Correlation between ethanol behavioral sensitization and midbrain dopamine neuron reactivity to ethanol

Effects of social housing conditions on ethanol-induced behavioral sensitization in Swiss mice

RATIONALE

In previous animal model studies, it was shown that drug sensitization is dependent upon physical environmental conditions. However, the effects of social housing conditions on drug sensitization is much less known.

OBJECTIVE

The aim of the present study was to investigate the effects of social conditions, through the size of housing groups, on ethanol stimulant effects and ethanol-induced behavioral sensitization in mice.

MATERIALS AND METHODS

Male and female Swiss mice were housed in groups of different sizes (isolated mice, two mice per cage, four mice per cage and eight mice per cage) during a six-week period. A standard paradigm of ethanol-induced locomotor sensitization was then started with one daily injection of 2.5 g/kg ethanol for 8 consecutive days.

RESULTS

The results show that social housing conditions affect the acute stimulant effects of ethanol. The highest stimulant effects were observed in socially isolated mice and then gradually decreased as the size of the group increased. Although the rate of ethanol sensitization did not differ between groups, the ultimate sensitized levels of ethanol-induced stimulant effects were significantly reduced in mice housed in groups of eight.

CONCLUSIONS

These results are consistent with the idea that higher levels of acute and sensitized ethanol stimulant effects are observed in mice housed in stressful housing conditions, such as social isolation.

Vulnerability to ethanol sensitization predicts higher intake and motivation to self-administer ethanol: Proof of the incentive salience sensitization theory?

Ethanol-induced behavioral sensitization (EIBS) is thought to play a key role in addiction. However, whether EIBS is linked to an increase in the motivation to self-administerethanol in an operant paradigm has never been demonstrated, and thus, the motivational sensitization theory (increase in drug wanting) has not been yet confirmed. We investigated using the operant ethanol self-administrationparadigm if the motivation to self-administerethanol (breakpoint) is increased in female mice prone to develop EIBS. Outbred female Swiss mice were treated once a day with 2.5-g ethanol per kilogram during 10 days and challenged with the same dose of ethanol 7 days later. EIBS-pronegroup was characterized by a significant increase in locomotion between the challenge day and day 1. When the difference was not significant, mice were considered as the "EIBS-resistant"group. Mice were then trained to nose poke for a 20% ethanol solution reinforcer under a FR1 and then a FR-2schedule of reinforcement. Motivation was assessed more directly with a progressive ratio schedule. Our results show that there is a positive correlation between EIBS and both the level of intake and motivation. Interestingly, acquisition of ethanol self-administrationwas faster in sensitized mice that also display a quick and long-lastingincrease in ethanol intake together with a lack of effect of alcohol challenge on c-Fosexpression restricted to the dorsolateral striatum. These results further support that EIBS vulnerability is crucial in the development of addictive behaviors and suggest a potential link with habit learning processes.

A history of ethanol drinking increases locomotor stimulation and blunts enhancement of dendritic dopamine transmission by methamphetamine

Ethanol and psychostimulant use disorders exhibit comorbidity in humans and cross-sensitization in animal models, but the neurobiological underpinnings of this are not well understood. Ethanol acutely increases dopamine neuron excitability, and psychostimulants such as cocaine or methamphetamine increase extracellular dopamine through inhibition of uptake through the dopamine transporter (DAT) and/or vesicular monoamine transporter 2 (VMAT2). Psychostimulants also depress dopamine neuron activity by enhancing dendritic dopamine neurotransmission. Here, we show that mice with a previous history of ethanol drinking are more sensitive to the locomotor-stimulating effects of a high dose (5 mg/kg), but not lower doses (1 and 3 mg/kg) of methamphetamine or any tested dose of cocaine (3, 10, and 18 mg/kg), compared with water-drinking controls. We next investigated the impact of a history of ethanol drinking, in a separate group of mice, on methamphetamine- or cocaine-induced enhancement of dendritic dopamine transmission using whole-cell voltage clamp electrophysiology in mouse brain slices. Methamphetamine, applied at a concentration (10 μM) that affects both DAT and VMAT2, enhanced D2 receptor-mediated inhibitory postsynaptic currents (D2-IPSCs) in both groups, but this effect was blunted in mice with a history of ethanol drinking. As methamphetamine action at VMAT2 disrupts dopamine neurotransmission, these results may suggest enhanced action of methamphetamine at VMAT2. Furthermore, there were no differences in low-dose methamphetamine or cocaine-induced enhancement of D2-IPSCs, suggesting intact DAT function. Disruption of methamphetamine-induced enhancement of dendritic dopamine transmission would result in decreased inhibition of dopamine neurons, ultimately increasing downstream release and the behavioral effects of methamphetamine.

