Strain differences in the acquisition of nicotine-induced conditioned taste aversion

Impact of the Aversive Effects of Drugs on Their Use and Abuse

Drug use and abuse are complex issues in that the basis of each may involve different determinants and consequences, and the transition from one to the other may be equally multifaceted. A recent model of the addiction cycle (as proposed by Koob and his colleagues) illustrates how drug-taking patterns transition from impulsive (acute use) to compulsive (chronic use) as a function of various neuroadaptations leading to the downregulation of DA systems, upregulation of stress systems, and the dysregulation of the prefrontal/orbitofrontal cortex. Although the nature of reinforcement in the initiation and mediation of these effects may differ (positive vs. negative), the role of reinforcement in drug intake (acute and chronic) is well characterized. However, drugs of abuse have other stimulus properties that may be important in their use and abuse. One such property is their aversive effects that limit drug intake instead of initiating and maintaining it. Evidence of such effects comes from both clinical and preclinical populations. In support of this position, the present review describes the aversive effects of drugs (assessed primarily in conditioned taste aversion learning), the fact that they occur concurrently with reward as assessed in combined taste aversion/place preference designs, the role of aversive effects in drug-taking (in balance with their rewarding effects), the dissociation of these affective properties in that they can be affected in different ways by the same manipulations, and the impact of various parametric, experiential, and subject factors on the aversive effects of drugs and the consequent impact of these factors on their use and abuse potential.

Anatabine, Nornicotine, and Anabasine Reduce Weight Gain and Body Fat through Decreases in Food Intake and Increases in Physical Activity

Obesity is a leading cause of preventable death in the United States. Currently approved pharmacotherapies for the treatment of obesity are associated with rebound weight gain, negative side effects, and the potential for abuse. There is a need for new treatments with fewer side effects. Minor tobacco alkaloids (MTAs) are potential candidates for novel obesity pharmacotherapies. These alkaloids are structurally related to nicotine, which can help reduce body weight, but without the same addictive potential. The purpose of the current study was to examine the effects of three MTAs (nornicotine, anatabine, and anabasine) and nicotine on weight gain, body composition, chow intake, and physical activity. We hypothesized that the MTAs and nicotine would reduce weight gain through reductions in chow intake and increases in physical activity. To test this, male Sprague Dawley rats were housed in metabolic phenotyping chambers. Following acclimation to these chambers and to (subcutaneous (sc)) injections of saline, animals received daily injections (sc) of nornicotine, anabasine, anatabine, or nicotine for one week. Compared to saline-injected animals that gained body weight and body fat during the treatment phase, injections of nornicotine and anatabine prevented additional weight gain, alongside reductions in body fat. Rats receiving anabasine and nicotine gained body weight at a slower rate relative to rats receiving saline injections, and body fat remained unchanged. All compounds reduced the intake of chow pellets. Nornicotine and nicotine produced consistent increases in physical activity 6 h post-injection, whereas anabasine’s and anatabine’s effects on physical activity were more transient. These results show that short-term, daily administration of nornicotine, anabasine, and anatabine has positive effects on weight loss, through reductions in body fat and food intake and increases in physical activity. Together, these findings suggest that MTAs are worthy of further investigations as anti-obesity pharmacotherapies.

Non-nicotine constituents in e-cigarette aerosol extract attenuate nicotine's aversive effects in adolescent rats

BACKGROUND

Development of preclinical methodology for evaluating the abuse liability of electronic cigarettes (ECs) in adolescents is urgently needed to inform FDA regulation of these products. We previously reported reduced aversive effects of EC liquids containing nicotine and a range of non-nicotine constituents (e.g., propylene glycol, minor tobacco alkaloids) compared to nicotine alone in adult rats as measured using intracranial self-stimulation. The goal of this study was to compare the aversive effects of nicotine alone and EC aerosol extracts in adolescent rats as measured using conditioned taste aversion (CTA), which can be conducted during the brief adolescent period.

METHODS AND RESULTS

In Experiment 1, nicotine alone (1.0 or 1.5 mg/kg, s.c.) produced significant CTA in adolescent rats in a two-bottle procedure, thereby establishing a model to study the effects of EC extracts. At a nicotine dose of 1.0 mg/kg, CTA to Vuse Menthol EC extract, but not Aroma E-Juice EC extract, was attenuated compared to nicotine alone during repeated two-bottle CTA tests (Experiment 2a). At a nicotine dose of 0.5 mg/kg, CTA to Vuse Menthol EC extract did not differ from nicotine alone during the first two-bottle CTA test but extinguished more rapidly across repeated two-bottle tests (Experiment 2b).

CONCLUSIONS

Non-nicotine constituents in Vuse Menthol EC extracts attenuated CTA in a two-bottle procedure in adolescents. This model may be useful for anticipating the abuse liability of ECs in adolescents and for modeling FDA-mandated changes in product standards for nicotine or other constituents in ECs.

