Alpha7-nicotinic receptors modulate nicotine-induced

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(ACb), is considered a core site for the processing of nicotine's reinforcing .... doses were tested: 35 ng and 70 ng/50 nL (as free base; the doses referring to the ...
Psychopharmacology DOI 10.1007/s00213-011-2422-1

ORIGINAL INVESTIGATION

Alpha7-nicotinic receptors modulate nicotine-induced reinforcement and extracellular dopamine outflow in the mesolimbic system in mice Morgane Besson & Vincent David & Mathieu Baudonnat & Pierre Cazala & Jean-Philippe Guilloux & Christelle Reperant & Isabelle Cloez-Tayarani & Jean-Pierre Changeux & Alain M. Gardier & Sylvie Granon

Received: 13 June 2011 / Accepted: 16 July 2011 # Springer-Verlag 2011

Abstract Rationale Nicotine is the main addictive component of tobacco and modifies brain function via its action on neuronal acetylcholine nicotinic receptors (nAChRs). The mesolimbic dopamine (DA) system, where neurons of the ventral tegmental area (VTA) project to the nucleus accumbens (ACb), is considered a core site for the processing of nicotine’s reinforcing properties. However, the precise subtypes of nAChRs that mediate the rewarding properties of nicotine and that contribute to the development of addiction remain to be identified. Objectives We investigated the role of the nAChRs containing the α7 nicotinic subunit (α7*nAChRs) in the reinforcing properties of nicotine within the VTA and in the nicotineinduced changes in ACb DA outflow in vivo. Methods We performed intra-VTA self-administration and microdialysis experiments in genetically modified mice

lacking the α7 nicotinic subunit or after pharmacological blockade of α7*nAChRs in wild-type mice. Results We show that the reinforcing properties of nicotine within the VTA are lower in the absence or after pharmacological blockade of α7*nAChRs. We also report that nicotineinduced increases in ACb DA extracellular levels last longer in the absence of these receptors, suggesting that α7*nAChRs regulate the action of nicotine on DA levels over time. Conclusions The present results reveal new insights for the role of α7*nAChRs in modulating the action of nicotine within the mesolimbic circuit. These receptors appear to potentiate the reinforcing action of nicotine administered into the VTAwhile regulating its action over time on DA outflow in the ACb. Keywords Nicotine . Nicotinic receptors . Knockout mice . Ventral tegmental area . Nucleus accumbens . Reinforcement . Dopamine

M. Besson and V. David equally contributed to the work. M. Besson : I. Cloez-Tayarani : J.-P. Changeux : S. Granon Unité Neurobiologie Intégrative des Systèmes Cholinergiques, Unité de Recherche Associée 2182, Centre National de la recherche Scientifique, Institut Pasteur, 75015 Paris, France

M. Besson (*) Unité Neurobiologie Intégrative des Systèmes Cholinergiques, Institut Pasteur, 25 rue du Dr. Roux, 75015 Paris, France e-mail: [email protected]

V. David : M. Baudonnat : P. Cazala Institut de Neurosciences Cognitives et Intégratives d’Aquitaine, CNRS UMR 5287, Université de Bordeaux, 33405 Talence, France

Present Address: I. Cloez-Tayarani Unité de Génétique Humaine et Fonctions Cognitives, Institut Pasteur, 75015 Paris, France

J.-P. Guilloux : C. Reperant : A. M. Gardier EA 3544, Faculté de Pharmacie, Université Paris-Sud, Châtenay-Malabry Cedex 92296, France

Present Address: S. Granon Centre de Neurosciences Paris Sud XI, CNRS UMR 8195, Université Paris-Sud, 91405 Orsay Cedex, France

