Increased caloric intake on a fat-rich diet: role of ovarian steroids and galanin in the medial preoptic and paraventricular nuclei and anterior pituitary of female rats

Sex Differences and Estrous Influences on Oxytocin Control of Food Intake

Central oxytocin potently reduces food intake and is being pursued as a clinical treatment for obesity. While sexually dimorphic effects have been described for the effects of oxytocin on several behavioral outcomes, the role of sex in central oxytocin modulation of feeding behavior is poorly understood. Here we investigated the effects of sex, estrous cycle stage, and female sex hormones (estrogen, progesterone) on central oxytocin-mediated reduction of food intake in rats. Results show that while intracerebroventricular (ICV) oxytocin potently reduces chow intake in both male and female rats, these effects were more pronounced in males than in females. We next examined whether estrous cycle stage affects oxytocin's food intake-reducing effects in females. Results show that ICV oxytocin administration significantly reduces food intake during all estrous cycle stages except proestrous, suggesting that female sex hormones may modulate the feeding effects of oxytocin. Indeed, additional results reveal that estrogen, but not progesterone replacement, in ovariectomized rats abolishes oxytocin-mediated reductions in chow intake. Lastly, oxytocin receptor mRNA (Oxtr) quantification (via quantitative PCR) and anatomical localization (via fluorescent in situ hybridization) in previously established sites of action for oxytocin control of food intake revealed comparable Oxtr expression between male and female rats, suggesting that observed sex and estrous differences may be based on variations in ligand availability and/or binding. Overall, these data show that estrogen reduces the effectiveness of central oxytocin to inhibit food intake, suggesting that sex hormones and estrous cycle should be considered in clinical investigations of oxytocin for obesity treatment.

Ion Channels in the Paraventricular Hypothalamic Nucleus (PVN); Emerging Diversity and Functional Roles

The paraventricular nucleus of the hypothalamus (PVN) is critical for the regulation of homeostatic function. Although also important for endocrine regulation, it has been referred to as the "autonomic master controller." The emerging consensus is that the PVN is a multifunctional nucleus, with autonomic roles including (but not limited to) coordination of cardiovascular, thermoregulatory, metabolic, circadian and stress responses. However, the cellular mechanisms underlying these multifunctional roles remain poorly understood. Neurones from the PVN project to and can alter the function of sympathetic control regions in the medulla and spinal cord. Dysfunction of sympathetic pre-autonomic neurones (typically hyperactivity) is linked to several diseases including hypertension and heart failure and targeting this region with specific pharmacological or biological agents is a promising area of medical research. However, to facilitate future medical exploitation of the PVN, more detailed models of its neuronal control are required; populated by a greater compliment of constituent ion channels. Whilst the cytoarchitecture, projections and neurotransmitters present in the PVN are reasonably well documented, there have been fewer studies on the expression and interplay of ion channels. In this review we bring together an up to date analysis of PVN ion channel studies and discuss how these channels may interact to control, in particular, the activity of the sympathetic system.

Modulation of nociception by medial pre-optic area orexin a receptors and its relation with morphine in male rats

INTRODUCTION

Recent studies have shown that medial pre-optic area (MPOA) of hypothalamus are involved in nociception. Orexin A (hypocretin 1) has been found to have numerous applications including pain modulation. However, the role of orexin A receptors in the MPOA on the nociception has not been yet studied. Therefore, the aim of the present study is to investigate the effect of orexin A microinjection on MPOA on the nociception transmission and morphine induced analgesia in adult male rats.

METHODS

Using stereotaxic surgery, a cannula was implanted at a site 1mm above the MPOA in the anesthetized rats. After the recovery period, tail-flick (TF) latency was measured as 0, 15, 30, 45 and 60min following the onset of two experimental protocols. Two experiments were carried out. Experiment 1: The male rats received intra-MPOA of 25, 100, 1000, 10000pmol/0.5μl orexin A or 0.5μl of aCSF (control, just 5min before the TF assay. Experiment 2: The aim of this experiment was to examine the effect of orexin microinjection into MPOA on morphine analgesia (3mg/kg,s.c). Morphine was administered 30min before orexin A intra-MPOA microinjection (four doses similar to experiment 1) or aCSF, then TF latency was measured.

