A role for the neuropeptide somatostatin in the neurobiology of behaviors associated with substances abuse and affective disorders

In recent years, neuropeptides which display potent regulatory control of stress-related behaviors have been extensively demonstrated to play a critical role in regulating behaviors associated with substance abuse and affective disorders. Somatostatin (SST) is one neuropeptide known to significantly contribute to emotionality and stress behaviors. However, the role of SST in regulating behavior has received relatively little attention relative to other stress-involved peptides, such as neuropeptide Y or corticotrophin releasing factor. This review characterizes our current understanding of the role of SST and SST-expressing cells in general in modulating several behaviors intrinsically linked to substance abuse and affective disorders, specifically: anxiety and fear; stress and depression; feeding and drinking; and circadian rhythms. We further summarize evidence of a direct role for the SST system, and specifically somatostatin receptors 2 and 4, in substance abuse disorders. This article is part of the special issue on 'Neuropeptides'

Somatostatin signaling modulates binge drinking behavior via the central nucleus of the amygdala

Somatostatin (SST) is a neuropeptide widely expressed in the central nervous system with dense expression in limbic regions such as the extended amygdala. It has recently gained attention for playing a role in modulating alcohol use disorders and co-morbid neuropsychiatric disorders. However, the role of SST in the central nucleus of the amygdala (CeA), a key region for neuropeptide regulation of alcohol and anxiety related behaviors, in alcohol consumption has not been assessed. In this work we perform an initial examination of the interaction between the CeA SST system and binge ethanol intake. Binge intake is a dangerous pattern of excessive ethanol consumption associated with health complications and the transition into alcohol dependence. We use the Drinking in the Dark (DID) model of binge intake in C57BL/6J male and female mice to examine: 1) the impact of 3 DID cycles on CeA SST expression; 2) the effect of intra-CeA SST injection on binge-like ethanol consumption; and 3) if the SST receptor 2 or 4 (SST2R or SST4R) mediate any effect on consumption. Our results show binge-like ethanol intake decreases SST expression in the CeA, but not neighboring basolateral amygdala. We further found intra-SST CeA administration reduces binge ethanol intake. This decrease was replicated by the administration of an SST4R agonist. These effects were not sex-dependent. Overall, this work lends further support for SST playing a role in alcohol related behaviors and as a potential therapeutic target

Commentary: Studies on Binge-Like Ethanol Drinking May Help to Identify the Neurobiological Mechanisms Underlying the Transition to Dependence

The goals of this commentary are to discuss the important contributions of the work by Kaur et al. titled “Corticotropin releasing factor acting on corticotropin releasing factor receptor type 1 is critical for binge alcohol drinking in mice”, published in this issue of Alcoholism: Clinical and Experimental Research, and to highlight the importance of pre-clinical research aimed at identifying the neurobiology of binge ethanol drinking. The work by Kaur et al. provides an important extension of previous pharmacological evidence implicating corticotropin releasing factor (CRF) type-1 receptors (CRF1R) in binge-like ethanol drinking by verifying the role of the CRF1R using genetic tools, and by establishing that CRF, but not urocortin 1 (Ucn1), is the primary neuropeptide associated with the CRF system that modulates binge-like ethanol drinking in C57BL/6J mice. It is suggested that the evidence for a critical role of the CRF1R in excessive ethanol intake observed in both models of binge-like ethanol drinking and dependence-like ethanol intake indicates that overlapping mechanisms may be involved, and that studies that employ models of binge-like ethanol drinking may provide insight into the neurobiological mechanisms that underlie the transition to ethanol dependence

"Drinking in the Dark" (DID): a simple mouse model of binge-like alcohol intake

One of the greatest challenges that scientists face when studying the neurobiology and/or genetics of alcohol (ethanol) consumption is that most preclinical animal models do not voluntarily consume enough ethanol to achieve pharmacologically meaningful blood ethanol concentrations (BECs). Recent rodent models have been developed that promote binge-like levels of ethanol consumption associated with high BECs (i.e., ≥100 mg/dl). This unit describes procedures for an animal model of binge-like ethanol drinking which has come to be called "drinking in the dark" (DID). The "basic" variation of DID involves replacing the water bottle with a bottle containing 20% ethanol for 2 to 4 hr, beginning 3 hr into the dark cycle, on cages of singly-housed C57BL/6J mice. Using this procedure, mice typically consume enough ethanol to achieve BECs >100 mg/dl and to exhibit behavioral evidence of intoxication. An alternative two-bottle (ethanol and water) procedure is also described

“Drinking in the dark” (DID) procedures: A model of binge-like ethanol drinking in non-dependent mice

