Nicotine enhances responding for conditioned reinforcement via α4β2 nicotinic acetylcholine receptors in the ventral tegmental area, but not the nucleus accumbens or the prefrontal cortex

Highlights

• Systemic administration of nicotine enhanced responding for conditioned reinforcement.

• This effect of systemic nicotine was attenuated by infusion of DHβE into the VTA, NAcc, and IL cortex, but not PrL cortex.

• Infusion of nicotine into the VTA, but not NAcc or IL cortex, enhanced responding for conditioned reinforcement.

Abstract

Nicotine enhances the conditioned reinforcing properties of reward-paired cues. We investigated the role of the ventral tegmental area (VTA), nucleus accumbens (NAcc), and prelimbic (PrL) and infralimbic (IL) cortices in mediating this enhancement. Male Long-Evans rats were implanted with bilateral guide cannulae aimed at either the VTA, NAcc, PrL or IL cortex. Next, rats underwent 12 sessions of Pavlovian conditioning. Each session consisted of 30 trials wherein a 5 s conditioned stimulus (CS) was paired with water (0.05 ml). Tests of responding for conditioned reinforcement were conducted during which presentation of the CS, now acting as a conditioned reinforcer (CRf), was contingent upon pressing one of two levers (CRf lever). Pressing the other lever had no consequences (NCRf lever). To determine if nicotinic acetylcholine receptors (nAChRs) in the VTA, NAcc, PrL cortex, or IL cortex mediate nicotine-enhanced responding for a CRf, the α4β2 nAChR antagonist Dihydro-Beta-Erythroidine (DHβE; 10 nmol/0.5 μL) was infused into the respective areas prior to a systemic nicotine injection (0.2 mg/kg; SC). DHβE infused into the VTA, NAcc, or IL cortex, but not PrL cortex, attenuated nicotine-enhanced responding for a CRf. Next, to confirm that nAChRs in the VTA, NAcc, or IL cortex mediate this enhancement, nicotine (8, 16, or 32 nmol/0.5 μL) was infused into the respective areas. Nicotine infused into the VTA, but not NAcc or IL cortex, enhanced responding for a CRf. These findings suggest that nicotine primarily acts on α4β2 nAChRs in the VTA to potentiate the conditioned reinforcing properties of reward-related cues.

Introduction

Tobacco use, mostly in the form of cigarette smoking, is associated with adverse health outcomes. Nicotine, the main psychoactive component in tobacco, possesses addictive properties that promote smoking and contribute to tobacco dependence (Stolerman and Jarvis, 1995). The mesolimbic dopamine (DA) system, originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAcc), has been implicated in the behavioral effects of nicotine. Lesions of this system or administration of DA receptor antagonists attenuate the locomotor stimulant effects of nicotine (Clarke et al., 1988) and nicotine self-administration (Corrigall and Coen, 1991; Corrigall et al., 1992). Nicotinic acetylcholine receptors (nAChRs) are found in DAergic neurons at the level of cell bodies in the VTA and terminal fields in the NAcc (Clarke and Pert, 1985; Clarke et al., 1984; Deutch et al., 1987; Swanson et al., 1987). Nicotine may then produce its reinforcement effects by acting on nAChRs in the VTA or the NAcc.

Nicotine either injected systemically (Imperato et al., 1986) or infused into the VTA (Nisell et al., 1994a) or the NAcc (Mifsud et al., 1989; Nisell et al., 1994a) increases extracellular DA levels in the NAcc. However, nAChR antagonists infused into the VTA, but not the NAcc, attenuate this increase in accumbal DA release (Nisell et al., 1994b). With respect to its behavioral effects, nicotine infused into the VTA, but not the NAcc, enhances locomotor activity (Reavill and Stolerman, 1990). Additionally, nAChR antagonists infused into the VTA, but not the NAcc, reduce nicotine self-administration (Corrigall et al., 1994). These findings suggest that nicotine primarily acts at nAChRs in the VTA to produce its neurochemical and behavioral effects.

