Orexigenic response to tail pinch: role of brain NPY(1) and corticotropin releasing factor receptors

Brain regulation of gastric dysfunction induced by stress

Psychological and physical stressors have been implicated in gastric disorders in humans. The mechanism coupling the brain to the stomach underlying stress-induced gastric dysfunction has remained elusive. Here, we show that the stomach directly receives acetylcholinergic inputs from the dorsal motor nucleus of the vagus (AChDMV), which are innervated by serotonergic neurons in the dorsal raphe nucleus (5-HTDRN). Microendoscopic calcium imaging and multi-tetrode electrophysiological recordings reveal that the 5-HTDRN → AChDMV → stomach circuit is inhibited with chronic stress accompanied by hypoactivate gastric function. Artificial activation of this circuit reverses the gastric dysfunction induced by chronic stress in both male and female mice. Our study demonstrates that this 5-HTDRN → AChDMV → stomach axis drives gastric dysfunction associated with stress, thus providing insights into the circuit basis for brain regulation of the stomach.

Binge eating, overeating and food addiction: Approaches for examining food overconsumption in laboratory rodents

Overeating ranges in severity from casual overindulgence to an overwhelming drive to consume certain foods. At its most extreme, overeating can manifest as clinical diagnoses such as binge eating disorder or bulimia nervosa, yet subclinical forms of overeating such as emotional eating or uncontrolled eating can still have a profoundly negative impact on health and wellbeing. Although rodent models cannot possibly capture the full spectrum of disordered overeating, studies in laboratory rodents have substantially progressed our understanding of the neurobiology of overconsumption. These experimental approaches range from simple food-exposure protocols that promote binge-like eating and the development of obesity, to more complex operant procedures designed to examine distinct 'addiction-like' endophenotypes for food. This review provides an overview of these experimental approaches, with the view to providing a comprehensive resource for preclinical investigators seeking to utilize behavioural models for studying the neural systems involved in food overconsumption.

Nociceptive Stimuli Activate the Hypothalamus-Habenula Circuit to Inhibit the Mesolimbic Reward System and Cocaine-Seeking Behaviors

Nociceptive signals interact with various regions of the brain, including those involved in physical sensation, reward, cognition, and emotion. Emerging evidence points to a role of nociception in the modulation of the mesolimbic reward system. The mechanism by which nociception affects dopamine (DA) signaling and reward is unclear. The lateral hypothalamus (LH) and the lateral habenula (LHb) receive somatosensory inputs and are structurally connected with the mesolimbic DA system. Here, we show that the LH-LHb pathway is necessary for nociceptive modulation of this system using male Sprague Dawley rats. Our extracellular single-unit recordings and head-mounted microendoscopic calcium imaging revealed that nociceptive stimulation by tail pinch excited LHb and LH neurons, which was inhibited by chemical lesion of the LH. Tail pinch increased activity of GABA neurons in ventral tegmental area, decreased the extracellular DA level in the nucleus accumbens ventrolateral shell in intact rats, and reduced cocaine-increased DA concentration, which was blocked by disruption of the LH. Furthermore, tail pinch attenuated cocaine-induced locomotor activity, 22 and 50 kHz ultrasonic vocalizations, and reinstatement of cocaine-seeking behavior, which was inhibited by chemogenetic silencing of the LH-LHb pathway. Our findings suggest that nociceptive stimulation recruits the LH-LHb pathway to inhibit mesolimbic DA system and drug reinstatement.SIGNIFICANCE STATEMENT The LHb and the LH have been implicated in processing nociceptive signals and modulating DA release in the mesolimbic DA system. Here, we show that the LH-LHb pathway is critical for nociception-induced modulation of mesolimbic DA release and cocaine reinstatement. Nociceptive stimulation alleviates extracellular DA release in the mesolimbic DA system, cocaine-induced psychomotor activities, and reinstatement of cocaine-seeking behaviors through the LH-LHb pathway. These findings provide novel evidence for sensory modulation of the mesolimbic DA system and drug addiction.

