Reward loss and addiction: Opportunities for cross-pollination

Frustrative nonreward: Detailed c-Fos expression patterns in the amygdala after consummatory successive negative contrast

The amygdala has been implicated in frustrative nonreward induced by unexpected reward downshifts, using paradigms like consummatory successive negative contrast (cSNC). However, existing evidence comes from experiments involving the central and basolateral nuclei on a broad level. Moreover, whether the amygdala's involvement in reward downshift requires a cSNC effect (i.e., greater suppression in downshifted animals than in unshifted controls) or just consummatory suppression without a cSNC effect, remains unclear. Three groups were exposed to (1) a large reward disparity leading to a cSNC effect (32-to-2% sucrose), (2) a small reward disparity involving consummatory suppression in the absence of a cSNC effect (8-to-2% sucrose), and (3) an unshifted control (2% sucrose). Brains obtained after the first reward downshift session were processed for c-Fos expression, a protein often used as a marker for neural activation. c-Fos-positive cells were counted in the anterior, medial, and posterior portions (A/P axis) of ten regions of the rat basolateral, central, and medial amygdala. c-Fos expression was higher in 32-to-2% sucrose downshift animals than in the other two groups in four regions: the anterior and the medial lateral basal amygdala, the medial capsular central amygdala, and the anterior anterio-ventral medial amygdala. None of the areas exhibited differential c-Fos expression between the 8-to-2% sucrose downshift and the unshifted conditions. Thus, amygdala activation requires exposure to a substantial reward disparity. This approach has identified, for the first time, specific amygdala areas relevant to understand the cSNC effect, suggesting follow-up experiments aimed at testing the function of these regions in reward downshift.

Instrumental successive negative contrast in rats: Trial distribution, reward magnitude, and prefrontal cortex activation

Successive negative contrast (SNC) has been used to study reward relativity, reward loss, and frustration for decades. In instrumental SNC (iSNC), the anticipatory performance of animals downshifted from a large reward to a small reward is compared to that of animals always reinforced with the small reward. iSNC involves a transient deterioration of anticipatory behavior in downshifted animals compared to unshifted controls. There is scattered information on the optimal parameters to produce this effect and even less information about its neural basis. Five experiments with rats trained in a runway to collect food pellets explored the effects of trial distribution (massed or spaced), amount of preshift training, reward disparity, and reward magnitude on the development of an iSNC effect. Start, run, and goal latencies were measured. Using spaced trials (one trial per day), evidence of the iSNC effect was observed with 24 preshift trials and a 32-to-4 pellet disparity. With massed trials (4 trials per session separated by 30-s intertrial intervals), evidence of iSNC was found with 12 preshift sessions (a total of 48 trials) and a 16-to-2 pellet disparity. The massed-training procedure was then used to assess neural activity in three prefrontal cortex areas using c-Fos expression in animals perfused after the first downshift session. There was evidence of increased activation in the anterior cingulate cortex and a trend toward increased activation in the infralimbic and prelimbic cortices. These procedures open a venue for studying the neural basis of the instrumental behavior of animals that experience reward loss.

Behavioral and neural correlates of licking for 66% alcohol in Wistar rats: Caloric balance or sensation/novelty seeking?

Whereas rodents generally reject high alcohol concentrations, access to 66% alcohol can reinforce operant licking in a progressive ratio situation. Three experiments were conducted to identify a potential mechanism underlying this effect. In Experiment 1, food-restricted male and female Wistar rats received access to either 66% alcohol or water in their home cage for one hour over four sessions. Consumption of alcohol and water was similar, showing that rats neither preferred nor rejected 66% alcohol. Peripheral (but not central) activity in an open field (OF) was higher after access to 66% alcohol than water, a result inconsistent with motor impairment. Blood alcohol concentration was higher after 66% alcohol than water and was positively correlated with fluid displacement and peripheral distance in the OF. c-Fos immunoreactivity after exposure to 66% alcohol vs. water showed increased activation in the nucleus accumbens shell, anterior cingulate cortex, and insular cortex. In Experiment 2, whether access to food was restricted (to an 81-84% of the ad libitum weight) or free (ad libitum), female Wistar rats licked at similar frequency from a sipper tube delivering 66% alcohol. This result is inconsistent with an account based on the caloric content of 66% alcohol. In Experiment 3, food-restricted male and female Wistar rats exhibited a positive correlation between activity in the central area of an OF (an index of sensation/novelty seeking) and licking for 66% alcohol. These results are consistent with the hypothesis that the reinforcing value of 66% alcohol is related to sensation/novelty seeking.

