Forebrain-Midbrain Circuits and Peptides Involved in Hyperalgesia After Chronic Alcohol Exposure

A role for circuitry of the cortical amygdala in excessive alcohol drinking, withdrawal, and alcohol use disorder

Alcohol use disorder (AUD) poses a significant public health challenge. Individuals with AUD engage in chronic and excessive alcohol consumption, leading to cycles of intoxication, withdrawal, and craving behaviors. This review explores the involvement of the cortical amygdala (CoA), a cortical brain region that has primarily been examined in relation to olfactory behavior, in the expression of alcohol dependence and excessive alcohol drinking. While extensive research has identified the involvement of numerous brain regions in AUD, the CoA has emerged as a relatively understudied yet promising candidate for future study. The CoA plays a vital role in rewarding and aversive signaling and olfactory-related behaviors and has recently been shown to be involved in alcohol-dependent drinking in mice. The CoA projects directly to brain regions that are critically important for AUD, such as the central amygdala, bed nucleus of the stria terminalis, and basolateral amygdala. These projections may convey key modulatory signaling that drives excessive alcohol drinking in alcohol-dependent subjects. This review summarizes existing knowledge on the structure and connectivity of the CoA and its potential involvement in AUD. Understanding the contribution of this region to excessive drinking behavior could offer novel insights into the etiology of AUD and potential therapeutic targets.

Alcohol use and the pain system

The World Health Organization's epidemiological data from 2016 revealed that while 57% of the global population aged 15 years or older had abstained from drinking alcohol in the previous year, more than half of the population in the Americas, Europe, and Western Pacific consumed alcohol. The spectrum of alcohol use behavior is broad: low-risk use (sensible and in moderation), at-risk use (e.g., binge drinking), harmful use (misuse) and dependence (alcoholism; addiction; alcohol use disorder). The at-risk use and misuse of alcohol is associated with the transition to dependence, as well as many damaging health outcomes and preventable causes of premature death. Recent conceptualizations of alcohol dependence posit that the subjective experience of pain may be a significant contributing factor in the transition across the spectrum of alcohol use behavior. This narrative review summarizes the effects of alcohol at all levels of the pain system. The pain system includes nociceptors as sensory indicators of potentially dangerous stimuli and tissue damage (nociception), spinal circuits mediating defensive reflexes, and most importantly, the supraspinal circuits mediating nocifensive behaviors and the perception of pain. Although the functional importance of pain is to protect from injury and further or future damage, chronic pain may emerge despite the recovery from, and absence of, biological damage (i.e., in the absence of nociception). Like other biological perceptual systems, pain is a construction contingent on sensory information and a history of individual experiences (i.e., learning and memory). Neuroadaptations and brain plasticity underlying learning and memory and other basic physiological functions can also result in pathological conditions such as chronic pain and addiction. Moreover, the negative affective/emotional aspect of pain perception provides embodied and motivational components that may play a substantial role in the transition from alcohol use to dependence.

Alcohol withdrawal and pain: Peripheral mechanisms join central circuits

Negative affective aspects of alcohol withdrawal and pain involve converging brain circuits. In this issue of Neuron, Son et al.1 identify a peripheral mechanism of an alcohol-withdrawal-induced headache-like condition, which is centered on mast-cell-specific receptor MrgprB2 activated by corticotropin-releasing factor (CRF) in dura mater to drive nociception.

Adolescent ethanol drinking promotes hyperalgesia, neuroinflammation and serotonergic deficits in mice that persist into adulthood

Adolescent alcohol use can permanently alter brain function and lead to poor health outcomes in adulthood. Emerging evidence suggests that alcohol use can predispose individuals to pain disorders or exacerbate existing pain conditions, but the underlying neural mechanisms are currently unknown. Here we report that mice exposed to adolescent intermittent access to ethanol (AIE) exhibit increased pain sensitivity and depressive-like behaviors that persist for several weeks after alcohol cessation and are accompanied by elevated CD68 expression in microglia and reduced numbers of serotonin (5-HT)-expressing neurons in the dorsal raphe nucleus (DRN). 5-HT expression was also reduced in the thalamus, anterior cingulate cortex (ACC) and amygdala as well as the lumbar dorsal horn of the spinal cord. We further demonstrate that chronic minocycline administration after AIE alleviated hyperalgesia and social deficits, while chemogenetic activation of microglia in the DRN of ethanol-naïve mice reproduced the effects of AIE on pain and social behavior. Chemogenetic activation of microglia also reduced tryptophan hydroxylase 2 (Tph2) expression and was negatively correlated with the number of 5-HT-immunoreactive cells in the DRN. Taken together, these results indicate that microglial activation in the DRN may be a primary driver of pain, negative affect, and 5-HT depletion after AIE.

2-Arachidonoylglycerol-mediated endocannabinoid signaling modulates mechanical hypersensitivity associated with alcohol withdrawal in mice

BACKGROUND

Alcohol use disorder (AUD) commonly occurs in patients with chronic pain, and a major barrier to achieving abstinence and preventing relapse is the emergence of hyperalgesia during alcohol withdrawal. Elucidating novel therapeutic approaches to target hyperalgesia associated with alcohol withdrawal could have important implications for treating AUD. Here, we examined the role of 2-arachidonoylglycerol (2-AG)-mediated endocannabinoid (eCB) signaling in the regulation of hyperalgesia associated with alcohol withdrawal in mice. We tested the hypothesis that pharmacological augmentation of 2-AG signaling could reduce hyperalgesia during withdrawal.

METHODS

Male and female C57BL/6J mice were tested during withdrawal from a continuous access two-bottle choice (2BC) paradigm to investigate how eCB signaling modulates mechanical and thermal sensitivity during withdrawal. Mice were pretreated with the monoacylglycerol lipase (MAGL) inhibitor JZL184 to elevate levels of 2-AG. Rimonabant or AM630 were given to block CB1 and CB2 receptor activity, respectively. DO34 was given to reduce 2-AG by inhibiting the 2-AG synthetic enzyme diacylglycerol lipase (DAGL).

RESULTS

After 72 h of withdrawal, male and female mice exhibited increased mechanical, but not thermal, hypersensitivity, which normalized by 7 days. This effect was reversed by pretreatment with JZL184. The effects of JZL184 were prevented by coadministration of either the CB1 or the CB2 antagonist. DO34, Rimonabant, and AM630 exacerbated mechanical hypersensitivity during alcohol withdrawal, causing an earlier onset and persistent hypersensitivity even 1 week into withdrawal.

CONCLUSIONS

Our findings demonstrate the critical role of 2-AG signaling in the bidirectional regulation of mechanical sensitivity during alcohol withdrawal, with enhancement of 2-AG levels reducing sensitivity, and inhibition of 2-AG signaling exacerbating sensitivity. These data suggest that 2-AG augmentation represents a novel approach to the treatment of alcohol withdrawal-associated hyperalgesia and AUD in patients with comorbid pain disorders.