The acute suckling stimulus induces expression of neuropeptide Y (NPY) in cells in the dorsomedial hypothalamus and increases NPY expression in the arcuate nucleus

Leptin regulates appetite-related neuropeptides in the hypothalamus of developing rats without affecting food intake

Leptin regulates food intake in adult mammals by stimulating hypothalamic anorexigenic pathways and inhibiting orexigenic ones. In developing rodents, fat stores are low, yet circulating leptin levels are high and do not appear to regulate food intake. We determined whether two appetite-related neuropeptides [neuropeptide Y (NPY) and proopiomelanocortin (POMC)] and food intake behavior are sensitive to leptin [3 mg/kg body weight (BW), ip] in neonates. We measured the effects of 1) acute leptin administration (3 mg/kg BW, ip, 3 h before testing) on food intake on postnatal day (PND) 5, 8, and 10; and 2) chronic leptin treatment (3 mg/kg BW, ip, daily PND3-PND10) on BW gain and fat pads weight on PND10. In addition to hypothalamic POMC and NPY expression, we determined the expression of suppressor of cytokine signaling-3, all subtypes of leptin receptors, and corticotropin-releasing factor receptor-2 mRNA in PND10 pups receiving either an acute (PND10) or a chronic (PND 3-10) leptin (3 mg/kg BW, ip) or vehicle treatment. Brains were removed 30 or 120 min after the last injection. Acute leptin administration did not affect food intake at any age tested. Chronic leptin treatment did not change BW but decreased fat pad weight significantly. In the arcuate nucleus (ARC), acute leptin increased SOCS-3 and POMC mRNA levels, but decreased NPY mRNA levels in the rostral part of ARC. Chronic leptin down-regulated all subtypes of leptin receptors mRNA and decreased NPY mRNA levels in the caudal ARC but had no further effect on POMC expression. Chronic leptin increased corticotropin-releasing factor receptor-2 mRNA levels in the ventromedial hypothalamus. We conclude that despite adult-like effects of leptin on POMC, NPY, and CRFR-2 expression in neonates, leptin does not regulate food intake during early development.

Plasma leptin-binding activity and hypothalamic leptin receptor expression during pregnancy and lactation in the rat

Leptin, the 16-kDa peptide hormone product of the ob gene, regulates body weight via the hypothalamus but also influences several aspects of reproductive function. Results of previous studies have suggested that pregnancy is a state of leptin resistance, because food consumption remains stable or increases despite a progressive rise in plasma leptin across most of gestation. In the present study, we assessed whether this apparent leptin resistance during rat pregnancy was due to either increased plasma leptin-binding activity and/or reduced expression of hypothalamic leptin receptor. Plasma leptin increased from 2.2 +/- 0.4 ng/ml before pregnancy to a maximum at midgestation (4.2 +/- 0.8 ng/ml on Day 12) and then fell by Day 22 and remained low throughout lactation. Despite the higher plasma leptin levels in pregnancy, food consumption increased from a minimum of 13.6 +/- 0.5 g/day before pregnancy to a peak of 21.9 +/- 0.6 g/day on Day 19, then fell before parturition (11.9 +/- 0.4 g/day on Day 22). At least part of the increase in plasma leptin during pregnancy was attributable to a marked increase (P < 0.001) in plasma leptin-binding activity between diestrus and late pregnancy, which then fell after birth but remained at midpregnancy levels to at least Day 12 of lactation. Hypothalamic expression of mRNA encoding the long form of the leptin receptor (Ob-Rb) was elevated in early pregnancy (Day 7) but returned to prepregnancy levels by midgestation and remained stable thereafter. The results of this study confirm that pregnancy in the rat is a state of relative leptin resistance, which is due primarily to increased plasma leptin-binding activity rather than to changes in hypothalamic Ob-Rb expression.