RESULTS

The results indicated that microinjection of orexin A into the MPOA showed anti-nociceptive effect in a time-dependent manner. Dose response curve results also revealed that the maximum effective dose of orexin A injection into MPOA for pain inhibition is 1000pmol/0.5μl. Co-administration of systemic morphine and orexin into the MPOA has additive analgesia with different time course compared morphine or orexin alone.

CONCLUSION

It can be concluded that MPOA OrexinA receptors play an important role in the modulation of pain in normal and morphine treated male rats.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Allopregnanolone preferentially induces energy-rich food intake in male Wistar rats

Obesity is an increasing problem and identification of the driving forces for overeating of energy‐rich food is important. Previous studies show that the stress and sex steroid allopregnanolone has a hyperphagic effect on both bland food and palatable food. If allopregnanolone induces a preference for more palatable or for more energy‐rich food is not known. The aim of this study was to elucidate the influence of allopregnanolone on food preference. Male Wistar rats were subjected to two different food preference tests: a choice between standard chow and cookies (which have a higher energy content and also are more palatable than chow), and a choice between a low caloric sucrose solution and standard chow (which has a higher energy content and is less palatable than sucrose). Food intake was measured for 1 h after acute subcutaneous injections of allopregnanolone. In the choice between cookies and chow allopregnanolone significantly increased only the intake of cookies. When the standard chow was the item present with the highest caloric load, the chow intake was increased and allopregnanolone had no effect on intake of the 10% sucrose solution. The increased energy intakes induced by the high allopregnanolone dose compared to vehicle were very similar in the two tests, 120% increase for cookies and 150% increase for chow. It appears that in allopregnanolone‐induced hyperphagia, rats choose the food with the highest energy content regardless of its palatability.

We show that allopregnanolone increased the consumption of the more calorie dense food offered in different food preference situations, while palatability was of secondary importance. Thus, it seems as allopregnanolone treatment could favor ingestion of a more energy‐rich diet.

Common effects of fat, ethanol, and nicotine on enkephalin in discrete areas of the brain

Fat, ethanol, and nicotine share a number of properties, including their ability to reinforce behavior and produce overconsumption. To test whether these substances act similarly on the same neuronal populations in specific brain areas mediating these behaviors, we administered the substances short-term, using the same methods and within the same experiment, and measured their effects, in areas of the hypothalamus (HYPO), amygdala (AMYG), and nucleus accumbens (NAc), on mRNA levels of the opioid peptide, enkephalin (ENK), using in situ hybridization and on c-Fos immunoreactivity (ir) to indicate neuronal activity, using immunofluorescence histochemistry. In addition, we examined for comparison another reinforcing substance, sucrose, and also took measurements of stress-related behaviors and circulating corticosterone (CORT) and triglycerides (TG), to determine if they contribute to these substances' behavioral and physiological effects. Adult Sprague-Dawley rats were gavaged three times daily over 5 days with 3.5 mL of water, Intralipid (20% v/v), ethanol (12% v/v), nicotine (0.01% w/v) or sucrose (22% w/v) (approximately 7 kcal/dose), and tail vein blood was collected for measurements of circulating CORT and TG. On day five, animals were sacrificed, brains removed, and the HYPO, AMYG, and NAc processed for single- or double-labeling of ENK mRNA and c-Fos-ir. Fat, ethanol, and nicotine, but not sucrose, increased the single- and double-labeling of ENK and c-Fos-ir in precisely the same brain areas, the middle parvocellular but not lateral area of the paraventricular nucleus, central but not basolateral nucleus of the AMYG, and core but not shell of the NAc. While having little effect on stress-related behaviors or CORT levels, fat, ethanol, and nicotine all increased circulating levels of TG. These findings suggest that the overconsumption of these three substances and their potential for abuse are mediated by the same populations of ENK-expressing neurons in specific nuclei of the hypothalamus and limbic system.

Effect of dietary fatty acid composition on food intake, triglycerides, and hypothalamic peptides

