Hypothalamic hormone deficiency enables physiological anorexia

Mammalian hibernators survive prolonged periods of cold and resource scarcity by temporarily modulating normal physiological functions, but the mechanisms underlying these adaptations are poorly understood. The hibernation cycle of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) lasts for 5-7 months and comprises weeks of hypometabolic, hypothermic torpor interspersed with 24-48-hour periods of an active-like interbout arousal (IBA) state. We show that ground squirrels, who endure the entire hibernation season without food, have negligible hunger during IBAs. These squirrels exhibit reversible inhibition of the hypothalamic feeding center, such that hypothalamic arcuate nucleus neurons exhibit reduced sensitivity to the orexigenic and anorexigenic effects of ghrelin and leptin, respectively. However, hypothalamic infusion of thyroid hormone during an IBA is sufficient to rescue hibernation anorexia. Our results reveal that thyroid hormone deficiency underlies hibernation anorexia and demonstrate the functional flexibility of the hypothalamic feeding center.

microRNA-33 controls hunger signaling in hypothalamic AgRP neurons

AgRP neurons drive hunger, and excessive nutrient intake is the primary driver of obesity and associated metabolic disorders. While many factors impacting central regulation of feeding behavior have been established, the role of microRNAs in this process is poorly understood. Utilizing unique mouse models, we demonstrate that miR-33 plays a critical role in the regulation of AgRP neurons, and that loss of miR-33 leads to increased feeding, obesity, and metabolic dysfunction in mice. These effects include the regulation of multiple miR-33 target genes involved in mitochondrial biogenesis and fatty acid metabolism. Our findings elucidate a key regulatory pathway regulated by a non-coding RNA that impacts hunger by controlling multiple bioenergetic processes associated with the activation of AgRP neurons, providing alternative therapeutic approaches to modulate feeding behavior and associated metabolic diseases.

Loss of cis-PTase function in the liver promotes a highly penetrant form of fatty liver disease that rapidly transitions to hepatocellular carcinoma

Obesity-linked fatty liver is a significant risk factor for hepatocellular carcinoma (HCC) 1,2 ; however, the molecular mechanisms underlying the transition from non-alcoholic fatty liver disease (NAFLD) to HCC remains unclear. The present study explores the role of the endoplasmic reticulum (ER)-associated protein NgBR, an essential component of the cis-prenyltransferases (cis-PTase) enzyme 3 , in chronic liver disease. Here we show that genetic depletion of NgBR in hepatocytes of mice (N-LKO) intensifies triacylglycerol (TAG) accumulation, inflammatory responses, ER/oxidative stress, and liver fibrosis, ultimately resulting in HCC development with 100% penetrance after four months on a high-fat diet. Comprehensive genomic and single cell transcriptomic atlas from affected livers provides a detailed molecular analysis of the transition from liver pathophysiology to HCC development. Importantly, pharmacological inhibition of diacylglycerol acyltransferase-2 (DGAT2), a key enzyme in hepatic TAG synthesis, abrogates diet-induced liver damage and HCC burden in N-LKO mice. Overall, our findings establish NgBR/cis-PTase as a critical suppressor of NAFLD-HCC conversion and suggests that DGAT2 inhibition may serve as a promising therapeutic approach to delay HCC formation in patients with advanced non-alcoholic steatohepatitis (NASH).

Ghrelin Predicts Stimulant and Sedative Effects of Alcohol in Heavy Drinkers

AIM

The aim of this study was to examine the relationship between ghrelin levels and the subjective effects of alcohol in heavy drinkers, and to compare them to healthy controls.

METHODS

Ghrelin levels were collected as part of two laboratory studies. Both groups received either IV infusion of saline or high dose of alcohol (100 mg%). In the study of heavy drinkers, ghrelin was gathered on all subjects, but data was analyzed only for participants who received placebo (N=12). Healthy controls (N=20) came from another study that collected data on family history. Ghrelin levels and measures of alcohol effects (BAES, VAS, NDS, YCS [see manuscript for details]) were collected at 4 timepoints: baseline, before infusion, during infusion and after infusion.

RESULTS

IV alcohol significantly reduced ghrelin levels and higher fasting ghrelin levels were associated with more intense subjective alcohol effects. There were no differences in fasting ghrelin levels or subjective effects between heavy drinkers and controls. However, while both groups showed similar decline in ghrelin levels following alcohol infusion, on the placebo day, ghrelin levels in the healthy subjects increased significantly and exponentially over time while for the heavy drinkers ghrelin levels remained flat.

CONCLUSIONS

Our findings support the role of ghrelin in reward mechanisms for alcohol. Contrary to others, we found no differences in fasting ghrelin levels or subjective experiences of alcohol between heavy drinkers and healthy controls. However, the group differences on the IV placebo day may be a possible indication of ghrelin abnormalities in heavy drinkers.

LINE-1 activation in the cerebellum drives ataxia

Dysregulation of long interspersed nuclear element 1 (LINE-1, L1), a dominant class of transposable elements in the human genome, has been linked to neurodegenerative diseases, but whether elevated L1 expression is sufficient to cause neurodegeneration has not been directly tested. Here, we show that the cerebellar expression of L1 is significantly elevated in ataxia telangiectasia patients and strongly anti-correlated with the expression of epigenetic silencers. To examine the role of L1 in the disease etiology, we developed an approach for direct targeting of the L1 promoter for overexpression in mice. We demonstrated that L1 activation in the cerebellum led to Purkinje cell dysfunctions and degeneration and was sufficient to cause ataxia. Treatment with a nucleoside reverse transcriptase inhibitor blunted ataxia progression by reducing DNA damage, attenuating gliosis, and reversing deficits of molecular regulators for calcium homeostasis in Purkinje cells. Our study provides the first direct evidence that L1 activation can drive neurodegeneration.

TET3 epigenetically controls feeding and stress response behaviors via AGRP neurons

The TET family of dioxygenases promote DNA demethylation by oxidizing 5-methylcytosine to 5-hydroxymethylcytosine (5hmC). Hypothalamic agouti-related peptide-expressing (AGRP-expressing) neurons play an essential role in driving feeding, while also modulating nonfeeding behaviors. Besides AGRP, these neurons produce neuropeptide Y (NPY) and the neurotransmitter GABA, which act in concert to stimulate food intake and decrease energy expenditure. Notably, AGRP, NPY, and GABA can also elicit anxiolytic effects. Here, we report that in adult mouse AGRP neurons, CRISPR-mediated genetic ablation of Tet3, not previously known to be involved in central control of appetite and metabolism, induced hyperphagia, obesity, and diabetes, in addition to a reduction of stress-like behaviors. TET3 deficiency activated AGRP neurons, simultaneously upregulated the expression of Agrp, Npy, and the vesicular GABA transporter Slc32a1, and impeded leptin signaling. In particular, we uncovered a dynamic association of TET3 with the Agrp promoter in response to leptin signaling, which induced 5hmC modification that was associated with a chromatin-modifying complex leading to transcription inhibition, and this regulation occurred in both the mouse models and human cells. Our results unmasked TET3 as a critical central regulator of appetite and energy metabolism and revealed its unexpected dual role in the control of feeding and other complex behaviors through AGRP neurons.