Leptin secretion and hypothalamic neuropeptide and receptor gene expression in sheep

Peripheral and hypothalamic mechanisms underlying the hyperphagia of lactation have been investigated in sheep. Sheep were fed ad libitum and killed at 6 and 18 days of lactation; ad libitum-fed nonlactating sheep were killed as controls. Despite increased food intake, lactating ewes were in negative energy balance. Lactation decreased plasma leptin and adipose tissue leptin mRNA concentrations. OB-Rb gene expression, determined by in situ hybridization, was increased in the hypothalamic arcuate nucleus (ARC) and ventromedial hypothalamic nucleus (VMH) at both stages of lactation. Neuropeptide Y (NPY) was increased by lactation in both the ARC and dorsomedial hypothalamus (DMH), although increased gene expression in the DMH was only apparent at day 18 of lactation. Gene expression was decreased for cocaine- and amphetamine-regulated transcript (CART) in the ARC and VMH and for proopiomelanocortin in ARC during lactation. Agouti-related peptide gene expression was increased in the ARC, and melanocortin receptor expression was unchanged in both the ARC and VMH with lactation. Thus the hypoleptinemia of lactation may activate NPY orexigenic pathways and attenuate anorexigenic melanocortin and CART pathways in the hypothalamus to promote the hyperphagia of lactation.

Urocortin III-immunoreactive projections in rat brain: partial overlap with sites of type 2 corticotrophin-releasing factor receptor expression

Urocortin (Ucn) III, or stresscopin, is a new member of the corticotropin-releasing factor (CRF) peptide family identified in mouse and human. Pharmacological studies showed that Ucn III is a high-affinity ligand for the type 2 CRF receptor (CRF-R2). To further understand physiological functions the peptide may serve in the brain, the distribution of Ucn III neurons and fibers was examined by in situ hybridization and immunohistochemistry in the rat brain. Ucn III-positive neurons were found predominately within the hypothalamus and medial amygdala. In the hypothalamus, Ucn III neurons were observed in the median preoptic nucleus and in the rostral perifornical area lateral to the paraventricular nucleus. The Ucn III fibers were distributed mainly in the hypothalamus and limbic structures. Hypothalamic regions that were innervated prominently by Ucn III fibers included the ventromedial nucleus, medial preoptic nucleus, and ventral premammillary nucleus. Outside the hypothalamus, the densest projections were found in the intermediate part of the lateral septum, posterior division of the bed nucleus stria terminalis, and the medial nucleus of the amygdala. Several major Ucn III terminal fields identified in the present study, including the lateral septum and the ventromedial hypothalamus, are known to express high levels of CRF-R2. Thus, these anatomical data strongly support the notion that Ucn III is an endogenous ligand for CRF-R2 in these areas. These results also suggest that Ucn III is positioned to play a role in mediating physiological functions, including food intake and neuroendocrine regulation.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

Novel expression of neuropeptide Y (NPY) mRNA in hypothalamic regions during development: region-specific effects of maternal deprivation on NPY and Agouti-related protein mRNA

During development there is novel expression of NPY mRNA in the dorsomedial hypothalamic nucleus (DMH) and perifornical region (PFR), in addition to the arcuate nucleus (ARH). Furthermore, NPY mRNA levels peak in all regions on postnatal d 16 (P16) and decrease to adult levels by P30. The purpose of the present study was to determine whether NPY and agouti-related protein (AGRP) mRNA expression in the different hypothalamic regions on P11 and P16 are similarly affected by fasting. An examination of the full rostral to caudal extent of the hypothalamus revealed two additional regions displaying novel NPY mRNA expression, the parvocellular division of the paraventricular nucleus (PVH) and lateral hypothalamus (LH). Maternal deprivation for 36 h, used to bring about a fast, similarly increased (23-29%) NPY and AGRP mRNA expression in the ARH on P11 and P16. In contrast, NPY expression in the DMH and PFR were significantly decreased (19-30% and 48-53%, respectively), whereas NPY mRNA levels in the PVH and LH were not altered by this treatment. The increase in NPY and AGRP mRNA expression in the ARH in response to maternal deprivation suggests that these neuronal populations respond to signals of energy balance. In contrast, NPY expression in the DMH, PFR, PVH, and LH is differentially regulated by maternal deprivation or other factors associated with maternal separation.

