Mechanisms of suppression of ovarian follicular development during lactation in the rat

Effects of pregnancy experience on ovarian senescence and longevity in Hatano rats bred for high- and low-avoidance learning

We investigated the effects of pregnancy experience on ovarian senescence and longevity using two inbred strains of Hatano rats. These strains have been selectively bred for high- and low-avoidance animals (HAA and LAA, respectively), but the HAA line has a slower onset of ovarian senescence and a shorter lifespan compared with the LAA line. The onset of abnormal estrous cycles and survival curves were compared between nulliparous and parous rats in each line. In the HAA line, pregnancy experience did not change the onset of ovarian senescence but increased longevity. This suggests that a pituitary tumor, which is a causal factor for accelerated mortality in this line, developed slowly in parous rats. In the LAA line, pregnancy experience delayed the onset of ovarian senescence and reduced the incidence of mammary tumors but did not increase longevity because of an increased frequency of constipation with megacolon. These data suggest that the effects of pregnancy experience on ovarian senescence and longevity depend on the reproductive characteristics of the rat strains.

Breastfed at Tiffany's

The importance of breast milk for the growing infant is undisputed; breastfeeding decreases infantile mortality by tenfold and decreases the incidence of infectious diseases. Despite its recognized benefits, the structural richness of breast milk has also impeded the characterization of the multiple effects of milk components on infant physiology. However, the important roles of some components of breast milk are beginning to be dissected. For instance, molecules such as immunoglobulin A (IgA) and milk oligosaccharides protect from gastrointestinal infections and influence the development of the gut microbiota. Deciphering the complex composition of breast milk brings to light multifaceted contributions that combine to make breast milk the ultimate personalized medicine.

Effect of prolactin acting on the coeliac ganglion via the superior ovarian nerve on ovarian function in the postpartum lactating and non-lactating rat

Whether prolactin (PRL) has a luteotrophic or luteolytic effect in the rat ovary depends on the nature of the corpora lutea present in the ovaries and the hormonal environment to which they are exposed. The aim was to investigate the effect of PRL acting on the coeliac ganglion (CG) on the function of the corpora lutea on day 4 postpartum under either lactating or non-lactating conditions, using the CG-superior ovarian nerve-ovary system. The ovarian release of progesterone (P), estradiol, PGF2α, and nitrites was assessed in the ovarian compartment at different incubation times. Luteal mRNA expression of 3β-HSD, 20α-HSD, aromatase, PGF2α receptor, iNOS, Bcl-2, Bax, Fas and FasL was analysed in the corpus luteum of pregnancy at the end of the experiments. Comparative analysis of control groups showed that the ovarian release of P, nitrites, and PGF2α, the expression of PGF2α receptor, and the Bcl-2/Bax ratio were lower in non-lactating rats, with increased release of estradiol, and higher expression of aromatase, Fas and FasL, demonstrating the higher luteal functionality in ovaries of lactating animals. PRL added to the CG compartment increased the ovarian release of P, estradiol, nitrites and PGF2α, and decreased the Bcl-2/Bax ratio in non-lactating rats; yet, with the exception of a reduction in the release of nitrites, such parameters were not modified in lactating animals. Together, these data suggest that the CG is able to respond to the effect of PRL and, via a neural pathway, fine-tune the physiology of the ovary under different hormonal conditions.

Galectin-1 and galectin-3 in the corpus luteum of mice are differentially regulated by prolactin and prostaglandin F2 α

Galectin-1 and galectin-3, β-galactoside-binding lectins, are specifically expressed in the regressing corpus luteum (CL) of mice; however, their function remains unclear. In this study, we examined the effects of prolactin (PRL) and prostaglandin F2 α (PGF2 α), two main regulatory molecules of mouse CL function, on galectin expression. In situ hybridization analysis clearly demonstrated an initial increase in galectin-1 in the newly formed CL (CLN) after postpartum ovulation 48 h after compulsory weaning. This was accompanied by a decline in 3β-hydroxysteroid dehydrogenase (3β-HSD) and LH receptor (LH-R) expression, suggesting a withdrawal of PRL stimulation. At 72 h after the weaning, the expression of both galectins in CLN was remarkably increased, being associated with an intense expression of progesterone degradation enzyme (20α-HSD). Compulsory weaning did not significantly alter both galectin expression in the remaining CL of pregnancy (CLP), while PGF2 α strongly upregulated both galectin expression only in the remaining CLP, which lacked LH-R in postpartum mice. Administration of bromocriptine, an antagonist for PRL secretion, to nonpregnant cyclic mice induced an accumulation of galectin-1 – but not galectin-3 – in all CL of various generations, and additional PRL treatment reduced its accumulation, suggesting a direct suppressive effect of PRL on galectin-1 expression. Although the function and regulatory mechanism of galectin in the CL is not fully understood, PGF2 α is an excellent candidate that regulates galectin expression, but its effect may be abolished by LH-R-mediated signal. PRL withdrawal seems to be necessary for an initiation of luteolysis and the following PGF2 α-induced galectin expression.