This review provides an overview of an animal model of binge-like ethanol drinking that has come to be called “drinking in the dark” (DID), a procedure that promotes high levels of ethanol drinking and pharmacologically relevant blood ethanol concentrations (BECs) in ethanol-preferring strains of mice. Originally described by Rhodes et al. (2005), the most common variation of the DID procedure, using singly housed mice, involves replacing the water bottle with a bottle containing 20% ethanol for 2 to 4 hours, beginning 3 hours into the dark cycle. Using this procedure, high ethanol drinking strains of mice (e.g., C57BL/6J) typically consume enough ethanol to achieve BECs greater than 100 mg/dL and to exhibit behavioral evidence of intoxication. This limited access procedure takes advantage of the time in the animal’s dark cycle in which the levels of ingestive behaviors are high, yet high ethanol intake does not appear to stem from caloric need. Mice have the choice of drinking or avoiding the ethanol solution, eliminating the stressful conditions that are inherent in other models of binge-like ethanol exposure in which ethanol is administered by the experimenter, and in some cases, potentially painful. The DID procedure is a high throughput approach that does not require extensive training or the inclusion of sweet compounds to motivate high levels of ethanol intake. The high throughput nature of the DID procedure makes it useful for rapid screening of pharmacological targets that are protective against binge-like drinking and for identifying strains of mice that exhibit binge-like drinking behavior. Additionally, the simplicity of DID procedures allows for easy integration into other paradigms, such as prenatal ethanol exposure and adolescent ethanol drinking. It is suggested that the DID model is a useful tool for studying the neurobiology and genetics underlying binge-like ethanol drinking, and may be useful for studying the transition to ethanol dependence

Corticotropin Releasing Factor Type 1 and 2 Receptor Signaling in the Medial Prefrontal Cortex Modulates Binge-Like Ethanol Consumption in C57BL/6J Mice

Corticotropin releasing factor (CRF) signaling via limbic CRF1 and 2 receptors (CRF1R and CRF2R, respectively) is known to modulate binge-like ethanol consumption in rodents. Though CRF signaling in the medial prefrontal cortex (mPFC) has been shown to modulate anxiety-like behavior and ethanol seeking, its role in binge ethanol intake is unknown. Here, we used "drinking-in-the-dark" (DID) procedures in male and female C57BL/6J mice to address this gap in the literature. First, the role of CRF1R and CRF2R signaling in the mPFC on ethanol consumption was evaluated through site-directed pharmacology. Next, we evaluated if CRF1R antagonist reduction of binge-intake was modulated in part through CRF2R activation by co-administration of a CRF1R and CRF2R antagonist. Intra-mPFC inhibition of CRF1R and activation of CRF2R resulted in decreased binge-like ethanol intake. Further, the inhibitory effect of the CRF1R antagonist was attenuated by co-administration of a CRF2R antagonist. We provide novel evidence that (1) inhibition of CRF1R or activation of CRF2R in the mPFC reduces binge-like ethanol intake; and (2) the effect of CRF1R antagonism may be mediated via enhanced CRF2R activation. These observations provide the first direct behavioral pharmacological evidence that CRF receptor activity in the mPFC modulates binge-like ethanol consumption

Pre-Clinical Evidence that Corticotropin-Releasing Factor (CRF) Receptor Antagonists are Promising Targets for Pharmacological Treatment of Alcoholism

Alcoholism is a chronic disorder characterized by cycling periods of excessive ethanol consumption, withdrawal, abstinence and relapse, which is associated with progressive changes in central corticotropin-releasing factor (CRF) receptor signaling. CRF and urocortin (Ucn) peptides act by binding to the CRF type 1 (CRF1R) or the CRF type 2 (CRF2R) receptors, both of which have been implicated in the regulation of neurobiological responses to ethanol. The current review provides a comprehensive overview of preclinical evidence from studies involving rodents that when viewed together, suggest a promising role for CRF receptor (CRFR) antagonists in the treatment of alcohol abuse disorders. CRFR antagonists have been shown to protect against excessive ethanol intake resulting from ethanol dependence without influencing ethanol intake in non-dependent animals. Similarly, CRFR antagonists block excessive binge-like ethanol drinking in non-dependent mice but do not alter ethanol intake in mice drinking moderate amounts of ethanol. CRFR antagonists protect against increased ethanol intake and relapse-like behaviors precipitated by exposure to a stressful event. Additionally, CRFR antagonists attenuate the negative emotional responses associated with ethanol withdrawal. The protective effects of CRFR antagonists are modulated by the CRF1R. Finally, recent evidence has emerged suggesting that CRF2R agonists may also be useful for treating alcohol abuse disorders