Neurobiological bases underlying the control of the onset of puberty in the rhesus monkey: a representative higher primate

The purpose of this article is to discuss our understanding of the neurobiological mechanisms that govern the timing of the onset of puberty in the rhesus monkey, a representative higher primate, and, whenever possible, to place findings obtained from studies of this macaque in perspective with those for the human situation. Specifically, the dynamics in the postnatal ontogeny of hypothalamic GnRH gene expression and release are described, and the roles of neuropeptide Y and gamma-aminobutyric acid in imposing the restraint on pulsatile GnRH release during juvenile development are examined. Finally, the hypothesis that circulating leptin provides the signal that times the reaugmentation of pulsatile GnRH release at the termination of juvenile development, and therefore triggers the onset of primate puberty, is discussed.

Differential regulation of leptin receptor but not orexin in the hypothalamus of the lactating rat

During lactation, hypothalamic levels of neuropeptide Y (NPY) and agouti related protein (AGRP) mRNA are increased, while pro-opiomelanocortin (POMC) mRNA is decreased. Serum leptin levels are also decreased during lactation. These changes may underlie the large increases of both food and water intake that occur in concert with milk production. However, additional hypothalamic substances, such as the novel peptide, orexin, may be involved. In addition, in the presence of chronically suppressed levels of serum leptin, there may be a change in leptin receptor expression in the hypothalamus. The objectives of the present study were to determine if orexin and leptin receptor mRNA levels were changed during lactation. Rats were studied on dioestrus of the oestrous cycle or on day 10 postpartum (the lactating animals were suckling eight pups). Orexin mRNA levels in the lateral hypothalamus did not differ between dioestrus and lactation. There was a significant increase in leptin receptor mRNA levels in the supraoptic nucleus during lactation compared to dioestrus. Furthermore, leptin receptor protein, as determined by immunocytochemistry, was colocalized in virtually all vasopressin and oxytocin cells in the supraoptic nucleus. Lactating animals exhibited a decrease in leptin receptor mRNA in the ventromedial hypothalamic nucleus whereas no change was apparent in other hypothalamic areas compared to the dioestrus animals. These results demonstrate that changes in orexin do not appear to contribute to the increase in food intake during lactation. It is likely that the increases in NPY and ARGP, coupled with the decrease in POMC, are primarily responsible for sustaining the chronic hyperphagia of lactation. The changes observed in leptin receptor expression in the hypothalamus, along with the suppression of serum leptin levels, also suggest that the leptin signalling system may play a significant role in the regulation of food and water intake during lactation.

The proglucagon-derived peptide, glucagon-like peptide-2, is a neurotransmitter involved in the regulation of food intake

The dorsomedial hypothalamic nucleus harbors leptin sensitive neurons and is intrinsically connected to hypothalamic nuclei involved in feeding behavior. However, it also receives ascending input from the visceroceptive neurons of the brainstem. We have identified a unique glucagon-like-peptide-2 containing neuronal pathway connecting the nucleus of the solitary tract with the dorsomedial hypothalamic nucleus. A glucagon-like-peptide-2 fiber plexus targets neurons expressing its receptor within the dorsomedial hypothalamic nucleus. Pharmacological and behavioral studies confirmed that glucagon-like-peptide-2 signaling is a specific transmitter inhibiting rodent feeding behavior and with potential long-term effects on body weight homeostasis. The glucagon-like-peptide-1 receptor antagonist, Exendin (9-39) is also a functional antagonist of centrally applied glucagon-like-peptide-2.

Corticotropin releasing hormone neurons in the paraventricular nucleus are direct targets for neuropeptide Y neurons in the arcuate nucleus: an anterograde tracing study