Effects of pantothenic acid supplement on secretion of steroids by the adrenal cortex in female rats

Purpose

The effect of pantothenic acid (PaA) supplementation on adrenal secretion of corticosterone and progesterone in female rats was investigated.

Methods

An in-vitro primary adrenal cell culture system was used. Pregnant rats were given 0.03% PaA in their drinking water throughout pregnancy and the period of lactation. In the first experiment, after weaning, female rats continued to receive 0.03% PaA treatment until 10 weeks of age. The animals were then decapitated and adrenal cells were cultured in the absence or presence of rat adrenocorticotropic hormone (ACTH) for 4 h. In the second experiment, adrenal cells from lactating rats on day 5 of lactation were cultured in the absence or presence of rat ACTH for 4 h.

Results

The effect of ACTH at 10^-10 m on corticosterone and progesterone release was greater for PaA-treated cyclic rats than for control cyclic rats. The effect of ACTH at 10^-10 m on corticosterone release was greater for PaA-treated lactating rats than for control lactating rats. Circulating ACTH and corticosterone levels in PaA-treated and control cyclic and lactating rats were no different.

Conclusions

These results indicate that PaA supplementation induced hyperresponsiveness to ACTH stimulation in cyclic and lactating female rats. These results clearly demonstrated that PaA is an essential factor in adrenal steroidogenesis of female rats.

Glucocorticoids disrupt neuroendocrine and behavioral responses during lactation

The influence of glucocorticoids on the neuroendocrine system and behavior of lactating rats is not well known. To evaluate the effects of glucocorticoids on the neuroendocrine system and maternal and aggressive behavior, lactating female rats were treated with dexamethasone or vehicle for 2 h before experiments. Blood samples were collected 15 min after the beginning of suckling to evaluate hormonal changes. To evaluate the maternal behavior of lactating rats, eight pups were placed in their home cages on the side opposite the location of the previous nest, and the resulting behavior of the lactating rat was filmed for 30 min. Aggressive behavior was evaluated by placing a male rat (intruder) in the home cage. Dexamethasone treatment reduced oxytocin and prolactin secretion during lactation and reduced pup weight gain. Relative to control treatment, dexamethasone treatment also adversely affected a variety of maternal behaviors; it increased the latency to build a new nest, decreased the number of pups gathered to the nest, increased the latency to retrieve the first pup, and decreased the percentage of time spent in the arched-nursing position. Dexamethasone treatment, compared with control, also reduced aggressive behavior, as evidenced by an increase in the latency to the first attack, a reduction in the number of front and side attacks, and a decrease in lateral threat and biting. Taken together, our results suggest dexamethasone treatment in lactating rats disrupts prolactin and oxytocin secretion, and this is followed by an attenuation of maternal and aggressive behavior.

Follicular turnover and hormonal association in postpartum goats during early and late lactation