The Antagonism of Corticotropin-Releasing Factor Receptor-1 in Brain Suppress Stress-Induced Propofol Self-Administration in Rats

Propofol addiction has been detected in humans and rats, which may be facilitated by stress. Corticotropin-releasing factor acts through the corticotropin-releasing factor (CRF) receptor-1 (CRF1R) and CRF2 receptor-2 (CRF2R) and is a crucial candidate target for the interaction between stress and drug abuse, but its role on propofol addiction remains unknown. Tail clip stressful stimulation was performed in rats to test the stress on the establishment of the propofol self-administration behavioral model. Thereafter, the rats were pretreated before the testing session at the bilateral lateral ventricle with one of the doses of antalarmin (CRF1R antagonist, 100–500 ng/site), antisauvagine 30 (CRF2R antagonist, 100–500 ng/site), and RU486 (glucocorticoid receptor antagonist, 100–500 ng/site) or vehicle. The dopamine D1 receptor (D1R) in the nucleus accumbens (NAc) was detected to explore the underlying molecular mechanism. The sucrose self-administration establishment and maintenance, and locomotor activities were also examined to determine the specificity. We found that the establishment of propofol self-administration was promoted in the tail clip treated group (the stress group), which was inhibited by antalarmin at the dose of 100–500 ng/site but was not by antisauvagine 30 or RU486. Accordingly, the expression of D1R in the NAc was attenuated by antalarmin, dose-dependently. Moreover, pretreatments fail to change sucrose self-administration behavior or locomotor activities. This study supports the role of CRF1R in the brain in mediating the central reward processing through D1R in the NAc and provided a possibility that CRF1R antagonist may be a new therapeutic approach for the treatment of propofol addiction.

Central somatostatin signaling and regulation of food intake

The discovery of somatostatin (SST) in the hypothalamus implicated the peptide in the inhibition of growth hormone release. However, as observed for numerous neuropeptides, SST was neither restricted to this one brain site nor to this one function. Subsequent studies established a widespread but specific expression of SST in the central nervous system of rodents and humans along with the expression patterns of five receptors (sst_1-5 ). Among biological actions, the activation of central SST signaling induced a robust stimulation of food and water intake, which is mediated by the sst₂ as assessed using selective sst agonists. The past years have witnessed the identification of brain SST circuitries involved using chemogenetic and optogenetic approaches and further established a physiological orexigenic role of brain SST signaling. The present review will discuss these recent findings.

Brain corticotropin-releasing factor signaling: Involvement in acute stress-induced visceral analgesia in male rats

BACKGROUND

Water avoidance stress (WAS) induces a naloxone-independent visceral analgesia in male rats under non-invasive conditions of monitoring. The objective of the study was to examine the role of brain CRF signaling in acute stress-induced visceral analgesia (SIVA).

METHODS

Adult male Sprague-Dawley rats were chronically implanted with an intracerebroventricular (ICV) cannula. The visceromotor response (VMR) to graded phasic colorectal distension (CRD: 10, 20, 40, 60 mm Hg, 20 seconds, 4 minutes intervals) was monitored using manometry. The VMR to a first CRD (baseline) was recorded 5 minutes after an ICV saline injection, followed 1 hour later by ICV injection of either CRF (30, 100, or 300 ng and 1, 3, or 5 μg/rat) or saline and a second CRD, 5 minutes later. Receptor antagonists against CRF₁ /CRF₂ (astressin-B, 30 μg/rat), CRF₂ (astressin₂ -B, 10 μg/rat), oxytocin (tocinoic acid, 20 μg/rat), or vehicle were injected ICV 5 minutes before CRF (300 ng/rat, ICV) or 15 minutes before WAS (1 hour).

KEY RESULTS

ICV CRF (100 and 300 ng) reduced the VMR to CRD at 60 mm Hg by -36.6% ± 6.8% and -48.7% ± 11.7%, respectively, vs baseline (P < 0.001), while other doses had no effect and IP CRF (10 µg/kg) induced visceral hyperalgesia. Astressin-B and tocinoic acid injected ICV induced hyperalgesia and prevented the analgesic effect of ICV CRF (300 ng/rat) and WAS, while astressin₂ -B only blocked WAS-induced SIVA.

