Differential role of D1 and D2 receptors in the perifornical lateral hypothalamus in controlling ethanol drinking and food intake: possible interaction with local orexin neurons.

BACKGROUND: The neurotransmitter dopamine (DA), acting in various mesolimbic brain regions, is well known for its role in promoting motivated behaviors, including ethanol (EtOH) drinking. Indirect evidence, however, suggests that DA in the perifornical lateral hypothalamus (PF/LH) has differential effects on EtOH consumption, depending on whether it acts on the DA 1 (D1) or DA 2 (D2) receptor subtype, and that these effects are mediated in part by local peptide systems, such as orexin/hypocretin (OX) and melanin-concentrating hormone (MCH), known to stimulate the consumption of EtOH. METHODS: The present study in brain-cannulated Sprague-Dawley rats measured the effects of dopaminergic compounds in the PF/LH on drinking behavior in animals trained to consume 7% EtOH and also on local peptide mRNA expression using digoxigenin-labeled in situ hybridization in EtOH-naïve animals. RESULTS: Experiments 1 and 2 showed that the D1 agonist SKF81297 (10.8 nmol/side) in the PF/LH significantly increased food intake, while tending to increase EtOH intake, and the D1 antagonist SCH23390 significantly decreased EtOH intake without affecting food intake. In contrast, the D2 agonist quinelorane (6.2 nmol/side) in the PF/LH significantly reduced EtOH consumption, while the D2 antagonist sulpiride increased it. Experiments 3 and 4 revealed differential effects of PF/LH injection of the DA agonists on local OX mRNA, which was increased by the D1 agonist and decreased by the D2 agonist. These DA agonists had no impact on MCH expression. CONCLUSIONS: These results support a stimulatory role of the PF/LH D1 receptor in promoting the consumption of both EtOH and food, in contrast to a suppressive effect of the D2 receptor on EtOH drinking. They further suggest that these receptors affect consumption, in part, through local OX-expressing neurons. These findings provide new evidence for the function of PF/LH DA receptor subtypes in controlling EtOH and food intake.

Computational Analysis of Multidimensional Behavioral Alterations After Chronic Social Defeat Stress.

BACKGROUND: The study of depression in humans depends on animal models that attempt to mimic specific features of the human syndrome. Most studies focus on one or a few behavioral domains, with time and practical considerations prohibiting a comprehensive evaluation. Although machine learning has enabled unbiased analysis of behavior in animals, this has not yet been applied to animal models of psychiatric disease. METHODS: We performed chronic social defeat stress (CSDS) in mice and evaluated behavior with PsychoGenics' SmartCube, a high-throughput unbiased automated phenotyping platform that collects >2000 behavioral features based on machine learning. We evaluated group differences at several times post-CSDS and after administration of the antidepressant medication imipramine. RESULTS: SmartCube analysis after CSDS successfully separated control and defeated-susceptible mice, and defeated-resilient mice more resembled control mice. We observed a potentiation of CSDS effects over time. Treatment of susceptible mice with imipramine induced a 40.2% recovery of the defeated-susceptible phenotype as assessed by SmartCube. CONCLUSIONS: High-throughput analysis can simultaneously evaluate multiple behavioral alterations in an animal model for the study of depression, which provides a more unbiased and holistic approach to evaluating group differences after CSDS and perhaps can be applied to other mouse models of psychiatric disease.

Differential patterns of alcohol consumption and dopamine-2 receptor binding in Wistar-Kyoto and Wistar rats.

The Wistar-Kyoto (WKY) rat strain has been described as an animal model of depressive behavior that consumes significantly greater amounts of alcohol compared to the Wistar (WIS) rat strain. Since the mesolimbic dopamine (DA) type-2 (D2) receptors mediate reward-related behaviors, the present study measured the binding of [(125)I]-Iodosulpiride to D2 receptors in the brains of WKY versus WIS rats following 24 days of voluntary alcohol or water consumption. Alcohol consuming WKY rats showed a significant increase in D2 receptor binding in several regions of the mesolimbic and nigrostriatal systems. In contrast, alcohol consuming WIS rats showed a reduction in D2 receptor binding in DA cell body areas. The differential regulation of D2 receptors by voluntary alcohol consumption in the two rat strains suggests that D2 receptor mediated neurotransmission may be playing a role in the increased alcohol drinking behavior reported in WKY rats.

