Conditioned taste aversion and drugs of abuse: History and interpretation

Dietary agents in the prevention of radiation-induced nausea and vomiting (RINV): review addressing the scientific observations, benefits, lacunae and future direction

PURPOSE

Radiation-induced Nausea and Vomiting (RINV) is an important side effect and conservative estimates are that 50-80% of the patients undergoing curative radiotherapy (RT) will experience some sought of retching, nausea, and/or vomiting during the course of their treatment. Conventionally, antiemetic drugs like the 5-hydroxytryptamine receptor antagonists and steroids are the mainstay of treatment. However, the use of these agents, especially steroids, can cause side effects and thereby negate the proposed benefits. The antiemetic effects of Centella asiatica (Indian pennywort), Hippophae rhamnoides (Sea buckthorn), oil of Mentha spicata (Spearmint) and the rhizomes of Zingiber officinale (ginger) have been addressed.

CONCLUSIONS

Results indicate that Indian pennywort, Sea buckthorn, Spearmint oil and ginger are beneficial in mitigating RINV. Also, of the four plants investigated in preclinical models of study, mint oil and ginger seem to be more useful and merit structured systematic translational studies to ascertain the benefit of these two agents.

Evaluating proxies for motion sickness in rodent

Motions sickness (MS) occurs when the brain receives conflicting sensory signals from vestibular, visual and proprioceptive systems about a person's ongoing position and/or motion in relation to space. MS is typified by symptoms such as nausea and emesis and implicates complex physiological aspects of sensations and sensorimotor reflexes. Use of animal models has been integral to unraveling the physiological causality of MS. The commonly used rodents (rat and mouse), albeit lacking vomiting reflex, reliably display phenotypic behaviors of pica (eating of non-nutritive substance) and conditioned taste aversion (CTAver) or avoidance (CTAvoi) which utilize neural substrates with pathways that cause gastrointestinal malaise akin to nausea/emesis. As such, rodent pica and CTAver/CTAvoi have been widely used as proxies for nausea/emesis in studies dealing with neural mechanisms of nausea/emesis and MS, as well as for evaluating therapeutics. This review presents the rationale and experimental evidence that support the use of pica and CTAver/CTAvoi as indices for nausea and emesis. Key experimental steps and cautions required when using rodent MS models are also discussed. Finally, future directions are suggested for studying MS with rodent pica and CTAver/CTAvoi models.

Western diet consumption does not impact the rewarding and aversive effects of morphine in male Sprague-Dawley rats

The impacts of high-fat and/or high-sugar diets on opioid-induced effects are well documented; however, little is known about the effect of such diet on the affective responses to opiates. To address this issue, in the present experiment male Sprague-Dawley rats were given ad libitum access to a western-style diet (high in saturated fat and sugar) or a standard laboratory chow diet beginning in adolescence and continuing into adulthood at which point they were trained in a combined conditioned taste avoidance (CTA)/conditioned place preference (CPP) procedure to assess the aversive and rewarding effects of morphine, respectively. On four conditioning cycles, animals were given access to a novel saccharin solution, injected with morphine (1 mg/kg or 5 mg/kg), and then placed on one side of a place preference chamber. Animals were then tested for place preference and saccharin preference. All subjects injected with morphine displayed significant avoidance of the morphine-associated solution (CTA) and preferred the side associated with the drug (CPP). Furthermore, there were no differences between the two diet groups, indicating that chronic exposure to the western diet had no impact on the affective properties of morphine (despite increasing caloric intake, body weight, body fat and lean body mass). Given previously reported increases in drug self-administration in animals with a history of western-diet consumption, this study suggests that western-diet exposure may increase drug intake via mechanisms other than changes in the rewarding or aversive effects of the drug.

Effects of Serial Polydrug Use on the Rewarding and Aversive Effects of the Novel Synthetic Cathinone Eutylone

BACKGROUND

As individual synthetic cathinones become scheduled and regulated by the Drug Enforcement Administration (DEA), new ones regularly are produced and distributed. One such compound is eutylone, a novel third-generation synthetic cathinone whose affective properties (and abuse potential) are largely unknown. The following experiments begin to characterize these effects and how they may be impacted by drug history (a factor affecting reward/aversion for other drugs of abuse).