CONCLUSIONS & INFERENCES

These data support a role for brain CRF signaling via CRF₂ in SIVA in a model of WAS and CRD likely mediated by the activation of brain oxytocin pathway.

The functional link between tail-pinch-induced food intake and emotionality and its possible role in stress coping in rats

Tail pinch facilitates eating in rats. We investigated an unidentified link between tail-pinch-induced eating behavior and individual emotionality in male Sprague-Dawley rats. Anxiety-like behavior was assessed on the elevated plus maze (EPM) and in the open field test (OFT). Tail-pinch-induced eating was observed as follows: After a 30-min habituation period, the tail pinch was applied for 5 min, followed by a 30-min recovery period. During the habituation and recovery periods, rats were allowed to access food ad libitum. During the recovery period, 14 of 24 rats ate more food than during the habituation period. Thus, we named them "high responders" and the others as "low responders". The food intake was significantly greater, while the times spent in the open arms in the EPM and in the center area in the OFT were significantly shorter in high responders than in low responders. This result suggests that the rats consuming more food after mild stress have higher anxiety.

Gut-Brain Neuroendocrine Signaling Under Conditions of Stress-Focus on Food Intake-Regulatory Mediators

The gut-brain axis represents a bidirectional communication route between the gut and the central nervous system comprised of neuronal as well as humoral signaling. This system plays an important role in the regulation of gastrointestinal as well as homeostatic functions such as hunger and satiety. Recent years also witnessed an increased knowledge on the modulation of this axis under conditions of exogenous or endogenous stressors. The present review will discuss the alterations of neuroendocrine gut-brain signaling under conditions of stress and the respective implications for the regulation of food intake.

Central Neuropeptide Y Plays an Important Role in Mediating the Adaptation Mechanism Against Chronic Stress in Male Rats

Exposure to continuous life stress often causes gastrointestinal (GI) symptoms. Studies have shown that neuropeptide Y (NPY) counteracts the biological actions of corticotrophin-releasing factor (CRF) and is involved in the termination of the stress response. However, in chronic repeated restraint stress (CRS) conditions, the actions of NPY on GI motility remain controversial. To evaluate the role of NPY in mediation of the adaptation mechanism and GI motility in CRS conditions, a CRS rat model was set up. Central CRF and NPY expression levels were analyzed, serum corticosterone and NPY concentrations were measured, and GI motor function was evaluated. The NPY Y1 receptor antagonist BIBP-3226 was centrally administered before stress loading, and on days 1 through 5 of repeated stress, the central CRF and the serum corticosterone concentrations were measured. In addition, gastric and colonic motor functions were evaluated. The elevated central CRF expression and corticosterone concentration caused by acute stress began to fall after 3 days of stress loading, whereas central NPY expression and serum NPY began to increase. GI dysmotility also returned to a normal level. Pretreatment with BIBP-3226 abolished the adaptation mechanism and significantly increased CRF expression and the corticosterone concentration, which resulted in delayed gastric emptying and accelerated fecal pellet output. Inhibited gastric motility and enhanced distal colonic motility were also recorded. CRS-produced adaptation, overexpressed central CRF, and GI dysmotility observed in acute restraint stress were restored to normal levels. Central NPY via the Y1 receptor plays an important role in mediating the adaptation mechanism against chronic stress.

Posterodorsal Medial Amygdala Mediates Tail-Pinch Induced Food Intake in Female Rats