In the present study, anterograde tracing combined with triple label immunofluorescent staining was conducted to examine the possible anatomical interactions between Neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus (ARH) and the corticotropin releasing hormone (CRH) system in the paraventricular nucleus of the hypothalamus (PVH). The anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was iontophresed into the ARH of female rats and triple label immunofluorescence staining with three different fluorophores was performed to visualize PHA-L, NPY and CRH, with the aid of confocal microscopy. In PVH, NPY and PHA-L double-labeled fibers were found mainly in the parvocellular part of the PVH (PVHp). Confocal analysis demonstrated that NPY/PHA-L double-labeled fibers came in close apposition to CRH perikarya. In the median eminence, NPY/PHA-L double-labeled fibers were found both in the inner and the outer zones of the median eminence. However, very few double-labeled fibers were found in the proximity of CRH neuronal fibers in the median eminence. Double label staining was also performed to determine if NPY Y1 receptors were expressed in CRH neurons. Two different fluorophores were used to visualize CRH neurons and Y1 receptor. No convincing Y1-positive staining was found in CRH cell bodies in the PVH, even though Y1-positive staining in numerous fibers and cell bodies was observed throughout the region. However, Y1-positive fibers were shown to make close contact with CRH cell bodies in the PVH. In the ME, the majority of the Y1-positive fibers were located in the lateral portion of the ME, whereas the CRH fibers were found mainly in the medial portion of the external zone of the ME. The results of the present study suggest that ARH NPY neurons provide direct input into CRH cell bodies in the PVH region. However, the direct effects of NPY must be mediated by some receptor subtype other than Y1. Y1 receptor involvement in NPY modulation of CRH neuronal function in the PVH appears to be indirect through modulation of neuronal afferents making contact with CRH neurons.

Morphological evidence for direct interaction between arcuate nucleus neuropeptide Y (NPY) neurons and gonadotropin-releasing hormone neurons and the possible involvement of NPY Y1 receptors

Neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus (ARH) have been shown to play an important role in modulating LH secretion. One mechanism by which the ARH NPY system may regulate LH secretion is by modulating GnRH neuronal function. Thus, the present study examined whether the ARH NPY system provided direct input to GnRH cell bodies in the preoptic area (POA), as well as to their nerve terminals in the median eminence (ME). The possible involvement of the NPY Y1 receptor subtype in mediating the effects of NPY was also investigated. Lactating rats were used in these studies because they have increased hypothalamic NPY content, especially in the ARH/ME areas, making it easier to detect NPY fibers and terminals. The anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was iontophoresed into the ARH of lactating rats; and triple-label immunofluorescence was performed, with the aid of confocal microscopy, to visualize NPY, PHA-L, and GnRH. GnRH cell bodies were found scattered throughout the organum vasculosum laminae terminalis (OVLT)/POA region, and NPY/ PHA-L double-labeled fibers were found in very close proximity to numerous GnRH perikarya. In the ME, double-labeled NPY/PHA-L fibers were found in the inner and external zones, and they were found in close proximity to GnRH neuronal fibers. Using a NPY Y1 specific antibody, double-label immunofluorescence was performed to examine whether the Y1 receptor subtype was expressed in GnRH neurons. No convincing Y1-positive staining was found in GnRH cell bodies in the OVLT/POA region. However, abundant Y1-positive fiber and cell staining were observed throughout the region, and Y1-positive fibers were found in close apposition to GnRH cell bodies. In contrast, numerous GnRH nerve fibers and terminals in both the OVLT and ME were colocalized with Y1-positive staining. The results of this study suggest that ARH NPY neurons come in close contact with GnRH neurons and may provide direct input to both GnRH cell bodies in the POA region and to their nerve terminals in the ME. The Y1 receptor subtype may be directly involved in NPY modulation of GnRH secretion from its nerve terminals.

Altered expression of agouti-related protein and its colocalization with neuropeptide Y in the arcuate nucleus of the hypothalamus during lactation