METHODS

Eutylone was assessed for its ability to induce conditioned taste avoidance (CTA; aversive effect) and conditioned place preference (CPP; rewarding effect) and their relationship (Experiment 1). Following this, the effects of exposure to cocaine or 3,4-methylenedioxymethamphetamine [MDMA] on eutylone's affective properties were investigated (Experiment 2).

RESULTS

Eutylone produced dose-dependent CTA and CPP (Experiment 1), and these endpoints were unrelated. Pre-exposure to cocaine and MDMA differentially impacted taste avoidance induced by eutylone (MDMA > cocaine) and did not impact eutylone-induced place preference.

CONCLUSIONS

These data indicate that eutylone, like other synthetic cathinones, has co-occurring, independent rewarding and aversive effects that may contribute to its abuse potential and that these effects are differentially impacted by drug history. Although these studies begin the characterization of eutylone, future studies should examine the impact of other factors on eutylone's affective properties and its eventual reinforcing effects (i.e., intravenous self-administration [IVSA]) to predict its use and abuse liability.

Taste aversion learning in the snail Cornu aspersum

The present study was conducted to provide evidence of conditioned taste aversion learning (CTA) in the snail Cornu aspersum, using quinidine as the aversive stimulus in a procedure of Pavlovian Conditioning of Tentacle Lowering. Subjects were split into two groups: paired and unpaired. During the devaluation phase, subjects from the "paired group" received the US followed by the quinidine exposure, while subjects from the "unpaired group" received the quinidine and, 30 min later, the US. Subjects which had received the US paired with the quinidine showed a decrease of the conditioned response (CR), in contrast to subjects which had received the quinidine and the US unpaired. These results provide a useful CTA procedure in terrestrial snails. The implication of the results for learning and the physiological correlates is discussed.

Comorbidity of Post-Traumatic Stress Disorder and Alcohol Use Disorder: Animal Models and Associated Neurocircuitry

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are prevalent neuropsychiatric disorders and frequently co-occur concomitantly. Individuals suffering from this dual diagnosis often exhibit increased symptom severity and poorer treatment outcomes than those with only one of these diseases. Lacking standard preclinical models limited the exploration of neurobiological mechanisms underlying PTSD and AUD comorbidity. In this review, we summarize well-accepted preclinical model paradigms and criteria for developing successful models of comorbidity. We also outline how PTSD and AUD affect each other bidirectionally in the nervous nuclei have been heatedly discussed recently. We hope to provide potential recommendations for future research.

Selective TAAR1 agonists induce conditioned taste aversion

RATIONALE

Trace amine-associated receptor 1 (TAAR1) is the best-studied receptor of trace amines, a group of biogenic amines expressed at a relatively low level in the mammalian brain. Growing evidence suggests that TAAR1 plays a critical role in various neuropsychiatric disorders. Given that selective TAAR1 agonists were shown to produce pro-cognition and antipsychotic-like effects as well as to suppress drug use and relapse, they have been proposed to be novel treatments for mental disorders such as schizophrenia and addiction. However, the aversive effects of selective TAAR1 agonists remain largely unknown.

OBJECTIVES

Here, we evaluated whether the selective TAAR1 full agonist RO5166017 and partial agonist RO5263397 could induce conditioned taste aversion (CTA).

RESULTS

We found that RO5166017 and RO5263397 produced significant aversions to both saccharin and NaCl taste novelty. Furthermore, RO5166017 produced CTA to saccharin in TAAR1 heterozygous knockout (taar1^±) and wild-type rats but not in TAAR1 homozygous knockout rats (taar1^-/-), suggesting that TAAR1 was sufficient for the taste aversive stimulus property of RO5166017.