A hypothalamic pathway for Augmentor α-controlled body weight regulation

Augmentor α and β (Augα and Augβ) are newly discovered ligands of the receptor tyrosine kinases Alk and Ltk. Augα functions as a dimeric ligand that binds with high affinity and specificity to Alk and Ltk. However, a monomeric Augα fragment and monomeric Augβ also bind to Alk and potently stimulate cellular responses. While previous studies demonstrated that oncogenic Alk mutants function as important drivers of a variety of human cancers, the physiological roles of Augα and Augβ are poorly understood. Here, we investigate the physiological roles of Augα and Augβ by exploring mice deficient in each or both Aug ligands. Analysis of mutant mice showed that both Augα single knockout and double knockout of Augα and Augβ exhibit a similar thinness phenotype and resistance to diet-induced obesity. In the Augα-knockout mice, the leanness phenotype is coupled to increased physical activity. By contrast, Augβ-knockout mice showed similar weight curves as the littermate controls. Experiments are presented demonstrating that Augα is robustly expressed and metabolically regulated in agouti-related peptide (AgRP) neurons, cells that control whole-body energy homeostasis in part via their projections to the paraventricular nucleus (PVN). Moreover, both Alk and melanocortin receptor-4 are expressed in discrete neuronal populations in the PVN and are regulated by projections containing Augα and AgRP, respectively, demonstrating that two distinct mechanisms that regulate pigmentation operate in the hypothalamus to control body weight. These experiments show that Alk-driven cancers were co-opted from a neuronal pathway in control of body weight, offering therapeutic opportunities for metabolic diseases and cancer.

AgRP neurons control compulsive exercise and survival in an activity-based anorexia model

Hypothalamic agouti-related peptide (AgRP) and neuropeptide Y-expressing neurons have a critical role in driving food intake, but also in modulating complex, non-feeding behaviours¹. We interrogated whether AgRP neurons are relevant to the emergence of anorexia nervosa symptomatology in a mouse model. Here we show, using in vivo fibre photometry, a rapid inhibition of AgRP neuronal activity following voluntary cessation of running. All AgRP neuron-ablated, food-restricted mice die within 72 h of compulsive running, while daily activation of AgRP neurons using a chemogenetic tool increases voluntary running with no lethality of food-restricted animals. Animals with impaired AgRP neuronal circuits are unable to properly mobilize fuels during food-restriction-associated exercise; however, when provided with elevated fat content through diet, their death is completely prevented. Elevated fat content in the diet also prevents the long-term behavioural impact of food-restricted fit mice with elevated exercise volume. These observations elucidate a previously unsuspected organizational role of AgRP neurons, via the mediation of the periphery, in the regulation of compulsive exercise and its related lethality with possible implications for psychiatric conditions, such as anorexia nervosa.

Nesfatin-1 decreases the motivational and rewarding value of food

Homeostatic and hedonic pathways distinctly interact to control food intake. Dysregulations of circuitries controlling hedonic feeding may disrupt homeostatic mechanisms and lead to eating disorders. The anorexigenic peptides nucleobindin-2 (NUCB2)/nesfatin-1 may be involved in the interaction of these pathways. The endogenous levels of this peptide are regulated by the feeding state, with reduced levels following fasting and normalized by refeeding. The fasting state is associated with biochemical and behavioral adaptations ultimately leading to enhanced sensitization of reward circuitries towards food reward. Although NUCB2/nesfatin-1 is expressed in reward-related brain areas, its role in regulating motivation and preference for nutrients has not yet been investigated. We here report that both dopamine and GABA neurons express NUCB2/nesfatin-1 in the VTA. Ex vivo electrophysiological recordings show that nesfatin-1 hyperpolarizes dopamine, but not GABA, neurons of the VTA by inducing an outward potassium current. In vivo, central administration of nesfatin-1 reduces motivation for food reward in a high-effort condition, sucrose intake and preference. We next adopted a 2-bottle choice procedure, whereby the reward value of sucrose was compared with that of a reference stimulus (sucralose + optogenetic stimulation of VTA dopamine neurons) and found that nesfatin-1 fully abolishes the fasting-induced increase in the reward value of sucrose. These findings indicate that nesfatin-1 reduces energy intake by negatively modulating dopaminergic neuron activity and, in turn, hedonic aspects of food intake. Since nesfatin-1´s actions are preserved in conditions of leptin resistance, the present findings render the NUCB2/nesfatin-1 system an appealing target for the development of novel therapeutical treatments towards obesity.

SUN-097 AgRP Neurons Determine Survival in Activity-Based Anorexia Model

Anorexia Nervosa (AN) is an eating disorder characterized by severe hypophagia, high levels of physical activity, harsh weight loss and an intense fear of weight gain. It has the highest mortality rate among psychiatric illnesses and, due to the unknown underlying neurobiology, it is challenging to treat. Agouti-related protein (AgRP) neurons, which are localized in the arcuate nucleus in the hypothalamus, are both necessary and sufficient or feeding in adult animals.To uncover new neural circuits that may contribute towards vulnerability to AN or might be affected by AN, we employed the specific diphtheria toxin receptor-expressing mice (AgRP-DTR) which, by the selective ablation of AgRP neurons, allow to test the impact of an impaired AgRP circuit function under the activity-based anorexia (ABA) paradigm. ABA is a bio-behavioral phenomenon described in rodents and refers to the weight loss, hypophagia and paradoxical hyperactivity that develops in rodents exposed to running wheels and restricted food access, and provides a model for the key symptoms of AN. Mice that express DTR only in AgRP neurons and subcutaneously injected with diphtheria toxin (DTX) at postnatal day 3 lost more than 50% of AgRP neurons on postnatal day 7 compared to control. The same percentage of neuronal loss was detected in 8 weeks old mice. In addition, the neonatal animals developed normally after AgRP ablation, did not show any phenotypic effects and maintained normal food intake and weight when fed ad libitum. On postnatal day 36 (P36), males and females animals were housed with access on a running wheel and fed ad libitum for 4 days (acclimation phase). On P40, for 72 hours animals were fed for only two hours daily. Following the fasting phase, free access to food was returned and the running wheel was removed. Continuous multi-day analysis of running wheel activity showed that both controls and AgRP-DTR mice kept constant weight and food intake during acclimation. In contrast, although mice became hyperactive within the 24 hours following the onset of food restriction (FR), we noted a 10% mortality on day 2 and 70% mortality at day 4 among the AgRP DTR mice. Moreover, the survived AgRP-DTR mice failed to return to normal food intake and weight even when ad libitum food was provided. Overall, AgRP neurons showed to be crucial for the full development of ABA symptoms. The results suggest that our new experimental setting is able to correlate particular neurons populationto vulnerability, onset and progression of anorexia nervosa.

Ghrelin is Related to Personality Differences in Reward Sensitivity and Impulsivity

Aims

Ghrelin, a feeding-related peptide mainly produced in the stomach, has been linked to reward mechanisms for food and drugs of abuse in addition to traits of impulsivity. This study is a secondary analysis of an existing data set designed to examine the direct relationships between fasting ghrelin levels and reward sensitivity/impulsivity in healthy social drinkers.

Methods

Participants (n = 20) were recruited from an original study examining the subjective effects of alcohol among social drinkers. Fasting ghrelin levels were collected at baseline. Personality measures (Behavioral Inhibition, Behavioral Activation, and Affective Response to Impending Reward and Punishment and Barratt Impulsiveness Scale) were administered at baseline to evaluate sensitivity to reward and punishment, and measure traits of impulsivity, respectively.

Results

Fasting ghrelin levels were significantly related to reward sensitivity and impulsivity traits. Specifically, those with higher ghrelin levels were more sensitive to reward and were more impulsive (have lower self-control).

Conclusions

The results indicate that individuals with higher levels of ghrelin are more sensitive to reward. In addition, they are less able to exercise self-control and to an extent more likely to act without thinking. This is the first study to report on the direct relationship between fasting ghrelin levels and personality characteristics such as reward sensitivity and aspects of impulsivity among healthy social drinkers.

Short summary

Individuals with higher levels of fasting ghrelin are more sensitive to reward, but less sensitive to punishment. Higher ghrelin levels are also related to some aspects of impulsivity such as decreased self-control and increased likelihood of acting without thinking.

Ghrelin is Supressed by Intravenous Alcohol and is Related to Stimulant and Sedative Effects of Alcohol

Aims

Evidence indicates that feeding-related peptides, such as ghrelin, have a role in the rewarding properties of addictive substances like alcohol. Oral alcohol administration significantly suppresses ghrelin. This study was designed to evaluate the effects of two doses of alcohol on ghrelin and examine if ghrelin levels predict the subjective effects of alcohol.