Differential effects of acute and chronic ethanol exposure on orexin expression in the perifornical lateral hypothalamus.

BACKGROUND: Recent reports support the involvement of hypothalamic orexigenic peptides in stimulating ethanol intake. Our previous studies have examined the effects of ethanol on hypothalamic peptide systems of the paraventricular nucleus (PVN) and identified a positive feedback loop in which PVN peptides, such as enkephalin and galanin, stimulate ethanol intake and ethanol, in turn, stimulates the expression of these peptides. Recently, orexin (OX), a peptide produced mainly by cells in the perifornical lateral hypothalamus (PFLH), has been shown to play an important role in mediating the rewarding aspects of ethanol intake. However, there is little evidence showing the effects that ethanol itself may have on the OX peptide system. In order to understand the feedback relationship between ethanol and the OX system, the current investigation was designed to measure OX gene expression in the PFLH following acute as well as chronic ethanol intake. METHODS: In the first experiment, Sprague-Dawley rats were trained to voluntarily consume a 2 or 9% concentration of ethanol, and the expression of OX mRNA in the PFLH was measured using quantitative real-time polymerase chain reaction (qRT-PCR). The second set of experiments tested the impact of acute oral gavage of 0.75 and 2.5 g/kg ethanol solution on OX expression in the PFLH using qRT-PCR, as well as radiolabeled in situ hybridization. Further tests using digoxigenin-labeled in situ hybridization and immunofluorescence histochemistry allowed us to more clearly distinguish the effects of acute ethanol on OX cells in the lateral hypothalamic (LH) versus perifornical (PF) regions. RESULTS: The results showed chronic consumption of ethanol versus water to dose-dependently reduce OX mRNA in the PFLH, with a larger effect observed in rats consuming 2.5 g/kg/d (-70%) or 1.0 g/kg/d (-50%) compared to animals consuming 0.75 g/kg/d (-40%). In contrast to chronic intake, acute oral ethanol compared to water significantly enhanced OX expression in the PFLH, and this effect occurred at the lower (0.75 g/kg) but not higher (2.5 g/kg) dose of ethanol. Additional analyses of the OX cells in the LH versus PF regions identified the former as the primary site of ethanol's stimulatory effect on the OX system. In the LH but not the PF, acute ethanol increased the density of OX-expressing and OX-immunoreactive neurons. The increase in gene expression was detected only at the lower dose of ethanol (0.75 g/kg), whereas the increase in OX peptide was seen only at the higher dose of ethanol (2.5 g/kg). CONCLUSION: These results lead us to propose that OX neurons, while responsive to negative feedback signals from chronic ethanol consumption, are stimulated by acute ethanol administration, most potently in the LH where OX may trigger central reward mechanisms that promote further ethanol consumption.

Role of melanin-concentrating hormone in drug use disorders.

Melanin-concentrating hormone (MCH) is a neuropeptide primarily transcribed in the lateral hypothalamus (LH), with vast projections to many areas throughout the central nervous system that play an important role in motivated behaviors and drug use. Anatomical, pharmacological and genetic studies implicate MCH in mediating the intake and reinforcement of commonly abused substances, acting by influencing several systems including the mesolimbic dopaminergic system, glutamatergic as well as GABAergic signaling and being modulated by inflammatory neuroimmune pathways. Further support for the role of MCH in controlling behavior related to drug use will be discussed as it relates to cerebral ventricular volume transmission and intracellular molecules including cocaine- and amphetamine-regulated transcript peptide, dopamine- and cAMP-regulated phosphoprotein 32 kDa. The primary goal of this review is to introduce and summarize current literature surrounding the role of MCH in mediating the intake and reinforcement of commonly abused drugs, such as alcohol, cocaine, amphetamine, nicotine and opiates.

Complementary roles of orexin and melanin-concentrating hormone in feeding behavior.

