Sex chromosome and gonadal hormone contributions to binge-like and aversion-resistant ethanol drinking behaviors in Four Core Genotypes mice

Introduction

While substantial research has focused on the contribution of sex hormones to driving elevated levels of alcohol drinking in female rodents, fewer studies have investigated how genetic influences may underlie sex differences in this behavior.

Methods

We used the Four Core Genotypes (FCG) mouse model to explore the contribution of sex chromosome complement (XX/XY) and gonad type [ovaries (Sry-)/testes (Sry+)] to ethanol (EtOH) consumption and quinine-resistant drinking across two voluntary self-administration tasks: limited access consumption in the home cage and an operant response task.

Results

For limited access drinking in the dark, XY/Sry + (vs. XX/Sry +) mice consumed more 15% EtOH across sessions while preference for 15% EtOH vs. water was higher in XY vs. XX mice regardless of gonad type. XY chromosomes promoted quinine-resistant drinking in mice with ovaries (Sry-) and the estrous cycle did not affect the results. In the operant response task, responding for EtOH was concentration dependent in all genotypes except XX/Sry + mice, which maintained consistent response levels across all concentrations (5-20%) of EtOH. When increasing concentrations of quinine (100-500 μM) were added to the solution, FCG mice were insensitive to quinine-punished EtOH responding, regardless of sex chromosome complement. Sry + mice were further found to be insensitive to quinine when presented in water. Importantly, these effects were not influenced by sensitivity to EtOH's sedative effect, as no differences were observed in the time to lose the righting reflex or the time to regain the righting reflex between genotypes. Additionally, no differences in EtOH concentration in the blood were observed between any of the genotypes once the righting reflex was regained.

Discussion

These results provide evidence that sex chromosome complement regulates EtOH consumption, preference, and aversion resistance and add to a growing body of literature suggesting that chromosomal sex may be an important contributor to alcohol drinking behaviors. Examination of sex-specific genetic differences may uncover promising new therapeutic targets for high-risk drinking.

Effects of chemotherapy on operant responding for palatable food in male and female mice

Patients treated with cancer chemotherapeutics frequently report chemotherapy-induced peripheral neuropathy (CIPN), changes in mood (depression and anxiety) and functional impairments. Rodent models of CIPN elicit limited alterations in functional behaviors, which pose challenges in developing preclinical models of chemotherapy-induced behavioral depression. The study examined the consequences of chemotherapy-induced mechanical hypersensitivity (paclitaxel: 32 or 64 mg/kg, cumulative; oxaliplatin: 30 mg/kg, cumulative) on behavioral depression, as measured with operant responding for palatable food during periods of food restriction and ad libitum chow, consumption of noncontingently available palatable food in the presence of ad libitum chow, and voluntary wheel running. The study employed two inbred mouse strains (C57BL/6J and Balb/cJ) and examined potential sex differences. All chemotherapeutic regimens caused profound mechanical hypersensitivity for the duration of the observation periods (up to 7 months), but no treatments changed voluntary wheel running or consumption of noncontingent palatable food. The high dose of paclitaxel temporarily reduced operant responding for palatable food in male C57BL/6J mice undergoing food restriction or maintained on ad libitum chow. However, paclitaxel failed to decrease operant responding for palatable food in free-feeding female C57BL/6J mice or Balb/cJ mice of either sex. Moreover, oxaliplatin did not significantly alter operant responding for palatable food in male or female C57BL/6J mice maintained on ad libitum chow. These findings demonstrate a dissociation between chemotherapy-induced mechanical hypersensitivity and behavioral depression. The transient effects of paclitaxel on operant responding in male C57BL/6J mice may represent a fleeting behavioral correlate of chemotherapy-associated pain-like behaviors.

Unexpected loss of sensitivity to the nicotinic acetylcholine receptor antagonist activity of mecamylamine and dihydro-β-erythroidine in nicotine-tolerant mice

OBJECTIVES

There is a long-standing interest in developing nicotinic acetylcholine receptor (nAChR) antagonists for concomitant use with nAChR agonists (e.g., nicotine replacement) as complementary smoking cessation aids. Previous studies demonstrate that daily nicotine treatment confers tolerance to some effects of nicotine, as well as cross-tolerance to other nAChR agonists. The current study assessed the extent to which antagonism of nicotine varies as a function of daily nicotine treatment.

