Sylvie Granon

Nicotine reinforcement and cognition restored by targeted expression of nicotinic receptors

Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, but nicotine is also known to enhance cognitive performance. Identifying the molecular mechanisms involved in nicotine reinforcement and cognition is a priority and requires the development of new in vivo experimental paradigms. The ventral tegmental area (VTA) of the midbrain is thought to mediate the reinforcement properties of many drugs of abuse. Here we specifically re-expressed the β2-subunit of the nicotinic acetylcholine receptor (nAChR) by stereotaxically injecting a lentiviral vector into the VTA of mice carrying β2-subunit deletions. We demonstrate the efficient re-expression of electrophysiologically responsive, ligand-binding nicotinic acetylcholine receptors in dopamine-containing neurons of the VTA, together with the recovery of nicotine-elicited dopamine release and nicotine self-administration. We also quantified exploratory behaviours of the mice, and showed that β2-subunit re-expression restored slow exploratory behaviour (a measure of cognitive function) to wild-type levels, but did not affect fast navigation behaviour. We thus demonstrate the sufficient role of the VTA in both nicotine reinforcement and endogenous cholinergic regulation of cognitive functions.

Reduced social interaction and ultrasonic communication in a mouse model of monogenic heritable autism.

Autism spectrum conditions (ASCs) are heritable conditions characterized by impaired reciprocal social interactions, deficits in language acquisition, and repetitive and restricted behaviors and interests. In addition to more complex genetic susceptibilities, even mutation of a single gene can lead to ASC. Several such monogenic heritable ASC forms are caused by loss-of-function mutations in genes encoding regulators of synapse function in neurons, including NLGN4. We report that mice with a loss-of-function mutation in the murine NLGN4 ortholog Nlgn4, which encodes the synaptic cell adhesion protein Neuroligin-4, exhibit highly selective deficits in reciprocal social interactions and communication that are reminiscent of ASCs in humans. Our findings indicate that a protein network that regulates the maturation and function of synapses in the brain is at the core of a major ASC susceptibility pathway, and establish Neuroligin-4-deficient mice as genetic models for the exploration of the complex neurobiological disorders in ASCs.

Working memory, response selection, and effortful processing in rats with medial prefrontal lesions.

This study examined the effects of lesions of the prelimbic area of the rat prefrontal cortex on acquisition and retention of nonmatching (NMTS) and matching-to-sample (MTS) tasks. Both tasks involved a reference and a working memory component, but only working memory was impaired by the lesions. A comparison of the 2 tasks revealed quantitatively similar deficits in postoperatively trained rats. In preoperatively trained rats, however, the deficits were more important in the MTS task than in the NMTS task. In addition, an effect of interference between successive trials was observed in the NMTS task but not in the MTS task. Perseverative tendencies were observed in the MTS task only. These results suggest that prefrontal lesions induce working memory deficits as a result of poor temporal encoding and increased susceptibility to interference and impair effortful processing, such as that engaged in response selection mechanisms.

Nicotinic and muscarinic receptors in the rat prefrontal cortex: Differential roles in working memory, response selection and effortful processing

The aim of the present study was to evaluate the effects of cholinergic receptor blockade in the rat prefrontal cortex on cognitive processes. The nicotinic antagonists neuronal bungarotoxin and dihydro-β-erythroidine and the muscarinic antagonist scopolamine were injected into the prelimbic area of the prefrontal cortex. Their behavioural effects were assessed in a T-maze to test reference memory (visual discrimination task) and working memory in delayed matching (MTS) and non-matching to sample (NMTS) tasks. Neuronal bungarotoxin produced a significant decrease in working memory performance in the MTS task but not in the NMTS task. In contrast, scopolamine impaired working memory in both MTS and NMTS tasks. Reference memory was not altered by any of the cholinergic antagonists. These results demonstrate a differential role of nicotinic and muscarinic receptors in the rat prefrontal cortex. Nicotinic transmission appears to be important in delayed response tasks requiring effortful processing for response selection, while the muscarinic system is involved in general working memory processes.

Executive and social behaviors under nicotinic receptor regulation

Nicotine enhances several cognitive and psychomotor behaviors, and nicotinic antagonists cause impairments in tasks requiring cognitive effort. To explore the contribution of nicotinic receptors to complex cognitive functions, we developed an automated method to investigate sequential locomotor behavior in the mouse and an analysis of social behavior. We show that, in the β2-/- mutant, the high-order spatiotemporal organization of locomotor behavior, together with conflict resolution and social interaction, is selectively dissociated from low-level, more automatic motor behaviors. Such deficits in executive functions resemble the rigid and asocial behavior found in some psychopathological disorders such as autism and attention deficit hyperactivity disorder.

Computerized video analysis of social interactions in mice

The study of social interactions in mice is used as a model for normal and pathological cognitive and emotional processes. But extracting comprehensive behavioral information from videos of interacting mice is still a challenge. We describe a computerized method and software, MiceProfiler, that uses geometrical primitives to model and track two mice without requiring any specific tagging. The program monitors a comprehensive repertoire of behavioral states and their temporal evolution, allowing the identification of key elements that trigger social contact. Using MiceProfiler we studied the role of neuronal nicotinic receptors in the establishment of social interactions and risk-prone postures. We found that the duration and type of social interactions with a conspecific evolves differently over time in mice lacking neuronal nicotinic receptors (Chrnb2−/−, here called β2−/−), compared to C57BL/6J mice, and identified a new type of coordinated posture, called back-to-back posture, that we rarely observed in β2−/− mice.