Psychopharmacology

Introduction Tobacco smoking is one of the major causes of disease in the world (Peto et al. 1996). Nicotine is considered as the main reinforcing component responsible for the addictive properties of tobacco (Stolerman and Jarvis 1995). Various neural pathways appear to encode the addictive properties of drugs. As for most drugs of abuse, an increasing body of evidence point out to a role of the mesolimbic dopamine system in the reinforcing properties of nicotine. Notably, systemic nicotine administration or local infusion into the ventral tegmental area (VTA) causes an increase in dopamine (DA) release in the nucleus accumbens (ACb) (di Chiara 2000; Imperato et al. 1986; Nisell et al. 1994; Schilström et al. 2000). DA neuronal activity is enhanced by nicotine (Grenhoff et al. 1986; Grenhoff and Svensson 1988), and nicotine lowers the threshold for intracranial self-stimulation into the VTA (Bespalov et al 1999). Accordingly, the VTA has been directly implicated in the reinforcing (Besson et al. 2006; David et al. 2006; Ikemoto et al. 2006; Laviolette and van der Kooy 2004; Maskos et al. 2005) and rewarding (Laviolette and van der Kooy 2003a) properties of nicotine. Long-term or repeated administration of nicotine (Belluardo et al. 2000; Brown and Kolb 2001; Pich et al. 1997; Shim et al. 2001; Visanji et al. 2006) and nicotine withdrawal (Gäddnäs et al. 2002; Hildebrand et al. 1999; Rada et al. 2001) also induce neuroadaptations within the reward system. After chronic exposure to nicotine, high affinity nicotinic receptors could remain in a desensitised state in animals (Benwell et al, 1995) and human smokers (Brody et al, 2006), which could account for tolerance to nicotine’s effect on dopamine activity (Benwell et al, 1995). Neuronal acetylcholine nicotinic receptors (nAChRs) are homo- and hetero-pentameric cationic ligand-gated ion channels composed of alpha and beta subunits (Changeux and Edelstein 2005). Various sub-types of nAChRs are expressed in the brain, which differ in their subunit composition and sensitivity to nicotine. The heteromeric receptors containing the β2 subunit (β2*nAChRs) and the homomeric receptors containing the α7 subunit (α7*nAChRs) are the most abundant nAChRs found in the brain. They are widely expressed in the mesolimbic system, notably in the VTA (Champtiaux et al. 2003; Jones and Wonnacott 2004; Pidoplichko et al. 1997; Zhou et al. 2002). The crucial role of β2*nAChRs in the reinforcing properties of nicotine is well established (Corrigall et al. 1994; Grabus et al. 2006; Walters et al. 2006). Genetically modified mice lacking the β2 subunit (β2−/− mice) (Picciotto et al. 1995) do not exhibit nicotine reinforcement (Besson et al. 2006; Grabus et al. 2006; Picciotto et al. 1998; Pons et al. 2008), and specific re-expression via a viral vector of the β2 subunit within the VTA restores nicotine self-administration in these mice (Besson et al.

2006; Maskos et al. 2005; Pons et al. 2008). Moreover, in β2−/− mice, nicotine neither elicit any increase in mesolimbic DA release nor modify DA neuronal activity in the VTA (Mameli-Engvall et al. 2006; Maskos et al. 2005; Picciotto et al. 1998). In contrast, the involvement of α7*nAChRs in the reinforcing properties of nicotine and in nicotine-induced functional modifications of the mesolimbic system has not been fully characterised. Systemic administration of methyllylcaconitine (MLA), an α7*nAChR antagonist, produces inconsistent effects on nicotine rewarding and reinforcing effects (Grabus et al. 2006; Grottick et al. 2000; Markou and Paterson 2001). Mice lacking the α7 subunit (α7−/− mice) displayed normal nicotine-induced conditioned place preference (CPP) (Walters et al. 2006). In α7−/− mice, DA neurons showed a large degree of variability in response to nicotine, as well as an attenuated increase in firing rate as compared to wild-type (WT) controls, with no changes in bursting (Mameli-Engvall et al. 2006). This contrasts with the lack of any DA neuronal response to nicotine in β2−/− mice observed in the same study. However, a role for α7*nAChRs in homeostatic maintenance of both mesolimbic DA function and exploratory behaviour after long-term exposure to nicotine has been identified in mice (Besson et al. 2007). Finally, using functional magnetic resonance imaging in knockout mice, β2*nAChRs and α7*nAChRs have been shown to exert complementary effects on nicotine-evoked brain activation (Suarez et al. 2009). In order to understand better the neurobiological mechanisms by which nicotine acts on the mesolimbic DA system to elicit its reinforcing effects, it is essential to further identify the nAChR subtypes involved in the action of nicotine on the mesolimbic system. For this purpose, we have investigated the role of α7*nAChRs in (a) the reinforcing properties of nicotine using an intra-VTA selfadministration procedure and (b) in the effects of nicotine on striatal extracellular levels of DA using microdialysis in freely moving animals.