METHODS

Schedule-controlled responding and hypothermia were selected for study because they have been previously used to examine the pharmacology of nicotine, and both are sensitive to the development nicotine tolerance. The rate-decreasing and hypothermic effects of nicotine, as well as antagonism of those effects, were examined in C57BL/6J mice before, during treatment with, and after discontinuation of three daily injections of 1.78 mg/kg nicotine. The nonselective nAChR antagonist mecamylamine and the β2 nAChR antagonist dihydro-β-erythroidine (DHβE) were studied in combination with nicotine.

RESULTS

The ED₅₀ values of nicotine to produce rate-decreasing and hypothermic effects were, respectively, 0.44 and 0.82 mg/kg prior, 1.6 and 3.2 mg/kg during, and 0.74 and 1.1 mg/kg after discontinuation of daily nicotine treatment. Prior to daily nicotine treatment, mecamylamine decreased response rate and rectal temperature. However, during daily nicotine, mecamylamine (up to 5.6 mg/kg) only decreased rectal temperature. DHβE (up to 5.6 mg/kg) when studied prior to daily nicotine decreased rectal temperature, but that decrease was abolished during chronic nicotine treatment. Mecamylamine and DHβE antagonized the rate-decreasing and hypothermic effects of nicotine before and after daily nicotine; however, during daily nicotine, mecamylamine and DHβE antagonized only the hypothermic effects of nicotine.

CONCLUSIONS

The differential antagonism of rate-decreasing and hypothermic effects implicates differential involvement of nAChR subtypes. The decreased capacity of mecamylamine and DHβE to antagonize nicotine during chronic nicotine treatment may indicate that their effectiveness as smoking cessations might vary as a function of nicotine tolerance and dependence.

Glucagon-Like Peptide-1 (GLP-1) and 5-Hydroxytryptamine 2c (5-HT2c) Receptor Agonists in the Ventral Tegmental Area (VTA) Inhibit Ghrelin-Stimulated Appetitive Reward

Current literature indicates that the orexigenic peptide ghrelin increases appetitive motivation via signaling in the mesolimbic reward system. Another gastric peptide, glucagon-like peptide-1 (GLP-1), and the neurotransmitter 5-hydroxytryptamine (5-HT), are both known to suppress operant responding for food by acting on key mesolimbic nuclei, including the ventral tegmental area (VTA). In order to investigate the interaction effects of ghrelin, GLP-1, and 5-HT within the VTA, we measured operant responding for sucrose pellets after the administration of ghrelin, the GLP-1 receptor agonist exendin-4 (Ex-4), and the 5-HT_2c receptor agonist Ro60-0175 in male Sprague-Dawley rats. Following training on a progressive ratio 3 (PR3) schedule, animals were first injected with ghrelin into the VTA at doses of 3 to 300 pmol. In subsequent testing, separate rats were administered intraperitoneal (IP) Ex-4 (0.1–1.0 µg/kg) or VTA Ex-4 (0.01–0.1 µg) paired with 300 pmol ghrelin. In a final group of rats, the 5-HT_2c agonist Ro60-0175 was injected IP (0.25–1.0 mg/kg) or into the VTA (1.5–3.0 µg), and under both conditions paired with 300 pmol ghrelin delivered into the VTA. Our results indicated that ghrelin administration increased operant responding for food reward and that this effect was attenuated by IP and VTA Ex-4 pretreatment as well as pre-administration of IP or VTA Ro60-0175. These data provide compelling evidence that mesolimbic GLP-1 and serotonergic circuitry interact with the ghrelinergic system to suppress ghrelin’s effects on the mediation of food reinforcement.