CONCLUSIONS

Taken together, our data indicate that selective TAAR1 agonists could produce strong CTA. Our study urges careful evaluations of the aversive effects of TAAR1 agonists before translating them to clinical use for the treatment of mental disorders.

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.

Stimulus functions of nicotine

Behavioral pharmacology has made vital contributions to the concepts and methods used in tobacco and other drug use research, and is largely responsible for the now generally accepted notion that nicotine is the primary component in tobacco that engenders and maintains tobacco use. One of the most important contributions of behavioral pharmacology to the science of drug use is the notion that drugs can act as environmental stimuli that control behavior in many of the same ways as other stimuli (e.g., visual, gustatory, olfactory). The purpose of this chapter is to provide an overview of research that illustrates the respondent and operant stimulus functions of nicotine, using a contemporary taxonomy of stimulus functions as a general framework. Each function is formally defined and examples from research on the behavioral pharmacology of nicotine are presented. Some of the factors that modulate each function are also discussed. The role of nicotine's stimulus functions in operant and respondent theories of tobacco use is examined and some suggestions for future research are presented. The chapter illustrates how a taxonomy of stimulus functions can guide conceptions of tobacco use and direct research and theory accordingly.

Genetic determinants of liking and intake of coffee and other bitter foods and beverages

Coffee is a widely consumed beverage that is naturally bitter and contains caffeine. Genome-wide association studies (GWAS) of coffee drinking have identified genetic variants involved in caffeine-related pathways but not in taste perception. The taste of coffee can be altered by addition of milk/sweetener, which has not been accounted for in GWAS. Using UK and US cohorts, we test the hypotheses that genetic variants related to taste are more strongly associated with consumption of black coffee than with consumption of coffee with milk or sweetener and that genetic variants related to caffeine pathways are not differentially associated with the type of coffee consumed independent of caffeine content. Contrary to our hypotheses, genetically inferred caffeine sensitivity was more strongly associated with coffee taste preferences than with genetically inferred bitter taste perception. These findings extended to tea and dark chocolate. Taste preferences and physiological caffeine effects intertwine in a way that is difficult to distinguish for individuals which may represent conditioned taste preferences.

Ethanol pre-exposure differentially impacts the rewarding and aversive effects of α-pyrrolidinopentiophenone (α-PVP): Implications for drug use and abuse

RATIONALE

Exposure to a drug can subsequently impact its own reactivity as well as that of other drugs. Given that users of synthetic cathinones, i.e., "bath salts", typically have extensive and varied drug histories, an understanding of the effects of drug history on the behavioral and physiological consequences of synthetic cathiones may be important to their abuse liability.

OBJECTIVES

The goal of the current work was to assess the effects of an ethanol pre-exposure on the rewarding and aversive effects of α-PVP.

METHODS

Adult male Sprague Dawley rats were exposed to ethanol prior to combined conditioned taste avoidance/conditioned place preference training in which rats were injected with 1.5, 3 or 5 mg/kg of racemic α-PVP or vehicle. Following a 7-day washout period, rats were then tested for thermoregulatory effects of α-PVP using subcutaneous probes to measure body temperature changes over the course of 8 h. This was followed 10 days later by assessments for α-PVP-induced locomotor activity and stereotypies over a 1-h session.

RESULTS

α-PVP induced significant dose- and trial-dependent taste avoidance that was significantly attenuated by ethanol history and dose- and time-dependent increases in locomotor activity that were significantly increased by ethanol. α-PVP also induced place preferences and dose- and time-dependent increases in body temperature, but these measures were unaffected by ethanol history.

CONCLUSIONS

α-PVP's aversive effects (as measured by taste avoidance) were attenuated, while its rewarding effects (as indexed by place preference conditioning) were unaffected, by ethanol pre-exposure. Such a pattern may indicate increased α-PVP abuse liability, as changes in the balance of aversion and reward may impact overall drug effects and likelihood of drug intake. Future self-administration studies will be necessary to explore this possibility.

