). The present study raises the question as to how an up-regulation of the mPOA prolactin receptor system might affect maternal behavior? It is well established in the female rat that the mPOA is an integral site for the regulation of ongoing maternal behavior (8) and a site of prolactin stimulation of maternal responsiveness (14). Other behavioral studies have shown that once established, maternal care or memory persists for at least 80 d (4). It is thought that the regulation of maternal behavior in the female rat undergoes a transition from a hormonal to nonhormonal dependency after parturition (1). Although unproven, it is possible that the central up-regulation of the neural prolactin receptor system that occurs as a function of reproductive experience helps account for this shift from an endocrine to a neural-hormone reliance. Specifically, it would be of interest to determine whether neural derived prolactin is produced within the mPOA, as appears the case within the PVN (49), and is up-regulated as a consequence of reproductive experience. An increase in sensitivity of the central maternal care mechanisms to low concentrations of prolactin may be a part of the transition of this system from predominantly endocrine to predominantly neural regulation that occurs once a female gains reproductive experience.

Lastly, it should be noted that prolactin exerts important influences on a variety of hypothalamic functions in addition to maternal behavior, such as modulation of reproductive function and stress responses (for review see Ref. 48). Beyond these central actions, the documented effects of prolactin on nonneural endocrine target tissues are numerous (22). The possibility that these systems might also exhibit enhanced prolactin responsiveness in parous females deserves investigation.


   Acknowledgments
 
Prolactin RIA reagents were provided by Dr. A. F. Parlow (National Hormone and Pituitary Program, National Institute of Diabetes and Digestive and Kidney Diseases, Torrance, CA).


   Footnotes
 
This work was supported by New Zealand Marsden Fund Grant UOO009 (to D.R.G.), a University of Otago William Evans Fellowship, and National Institutes of Health Grant HD39895 (to R.S.B.).

Disclosure statement: the authors have nothing to disclose.

First Published Online July 6, 2006

Abbreviations: ARC, Arcuate nucleus; CV, coefficient of variation; mPOA, medial preoptic area; PVN, paraventricular nucleus; SOCS, suppressors of cytokine signaling; STAT, signal transducer and activator of transcription; TIDA, tuberoinfundibular dopamine.

Received May 5, 2006.

Accepted for publication June 27, 2006.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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G. M. Anderson, D. C. Kieser, F. J. Steyn, and D. R. Grattan
Hypothalamic Prolactin Receptor Messenger Ribonucleic Acid Levels, Prolactin Signaling, and Hyperprolactinemic Inhibition of Pulsatile Luteinizing Hormone Secretion Are Dependent on Estradiol
Endocrinology, April 1, 2008; 149(4): 1562 - 1570.
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