To clarify the effect of lactation period on ovarian follicular activity and associated hormonal levels in goats, six goats were monitored daily by ultrasonographic examination with blood sampling during early (Days 5 to 25; Day 0 was the day of kidding) and late (Days 40 to 60) lactation. While the presence of a corpus luteum of pregnancy retarded follicular growth in the ipsilateral ovary until Days 11-13 postpartum, the total follicular number (TFN) and area (TFA) increased during late lactation due to the significant increase in the number of medium- and large-sized follicles and decrease in the number of small follicles. Four goats showed a similar pattern of follicular development during the period studied characterized by the emergence of five and six waves during the early and late lactation, respectively. The largest follicle diameter of the first three waves monitored during early lactation was significantly smaller as compared with the diameter of those existing during late lactation. TFN showed a positive correlation with FSH but showed a negative correlation with immunoreactive (ir-) inhibin and estradiol during the postpartum period. TFA was positively correlated with ir-inhibin, estradiol and PRL and negatively correlated with FSH during the monitored periods. The plasma levels of ir-inhibin and progesterone were significantly higher during late lactation compared with the levels recorded during early lactation. Ir-inhibin levels showed a significant positive correlation with LH and estradiol during early and late lactation but showed a negative correlation with FSH during the whole lactation period. LH was positively correlated with estradiol and PRL during early and late lactation, respectively. These results suggest that the lactation period has a detrimental effect on ovarian activity during the early postpartum period in goats.

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.

Structural regression of the rat corpus luteum of pregnancy: relationship with functional regression, apoptotic cell death, and the suckling stimulus

The nature of the structural regression of the corpus luteum of pregnancy (PCL) was investigated in rats from day 19 of pregnancy (PRG19) through day 6 postpartum (PP6). A pre-partum decrease in tissue weight of the PCL during the last 4 days of pregnancy was apparent (nearly a 40% decrease) and seemed to parallel the fall in plasma progesterone. The PCL in normally lactating rats further experienced a gradual decrease in the postpartum period, but the reduction was significantly slowed by PP6 in non-lactating rats that were forced to wean from the parturient day (PPO). TUNEL analysis revealed that apoptotic cell death in the regressing PCL showed a 3-4 fold increase in signals after parturition (on PP3) but was fairly limited. Instead, luteal steroidogenic cells came to have cytoplasmic vacuoles by PRG21, and vacuoles increased through PP3. A decelerated decrease in PCL weight following forced weaning was potentially reversed by repeated administration of exogenous prolactin (10 IU) twice daily. This study adds evidence that structural regression of the rat PCL 1) starts to occur before the completion of functional regression, 2) involves a modest incidence of TUNEL-reactive apoptosis, and 3) is promoted by the suckling stimulus, presumably through the effects of prolactin.

Differential regulation of phospholipase A(2) activity and prostaglandin E(2) synthesis in activated and non-activated corpora lutea of lactation by the presence and absence of suckling stimulus in rats

We investigated content of prostaglandin (PG) E(2), a luteotropic eicosanoid, and phospholipase A(2) (PLA(2)) activity in two distinct states of rat corpora lutea of lactation (LCL). Rats subjected to forced weaning on day 0 postpartum (PP0) had a significant attenuation of progesterone synthesis and LCL weight on PP6 and/or PP3 compared with normally lactating rats. Repeated administration of prolactin (10 IU, twice daily) to weaned rats reversed impaired LCL function and structural development beyond the normal level of lactating animals. From PP3 to PP6, PGE(2) level in lactating LCL was increased while it was not altered in non-lactating LCL. In contrast, PLA(2) activity in the cytosol plus microsome fractions was decreased in lactating LCL and was up-regulated in non-lactating LCL as early as on PP3. This study provides the first evidence for differential regulation of PLA(2) activity and PGE(2) synthesis in activation and activation failure of postpartum CL in rats.

Dissecting natural sensory plasticity: hormones and experience in a maternal context

There is a growing consensus that the auditory system is dynamic in its representation of behaviorally relevant sounds. The auditory cortex in particular seems to be an important locus for plasticity that may reflect the memory of such sounds, or functionally improve their processing. The mechanisms that underlie these changes may be either intrinsic because they depend on the receiver's physiological state, or extrinsic because they arise from the context in which behavioral relevance is gained. Research in a mouse model of acoustic communication between offspring and adult females offers the opportunity to explore both of these contributions to auditory cortical plasticity in a natural context. Recent works have found that after the vocalizations of infant mice become behaviorally relevant to mothers, auditory cortical activity is significantly changed in a way that may improve their processing. Here we consider the hypothesis that maternal hormones (intrinsic factor) and sensory experience (extrinsic factor) contribute together to drive these changes, focusing specifically on the evidence that well-known experience-dependent mechanisms of cortical plasticity can be modulated by hormones.