Effects of Ethanol Exposure During Adolescence or Adulthood on Locomotor Sensitization and Dopamine Levels in the Reward System

Behavioral sensitization is a process of neuroadaptation characterized by a gradual increase in motor behaviors. The major neural substrates involved in the behavioral sensitization lie on the dopaminergic mesocorticolimbic pathway, which is still under development during adolescence. To investigate age-differences in ethanol behavioral sensitization and dopamine levels in distinct brain regions of the reward system, adolescent and adult mice were repeatedly pretreated with saline or ethanol (2.0 g/kg i.p.) during 15 consecutive days and challenged with saline or ethanol 5 days after pretreatment. Dopamine and its metabolites were measured in tissue samples of the prefrontal cortex (PFC), nucleus accumbens (NAc) and striatum by HPLC analysis. While repeated ethanol administration resulted in the development of locomotor sensitization in both adult and adolescent mice, only the adults expressed sensitization to a subsequent ethanol challenge injection. Neurochemical results showed reduced dopamine levels in adolescents compared to adults. Specifically, mice pretreated with ethanol during adolescence displayed lower dopamine levels in the PFC compared to the respective adult group in response to an ethanol challenge injection, and preadolescent mice exhibited lower dopamine levels in the NAc following an acute ethanol treatment compared to adults. These findings suggest that adolescent mice are not only less sensitive to the expression of ethanol-induced sensitization than adults, but also show lower dopamine content after ethanol exposition in the PFC and NAc.

Investigating the reciprocal relationships between locomotor sensitization to ethanol and PTSD-like clusters in DBA/2J mice

BACKGROUND

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are two conditions that co-occur frequently. The mechanistic explanations of this co-morbidity are still unclear. The goal of this study was twofold. First to investigate whether PTSD reduces the threshold for the acquisition of ethanol sensitization in an animal model of PTSD. Then to investigate whether ethanol sensitization modulates the expression of PTSD.

METHODS

152 female inbred DBA/2 J mice were submitted to an inescapable footshock paradigm to induce a PTSD-like condition (PTSDLC) and to a paradigm of locomotor sensitization to ethanol. In a first experiment, mice were submitted to the PTSDLC and then repeatedly injected with either saline, 1 g/kg ethanol or 2 g/kg ethanol. Their sensitization to the locomotor stimulant effects of ethanol was then tested in an open field. In a second experiment, mice were first sensitized to the locomotor stimulant effects of ethanol and then tested for their behavioral response to PTSDLC.

RESULTS

In the first experiment, PTSDLC failed to induce a significant locomotor sensitization at the subthreshold dose of 1 g/kg ethanol. However, with 2 g/kg ethanol, a stronger ethanol sensitization was observed in mice submitted to the footshock relative to the control group. In the second experiment, ethanol sensitization increased only some of the behavioral clusters of PTSDLC, namely the fear generalization in a new context.

CONCLUSION

PTSDLC did not reduce the dose threshold for the acquisition of ethanol sensitization but strengthened the development of ethanol sensitization with effective doses. This suggests that PTSD might interact with one of the mechanisms underlying the development of alcohol sensitization. When the relationship between ethanol sensitization and PTSDLC is tested in the reverse direction, the present study only shows a significant effect of ethanol administration on the "sensitized fear" PTSD cluster.