Effects of pharmacological manipulation of the kappa opioid receptors on the aversive effects of nicotine

Nicotine, an addictive component of tobacco smoke, produces both rewarding and aversive effects. Increasing the aversive effects of nicotine may help in promoting smoking cessation. However, neural targets mediating the aversive effects of nicotine have not been fully identified. In this study, we evaluated the role of kappa opioid receptors (KORs) in the aversive effects of nicotine (0.4 mg/kg, base; s.c.) using the nicotine-induced conditioned taste aversion (CTA) model in Wistar rats. The KORs were activated using the selective KOR agonist (±)U-50,488H (0, 0.03, 0.15 & 0.3mg/kg; s.c.) and inhibited using the KOR antagonist nor-binaltorphimine (nor-BNI; 0, 15 & 30mg/kg; s.c.) in separate groups of rats using a between-subjects design. Pretreatment with the KOR agonist (±)U-50,488H (0.3mg/kg) significantly increased aversion for the nicotine-associated solution. Additionally, (±)U-50,488H (0.3mg/kg) on its own induced aversion to the flavored solution associated with it even in the absence of nicotine, suggesting that the KOR agonist induced increase in nicotine-induced aversion was an additive effect. Interestingly, administration of the KOR antagonist nor-BNI (30mg/kg) prior to conditioning with nicotine/saline, but not after conditioning with nicotine/saline, attenuated nicotine-induced aversive effects compared to saline controls. Taken together, these data suggest a role for KORs in the aversive effects of nicotine.

The Affective Properties of Synthetic Cathinones: Role of Reward and Aversion in Their Abuse

The drug class known as synthetic cathinones has gained significant attention in the last few years as a result of increased use and abuse. These compounds have been shown to possess reinforcing efficacy in that they are abused in human populations and are self-administered in animal models. The present chapter outlines the affective properties of synthetic cathinones that are thought to impact drug self-administration in general and presents research confirming that these drugs have both rewarding and aversive effects in standalone and concurrent assessments. The implications of these affective properties for the overall abuse potential of these compounds are discussed along with directions for future research.

Concurrent access to nicotine and sucrose in rats

BACKGROUND

Animal models that allow concurrent access to drug and nondrug reinforcers provide unique insight into the etiology, maintenance, and treatment of drug use.

OBJECTIVES

We sought to develop and utilize a concurrent access procedure with nicotine and sucrose in rats.

METHODS

Pressing one lever delivered intravenous nicotine, and pressing another lever delivered sucrose pellets, with both reinforcers freely available throughout daily sessions.

RESULTS

Rats that had been pretrained with nicotine on some days and sucrose on other days responded on both levers when subsequently given concurrent access, but almost all responded at substantially higher rates on the sucrose lever. In contrast, rats pretrained exclusively with nicotine before being given concurrent access showed individual differences, with about half responding more on the nicotine lever. Treatment with the nicotinic receptor partial agonist varenicline selectively decreased nicotine self-administration. Food restriction and removal of the sucrose lever both increased nicotine self-administration.

CONCLUSIONS

The finding that rats continue to take nicotine when sucrose is concurrently available-and in many cases take it more frequently than sucrose-demonstrates that nicotine self-administration does not only occur in the absence of alternative reinforcement options. As a model of human nicotine use, concurrent access is more naturalistic and has higher face validity than procedures in which only one reinforcer is available or choosing one reinforcer precludes access to other reinforcers. As such, this procedure could be useful for evaluating therapeutic agents and improving our understanding of environmental conditions that promote or discourage nicotine use.

3,4-Methylenedioxypyrovalerone (MDPV)-induced conditioned taste avoidance in the F344/N and LEW rat strains

The inbred Fischer (F344) and Lewis (LEW) rats, while originally developed as animal models for cancer and tissue transplantation research, have since been used to study genetic differences in a variety of physiological and behavioral endpoints. In this context, LEW rats show greater sensitivity to the aversive effects of cocaine as compared to F344 rats in a conditioned taste avoidance procedure. Like cocaine, 3,4-methylenedioxypyrovalerone (MDPV; "bath salts") acts as a dopamine transport blocker and possesses aversive properties, making it a good candidate for assessing whether the aforementioned strain differences with cocaine would generalize to drugs with similar biochemical action. Accordingly, male F344 and LEW rats were exposed to a novel saccharin solution followed by injections of one of four doses of MDPV in a taste avoidance procedure. Over the four saccharin/MDPV pairings during conditioning, core body temperatures were also assessed. Similar to previous research, MDPV induced robust dose-dependent taste avoidance, although no effect of strain was observed. MDPV also produced hyperthermia that was independent of strain and unrelated to the conditioned taste avoidance. These findings argue for a complex influence of multiple (and likely interacting) monoaminergic systems mediating MDPV-induced taste avoidance in the two strains and suggest different mechanisms of avoidance learning for cocaine and MDPV.