Vaginal oestrus during the reproductive and non-reproductive period in European ground squirrels

Most hibernating animals are thought to be monoestrous because reproductive activity is subject to strong time constraints. In previous studies, female European ground squirrels (Spermophilus citellus) turned out to have elevated oestradiol levels during late lactation and after weaning, indicating ovarian activity during summer. Therefore, we monitored vaginal cytology, endocrine changes, and vulval development in semi-free-living female European ground squirrels throughout one active season. Vaginal oestrus, defined by the predominance of cornified cells in smear samples, was found during the mating period shortly after vernal emergence. This phase was followed by metoestrus, characterised by the appearance of leukocytes, and a subsequent anoestrous phase. During weaning or postlactation, a second vaginal oestrus was documented in all experimental females, again followed by a metoestrous and an anoestrous phase lasting until hibernation. In line with the second vaginal oestrus, plasma oestradiol concentrations peaked during postlactation. Progesterone levels were elevated from gestation to postlactation, and titres were marginally higher during vaginal oestrus in summer than in spring. Vulval swelling was more pronounced during the first than the second vaginal oestrus. The second oestrous cycle was non-reproductive, as males were sexually inactive with regressed testes during summer. We assume that the second oestrous cycle and the accompanying endocrine changes have beneficial effects on prehibernatory fattening and reproductive performance in the subsequent season. This might allow females to become oestrous immediately after emergence from hibernation in spring.

Follicular development in European ground squirrels (Spermophilus citellus) in different phases of the annual cycle

In seasonally breeding mammals, in particular hibernators, reproduction underlies severe energetic and temporal constraints to enable the allocation of sufficient body fat reserves before winter. Thus, the timing of conception in spring can be crucial in terms of reproductive success. This study investigates follicular development in European ground squirrels (Spermophilus citellus) in three phases of the annual cycle: at vernal emergence, after weaning the offspring and shortly before hibernation. The animals were kept in outdoor enclosures within the natural habitat of the species. They were captured in weekly intervals, weighed and reproductive status was determined. Unilateral ovariectomy was scheduled such that the three periods were sampled. Numbers and diameters of tertiary follicles (TF) and corpora lutea (CL) in each ovary were determined, and plasma oestradiol and progesterone levels were analysed. The highest numbers of TF, including Graafian follicles, were found in ovaries at vernal emergence. During post-lactation, the number of TF was lower and active CL appeared in the investigated ovaries. Shortly before hibernation, active CL were present, but luteolysis had started in some individuals. Both oestradiol and progesterone secretion peaked after the termination of lactation and decreased before hibernation. The results demonstrate a second oestrus cycle in European ground squirrels after weaning, including an active luteal phase. This non-reproductive oestrus cycle with its endocrine output is an intriguing phenomenon. It may positively affect both prehibernatory fattening and reproduction in the subsequent season.

Superovulation of Holstein heifers by a single subcutaneous injection of FSH dissolved in polyvinylpyrrolidone

This study was undertaken to determine whether a single injection of porcine FSH (pFSH) would induce a superovulatory response in cattle. Holstein heifers were given a single injection of pFSH (30 mg, s.c.) dissolved in saline (Group 1, n = 5); 50% polyvinylpyrrolidone (PVP; Group 2, n = 5); or 25% PVP (Group 3, n = 4). Group-4 heifers (n = 5) were given multiple intramuscular injections of pFSH every 12 h for 3 d at decreasing doses, for a total of 30 mg. All animals received a single injection of 750 microg PGF2 alpha 48 h after the initiation of pFSH treatment. Animals exhibiting estrus were artificially inseminated twice throughout estrus. Ova and embryos were recovered nonsurgically. Ovaries were examined by transrectal ultrasonography or by palpation per rectum on Day 7 or 8 of estrus. Plasma concentrations of pFSH, bovine FSH progesterone, estradiol-17 beta and inhibin were determined by specific radioimmunoassays. The number of corpora lutea (CL) and the numbers of total and transferable embryos which were detected and recovered in Groups 2 and 3 were equivalent to the numbers detected and recovered in Group 4. In Group 1, however, only 1 of 5 animals ovulated even a single oocyte. The present study demonstrated that only a single injection of pFSH dissolved in PVP was capable of inducing a superovulatory response by maintaining a high plasma FSH concentration to allow for the recovery of a sufficient number of embryos for transplantation.