Transcribed within the lateral hypothalamus, the neuropeptides orexin/hypocretin (OX) and melanin-concentrating hormone (MCH) both promote palatable food intake and are stimulated by palatable food. While these two neuropeptides share this similar positive relationship with food, recent evidence suggests that this occurs through different albeit complementary effects on behavior, with OX promoting food seeking and motivation for palatable food and MCH functioning during ongoing food intake, reinforcing the consumption of calorically dense foods. Further differences are evident in their effects on physiological processes, which are largely opposite in nature. For example, activation of OX receptors, which is neuronally excitatory, promotes waking, increases energy expenditure, and enhances limbic dopamine levels and reward. In contrast, activation of MCH receptors, which is neuronally inhibitory, promotes paradoxical sleep, enhances energy conservation, reduces limbic dopamine, and increases depressive behavior. This review describes these different effects of the neuropeptides, developing the hypothesis that they stimulate the consumption of palatable food through excessive seeking in the case of OX and through excessive energy conservation in the case of MCH. It proposes that OX initiates food intake and subsequently stimulates MCH which then acts to prolong the consumption of palatable, energy-dense food.

Stimulation of nicotine reward and central cholinergic activity in Sprague-Dawley rats exposed perinatally to a fat-rich diet.

RATIONALE: While clinical studies show maternal consumption of palatable fat-rich diets during pregnancy to negatively impact the children's behaviors and increase their vulnerability to drug abuse, the precise behavioral and neurochemical mechanisms mediating these phenomena have yet to be examined. OBJECTIVE: The study examined in rats whether gestational exposure to a high-fat diet (HFD) can increase the offspring's propensity to use nicotine and whether disturbances in central nicotinic cholinergic signaling accompany this behavioral effect. METHODS: Rat offspring exposed perinatally to a HFD or chow diet were characterized in terms of their nicotine self-administration behavior in a series of operant response experiments and the activity of acetylcholinesterase (AChE) and density of nicotinic ACh receptors (nAChRs) in different brain areas. RESULT: Perinatal HFD compared to chow exposure increased nicotine-self administration behavior during fixed ratio and dose-response testing and caused an increase in breakpoint using progressive ratio testing, while nicotine seeking in response to nicotine prime-induced reinstatement was reduced. This behavioral change induced by the HFD was associated with a significant reduction in activity of AChE in the midbrain, hypothalamus, and striatum and increased density of ß2-nAChRs in the ventral tegmental area and substantia nigra and of α7-nAChRs in the lateral and ventromedial hypothalamus. CONCLUSIONS: Perinatal exposure to a HFD increases the vulnerability of the offspring to excessive nicotine use by enhancing its reward potential, and these behavioral changes are accompanied by a stimulation of nicotinic cholinergic signaling in mesostriatal and hypothalamic brain areas important for reinforcement and consummatory behavior.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use.

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

Disturbances in behavior and cortical enkephalin gene expression during the anticipation of ethanol in rats characterized as high drinkers.

The process of ethanol anticipation is a particularly important phenomenon that can determine subsequent drug-taking behavior. Recent studies suggest that systems within the medial prefrontal cortex (mPFC), during anticipation, may contribute to the goal-directed seeking of ethanol. The current investigation examined the possibility that the opioid peptide enkephalin (ENK), known to mediate some of the reinforcing properties of ethanol, may function in the mPFC during the anticipation of ethanol access. Using a limited access (3 h/d) paradigm for 10 days with 20% ethanol, Sprague-Dawley rats were first identified either as low drinkers (LD, <1.0 g/kg/3 h) or as high drinkers (HD, >2.0 g/kg/3 h) that exhibited a long-term phenotype of high ethanol consumption and a significant ethanol deprivation effect. During the anticipation period immediately preceding daily ethanol access, the HD rats compared to LD or Control animals with ad libitum ethanol access exhibited increased anticipatory behaviors, including greater exploratory behavior in a novel open field as revealed by significantly more time spent in the rearing position (+53-65%, p < 0.05) and increased number of rears made (+33-44%, p < 0.05) and greater novelty-seeking behavior in a hole-board apparatus revealed by an increase in total (+50-52%, p < 0.05) and novel nose pokes (+45-48%, p < 0.05). In the HD rats, analysis of the mPFC using real-time quantitative PCR showed significantly greater mRNA levels of ENK (p < 0.05) and the mu-opioid receptor (MOR) (p < 0.05), but not delta-opioid receptor (DOR), and this increase in ENK expression was found, using in situ hybridization, to occur specifically in the prelimbic (PrL) subregion of the mPFC. When injected into the PrL during the anticipation period, a MOR agonist but not DOR agonist significantly increased consumption of 20% ethanol (p < 0.05). These findings support the role of ENK, acting through MOR within the PrL to promote the anticipation and excessive consumption of ethanol.