Conditioned taste avoidance induced by Δ(9)-tetrahydrocannabinol in the Fischer (F344) and Lewis (LEW) rat strains

Although Fischer (F344) and Lewis (LEW) rats differ in their sensitivity to the rewarding effects of ∆(9)-tetrahydrocannabinol (THC), no data have been reported on differences in their sensitivity to the drug's aversive effects, a limiting factor in drug use and abuse. Examining the degree of differences (if any) in such effects in these strains may help further characterize possible genetic factors important to abuse vulnerability. Accordingly, the aversive effects of THC (1-5.6 mg/kg; intraperitoneal) were examined in 32F344 and 32 LEW subjects using the conditioned taste avoidance (CTA) procedure. Thermoregulation was assessed following an acute injection of THC (same as CTA groups) after a week washout period following the last trial. Subjects in both strains displayed dose-dependent THC-induced taste avoidance, with no significant strain difference. THC induced dose-dependent decreases in core body temperature in both strains. LEW subjects displayed lower core body temperatures than F344 rats, although this effect was independent of THC and was likely stress related. These results were discussed in terms of the nature of THC-induced taste avoidance and the basis of strain differences in the aversive effects of drugs of abuse.

The role of the aversive effects of drugs in self-administration: assessing the balance of reward and aversion in drug-taking behavior

Since the first experimental demonstration that a drug of abuse supports instrumental behavior, drugs have been discussed in the context of their rewarding effects, which are assumed to drive and maintain drug-taking behavior. Indeed, drug reward has been fundamental in the formulation of most models of drug use, abuse, and addiction. Over the last several decades, however, drugs of abuse have been increasingly recognized as complex pharmacological compounds producing multiple stimulus effects, not all of which are rewarding. The aversive effects of such drugs, for example, have been described by a number of researchers working in the field, although few attempts have been made to investigate the role of these aversive effects in drug taking. The present paper offers a historical perspective on the view that drugs of abuse are complex pharmacological compounds with multiple stimulus effects. In doing so, we argue that the discussion of drug reward only may be insufficient in accounting for drug taking and we present evidence for the theoretical position that both the rewarding and the aversive effects of drugs should be taken into consideration in ongoing attempts to model drug-taking behavior. The present review summarizes several decades of research characterizing the aversive effects of major drugs of abuse, as well as more recent studies seeking to assess directly the role of drug aversion in drug taking.

Prepubertal Fischer 344 rats display stronger morphine-induced taste avoidance than prepubertal Lewis rats

The present report asked if the previously reported differences in morphine-induced conditioned taste avoidance between adult F344 and LEW rats (F344 > LEW) are also evident in prepubescence (early adolescence). To assess this possibility, adult (Experiment 1) and prepubertal (Experiment 2) F344 and LEW rats were assessed for their ability to acquire morphine-induced taste avoidance (0, 3.2, 10, or 18 mg/kg) in a modified taste avoidance procedure. In each experiment, rats of both strains were given repeated pairings of saccharin and morphine followed by a final two-bottle avoidance test. Adult and prepubertal F344 subjects displayed a more rapid acquisition of the avoidance response as well as stronger suppression of consumption than their LEW counterparts. These data suggest the strains differ in their sensitivity to the aversive effects of morphine and that this differential sensitivity is evident early in development and is developmentally stable. The basis for these strain differences in morphine-induced avoidance was discussed.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Role of α5* nicotinic acetylcholine receptors in the effects of acute and chronic nicotine treatment on brain reward function in mice

OBJECTIVE

Allelic variation in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene, CHRNA5, increases vulnerability to tobacco addiction. Here, we investigated the role of α5* nAChRs in the effects of nicotine on brain reward systems.

MATERIALS AND METHODS

Effects of acute (0.03125-0.5 mg/kg SC) or chronic (24 mg/kg per day; osmotic minipump) nicotine and mecamylamine-precipitated withdrawal on intracranial self-stimulation (ICSS) thresholds were assessed in wild-type and α5 nAChR subunit knockout mice. Noxious effects of nicotine were further investigated using a conditioned taste aversion procedure.

RESULTS

Lower nicotine doses (0.03125-0.125 mg/kg) decreased ICSS thresholds in wild-type and α5 knockout mice. At higher doses (0.25-0.5 mg/kg), threshold-lowering effects of nicotine were diminished in wild-type mice, whereas nicotine lowered thresholds across all doses tested in α5 knockout mice. Nicotine (1.5 mg/kg) conditioned a taste aversion to saccharine equally in both genotypes. Mecamylamine (5 mg/kg) elevated ICSS thresholds by a similar magnitude in wild-type and α5 knockout mice prepared with minipumps delivering nicotine. Unexpectedly, mecamylamine also elevated thresholds in saline-treated α5 knockout mice.

CONCLUSION

α5* nAChRs are not involved in reward-enhancing effects of lower nicotine doses, the reward-inhibiting effects of nicotine withdrawal, or the general noxious effects of higher nicotine doses. Instead, α5* nAChRs regulate the reward-inhibiting effects nicotine doses that oppose the reward-facilitating effects of the drug. These data suggest that disruption of α5* nAChR signaling greatly expands the range of nicotine doses that facilitate brain reward activity, which may help explain the increased tobacco addiction vulnerability associated with CHRNA5 risk alleles.

CREB involvement in the regulation of striatal prodynorphin by nicotine

RATIONALE

The transcription factor cAMP response element binding (CREB) protein plays a pivotal role in drug-dependent neuronal plasticity. CREB phosphorylation at Ser133 is enhanced by drugs of abuse, including nicotine. Dynorphin (Dyn) contributes to the addictive process and its precursor gene prodynorphin (PD) is regulated by CREB. PD mRNA and Dyn synthesis were enhanced in the striatum following acute nicotine, suggesting genomic regulation.

