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

Inactivation of the thalamic paraventricular nucleus promotes place preference and sucrose seeking in male rats

RATIONALE

The experience of reward entails both positive affect and motivation. While the brain regions responsible for these distinct aspects of reward are dissociable from each other, the paraventricular nucleus of the thalamus (PVT) may play a role in both.

OBJECTIVES

To investigate the role of the PVT in both affect and motivation, and to identify neuropeptides that might mediate these effects.

METHODS

Male rats were tested for conditioned place preference following temporary inactivation of the anterior or posterior PVT with local injections of the GABA_B and GABA_A agonists, baclofen + muscimol. They were tested for sucrose seeking under a fixed ratio 3 (FR3) schedule of reinforcement and after extinction, following injection into the posterior PVT of baclofen + muscimol or saline vehicle. Finally, quantitative real-time PCR was used to examine local neuropeptide gene expression following injection into the posterior PVT of baclofen + muscimol or saline vehicle.

RESULTS

Conditioned place preference was induced by temporary inactivation of the posterior but not anterior PVT. While sucrose seeking under an FR3 schedule of reinforcement was unaffected by inactivation of the posterior PVT, reinstatement of sucrose seeking was promoted by posterior PVT inactivation. Local gene expression of pituitary adenylate cyclase-activating polypeptide (PACAP), but not enkephalin or neurotensin, was reduced following inactivation of the posterior PVT.

CONCLUSIONS

Temporary inactivation of the posterior PVT affects both affect and motivation as well as local gene expression of PACAP. These results suggest that the posterior PVT is one brain region that may participate in both major aspects of reward.

Blockade of Opiodergic System During Early Weaning Reverts Feeding Behavior Altered Patterns

Adverse experiences that occur during the early stages of life can have permanent repercussions in adulthood. Among these experiences, early weaning is one that can alter the molecular, cellular, and behavior patterns in later life. Centered on this fact, the objective of the current study was to evaluate the effect of early weaning at 15 days of life of Wistar rats on their feeding behavior and if the opioidergic system blockade would cause a reversal of these outcomes. Experimental groups were formed based on the weaning period of each litter. On postnatal day 15, the group D15 was weaned and, on postnatal day 30 (natural weaning), the group D30 was weaned. The rats weaned on postnatal day 15, and administered subcutaneous Naltrexone (3 mg/kg) were from group D15 + NTX. Those weaned at 15 days of age exhibited higher depressive-like behavior, lesser reactivity time to sucrose, and higher intake of palatable food than the control group. The Naltrexone administration was observed to reverse some outcomes, such as increasing the reactivity time to sucrose and decreasing the quantity of palatable food consumed, to levels similar to those of the control group. Together, the findings of the present study are indicative of the vital role played by the opioidergic system in inducing the changes noted in the eating behavior patterns during adulthood, post early weaning.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Functional μ-Opioid-Galanin Receptor Heteromers in the Ventral Tegmental Area

The neuropeptide galanin has been shown to interact with the opioid system. More specifically, galanin counteracts the behavioral effects of the systemic administration of μ-opioid receptor (MOR) agonists. Yet the mechanism responsible for this galanin-opioid interaction has remained elusive. Using biophysical techniques in mammalian transfected cells, we found evidence for selective heteromerization of MOR and the galanin receptor subtype Gal1 (Gal1R). Also in transfected cells, a synthetic peptide selectively disrupted MOR-Gal1R heteromerization as well as specific interactions between MOR and Gal1R ligands: a negative cross talk, by which galanin counteracted MAPK activation induced by the endogenous MOR agonist endomorphin-1, and a cross-antagonism, by which a MOR antagonist counteracted MAPK activation induced by galanin. These specific interactions, which represented biochemical properties of the MOR-Gal1R heteromer, could then be identified in situ in slices of rat ventral tegmental area (VTA) with MAPK activation and two additional cell signaling pathways, AKT and CREB phosphorylation. Furthermore, in vivo microdialysis experiments showed that the disruptive peptide selectively counteracted the ability of galanin to block the dendritic dopamine release in the rat VTA induced by local infusion of endomorphin-1, demonstrating a key role of MOR-Gal1R heteromers localized in the VTA in the direct control of dopamine cell function and their ability to mediate antagonistic interactions between MOR and Gal1R ligands. The results also indicate that MOR-Gal1R heteromers should be viewed as targets for the treatment of opioid use disorders.

