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.

Resistance to ethanol sensitization is associated with a loss of synaptic plasticity in the hippocampus

Behavioral sensitization to repeated ethanol (EtOH) exposure induces an increase in locomotor activity in mice. However, not all animals express such sensitization. Although the literature indicated that the hippocampus may play a role in EtOH sensitization, it is not known whether behavioral sensitization to EtOH is associated with preferential changes in bidirectional synaptic plasticity, i.e., LTP and LTD, two markers of learning capabilities that have also been shown to be involved in addictive behavior. In the present study, we examined whether the vulnerability to develop and express behavioral sensitization to EtOH is associated with altered bidirectional synaptic plasticity in the CA1 area of the dorsal hippocampus. For this purpose, we analyzed both LTP and LTD in resistant and sensitized mice during the expression phase, i.e., 7 days after 10 days of repeated EtOH i.p. administration. We found that resistant mice showed a lack of LTD without changes in LTP. The lack of LTD was associated with an increase in GluN2A protein level and was not due to an altered level of neuronal activity, since no difference was observed between the number of c-FOS positive neurons in sensitized and resistant mice. Given that both types of synaptic plasticity signals may have distinct roles in specific learning and behaviors, our results suggest that resistant mice could exhibit different phenotypes in terms of learning/memory and addictive behaviors compared to sensitized ones. Synapse 71:e21899, 2017. © 2016 Wiley Periodicals, Inc.

Rôle de la sensibilisation aux effets stimulants de l’alcool dans l’addiction et implication de la plasticité synaptique hippocampique

Parmi les théories de l’addiction, celle de la sensibilisation comportementale tient une place importante avec l’augmentation de la motivation à consommer la drogue au fil des expositions répétées (sensibilisation de la valeur incitatrice de la drogue encore appelée sensibilisation motivationnelle). Cependant, il existe très peu de preuves du rôle de cette sensibilisation dans le comportement addictif. La sensibilisation pourrait jouer un rôle majeur non seulement dans l’escalade de la consommation mais aussi dans le phénomène de rechute et le craving, une composante majeure de l’addiction. Nous avons déjà démontré une corrélation inverse entre comportement de type anxieux et vulnérabilité à développer la sensibilisation. Toujours dans une approche heuristique du phénomène, nous avons réalisé des expériences chez la souris pour déterminer le rôle de ce phénomène de sensibilisation dans la motivation à consommer de l’alcool. Nous avons aussi voulu savoir si des modifications de plasticité synaptique hippocampique pouvaient être associées à vulnérabilité au phénomène de sensibilisation. Nos résultats démontrent une vulnérabilité individuelle face au phénomène de sensibilisation aux effets stimulants moteurs de l’alcool : des souris présentant une forte sensibilisation alors que d’autres sont complètement résistantes. Les souris sensibilisées sont celles qui présentent la plus forte consommation d’alcool dans une procédure d’auto-administration opérante d’alcool et surtout celles qui présentent la plus forte motivation à consommer de l’alcool dans un paradigme de ratio progressif où le prix à payer pour obtenir de l’alcool augmente lors d’une même session. Enfin nos résultats d’électrophysiologie démontrent des atteintes spécifiques de la dépression synaptique à long terme chez les souris résistantes et ouvrent donc des perspectives intéressantes en termes de bases neurobiologiques. L’ensemble de ces résultats a une incidence majeure dans le champ de l’addiction à l’alcool car c’est la première fois que la relation entre vulnérabilité à la sensibilisation et motivation à consommer de l’alcool est établie.

