Leptin concentrations increase during pregnancy, but this does not prevent the pregnancy-induced increase in food intake, suggesting a state of leptin resistance. This study investigated the response to intracerebroventricular leptin administration in pregnant rats. After fasting, nonpregnant, d-7 and d-14 pregnant rats received leptin (4 microg) or vehicle, then food intake was measured. Serial blood samples were collected in another group of rats to determine plasma leptin concentrations. Further groups of d-14 pregnant and nonpregnant rats were killed after leptin or vehicle treatment, and brains were collected. Hypothalamic nuclei were microdissected, and levels of signal transducer and activator of transcription (STAT)3 phosphorylation were measured using Western blot analysis. Fasting decreased leptin concentrations in both pregnant and nonpregnant rats. Leptin treatment significantly reduced food intake in nonpregnant and d-7 pregnant rats but not in d-14 pregnant rats. In addition, there was no postfasting hyperphagic response in the pregnant rats. In the pregnant rats, leptin-induced STAT3 phosphorylation was suppressed in the arcuate nucleus and, to a lesser extent, in the ventromedial hypothalamus (VMH), compared with nonpregnant rats. Unstimulated STAT3 levels were also decreased in the VMH during pregnancy. Leptin-induced phosphorylation of STAT3 in the dorsomedial and lateral hypothalamus was not different between pregnant and nonpregnant rats. These data indicate that pregnant rats become resistant to the satiety action of leptin. Furthermore, leptin-induced activation of the STAT3 is impaired during pregnancy, specifically in the arcuate nucleus and VMH. These data support the hypothesis that pregnancy is a state of hypothalamic leptin resistance.

Pregnancy hormones, such as prolactin, sensitize neural circuits controlling parental interactions to induce timely activation of maternal behaviors immediately after parturition. While the medial preoptic area (MPOA) is known to be critical for maternal behavior, the specific role of prolactin in this brain region has remained elusive. Here, we evaluated the role of prolactin action in the MPOA using complementary genetic strategies in mice. We characterized prolactin-responsive neurons within the MPOA at different hormonal stages and delineated their projections in the brain. We found that MPOA neurons expressing prolactin receptors (Prlr) form the nexus of a complex prolactin-responsive neural circuit, indicating that changing prolactin levels can act at multiple sites and thus, impinge on the overall activity of a distributed network of neurons. Conditional KO of Prlr from neuronal subpopulations expressing the neurotransmitters GABA or glutamate within this circuit markedly reduced the capacity for prolactin action both in the MPOA and throughout the network. Each of these manipulations, however, produced only subtle impacts on maternal care, suggesting that this distributed circuit is robust with respect to alterations in prolactin signaling. In contrast, acute deletion of Prlr in all MPOA neurons of adult female mice resulted in profound deficits in maternal care soon after birth. All mothers abandoned their pups, showing that prolactin action on MPOA neurons is necessary for the normal expression of postpartum maternal behavior in mice. Our data establish a critical role for prolactin-induced behavioral responses in the maternal brain, ensuring survival of mammalian offspring.maternal behavior | prolactin | prolactin receptor | medial preoptic area M aternal care is critical to survival of dependent offspring in mammals. Seminal work from Rosenblatt (1), published 50 y ago, showed that maternal behavior can be exhibited in ovariectomized, hypophysectomized rats, suggesting an underlying neural basis that was not dependent on hormonal inputs. Subsequent studies have characterized a complex neural circuitry controlling parental interactions (2, 3), with distributed sites mediating different components of the behavior (4). While the neural circuit controlling maternal behavior is not thought to be dependent on hormonal inputs, it is clear that pregnancy hormones, particularly rising levels of estradiol, oxytocin, prolactin, and placental lactogen, coupled with an abrupt decrease in progesterone can sensitize the underlying circuitry to induce timely activation of maternal behaviors immediately after parturition (5). The medial preoptic area (MPOA) forms a critical nexus (2, 3), integrating a range of hormonal and sensory inputs into the maternal circuit.Within the complex hormonal milieu of pregnancy, the specific role of prolactin in maternal behavior has remained elusive. This is largely because prolactin and the related placental lactogen have an obligate role in sustaining ovarian progesterone productio...