Frustrative nonreward and the basal ganglia: Chemogenetic inhibition and excitation of the nucleus accumbens and globus pallidus externus during reward downshift

Frustrative nonreward contributes to anxiety disorders and addiction, and is included in the Research Domain Criteria initiative as a relevant endophenotype. These experiments explored the role of the basal ganglia in consummatory reward downshift (cRD) using inhibitory and excitatory DREADDs (designer receptors exclusively activated by designer drugs) infused in either the nucleus accumbens (NAc) or one of its downstream targets, the globus pallidus externus (GPe). NAc inhibition did not disrupt consummatory suppression during a 32-to-2% (Experiment 1) or 8-to-2% sucrose downshift (Experiment 2). However, NAc excitation enhanced consummatory suppression during a 32-to-2% sucrose downshift (Experiment 1). GPe inhibition caused a trend toward increased consummatory suppression after a 32-to-2% sucrose downshift, whereas GPe excitation eliminated consummatory suppression after an 8-to-2% sucrose downshift (Experiment 3). Chemogenetic manipulations of NAc and GPe had no detectable effects on open field activity. The effects of DREADD activation via clozapine N-oxide (CNO) administration were compared to controls that carried the DREADDs, but received vehicle injections. There was no evidence that CNO or vehicle injections in virus vector control (VVC) animals affected cRD or OF activity after either CNO or vehicle injections. NAc and GPe excitation led to opposite results in the cRD task, providing evidence that the basal ganglia circuit has a function in frustrative nonreward in the absence of detectable motor effects.

Evidence of successive negative contrast in terrestrial toads (Rhinella arenarum): central or peripheral effect?

Prior research with terrestrial toads (Rhinella arenarum) in a water-reinforced instrumental situation indicated a direct relationship between acquisition rate and reward magnitude. However, a reward downshift produced a gradual adjustment of instrumental performance and a rapid adjustment of consummatory performance, rather than the abrupt and transient deterioration of behavior typical of a successive negative contrast effect. In Experiment 1, using a two-chamber box, a downshift from deionized water (which supports maximal rehydration) to 250-mM sodium chloride solution (which supports a lower rehydration), also yielded a gradual adjustment of instrumental behavior. In this experiment, animals received one trial per day and were allowed 300 s of access to the reward in the goal box. Experiment 2 used the same procedure, except that animals were allowed access to the solution in the goal box for 600 s. Under these conditions, reward downshift led to longer latencies (instrumental) and lower rehydration levels (consummatory) than those of unshifted controls, providing evidence for successive negative contrast. Unlike in similar experiments with mammals, the effect was not transient, but persisted relatively unmodified over twelve daily postshift trials. In this case, the possibility of adaptation of the peripheral mechanisms for water uptake is considered. The comparative relevance of these results is discussed in terms of habit formation versus expectancy-guided behavior in vertebrate learning.

Frustrative nonreward and cannabinoid receptors: Chronic (but not acute) WIN 55,212-2 treatment increased resistance to change in two reward downshift tasks

Assessing the role of cannabinoid (CB) receptors in behavior is relevant given the trend toward the legalization of medicinal and recreational marijuana. The present research aims at bridging a gap in our understanding of CB-receptor function in animal models of frustrative nonreward. These experiments were designed to (1) determine the effects of chronic administration of the nonselective CB1-receptor agonist WIN 55,212-2 (WIN) on reward downshift in rats and (2) determine whether the effects of chronic WIN were reducible to acute effects. In Experiment 1, chronic WIN (7 daily injections, 10 mg/kg, ip) accelerated the recovery of consummatory behavior after a 32-to-4% sucrose downshift relative to vehicle controls. In addition, chronic WIN eliminated the preference for an unshifted lever when the other lever was subject to a 12-to-2 pellet downshift in free-choice trials, but only in animals with previous experience with a sucrose downshift. In Experiment 2, acute WIN (1 mg/kg, ip) reduced consummatory behavior, but did not affect recovery from a 32-to-4% sucrose downshift. The antagonist SR 141716A (3 mg/kg, ip) also failed to interfere with recovery after the sucrose downshift. In Experiment 3, acute WIN administration (1 mg/kg, ip) did not affect free-choice behavior after a pellet downshift, although it reduced lever pressing and increased magazine entries relative to vehicle controls. The effects of chronic WIN on frustrative nonreward were not reducible to acute effects of the drug. Chronic WIN treatment in rats, like chronic marijuana use in humans, seems to increase resistance to the effects of frustrative nonreward.