Prolactin and the hyperphagia of lactation

The nutritional needs of developing young place a large energetic demand on lactating females. In this paper some of the mechanisms through which lactating rats meet these demands are described. Emphasis is placed on the mechanisms that drive increased food intake in suckled rats that are independent of milk delivery and negative energy balance. Evidence is presented to suggest that prolactin (PRL), released from the pituitary in response to suckling stimulation, acts centrally to stimulate food intake during lactation. Brain areas in which PRL may act to stimulate food intake as well as its interactions with other hormones and neuropeptides involved in energy balance are discussed.

Cloning and characterization of a novel zinc finger protein (rZFP96) in the rat corpus luteum

The corpus luteum (CL) is a temporary organ involved in the maintenance of pregnancy. In the course of its life-cycle, the CL undergoes two distinct and consecutive processes for its inevitable removal through apoptosis: functional and structural luteolysis. We isolated a gene encoding for a novel rat zinc finger protein (ZFP), named rat ZFP96 (rZFP96) from an ovarian lambda cDNA library. Sequence analysis revealed close sequence and structural similarity to mouse ZFP96 and human zinc finger protein 305 (ZNF305). Quantitative reverse transcription-polymerase chain reaction analysis revealed a positive correlation with the end of pregnancy, that is, the onset of structural luteolysis of the CL. Messenger RNA levels increased 3-fold (P < 0.01) between days 13 and 22 of pregnancy and 8-fold (P < 0.01) between day 13 of pregnancy and day 1 post-partum. In addition, we detected rZFP96 expression in mammary, placenta, heart, kidney and skeletal muscle. Sequence analysis predicted that rZFP96 has a high probability of localizing to the nuclear compartment. The presence of both a perfect consensus TGEKP linker sequence between zinc fingers 2 and 3 as well as several similar sequences between the other zinc fingers suggests physical interaction with DNA. Speculatively, rZFP96 may therefore function as a transcription factor, switching-off pro-survival genes and/or upregulating pro-apoptotic genes and thereby contributing to the demise of the CL.

Reduced anxiety in postpartum rats requires recent physical interactions with pups, but is independent of suckling and peripheral sources of hormones

Changes in emotional behavior occur across the reproductive cycle in female rodents, with reduced anxiety found during the postpartum period, but relatively little is known about factors contributing to this decreased anxiety. Using increased duration of time spent in the open arms of an elevated plus-maze as an indicator of reduced anxiety, it was found in a series of experiments that (1) anxiety is significantly reduced in Long-Evans females during the first week of lactation, but not thereafter, (2) relatively recent contact with pups before testing (within 4 h) is necessary for their reduced anxiety, (3) dams that receive only distal sensory cues from pups for the 4 h prior to testing do not show reduced anxiety, (4) the absence of nipples, and therefore a lack of suckling by pups, has no effect on dams' anxiety, (5) cesarean delivery of pups 2 days prior to expected parturition did not alter later anxiety in dams, (6) hypophysectomy during mid-pregnancy or ovariectomy within 24 h after parturition also did not prevent reduced anxiety in dams, and (7) differences in anxiety between lactating and virgin females are greatest 4-8 min after being placed in the plus-maze. Therefore, exposure to their own peripheral hormones through mid-pregnancy is sufficient to prime female rats to show reduced anxiety, but only if they later have recent physical interaction with pups. Furthermore, because suckling and the peripheral hormones released during suckling appear to be unnecessary, decreased anxiety in maternal rats may instead be regulated by the transient intracerebral release of neuropeptides or neurotransmitters while dams receive other types of tactile inputs from their infants.

Differential Regulation of Apoptosis in the Corpus Luteum of Pregnancy and Newly Formed Corpus Luteum after Parturition in Rats1

Apoptosis contributes to luteal regression in many species. In the postpartum rat, there are two different types of corpora lutea (CL) in the ovary: CL of pregnancy (CLP) and newly formed CL (NCL). To investigate the regulation of apoptosis in the two different types of CL during luteal regression, apoptosis and caspase-3 activity were examined in the CL obtained on Days 7, 15, and 21 of pregnancy and Days 0, 1, 3, 5, 7, and 9 postpartum. Furthermore, the effect of lactation on apoptosis in the CL was examined in two groups of postpartum rats: lactating rats that nurse more than 10 pups, and nonlactating rats that nurse no pups. Apoptotic cells were detected after Day 21 of pregnancy. In the CLP, remarkable increases in the number of apoptotic cells on Days 5 and 9 postpartum were observed in nonlactating rats (P < 0.01), but not in lactating rats. Changes in caspase-3 activity in the CLP were not consistent with those in number of apoptotic cells. In the NCL, an increase in apoptosis was found only on Day 5 postpartum in nonlactating rats (P < 0.01), but not in lactating rats. Changes in caspase-3 activity in the NCL were consistent with those in number of apoptotic cells. In conclusion, apoptosis is, at least in part, involved in luteal regression after parturition, and lactation appears to inhibit apoptosis. This study also suggests the presence of a caspase-3-independent mechanism for apoptosis in CLP regression in the rat.