Materials and methods Ethical statement All procedures were carried out in accordance with the guidelines of the European Communities Council Directive of 24 November 1986 (86/609/ECC). Subjects Experiments were performed on wild-type (α7+/+) (C57BL6/J strain, Charles River) and α7−/− mutant male mice, between 3 and 6 months of age. The construction of

Psychopharmacology

the α7−/− mice was previously described (Orr-Urtreger et al. 1997). The α7−/− mice had been back-crossed for nine generations to the C57BL6/J parental strain. Mice were housed in cages containing either a maximum of six animals or individually (after surgery), under a 12-h light– dark cycle in rooms at a controlled temperature (21°C) with free access to water and food. All experiments were carried out between 10 A.M. and 6 P.M. Drugs For self-administration experiments, (−)nicotine hydrogene tartrate (Sigma) was dissolved in artificial cerebrospinal fluid (aCSF; microdialysis perfusion fluid, Phymep®). Two doses were tested: 35 ng and 70 ng/50 nL (as free base; the doses referring to the weight of the salt were 100 and 200 ng). The choice of the dose was based on our previous studies showing that the dose of 35 ng induces optimal selfadministration performance in WT mice (Besson et al. 2006; David et al. 2006). Methyllycaconitine citrate (Sigma) was dissolved in sterile saline solution (0.9% NaCl), and a dose of 1 mg/kg (free base) was used. For microdialysis experiments, nicotine was dissolved in 0.9% NaCl, and a dose of 1 mg/kg (free base) was used, based on previous studies (Champtiaux et al. 2003; Maskos et al. 2005). The pH was adjusted with NaOH to reach NaCl or aCSF pH (7.4). Intracranial nicotine self-administration Surgery Mice were anaesthetised with ketamine (Virbac, France) at 100 mg/kg and xylazine (Rompun 2%) at 8 mg/kg and unilaterally implanted with a guide cannula (outer diameter, 0.460 mm; inner diameter, 0.255 mm) into the VTA or the substantia nigra (SN) in a counterbalanced left and right order. Lidocaine HCl (Xylocaine®, 5%) was applied locally before opening the scalp. Stereotaxic coordinates were taken from the mouse brain atlas (Paxinos and Franklin 2004) (from Bregma: VTA, posterior, −3.34 mm; lateral, ±0.30 mm; ventral, −3.30 mm; SN: posterior, −3.34 mm; lateral, ±0.80 mm; ventral, −3.30 mm). Mice were allowed to recover from surgery for 1 week and received a daily systemic administration of Fynadine® (1 mg/kg), a nonopioid/non-steroid analgesic for five consecutive days.