Methods

Healthy social drinkers (N = 20) participated in three, randomly assigned, and counterbalanced laboratory sessions. During each session they received a continuous IV infusion of either placebo (saline), low dose (40 mg%) or high dose (100 mg%) of alcohol. Participants were given a standardized, light breakfast 90 min before the start of the infusion. Ghrelin levels [acyl ghrelin (AG) and total ghrelin (TG)] were collected at four time points before, during and after the infusion. Subjective effects of alcohol, using the BAES, were evaluated before, during and after alcohol infusion.

Results

IV alcohol significantly reduced AG but not TG levels with no difference between the two doses of alcohol. The percent change (%∆) in AG suppression was substantial in both high dose (43.4%∆), and low dose (39.5%∆) of alcohol. Also, fasting AG and TG levels were significant predictors of alcohol stimulant and sedative effects. Higher fasting ghrelin levels were associated with longer and more intense subjective effects.

Conclusions

The results provide evidence that IV alcohol suppresses ghrelin levels similarly to oral alcohol. This study is the first to show that ghrelin predicts subjective effects of alcohol, suggesting that ghrelin may have a role in the rewarding mechanisms for alcohol.

Short summary

Intravenous alcohol infusion (low dose and high dose of alcohol) when compared to placebo (saline) significantly suppresses ghrelin in healthy social drinkers. Fasting ghrelin levels also predict subjective behavioral effects of alcohol. Those with higher fasting ghrelin levels tend to experience alcohol effects longer and more intensely.

Endothelial HIF-1α Enables Hypothalamic Glucose Uptake to Drive POMC Neurons

Glucose is the primary driver of hypothalamic proopiomelanocortin (POMC) neurons. We show that endothelial hypoxia-inducible factor 1α (HIF-1α) controls glucose uptake in the hypothalamus and that it is upregulated in conditions of undernourishment, during which POMC neuronal activity is decreased. Endothelium-specific knockdown of HIF-1α impairs the ability of POMC neurons to adapt to the changing metabolic environment in vivo, resulting in overeating after food deprivation in mice. The impaired functioning of POMC neurons was reversed ex vivo or by parenchymal glucose administration. These observations indicate an active role for endothelial cells in the central control of metabolism and suggest that central vascular impairments may cause metabolic disorders.

CD301b(+) Mononuclear Phagocytes Maintain Positive Energy Balance through Secretion of Resistin-like Molecule Alpha

Mononuclear phagocytes (MNPs) are a highly heterogeneous group of cells that play important roles in maintaining the body's homeostasis. Here, we found CD301b (also known as MGL2), a lectin commonly used as a marker for alternatively activated macrophages, was selectively expressed by a subset of CD11b(+)CD11c(+)MHCII(+) MNPs in multiple organs including adipose tissues. Depleting CD301b(+) MNPs in vivo led to a significant weight loss with increased insulin sensitivity and a marked reduction in serum Resistin-like molecule (RELM) α, a multifunctional cytokine produced by MNPs. Reconstituting RELMα in CD301b(+) MNP-depleted animals restored body weight and normoglycemia. Thus, CD301b(+) MNPs play crucial roles in maintaining glucose metabolism and net energy balance.

Bcl-xL is necessary for neurite outgrowth in hippocampal neurons

AIMS

B-cell lymphoma-extra large (Bcl-xL) protects survival in dividing cells and developing neurons, but was not known to regulate growth. Growth and synapse formation are indispensable for neuronal survival in development, inextricably linking these processes. We have previously shown that, during synaptic plasticity, Bcl-xL produces changes in synapse number, size, activity, and mitochondrial metabolism. In this study, we determine whether Bcl-xL is required for healthy neurite outgrowth and whether neurite outgrowth is necessary for survival in developing neurons in the presence or absence of stress.

RESULTS

Depletion of endogenous Bcl-xL impairs neurite outgrowth in hippocampal neurons followed by delayed cell death which is dependent on upregulation of death receptor 6 (DR6), a molecule that regulates axonal pruning. Under hypoxic conditions, Bcl-xL-depleted neurons demonstrate increased vulnerability to neuronal process loss and to death compared with hypoxic controls. Endogenous DR6 expression and upregulation during hypoxia are associated with worsened neurite damage; depletion of DR6 partially rescues neuronal process loss, placing DR6 downstream of the effects of Bcl-xL on neuronal process outgrowth and protection. In vivo ischemia produces early increases in DR6, suggesting a role for DR6 in brain injury.

INNOVATION

We suggest that DR6 levels are usually suppressed by Bcl-xL; Bcl-xL depletion leads to upregulation of DR6, failure of neuronal outgrowth in nonstressed cells, and exacerbation of hypoxia-induced neuronal injury.

CONCLUSION

Bcl-xL regulates neuronal outgrowth during development and protects neurites from hypoxic insult, as opposed by DR6. Factors that enhance neurite formation may protect neurons against hypoxic injury or neurodegenerative stimuli.

Antibodies to cannabinoid type 1 receptor co-react with stomatin-like protein 2 in mouse brain mitochondria

Anti-cannabinoid type 1 receptor (CB1 ) polyclonal antibodies are widely used to detect the presence of CB1 in a variety of brain cells and their organelles, including neuronal mitochondria. Surprisingly, we found that anti-CB1 sera, in parallel with CB1 , also recognize the mitochondrial protein stomatin-like protein 2. In addition, we show that the previously reported effect of synthetic cannabinoid WIN 55,212-2 on mitochondrial complex III respiration is not detectable in purified mitochondrial preparations. Thus, our study indicates that a direct relationship between endocannabinoid signaling and mitochondrial functions in the cerebral cortex seems unlikely, and that caution should be taken interpreting findings obtained using anti-CB1 antibodies.

Peroxisome proliferation-associated control of reactive oxygen species sets melanocortin tone and feeding in diet-induced obesity

Previous studies have proposed roles for hypothalamic reactive oxygen species (ROS) in the modulation of circuit activity of the melanocortin system. Here we show that suppression of ROS diminishes pro-opiomelanocortin (POMC) cell activation and promotes the activity of neuropeptide Y (NPY)- and agouti-related peptide (AgRP)-co-producing (NPY/AgRP) neurons and feeding, whereas ROS-activates POMC neurons and reduces feeding. The levels of ROS in POMC neurons were positively correlated with those of leptin in lean and ob/ob mice, a relationship that was diminished in diet-induced obese (DIO) mice. High-fat feeding resulted in proliferation of peroxisomes and elevated peroxisome proliferator-activated receptor γ (PPAR-γ) mRNA levels within the hypothalamus. The proliferation of peroxisomes in POMC neurons induced by the PPAR-γ agonist rosiglitazone decreased ROS levels and increased food intake in lean mice on high-fat diet. Conversely, the suppression of peroxisome proliferation by the PPAR antagonist GW9662 increased ROS concentrations and c-fos expression in POMC neurons. Also, it reversed high-fat feeding-triggered elevated NPY/AgRP and low POMC neuronal firing, and resulted in decreased feeding of DIO mice. Finally, central administration of ROS alone increased c-fos and phosphorylated signal transducer and activator of transcription 3 (pStat3) expression in POMC neurons and reduced feeding of DIO mice. These observations unmask a previously unknown hypothalamic cellular process associated with peroxisomes and ROS in the central regulation of energy metabolism in states of leptin resistance.