OBJECTIVE

These studies investigated PD transcription in mice acutely treated with nicotine, determined the role of CREB, and characterized the receptors involved.

RESULTS

Acute nicotine increased adenylyl cyclase activity, cAMP, and pCREB Ser133 levels in striatum and enhanced CREB binding to CRE elements (DynCREs) of the PD promoter, preferentially DynCRE3. DynCRE3 binding was dose dependent with 1 mg of nicotine giving a maximal response. Additionally, DynCRE binding was time dependent, rising by 15 min, reaching a maximum at 1 h, and returning to control by 3 h, a temporal pattern similar to that of cAMP and pCREB. Supershift experiments showed that CREB and pCREB Ser133 were the major contributors to DynCRE3 binding complex. The nAChR antagonist mecamylamine and the dopamine D1-like receptor antagonist SCH 23390 prevented the nicotine-induced increase of pCREB and nuclear protein binding to DynCRE3.

CONCLUSIONS

Our findings suggest that nicotine regulates PD expression in striatum at the transcriptional level and CREB is involved. Dopamine D1 receptor stimulation by nAChR-released dopamine appears to be an underlying mechanism. Altered Dyn synthesis might be relevant for the behavioral actions of nicotine and especially its aversive properties.

Naloxone-induced taste aversions in opiate-naïve Lewis and Fischer 344 rat strains

BACKGROUND

The Lewis (LEW) and Fischer (F344) rat strains appear differentially sensitive to the aversive effects of several used and abused drugs. Naloxone, a mu opioid receptor antagonist that induces aversions in outbred rats but has no abuse potential, was assessed to determine the characteristics of compounds for which the strains differ.

METHODS

Opioid-naïve male LEW and F344 rats were given access to saccharin followed by low (Experiment 1) and high (Experiment 2) doses of naloxone every 4th day for five pairings. Aversions were assessed in both one-bottle and two-bottle tests.

RESULTS

In Experiment 1, aversions were evident at 10mg/kg (one-bottle) and at 5.6 and 10mg/kg (two-bottle) with no apparent strain difference for either assessment. In Experiment 2, aversions were evident for LEW animals (but not F344) at 18 and 32 mg/kg (one-bottle). LEW animals injected with 32 mg/kg displayed greater aversions than F344 animals receiving the same dose. Both strains displayed aversions at all doses in the two-bottle test with no strain difference.

CONCLUSIONS

Naloxone induced aversions that were strain dependent only at specific doses and under the one-bottle testing condition. These results parallel those of several other used and abused drugs but differ dramatically from those seen with morphine in the two strains (F344>LEW). Further assessments utilizing the LEW-F344 model should investigate other drugs to establish the set of compounds for which the strains differ and to characterize the mechanism underlying the observed differences.

Assessment of the aversive effects of peripheral mu opioid receptor agonism in Fischer 344 and Lewis rats

The Fischer 344 (F344) and Lewis (LEW) inbred rat strains differ on a host of biochemical, neuroanatomical, immunological and behavioral endpoints. One behavioral difference of interest is their differential reactivity to the aversive effects of morphine as indexed by the conditioned taste aversion preparation (aversions acquired by F344 rats are significantly greater than those acquired by the LEW strain). This differential effect appears to be specific to opioids that work primarily on the mu opioid receptor. Given that morphine works systemically, it is unknown whether these differential effects in F344 and LEW animals are centrally or peripherally mediated. To address this issue, the present study investigated the ability of the peripherally acting mu preferring opioid agonist loperamide to induce differential taste aversions in F344 and LEW animals. Both F344 and LEW animals acquired dose-dependent taste aversions to the loperamide-associated solution with no difference between them. Additionally, control animals initially injected with vehicle during aversion training with loperamide and subsequently conditioned with morphine displayed the typical aversive profile to morphine (F344>LEW). Although the basis for the present data is unknown, their relation to morphine-induced taste aversions and the role of the interaction of stimulus effects of drugs that produce differential abuse liability were discussed.

The effects of haloperidol on cocaine-induced conditioned taste aversions

Although the mechanism underlying the rewarding effects of cocaine has been well characterized, little is known about the mechanism underlying its aversive effects. Several reports have indicated a possible role of dopamine (DA) in the aversive effects; however, several procedural issues limit any conclusions regarding its specific role. In order to investigate a possible dopaminergic role in cocaine-induced CTAs using procedures that circumvented these possible issues, the present series of investigations assessed the aversive effects of the DA antagonist haloperidol alone (Experiment 1) and in combination with cocaine (Experiment 2). Haloperidol, at doses that were determined to be non-aversive, yet behaviorally active in a locomotor assessment, attenuated cocaine-induced taste aversions, suggesting that cocaine's aversive effects are mediated in part by dopaminergic activity. These findings were discussed in consideration with other evidence implicating DA and other neurotransmitter systems in cocaine-induced CTAs.