The galanin receptor-3 antagonist, SNAP 37889, inhibits cue-induced reinstatement of alcohol-seeking and increases c-Fos expression in the nucleus accumbens shell of alcohol-preferring rats

INTRODUCTION

This study aimed to investigate the effects of the galanin-3 receptor antagonist, SNAP 37889, on c-Fos protein expression after cue-induced reinstatement of alcohol-seeking in the brains of alcohol-preferring rats.

METHODS

Eighteen alcohol-preferring rats were trained to self-administer 10% v/v ethanol in the presence of response-contingent cues, which was followed by extinction. Rats were then treated with SNAP 37889 (30 mg/kg, i.p.) or vehicle, before being tested for cue-induced reinstatement. Administration of SNAP 37889 reduced cue-induced reinstatement of ethanol-seeking behaviour. To examine the effect of SNAP 37889 and cue-induced reinstatement on neuronal activation, c-Fos expression was measured in subregions of the medial prefrontal cortex and nucleus accumbens.

RESULTS

SNAP 37889 administration increased c-Fos immunoreactivity in the nucleus accumbens shell, but was without effect in the nucleus accumbens core and the medial prefrontal cortex. Dual-label Fos/tyrosine hydroxylase immunohistochemistry was used to examine the effects of SNAP 37889 on dopamine neurons in the ventral tegmental area; however, no differences between SNAP 37889 and vehicle-treated rats were found.

CONCLUSIONS

These data support previous findings of galanin-3 receptor involvement in cue-induced reinstatement of alcohol-seeking behaviour, and provide novel evidence that the ability of galanin-3 receptor antagonism to attenuate cue-induced reinstatement relates to activation of the nucleus accumbens shell.

Effect of Interpersonal and Cognitive Stressors on Habituation and the Utility of Heart Rate Variability to Measure Habituation

Habituation is a decrease in responding to a repeated stimulus. Operant responding and salivation measure habituation in eating behaviour research. Stress may increase eating by acting as a distractor, yielding spontaneous recovery and prolonging responding for food. Our research tested differences in the ability of cognitive and interpersonal stressors to recover responding for food. We also tested heart rate variability (HRV) as a measure of habituation. Twenty women worked for portions of macaroni and cheese for 15 trials on three separate laboratory visits. Between the 12th and 13th trial, one of three different stressor types (speech, stroop and subtraction) was presented during each visit. HRV was measured continuously throughout the laboratory visits. Responding for food declined across the 12 trials with no difference in rate of habituation by visit (p > 0.8) There was no difference between stressor type in the magnitude of spontaneous recovery after each stressor (p > 0.8). Rates of habituation of HRV variables correlated (p < 0.02) with the rate of operant responding habituation. Cognitive and interpersonal stressors do not differ in their ability to recover reduced responding for food. HRV variables may measure habituation to food similar to operant responding. Copyright © 2014 John Wiley & Sons, Ltd.

Deep brain stimulation in the central nucleus of the amygdala decreases 'wanting' and 'liking' of food rewards

We investigated the potential of deep brain stimulation (DBS) in the central nucleus of the amygdala (CeA) in rats to modulate functional reward mechanisms. The CeA is the major output of the amygdala with direct connections to the hypothalamus and gustatory brainstem, and indirect connections with the nucleus accumbens. Further, the CeA has been shown to be involved in learning, emotional integration, reward processing, and regulation of feeding. We hypothesized that DBS, which is used to treat movement disorders and other brain dysfunctions, might block reward motivation. In rats performing a lever-pressing task to obtain sugar pellet rewards, we stimulated the CeA and control structures, and compared stimulation parameters. During CeA stimulation, animals stopped working for rewards and rejected freely available rewards. Taste reactivity testing during DBS exposed aversive reactions to normally liked sucrose tastes and even more aversive taste reactions to normally disliked quinine tastes. Interestingly, given the opportunity, animals implanted in the CeA would self-stimulate with 500 ms trains of stimulation at the same frequency and current parameters as continuous stimulation that would stop reward acquisition. Neural recordings during DBS showed that CeA neurons were still active and uncovered inhibitory-excitatory patterns after each stimulus pulse indicating possible entrainment of the neural firing with DBS. In summary, DBS modulation of CeA may effectively usurp normal neural activity patterns to create an 'information lesion' that not only decreased motivational 'wanting' of food rewards, but also blocked 'liking' of rewards.