Appetite and food intake are increased during pregnancy, comprising an adaptive response that facilitates energy storage in preparation for the high metabolic demands of pregnancy and subsequent lactation. To maintain the increased energy intake in the face of increased adiposity and rising leptin levels, pregnant females become resistant to the central anorectic actions of leptin. In rats, pregnancy‐induced leptin resistance is characterised by elevated neuropeptide Y and reduced pro‐opiomelanocortin expression in the arcuate nucleus, reduced leptin receptor mRNA levels and suppression of leptin‐induced phosphorylated signal transducer and activator of transcription‐3 protein in the ventromedial hypothalamic nucleus, as well as a loss of anorectic responses to both leptin and α‐melantocyte‐stimulating hormone. Our recent data suggest that this leptin‐resistance may also cause central insulin resistance and an altered peripheral glucose homeostasis. The specific hormone changes during pregnancy that might mediate these effects on leptin signalling are a current focus of investigation. In pseudopregnant rats, chronic i.c.v. infusion of ovine prolactin to mimic patterns of placental lactogen secretion that occur during pregnancy completely blocked the ability of leptin to suppress food intake. These data suggest that placental lactogen secretion may mediate the hormone‐induced loss of response to leptin during pregnancy. This action of prolactin/placental lactogen appears to be mediated downstream of the primary leptin‐responsive neurones in the mediobasal hypothalamus, possibly in the paraventricular nucleus. Our studies show complex hormone‐induced adaptations in the normal hypothalamic pathways regulating body weight homeostasis during pregnancy.

Pregnancy in the rat is a state of leptin resistance associated with impaired leptin signal transduction in the hypothalamus. The aim of this study was to determine whether this leptin-resistant state is mediated by a change in the level of leptin receptors in the hypothalamus. Real-time RT-PCR was used to determine levels of mRNA for the various leptin receptor isoforms in a number of microdissected hypothalamic nuclei and the choroid plexus. To investigate the functional activation of the leptin receptor, immunohistochemistry for phosphorylated signal transducer and activator of transcription 3 (pSTAT3) was examined in the arcuate nucleus and the ventromedial nucleus of the hypothalamus (VMH) of fasted diestrous and d-14 pregnant rats after an intracerebroventricular (i.c.v.) injection of either leptin (4 mug) or vehicle. A significant reduction of Ob-Rb mRNA levels was observed in the VMH during pregnancy compared with the nonpregnant controls. Furthermore, in pregnant rats the number of cells positive for leptin-induced pSTAT3 in the VMH was greatly reduced during pregnancy compared with nonpregnant rats. There were no differences in the level of Ob-Rb mRNA or in the number of leptin-induced pSTAT3-positive cells in the arcuate nucleus of nonpregnant and pregnant rats. These data implicate the VMH as a key hypothalamic site involved in pregnancy-induced leptin resistance. There were also reduced levels of mRNA for Ob-Ra, a proposed leptin transporter molecule, in the choroid plexus on d 7 and 21 of pregnancy. Hence, diminished transport of leptin into the brain may also contribute to pregnancy-induced leptin resistance.