The neurobiology of binge-like ethanol drinking: Evidence from rodent models

Binge alcohol (ethanol) drinking is a destructive pattern of ethanol consumption that may precipitate ethanol dependence, a chronic, debilitating, and prevalent health problem. While an abundance of research has focused on the neurochemical underpinnings of ethanol dependence, relatively little is known about the mechanisms underlying the heavy consumption characteristic of binge ethanol drinking. Recently, a simple preclinical model termed “drinking in the dark” (DID) was developed to examine binge-like ethanol consumption in a rodent population. This assay capitalizes on the predisposition of C57BL/6J mice to voluntarily consume substantial quantities of a high concentration (20% v/v) ethanol solution, resulting in pharmacologically relevant blood ethanol concentrations (BECs). This review provides a comprehensive overview of recent literature utilizing this model to investigate the neuromodulatory systems that may influence binge ethanol drinking. Studies examining the glutamatergic and opioidergic systems not only provide evidence for these systems in the modulation of binge-like ethanol consumption, but also suggest this preclinical model has predictive validity and may be an appropriate tool for screening novel pharmacological compounds aimed at treating binge ethanol drinking in the human population. Additionally, this review presents evidence for the involvement of the GABAergic, dopaminergic, nicotinic, and endocannabinoid systems in modulating binge-like ethanol consumption. Finally, recent evidence shows that corticotropin-releasing factor (CRF), agouti-related protein (AgRP), neuropeptide Y (NPY), and ghrelin are also implicated as impacting this pattern of ethanol consumption

Neuropeptide Y conjugated to saporin alters anxiety-like behavior when injected into the central nucleus of the amygdala or basomedial hypothalamus in BALB/cJ mice

Neuropeptide Y (NPY) is a 36-amino-acid neuromodulator that is distributed throughout the central nervous system and has been implicated in a wide range of neurobiological responses including the integration of emotional behavior. The anxiolytic properties of NPY are modulated by NPY signaling in the hippocampus and in the central (CeA) and basolateral (BLA) nuclei of the amygdala. Recently, the neurotoxin saporin, when conjugated to NPY (NPY-SAP), was shown to selectively kill NPY receptor-expressing neurons and has been used as a tool to study the central NPY neurocircuitry involved with feeding behaviors. Here we determined if NPY-SAP can be used as a tool to study the central NPY neurocircuitry that modulates anxiety-like behaviors. BALB/cJ mice were given injection of either NPY-SAP or a control blank saproin (B-SAP) into the CeA or the basomedial hypothalamus (BMH) as a control injection site. The elevated zero maze test was used to assess anxiety-like behavior and NPY-SAP-induced lesions were verified using NPY Y1 receptor (Y1R) immunoreactivity (IR). Results showed that injection of NPY-SAP into the CeA site-specifically blunted Y1R IR in the CeA which was associated with a significant increase in anxiety-like behavior. Injection of NPY-SAP into the BMH, while locally blunting Y1R IR, promoted a compensatory increase of Y1R IR in the BLA and the CA3 region of the hippocampus which was associated with a significant reduction of anxiety-like behavior. The present set of experiments suggest that the NPY-SAP neurotoxin may be a useful tool for studying the NPY neurocircuitry that modulates anxiety-like behaviors

Basolateral amygdala neuropeptide Y system modulates binge ethanol consumption

Neuropeptide Y (NPY) signaling regulation of corticolimbic communication is known to modulate binge-like ethanol consumption in rodents. In this work we sought to assess the impact of intra-BLA NPY system modulation on binge-like ethanol intake and to assess the role of the NPY1R+ projection from the BLA to the mPFC in this behavior. We used “drinking-in-the-dark” (DID) procedures in C57BL6J mice to address these questions. First, the impact of intra-BLA administration of NPY on binge-like ethanol intake was assessed. Next, the impact of repeated cycles of DID intake on NPY1R expression in the BLA was assessed with use of immunohistochemistry (IHC). Finally, chemogenetic inhibition of BLA→mPFC NPY1R+ projections was assessed to determine if limbic communication with the mPFC was specifically involved in binge-like ethanol intake. Importantly, as both the BLA and NPY system are sexually dimorphic, both sexes were assessed in these studies. Intra-BLA NPY dose-dependently decreased binge-like ethanol intake in males only. Repeated DID reduced NPY1R expression in the BLA of both sexes. Silencing of BLA→mPFC NPY1R+ neurons significantly reduced binge-like ethanol intake in both sexes in a dose-dependent manner. We provide novel evidence that (1) intra-BLA NPY reduces binge-like ethanol intake in males; (2) binge-like ethanol intake reduces NPY1R levels in the BLA; and (3) chemogenetic inhibition of BLA→mPFC NPY1R+ neurons blunts binge-like drinking in male and female mice. These observations provide the first direct evidence that NPY signaling in the BLA, and specifically BLA communication with the mPFC, modulates binge-like ethanol consumption