Peripartum interneuronal coupling in the supraoptic nucleus

In the supraoptic nucleus (SON), the incidence of conducting gap junctions (gjs), as indicated by dye coupling, is low in cycling females, but dramatically elevated in nursing mothers. Functionally, this is consistent with the well-established presence of synchronous milk ejection bursts among oxytocin neurons only in the lactating rat. In situ hybridization data, however, revealed elevated gj mRNA expression on the last day of pregnancy, a time when burst firing by putative oxytocin neurons is absent. Using Lucifer Yellow dye coupling, we determined the incidence of high conductance gjs in SONs of proestrous, immediately prepartum, postpartum non-lactating, lactating day 1, and lactating day 9-10 rats. Results indicate that coupling incidence is high only at times when milk ejection bursts are known to occur, and that the elevated gj mRNA expression seen on the last day of pregnancy does not indicate conducting gjs. It is suggested that gj conductance states, but not gj expression, are modulated by plasma estradiol titers.

Changes in leptin levels during lactation: implications for lactational hyperphagia and anovulation

In these studies we investigated the time course of changes in circulating leptin levels in lactating rats and the dependence of these changes on the energetic cost of lactation and evaluated the contribution of changes in leptin levels to lactational hyperphagia and infertility. In the first experiment, plasma leptin levels were measured on Days 5, 10, 15, 20, and 25 postpartum in freefeeding lactating rats and age-matched virgin females. Retroperitoneal and parametrial fat pads weights were obtained from the same females. In the second experiment the same measures, together with plasma insulin and prolactin levels, were taken on Days 15 and 20 postpartum from galactophore-cut and sham-operated females. In Experiments 3 and 4, the effects of exogenous leptin administration, either subcutaneously (sc) or intracerebroventricularly (icv), on lactational anovulation, maternal food intake, and dam and litter weights were examined. Circulating leptin levels decreased in lactating rats. Leptin levels were highly positively correlated with fat pad weight. Eliminating the energetic costs of lactation by preventing milk delivery induced dramatic increases in plasma leptin and insulin levels and also increased adiposity. Exogenous leptin administration did not affect length of lactational anovulation but reduced food intake, maternal body weight, and litter weight gain when given centrally and maternal body weight when given systemically. Together, these data show that the energetic costs of lactation are associated with a fall in circulating leptin levels but that these do not make a major contribution to the suppression of reproduction in lactating rats; however, they may be permissive to the hyperphagia of lactation.

Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons

Unlike many neuron populations, supraoptic nucleus (SON) neurons are rich in both nitric oxide synthase (NOS) and the NO receptor-soluble guanylyl cyclase (GC), the activation of which leads to cGMP accumulation. Elevations in cGMP result in increased coupling among SON neurons. We investigated the effect of NO on dye coupling in SONs from male, proestrus virgin female, and lactating rats. In 167 slices 263 SON neurons were recorded; 210 of these neurons were injected intracellularly (one neuron per SON) with Lucifer yellow (LY). The typically minimal coupling seen in virgin females was increased nearly fourfold by the NO precursor, L-arginine, or the NO donor, sodium nitroprusside (SNP). L-Arginine-induced coupling was abolished by a NOS inhibitor. In slices from male and lactating rats who have a higher basal incidence of coupling, SNP increased coupling by approximately twofold over control (p < 0.03). SNP effects were prevented by the NO scavenger hemoglobin (20 microM) and by the selective blocker of NO-activated GC, ODQ (10 microM). These results suggest that NO released from cells within the SON can expand the coupled network of neurons and that this action occurs via cGMP-dependent processes. Because increased coupling is associated with elevated SON neuronal excitability, we also studied the effects of 8-bromo-cGMP on excitability. In both phasically and continuously firing neurons 8-bromo-cGMP (1-2 mM), but not cGMP, produced membrane depolarizations accompanied by membrane conductance increases. Conductance increases remained when depolarizations were eliminated by current-clamping the membrane potential. Thus, NO-induced cGMP increases SON neuronal coupling and excitability.