During lactation, the levels of neuropeptide Y (NPY), which plays an important role in mediating food intake, are significantly elevated in a number of hypothalamic areas, including the arcuate nucleus (ARH). To identify additional hypothalamic systems that might be important in mediating the increase in food intake and alterations in energy homeostasis during lactation, the present studies examined the expression of agouti-related protein (AGRP), a recently described homologue of the skin agouti protein. AGRP is found in the hypothalamus and has been suggested to play an important role in the regulation of food intake. In the first experiment, animals were studied during diestrus of the estrous cycle, a stage of the cycle when estrogen levels are basal and similar to lactation, or during days 12-13 postpartum. Lactating animals had their litters adjusted to eight pups on day 2 postpartum. Brain tissue sections were used to measure AGRP messenger RNA (mRNA) levels by in situ hybridization. AGRP mRNA signal was found mostly in the ventromedial portion of the ARH, which has been shown to contain a high density of NPY neurons. A significant increase in AGRP mRNA content was observed in the mid- to caudal portion of the ARH of lactating animals compared with diestrous females. No difference was found in the rostral portion of the ARH. In the second experiment, double-label in situ hybridization for AGRP and NPY was performed in lactating animals to determine the extent of colocalization of the two peptides in the ARH, using 35S-labeled and digoxigenin-labeled antisense complementary RNA probes. It was found that almost all of the NPY-positive neurons throughout the ARH also expressed AGRP mRNA signal. Furthermore, AGRP expression was confined almost exclusively to NPY-positive neurons. Thus, the present study showed that during lactation, AGRP gene expression was significantly elevated in a subset of the AGRP neurons in the ARH. The high degree of colocalization of AGRP and NPY, coupled with previous reports from our laboratory demonstrating increased NPY expression in the ARH in response to suckling, suggests that AGRP and NPY are coordinately regulated and may be involved in the increase in food intake during lactation.

Identification of neuronal input to the arcuate nucleus (ARH) activated during lactation: implications in the activation of neuropeptide Y neurons

Activation of the neuropeptide Y (NPY) neuronal system in the arcuate nucleus of the hypothalamus (ARH) during lactation in the rat is likely due to the neural impulses arising from the suckling stimulus. However, the afferent neuronal input to the ARH that is activated during lactation and is responsible for activation of NPY neurons is currently unknown. Previously, using cFos as a marker for neuronal activation, we identified several brain areas in the lactating animals that were activated by the suckling stimulus. Thus, the objective of the present study was to determine if these activated areas observed in the lactating animals project directly into the ARH. The retrograde tracer, fluorogold (FG), was injected into the ARH on day 4 postpartum. Chronically suckled rats were then deprived of their eight-pup litters on day 9 postpartum, and 48 h later, the pups were returned to the females to reinitiate the suckling stimulus for 90 min to induce cFos expression. The animals were then perfused and the brains were subjected to double-label immunohistochemistry to visualize both FG- and cFos-positive cells. Substantial FG/cFos double-labeled cells were found in forebrain regions, including the medial preoptic area, periventricular preoptic area, bed nucleus of the stria terminalis, and the medial amygdala, and in brainstem regions including the lateral parabrachial nucleus, peripeduncular area and ventrolateral medulla. The results of the present study demonstrate that specific areas in the brain are activated during lactation and send direct projections to the ARH. Thus, these areas are potentially important candidates for mediating the activation of the NPY neuronal system in the ARH during lactation.

Prenatal nicotine increases pulmonary alpha7 nicotinic receptor expression and alters fetal lung development in monkeys

It is well established that maternal smoking during pregnancy is a leading preventable cause of low birth weight and prematurity. Less appreciated is that maternal smoking during pregnancy is also associated with alterations in pulmonary function at birth and greater incidence of respiratory illnesses after birth. To determine if this is the direct result of nicotine interacting with nicotinic cholinergic receptors (nAChRs) during lung development, rhesus monkeys were treated with 1 mg/kg/day of nicotine from days 26 to 134 of pregnancy. Nicotine administration caused lung hypoplasia and reduced surface complexity of developing alveoli. Immunohistochemistry and in situ alpha-bungarotoxin (alphaBGT) binding showed that alpha7 nAChRs are present in the developing lung in airway epithelial cells, cells surrounding large airways and blood vessels, alveolar type II cells, free alveolar macrophages, and pulmonary neuroendocrine cells (PNEC). As detected both by immunohistochemistry and by alphaBGT binding, nicotine administration markedly increased alpha7 receptor subunit expression and binding in the fetal lung. Correlating with areas of increased alpha7 expression, collagen expression surrounding large airways and vessels was significantly increased. Nicotine also significantly increased numbers of type II cells and neuroendocrine cells in neuroepithelial bodies. These findings demonstrate that nicotine can alter fetal monkey lung development by crossing the placenta to interact directly with nicotinic receptors on non-neuronal cells in the developing lung, and that similar effects likely occur in human infants whose mothers smoke during pregnancy.