Comfort eating during periods of stress is a common phenomenon observed in both animals and humans. However, the underlying mechanisms of stress-induced food intake remain elusive. The amygdala plays a central role in higher-order emotional processing and the posterodorsal subnucleus of the medial amygdala (MePD), in particular, is involved in food intake. Extra-hypothalamic corticotrophin-releasing factor (CRF) is well recognised for mediating behavioural responses to stress. To explore the possible role of amygdala CRF receptor activation in stress-induced food intake, we evaluated whether a stressor such as tail-pinch, which reliably induces food intake, would fail to do so in animals bearing bilateral neurotoxic lesions of the MePD. Our results showed that ibotenic acid induced lesions of the MePD markedly reduced tail-pinch induced food intake in ovariectomised, 17β-oestradiol replaced rats. In addition, intra-MePD (right side only) administration of CRF (0.002 or 0.02 ng) via chronically implanted cannulae resulted in a dose-dependent increase in food intake, although higher doses of 0.2 and 2 ng CRF had less effect, producing a bell shaped curve. Furthermore, intra-MePD (bilateral) administration of the CRF receptor antagonist, astressin (0.3 μg per side) effectively blocked tail-pinch induced food intake. These data suggest that the MePD is involved in stress-induced food intake and that the amygdala CRF system may be a mediator of comfort eating.

A RAPID Method for Blood Processing to Increase the Yield of Plasma Peptide Levels in Human Blood

Research in the field of food intake regulation is gaining importance. This often includes the measurement of peptides regulating food intake. For the correct determination of a peptide's concentration, it should be stable during blood processing. However, this is not the case for several peptides which are quickly degraded by endogenous peptidases. Recently, we developed a blood processing method employing Reduced temperatures, Acidification, Protease inhibition, Isotopic exogenous controls and Dilution (RAPID) for the use in rats. Here, we have established this technique for the use in humans and investigated recovery, molecular form and circulating concentration of food intake regulatory hormones. The RAPID method significantly improved the recovery for (125)I-labeled somatostatin-28 (+39%), glucagon-like peptide-1 (+35%), acyl ghrelin and glucagon (+32%), insulin and kisspeptin (+29%), nesfatin-1 (+28%), leptin (+21%) and peptide YY3-36 (+19%) compared to standard processing (EDTA blood on ice, p <0.001). High performance liquid chromatography showed the elution of endogenous acyl ghrelin at the expected position after RAPID processing, while after standard processing 62% of acyl ghrelin were degraded resulting in an earlier peak likely representing desacyl ghrelin. After RAPID processing the acyl/desacyl ghrelin ratio in blood of normal weight subjects was 1:3 compared to 1:23 following standard processing (p = 0.03). Also endogenous kisspeptin levels were higher after RAPID compared to standard processing (+99%, p = 0.02). The RAPID blood processing method can be used in humans, yields higher peptide levels and allows for assessment of the correct molecular form.

Neuropeptide Y: A stressful review

Stress is defined as an adverse condition that disturbs the homeostasis of the body and activates adaptation responses. Among the many pathways and mediators involved, neuropeptide Y (NPY) stands out due to its unique stress-relieving, anxiolytic and neuroprotective properties. Stress exposure alters the biosynthesis of NPY in distinct brain regions, the magnitude and direction of this effect varying with the duration and type of stress. NPY is expressed in particular neurons of the brainstem, hypothalamus and limbic system, which explains why NPY has an impact on stress-related changes in emotional-affective behaviour and feeding as well as on stress coping. The biological actions of NPY in mammals are mediated by the Y1, Y2, Y4 and Y5 receptors, Y1 receptor stimulation being anxiolytic whereas Y2 receptor activation is anxiogenic. Emerging evidence attributes NPY a role in stress resilience, the ability to cope with stress. Thus there is a negative correlation between stress-induced behavioural disruption and cerebral NPY expression in animal models of post-traumatic stress disorder. Exogenous NPY prevents the negative consequences of stress, and polymorphisms of the NPY gene are predictive of impaired stress processing and increased risk of neuropsychiatric diseases. Stress is also a factor contributing to, and resulting from, neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's disease, in which NPY appears to play an important neuroprotective role. This review summarizes the evidence for an implication of NPY in stress-related and neurodegenerative pathologies and addresses the cerebral NPY system as a therapeutic target.

