Benjamin Boutrel

Neurobiological mechanisms in the transition from drug use to drug dependence

Drug addiction is a chronic relapsing disorder characterized by compulsive drug intake, loss of control over intake, and impairment in social and occupational function. Animal models have been developed for various stages of the addiction cycle with a focus in our work on the motivational effects of drug dependence. A conceptual framework focused on allostatic changes in reward function that lead to excessive drug intake provides a heuristic framework with which to identify the neurobiologic mechanisms involved in the development of drug addiction. Neuropharmacologic studies in animal models have provided evidence for the dysregulation of specific neurochemical mechanisms in specific brain reward and stress circuits that provide the negative motivational state that drives addiction. The allostatic model integrates molecular, cellular and circuitry neuroadaptations in brain motivational systems produced by chronic drug ingestion with genetic vulnerability, and provides a new opportunity to translate advances in animal studies to the human condition.

Optogenetic Interrogation of Dopaminergic Modulation of the Multiple Phases of Reward-Seeking Behavior

Phasic activation of dopaminergic neurons is associated with reward-predicting cues and supports learning during behavioral adaptation. While noncontingent activation of dopaminergic neurons in the ventral tegmental are (VTA) is sufficient for passive behavioral conditioning, it remains unknown whether the phasic dopaminergic signal is truly reinforcing. In this study, we first targeted the expression of channelrhodopsin-2 to dopaminergic neurons of the VTA and optimized optogenetically evoked dopamine transients. Second, we showed that phasic activation of dopaminergic neurons in freely moving mice causally enhances positive reinforcing actions in a food-seeking operant task. Interestingly, such effect was not found in the absence of food reward. We further found that phasic activation of dopaminergic neurons is sufficient to reactivate previously extinguished food-seeking behavior in the absence of external cues. This was also confirmed using a single-session reversal paradigm. Collectively, these data suggest that activation of dopaminergic neurons facilitates the development of positive reinforcement during reward-seeking and behavioral flexibility.

Stress and arousal: the corticotrophin-releasing factor/hypocretin circuitry.

The hypocretins (also know as orexins) are two neuropeptides now commonly described as critical components for maintaining and regulating the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. New data indicate that the corticotrophin-releasing factor (CRF) peptidergic system directly innervates hypocretin-expressing neurons. CRF depolarizes hypocretin neurons, and this effect is blocked by a CRF-R1 antagonist. Furthermore, activation of hypocretinergic neurons by stress is impaired in CRF-R1 knockout mice. These data suggest that CRF-R1 receptor mediates the stress-induced activation of the hypocretinergic system. A significant amount of evidence also indicates that hypocretin cells connect reciprocally to the CRF system. We propose that upon stressor stimuli, CRF activates the hypocretin system, which relays these signals to brain stem nuclei involved in the modulation of arousal as well as to the extended amygdala, a structure involved in the negative motivational state that drives addiction.

Insulin induces long-term depression of ventral tegmental area dopamine neurons via endocannabinoids

The prevalence of obesity has markedly increased over the past few decades. Exploration of how hunger and satiety signals influence the reward system can help us understand non-homeostatic feeding. Insulin may act in the ventral tegmental area (VTA), a critical site for reward-seeking behavior, to suppress feeding. However, the neural mechanisms underlying insulin effects in the VTA remain unknown. We demonstrate that insulin, a circulating catabolic peptide that inhibits feeding, can induce long-term depression (LTD) of mouse excitatory synapses onto VTA dopamine neurons. This effect requires endocannabinoid-mediated presynaptic inhibition of glutamate release. Furthermore, after a sweetened high-fat meal, which elevates endogenous insulin, insulin-induced LTD is occluded. Finally, insulin in the VTA reduces food anticipatory behavior in mice and conditioned place preference for food in rats. Taken together, these results suggest that insulin in the VTA suppresses excitatory synaptic transmission and reduces anticipatory activity and preference for food-related cues.

The role of hypocretin in driving arousal and goal-oriented behaviors

The hypocretins (Hcrts), also called orexins, are two neuropeptides secreted by a few thousand neurons restricted to the lateral hypothalamus. The Hcrt peptides bind to two receptors located in nuclei associated with diverse cognitive and physiological functions. Experimental evidence has demonstrated that the physiological roles of hypocretins extend far beyond its initial role in food consumption and has emerged as a key system in the fields of sleep disorders and drug addiction. Here, we discuss recent evidence demonstrating a key role of hypocretin in the motivation for reward seeking in general, and drug taking in particular, and we delineate a physiological framework for this peptidergic system in orchestrating the appropriate levels of alertness required for the elaboration and the execution of goal-oriented behaviors. We propose a general role for hypocretins in mediating arousal, especially when an organism must respond to unexpected stressors and environmental challenges, which serve to shape survival behaviors. We also discuss the limit of the current experimental paradigms to address the question of how a system normally involved in the regulation of vigilance states and hyperarousal may promote a pathological state that elicits compulsive craving and relapse to drug seeking.

Addiction and arousal: The hypocretin connection

The hypocretins, also known as orexins, are two neuropeptides now commonly described as critical components to maintain and regulate the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. Intracerebral administration of hypocretin leads to a dose-related reinstatement of drug and food seeking behaviors. Furthermore, stress-induced reinstatement can be blocked with hypocretin receptor 1 antagonism. These results, together with recent data showing that hypocretin is critically involved in cocaine sensitization through the recruitment of NMDA receptors in the ventral tegmental area, strongly suggest that activation of hypocretin neurons play a critical role in the development of the addiction process. The activity of hypocretin neurons may affect addictive behavior by contributing to brain sensitization or by modulating the brain reward system. Hypocretinergic cells, in coordination with brain stress systems may lead to a vulnerable state that facilitates the resumption of drug seeking behavior. Hence, the hypocretinergic system is a new drug target that may be used to prevent relapse of drug seeking.