Chronic Fentanyl Self-Administration Generates a Shift toward Negative Affect in Rats during Drug Use

Drug addiction is thought to be driven by negative reinforcement, and it is thought that a shift from positive affect upon initial exposure to negative affect after chronic exposure to a drug is responsible for maintaining self-administration (SA) in addicted individuals. This can be modeled in rats by analyzing ultrasonic vocalizations (USVs), a type of intraspecies communication indicative of affective state based on the frequency of the emission: calls in the 22 kHz range indicate negative affect, whereas calls in the 50 kHz range indicate positive affect. We employed a voluntary chronic, long-access model of fentanyl SA to analyze affective changes in the response to chronic fentanyl exposure. Male Sprague-Dawley rats self-administered either fentanyl (N = 7) or saline (N = 6) for 30 consecutive days and USVs were recorded at four different time points: the day before the first SA session (PRE), the first day of SA (T01), the last day of SA (T30), and the first day of abstinence (ABS). At T01, the ratio of 50 to 22 kHz calls was similar between the fentanyl and saline groups, but at T30, the ratio differed between groups, with the fentanyl group showing significantly fewer 50 kHz calls and more 22 kHz calls relative to saline animals. These results indicate a shift toward a negative affect during drug use after chronic exposure to fentanyl and support negative reinforcement as a main driving factor of opioid addiction.

The Paradoxical Effect Hypothesis of Abused Drugs in a Rat Model of Chronic Morphine Administration

A growing body of studies has recently shown that abused drugs could simultaneously induce the paradoxical effect in reward and aversion to influence drug addiction. However, whether morphine induces reward and aversion, and which neural substrates are involved in morphine’s reward and aversion remains unclear. The present study first examined which doses of morphine can simultaneously produce reward in conditioned place preference (CPP) and aversion in conditioned taste aversion (CTA) in rats. Furthermore, the aversive dose of morphine was determined. Moreover, using the aversive dose of 10 mg/kg morphine tested plasma corticosterone (CORT) levels and examined which neural substrates were involved in the aversive morphine-induced CTA on conditioning, extinction, and reinstatement. Further, we analyzed c-Fos and p-ERK expression to demonstrate the paradoxical effect—reward and aversion and nonhomeostasis or disturbance by morphine-induced CTA. The results showed that a dose of more than 20 mg/kg morphine simultaneously induced reward in CPP and aversion in CTA. A dose of 10 mg/kg morphine only induced the aversive CTA, and it produced higher plasma CORT levels in conditioning and reacquisition but not extinction. High plasma CORT secretions by 10 mg/kg morphine-induced CTA most likely resulted from stress-related aversion but were not a rewarding property of morphine. For assessments of c-Fos and p-ERK expression, the cingulate cortex 1 (Cg1), prelimbic cortex (PrL), infralimbic cortex (IL), basolateral amygdala (BLA), nucleus accumbens (NAc), and dentate gyrus (DG) were involved in the morphine-induced CTA, and resulted from the aversive effect of morphine on conditioning and reinstatement. The c-Fos data showed fewer neural substrates (e.g., PrL, IL, and LH) on extinction to be hyperactive. In the context of previous drug addiction data, the evidence suggests that morphine injections may induce hyperactivity in many neural substrates, which mediate reward and/or aversion due to disturbance and nonhomeostasis in the brain. The results support the paradoxical effect hypothesis of abused drugs. Insight from the findings could be used in the clinical treatment of drug addiction.

Relationship between voluntary ethanol drinking and approach-avoidance biases in the face of motivational conflict: novel sex-dependent associations in rats

RATIONALE

Aberrant approach-avoidance conflict processing may contribute to compulsive seeking that characterizes addiction. Exploration of the relationship between drugs of abuse and approach-avoidance behavior remains limited, especially with ethanol.

OBJECTIVES

To investigate the effects of voluntary ethanol consumption on approach-avoidance conflict behavior and to examine the potential approach/avoidance bias to predict drinking in male and female rats.