While a high-fat diet when compared to low-fat diet is known to produce overeating and health complications, less is known about the effects produced by fat-rich diets differing in their specific composition of fat. This study examined the effects of a high-fat diet containing relatively high levels of saturated compared to unsaturated fatty acids (HiSat) to a high-fat diet with higher levels of unsaturated fatty acids (USat). A HiSat compared to USat meal caused rats to consume more calories in a subsequent chow test meal. The HiSat meal also increased circulating levels of triglycerides (TG) and expression of the orexigenic peptides, galanin (GAL) in the hypothalamic paraventricular nucleus (PVN) and orexin (OX) in the perifornical lateral hypothalamus (PFLH). A similar increase in TG levels and PVN GAL and PFLH OX was also seen in rats given chronic access to the HiSat compared to USat diet, while neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus showed decreased expression. The importance of TG in producing these changes was supported by the finding that the TG-lowering medication gemfibrozil as compared to vehicle, when peripherally administered before consumption of a HiSat meal, significantly decreased the expression of OX, while increasing the expression of NPY and AgRP. These findings substantiate the importance of the fat composition in a diet, indicating that those rich in saturated compared to unsaturated fatty acids may promote overeating by increasing circulating lipids and specific hypothalamic peptides, GAL and OX, known to preferentially stimulate the consumption of a fat-rich diet.

The Impact of Maternal High-Fat Diet Consumption on Neural Development and Behavior of Offspring

Maternal diet and metabolic state are important factors in determining the environment experienced during perinatal development. Epidemiological studies and evidence from animal models provide evidence that a mother's diet and metabolic condition are important in programming the neural circuitry that regulates behavior, resulting in a persistent impact on the offspring's behavior. Potential mechanisms by which maternal diet and metabolic profile influence the perinatal environment include placental dysfunction and increases in circulating factors such as inflammatory cytokines, nutrients (glucose and fatty acids) and hormones (insulin and leptin). Maternal obesity and high-fat diet (HFD) consumption exposure during development have been observed to increase the risk of developing serious mental health and behavioral disorders including anxiety, depression, attention deficit hyperactivity disorder and autism spectrum disorder. The increased risk of developing these behavioral disorders is postulated to be due to perturbations in the development of neural pathways that regulate behavior, including the serotonergic, dopaminergic and melanocortinergic systems. It is critical to examine the influence that a mother's nutrition and metabolic profile have on the developing offspring considering the current and alarmingly high prevalence of obesity and HFD consumption in pregnant women.

Decreased galanin serum levels are associated with alcohol-craving during withdrawal

BACKGROUND

The hypothalamic galanin expression has been associated with increased intake of carbohydrates and fats in preclinical studies. The appetite stimulating effect of galanin is thought to underlie the positive association between alcohol consumption and hypothalamic galanin expression observed in preclinical studies.

METHODS

In this pilot study we investigated alterations in galanin serum levels (33 male patients) in alcohol-dependent patients during alcohol withdrawal (days 1, 7 and 14) in comparison to healthy controls (19 male controls). In order to assess the putative association between appetite regulation, galanin serum levels and alcohol consumption we additionally investigated the serum levels of insulin, glucose and triglycerides.

RESULTS

The galanin serum levels on day 1 of alcohol withdrawal were significantly reduced in the alcohol-dependent patients (T=-3.302, p=0.002) and increased significantly from day 1 to day 14 of alcohol withdrawal (F=6.437, p=0.002). We found a significant negative association between the galanin serum levels and alcohol craving measured by the Obsessive Compulsive Drinking Scale (OCDS) (r=-0.449, p=0.009) and the obsessive subscale of the OCDS (r=-0.521, p=0.002) on day 1 of alcohol withdrawal. There was no association between the galanin serum levels and the parameters of energy homeostasis (triglycerides, cholesterol, insulin, and glucose) investigated.

CONCLUSIONS

Acute alcohol withdrawal was associated with decreased galanin serum levels in this pilot study. There was no association between the galanin serum levels and the parameters of energy homeostasis. Further research of galanin serum levels in active drinkers will be necessary to clarify the putative association between galanin serum levels, appetite regulation and alcohol consumption.

Differential effects of allopregnanolone on the escalation of cocaine self-administration and sucrose intake in female rats

RATIONALE

Evidence suggests that the progesterone metabolite allopregnanolone (ALLO) decreases cocaine seeking in animal models of relapse.

OBJECTIVE

The purpose of this study was to examine the effects of ALLO on an animal model of cocaine and sucrose bingeing (escalation). Allopregnanolone's effects on yohimbine-induced sucrose intake were also examined. In a separate group of animals, dose interactions between ALLO and cocaine were examined with an abbreviated procedure, a short access progressive ratio (PR) schedule for cocaine reinforcement.