The paradox of drug taking: the role of the aversive effects of drugs

In 1991, Woods described the paradoxical nature of eating, specifically, that it produced aversive and negative effects. He noted in this analysis the multiple physiological and behavior adaptations, both learned and unlearned, that were aimed at regulating food intake and reducing its aversive, disruptive effects. From this position, he argued that consumption reflected a balance of the positive and aversive effects of eating. The present review extends this analysis to drug use and abuse, i.e., that drug taking itself also is a balance of reward and aversion. Although traditionally the analysis of drug use and abuse has focused on a drug's positive and negative rewarding effects, the present review highlights the aversive effects of these same drugs, e.g., cocaine, morphine, alcohol, and describes such effects as protective in nature. This balance and the manner by which it can be impacted by subject and experiential factors are described with a focus on genetic models of drug abuse using the Lewis and Fischer inbred rat strains.

Conditioned taste aversion learning: implications for animal models of drug abuse

Drugs of abuse are typically discussed in terms of their rewarding effects and how these effects mediate drug taking. However, these drugs produce aversive effects that could have an important role in the overall acceptability of a drug and its likelihood of being self-administered. Rewarding and aversive effects, then, could be interpreted as separate behavioral effects, with the balance of the two determining overall drug acceptability. Interestingly, the role of aversive effects on drug acceptability in the self-administration preparation has received limited attention in this context. This chapter examines the aversive effects of drugs and discusses their role in drug taking. If these aversive effects serve a protective function, manipulations that alter or decrease these effects could have implications for drug taking. Several factors have been reported to alter conditioned taste aversion (CTA) learning, a preparation used in the assessment of the aversive effects of drugs in general. Two of these factors, drug history and strain, are reviewed here. By reviewing these, we intend to demonstrate the protective nature of aversive effects in the initiation and escalation of drug taking and to provide evidence that reductions in aversive effects could produce changes in patterns of drug self-administration that could lead to an increased vulnerability to abuse drugs by altering the reward-aversion balance. The aim of this chapter is not to question the importance of rewarding effects in self-administration but rather to provide evidence that aversive effects are an important factor that needs to be considered in discussions of drug-taking behavior.

Opiate-agonist induced taste aversion learning in the Fischer 344 and Lewis inbred rat strains: evidence for differential mu opioid receptor activation

The Fischer 344 (F344) and Lewis (LEW) inbred rat strains react differently to morphine in a number of behavioral and physiological preparations, including the acquisition of aversions induced by this compound. The present experiment tested the ability of various compounds with relative selectivity at kappa, delta and mu receptor subtypes to assess the relative roles of these subtypes in mediating the differential aversive effects of morphine in the two strains. In the assessment of the role of the kappa receptor in morphine-induced aversions, animals in both strains were given access to saccharin followed by varying doses of the kappa agonist (-)-U50,488H (0.0, 0.28, 0.90 and 1.60 mg/kg). Although (-)-U50,488H induced aversions in both strains, no strain differences emerged. A separate subset of subjects was trained with the selective delta opioid agonist, SNC80 (0.0, 5.6, 10.0 and 18.0 mg/kg), and again although SNC80 induced aversions, there were no strain differences. Finally, a third subset of subjects was trained with heroin (0.0, 3.2, 5.6 and 10.0 mg/kg), a compound with activity at all three opiate receptor subtypes. Although heroin induced aversions in both strains, the aversions were significantly greater in the F344 strain, suggesting that differential activation of the mu opioid receptor likely mediates the reported strain differences in morphine-induced aversion learning. These data were discussed in terms of strain differences in opioid system functioning and the implications of such differences for other morphine-induced behavioral effects reported in F344 and LEW rats.

Ethanol-induced conditioned taste avoidance: reward or aversion?

BACKGROUND

Rats avoid intake of a palatable taste cue when paired with all drugs of abuse tested. Evidence suggests that, at least for morphine and cocaine, rats avoid the taste cue because they are anticipating the rewarding properties of the drug. Thus, the suppressive effects of a rewarding sucrose solution and cocaine, but not those of the putatively aversive agent, lithium chloride (LiCl), are exaggerated in drug-sensitive Lewis rats. Likewise, the suppressive effects of sucrose and morphine, but not those of LiCl, are eliminated by bilateral lesions of the gustatory thalamus. Unlike morphine and cocaine, it is less clear whether rewarding or aversive drug properties are responsible for ethanol-induced suppression of intake of a taste cue. The present set of studies tests whether, like cocaine, ethanol-induced suppression of intake of a taste cue also is greater in the drug-sensitive Lewis rats and whether the suppressive effects of the drug are prevented by bilateral lesions of the taste thalamus.

METHODS

In Experiment 1, fluid-deprived Lewis and Fischer rats were given 5-minute access to 0.15% saccharin and then injected with saline or a range of doses of ethanol (0.5, 0.75, 1.0, or 1.5 g/kg). There was a total of 6 such pairings. In Experiments 2 and 3, Sprague-Dawley rats received bilateral electrophysiologically guided lesions of the gustatory thalamus. After recovery, suppression of intake of the saccharin cue was evaluated following repeated daily pairings with either a high (1.5 g/kg) or a low (0.75 g/kg) dose of ethanol.