A new gustometer for taste testing in rodents

In recent years, to circumvent the interpretive limitations associated with intake tests commonly used to assess taste function in rodents, investigators have developed devices called gustometers to deliver small volumes of taste samples and measure immediate responses, thereby increasing confidence that the behavior of the animal is under orosensory control. Most of these gustometers can be used to measure unconditioned licking behavior to stimuli presented for short durations and/or can be used to train the animal to respond to various fluid stimuli differentially so as to obtain a reward and/or avoid punishment. Psychometric sensitivity and discrimination functions can thus be derived. Here, we describe a new gustometer design, successfully used in behavioral experiments, that was guided by our experience with an older version used for over 2 decades. The new computer-controlled gustometer features no dead space in stimulus delivery lines, effective cleaning of the licking substrate, and the ability to measure licking without passing electrical current through the animal. The parts and dimensions are detailed, and the benefits and limitations of certain design features are discussed. Schematics for key circuits are provided as supplemental information. Accordingly, it should be possible to fabricate this device in a fashion customized for one's needs.

GOAT induced ghrelin acylation regulates hedonic feeding

Ghrelin is an orexigenic hormone that regulates homeostatic and reward-related feeding behavior. Recent evidence indicates that acylation of ghrelin by the gut enzyme ghrelin O-acyl transferase (GOAT) is necessary to render ghrelin maximally active within its target tissues. Here we tested the hypothesis that GOAT activity modulates food motivation and food hedonics using behavioral pharmacology and mutant mice deficient for GOAT and the ghrelin receptor (GHSR). We evaluated operant responding following pharmacological administration of acyl-ghrelin and assessed the necessity of endogenous GOAT activity for operant responding in GOAT and GHSR-null mice. Hedonic-based feeding behavior also was examined in GOAT-KO and GHSR-null mice using a "Dessert Effect" protocol in which the intake of a palatable high fat diet "dessert" was assessed in calorically-sated mice. Pharmacological administration of acyl-ghrelin augmented operant responding; notably, this effect was dependent on intact GHSR signaling. GOAT-KO mice displayed attenuated operant responding and decreased hedonic feeding relative to controls. These behavioral results correlated with decreased expression of the orexin-1 receptor in reward-related brain regions in GOAT-KO mice. In summary, the ability of ghrelin to stimulate food motivation is dependent on intact GHSR signaling and modified by endogenous GOAT activity. Furthermore, GOAT activity is required for hedonic feeding behavior, an effect potentially mediated by forebrain orexin signaling. These data highlight the significance of the GOAT-ghrelin system for the mediation of food motivation and hedonic feeding.

Leptin regulates energy balance and motivation through action at distinct neural circuits

BACKGROUND

Overconsumption of calorically dense foods contributes substantially to the current obesity epidemic. The adiposity hormone leptin has been identified as a potential modulator of reward-induced feeding. The current study asked whether leptin signaling within the lateral hypothalamus (LH) and midbrain is involved in effort-based responding for food rewards and/or the modulation of mesolimbic dopamine.

METHODS

The contribution of endogenous leptin signaling for food motivation and mesolimbic dopamine tone was examined after viral-mediated reduction of the leptin receptor within LH and midbrain neurons in male rats.

RESULTS

Knockdown of leptin receptors selectively in the LH caused increased body weight, caloric consumption, and body fat in rats maintained on a calorically dense diet. Knockdown of leptin receptors selectively in midbrain augmented progressive ratio responding for sucrose and restored high-fat, diet-induced suppression of dopamine content in the nucleus accumbens.

CONCLUSIONS

In summary, endogenous leptin signaling in the hypothalamus restrains the overconsumption of calorically dense foods and the consequent increase in body mass, whereas leptin action in the midbrain regulates effort-based responding for food rewards and mesolimbic dopamine tone. These data highlight the ability of leptin to regulate overconsumption of palatable foods and food motivation through pathways that mediate energy homeostasis and reward, respectively.