Plasma progesterone concentrations during pregnancy and lactation in Mongolian gerbils (Meriones unguiculatus)

Plasma progesterone (P4) concentrations during pregnancy and lactation in Mongolian gerbils were measured by enzymeimmunoassay (EIA). The P4 concentration was found to increase rapidly after mating and reach a peak on day 6 of pregnancy. The maximum level (166.0 to 184.8 ng/ml) was maintained to day 12 and then decreased to a plateau at a moderate level until day 21. The P4 concentration declined to its lowest level on day 24 of pregnancy, one day before parturition. The P4 concentration on the first day of lactation was similar to that on the last day of pregnancy. The concentration then increased to a significantly higher level on day 4 of lactation, remained relatively stable until day 19, and then significantly dropped on day 22. The present study in gerbils showed that the plasma P4 concentration during pregnancy was higher in the first half than in the second half of gestation and that the P4 level during lactation was fairly constant. The pattern of changes in the P4 level during pregnancy and lactation is therefore quite different from that of other rodent species, such as rats, mice and hamsters.

Function-related plasticity in hypothalamus

Physiological activation of the magnocellular hypothalamo-neurohypophysial system induces a coordinated astrocytic withdrawal from between the magnocellular somata and the parallel-projecting dendrites of the supraoptic nucleus. Neural lobe astrocytes release engulfed axons and retract from their usual positions along the basal lamina. Occurring on a minutes-to-hours time scale, these changes are accompanied by increased direct apposition of both somatic and dendritic membrane, the formation of dendritic bundles, the appearance of novel multiple synapses in both the somatic and dendritic zones, and increased neural occupation of the perivascular basal lamina. Reversal, albeit with varying time courses, is achieved by removing the activating stimuli. Additionally, activation results in interneuronal coupling increases that are capable of being modulated synaptically via second messenger-dependent mechanisms. These changes appear to play important roles in control and coordination of oxytocin and vasopressin release during such conditions as lactation and dehydration.

Food restriction, gonadotropins, and behavior in the lactating rat

This study sought to quantify effects of undernutrition on behaviors and to relate these to gonadotropin and prolactin concentrations in the lactating dam. Dams were studied in a 2 x 3 factorial design with litter size and food intake as the two factors. Behavioral data were collected from each dam and her litter on day 9, day 14, and day 19 of lactation, and maternal blood samples collected. Plasma was analyzed for luteinizing hormone, follicle stimulating hormone and prolactin. On day 15 of lactation, percent time nursing, number of pups actively nursing, total number of pups nursing and dam location acted as mediating factors of the effect of diet group on plasma luteinizing hormone concentration. No such relationships were seen for plasma follicle stimulating hormone, and only nest condition score appeared to be a mediator for plasma prolactin concentration. In conclusion, this analysis suggests that food restriction indirectly influences plasma concentration of luteinizing hormone, but not follicle stimulating hormone, by changing maternal and pup behaviors. The relationship among diet, behavior and circulating prolactin was less clear.

Incidence of neuronal coupling in supraoptic nuclei of virgin and lactating rats: estimation by neurobiotin and lucifer yellow

Dye coupling among neurons has been shown to reflect electrotonic coupling. Recent work in retina has revealed that the incidence of coupling is greater when estimated by neurobiotin (NB) transfer than by Lucifer yellow (LY). Several previous studies have shown that the incidence of LY coupling among supraoptic nucleus (SON) neurons of lactating rats is 2- to 4-fold higher than is observed in virgin females. We compared the incidence of coupling among SON neurons following simultaneous injections of LY and NB into the same cells in slices from virgin or lactating rats. As seen in previous studies, there were 4-fold more LY-coupled neurons per injection in lactating as compared to virgin rats. Under both conditions, the numbers of NB-coupled neurons per injection were 4-fold greater than was observed for LY; possible mechanisms are discussed. Individual NB-filled neurons were coupled to as many as 10 other cells distributed over a large area of the SON. These results confirm previous findings of more coupling in lactating than virgin SONs, and suggest that both the incidence and spatial extent of interneuronal coupling are greater and thus probably more important functionally than has been heretofore appreciated.