SIGNIFICANCE STATEMENT

The μ-opioid receptor (MOR) localized in the ventral tegmental area (VTA) plays a key role in the reinforcing and addictive properties of opioids. With parallel in vitro experiments in mammalian transfected cells and in situ and in vivo experiments in rat VTA, we demonstrate that a significant population of these MORs form functional heteromers with the galanin receptor subtype Gal1 (Gal1R), which modulate the activity of the VTA dopaminergic neurons. The MOR-Gal1R heteromer can explain previous results showing antagonistic galanin-opioid interactions and offers a new therapeutic target for the treatment of opioid use disorder.

Interacting Neural Processes of Feeding, Hyperactivity, Stress, Reward, and the Utility of the Activity-Based Anorexia Model of Anorexia Nervosa

Anorexia nervosa (AN) is a psychiatric illness with minimal effective treatments and a very high rate of mortality. Understanding the neurobiological underpinnings of the disease is imperative for improving outcomes and can be aided by the study of animal models. The activity-based anorexia rodent model (ABA) is the current best parallel for the study of AN. This review describes the basic neurobiology of feeding and hyperactivity seen in both ABA and AN, and compiles the research on the role that stress-response and reward pathways play in modulating the homeostatic drive to eat and to expend energy, which become dysfunctional in ABA and AN.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Neurochemical heterogeneity of rats predicted by different measures to be high ethanol consumers

BACKGROUND

Alcoholism is a heterogeneous disease, with subjects possibly differing both in the best measure that predicts their excess consumption and in their most effective pharmacotherapy. Two different measures, high novelty-induced activity and high-fat-induced triglycerides (TGs), are known to identify subgroups of animals prone to consuming higher amounts of ethanol (EtOH). The question investigated here is whether these subgroups are, in fact, similar in their neurochemical phenotype that may contribute to their overconsumption.

METHODS

EtOH-naïve, Sprague-Dawley rats were subgrouped based on the 2 predictor measures of activity or TG levels, and then quantitative real-time polymerase chain reaction and digoxigenin-labeled in situ hybridization were used to measure their expression of hypothalamic peptides that affect EtOH intake. In additional subgroups subsequently trained to drink 9% EtOH, the opioid antagonist and alcoholism medication, naltrexone, was tested at a low dose (0.02 mg/kg, s.c.) to determine the rats' sensitivity to its effects.

RESULTS

The 2 measures, while both effective in predicting amount of EtOH intake, were found to identify distinctive subgroups. Rats with high compared to low activity exhibited significantly greater expression of galanin and enkephalin in the paraventricular nucleus (PVN) and of orexin in the perifornical lateral hypothalamus (PFLH), but no difference in melanin-concentrating hormone in PFLH or neuropeptide Y in arcuate nucleus. This contrasts with rats having high TG, which exhibited greater expression only of PVN galanin, along with reduced PFLH orexin. The high-activity rats with elevated enkephalin, but not high-TG rats, were also unusually sensitive to naltrexone, which significantly reduced their alcohol intake.

CONCLUSIONS

In addition to revealing differences in endogenous peptides and drug responsiveness in predicted high EtOH drinkers, this study demonstrates that these disturbances differ markedly between the 2 at-risk subgroups. This indicates that simple tests may be effective in identifying subjects most responsive to a specific pharmacotherapy.

The opioid system and food intake: homeostatic and hedonic mechanisms

Opioids are important in reward processes leading to addictive behavior such as self-administration of opioids and other drugs of abuse including nicotine and alcohol. Opioids are also involved in a broadly distributed neural network that regulates eating behavior, affecting both homeostatic and hedonic mechanisms. In this sense, opioids are particularly implicated in the modulation of highly palatable foods, and opioid antagonists attenuate both addictive drug taking and appetite for palatable food. Thus, craving for palatable food could be considered as a form of opioid-related addiction. There are three main families of opioid receptors (µ, ĸ, and δ) of which µ-receptors are most strongly implicated in reward. Administration of selective µ-agonists into the NAcc of rodents induces feeding even in satiated animals, while administration of µ-antagonists reduces food intake. Pharmacological studies also suggest a role for ĸ- and δ-opioid receptors. Preliminary data from transgenic knockout models suggest that mice lacking some of these receptors are resistant to high-fat diet-induced obesity.