Maternal ghrelin deficiency compromises reproduction in female progeny through altered uterine developmental programming

Ghrelin has a well-known role in the regulation of appetite, satiety, energy metabolism, and reproduction; however ghrelin has not been implicated in reproductive tract development. We examined the effect of ghrelin deficiency on the developmental programming of female fertility. We observed that female wild-type mice born of ghrelin heterozygote dams (i.e. exposed in utero to ghrelin deficiency) had diminished fertility and produced smaller litters. We demonstrate that exposure to in utero ghrelin deficiency led to altered developmental programming of the reproductive tract. The number of ovarian follicles, corpora lutea, and embryos produced were identical in both exposed and unexposed mice. However wild-type embryos transferred to uteri of mice exposed to in utero ghrelin deficiency had a 60% reduction in the rate of embryo implantation compared with those transferred to wild-type unexposed uteri. We identified significant alterations in the uterine expression of four genes critical for implantation and a defect in uterine endometrial proliferation. Taken together, these results demonstrate that the mechanism of subfertility was abnormal endometrial function. In utero exposure to decreased levels of ghrelin led to defects in developmental programming of the uterus and subsequent subfertility in wild-type offspring.

Effects of chronic weight perturbation on energy homeostasis and brain structure in mice

Maintenance of reduced body weight in lean and obese human subjects results in the persistent decrease in energy expenditure below what can be accounted for by changes in body mass and composition. Genetic and developmental factors may determine a central nervous system (CNS)-mediated minimum threshold of somatic energy stores below which behavioral and metabolic compensations for weight loss are invoked. A critical question is whether this threshold can be altered by environmental influences and by what mechanisms such alterations might be achieved. We examined the bioenergetic, behavioral, and CNS structural responses to weight reduction of diet-induced obese (DIO) and never-obese (CON) C57BL/6J male mice. We found that weight-reduced (WR) DIO-WR and CON-WR animals showed reductions in energy expenditure, adjusted for body mass and composition, comparable (-10-15%) to those seen in human subjects. The proportion of excitatory synapses on arcuate nucleus proopiomelanocortin neurons was decreased by ∼50% in both DIO-WR and CON-WR mice. These data suggest that prolonged maintenance of an elevated body weight (fat) alters energy homeostatic systems to defend a higher level of body fat. The synaptic changes could provide a neural substrate for the disproportionate decline in energy expenditure in weight-reduced individuals. This response to chronic weight elevation may also occur in humans. The mouse model described here could help to identify the molecular/cellular mechanisms underlying both the defense mechanisms against sustained weight loss and the upward resetting of those mechanisms following sustained weight gain.

Enhanced anorexigenic signaling in lean obesity resistant syndecan-3 null mice

Obesity is associated with increased risk of diabetes, cardiovascular disease and several types of cancers. The hypothalamus is a region of the brain critical in the regulation of body weight. One of the critical and best studied hypothalamic circuits is comprised of the melanocortinergic orexigenic agouti-related protein (AgRP) and anorexigenic α-melanocyte stimulating hormone (α-MSH) neurons. These neurons project axons to the same hypothalamic target neurons and balance each other's activity leading to body weight regulation. We previously showed that the brain proteoglycan syndecan-3 regulates feeding behavior and body weight, and syndecan-3 null (SDC-3(-/-)) mice are lean and obesity resistant. Here we show that the melanocortin agonist Melanotan II (MTII) potently suppresses food intake and activates the hypothalamic paraventricular nuclei (PVN) in SDC-3(-/-) mice based on c-fos immunoreactivity. Interestingly, we determined that the AgRP neuropeptide is reduced in the PVN of SDC-3(-/-) mice compared to wild type mice. In contrast, neuropeptide Y, coexpressed in the AgRP neuron, is not differentially expressed nor is the counteracting neuropeptide α-MSH. These findings are unprecedented and indicate that AgRP protein localization can be selectively regulated within the hypothalamus resulting in altered neuropeptide response and tone.

Divergent regulation of energy expenditure and hepatic glucose production by insulin receptor in agouti-related protein and POMC neurons

OBJECTIVE

The sites of insulin action in the central nervous system that regulate glucose metabolism and energy expenditure are incompletely characterized. We have shown that mice with hypothalamic deficiency (L1) of insulin receptors (InsRs) fail to regulate hepatic glucose production (HGP) in response to insulin.

RESEARCH DESIGN AND METHODS

To distinguish neurons that mediate insulin's effects on HGP from those that regulate energy homeostasis, we used targeted knock-ins to express InsRs in agouti-related protein (AgRP) or proopiomelanocortin (POMC) neurons of L1 mice.

RESULTS

Restoration of insulin action in AgRP neurons normalized insulin suppression of HGP. Surprisingly, POMC-specific InsR knock-in increased energy expenditure and locomotor activity, exacerbated insulin resistance and increased HGP, associated with decreased expression of the ATP-sensitive K(+) channel (K(ATP) channel) sulfonylurea receptor 1 subunit, and decreased inhibitory synaptic contacts on POMC neurons.

CONCLUSIONS

The contrasting phenotypes of InsR knock-ins in POMC and AgRP neurons suggest a branched-pathway model of hypothalamic insulin signaling in which InsR signaling in AgRP neurons decreases HGP, whereas InsR activation in POMC neurons promotes HGP and activates the melanocortinergic energy expenditure program.

Early-life experience reduces excitation to stress-responsive hypothalamic neurons and reprograms the expression of corticotropin-releasing hormone

Increased sensory input from maternal care attenuates neuroendocrine and behavioral responses to stress long term and results in a lifelong phenotype of resilience to depression and improved cognitive function. Whereas the mechanisms of this clinically important effect remain unclear, the early, persistent suppression of the expression of the stress neurohormone corticotropin-releasing hormone (CRH) in hypothalamic neurons has been implicated as a key aspect of this experience-induced neuroplasticity. Here, we tested whether the innervation of hypothalamic CRH neurons of rat pups that received augmented maternal care was altered in a manner that might promote the suppression of CRH expression and studied the cellular mechanisms underlying this suppression. We found that the number of excitatory synapses and the frequency of miniature excitatory synaptic currents onto CRH neurons were reduced in "care-augmented" rats compared with controls, as were the levels of the glutamate vesicular transporter vGlut2. In contrast, analogous parameters of inhibitory synapses were unchanged. Levels of the transcriptional repressor neuron-restrictive silencer factor (NRSF), which negatively regulates Crh gene transcription, were markedly elevated in care-augmented rats, and chromatin immunoprecipitation demonstrated that this repressor was bound to a cognate element (neuron-restrictive silencing element) on the Crh gene. Whereas the reduced excitatory innervation of CRH-expressing neurons dissipated by adulthood, increased NRSF levels and repression of CRH expression persisted, suggesting that augmented early-life experience reprograms Crh gene expression via mechanisms involving transcriptional repression by NRSF.

UCP2 mediates ghrelin's action on NPY/AgRP neurons by lowering free radicals

The gut-derived hormone ghrelin exerts its effect on the brain by regulating neuronal activity. Ghrelin-induced feeding behaviour is controlled by arcuate nucleus neurons that co-express neuropeptide Y and agouti-related protein (NPY/AgRP neurons). However, the intracellular mechanisms triggered by ghrelin to alter NPY/AgRP neuronal activity are poorly understood. Here we show that ghrelin initiates robust changes in hypothalamic mitochondrial respiration in mice that are dependent on uncoupling protein 2 (UCP2). Activation of this mitochondrial mechanism is critical for ghrelin-induced mitochondrial proliferation and electric activation of NPY/AgRP neurons, for ghrelin-triggered synaptic plasticity of pro-opiomelanocortin-expressing neurons, and for ghrelin-induced food intake. The UCP2-dependent action of ghrelin on NPY/AgRP neurons is driven by a hypothalamic fatty acid oxidation pathway involving AMPK, CPT1 and free radicals that are scavenged by UCP2. These results reveal a signalling modality connecting mitochondria-mediated effects of G-protein-coupled receptors on neuronal function and associated behaviour.