Light alcohol intake during adolescence induces alcohol addiction in a neurodevelopmental model of schizophrenia

Schizophrenia is a mental disorder characterized by a series of positive, negative or cognitive symptoms but with also the particularity of exhibiting a high rate of co-morbid use of drugs of abuse. While more than 80% of schizophrenics are smokers, the second most consumed drug is alcohol, with dramatic consequences on frequency and intensity of psychotic episodes and on life expectancy. Here we investigated the impact of light alcohol intake during adolescence on the subsequent occurrence of alcohol addiction-like behavior in neonatal ventral hippocampal lesion (NVHL) rats, a neurodevelopmental model of schizophrenia. Our findings demonstrated an increased liability to addictive behaviors in adult NVHL rats after voluntary alcohol intake during adolescence. NVHL rats displayed several signs of alcohol use disorder such as a loss of control over alcohol intake and high motivation to consume alcohol, associated with a higher resistance to extinction. In addition, once NVHL rats relapsed, they maintained higher drinking levels than controls. We finally showed that the anti-addictive drug naltrexone is efficient in reducing excessive alcohol intake in NVHL rats. Our results are in accordance with epidemiological studies underlying the particular vulnerability to alcohol addiction after adolescent exposure to alcohol and highlight the fact that schizophrenic subjects may be particularly at risk even after light alcohol consumption. Based on these results, it seems particularly relevant to prevent early onset of alcohol use in at-risk subjects and thus to reduce the incidence of co-morbid alcohol abuse in psychotic patients.

Brain-derived neurotrophic factor mediates the suppression of alcohol self-administration by memantine

Brain-derived neurotrophic factor (BDNF) within the striatum is part of a homeostatic pathway regulating alcohol consumption. Memantine, a non-competitive antagonist of N-methyl-D-aspartate receptors, induces expression of BDNF in several brain regions including the striatum. We hypothesized that memantine could decrease ethanol (EtOH) consumption via activation of the BNDF signalling pathway. Effects of memantine were evaluated in Long-Evans rats self-administering moderate or high amounts of EtOH 6, 30 and 54 hours after an acute injection (12.5 and 25 mg/kg). Motivation to consume alcohol was investigated through a progressive ratio paradigm. The possible role for BDNF in the memantine effect was tested by blockade of the TrkB receptor using the pharmacological agent K252a and by the BDNF scavenger TrkB-Fc. Candidate genes expression was also assessed by polymerase chain reaction array 4 and 28 hours after memantine injection. We found that memantine decreased EtOH self-administration and motivation to consume EtOH 6 and 30 hours post-injection. In addition, we found that inhibition or blockade of the BDNF signalling pathway prevented the early, but not the delayed decrease in EtOH consumption induced by memantine. Finally, Bdnf expression was differentially regulated between the early and delayed timepoints. These results demonstrate that an acute injection of memantine specifically reduces EtOH self-administration and motivation to consume EtOH for at least 30 hours. Moreover, we showed that BDNF was responsible for the early effect, but that the delayed effect was BDNF-independent.

Deciphering the relationship between vulnerability to ethanol-induced behavioral sensitization and ethanol consumption in outbred mice

Ethanol (EtOH)-induced behavioral sensitization (EIBS) is proposed to play a role in early and recurring steps of alcohol dependence, but its impact on alcohol abuse is not clear. EIBS development is dependent upon animal species, strain and also individual factors. We proposed here to decipher the co-expression of EIBS and EtOH intake in individual animals among outbred Swiss mice, which exhibit heterogeneity that parallels what may occur in humans. To do so, mice were exposed to a two-bottle choice with free access to water or 10% EtOH for 6 days just before and immediately after chronic intraperitoneal 2.5 g/kg ethanol injections once a day for 10 consecutive days. Based on their sensitization scores, mice were split into resistant and sensitized animals. First, we showed that individual susceptibility to EIBS is inversely correlated with voluntary EtOH consumption. Exposure to repeated EtOH during EIBS development increased subsequent EtOH intake among the entire population. Very interestingly, subsequent analyses suggested that the less the mice are sensitized the more they increase their EtOH intake; however, resistant mice were sensitive to EtOH adulteration with quinine, whereas sensitized ones maintained their EtOH intake levels, therefore exhibiting a compulsive-like drinking pattern. In addition, we showed that resistant mice do not exhibit a weaker sensitivity to the aversive properties of EtOH that may contribute to their higher level of EtOH intake compared to sensitized mice. This study confirms and extends previous data showing a deep relationship between propensity for EtOH consumption and susceptibility to EIBS in Swiss mice.