Recombinant CART peptide induces c-Fos expression in central areas involved in control of feeding behaviour

We have recently shown that the hypothalamic neuropeptide CART (cocaine-amphetamine-regulated-transcript) is a leptin dependent endogenous satiety factor in the rat. In the present study we confirm and extend our previous observations by showing that intracerebroventricular (i.c.v.) administered CART(42-89) dose-dependently inhibits 3-h food intake in food restricted rats with a lowest effective dose of 0.5 microgram. CART also potently inhibits NPY-induced food intake in satiated rats as well as nighttime food intake in free feeding animals. To identify brain areas potentially involved in mediating the anorectic effects of CART, the temporal expression pattern of the immediate early gene c-fos was examined in the central nervous system by immunohistochemistry in rats receiving recombinant CART. Compared to vehicle, CART induced c-Fos expression in several hypothalamic and brainstem structures implicated in the central control of food intake. In the hypothalamus, high numbers of c-Fos immunoreactive (-ir) cells were observed in the medial parvocellular part of the paraventricular nucleus and in the posterior part of the dorsomedial nucleus. Lower numbers of c-Fos positive nuclei were found in the supraoptic and arcuate nuclei. A relatively high number of c-Fos-ir cells was found in the central nucleus of the amygdala. In the brainstem, c-Fos-positive nuclei were found in the parabrachial nucleus, and in the nucleus of the solitary tract. Notably both the area postrema and the dorsal motor nucleus of the vagus were virtually devoid of c-Fos-ir cells. The present experiments suggest that CART peptide exerts its inhibitory effects on appetite by activating hypothalamic and brainstem neurones implicated in the central control of feeding behaviour and metabolism.

Neuropeptide Y and tuberoinfundibular dopamine activities are altered during lactation: role of prolactin

During lactation the suckling stimulus increases the activity of two populations of neuropeptide Y (NPY) neurons in the hypothalamus, the caudal portion of the arcuate nucleus (ARH) and the dorsomedial hypothalamus (DMH), and suppresses the activity of TIDA neurons in the ARH. In the present study, an acute resuckling model was used to examine the role of suckling-induced hyperprolactinemia in modulating the activity of these systems. Lactating rats were deprived of their eight-pup litters on day 9 postpartum, and 48 h later, the animals served either as nonsuckled controls (0 pups) or were suckled for 24 h. In addition, some of the resuckled animals received two s.c. injections of bromocriptine (0.5 mg/rat x injection), a dopamine D2 agonist, to inhibit suckling-induced PRL secretion. In situ hybridization was performed for rat NPY messenger RNA (mRNA) and tyrosine hydroxylase (TH) mRNA to provide an index for NPY and TIDA neuronal activities, respectively. Resuckling for 24 h induced a significant increase in NPY mRNA levels in the caudal portion of the ARH and in the DMH. Bromocriptine treatment did not alter the increase in NPY mRNA levels in the ARH, whereas the treatment greatly attenuated the increase in NPY mRNA in the DMH. TH mRNA levels in the rostral ARH area returned to basal levels in the nonsuckled control animals, and 24 h of resuckling significantly suppressed TH mRNA expression in this area. Bromocriptine treatment caused a significant increase in TH mRNA levels compared with those in the eight-pup suckled group. Thus, the results from the present study demonstrate that the suckling stimulus activated the two populations of NPY neurons and suppressed TIDA activity. Suckling-induced hyperprolactinemia did not participate in the increase in ARH NPY activity, whereas it played a major stimulatory role in suckling-induced activation of NPY neurons in the DMH and an inhibitory role in suckling-induced suppression of TIDA activity. The increase in TIDA activity after bromocriptine treatment was unexpected and suggests that the role of PRL in the regulation of TIDA activity is significantly altered during lactation.