Frustrative nonreward and emotional self-medication:Factors modulating alcohol consumption following reward downshift in rats

Life experience involving unexpected incentive loss (e.g., loss of job or a significant other) may result in negative emotional reactions (frustration) and promote alcohol drinking. Similarly, animals exposed to a frustrative 32-to-4% sucrose downshift increase their preference for alcohol (2%) vs. water. This result was interpreted as reflecting emotional self-medication-the consumption of substances that reduce negative emotions. We conducted three experiments examining parametric manipulations of the animal model: (1) effects of a severe reward downshift (32-to-4% sucrose) on consumption of various alcohol concentrations (Experiment 1); (2) effects of different magnitudes of reward downshifts on consumption of 32% alcohol (Experiment 2); and (3) effects of partial reinforcement (an intervention that increases resistance to frustration) on 2% alcohol intake induced by a 32-to-4% sucrose downshift (Experiment 3). The results show that (1) a 32-to-4% sucrose downshift leads to an increase in alcohol intake over a wide range of alcohol concentrations; (2) the greater the reward downshift, the higher the relative increase in alcohol consumption; and (3) a treatment that increases resistance to frustration (partial reinforcement) also attenuates alcohol consumption after a sucrose downshift. These data are discussed in relation to the role of frustrative nonreward in alcohol consumption.

Facilitating mGluR4 activity reverses the long-term deleterious consequences of chronic morphine exposure in male mice

Understanding the neurobiological underpinnings of abstinence from drugs of abuse is critical to allow better recovery and ensure relapse prevention in addicted subjects. By comparing the long-term transcriptional consequences of morphine and cocaine exposure, we identified the metabotropic glutamate receptor subtype 4 (mGluR4) as a promising pharmacological target in morphine abstinence. We evaluated the behavioral and molecular effects of facilitating mGluR4 activity in abstinent mice. Transcriptional regulation of marker genes of medium spiny neurons (MSNs) allowed best discriminating between 4-week morphine and cocaine abstinence in the nucleus accumbens (NAc). Among these markers, Grm4, encoding mGluR4, displayed down-regulated expression in the caudate putamen and NAc of morphine, but not cocaine, abstinent mice. Chronic administration of the mGluR4 positive allosteric modulator (PAM) VU0155041 (2.5 and 5 mg/kg) rescued social behavior, normalized stereotypies and anxiety and blunted locomotor sensitization in morphine abstinent mice. This treatment improved social preference but increased stereotypies in cocaine abstinent mice. Finally, the beneficial behavioral effects of VU0155041 treatment in morphine abstinent mice were correlated with restored expression of key MSN and neural activity marker genes in the NAc. This study reports that chronic administration of the mGluR4 PAM VU0155041 relieves long-term deleterious consequences of morphine exposure. It illustrates the neurobiological differences between opiate and psychostimulant abstinence and points to pharmacological repression of excessive activity of D2-MSNs in the NAc as a promising therapeutic lever in drug addiction.

High Alcohol-Preferring Mice Show Reaction to Loss of Ethanol Reward Following Repeated Binge Drinking

BACKGROUND

Beyond yielding high blood ethanol (EtOH) concentrations (BECs), binge-drinking models allow examination of drinking patterns which may be associated with EtOH's rewarding effects, including front-loading and consummatory successive negative contrast (cSNC), a decrease in intake when only water is available to subjects expecting EtOH. The goals of the current study were to broaden our understanding of these reward-related behaviors during binge EtOH access in high alcohol-preferring (HAP) replicate lines (HAP2 and HAP3) of mice selectively bred to prefer alcohol. We hypothesized that both lines would show evidence of front-loading during binge EtOH access and that we would find a cSNC effect in groups where EtOH was replaced with water, as these results have been shown previously in HAP1 mice.

METHODS

HAP replicate 2 and replicate 3 female and male mice were given 2 hours of EtOH or water access in the home cage for 15 consecutive days using "drinking in the dark" (DID) procedures. Mice received the same fluid (either 20% unsweetened EtOH or water) for the first 14 days. However, on the 15th day, half of the mice from these 2 groups were provided with the opposite assigned fluid (EtOH groups received water and vice versa). Intake was measured in 1-minute bins using specialized sipper tubes, which allowed within-session analyses of binge-drinking patterns.

