The role of nicotinic receptor beta-2 subunits in nicotine discrimination and conditioned taste aversion

Abstract

The subtypes of nicotinic receptors at which the behavioural effects of nicotine originate are not fully understood. These experiments use mice lacking the β2 subunit of nicotinic receptors to investigate its role in nicotine discrimination and conditioned taste aversion (CTA). Wild-type and mutant mice were trained either in a two-lever nicotine discrimination procedure using a tandem schedule of food reinforcement, or in a counterbalanced two-flavour CTA procedure. Rates of lever-pressing of wild-type and mutant mice did not differ. Wild-type mice acquired discrimination of nicotine (0.4 or 0.8 mg/kg) rapidly and exhibited steep dose–response curves. Mutant mice failed to acquire these nicotine discriminations and exhibited flat dose–response curves. Both wild-type and mutant mice acquired discrimination of nicotine (1.6 mg/kg) although discrimination performance was weak in the mutants. Nicotine initially reduced response rates in wild-type and mutant mice, and tolerance developed to this effect in each genotype. Both genotypes acquired discrimination of morphine (3 mg/kg) with similar degrees of accuracy, and dose–response curves for morphine discrimination in the two genotypes were indistinguishable. Nicotine produced dose-related CTA in both genotypes, but the magnitude of the effect was less in the mutants than in the wild-type controls. It is concluded that nicotinic receptors containing the β2 subunit play a major role in the discriminative stimulus and taste aversion effects of nicotine that may reflect psychological aspects of tobacco dependence. Such receptors appear to have a less crucial role in the response-rate, reducing effects of nicotine and in nicotine tolerance.

Introduction

Nicotine produces a wide range of behavioural effects that may reflect the diversity of neuronal nicotinic receptors. The 12 known subunits of neuronal nicotinic receptors form pentameric structures associated with a wide spectrum of physiological and pharmacological functions (Corringer et al., 2000). Studies such as those described below have attempted to relate specific subtypes of these receptors to the behavioural effects of nicotine and to nicotine dependence as studied in animal models. This study uses mice with targeted deletions of the gene for the β2 subunit of nicotine receptors to investigate the receptor subtypes mediating the discriminative and aversive stimulus properties of nicotine.

The positive reinforcing stimulus property of nicotine and its associated subjective effects for which drug discrimination experiments serve as a model play a central role in tobacco dependence. This postulated relationship between the discriminative stimulus effects of drugs and their effects on subjective states (mood) has been discussed many times (e.g. Schuster et al., 1981). Several nicotinic receptor subunits can be identified in mesocortico-limbic regions that are implicated in drug dependence (Le Novère et al., 1996, Picciotto et al., 1998, Klink et al., 2001). Experiments with nicotinic agonists and antagonists have supported the view that the stimulus properties of nicotine are mediated by heteromeric high-affinity receptors such as those of the α4β2 subtype, rather than by α7 receptors. For example, in early studies, the potency of nicotinic agonists in nicotine discrimination experiments in rats was correlated with their potency as inhibitors of high-affinity tritiated nicotine binding (Romano et al., 1981, Reavill et al., 1988) that is now considered to mark mainly α4β2 receptors (Flores et al., 1992). Dihydro-β-erythroidine (DHβE) is a potent antagonist at most heteromeric nicotinic receptor subtypes, but it has low potency at homomeric α7-containing receptors; it can therefore be used to distinguish between heteromeric and α7-containing receptors but it cannot distinguish between, for example, α4β2 and α3β4 subtypes. DHβE antagonised the positive reinforcing effect of nicotine in rats (Watkins et al., 1999) and the discriminative stimulus effect of nicotine in both rats and mice (Stolerman et al., 1997, Gommans et al., 2000, Shoaib et al., 2000). In contrast, the potent α7 antagonist methyllycaconitine (MLA) appears unable to block the stimulus properties of nicotine in drug discrimination experiments in rats or mice (Brioni et al., 1996, Gommans et al., 2000).

Conditioned taste aversions produced by abused drugs provide a potential model for their aversive effects that may set an upper limit to the amounts consumed (Cappell and Le Blanc, 1975, Kumar and Stolerman, 1977). DHβE antagonised the conditioned taste aversions produced by nicotine in rats and mice, whereas MLA did not block the effect in mice but was not tested in rats (Gommans et al., 2000, Shoaib et al., 2000). These studies, like those considered above on nicotine self-administration and discrimination, all supported the view that heteromeric receptors such as those of the α4β2 subtype play a critical role in the stimulus properties of nicotine. However, the interpretation of conditioned taste aversions produced by psychoactive drugs is controversial because such effects may reflect positive conditioned suppression and may be more closely related to the rewarding than to the aversive properties of the substances (Stolerman and D’Mello, 1981a, Grigson, 1997).

While the pharmacological analyses of the stimulus properties of nicotine considered above were largely in agreement with each other, they were limited by reliance upon a small number of nicotinic agonists and antagonists that have limited selectivity for receptor subtypes. More recently, the construction of mice with targeted deletion of genes for β2 and other nicotinic receptor subunits has led to another avenue of investigation that complements the classical approach. In the β2 knockout mouse, the binding of tritiated nicotine was almost eliminated whereas there were normal levels of messenger RNA encoding the α2–α7 and β3–β4 subunits (Picciotto et al., 1995, Cordero-Erausquin et al., 2000). Nicotine-induced release of dopamine in the striatum was eliminated in β2 knockout mice and such mice also ceased responding when intravenous infusions of nicotine were substituted for cocaine in a self-administration experiment (Picciotto et al., 1998, Grady et al., 2001).

The present studies extended the investigations in genetically modified mice to the discriminative stimulus effect of nicotine and to its ability to produce conditioned taste aversions. The procedures for nicotine discrimination also provided an opportunity to examine the effects of the β2 knockout on other parameters such as the acquisition of the lever-pressing task used to establish the behavioural baseline for drug discrimination, the response-rate reducing effect of nicotine on this baseline, and the development of tolerance to the latter effect. To test whether differences between wild-type and mutant mice were specific to nicotine, some animals were retrained to discriminate morphine from saline. Other animals were retrained to explore the effect of training with a larger dose of nicotine.