Involvement of cholinergic mechanisms in the behavioral effects of dietary fat consumption.

Clinical reports suggest a positive association between fat consumption and the incidence of hyperactivity, impulsivity and cognitive abnormalities. To investigate possible mechanisms underlying these disturbances under short-term conditions, we examined in Sprague-Dawley rats the influence of 7-day consumption of a high-fat diet (HFD) compared to chow on anxiety, novelty-seeking and exploratory behaviors and also on acetylcholine (ACh) neurotransmission that may mediate these behaviors. The HFD consumption, which elevated circulating fatty acids but produced no change in caloric intake or body weight, stimulated novelty-seeking and exploration in an open field, while reducing anxiety in an elevated plus maze. Using the Ellman assay to measure ACh esterase (AChE) activity that breaks down ACh, the second experiment showed HFD consumption to significantly reduce AChE activity in the frontal cortex, hypothalamus and midbrain. With measurements of [¹²5I]-epibatidine or [¹²5I]-bungarotoxin binding to nicotinic ACh receptors (nAChRs) containing ß2 or α7 subunits, respectively, the results also showed HFD consumption to increase both ß2-nAChR binding in the medial prefrontal cortex and substantia nigra and α7-nAChR binding in the lateral and ventromedial hypothalamus. When treated with an acute dose of the nicotinic antagonist, mecamylamine (0.5 mg/kg, sc), the HFD animals responded with significantly reduced exploratory and novelty-seeking behaviors, whereas the chow-consuming rats exhibited no response. These findings suggest that the exploratory and novelty-seeking behaviors induced by dietary fat may be mediated by enhanced nicotinic cholinergic activity, which is accompanied by increased density of ß2-nAChRs in cortical and midbrain regions associated with impulsivity and locomotor activity and of α7-nAChRs in hypothalamic regions associated with arousal and energy balance.

Regulation of drug and palatable food overconsumption by similar peptide systems.

This review is aimed at understanding some of the common neurochemical, behavioral and physiological determinants of drug and food overconsumption. Much current work has been devoted to determining the similarities between the brain circuits controlling excessive use of addictive drugs and the overconsumption of palatable foods. The brain systems involved likely include peptides of both mesolimbic and hypothalamic origin. Evidence gathered from expression and injection studies suggests that the consumption of drugs, such as ethanol and nicotine, and also of palatable foods rich in fat is stimulated by different orexigenic peptides, such as enkephalin, galanin, orexin, and melaninconcentrating hormone, acting within the hypothalamus or various limbic structures, while another peptide, neuropeptide Y, is closely related to carbohydrate consumption and shows an inverse relationship with ethanol and nicotine consumption. Moreover, studies in animal models suggest that a propensity to overconsume these reinforcing substances may result from preexisting disturbances in these same peptide systems. These neurochemical disturbances, in turn, may also be closely linked to specific behaviors associated with excessive consummatory behavior, such as hyperactivity or novelty-seeking, palatable food preference, and also fluctuations in circulating lipid levels. Clear understanding of the relationship between these various determinants of consummatory behavior will allow researchers to effectively predict and examine at early stages of exposure animals that are prone to drug and food overconsumption. This work may ultimately aid in the identification of inherent traits that increase the risk for drug abuse and palatable food overconsumption.

Similarities in hypothalamic and mesocorticolimbic circuits regulating the overconsumption of food and alcohol.