Overweight children habituate slower than non-overweight children to food

We have shown that physiological and behavioral responses habituate to food stimuli and recover when novel stimuli are presented. In addition, physiological responses in obese adults habituate slower to repeated food stimuli than those in non-obese individuals, which is related to greater energy intake. The purpose of this study was to test the hypothesis that instrumental responding in overweight children habituates slower to food cues than in their non-overweight peers. Children were provided the opportunity to work for access to cheeseburger for 10 2-min trials, followed by French fries for 3 2-min trials. Results showed that children who had a body mass index (BMI) at or above the 85th BMI percentile (at risk for overweight; n=17) habituated slower than those with a BMI percentile less than the 85th BMI percentile (non-overweight; n=17). Response recovery to French fries did not differ between groups. Overweight children consumed significantly more grams of food and more energy than non-overweight children. When taken together, these data show that habituation may be an important individual difference characteristic that differentiates overweight from non-overweight children. Implications of this for prevention and treatment of obesity are discussed.

Rostral-caudal differences in the effects of intra-VTA muscimol on cocaine self-administration

We have found that dopamine (DA) in the ventral tegmental area (VTA) plays an important role in cocaine self-administration. DA in the VTA acts at D1-type receptors on the terminals of GABA afferents causing release of this neurotransmitter. Thus, the neurochemical pathways whereby VTA DA might be involved in cocaine self-administration may include GABA neurotransmission. In the present study, we investigated this possibility. Rats were prepared with intravenous catheters and bilateral guide cannulae positioned to allow microinjections directly into the VTA or a site 1 mm dorsal to it. The rats were then trained to self-administer cocaine (1.0 mg/kg/injection) under a fixed-ratio 1 schedule of reinforcement and tested with microinjections of muscimol (0, 0.05 and 0.1 microg/0.25 microl) or picrotoxin (0, 0.025 and 0.05 microg/0.25 microl) or trained under a progressive ratio (PR) schedule and tested with vehicle and 0.05 microg/0.25 microl muscimol. Muscimol in the VTA, but not immediately dorsal to it, significantly reduced cocaine intake under the FR1 schedule. Furthermore, when analyzed by rostral/caudal site of injection, it was found that rostral injections of muscimol significantly reduced cocaine self-administration whereas caudal injections produced non-significant decreases in self-administration. Inspection of individual records revealed no signs of non-specific behavioral effects of the muscimol treatments. Muscimol in the rostral VTA also significantly increased break points in responding under the PR schedule. Intra-VTA picrotoxin did not significantly affect cocaine self-administration. These data suggest that stimulation of GABA-A receptors in the VTA is involved in cocaine self-administration and reward and that this involvement is more pronounced in the rostral than in the caudal VTA.

Appetitive instrumental learning requires coincident activation of NMDA and dopamine D1 receptors within the medial prefrontal cortex

Through its complex role in cognition, memory, and emotion, the mammalian prefrontal cortex is thought to contribute to the organization of adaptive behavioral actions. In the present studies we examined the role of dopaminergic D1 and glutamatergic NMDA receptors within the prefrontal cortex of the rat during the development of adaptive instrumental learning. Hungry rats with bilateral indwelling cannulas aimed at the medial prefrontal cortex were trained to lever-press for food. Infusion of the selective D1 antagonist SCH-23390 (0.15, 0.3, 3.0 nmol) dose-dependently impaired acquisition of this behavior. Higher doses also impaired expression of this task. Co-infusion of the lowest dose of SCH 23390 with a low dose of the NMDA antagonist AP-5 (0.5 nmol), each of which had no effect on learning when infused alone, potently reduced the ability to acquire the response. Inhibition of intracellular protein kinase A with the selective PKA inhibitor Rp-cAMPS also disrupted acquisition, suggesting that PKA is an intracellular substrate for a D1-NMDA receptor interaction. In control experiments, drug infusions that impaired learning did not affect food intake or locomotion, suggesting a specific effect on learning. We hypothesize that coincident detection of D1-NMDA receptor activation and its transcriptional consequences, within multiple sites of a distributed corticostriatal network, may represent a conserved molecular mechanism for instrumental learning.