METHODS

Female rats were treated with ALLO (15 mg/kg, s.c.) or vehicle (VEH) and trained to lever press for cocaine infusions (0.4 mg/kg) under an extended-access procedure. In a separate condition, other ALLO- and VEH-treated female rats self-administered orally delivered liquid sucrose. Allopregnanolone and VEH treatment was then discountinued and the sucrose-maintained rats were administered priming injections of saline, yohimbine, or yohimbine + ALLO. For the PR condition, rats were first treated with VEH until reaching stability at four doses of cocaine (0.2, 0.4, 0.8, and 1.6 mg/kg in mixed order). Subsequently, rats re-established their baseline cocaine intake at the four cocaine doses following treatment with each of two counterbalanced doses of ALLO (15 and 30 mg/kg).

RESULTS

ALLO significantly blocked the escalation of cocaine self-administration but did not reliably affect intake of sucrose under a similar condition or affect cocaine intake at several doses under a PR schedule. Yohimbine significantly increased sucrose intake while ALLO failed to attenuate this increase.

CONCLUSION

These findings indicate that ALLO protects against binge-like patterns of cocaine intake but does not reduce sugar intake that is acutely increased by yohimbine in females.

Effect of chronic ethanol on enkephalin in the hypothalamus and extra-hypothalamic areas

BACKGROUND

Ethanol may be consumed for reasons such as reward, anxiety reduction, or caloric content, and the opioid enkephalin (ENK) appears to be involved in many of these functions. Previous studies in Sprague-Dawley rats have demonstrated that ENK in the hypothalamic paraventricular nucleus (PVN) is stimulated by voluntary consumption of ethanol. This suggests that this opioid peptide may be involved in promoting the drinking of ethanol, consistent with our recent findings that PVN injections of ENK analogs stimulate ethanol intake. To broaden our understanding of how this peptide functions throughout the brain to promote ethanol intake, we measured, in rats trained to drink 9% ethanol, the expression of the ENK gene in additional brain areas outside the hypothalamus, namely, the ventral tegmental area (VTA), nucleus accumbens shell (NAcSh) and core (NAcC), medial prefrontal cortex (mPFC), and central nucleus of the amygdala (CeA).

METHODS

In the first experiment, the brains of rats chronically drinking 1 g/kg/d ethanol, 3 g/kg/d ethanol, or water were examined using real-time quantitative polymerase chain reaction (qRT-PCR). In the second experiment, a more detailed, anatomic analysis of changes in gene expression, in rats chronically drinking 3 g/kg/d ethanol compared to water, was performed using radiolabeled in situ hybridization (ISH). The third experiment employed digoxigenin-labeled ISH (DIG) to examine changes in the density of cells expressing ENK and, for comparison, dynorphin (DYN) in rats chronically drinking 3 g/kg/d ethanol versus water.

RESULTS

With qRT-PCR, the rats chronically drinking ethanol plus water compared to water alone showed significantly higher levels of ENK mRNA, not only in the PVN but also in the VTA, NAcSh, NAcC, and mPFC, although not in the CeA. Using radiolabeled ISH, levels of ENK mRNA in rats drinking ethanol were found to be elevated in all areas examined, including the CeA. The experiment using DIG confirmed this effect of ethanol, showing an increase in density of ENK-expressing cells in all areas studied. It additionally revealed a similar change in DYN mRNA in the PVN, mPFC, and CeA, although not in the NAcSh or NAcC.

CONCLUSIONS

While distinguishing the NAc as a site where ENK and DYN respond differentially, these findings lead us to propose that these opioids, in response to voluntary ethanol consumption, are generally elevated in extra-hypothalamic as well as hypothalamic areas, possibly to carry out specific area-related functions that, in turn, drive animals to further consume ethanol. These functions include calorie ingestion in the PVN, reward and motivation in the VTA and NAcSh, response-reinforcement learning in the NAcC, stress reduction in the CeA, and behavioral control in the mPFC.

Increased intake of ethanol and dietary fat in galanin overexpressing mice

Evidence suggests that the orexigenic peptide, galanin (GAL), in the hypothalamic paraventricular nucleus (PVN) has a role in stimulating the consumption of ethanol, in addition to a high-fat diet. This possibility was further examined in mutant mice that overexpress the GAL gene. Two sets of GAL-overexpressors (GALOE) compared with wild-type (WT) controls, maintained on laboratory chow and water, were trained to voluntarily drink increasing concentrations of ethanol, from 3 to 15%. In the GALOE versus WT mice, the results revealed the following: (1) a 35-40% increase in ethanol intake and ethanol preference, which was evident only at the highest (15%) ethanol concentration, in male but not female mice, and was seen with comparisons to littermate and nonlittermate WT controls, (2) a significantly larger, 60-75% increase in ethanol intake and ethanol preference after a day of food deprivation, again only in male GALOE mice, (3) no change in consumption of sucrose or quinine solutions in preference tests, and (4) a 55% increase in consumption of a fat-rich diet during a 2-h test period, in both male and female GALOE mice. These results obtained with overexpression of the GAL gene provide strong support for a physiological role of this peptide in stimulating the consumption of ethanol and a fat-rich diet. They reveal gender differences in the behavioral phenotype, which may reflect GAL's functional relationship to reproductive hormones in the stimulation of consummatory behavior.