Differential activity by polymorphic variants of a remote enhancer that supports galanin expression in the hypothalamus and amygdala: implications for obesity, depression and alcoholism

The expression of the galanin gene (GAL) in the paraventricular nucleus (PVN) and in the amygdala of higher vertebrates suggests the requirement for highly conserved, but unidentified, regulatory sequences that are critical to allow the galanin gene to control alcohol and fat intake and modulate mood. We used comparative genomics to identify a highly conserved sequence that lay 42 kb 5' of the human GAL transcriptional start site that we called GAL5.1. GAL5.1 activated promoter activity in neurones of the PVN, arcuate nucleus and amygdala that also expressed the galanin peptide. Analysis in neuroblastoma cells demonstrated that GAL5.1 acted as an enhancer of promoter activity after PKC activation. GAL5.1 contained two polymorphisms; rs2513280(C/G) and rs2513281(A/G), that occurred in two allelic combinations (GG or CA) where the dominant GG alelle occurred in 70-83 % of the human population. Intriguingly, both SNPs were found to be in LD (R(2) of 0.687) with another SNP (rs2156464) previously associated with major depressive disorder (MDD). Recreation of these alleles in reporter constructs and subsequent magnetofection into primary rat hypothalamic neurones showed that the CA allele was 40 % less active than the GG allele. This is consistent with the hypothesis that the weaker allele may affect food and alcohol preference. The linkage of the SNPs analysed in this study with a SNP previously associated with MDD together with the functioning of GAL5.1 as a PVN and amygdala specific enhancer represent a significant advance in our ability to understand alcoholism, obesity and major depressive disorder.

Galanin and the orexin 2 receptor as possible regulators of enkephalin in the paraventricular nucleus of the hypothalamus: relation to dietary fat

Recent studies show that the non-opioid peptides, galanin (GAL) and orexin (OX), are similar to the opioid enkephalin (ENK) in being stimulated by dietary fat and also in enhancing the consumption of a high-fat diet (HFD). This suggests that, when an HFD is provided, these non-opioids may stimulate the opioid system to promote excess consumption of this diet. Using single- and double-labeling immunohistochemistry, the present study sought to identify possible neuroanatomical substrates for this close relationship. Focusing on the hypothalamic paraventricular nucleus (PVN), and particularly its anterior (aPVN), middle (mPVN) and posterior (pPVN) parts, the experiments examined whether GAL itself or the receptors for GAL and OX are stimulated by an HFD in the same areas and possibly the same neurons as ENK. Compared to animals fed a standard chow diet, rats consuming an HFD exhibited an increased density of medial parvocellular neurons immunoreactive (IR) for GAL in the mPVN and aPVN and for ENK in the mPVN and pPVN, distinguishing the mPVN as an area where both peptides were affected. While showing little evidence for GAL and ENK colocalization with a chow diet, double-labeling studies in HFD-fed rats revealed significant colocalization specifically in medial parvocellular neurons of the mPVN. Immediately posterior to this site, further analyses revealed a similar relationship between the OX 2 receptor (OX(2)R) and ENK in HFD-treated animals. While increasing the density of neurons immunoreactive for OX(2)R as well as for the GAL 1 receptor but not OX 1 receptor, HFD consumption increased the colocalization only of OX(2)R and ENK, specifically in the medial parvocellular neurons of the pPVN. These changes in HFD-fed rats, showing GAL and OX(2)R to colocalize with ENK exclusively in neurons of the medial parvocellular mPVN and pPVN, respectively, suggest possible neural substrates through which the non-opioid peptides may functionally interact with ENK when exposed to an HFD.

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.

Lesions of orexin neurons block conditioned place preference for sexual behavior in male rats

The hypothalamic neuropeptide orexin (hypocretin) mediates reward related to drugs of abuse and food intake. However, a role for orexin in sexual reward has yet to be investigated. Orexin neurons are activated by sexual behavior, but endogenous orexin does not appear to be essential for sexual performance and motivation in male rats. Therefore, the goal of the current study was to test the hypothesis that orexin is critically involved in processing of sexual reward in male rats. First, it was demonstrated following exposure to conditioned contextual cues associated with sexual behavior in a conditioned place preference paradigm that cFos expression is induced in orexin neurons, indicating activation of orexin neurons by cues predicting sexual reward. Next, orexin-cell specific lesions were utilized to determine the functional role of orexin in sexual reward processing. Hypothalami of adult male rats were infused with orexin-B-conjugated saporin, resulting in greater than 80% loss of orexin neurons in the perifornical-dorsomedial and lateral hypothalamus. Orexin lesions did not affect expression of sexual behavior, but prevented formation of conditioned place preference for a sexual behavior paired chamber. In contrast, intact sham-treated males or males with partial lesions developed a conditioned place preference for mating. Orexin lesioned males maintained the ability to form a conditioned place aversion to lithium chloride-induced visceral illness, indicating that orexin lesions did not disrupt associative contextual memory. Overall, these findings suggest that orexin is not essential for sexual performance or motivation, but is critical for reward processing and conditioned cue-induced seeking of sexual behavior.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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.

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.