Susceptibility of lactating rat mammary glands to gamma-ray-irradiation-induced tumorigenesis

Lactating rats of the Wistar-MS strain were irradiated with 260 cGy of gamma rays 21 days after parturition (day 21). Diethylstilbestrol (DES) pellets were implanted one month after termination of nursing and were allowed to remain for one year. A significantly higher incidence (96.4%) of mammary tumors was observed in these rats irradiated during late lactation than in virgin irradiated animals (30.4%). A control group of lactating animals irradiated during late lactation but not treated with DES was also observed for one year; the final incidence of mammary tumors in this group was 35.3%. The latency period was shortest in the DES-treated group irradiated during late lactation. Histological examination showed that the mammary glands of lactating rats were highly developed, with alveoli filled with milk. Five days after weaning, there was degeneration of alveolar tissue, concomitant with a marked decrease in the concentration of estrogen and prolactin receptors. A considerable amount of epithelial tissue remained in the mammary glands during the process of atrophy. When the rats were irradiated 5 days after weaning, and then were treated with DES for one year, the incidence of mammary tumors was 73.3%, significantly higher than that in virgin irradiated rats. However, this incidence was not significantly different from that in animals irradiated during late lactation. These results suggested that the induction of mammary tumors by gamma irradiation before or after weaning was more dependent upon the stage of differentiation in mammary glands than upon the proliferative activity of epithelial cells, and that DES is essential as a promoter for radiation-induced mammary tumorigenesis.

Effects of ovariectomy and estrogen replacement on dye coupling among rat supraoptic nucleus neurons

Among magnocellular neurosecretory neurons (MNCs), the frequency of dye coupling, and thus also of electrotonic coupling, is reduced in male rats following castration. Testosterone replacement prevented this reduction suggesting a modulatory role for gonadal steroids. To determine whether gonadal steroids in females influenced coupling incidence, Lucifer yellow CH injections were made in MNCs in slices taken from ovariectomized rats, either untreated or implanted with capsules containing estradiol-17 beta or estradiol-17 alpha, or from sham operated rats. In groups without biologically active estradiol, incidence of dye coupling was increased by 138-169% over those with normal plasma levels, as measured by radioimmunoassay. We conclude that estradiol and testosterone have opposite effects on coupling frequency among MNCs and that the facilitatory effects of testosterone in males are unlikely to be via its aromatization to estrogen.

An ultrastructural morphometric analysis of the adenohypophysis of lactating rats

A morphometric analysis of the adenohypophysis (pars distalis) of lactating rats was carried out by a semi-automated method at the ultrastructural level. The cellular elements were identified by their ultrastructural morphology. The following values were considered for the morphometric study: numerical density of cells/mm3 of tissue and the percentage of parenchymal volume occupied by every cell type. Mammotropes (PRL cells) numbered 624 X 10(3)/mm3 and occupied 59.9% of the parenchymal volume (p.v.). Somatotropes (GH cells) numbered 206 X 10(3)/mm3 and occupied 15.0% of the p.v. Folliculo-stellate cells (FS cells) numbered 128 X 10(3)/mm3 and occupied 8.1% of the p.v. Gonadotropes (GN cells) numbered 47 X 10(3)/mm3 and occupied 6.0% of the p.v. Adrenocorticotropes (ACTH cells) numbered 45 X 10(3)/mm3 and occupied 3.8% of the p.v. Thyrotropes (TSH cells) numbered 36 X 10(3)/mm3 and occupied 3.4% of the p.v. PRL cells were characterized by aspects compatible with intense hormone production. GH cells did not show differences with those of nonlactating animals. Folliculo-stellate elements appeared hypertrophic with abundant cytoplasm, enlarged Golgi complex, and dilation of the follicular lumina. GN cells had abundant cytoplasm with a well-developed and dilated ergastoplasm, particularly in type II GN cells. ACTH cells did not show differences with those of nonlactating animals. TSH cells showed moderate nucleocytoplasmic activation. These fine structural morphometric findings are discussed in relation to other studies regarding nonlactating adenohypophysis and hormone changes during lactation.