METHODS

Long-Evans rats consumed ethanol for 5 weeks under the intermittent access two-bottle choice (IA2BC) paradigm. Approach-avoidance tendencies were assessed before and after IA2BC drinking using a previously established cued approach-avoidance conflict maze task and the elevated plus maze (EPM).

RESULTS

Female rats displayed higher consumption of and preference for ethanol than males. In the conflict task, males showed greater approach bias towards cues predicting conflict than females. In females only, a median split and regression analysis of cued-conflict preference scores revealed that the more conflict-avoidant group displayed higher intake and preference for ethanol in the first few weeks of drinking. In both sexes, ethanol drinking did not affect cued-conflict preference, but ethanol exposure led to increased time spent in the central hub in the males only. Finally, anxiety levels in EPM predicted subsequent onset of ethanol drinking in males only.

CONCLUSIONS

Our results highlight sex and individual differences in both drinking and approach-avoidance bias in the face of cued conflict and further suggest that cued-conflict preference should be examined as a potential predictor of ethanol drinking. Ethanol exposure may also affect the timing of decision-making in the face of conflict.

Assessment of aversive effects of methylone in male and female Sprague-Dawley rats: Conditioned taste avoidance, body temperature and activity/stereotypies

Methylone's rewarding effects have been well characterized; however, little is known about its aversive effects and how such effects may be impacted by sex. In this context, the present study investigated the aversive effects of methylone (vehicle, 5.6, 10 or 18 mg/kg, IP) in 35 male and 31 female Sprague-Dawley rats assessed by conditioned taste avoidance and changes in body temperature and activity/stereotypies. Methylone induced significant taste avoidance, changes in temperature and increased activity and stereotypies in both males and females. Similar to work with other synthetic cathinones, methylone has aversive effects as indexed by significant taste avoidance and changes in temperature and activity (two characteristics of methylone overdose in humans). The only endpoint for which there were significant sex differences was in general activity with males displaying a faster onset and females displaying a longer duration. Although sex was not a factor with taste avoidance and temperature, separate analyses for males and females revealed different patterns, e.g., males displayed a more rapid acquisition of taste avoidance and females displayed changes in temperature at lower doses. Males displayed a faster onset and females displayed a longer duration of activity (consistent with the analyses considering sex as a factor), while time- and dose-dependent stereotypies did not show consistent pattern differences. Although sex differences were relatively limited when sex was specifically assessed as a factor (or only evident when sex comparisons were made in the patterns of effects), sex as a biological variable in the study of drugs should be made to determine if differences exist and, if evident, the basis for these differences.

Hippocampal-Dependent Inhibitory Learning and Memory Processes in the Control of Eating and Drug Taking

As manifestations of excessive and uncontrolled intake, obesity and drug addiction have generated much research aimed at identifying common neuroadaptations that could underlie both disorders. Much work has focused on changes in brain reward and motivational circuitry that can overexcite eating and drug-taking behaviors. We suggest that the regulation of both behaviors depends on balancing excitation produced by stimuli associated with food and drug rewards with the behavioral inhibition produced by physiological "satiety" and other stimuli that signal when those rewards are unavailable. Our main hypothesis is that dysregulated eating and drug use are consequences of diet- and drug-induced degradations in this inhibitory power. We first outline a learning and memory mechanism that could underlie the inhibition of both food and drug-intake, and we describe data that identifies the hippocampus as a brain substrate for this mechanism. We then present evidence that obesitypromoting western diets (WD) impair the operation of this process and generate pathophysiologies that disrupt hippocampal functioning. Next, we present parallel evidence that drugs of abuse also impair this same learning and memory process and generate similar hippocampal pathophysiologies. We also describe recent findings that prior WD intake elevates drug self-administration, and the implications of using drugs (i.e., glucagon-like peptide- 1 agonists) that enhance hippocampal functioning to treat both obesity and addiction are also considered. We conclude with a description of how both WD and drugs of abuse could initiate a "vicious-cycle" of hippocampal pathophysiology and impaired hippocampal-dependent behavioral inhibition.