one of the most relevant methods to assess reward effectiveness (Mason et al. 1985). The stem and the arms were 31 cm long and 12 cm high. The starting box (14×8cm) was separated from the stem by a sliding door. Each arm was composed of a sliding door at its entrance and a photoelectric cell 6 cm from its end. Arms were separated by an angle of 90°. Before starting the experiment, mice were habituated to the maze for one session but with no injection available. Afterwards, acquisition of nicotine self-administration was tested for seven consecutive days. Immediately before each daily training session, a stainless-steel injection cannula (outer diameter of 0.299 mm or gauge of 31, inner diameter of 0.127 mm or gauge of 36) was inserted into the VTA or the SN and was held in a fixed position by means of a small connector. The injection cannula was connected by a flexible polyethylene tubing to the microinjection system, which housed a 5-μL Hamilton syringe. The tip of the injection cannula projected beyond the guide cannula by 1.5 mm, so that minimum damage was caused to the brain area targeted by nicotine infusion. Syringe and catheter are filled with noncompressible extra-purified paraffin oil, so that each computer-controlled step of the motor drives the syringe piston pushing instantly and proportionally the nicotine solution. By interrupting photocell beams at the end of one arm of the Y-maze, mice could trigger an injection of drug. Each self-injection lasted 4.8 s. Normal drug flow was verified visually both before and after each session for each animal. If they chose the other arm, they received no injection. As soon as the mouse interrupted the photocell beams at the end of either the right or the left arm and after a 30-s confinement, the trial ended, and the mouse was manually replaced in the starting box. The following trial was then started with the same procedure until completion of the ten daily successive trials. The reinforced arm was the right one for half of the mice, and the left one for the other half. The number of runs into the reinforced arm (that is, the number of self-administrations) and the latency to trigger the injection (the self-injection latency, calculated as the mean latency to enter the reinforced arm over the reinforced trials) per daily session were recorded. After the acquisition phase, one group of mice received intraperitoneal (i.p.) injections of MLA 30 min before the selfadministration session. The effects of such treatment were assessed for five daily sessions. MLA was then replaced by NaCl injections for three additional daily sessions. Histology

Behavioural procedure As described previously (David et al. 2006; Maskos et al. 2005), behavioural experiments were conducted in a Y-maze using a spatial discrimination task between a nicotinereinforced and a neutral arm. Choice-based paradigms are

Mice were killed by an overdose of tribromoethanol (Avertin, 600 mg/kg). The head was placed in 10% formol for a 72-h period. The guide cannula was then withdrawn, and the brain was dissected and placed in a solution of formalin containing 30% sucrose for a further week. Frozen

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brains were cut on a microtome to provide 50-μm sections stained using 0.1% of thionine to identify the injection site. In vivo microdialysis Microdialysis and DA measurements were made as previously described (Malagié et al. 2001; Marubio et al. 2003; Maskos et al. 2005). Surgery Mice were anaesthetised with chloral hydrate (400 mg/kg, i.p.) and unilaterally implanted with a concentric microdialysis probe made of cuprophan fibres (active length of 1.0 mm, outer diameter of 0.30 mm) into the ACb, in a counterbalanced left and right order. Stereotaxic coordinates were taken from the mouse brain atlas (Paxinos and Franklin 2004) (from Bregma: anterior, 1.34 mm; lateral, ±0.7 mm; ventral, −5.4 mm). Procedure After approximately 20 h recovery from surgery, the probe was perfused continuously with artificial cerebrospinal fluid at a flow rate of 1.5 μL/min (composition in mM: NaCl, 147; KCl, 3.5; CaCl2, 1.26; MgCl2, 1.2; NaH2PO4, 1.0; NaHCO3, 25.0; pH 7.4±0.2). Dialysate samples were collected every 15 min, and DA content was measured using high-performance liquid chromatography (XL-ODS, 4.6×7.0 mm, particle size 3 m; Beckman, Roissy-Charlesde-Gaulle, France) coupled to amperometric detection (1049A, Hewlett-Packard, Les Ulis, France). Eight samples were collected before drug administration to determine basal extracellular levels of DA. Mice then received an i.p. injection of either saline or nicotine (1 mg/kg), and the response to drug administration was measured over a 2-h period (eight samples). The limit of sensitivity for [DA]ext was 0.5 fmol per sample (signal/noise ratio=2).

of acquisition of nicotine self-administration using the mean number of self-administrations per session and the selfinjection latencies as the dependent variables, training session (training day) as the main within-subject factor, and drug (aCSF, nicotine 35 ng or nicotine 70 ng), genotype (α7+/+ or α7−/−) and brain area (VTA or SN) as the main between-subject factors. Upon obtaining a significant main effect (p