Addiction and Arousal: Alternative Roles of Hypothalamic Peptides

The importance of the lateral hypothalamus in the regulation of reward and motivation has long been recognized. However, the neuronal network involved in such a hypothalamic regulation of reward remains essentially unknown. Recently, hypocretin-containing neurons, a group of hypothalamic neurons known to be associated with the stability of arousal, have emerged as important structures in the control of brain reward function. This review summarizes a Mini-Symposium presented at the 2006 Annual Meeting of the Society for Neuroscience.

5-HT 1A/1B receptor-mediated effects of the selective serotonin reuptake inhibitor, citalopram, on sleep: studies in 5-HT 1A and 5-HT 1B knockout mice.

Selective serotonin reuptake inhibitors (SSRIs) are extensively used for the treatment of depression. Aside from their antidepressant properties, they provoke a deficit in paradoxical sleep (PS) that is most probably mediated by the transporter blockade-induced increase in serotonin concentration in the extracellular space. Such an effect can be accounted for by the action of serotonin at various types of serotonergic receptors involved in PS regulation, among which the 5-HT1A and 5-HT1B types are the best candidates. According to this hypothesis, we examined the effects of citalopram, the most selective SSRI available to date, on sleep in the mouse after inactivation of 5-HT1A or 5-HT1B receptors, either by homologous recombination of their encoding genes, or pharmacological blockade with selective antagonists. For this purpose, sleep parameters of knockout mice that do not express these receptors and their wild-type counterparts were monitored during 8 h after injection of citalopram alone or in association with 5-HT1A or 5-HT1B receptor antagonists. Citalopram induced mainly a dose-dependent inhibition of PS during 2–6 h after injection, which was observed in wild-type and 5-HT1B−/− mice, but not in 5-HT1A−/− mutants. This PS inhibition was fully antagonized by pretreatment with the 5-HT1A antagonist WAY 100635, but only partially with the 5-HT1B antagonist GR 127935. These data indicate that the action of the SSRI citalopram on sleep in the mouse is essentially mediated by 5-HT1A receptors. Such a mechanism of action provides further support to the clinical strategy of antidepressant augmentation by 5-HT1A antagonists, because the latter would also counteract the direct sleep-inhibitory side-effects of SSRIs.

Orexin/hypocretin (Orx/Hcrt) transmission and drug-seeking behavior: is the paraventricular nucleus of the thalamus (PVT) part of the drug seeking circuitry?

The orexin/hypocretin (Orx/Hcrt) system has long been considered to regulate a wide range of physiological processes, including feeding, energy metabolism, and arousal. More recently, concordant observations have demonstrated an important role for these peptides in the reinforcing properties of most drugs of abuse. Orx/Hcrt neurons arise in the lateral hypothalamus (LH) and project to all brain structures implicated in the regulation of arousal, stress, and reward. Although Orx/Hcrt neurons have been shown to massively project to the paraventricular nucleus of the thalamus (PVT), only recent evidence suggested that the PVT may be a key relay of Orx/Hcrt-coded reward-related communication between the LH and both the ventral and dorsal striatum. While this thalamic region was not thought to be part of the “drug addiction circuitry,” an increasing amount of evidence demonstrated that the PVT—particularly PVT Orx/Hcrt transmission—was implicated in the modulation of reward function in general and several aspects of drug-directed behaviors in particular. The present review discusses recent findings that suggest that maladaptive recruitment of PVT Orx/Hcrt signaling by drugs of abuse may promote persistent compulsive drug-seeking behavior following a period of protracted abstinence and as such may represent a relevant target for understanding the long-term vulnerability to drug relapse after withdrawal.

Influence of ethanol dose and pigmentation on the incorporation of ethyl glucuronide into rat hair.

Ethyl glucuronide (EtG) is a minor and specific metabolite of ethanol. It is incorporated into growing hair, allowing a retrospective detection of alcohol consumption. However, the suitability of quantitative EtG measurements in hair to determine the quantity of alcohol consumed has not clearly been demonstrated yet. The purpose of this study was to evaluate the influence of ethanol dose and hair pigmentation on the incorporation of EtG into rat hair. Ethanol and EtG kinetics in blood were investigated after a single administration of ethanol. Eighteen rats were divided into four groups receiving 0 (control group), 1, 2, or 3g ethanol/kg body weight. Ethanol was administered on 4 consecutive days per week for 3 weeks by intragastric route. Twenty-eight days after the initial ethanol administration, newly grown hair was shaved. Pigmented and nonpigmented hair were analyzed separately by gas chromatography coupled to tandem mass spectrometry. Blood samples were collected within 12h after the ethanol administration. EtG and ethanol blood levels were measured by liquid chromatography coupled to tandem mass spectrometry and headspace gas chromatography-flame ionization detector, respectively. No statistically significant difference was observed in EtG concentrations between pigmented and nonpigmented hair (Spearmans rho=0.95). Thus, EtG incorporation into rat hair was not affected by hair pigmentation. Higher doses of ethanol resulted in greater blood ethanol area under the curve of concentration versus time (AUC) and in greater blood EtG AUC. A positive correlation was found between blood ethanol AUC and blood EtG AUC (Spearmans rho=0.84). Increased ethanol administration was associated with an increased EtG concentration in hair. Blood ethanol AUC was correlated with EtG concentration in hair (Pearsons r=0.89). EtG concentration in rat hair appeared to reflect the EtG concentration in blood. Ethanol was metabolized at a median rate of 0.22 g/kg/h, and the median elimination half-life of EtG was 1.21 h. This study supports that the bloodstream is likely to display a major role in the hair EtG incorporation.