Adult Male Mice Emit Context-Specific Ultrasonic Vocalizations That Are Modulated by Prior Isolation or Group Rearing Environment

Social interactions in mice are frequently analysed in genetically modified strains in order to get insight of disorders affecting social interactions such as autism spectrum disorders. Different types of social interactions have been described, mostly between females and pups, and between adult males and females. However, we recently showed that social interactions between adult males could also encompass cognitive and motivational features. During social interactions, rodents emit ultrasonic vocalizations (USVs), but it remains unknown if call types are differently used depending of the context and if they are correlated with motivational state. Here, we recorded the calls of adult C57BL/6J male mice in various behavioral conditions, such as social interaction, novelty exploration and restraint stress. We introduced a modulator for the motivational state by comparing males maintained in isolation and males maintained in groups before the experiments. Male mice uttered USVs in all social and non-social situations, and even in a stressful restraint context. They nevertheless emitted the most important number of calls with the largest diversity of call types in social interactions, particularly when showing a high motivation for social contact. For mice maintained in social isolation, the number of calls recorded was positively correlated with the duration of social contacts, and most calls were uttered during contacts between the two mice. This correlation was not observed in mice maintained in groups. These results open the way for a deeper understanding and characterization of acoustic signals associated with social interactions. They can also help evaluating the role of motivational states in the emission of acoustic signals.

Reinforcing effects of nicotine microinjections into the ventral tegmental area of mice: Dependence on cholinergic nicotinic and dopaminergic D1 receptors

We used an intracranial self-administration (ICSA) procedure to assess the involvement of the ventral tegmental area (VTA) nicotinic receptors in the rewarding effects of nicotine. We then challenged intra-VTA nicotine self-administration via systemic or local injections of dopamine (DA)-D1 and nicotinic receptor antagonists. C57BL/6J mice were stereotaxically implanted unilaterally with a guide cannula above the VTA. After 1 week of recovery, mice were allowed to discriminate between two arms of a Y-maze over seven daily sessions, one arm being reinforced by intracranial nicotine microinjection. Mice exhibited nicotine self-administration at both doses tested, i.e. 10 ng (21.6 pmol) and 100 ng (216 pmol)/50-nl injection. In contrast, mice receiving a 216-pmol nicotine dose 0.8 mm above VTA performed at chance level. Once the ICSA response was acquired, systemic pretreatment with the DA-D1 receptor antagonist SCH 23390 (25 microg/kg i.p.) or co-infusion of the nAChR antagonist DHbetaE with nicotine disrupted ICSA. Replacement of SCH 23390 by vehicle, or withdrawal of DHbetaE from nicotine/DHbetaE mixed solutions led to recovery of intra-VTA nicotine self-administration. We conclude that nicotinic receptors in the VTA, presumably alpha4beta2 nAChRs are critically to mediate the rewarding effects of nicotine and that DA-D1 receptors are also directly implicated.

Monoamine Oxidase Inhibitors Allow Locomotor and Rewarding Responses to Nicotine

Although nicotine is generally considered to be the main compound responsible for the addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other abused substances, such as psychostimulants and opiates. For example, nicotine is only a weak locomotor enhancer in rats and generally fails to induce a locomotor response in mice. This observation contradicts the general consensus that all drugs of abuse release dopamine in the nucleus accumbens, a subcortical structure, and thus increase locomotor activity in rodents. Because tobacco smoke contains monoamine oxidase inhibitors (MAOIs) and decreases MAO activity in smokers, we have combined MAOIs with nicotine to determine whether it is possible to obtain a locomotor response to nicotine in C57Bl6 mice. Among 15 individual or combined MAOIs, including harmane, norharmane, moclobemide, selegiline, pargyline, clorgyline, tranylcypromine and phenelzine, only irreversible inhibitors of both MAO-A and -B (tranylcypromine, phenelzine, and clorgyline+selegiline) allowed a locomotor response to nicotine. The locomotor stimulant interaction of tranylcypromine and nicotine was absent in β2-nicotinic acetylcholine receptor subunit knockout mice. Finally, it was found that, whereas naïve rats did not readily self-administer nicotine (10 μg/kg/injection), a robust self-administration of nicotine occurred when animals were pretreated with tranylcypromine (3 mg/kg). Our data suggest that MAOIs contained in tobacco and tobacco smoke act in synergy with nicotine to enhance its rewarding effects.

Long-term effects of chronic nicotine exposure on brain nicotinic receptors

Chronic nicotine exposure results in long-term homeostatic regulation of nicotinic acetylcholine receptors (nAChRs) that play a key role in the adaptative cellular processes leading to addiction. However, the relative contribution of the different nAChR subunits in this process is unclear. Using genetically modified mice and pharmacological manipulations, we provide behavioral, electrophysiological, and pharmacological evidence for a long-term mechanism by which chronic nicotine triggers opposing processes differentially mediated by β2*- vs. α7*nAChRs. These data offer previously undescribed insights into the understanding of nicotine addiction and the treatment of several human pathologies by nicotine-like agents chronically acting on β2*- or α7*nAChRs.