RESULTS

Ethanol-induced suppression of intake of the saccharin conditioned stimulus (CS) did not differ between the drug-sensitive Lewis rats relative to the less-sensitive Fischer rats. Lesions of the taste thalamus, however, prevented the suppressive effect of the 0.75 g/kg dose of the drug, but had no impact on the suppressive effect of the 1.5 g/kg dose of ethanol.

CONCLUSION

The results suggest that the suppressive effects of ethanol on CS intake are mediated by both rewarding and aversive consequences, varying as a function of dose.

Acute nicotine changes dynorphin and prodynorphin mRNA in the striatum

RATIONALE

Nicotine displays rewarding and aversive effects, and while dopamine has been linked with nicotine's reward, the neurotransmitter(s) involved with aversion remains speculative. The kappa-dynorphinergic system has been associated with negative motivational and affective states, and whether dynorphin (Dyn) contributes to the behavioral pharmacology of nicotine is a pertinent question.

OBJECTIVE

We determined whether administration of a single dose of nicotine alters the biosynthesis of Dyn in the striatum of mice.

RESULTS

Nicotine free base, 1 mg/kg, sc, induced a biphasic, protracted increase of striatal Dyn, an initial rise by 1 h, which declined to control levels by 2 h, and a subsequent increase, between 6 and 12 h, lasting over 24 h. At 1 h, the nicotine effect was dose dependent, with doses>or=0.5 mg/kg inducing a response. Prodynorphin mRNA increased by 30 min for over 24 h, and in situ hybridization demonstrated elevated signal in caudate/putamen and nucleus accumbens. The nicotinic antagonist mecamylamine prevented the Dyn response, and a similar effect was observed with D1- and D2-like dopamine receptor antagonists, SCH 23390, sulpiride, and haloperidol. The glutamate NMDA receptor antagonist MK-801 reversed the nicotine-induced increase of Dyn, while the AMPA antagonist NBQX had a marginal effect.

CONCLUSIONS

We interpret our findings to indicate that acute nicotine enhances the synthesis and release of striatal Dyn. We propose that nicotine influences Dyn primarily through dopamine release and that glutamate plays a modulatory role. A heightened dynorphinergic tone may contribute to the aversive effects of nicotine in naive animals and first-time tobacco smokers.

Bupropion differentially alters the aversive, locomotor and rewarding properties of nicotine in CD-1 mice

The present experiments determined the effects of bupropion on the motivational (aversive and rewarding) and locomotor properties of nicotine in CD-1 mice. Preliminary experiments determined effective nicotine doses (0.1-2.0 mg/kg) to produce a conditioned taste aversion (CTA) or conditioned place preference (CPP; Experiments 1a and 2a, respectively). Mice were administered vehicle or bupropion (1-20 mg/kg) followed by vehicle or nicotine after drinking saccharin during CTA training (Experiment 1b). Mice were administered vehicle or bupropion (1-20 mg/kg) 15 (Experiment 2b) or 30 (Experiment 2c) minutes (min) prior to vehicle or nicotine during CPP training. The two highest nicotine doses produced CTAs and a moderate nicotine dose (0.4 mg/kg) produced a CPP. Bupropion dose-dependently blocked nicotine CTA. For the 15-min pretreatment interval, bupropion dose-dependently increased locomotor activity and produced CPPs when administered alone; whereas for the 30-min pretreatment interval, only the highest bupropion dose increased locomotor activity and produced a CPP. However, bupropion failed to alter nicotine CPP and the co-administration of bupropion and nicotine did not increase locomotor activity more so than when bupropion was administered alone regardless as to the pretreatment interval. Thus, bupropion selectively altered the aversive properties of nicotine in CD-1 mice.

Acute nicotine reduces and repeated nicotine increases spontaneous activity in male and female Lewis rats

The Lewis (LEW) strain of rat appears more sensitive to nicotine than other strains in self-administration, conditioned place preference, and drug discrimination behavioral studies. The present study sought to further evaluate the behavioral effects of chronic nicotine treatment in the LEW strain by assessing spontaneous activity, which has consistently revealed sensitization to chronic nicotine administration in Sprague Dawley (SD) rats. High active and low active male and female LEW rats (N=8 per group) were treated twice daily with either nicotine (0.4 mg/kg, sc) or vehicle for 14 consecutive days. Regardless of baseline activity level or sex, spontaneous activity was significantly decreased, compared to saline-treated rats, after a single nicotine injection. However, spontaneous activity increased in both low- and high-activity rats (both sexes) over the two weeks of nicotine administration to levels that were significantly higher than saline-treated rats. Based on these findings, acute and chronic nicotine administration had greater suppressive and enhancing effects on spontaneous activity in LEW rats compared to other strains of rats previously studied. These results further clarify the behavioral sensitivity of the LEW strain of rat to nicotine exposure and lend credence to the role of genetics in the individual susceptibility to nicotine dependence.