Hypothalamic injection of non-opioid peptides increases gene expression of the opioid enkephalin in hypothalamic and mesolimbic nuclei: Possible mechanism underlying their behavioral effects

The peptides galanin (GAL) and orexin (OX) share common features with the opioid enkephalin (ENK) in their relationship to ingestive behavior, stimulating consumption of a fat-rich diet and ethanol when injected into the hypothalamus. Since receptors for GAL and OX are dense in areas where ENK-expressing neurons are concentrated, these non-opioid peptides may exert their effects, in part, through the stimulation of endogenous ENK. This study was conducted to determine whether injection of GAL or OX affects the expression of ENK in hypothalamic and mesolimbic nuclei involved in consummatory behavior. Rats were injected with GAL (1 microg), OX-A (1 microg), or saline vehicle just dorsal to the hypothalamic paraventricular nucleus (PVN). They were sacrificed 1h later for analysis of ENK mRNA levels in the PVN, ventral tegmental area (VTA), central nucleus of the amygdala (CeA), and nucleus accumbens (NAc). Both GAL and OX had similar effects, significantly increasing ENK mRNA expression in each of these areas, except for the NAc. This enhanced ENK expression in the PVN, VTA and CeA was demonstrated with real-time quantitative polymerase chain reaction and confirmed in separate groups using radiolabeled and digoxigenin-labeled in situ hybridization. These findings demonstrate that the non-opioid peptides, GAL or OX, which have similar effects on consummatory behavior, are also similar in their effect on endogenous ENK. In light of published findings showing an opioid antagonist to block GAL- and OX-induced feeding, these results provide additional evidence that ENK is involved in mediating the common behavioral effects of these peptides.

Adult consequences of post-weaning high fat feeding on the limbic-HPA axis of female rats

The peripubertal period is critical for the final maturation of circuits controlling energy homeostasis and stress response. However, the consequence of juvenile fat consumption on adult physiology is not clear. This study analyzed the adult consequences of post-weaning fat feeding on limbic-hypothalamic-pituitary-adrenal (HPA) axis components and on metabolic regulators of female rats. Wistar rats were fed either a high fat (HF) diet or the normal chow from weaning to puberty or to 3 months of age. Additional groups crossed their diets at puberty onset. Plasma leptin, insulin, and corticosterone levels were determined by radioimmunoassay and their brain receptors by western blot analysis. Adult HF-fed animals though not overweight, had higher corticosterone and reduced glucocorticoid receptor levels in the hypothalamus and hippocampus, compared to the controls. The alterations in HPA axis emerged already at puberty onset. Leptin receptor levels in the hypothalamus were reduced only by continuous fat feeding from weaning to adulthood. The pre-pubertal period appeared more vulnerable to diet-induced alterations in adulthood than the post-pubertal one. Switching from fat diet to normal chow at puberty onset restored most of the diet-induced alterations in the HPA axis. The corticosteroid circuit rather than the leptin or insulin system appears as the principal target for the peripubertal fat diet-induced effects in adult female rats.

Galanin knockout mice show disturbances in ethanol consumption and expression of hypothalamic peptides that stimulate ethanol intake

BACKGROUND

There is growing evidence suggesting that hypothalamic galanin (GAL), which is known to stimulate intake of a fat-rich diet, has a role in promoting the consumption of ethanol. The present study further examined this possibility in GAL knockout (GALKO) mice.

METHODS

Two groups of female and male GALKO mice, compared to wild-type (WT) controls, were trained to voluntarily drink increasing concentrations of ethanol, while maintained on lab chow and water. They were examined in terms of their daily ethanol intake and preference, acute consumption of a high-fat diet, preference for flavored solutions, and expression of different peptides shown to stimulate ethanol intake.