Conditioned taste avoidance induced by the combination of heroin and cocaine: Implications for the use of speedball

Speedball (heroin + cocaine) is a prevalent drug combination among intravenous drug users. Although its use is generally discussed to be a function of changes in the rewarding effects of either or both drugs, changes in the aversive effects of either drug may also be impacted (weakened) by the combination. To address this latter possibility and its potential role in the use of speedball, the present studies examined the interaction of cocaine and heroin in taste avoidance conditioning. In Experiment 1, male Sprague-Dawley rats were given access to a novel saccharin solution and then injected with either vehicle or heroin (3.2 mg/kg, IP) followed immediately by various doses of cocaine (10, 18 or 32 mg/kg, SC). At the two lowest doses of cocaine, only animals injected with the drug combination (H + C) displayed a taste avoidance relative to control subjects (taste avoidance was induced with both the combination and the high dose of cocaine). At no dose did animals injected with the combination of heroin and cocaine drink more than animals injected with cocaine alone. In Experiment 2, male Sprague-Dawley rats were similarly treated but injected with vehicle or cocaine (10 mg/kg) followed by injections of various doses of heroin (1.8, 3.2, 5.6 or 10 mg/kg). At the three highest doses of heroin, only animals injected with the drug combination (C + H) displayed significant avoidance relative to control subjects (no avoidance was evident with the combination of cocaine and the low dose of heroin). At no dose did animals injected with the combination of cocaine and heroin drink more than animals injected with heroin alone. Together, these results suggest that the aversive effects of heroin and cocaine are not attenuated by co-administration by cocaine and heroin, respectively. The importance of this for the use of speedball was discussed.

Ethanol-induced conditioned place preference and aversion differentially alter plasticity in the bed nucleus of stria terminalis

Contextual cues associated with drugs of abuse, such as ethanol, can trigger craving and drug-seeking behavior. Pavlovian procedures, such as place conditioning, have been widely used to study the rewarding/aversive properties of drugs and the association between environmental cues and drug seeking. Previous research has shown that ethanol as an unconditioned stimulus can induce a strong conditioned place preference (CPP) or aversion (CPA) in rodents. However, the neural mechanisms underlying ethanol-induced reward and aversion have not been thoroughly investigated. The bed nucleus of the stria terminalis (BNST), an integral part of the extended amygdala, is engaged by both rewarding and aversive stimuli and plays a role in ethanol-seeking behavior. Here, we used ex-vivo slice physiology to probe learning-induced synaptic plasticity in the BNST following ethanol-induced CPP and CPA. Male DBA/2 J mice (2-3 months old) were conditioned using previously reported ethanol-induced CPP/CPA procedures. Ethanol-induced CPP was associated with increased neuronal excitability in the ventral BNST (vBNST). Conversely, ethanol-induced CPA resulted in a significant decrease in spontaneous glutamatergic transmission without alterations in GABAergic signaling. Ethanol-CPA also led to a significant increase in the paired-pulse ratio at excitatory synapses, suggestive of a decrease in presynaptic glutamate release. Collectively, these data demonstrate that the vBNST is involved in the modulation of contextual learning associated with both the rewarding and the aversive properties of ethanol in mice.

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.

Further demonstration of running-based food avoidance learning in laboratory mice (Mus musculus)

Voluntary wheel running has hedonically bivalent properties in laboratory rats and mice. While it works as a reward for instrumental performance such as bar pressing, it also functions as an aversive stimulus to establish Pavlovian conditioned avoidance of the paired stimulus. The present study focused on the latter case. Running in closed wheels hampered habituation of a reluctance to eat a target snack in rats (Experiment 1A) and mice (Experiment 1B) trained by pairing access to a target snack with confinement to a wheel attached to the cage. Experiment 2 successfully confirmed and extended this finding with mice running in both open and closed wheels. A differential conditioning procedure employed in Experiment 3 ensured that this phenomenon is specific to the snack paired with running, implying that it reflects Pavlovian conditioned flavor avoidance (CFA). Free exploration in cages without wheels, however, did not results in a CFA.