Cued for risk: Evidence for an incentive sensitization framework to explain the interplay between stress and anxiety, substance abuse, and reward uncertainty in disordered gambling behavior

Gambling disorder is an impairing condition confounded by psychiatric co-morbidity, particularly with substance use and anxiety disorders. Yet, our knowledge of the mechanisms that cause these disorders to coalesce remains limited. The Incentive Sensitization Theory suggests that sensitization of neural "wanting" pathways, which attribute incentive salience to rewards and their cues, is responsible for the excessive desire for drugs and cue-triggered craving. The resulting hyper-reactivity of the "wanting' system is believed to heavily influence compulsive drug use and relapse. Notably, evidence for sensitization of the mesolimbic dopamine pathway has been seen across gambling and substance use, as well as anxiety and stress-related pathology, with stress playing a major role in relapse. Together, this evidence highlights a phenomenon known as cross-sensitization, whereby sensitization to stress, drugs, or gambling behaviors enhance the sensitivity and dopaminergic response to any of those stimuli. Here, we review the literature on how cue attraction and reward uncertainty may underlie gambling pathology, and examine how this framework may advance our understanding of co-mordidity with substance-use disorders (e.g., alcohol, nicotine) and anxiety disorders. We argue that reward uncertainty, as seen in slot machines and games of chance, increases dopaminergic activity in the mesolimbic pathway and enhances the incentive value of reward cues. We propose that incentive sensitization by reward uncertainty may interact with and predispose individuals to drug abuse and stress, creating a mechanism through which co-mordidity of these disorders may emerge.

Chemogenetic stimulation of the infralimbic cortex reverses alcohol-induced fear memory overgeneralization

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are often comorbid. Drinking tends to increase following trauma, which may exacerbate PTSD-related symptoms. Despite a clear relationship between excessive alcohol use and PTSD, how alcohol impacts the expression of traumatic fear remains unclear. This study aims to determine the neurobehavioral impact of chronic alcohol (ethanol; EtOH) on the expression of established fear memories in C57BL/6 N mice. We show that chronic EtOH selectively augments cued fear memory generalization and impairs fear extinction retrieval, leaving the expression of the original cued response intact. Immunohistochemistry for Arc/arg3.1 (Arc) revealed EtOH-induced decreases in Arc expression in the infralimbic cortex (IL) and basolateral amygdala complex (BLA) that were associated with cued fear memory overgeneralization. Chemogenetic stimulation of IL pyramidal neurons reversed EtOH-driven fear memory overgeneralization, identifying a role for the IL in cued fear memory precision. Considering the modulatory influence of the IL over conditioned fear expression, these data suggest a model whereby chronic EtOH-driven neuroadaptations in the IL promote fear memory overgeneralization. These findings provide new mechanistic insight into how excessive alcohol use, following exposure to a traumatic event, can exacerbate symptoms of traumatic fear.

The Urge to Fight: Persistent Escalation by Alcohol and Role of NMDA Receptors in Mice

Alcohol drinking, in some individuals, culminates in pathologically aggressive and violent behaviors. Alcohol can escalate the urge to fight, despite causing disruptions in fighting performance. When orally administered under several dosing conditions the current study examined in a mouse model if repeated alcohol escalates the motivation to fight, the execution of fighting performance, or both. Specifically, seven daily administrations of alcohol (0, 1.8, or 2.2 g/kg) determined if changes in the motivation to initiate aggressive acts occur with, or without, shifts in the severity of fighting behavior. Responding under the control of a fixed interval (FI) schedule for aggression reinforcements across the initial daily sessions indicated the development of tolerance to alcohol's sedative effect. By day 7, alcohol augmented FI response rates for aggression rewards. While alcohol escalated the motivation to fight, fighting performance remained suppressed across the entire 7 days. Augmented FI responding for aggression rewards in response to a low dose of alcohol (1.0 g/kg) proved to be persistent, as we observed sensitized rates of responding for more than a month after alcohol pretreatment. In addition, this sensitization of motivated aggression did not occur with a general enhancement of motor activity. Antagonism of NMDA or AMPA receptors with ketamine, dizocilpine, or NBQX during later challenges with alcohol were largely serenic without having any notable impact on the expression of alcohol-escalated rates of FI responding. The current dissociation of appetitive and performance measures indicates that discrete neural mechanisms controlling aggressive arousal can be distinctly sensitized by alcohol.