RESULTS

In the GALKO mice compared to WT, the results revealed: (i) a 35 to 45% decrease in ethanol intake and preference, which was evident only at the highest (15%) ethanol concentration, was stronger in female than in male mice, and was seen with comparisons to littermate as well as nonlittermate WT mice; (ii) a 48% decrease in acute intake of a fat-rich diet, again stronger in female than male mice; (iii) no difference in consumption of sucrose or quinine solutions in preference tests; (iv) a total loss of GAL mRNA in the hypothalamic paraventricular nucleus (PVN) of female and male mice; and (v) a gender-specific change in mRNA levels of peptides in the perifornical lateral hypothalamus (PFLH), orexin and melanin-concentrating hormone, which are known to stimulate ethanol and food intake and were markedly decreased in females while increased in males.

CONCLUSIONS

These results provide strong support for a physiological role of PVN GAL in stimulating the consumption of ethanol, as well as a fat-rich diet. Ablation of the GAL gene produced a behavioral phenotype, particularly in females, which may reflect the functional relationship of galanin to ovarian steroids. It also altered the peptides in the PFLH, with their reduced expression contributing to the larger behavioral effects observed in females and their increased expression attenuating these effects in males.

Puberty onset in female rats: relationship with fat intake, ovarian steroids and the peptides, galanin and enkephalin, in the paraventricular and medial preoptic nuclei

Puberty is a time of rapid change, including a marked increase in fat consumption and body fat accrual, particularly in females. The mechanisms underlying these changes are unknown. Building on the results obtained in adult rats, the present study in pubertal rats focused on the orexigenic peptides, galanin (GAL) and enkephalin (ENK), in the paraventricular nucleus (PVN) and medial preoptic nucleus (MPN), which are known to be responsive to female steroids and have a role in both energy balance and reproductive function. The present study examined female rats maintained on pure macronutrient diets from before weaning (day 15) to day 70. After an initial burst in protein intake (days 21-35), rats showed an increase, specifically in preference for fat, from 15% to 30%. In rats examined at different ages before (day 30) and after (days 45 and 60) puberty, this rise in fat intake was associated with a marked increase, from days 30-45, in levels of oestradiol and progesterone and in GAL and ENK mRNA or peptide levels, specifically in the PVN and MPN, but not other hypothalamic areas examined. This positive relationship with increased fat intake, steroids and peptides across ages was also observed when comparing pubertal rats that naturally preferred fat (> 25% of total diet) with those consuming little fat (< 15%) or rats that reached puberty at an early age (days 30-34) with those that were late (days 37-40). These rats with early puberty onset exhibited a strong fat preference 3-4 days before vaginal opening, which was positively related to steroid levels, GAL, fat intake and body fat accrual after puberty. These findings suggest that, in addition to providing a signal for puberty onset, early fat ingestion acting through mechanisms involving the steroids and orexigenic peptides may be related to long-term patterns of eating and body weight regulation.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Maternal high-fat diet and fetal programming: increased proliferation of hypothalamic peptide-producing neurons that increase risk for overeating and obesity

Recent studies in adult and weanling rats show that dietary fat, in close association with circulating lipids, can stimulate expression of hypothalamic peptides involved in controlling food intake and body weight. In the present study, we examined the possibility that a fat-rich diet during pregnancy alters the development of these peptide systems in utero, producing neuronal changes in the offspring that persist postnatally in the absence of the diet and have long-term consequences. The offspring of dams on a high-fat diet (HFD) versus balanced diet (BD), from embryonic day 6 to postnatal day 15 (P15), showed increased expression of orexigenic peptides, galanin, enkephalin, and dynorphin, in the paraventricular nucleus and orexin and melanin-concentrating hormone in the perifornical lateral hypothalamus. The increased density of these peptide-expressing neurons, evident in newborn offspring as well as P15 offspring cross-fostered at birth to dams on the BD, led us to examine events that might be occurring in utero. During gestation, the HFD stimulated the proliferation of neuroepithelial and neuronal precursor cells of the embryonic hypothalamic third ventricle. It also stimulated the proliferation and differentiation of neurons and their migration toward hypothalamic areas where ultimately a greater proportion of the new neurons expressed the orexigenic peptides. This increase in neurogenesis, closely associated with a marked increase in lipids in the blood, may have a role in producing the long-term behavioral and physiological changes observed in offspring after weaning, including an increase in food intake, preference for fat, hyperlipidemia, and higher body weight.