Prolonged wakefulness induces experience-dependent synaptic plasticity in mouse hypocretin/orexin neurons

Sleep is a natural process that preserves energy, facilitates development, and restores the nervous system in higher animals. Sleep loss resulting from physiological and pathological conditions exerts tremendous pressure on neuronal circuitry responsible for sleep-wake regulation. It is not yet clear how acute and chronic sleep loss modify neuronal activities and lead to adaptive changes in animals. Here, we show that acute and chronic prolonged wakefulness in mice induced by modafinil treatment produced long-term potentiation (LTP) of glutamatergic synapses on hypocretin/orexin neurons in the lateral hypothalamus, a well-established arousal/wake-promoting center. A similar potentiation of synaptic strength at glutamatergic synapses on hypocretin/orexin neurons was also seen when mice were sleep deprived for 4 hours by gentle handling. Blockade of dopamine D1 receptors attenuated prolonged wakefulness and synaptic plasticity in these neurons, suggesting that modafinil functions through activation of the dopamine system. Also, activation of the cAMP pathway was not able to further induce LTP at glutamatergic synapses in brain slices from mice treated with modafinil. These results indicate that synaptic plasticity due to prolonged wakefulness occurs in circuits responsible for arousal and may contribute to changes in the brain and body of animals experiencing sleep loss.

Enhanced leptin-stimulated Pi3k activation in the CNS promotes white adipose tissue transdifferentiation

The contribution of different leptin-induced signaling pathways in control of energy homeostasis is only partly understood. Here we show that selective Pten ablation in leptin-sensitive neurons (Pten(DeltaObRb)) results in enhanced Pi3k activation in these cells and reduces adiposity by increasing energy expenditure. White adipose tissue (WAT) of Pten(DeltaObRb) mice shows characteristics of brown adipose tissue (BAT), reflected by increased mitochondrial content and Ucp1 expression resulting from enhanced leptin-stimulated sympathetic nerve activity (SNA) in WAT. In contrast, leptin-deficient ob/ob-Pten(DeltaObRb) mice exhibit unaltered body weight and WAT morphology compared to ob/ob mice, pointing to a pivotal role of endogenous leptin in control of WAT transdifferentiation. Leanness of Pten(DeltaObRb) mice is accompanied by enhanced sensitivity to insulin in skeletal muscle. These data provide direct genetic evidence that leptin-stimulated Pi3k signaling in the CNS regulates energy expenditure via activation of SNA to perigonadal WAT leading to BAT-like differentiation of WAT.

Ghrelin modulates the activity and synaptic input organization of midbrain dopamine neurons while promoting appetite

The gut hormone ghrelin targets the brain to promote food intake and adiposity. The ghrelin receptor growth hormone secretagogue 1 receptor (GHSR) is present in hypothalamic centers controlling energy metabolism as well as in the ventral tegmental area (VTA), a region important for motivational aspects of multiple behaviors, including feeding. Here we show that in mice and rats, ghrelin bound to neurons of the VTA, where it triggered increased dopamine neuronal activity, synapse formation, and dopamine turnover in the nucleus accumbens in a GHSR-dependent manner. Direct VTA administration of ghrelin also triggered feeding, while intra-VTA delivery of a selective GHSR antagonist blocked the orexigenic effect of circulating ghrelin and blunted rebound feeding following fasting. In addition, ghrelin- and GHSR-deficient mice showed attenuated feeding responses to restricted feeding schedules. Taken together, these data suggest that the mesolimbic reward circuitry is targeted by peripheral ghrelin to influence physiological mechanisms related to feeding.

Orexin neuronal changes in the locus coeruleus of the aging rhesus macaque

Orexin neuropeptides regulate arousal state and excite the noradrenergic locus coeruleus (LC), so it is plausible that an age-related loss of orexin neurons and projections to the LC contributes to poor sleep quality in elderly humans and nonhuman primates. To test this hypothesis we examined orexin B-immunoreactivity in the lateral hypothalamic area (LHA) and the LC of male rhesus macaques (Macaca mulatta) throughout the life span. Orexin perikarya, localized predominantly in the LHA, showed identical distribution patterns irrespective of age. Similarly, orexin neuron number and serum orexin B concentrations did not differ with age. In contrast, orexin B-immunoreactive axon density in the LC of old animals was significantly lower than that observed in the young or adult animals. Furthermore, the age-related decline was associated with a significant decrease in tyrosine hydroxylase (TH) mRNA in the LC, despite no change in TH-immunoreactive neuron number. Taken together, these data suggest that age-related decreases in excitatory orexin innervation to the noradrenergic LC may contribute to the etiology of poor sleep quality in the elderly.

gp130 signaling in proopiomelanocortin neurons mediates the acute anorectic response to centrally applied ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) exerts anorectic effects by overcoming leptin resistance via activation of hypothalamic neurons. However, the exact site of CNTF action in the hypothalamus has not yet been identified. Using Cre-loxP-mediated recombination in vivo, we have selectively ablated the common cytokine signaling chain gp130, which is required for functional CNTF signaling, in proopiomelanocortin (POMC)-expressing neurons. POMC-specific gp130 knockout mice exhibit unaltered numbers of POMC cells and normal energy homeostasis under standard and high fat diet. Endotoxin (LPS) and stress-induced anorexia and adrenocorticotropin regulation were unaffected in these animals. Strikingly, the anorectic effect of centrally administered CNTF was abolished in POMC-specific gp130 knockout mice. Correspondingly, in these animals, CNTF failed to activate STAT3 phosphorylation in POMC neurons and to induce c-Fos expression in the paraventricular nucleus. These data reveal POMC neurons as a critical site of CNTF action in mediating its anorectic effect.

Enhanced PIP3 signaling in POMC neurons causes KATP channel activation and leads to diet-sensitive obesity

Leptin and insulin have been identified as fuel sensors acting in part through their hypothalamic receptors to inhibit food intake and stimulate energy expenditure. As their intracellular signaling converges at the PI3K pathway, we directly addressed the role of phosphatidylinositol3,4,5-trisphosphate-mediated (PIP3-mediated) signals in hypothalamic proopiomelanocortin (POMC) neurons by inactivating the gene for the PIP3 phosphatase Pten specifically in this cell type. Here we show that POMC-specific disruption of Pten resulted in hyperphagia and sexually dimorphic diet-sensitive obesity. Although leptin potently stimulated Stat3 phosphorylation in POMC neurons of POMC cell-restricted Pten knockout (PPKO) mice, it failed to significantly inhibit food intake in vivo. POMC neurons of PPKO mice showed a marked hyperpolarization and a reduction in basal firing rate due to increased ATP-sensitive potassium (KATP) channel activity. Leptin was not able to elicit electrical activity in PPKO POMC neurons, but application of the PI3K inhibitor LY294002 and the KATP blocker tolbutamide restored electrical activity and leptin-evoked firing of POMC neurons in these mice. Moreover, icv administration of tolbutamide abolished hyperphagia in PPKO mice. These data indicate that PIP3-mediated signals are critical regulators of the melanocortin system via modulation of KATP channels.

Agouti-related peptide–expressing neurons are mandatory for feeding

Multiple hormones controlling energy homeostasis regulate the expression of neuropeptide Y (NPY) and agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus. Nevertheless, inactivation of the genes encoding NPY and/or AgRP has no impact on food intake in mice. Here we demonstrate that induced selective ablation of AgRP-expressing neurons in adult mice results in acute reduction of feeding, demonstrating direct evidence for a critical role of these neurons in the regulation of energy homeostasis.

Rapid rewiring of arcuate nucleus feeding circuits by leptin

The fat-derived hormone leptin regulates energy balance in part by modulating the activity of neuropeptide Y and proopiomelanocortin neurons in the hypothalamic arcuate nucleus. To study the intrinsic activity of these neurons and their responses to leptin, we generated mice that express distinct green fluorescent proteins in these two neuronal types. Leptin-deficient (ob/ob) mice differed from wild-type mice in the numbers of excitatory and inhibitory synapses and postsynaptic currents onto neuropeptide Y and proopiomelanocortin neurons. When leptin was delivered systemically to ob/ob mice, the synaptic density rapidly normalized, an effect detectable within 6 hours, several hours before leptin's effect on food intake. These data suggest that leptin-mediated plasticity in the ob/ob hypothalamus may underlie some of the hormone's behavioral effects.