Adolescent exposure to nicotine alters the aversive effects of cocaine in adult rats

Nicotine is one of the most commonly used drugs in adolescence and has been shown to alter the rewarding effects of cocaine when administered in adulthood. Although the abuse potential of a drug has been suggested to be a balance between its rewarding and aversive effects, the long-term effects of nicotine on the aversive properties of other drugs had not been studied. To that end, in the present study rats exposed to nicotine (0.4 mg/kg) during adolescence (postnatal days 35-44) were tested for the acquisition and extinction of a cocaine-induced conditioned taste aversion (10, 18 or 32 mg/kg) in adulthood. Conditioning consisted of four saccharin-drug pairings followed by six extinction trials. Although cocaine-induced aversions at all doses, no effect of nicotine preexposure was seen during acquisition. During extinction, the nicotine-preexposed groups conditioned with 10 and 18 mg/kg cocaine displayed a decreased rate of extinction compared to their respective controls. These results suggest that while adolescent nicotine exposure does not appear to directly alter the aversive properties of cocaine it may affect other processes related to the response to drugs given in adulthood.

The effects of nicotine on ethanol-induced conditioned taste aversions in Long-Evans rats

RATIONALE

Overall drug acceptability is thought to be a function of the balance between its rewarding and aversive effects, the latter of which is reportedly affected by polydrug use.

OBJECTIVES

Given that nicotine and alcohol are commonly co-used, the present experiments sought to assess nicotine's impact on ethanol's aversive effects within a conditioned taste aversion design.

MATERIALS AND METHODS

Experiment 1 examined various doses of nicotine (0, 0.4, 0.8, 1.2 mg/kg) to determine a behaviorally active dose, and experiment 2 examined various doses of ethanol (0, 0.5, 1.0, 1.5 g/kg) to determine a dose that produced intermediate aversions. Experiment 3 then examined the aversive effects of nicotine (0.8 mg/kg) and ethanol (1.0 g/kg) alone and in combination. Additionally, nicotine's effects on blood alcohol concentrations (BAC) and ethanol-induced hypothermia were examined.

RESULTS

Nicotine and ethanol combined produced aversions significantly greater than those produced by either drug alone or the summed aversive effects of the individual compounds. These effects were unrelated to changes in BAC, but nicotine and ethanol combined produced a prolonged hypothermic effect which may contribute to the increased aversions induced by the combination.

CONCLUSIONS

These data demonstrate that nicotine may interact with ethanol, increasing ethanol's aversive effects. Although the rewarding effects of concurrently administered nicotine and ethanol were not assessed, these data do indicate that the reported high incidence of nicotine and ethanol co-use is unlikely due to reductions in the aversiveness of ethanol with concurrently administered nicotine. It is more likely attributable to nicotine-related changes in ethanol's rewarding effects.

Wistar Kyoto and Wistar rats differ in the affective and locomotor effects of nicotine

Anhedonia is a characteristic of clinical depression and has been associated with dysfunction of the mesolimbic dopaminergic system, a system also involved in mediating nicotine reward. To further examine the relationship between anhedonia, clinical depression and nicotine reward, the present experiment determined if Wistar Kyoto (WKY) rats, an animal model of clinical depression, differed from Wistar rats in nicotine conditioned place preference (CPP). Strain differences in nicotine-induced changes in locomotor activity also were determined simultaneously. To determine if strain differences were specific to reward-based learning, nicotine or lithium chloride (LiCl) conditioned taste avoidance (CTA) experiments were conducted. Rats received vehicle or nicotine (0.4 or 0.8 mg/kg) during a multi-trial, biased CPP training procedure or received vehicle, nicotine (0.2, 0.4 or 0.8 mg/kg) or lithium chloride (LiCl; 0.0375, 0.075 or 0.15 M) during a multi-trial CTA training procedure. Whereas both nicotine doses (0.4 and 0.8 mg/kg) initially induced hypoactivity, only the moderate nicotine dose (0.4 mg/kg) induced hyperactivity with repeated administration and produced a CPP in Wistar rats. Both nicotine doses failed to alter locomotor activity or produce a CPP in WKY rats. WKY rats also acquired a LiCl CTA more slowly and less robustly compared to Wistar rats. In contrast, nicotine dose-dependently produced a CTA in both strains and WKY rats were more sensitive to the avoidance effects of nicotine compared to Wistar rats. Collectively, these results suggest that WKY rats show deficits in nicotine reward and specific aversive drug stimuli compared to Wistar rats.

The effects of cocaine preexposure on cocaine-induced taste aversion learning in Fischer and Lewis rat strains

The Fischer (F344) and Lewis (LEW) inbred rat strains differ on a number of behaviors, including those induced by a variety of drugs of abuse. Although a number of physiological and biochemical differences between the strains have been reported following both single and repeated drug administration, studies assessing changes in the affective properties of drugs after repeated exposure are limited. To that end, using the F344 and LEW strains, the present study examined the effects of repeated exposure to cocaine on the subsequent acquisition of cocaine-induced conditioned taste aversions, a preparation often used in assessing the development of tolerance to the drug's aversive effects. Specifically, separate groups of male F344 and LEW rats received five injections of 32 mg/kg cocaine (or vehicle) prior to taste aversion conditioning with 32 mg/kg cocaine (or vehicle). Vehicle-preexposed subjects of both strains acquired aversions to the cocaine-associated taste with no differences in the strength of the aversions. Further, cocaine-preexposed subjects displayed significantly attenuated aversions, an effect consistent with prior work with outbred animals. There was no difference between the two strains in this attenuation, suggesting that there were no genotype-specific differences in tolerance to cocaine's aversive effects. The data were discussed in relation to genetic/environmental interactions in the vulnerability to drugs of abuse.