Historically, studies of food intake regulation started with the hypothalamus and gradually expanded to mesocorticolimbic regions, while studies of drug use began with mesocorticolimbic regions and now include the hypothalamus. As research on ingestive behavior has progressed, it has uncovered more and more similarities between the regulation of palatable food and drug intake. It has also identified specific neurochemicals involved in palatable food and drug intake. Hypothalamic orexigenic neurochemicals specifically involved in controlling fat ingestion, including galanin, enkephalin, orexin and melanin-concentrating hormone, show positive feedback with this macronutrient, with these peptides both increasing fat intake and being further stimulated by its intake. This positive relationship offers some explanation for why foods high in fat are so often overconsumed. Research in Bart Hoebel's laboratory in conjunction with our own has shown that consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by these neurochemical systems involved in fat intake, consistent with evidence closely relating fat and ethanol consumption. Both fat and ethanol intake are also regulated by dopamine and acetylcholine acting in mesocorticolimbic nuclei. This close relationship of fat and ethanol is likely driven in part by circulating lipids, which are increased by fat and ethanol intake, known to increase expression and levels of the neurochemicals, and found to promote further intake of fat and ethanol. Compellingly, recent studies suggest that these systems may already be dysregulated in animals prone to consuming excess fat or ethanol, even before they have ever been exposed to these substances. Further understanding of these systems involved in consummatory behavior will allow researchers to develop effective therapies for the treatment of overeating as well as drug abuse.

Galanin and consummatory behavior: special relationship with dietary fat, alcohol and circulating lipids.

Galanin (GAL) plays an integral role in consummatory behavior. In particular, hypothalamic GAL has a positive, reciprocal relationship with dietary fat and alcohol. In this relationship, GAL increases the consumption of fat or alcohol which, in turn, stimulates the expression of GAL, ultimately leading to overconsumption. Through actions in the amygdala, this relationship may become especially important in stress-induced food or drug intake. These effects of GAL in promoting overconsumption may involve various neurotransmitters, with GAL facilitating intake by stimulating norepinephrine and dopamine and reducing satiety by decreasing serotonin and acetylcholine. In addition, GAL in the hypothalamus stimulates the opioid, enkephalin, throughout the brain, which also promotes overconsumption. The relationship between GAL, fat, and alcohol may involve triglycerides, circulating lipids that are released by fat or alcohol and that correlate positively with hypothalamic GAL expression. In females, levels of endogenous GAL also fluctuate across the reproductive cycle, driven by a rise in the ovarian steroids, estrogen, and progesterone. They peak during the proestrous phase and also at puberty, simultaneous to a sharp increase in preference for fat to meet energy demands. Prenatal exposure to a high-fat diet also enhances hypothalamic expression of GAL into adulthood because of an increase in neurogenesis and proliferation of GAL-expressing neurons in this region. This organizational change may reflect the role of GAL in neuronal development, including neurite growth in adulthood, cell survival in aging, and cell stability in the disease state. By responding positively to fat and alcohol and guiding further neuronal development, GAL potentiates a long-term propensity to overconsume fat and alcohol.

Increased orexin and melanin-concentrating hormone expression in the perifornical lateral hypothalamus of rats prone to overconsuming a fat-rich diet.

The goal of this study is to examine the expression pattern of orexigenic peptides, orexin (OX) and melanin-concentrating hormone (MCH), in the perifornical lateral hypothalamus (PFLH) in subpopulations of Sprague-Dawley rats differing in their propensity to overconsume a high-fat diet. Immediately after an initial 5-day screening test that predicts long-term consumption, rats identified as high-fat consumers (HFC), ingesting 35% more calories of a high-fat relative to low-fat chow diet, had significantly elevated mRNA expression of OX in the perifornical but not lateral hypothalamic area and of MCH mRNA in both areas, when compared to control rats that consume similar amounts of these diets. This same OX and MCH expression pattern was seen in HFC rats maintained for two weeks on a low-fat chow diet, indicating that increased expression of these orexigenic peptides, occurring independently of the high-fat diet, may be an inherent characteristic of these rats. These HFC rats were also more active and slightly more anxious than controls, as measured by line crossings and time spent in the periphery or middle segments of an open field. Together, these results demonstrate that animals prone to overeating a high-fat diet show a baseline increase in orexigenic peptide expression in the PFLH along with higher behavioral arousal, which together may contribute to their increased consummatory behavior.