Effects of Oral Calcium Dosage and Timing on Ethanol-Induced Sensitization of Locomotion in DBA/2 Mice

Ethanol (EtOH) dosage, frequency, and paired associative learning affect the risk of alcoholism. Recently, Spanagel et al. reported that acamprosate calcium (Acam Ca) prescribed for alcoholism exerts an anti-relapse effect via Ca. Ca is contained in foods, sometimes consumed with alcohol. Therefore, we investigated the association among oral Ca ingestion, EtOH-induced locomotor sensitization, and plasma Ca levels on how to consume Ca for moderate drinking. We used DBA/2 CrSlc mice, and CaCl₂ as water-soluble Ca salts. For pre-administration, elemental Ca (50, 75, 100, or 150 mg/kg, per os (p.o.)) or water for control was administered 1 h before EtOH (2 g/kg, 20 v/v (%) EtOH in saline) administration intraperitoneal (i.p.) for locomotor sensitization or for plasma Ca level changes. For post-administration, elemental Ca (100 mg/kg) was administered 1 h after EtOH. Moreover, we employed bepridil and the dopamine D1 antagonist, SCH-23390 to further examine the mechanism of EtOH-induced sensitization. The locomotor sensitization segmentalized for 300 s had two peaks (0-90 s and 180-300 s). Pre-administration of Ca (50, 75, and 100 mg/kg) significantly reduced the 0-90-s peak, selectively blocked by SCH-23390, but "non-dose dependently" as Ca 150 mg/kg did not have this effect. Bepridil blocked the suppressive effect of pre-administration of Ca (100 mg/kg). The effective pre-doses of Ca (50-100 mg/kg) maintained plasma Ca basal levels against EtOH-induced decrease of Ca. On the contrary, post-administration of Ca inversely led to significant promotion of sensitization of both locomotor peaks. Oral Ca intake had diverse effects on EtOH-induced sensitization depending on Ca dosage and timing.

Increased ethanol consumption after interruption of fat bingeing

There is a marked comorbidity between alcohol abuse and eating disorders, especially in the young population. We have previously reported that bingeing on fat during adolescence increases the rewarding effects of ethanol (EtOH). The aim of the present work was to study if vulnerability to EtOH persists after cessation of binge eating. OF1 mice binged on fat (HFB: high-fat binge) during adolescence (PND 25-43) and were tested for 15 days after the last access to HFB (on PND 59) using the self-administration paradigm, the conditioned place preference (CPP) and locomotor sensitization to ethanol. Our results showed that after 15 days of cessation of fat ingestion, mice increased their consumption of ethanol and showed greater motivation to obtain ethanol. On the other hand, no effects were observed in the CPP, while an increased locomotor response to ethanol was detected. The present results confirm and extend our previous study demonstrating that the compulsive intake of fat induces long-lasting effects on the reward system that lead to an increased consumption of EtOH.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

The new kisspeptin derivative - kissorphin (KSO) - attenuates acute hyperlocomotion and sensitization induced by ethanol and morphine in mice

Kissorphin (KSO) is a new peptide derived from kisspeptin-10. This peptide possesses neuropeptide FF (NPFF)-like biological activity in vitro; NPFF, in many cases, inhibits opioid and ethanol effects in rodents. Therefore, the current study explored the influence of KSO on acute ethanol- and morphine-induced hyperactivity, and on the development and expression of locomotor sensitization induced by these drugs. In the present study, sensitization to locomotor effects was induced by repeated exposure to ethanol (2.4 g/kg, intraperitoneally [i.p.], 1 × 4 days) or morphine (10 mg/kg, subcutaneously [s.c.], 1 × 7 days). We found that KSO (1-10 nmol/300 μL, intravenously [i.v.]) did not have an impact on locomotor activity of naïve mice. However, it reduced both acute ethanol- (10 nmol/300 μL) and morphine-induced hyperactivity (3 and 10 nmol/300 μL). Pretreatment of animals with KSO (10 nmol/300 μL), before every ethanol or morphine injection during development of sensitization or before the ethanol or morphine challenge, attenuated the development, as well as the expression of locomotor sensitization to both substances. Moreover, prior administration of the NPFF receptor antagonist RF9 (10 nmol/300 μL, i.v.) inhibited the ability of KSO (10 nmol/300 μL) to reduce the expression of ethanol and morphine sensitization. KSO given alone, at all used doses, did not influence the motor coordination measured via the rotarod test. The results from this study show that KSO effectively attenuated acute and repeated effects of ethanol and morphine. Thus, KSO possesses NPFF-like anti-opioid activity in these behavioral studies.