Morphine-induced place conditioning in Fischer and Lewis rats: acquisition and dose-response in a fully biased procedure

The Fischer (F344) and Lewis (LEW) rat strains differ on a variety of behavioral assays examining the effects of morphine, with many of the differences observed during acquisition of behavioral responses. The results of these studies and others examining endogenous physiology and the biochemical effects of morphine suggest that F344 rats are more sensitive to morphine than LEW rats. However, LEW animals have shown greater conditioned place preferences (CPP) to 4 mg/kg than F344 rats. CPP is a popular assay of drug reward in which acquisition of the preference can be measured across multiple conditioning cycles, yet this aspect of CPP has not been assessed in F344 and LEW rats. As part of an ongoing effort to fully characterize the conditioned rewarding effects of abused drugs in these strains, the present study assessed the effects of 0, 1, 4 and 10 mg/kg subcutaneous (SC) morphine in adult male F344 and LEW rats (n=12/strain/dose). A fully biased place conditioning procedure was employed where morphine's effects were paired with the initially non-preferred chamber on Day 1, saline was paired with the preferred chamber on Day 2 and drug-free access to the entire apparatus was allowed on Day 3. This conditioning and testing regimen was repeated for four consecutive cycles. The F344 animals acquired CPP at 1 mg/kg only; this effect emerged after only two conditioning cycles. LEW rats never acquired a CPP at any dose tested. Peak blood morphine levels following SC injections of 1, 4 or 10 mg/kg revealed no significant strain or dose effects. These behavioral data are consistent with the hypothesis that F344 rats are more sensitive to the rewarding effects of morphine than LEW rats. Additional implications for the Fischer-Lewis model of drug abuse and the utility of CPP acquisition procedures are discussed.

Assessment of the aversive and rewarding effects of alcohol in Fischer and Lewis rats

RATIONALE

Application of the Fischer-Lewis genetic model of drug abuse to the study of alcohol's motivational properties has been limited.

OBJECTIVES

To assess the aversive and rewarding effects of ethanol in Fischer and Lewis rats.

MATERIALS AND METHODS

Fischer and Lewis rats underwent a four-trial combined conditioned taste aversion/conditioned place preference procedure (CTA/CPP; 0, 1, 1.25, or 1.5 g/kg IP ethanol). Others received 0, 1, or 1.5 g/kg followed by tail blood sampling at 15-, 60- and 180-min post-injection. In additional groups, hypothermia to 0, 1.5, and 3 g/kg was assessed before and 30- and 60-min post-injection.

RESULTS

All alcohol-treated groups except low-dose Lewis acquired CTA after one trial. Fischer rats developed stronger CTAs than Lewis at 1.25 and 1.5 g/kg. Ethanol-induced reward in taste or place conditioning was not evident in either strain. Lewis animals showed overall higher peak blood alcohol concentrations, but hypothermia did not vary by strain.

CONCLUSION

Compared to Fischer, Lewis rats are less sensitive to alcohol's aversive effects as assessed in the CTA paradigm. The behavioral differences observed are not due to hypothermia, but pharmacokinetic differences may contribute. These data underscore the importance of genetic factors and the aversive effects of initial drug exposures in modeling vulnerability to abuse. In addition to its application with other drugs, the Fischer-Lewis model may be useful for investigating the biobehavioral bases of alcohol abuse.

Naloxone, not proglumide or MK-801, alters effects of morphine preexposure on morphine-induced taste aversions

Both cholecystokinin (CCK) antagonists and N-methyl-D-aspartate (NMDA) antagonists block or reduce the development of morphine tolerance in several analgesic assays. The present experiments were performed to assess the ability of the CCK antagonist proglumide and the NMDA antagonist MK-801 to affect tolerance to the aversive properties of morphine as indexed by conditioned taste aversion (CTA) learning. Specifically, male Sprague-Dawley rats were exposed to either vehicle or morphine (5 mg/kg) in combination with either proglumide (5 mg/kg; Experiment 1), MK-801 (0.1 mg/kg; Experiment 2) or naloxone (1, 3.2 mg/kg; Experiment 3). Saccharin was then presented and was followed by an injection of either vehicle or morphine (10 mg/kg). Animals preexposed to and conditioned with morphine acquired an attenuated morphine-induced aversion to saccharin. While neither proglumide nor MK-801 had an effect on this attenuation, naloxone blocked the effects of morphine preexposure, suggesting that neither CCK nor NMDA may be involved in the aversive effects of morphine (or their modulation by drug exposure). That the attenuating effects of morphine preexposure on a morphine-induced CTA can be blocked suggests that the weakening of the aversive effects of morphine with chronic use can be prevented, an effect that may have implications for overall drug acceptability.