Coenzyme Q induces nigral mitochondrial uncoupling and prevents dopamine cell loss in a primate model of Parkinson's disease

Parkinson's disease is characterized by dopamine cell loss of the substantia nigra. Parkinson's disease and the neurotoxin 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine may destroy dopamine neurons through oxidative stress. Coenzyme Q is a cofactor of mitochondrial uncoupling proteins that enhances state-4 respiration and eliminate superoxides. Here we report that short-term oral administration of coenzyme Q induces nigral mitochondrial uncoupling and prevents dopamine cell loss after 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine administration in monkeys.

New approaches to the understanding of tamoxifen action and resistance

Tamoxifen (TAM) provides an effective agent for treatment of hormone-dependent breast cancer but resistance uniformly ensues upon continued use. Additional studies are required to define more precisely the mechanisms involved in development of resistance. We conducted systematic experimental and clinical studies based on the hypothesis that tumors exposed to TAM long-term may develop resistance by becoming hypersensitive to its estrogenic effects. These investigations uncovered new features of the TAM resistance (TR) phenomenon and identified possible means for its prevention and/or elimination. Initially we confirmed that TR may be divided into two subtypes, primary and acquired resistance, and that these differ by certain important characteristics including the level of the possible involvement of adaptive and genetic components. Then we distinguished at least three consequent stages of this phenomenon: stage I when TAM behaves as an antiestrogen, stage II with development of increased sensitivity to the agonistic (pro-estrogenic) properties of TAM and stage III with an adaptive increase in sensitivity to estradiol (E(2)). During this evolutionary process, as shown in vitro, MAP kinase (MAPK) and aromatase activities increase. The time frame of the increase in MAPK activity as a rule outpaces the increase in aromatase activity during the course of the development of TR. This may occur as a response to estrogen deprivation or interruption of the process of estrogen signaling and can be one of the promoting factors of increased aromatase activation. On the other hand, the chronology of these events indicates that changes in the MAPK cascade can be more important for the early steps of the development and maintenance of the TR state. Changes in local estrogen production/sensitivity to E(2) are perhaps essential for the later steps of this phenomenon. We have explored the use of a growth factor-blocking agent to abrogate the adaptive changes in sensitivity. Farnesylthiosalicylic acid (FTS), an inhibitor of GTP-Ras binding to its membrane acceptor site, reduces the increase in the number of MCF-7 cells induced by long-term TAM treatment. It also decreases MAPK activity in TAM-treated MCF-7 cells and in established TR cell lines. Alone or in combination with letrozole (presumably, through the influence on MAPK pathway) FTS exerts moderate inhibitory effects on aromatase activity in estrogen-deprived or estrogen-exposed MCF-7 cells. Taken together, our observations suggest that FTS is a 'candidate drug' for the treatment of TR. Both the adaptive and genetic types of resistance may be amenable to this approach. Our studies underline the possible importance of starting the treatment/prevention of TR early on. From our clinical studies using immunohistochemistry, there is a rather strong rationale to include as a predisposing factor in the development of TR the increase in MAPK and aromatase activities in human primary breast tumors. In summary, data obtained during the course of this project may be considered as evidence supporting the principle that processes resulting in responses to TAM as an agonist and the development of estrogen hypersensitivity of breast cancer cells could potentially be mechanistically linked.

Estrogen, synaptic plasticity and hypothalamic reproductive aging

Unlike primates who undergo ovarian failure and loss of sex steroids at the end of reproduction, aging rodents undergo constant vaginal estrus followed by constant diestrus and finally anestrus, which indicates the absence of responsive ovarian follicles. The latter state is analogous to menopause in women. The timing of the appearance of constant estrus is determined by many factors including estrogen exposure in the brain during development and the number of times that the animal gets pregnant. The chief site of this reproductive aging in rat brains is the arcuate nucleus of the hypothalamus. The transition from normal cycles to constant estrus parallels the females' gradually decreased ability to respond to administered estradiol with a cycle of inhibition followed by disinhibition of gonadotrophin-releasing hormone. Evidence has accumulated indicating this to be due to a loss of the rat's ability to respond to markedly elevated estradiol with the usual arcuate nucleus neuro-glial plasticity that supports the estrogen-induced gonadotrophin surge (EIGS). Just as male rats are not capable of an EIGS, aged females loose this ability through repeated EIGS. Experiments indicate that in male rats the hypothalamic synaptology that develops as a result of exposure to testicular androgens in the perinatal period (brain sexual differentiation) is a result of conversion of testosterone from the testes to estrogen in the brain and is therefore due to early estrogen exposure. Aging females appear to reach a synaptology similar to males and constant estrus as a result of repeated exposure to ovarian estrogens during their reproductive careers. The relative role of aging and hormonal factors remains unclear. Morphological evidence is presented that indicates the above effects of estrogen involve changes in hypothalamic arcuate nucleus neurons and glia, including changes in the organization of perikaryal membranes as well as arcuate nucleus synaptology and the load of peroxidase in the astroglia. A possible role for free radicals (reactive oxygen species) in hypothalamic reproductive aging has been proposed. Such a mechanism is supported by evidence that the anti-oxidant vitamin E delays the onset of constant estrus and the accumulation of glial peroxidase in aging female rats. However, since the synaptology and peroxidase load in constant estrus females is independent of the age at which the constant estrus occurs, it appears that the role of (repeated) estradiol exposure is more deterministic of hypothalamic failure than is aging, per se.

Gonadal hormones are responsible for maintaining the integrity of spine synapses in the CA1 hippocampal subfield of female nonhuman primates

It is well established that gonadal hormonal manipulation results in morphologic changes in the rat hippocampus. The great similarities in the hippocampal formation between nonhuman primates and humans, as well as the differences in this structure between humans and rats, led to this investigation of whether hormonal manipulation in female subhuman primates influences pyramidal cell spine density in the CA1 hippocampal subfield, as it does in rats. African green monkeys (Cercopithecus aethiops sabaeus) were ovariectomized, and half of the animals received estrogen replacement therapy. One month later, the monkeys were killed. In the first group of experiments, pyramidal cell spines were analyzed on Golgi-impregnated material taken from the CA1 hippocampal subfield. In the second experiment, unbiased electron microscopic stereologic calculations were performed to estimate the volumetric density of spine synapses in the same hippocampal subfield. Analysis of the Golgi-impregnated material showed that the spine density of CA1 pyramidal cells is much lower in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. The unbiased, electron microscopic, stereologic calculation confirmed the light microscopic observation. The volumetric density (number of spine synapses/microm(3)) of spine synapses was significantly lower (43.33%) in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. Because the hippocampus is involved in specific mnemonic functions, this observation highlights the importance of hormone replacement therapy in postmenopausal conditions.

Hormonal regulation of hippocampal spine synapse density involves subcortical mediation

It is well established that estrogen has positive effects on the density of pyramidal cell spines in the hippocampal CA1 subfield. This study explored whether afferent connections of the hippocampus that come from estrogen-sensitive subcortical structures, including the septal complex, median raphe and supramammillary area, play a role in this estrogen-induced hippocampal synaptic plasticity. These particular subcortical structures have major influences on hippocampal activity, including theta rhythm and long-term potentiation. The latter also promotes the formation of new synapses. All of the rats were ovariectomized; the fimbria/fornix, which contains the majority of subcortical efferents to the hippocampus, was transected unilaterally in each, and half of the animals received estrogen replacement. Using unbiased electron microscopic stereological methods, the CA1 pyramidal cell spine synapse density was calculated. In the estrogen-treated rats, contralateral to the fimbria/fornix transection, the spine density of CA1 pyramidal cells increased dramatically, compared to the spine density values of both the ipsilateral and contralateral hippocampi of non-estrogen-treated animals and to that of the ipsilateral hippocampus of the estrogen replaced rats. These observations indicate that fimbria/fornix transection itself does not considerably influence CA1 area pyramidal cell spine density and, most importantly, that the estrogenic effect on hippocampal morphology, in addition to directly affecting the hippocampus, involves subcortical mediation.