RESULTS

EtOH front-loading was observed in both replicates. HAP3 mice displayed front-loading on the first day of EtOH access, whereas front-loading developed following alcohol experience in HAP2 mice, which may suggest differences in initial sensitivity to EtOH reward. Consummatory SNC, which manifests as lower water intake in mice expecting EtOH as compared to mice expecting water, was observed in both replicates.

CONCLUSIONS

These findings increase confidence that defined changes in home cage consummatory behavior are driven by the incentive value of EtOH. The presence of cSNC across HAP replicates indicates that this reaction to loss of reward is genetically mediated, which suggests that there is a biological mechanism that might be targeted.

Frustrative nonreward: Chemogenetic inactivation of the central amygdala abolishes the effect of reward downshift without affecting alcohol intake

The role of the central amygdala (CeA) in the adjustment to a 32-to-2% sucrose downshift in the consummatory successive negative contrast (cSNC) task and in a free-choice 10% alcohol-water preference task (PT) was studied using chemogenetic inactivation. cSNC is a model of frustrative nonreward that enhances alcohol consumption. In Experiment 1, sessions 1-10 involved 5-min access to 32% sucrose and sessions 11-12 involved access to 2% sucrose. Vehicle or clozapine N-oxide (CNO; 1 or 3 mg/kg, ip), used later to activate the inhibitory designer receptor, was administered 30 min before sessions 11-12. There was no evidence that CNO affected consummatory behavior after the sucrose downshift. In Experiment 2, all animals received an infusion of the inhibitory designer receptor hM4D(Gi) into the CeA. After recovery, animals received access to either 32% or 2% sucrose on sessions 1-10, followed by 2% sucrose on sessions 11-12. Immediately after each 5-min sucrose session, animals received a 2-bottle, 1-h PT with 10% alcohol and water. CNO (3 mg/kg, ip) or vehicle was administered 30 min before sessions 11-12. CeA inactivation prior to sucrose downshift eliminated the cSNC effect, which was observed in vehicle controls. However, there was no evidence that CeA inactivation affected preference for 10% alcohol over water. These results support the hypothesis that CeA activity is critical for cSNC effect, an outcome consistent with the view that the amygdala plays a central role in frustrative nonreward.

Lateral habenula lesions disrupt appetitive extinction, but do not affect voluntary alcohol consumption

This study analyzed the effects of LHb lesions on appetitive extinction and alcohol consumption. Eighteen male Wistar rats received neurochemical lesions of the LHb (quinolinic acid) and 12 received a vehicle infusion (PBS). In a runway instrumental task, rats received acquisition (12 pellets/trial, 6 trials/session, 10 sessions) and extinction training (5 sessions). In a consummatory task, rats had daily access to 32% sucrose (5 min, 10 sessions) followed by access to water (5 sessions). Then, animals received 2 h preference tests with escalating alcohol concentrations (2%-24%), followed by two 24 h preference tests with 24% alcohol. Relative to Shams, LHb lesions delayed extinction, as indicated by lower response latencies (instrumental task) and higher fluid consumption (consummatory task). LHb lesions did not affect alcohol consumption regardless of alcohol concentration or test duration. The LHb modulates appetitive extinction and needs to be considered as part of the brain circuit underlying reward loss.

Emotional Reactivity to Incentive Downshift in Adult Rats Exposed to Binge-Like Ethanol Exposure During Adolescence

Alcohol use in adolescents is often characterized by binge-like ethanol consumption pattern, which is associated with long-term health consequences and even with important harms to his developing brain. Among this, ethanol exposure induces long-lasting alterations in anxiety-related neurobiological systems such as corticotropin releasing factor (CRF) or melanocortin system (MC). Recently, it has been demonstrated that adult rats exposed to adolescent intermittent ethanol (AIE) exposure exhibited anxiogenic-like behavior. Given that it has been demonstrated that negative affective state is relevant to development of addictive behavior, it is tempting to suggest that increased risk of adult abusive alcohol use exhibited in rats exposed to ethanol during adolescence may be related with differences in anxiety-related behavior. We conducted a study investigating the emotional reactivity after a reward devaluation (12-to-1 pellet or 32-to-4% sucrose downshift) in adult rats exposed to binge-like ethanol exposure during adolescence. For this aim, adolescent Sprague-Dawley rats were treated with ethanol (2.5 g/kg ip; AIE) or saline (AIS) for 2 consecutive days at 48-h intervals over a 14-day period (PND30-PND43). Following 25 free-ethanol days, adult rats were trained in consummatory and instrumental successive negative contrast task (cSNC and iSNC). Our data shows that both AIE and AIS groups exhibited suppression of the consummatory and instrumental behavior after reward devaluation relative to unshifthed control. Also, adult rats exposed to alcohol during adolescence exhibited a particularly strong negative affective state (lower sucrose consumption) with regards to the AIS group in the cSNC. This data demonstrated that adolescent binge-like ethanol exposure might trigger a greater emotional reactivity following incentive downshift, which might be linked to higher vulnerability to substance use disorder.