Influence of Aging and Gender Differences on Feeding Behavior and Ghrelin-Related Factors during Social Isolation in Mice

Psychological stress due to social isolation is known to cause abnormal feeding behaviors, but the influences of gender and aging on subchronic stress-induced changes in feeding behaviors are unknown. Thus, we examined the changes in body weight, food intake, and orexigenic ghrelin-related factors during 2 weeks of isolation stress in young and aged mice. Food intake increased significantly in young mice in the isolation group compared with the group-housed control throughout the experimental period. This isolation-induced increase in food intake was not observed in aged mice. In young mice, there were no significant differences in body weight between the isolated group and group-housed control up to 2 weeks. However, aged male mice exhibited significant weight loss at 2 weeks and a similar tendency was observed in aged female mice. Young male mice, but not female mice, had significantly increased (2.2-fold) plasma acylated ghrelin levels after 1 week of isolation compared with the group-housed control. A significant but lower increase (1.3-fold) was also observed in aged male mice. Hypothalamic preproghrelin gene expression decreased significantly with isolation in young male mice, whereas it increased significantly in female mice. The expression levels of NPY and AGRP in the hypothalamus, which are transmitted by elevated peripheral ghrelin signals, increased significantly in isolated young male mice, whereas the AGRP expression levels decreased significantly in young female mice. Isolation caused no significant differences in the expression levels of these genes in aged mice. In isolation, young female mice exhibited markedly increased dark- and light-phase locomotor activities compared with male mice, whereas male and female aged mice exhibited no obvious increases in activity immediately after the dark phase started. We conclude that the gender-specific homeostatic regulatory mechanisms required to maintain body weight operated during subchronic psychological stress in young mice but not in aged mice.

The role of brain somatostatin receptor 2 in the regulation of feeding and drinking behavior

Somatostatin was discovered four decades ago as hypothalamic factor inhibiting growth hormone release. Subsequently, somatostatin was found to be widely distributed throughout the brain and to exert pleiotropic actions via interaction with five somatostatin receptors (sst1-5) that are also widely expressed throughout the brain. Interestingly, in contrast to the predominantly inhibitory actions of peripheral somatostatin, the activation of brain sst2 signaling by intracerebroventricular injection of stable somatostatin agonists potently stimulates food intake and independently, drinking behavior in rodents. The orexigenic response involves downstream orexin-1, neuropeptide Y1 and μ receptor signaling while the dipsogenic effect is mediated through the activation of the brain angiotensin 1 receptor. Brain sst2 activation is part of mechanisms underlying the stimulation of feeding and more prominently water intake in the dark phase and is able to counteract the anorexic response to visceral stressors.

Chronic and acute effects of stress on energy balance: are there appropriate animal models?

Stress activates multiple neural and endocrine systems to allow an animal to respond to and survive in a threatening environment. The corticotropin-releasing factor system is a primary initiator of this integrated response, which includes activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. The energetic response to acute stress is determined by the nature and severity of the stressor, but a typical response to an acute stressor is inhibition of food intake, increased heat production, and increased activity with sustained changes in body weight, behavior, and HPA reactivity. The effect of chronic psychological stress is more variable. In humans, chronic stress may cause weight gain in restrained eaters who show increased HPA reactivity to acute stress. This phenotype is difficult to replicate in rodent models where chronic psychological stress is more likely to cause weight loss than weight gain. An exception may be hamsters subjected to repeated bouts of social defeat or foot shock, but the data are limited. Recent reports on the food intake and body composition of subordinate members of group-housed female monkeys indicate that these animals have a similar phenotype to human stress-induced eaters, but there are a limited number of investigators with access to the model. Few stress experiments focus on energy balance, but more information on the phenotype of both humans and animal models during and after exposure to acute or chronic stress may provide novel insight into mechanisms that normally control body weight.

Integrated circuits and molecular components for stress and feeding: implications for eating disorders

Eating disorders are complex brain disorders that afflict millions of individuals worldwide. The etiology of these diseases is not fully understood, but a growing body of literature suggests that stress and anxiety may play a critical role in their development. As our understanding of the genetic and environmental factors that contribute to disease in clinical populations like anorexia nervosa, bulimia nervosa and binge eating disorder continue to grow, neuroscientists are using animal models to understand the neurobiology of stress and feeding. We hypothesize that eating disorder clinical phenotypes may result from stress-induced maladaptive alterations in neural circuits that regulate feeding, and that these circuits can be neurochemically isolated using animal model of eating disorders.