Supramammillary area mediates subcortical estrogenic action on hippocampal synaptic plasticity

It is well established that systemically administered estrogen to ovariectomized rats positively affects the density of pyramidal cell spines in the hippocampal CA1 subfield and intact subcortical connections of the hippocampus are essential in this hormonal action. This study explored whether local estrogen administration into the supramammillary area influences the density of CA1 pyramidal cell spine synapses in ovariectomized rats. The first group of experiments using a combination of retrograde tracer technique and immunostaining for estrogen receptor-alpha demonstrated that a large population of supramammillary area estrogen receptor-alpha-containing neurons projects to the hippocampus. Animals belonging to the second experimental group were ovariectomized and received cannulae filled with 0.4% 17 beta-estradiol placed unilaterally into the supramammillary area. Control animals received a cholesterol-containing cannula into the supramammillary area or an estrogen-filled cannula implanted into the head of the caudate nucleus. One week later, rats were killed and CA1 pyramidal cell spine synapse density was determined using electron microscopic unbiased stereological procedures. Animals that received an estrogen-filled cannula into the supramammillary area exhibited a significantly higher (37%) density of CA1 pyramidal cell spine synapses than both other control groups. These observations indicate that the supramammillary area is involved in mediating synaptoplastic, estrogenic effects to the hippocampus.

Muscarinic tone sustains impulse flow in the septohippocampal GABA but not cholinergic pathway: implications for learning and memory

Systemic infusions of the muscarinic cholinergic receptor antagonists atropine and scopolamine (atr/scop) produce an amnesic syndrome in humans, subhuman primates, and rodents. In humans, this syndrome may resemble early symptoms of Alzheimer's disease. Behavioral studies in rats have demonstrated that the medial septum/diagonal band of Broca (MSDB), which sends cholinergic and GABAergic projections to the hippocampus, is a critical locus in mediating the amnesic effects of atr/scop. The amnesic effects of atr/scop in the MSDB have been presumed but not proven to be caused by a decrease in hippocampal acetylcholine (ACh) release after blockade of a muscarinic tone in the MSDB. Using electrophysiological recordings and fluorescent-labeling techniques to identify living septohippocampal neurons in rat brain slices, we now report that, contrary to current belief, a blockade of the muscarinic tone in the MSDB does not decrease impulse flow in the septohippocampal cholinergic pathway; instead, it decreases impulse flow in the septohippocampal GABAergic pathway via M(3) muscarinic receptors. We also report that the muscarinic tone in the MSDB is maintained by ACh that is released locally, presumably via axon collaterals of septohippocampal cholinergic neurons. As such, cognitive deficits that occur in various neurodegenerative disorders that are associated with a loss or atrophy of septohippocampal cholinergic neurons cannot be attributed solely to a decrease in hippocampal acetylcholine release. An additional, possibly more important mechanism may be the concomitant decrease in septohippocampal GABA release and a subsequent disruption in disinhibitory mechanisms in the hippocampus. Restoration of impulse flow in the septohippocampal GABA pathway, possibly via M(3) receptor agonists, may, therefore, be critical for successful treatment of cognitive deficits associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]D-aspartate autoradiographic and immunohistochemical study

It is well established that the supramammillary nucleus plays a critical role in hippocampal theta rhythm generation/regulation by its direct and indirect (via the septal complex) connections to the hippocampus. Previous morphological and electrophysiological studies indicate that both the supramammillo-hippocampal and supramammillo-septal efferents contain excitatory transmitter. To test the validity of this assumption, transmitter specific retrograde tracer experiments were performed. [3H]D-aspartate was injected into different locations of the hippocampus (granular and supragranular layers of the dentate gyrus and CA2 and CA3a areas of the Ammon's horn) and septal complex (medial septum and the area between the medial and lateral septum) that are known targets of the supramammillary projection. Consecutive vibratome sections prepared from the entire length of the posterior hypothalamus, including the supramammillary area, were immunostained for calretinin, tyrosine hydroxylase, or calbindin, and further processed for autoradiography. Radiolabeled, radiolabeled plus calretinin-containing, and calretinin-immunoreactive neurons were plotted at six different oro-caudal levels of the supramammillary area. The results demonstrated that following both hippocampal and septal injection of the tracer, the majority of the retrogradely radiolabeled (glutamatergic/aspartatergic) cells are immunoreactive for calretinin. However, non-radiolabeled calretinin-containing neurons and radiolabeled calretinin-immunonegative cells were also seen, albeit at a much lower density. These observations clearly indicate the presence of glutamatergic/aspartatergic projections to both the hippocampus and septal complex. It may be assumed that this transmitter could play a role in hippocampal theta rhythm generation/regulation.

Cholinergic excitation of septohippocampal GABA but not cholinergic neurons: implications for learning and memory

The medial septum/diagonal band (MSDB), which gives rise to the septohippocampal pathway, is a critical locus for the mnemonic effects of muscarinic drugs. Infusion of muscarinic cholinergic agonists into the MSDB enhance learning and memory processes both in young and aged rats and produce a continuous theta rhythm in the hippocampus. Intraseptal muscarinic agonists also alleviate the amnesic syndrome produced by systemic administration of muscarinic receptor antagonists. It has been presumed, but not proven, that the cellular mechanisms underlying the effects of muscarinic agonists in the MSDB involve an excitation of septohippocampal cholinergic neurons and a subsequent increase in acetylcholine (ACh) release in the hippocampus. Using a novel fluorescent labeling technique to selectively visualize live septohippocampal cholinergic neurons in rat brain slices, we have found that muscarinic agonists do not excite septohippocampal cholinergic neurons, instead they inhibit a subpopulation of cholinergic neurons. In contrast, unlabeled neurons, confirmed to be noncholinergic, septohippocampal GABA-type neurons using retrograde marking and double-labeling techniques, are profoundly excited by muscarine. Thus, the cognition-enhancing effects of muscarinic drugs in the MSDB cannot be attributed to an increase in hippocampal ACh release. Instead, disinhibitory mechanisms, caused by increased impulse flow in the septohippocampal GABAergic pathway, may underlie the cognition-enhancing effects of muscarinic agonists.

Opioids suppress IPSCs in neurons of the rat medial septum/diagonal band of Broca: involvement of mu-opioid receptors and septohippocampal GABAergic neurons

The medial septum/diagonal band region (MSDB), which provides a major cholinergic and GABAergic input to the hippocampus, expresses a high density of opioid receptors. Behaviorally, intraseptal injections of opioids produce deficits in spatial memory, however, little is known about the electrophysiological effects of opioids on MSDB neurons. Therefore, we investigated the electrophysiological effects of opioids on neurons of the MSDB using rat brain slices. In voltage-clamp recordings with patch electrodes, bath-applied met-enkephalin, a nonselective opioid receptor agonist, decreased the number of tetrodotoxin and bicuculline-sensitive inhibitory synaptic currents in cholinergic- and GABA-type MSDB neurons. A similar effect occurred in brain slices containing only the MSDB, suggesting that opioids decrease GABA release primarily by inhibiting spontaneously firing GABAergic neurons located within the MSDB. Accordingly, in extracellular recordings, opioid-sensitive, spontaneously firing neurons could be found within the MSDB. Additionally, in intracellular recordings a subpopulation of GABA-type neurons were directly inhibited by opioids. All effects of met-enkephalin were mimicked by a mu receptor agonist, but not by delta or kappa agonists. In antidromic activation studies, mu-opioids inhibited a subpopulation of septohippocampal neurons with high conduction velocity fibers, suggestive of thickly myelinated GABAergic fibers. Consistent with the electrophysiological findings, in double-immunolabeling studies, 20% of parvalbumin-containing septohippocampal GABA neurons colocalized the mu receptor, which at the ultrastructural level, was found to be associated with the neuronal cell membrane. Thus, opioids, via mu receptors, inhibit a subpopulation of MSDB GABAergic neurons that not only make local connections with both cholinergic and noncholinergic-type MSDB neurons, but also project to the hippocampus.