Voluntary adolescent drinking enhances excitation by low levels of alcohol in a subset of dopaminergic neurons in the ventral tegmental area

Enhanced dopamine (DA) neurotransmission from the ventral tegmental area (VTA) to the ventral striatum is thought to drive drug self-administration and mediate positive reinforcement. We examined neuronal firing rates in slices of mouse midbrain following adolescent binge-like alcohol drinking and find that prior alcohol experience greatly enhanced the sensitivity to excitation by ethanol itself (10-50 mM) in a subset of ventral midbrain DA neurons located in the medial VTA. This enhanced response after drinking was not associated with alterations of firing rate or other measures of intrinsic excitability. In addition, the phenomenon appears to be specific to adolescent drinking, as mice that established a drinking preference only after the onset of adulthood showed no change in alcohol sensitivity. Here we demonstrate not only that drinking during adolescence induces enhanced alcohol sensitivity, but also that this DA neuronal response occurs over a range of alcohol concentrations associated with social drinking in humans.

Label-Free Proteomic Analysis of Protein Changes in the Striatum during Chronic Ethanol Use and Early Withdrawal

The molecular mechanisms underlying the neuronal signaling changes in alcohol addiction and withdrawal are complex and multifaceted. The cortico-striatal circuit is highly implicated in these processes, and the striatum plays a significant role not only in the early stages of addiction, but in the developed-addictive state as well, including withdrawal symptoms. Transcriptional analysis is a useful method for determining changes in gene expression, however, the results do not always accurately correlate with protein levels. In this study, we employ label-free proteomic analysis to determine changes in protein expression within the striatum during chronic ethanol use and early withdrawal. The striatum, composed primarily of medium spiny GABAergic neurons, glutamatergic and dopaminergic nerve terminals and astrocytes, is relatively homogeneous for proteomic analysis. We were able to analyze more than 5000 proteins from both the dorsal (caudate and putamen) and ventral (nucleus accumbens) striatum and identified significant changes following chronic intermittent ethanol exposure and acute (8 h) withdrawal compared to ethanol naïve and ethanol exposure groups respectively. Our results showed significant changes in proteins involved in glutamate and opioid peptide signaling, and also uncovered novel pathways including mitochondrial function and lipid/cholesterol metabolism, as revealed by changes in electron transport chain proteins and RXR activation pathways. These results will be useful in the development of novel treatments for alcohol withdrawal and thereby aid in recovery from alcohol use disorder.

Response to novelty and cocaine stimulant effects: lack of stability across environments in female Swiss mice

RATIONALE

In humans, novelty/sensation seeking is seen as a personality trait with a positive relationship with addiction vulnerability. In animal studies, one of the standard procedures to model novelty seeking is the "response to novelty," i.e., the levels of locomotor activity in a new environment. In rodents, a positive correlation was demonstrated between the response to novelty and several effects of drugs, especially the locomotor stimulant effects of cocaine.

OBJECTIVES

The present study was designed to test in mice whether the response to novelty is stable across environments and whether its relationship with the stimulant effects of cocaine is altered by environmental changes. Experiment 1 assessed the responses to novelty of the same mice in two different novel environments. Experiment 2 tested the correlation between response to novelty and acute stimulant effects of cocaine recorded in two distinct environments.

RESULTS

The results show a weak correlation only during the first 5 min of the session between the responses to novelty measured in two distinct environments. Experiment 2 demonstrates that novelty responses and stimulant effects of cocaine are positively correlated only when both behavioral responses are measured in the same environment. In contrast, the relationship between response to novelty and acute stimulant effects of cocaine is completely lost when the behavioral responses are recorded in two different environments.

CONCLUSIONS

The present results question the usual interpretation of the correlation between the response to novelty and the stimulant effects of cocaine as reflecting a relationship between two underlying individual stable characteristics.