Tuberoinfundibular dopamine (TIDA) neurons, known as neuroendocrine regulators of prolactin secretion from the pituitary gland, also release GABA within the hypothalamic arcuate nucleus. As these neurons express prolactin receptors (Prlr), prolactin may regulate GABA secretion from TIDA neurons, potentially mediating actions of prolactin on hypothalamic function. To investigate whether GABA is involved in feedback regulation of TIDA neurons, we examined the physiological consequences of conditional deletion of Prlr in GABAergic neurons. For comparison, we also examined mice in which Prlr were deleted from most forebrain neurons. Both neuron-specific and GABA-specific recombination of the Prlr gene occurred throughout the hypothalamus and in some extrahypothalamic regions, consistent with the known distribution of Prlr expression, indicative of knock-out of Prlr. This was confirmed by a significant loss of prolactininduced phosphorylation of STAT5, a marker of prolactin action. Several populations of GABAergic neurons that were not previously known to be prolactin-sensitive, notably in the medial amygdala, were identified. Approximately 50% of dopamine neurons within the arcuate nucleus were labeled with a GABA-specific reporter, but Prlr deletion from these dopamine/GABA neurons had no effect on feedback regulation of prolactin secretion. In contrast, Prlr deletion from all dopamine neurons resulted in profound hyperprolactinemia. The absence of coexpression of tyrosine hydroxylase, a marker for dopamine production, in GABAergic nerve terminals in the median eminence suggested that rather than a functional redundancy within the TIDA population, the dopamine/GABA neurons in the arcuate nucleus represent a subpopulation with a functional role distinct from the regulation of prolactin secretion.

Pregnancy is associated with hyperphagia, increased fat mass, hyperleptinaemia and hyperprolactinaemia. The neuroendocrine control of bodyweight involves appetite-regulating centres in the hypothalamus, containing both orexigenic and anorexigenic neurons that express leptin receptors (LepR). In the rat, central leptin resistance develops during mid pregnancy, well after hyperphagia becomes apparent, to negate the appetite suppressing effects of leptin. We have investigated the hypothalamic response to leptin during pregnancy and examined the role of pregnancy hormones in inducing these changes. We have shown that there are multiple levels of leptin resistance during pregnancy. Despite elevated serum leptin, neuropeptide Y and agouti related peptide mRNA in the arcuate nucleus are not suppressed and may even be increased during pregnancy. LepR mRNA and leptin-induced pSTAT3 expression, however, are relatively normal in the arcuate nucleus. In contrast, both LepR and leptin-induced pSTAT3 are reduced in the ventromedial hypothalamic nucleus. Injecting α-melanocyte-stimulating hormone (α-MSH) into the brain, to bypass the first-order leptin-responsive neurons in the arcuate nucleus, also fails to suppress food intake during pregnancy, suggesting that pregnancy is also a melanocortin-resistant state. Using a pseudopregnant rat model, we have demonstrated that in addition to the changes in maternal ovarian steroid secretion, placental lactogen production is essential for the induction of leptin resistance in pregnancy. Thus, hormonal changes associated with pregnancy induce adaptive changes in the maternal hypothalamus, stimulating food intake and then allowing elevated food intake to be maintained in the face of elevated leptin levels, resulting in fat deposition to provide energy stores in preparation for the high metabolic demands of late pregnancy and lactation.

Hyperphagia during pregnancy, despite rising concentrations of the satiety hormone leptin, suggests that a state of leptin resistance develops. This study investigated the satiety response and hypothalamic responses to leptin during pregnancy in the mouse. Pregnant (day 13) and nonpregnant mice received an i.p. injection of either leptin or vehicle and then 24-h food intake was measured. Further groups of pregnant and nonpregnant mice were perfused 2 h after leptin or vehicle injections and brains were processed for pSTAT3 and pSTAT5 immunohistochemistry. Leptin treatment significantly decreased food intake in nonpregnant mice. In pregnant mice, however, leptin treatment did not suppress food intake, indicating a state of leptin resistance. In the arcuate nucleus, leptin treatment increased the number of cells positive for pSTAT3, a marker of leptin activity, to a similar degree in both nonpregnant and pregnant mice. In the ventromedial nucleus (VMN), the leptin-induced increase in pSTAT3-positive cell number was significantly reduced in pregnant mice compared to that in nonpregnant mice. In nonpregnant mice, leptin treatment had no effect on the number of pSTAT5-positive cells, suggesting that in this animal model, leptin does not act through STAT5. In pregnant mice, basal levels of pSTAT5 were higher than in nonpregnant mice, and leptin treatment led to a decrease in the number of pSTAT5-positive cells in the hypothalamus. Overall, these results demonstrate that during pregnancy in the mouse, a state of leptin resistance develops, and this is associated with a reduced sensitivity of the VMN to leptin.Reproduction (2012) 144 83-90

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