Environmental cues paired with smoking can evoke subjective and physiological responses indicative of craving, which in turn can increase the likelihood of smoking (Carter and Tiffany, 1999; Payne et al., 1991; Rose and Levin, 1991). In laboratory animal models, sensory cues paired with nicotine intake, such as a light or tone, can increase nicotine self-administration (Caggiula et al., 2002) and reinstate extinguished nicotine self-administration (Liu et al., 2005). The conditioned reinforcing properties of such stimuli may then play a role in maintaining nicotine-seeking behaviors. These properties can be examined in a test of responding for conditioned reinforcement. During an initial Pavlovian conditioning phase, animals learn to associate the presentation of a sensory stimulus with the delivery of an appetitive unconditioned stimulus (US). If they then perform an operant response for this conditioned stimulus (CS) in the absence of US delivery, the rate of responding provides a measure of its conditioned reinforcing properties (Mackintosh, 1974). Using this test, nicotine has been shown to systemically enhance responding for a conditioned reinforcer (CRf) (Guy and Fletcher, 2013; Olausson et al., 2004a, 2004b). This enhancement is attenuated by DA receptor antagonists (Guy and Fletcher, 2014), suggesting that the DA system mediates nicotine-enhanced responding for a CRf. Extensive evidence implicates nAChRs in the VTA, but not the NAcc, in the primary reinforcing effects of nicotine (Corrigall et al., 1994; Nisell et al., 1994b; Reavill and Stolerman, 1990). To the best of our knowledge, the neuroanatomical substrates mediating the reinforcement-enhancing effects of nicotine are not known. Therefore, one aim of this work was to examine the role of nAChRs in the VTA and NAcc in mediating nicotine-enhanced responding for a CRf.

The prefrontal cortex (PFC) is another area that receives DAergic projections from the VTA (Fuxe et al., 1974; Thierry et al., 1973). It is involved in higher-order processing, including goal-directed behavior (Ostlund and Balleine, 2005), attention (Passetti et al., 2002), and response inhibition (Chudasama et al., 2003). Nicotine enhances goal-directed behavior (Olausson et al., 2003), facilitates attention (Mirza and Stolerman, 1998), and increases impulsivity (Blondel et al., 2000). Additionally, some evidence suggests that nAChRs in the medial PFC (mPFC) mediate the effects of nicotine on attention (Hahn et al., 2003) and impulsivity (Tsutsui-Kimura et al., 2010). Since these processes may influence the ability of nicotine to enhance responding for reward-paired cues, another aim of this study was to examine the potential role of the mPFC in mediating the reinforcement-enhancing effects of nicotine. Specifically, we examined the role of the PrL and IL cortices in mediating nicotine-enhanced responding for a CRf. These sub-regions of the mPFC have been shown to have distinct efferent projections in the rat (Vertes, 2004) and play different roles in reward-related behavior (Balleine and Dickinson, 1998; Killcross and Coutureau, 2003) and impulse control (Chudasama et al., 2003).

The overall objective of these experiments was to identify brain regions mediating the effects of nicotine on responding for conditioned reinforcement. Specifically, we examined the role of α4β2 nAChRs in the VTA, NAcc, PrL cortex, and IL cortex in nicotine-enhanced responding for a CRf using two complementary approaches. The first approach consisted of infusing Dihydro-βeta-Erythroidine (DHβE), a selective α4β2 nAChR antagonist, into the VTA, NAcc, PrL cortex, or IL cortex to determine whether it attenuated the enhancement effects of systemically administered nicotine on responding for a CRf. DHβE was chosen given extensive literature implicating α4β2 nAChRs in mediating nicotine reinforcement (Brunzell et al., 2006; Guy and Fletcher, 2013; Kenny and Markou, 2006; Liu et al., 2007; Picciotto et al., 1998; Walters et al., 2006). DHβE attenuated nicotine-enhanced responding in all brain regions, except the PrL cortex. As such, the second approach consisted of infusing nicotine into the VTA, NAcc, or IL cortex to determine whether a local action within these brain regions was sufficient to enhance responding for a CRf.