Estrogen receptor-alpha in the raphe serotonergic and supramammillary area calretinin-containing neurons of the female rat

It is well established that estrogen affects hippocampal long-term potentiation and hippocampus-related memory processes. Furthermore, theta rhythm, in conjunction with long-term potentiation, influences memory and is regulated by subcortical structures, including the median raphe and supramammillary area. To test the validity of the hypothesis that the effects of estrogen on the hippocampus are mediated, at least partly, via these subcortical structures, it must first be determined whether the neurons of the median raphe and supramammillary area contain estrogen receptors. Light and electron microscopic double immunostaining for estrogen receptor-alpha plus serotonin and estrogen receptor-alpha plus calretinin on vibratome sections of the median raphe and supramammillary area, respectively, demonstrated that large populations of the median raphe serotonin and supramammillary area calretinin neurons exhibit estrogen receptor-immunoreactive nuclei. These observations indicate that circulating gonadal hormones can affect hippocampal electric activity indirectly, via those subcortical structures that are involved in theta rhythm regulation.

A novel method for concurrent visualization of immunostain under light and electron microscopy in pancreatic islets

We developed a simple method employing the use of flat-embedding techniques on thick frozen sections which allows correlation of light and electron microscopic immunohistochemistry. This method has been particularly useful in visualization of pancreas sections, an adaptation especially important because this tissue is not amenable to conventional vibratome sectioning. In this study we demonstrate the use of this technique to examine the same tissue section at the light and the electron microscopic level while maintaining morphology.

Extrinsic and intrinsic substance P innervation of the rat lateral septal area calbindin cells

The electrophysiological observations that substance P administration to the lateral septal area elicits both excitatory and inhibitory responses, together with earlier reports on the multiple sources of substance P innervation of the septum, implies that these axons with distinct origins have different functions. This prompted us to examine the origin and neurochemical character of substance P afferents to the lateral septal area. Chronic surgical isolation of the septum from its ventral afferents and retrograde tracer experiments using wheat germ agglutinin-conjugated horseradish peroxidase, both followed by an immunostaining for substance P, were employed to elucidate the origin of these axon terminals. In order to assess the possible co-existence of substance P with other neurotransmitter substances in the parent cells of the septopetal projections, co-localization studies for substance P and choline acetyltransferase, as well as substance P and GABA, were performed. The comparative distribution of substance P fibers and septal calbindin-containing neurons was also investigated using correlated light and electron microscopic double immunostaining. The results are summarized as follows: (i) the substance P innervation of the lateral septal area derives from several hypothalamic nuclei (including the lateral and lateroanterior hypothalamic area, tuber cinereum and ventromedial hypothalamic nucleus) and tegmental nuclei (the majority of fibers from the laterodorsal and a few from the pedunculopontine tegmental nucleus), as well as intrinsic septal cells; (ii) the septopetal substance P fibers of tegmental origin are cholinergic; intraseptal substance P neurons located in the dorsolateral part of the lateral septum also contain GABA, while substance P neurons seen on the border between the medial and lateral septal area and septopetal hypothalamic substance P cells do not contain GABA or acetylcholine; (iii) substance P fibers from pericellular baskets around calbindin-containing lateral septal neurons with a high degree of selectivity; (iv) approximately 90% of the entire calbindin cell population are postsynaptic targets of substance P axons; (v) their terminals contact the soma and the dendrites of these cells, among them the somatospiny neurons; and (vi) the extrinsic substance P boutons establish asymmetric, while the intrinsic substance P axon terminals form symmetric membrane specializations. Because neurons in the lateral septal area receive hippocampal input and project massively to hypothalamic areas, the different types of substance P input on these neurons can modify the information flow arriving from the hippocampus to diencephalic brain structures at the level of the lateral septal area.

Hypothalamic Leu-enkephalin-immunoreactive fibers terminate on calbindin-containing somatospiny cells in the lateral septal area of the rat

Correlated light and electron microscopic double-immunostaining experiments for Leu-enkephalin and calbindin were employed to determine the postsynaptic targets in the septal complex of Leu-enkephalin fibers. Chronic surgical isolation of the septal complex from its hypothalamic afferents and retrograde tracer studies using wheat germ agglutinin-conjugated horseradish peroxidase, both followed by an immunostaining for Leu-enkephalin, were performed to elucidate the location of the origin of these axon terminals. Furthermore, a colocalization study for glutamic acid decarboxylase and Leu-enkephalin was carried out on hypothalamic sections to determine their possible coexistence in cells projecting to the lateral septum. These studies revealed that 1) Leu-enkephalin-immunoreactive axons form pericellular baskets around a population of lateral septal area neurons; 2) they establish exclusively asymmetric synaptic contacts on their soma and initial dendritic segments; 3) 10% of the lateral septal area calbindin-containing cells, which are all of the gamma-aminobutyric acid (GABA)-ergic somatospiny type, are innervated by Leu-enkephalin-immunoreactive baskets; 4) only 40% of the Leu-enkephalin target neurons are calbindin immunopositive; 5) the septopetal Leu-enkephalin fibers derive from neurons located in the ipsilateral perifornical area and anterior hypothalamus; and 6) none of their cells of origin cocontains the inhibitory transmitter GABA. These observations indicate that hypothalamic Leu-enkephalin-containing neurons are non-GABAergic excitatory cells. Hence, they can effectively stimulate a population of lateral septal area neurons, including the somatospiny cells, which are all GABAergic. Therefore, after stimulatory Leu-enkephalin action, these neurons can inhibit their postsynaptic targets, including other projective lateral septal neurons.

Neuropeptide-Y innervation of estrogen-induced progesterone receptor-containing dopamine cells in the monkey hypothalamus: a triple labeling light and electron microscopic study

Light and electron microscopic triple immunostaining was performed on coronal vibratome sections prepared from the hypothalamus of ovariectomized (OVX) and OVX plus estrogen-treated African green monkeys (Cercopithecus aethiops). Immunoreactivity for progesterone receptors (PRs) and neuropeptide-Y (NPY) was visualized by a dark blue to black nickel diaminobenzidine reaction, while the tyrosine hydroxylase-containing perikarya were labeled with a light brown diaminobenzidine reaction. In the OVX plus estrogen-treated material, 30% of the tyrosine hydroxylase-immunoreactive neurons contained PR-immunopositive nuclei. The majority of these cells were found in the central portion of the periventricular area, and a few could be observed in the anterior hypothalamus and the arcuate and dorsomedial hypothalamic nuclei. These tyrosine hydroxylase-immunoreactive PR-containing cells were surrounded with NPY-immunoreactive axon terminals. A correlated electron microscopic analysis of the same sections revealed synaptic contacts between these NPY-immunoreactive boutons and the PR-containing tyrosine hydroxylase-immunoreactive neurons. In contrast, in the OVX animals, no PR-containing tyrosine hydroxylase-immunoreactive neurons could be detected. In these monkeys, the frequency of synaptic contacts between the NPY-immunoreactive axon terminals and tyrosine hydroxylase-immunopositive cells was similar to that in the OVX plus estrogen-treated monkeys. These observations indicate that in a population of hypothalamic dopamine cells, the presence of nuclear PRs is estrogen dependent, show that these cells are innervated by NPY axons, and suggest that these estrogen-induced PR-containing dopamine neurons are involved in mediation of the effect of NPY on hypophyseal hormone secretion, including ovarian steroid hormone-dependent LH and PRL release.