GABRA2 rs279858-linked variants are associated with disrupted structural connectome of reward circuits in heroin abusers

The reward system plays a vital role in drug addiction. The purpose of this study is to investigate the structural connectivity characteristics and driving-control subnetwork patterns of reward circuits in heroin abusers and assess the genetic modulation on the reward network. We first defined the reward network based on systematic literature review, and built the reward network based on diffusion tensor imaging data of 78 heroin abusers (HAs) and 79 healthy controls (HCs) using structural connectomics. Then we assessed genetic factors that might modulate changes in the reward network by performing imaging-genetic screening for 22 addiction-related polymorphisms. The genetic association was validated by performing genetic associations (1032 HAs and 2863 HCs) and expanded-variant analysis. Finally, we estimated the association between these genetic variations, reward network, and clinical performance. We found that HAs had widespread deficiencies in the structural connectivity of the reward circuit (center in VTA-linked connections), which correlated with cognition deficiency. The disruptions synchronously were shown on the reward driving system and reward control system. GABRA2 rs279858-linked variants might be a key genetic modulator for heroin vulnerability by affecting the connections of reward network and cognition. The role of the reward network connections that mediates the effects of rs279858 on cognition would be disrupted by heroin addiction. These findings provide new insights into the neurocircuitry and genetic mechanisms of addiction.

Metabotropic Glutamate Receptor 5 as a Target for the Treatment of Depression and Smoking: Robust Preclinical Data but Inconclusive Clinical Efficacy

The ability of novel pharmacological compounds to improve outcomes in preclinical models is often not translated into clinical efficacy. Psychiatric disorders do not have biological boundaries, and identifying mechanisms to improve the translational bottleneck between preclinical and clinical research domains is an important and challenging task. Glutamate transmission is disrupted in several neuropsychiatric disorders. Metabotropic glutamate (mGlu) receptors represent a diverse class of receptors that contribute to excitatory neurotransmission. Given the wide, yet region-specific manner of expression, developing pharmacological compounds to modulate mGlu receptor activity provides an opportunity to subtly and selectively modulate excitatory neurotransmission. This review focuses on the potential involvement of mGlu5 receptor disruption in major depressive disorder and substance and/or alcohol use disorders. We provide an overview of the justification of targeting mGlu5 receptors in the treatment of these disorders, summarize the preclinical evidence for negatively modulating mGlu5 receptors as a therapeutic target for major depressive disorders and nicotine dependence, and highlight the outcomes of recent clinical trials. While the evidence of mGlu5 receptor negative allosteric modulation has been promising in preclinical investigations, these beneficial effects have not translated into clinical efficacy. In this review, we identify key challenges that may contribute to poor clinical translation and provide suggested approaches moving forward to potentially improve the translation from preclinical to clinical domains. Such approaches may increase the success of clinical trials and may reduce the translational bottleneck that exists in drug discovery for psychiatric disorders.

Reward devaluation disrupts latent inhibition in fear conditioning

Three experiments explored the link between reward shifts and latent inhibition (LI). Using consummatory procedures, rewards were either downshifted from 32% to 4% sucrose (Experiments 1-2), or upshifted from 4% to 32% sucrose (Experiment 3). In both cases, appropriate unshifted controls were also included. LI was implemented in terms of fear conditioning involving a single tone-shock pairing after extensive tone-only preexposure. Nonpreexposed controls were also included. Experiment 1 demonstrated a typical LI effect (i.e., disruption of fear conditioning after preexposure to the tone) in animals previously exposed only to 4% sucrose. However, the LI effect was eliminated by preexposure to a 32%-to-4% sucrose devaluation. Experiment 2 replicated this effect when the LI protocol was administered immediately after the reward devaluation event. However, LI was restored when preexposure was administered after a 60-min retention interval. Finally, Experiment 3 showed that a reward upshift did not affect LI. These results point to a significant role of negative emotion related to reward devaluation in the enhancement of stimulus processing despite extensive nonreinforced preexposure experience.