Involvement of neural substrates in reward and aversion to methamphetamine addiction: Testing the reward comparison hypothesis and the paradoxical effect hypothesis of abused drugs

Clinical studies of drug addiction focus on the reward impact of abused drugs that produces compulsive drug-seeking behavior and drug dependence. However, a small amount of research has examined the opposite effect of aversion to abused drugs to balance the reward effect for drug taking. An aversive behavioral model of abused drugs in terms of conditioned taste aversion (CTA) was challenged by the reward comparison hypothesis (Grigson, 1997). To test the reward comparison hypothesis, the present study examined the rewarding or aversive neural substrates involved in methamphetamine-induced conditioned suppression. The behavioral data showed that methamphetamine induced conditioned suppression on conditioning and reacquisition but extinguished it on extinction. A higher level of stressful aversive corticosterone occurred on conditioning and reacquisition but not extinction. The c-Fos or p-ERK immunohistochemical activity showed that the cingulated cortex area 1 (Cg1), infralimbic cortex (IL), prelimbic cortex (PrL), basolateral amygdala (BLA), nucleus accumbens (NAc), and dentate gyrus (DG) of the hippocampus were overexpressed in aversive CTA induced by methamphetamine. These data may indicate that the Cg1, IL, PrL, BLA, NAc, and DG probably mediated the paradoxical effect-reward and aversion. Altogether, our data conflicted with the reward comparison hypothesis, and methamphetamine may simultaneously induce the paradoxical effect of reward and aversion in the brain to support the paradoxical effect hypothesis of abused drugs. The present data implicate some insights for drug addiction in clinical aspects.

Sex differences in α-pyrrolidinopentiophenone (α-PVP)-induced taste avoidance, place preference, hyperthermia and locomotor activity in rats

RATIONALE

The majority of synthetic cathinone research has used only male subjects, and as a result there are few studies assessing the impact of biological sex on their effects.

OBJECTIVES

The current work extends the characterization of the second-generation synthetic cathinone, α-PVP, by investigating how biological sex impacts α-PVP's aversive and rewarding effects important to its use and potential abuse.

METHODS

A combined conditioned taste avoidance/conditioned place preference preparation was utilized in which adult male and female Sprague Dawley rats were injected with 1.5, 3 or 6 mg/kg of racemic α-PVP or vehicle (saline) (IP). Following a 24-day washout period, rats were then tested for thermoregulatory effects of α-PVP using subcutaneous microchips to measure body temperature changes over the course of 8 h. This was followed 21 days later by assessments for α-PVP-induced locomotor activity and stereotypies over a 1-h session.

RESULTS

Dose-dependent conditioned taste avoidance was evident in both males and females, although females displayed weaker avoidance at 3 mg/kg compared to males. Males displayed a dose-dependent conditioned place preference, while females did not form a place preference at any dose. α-PVP elicited dose- and time-dependent hyperthermia, with males displaying a faster on-set and delayed off-set compared to females. α-PVP also produced dose- and time-dependent increases in locomotor activity (F > M) and stereotypies (M > F).

CONCLUSIONS

As described, males displayed greater rewarding (as indexed by place preference conditioning) and aversive (as indexed by taste avoidance, hyperthermia and stereotypies) effects of α-PVP. Although comparisons between males and females in α-PVP self-administration have not been reported, these data suggest that males may be more likely to use the drug. The implications for sex differences in human use of α-PVP were discussed.

The impact of low to moderate chronic intermittent ethanol exposure on behavioral endpoints in aged, adult, and adolescent rats

The average age of the population in the United States and other countries is increasing. Understanding the health consequences in the aged population is critical. Elderly individuals consume ethanol, often at elevated rates, and in some cases in a binge episode. The present study sought to investigate whether binge-like ethanol exposure in aged male rats produced differential health and behavioral effects compared to adult male and adolescent male rats. Subjects were exposed to either 1.0 g/kg or 2.0 g/kg ethanol every other day via intraperitoneal injection for 20 days, and tested on a variety of behavioral measures and body weight. Binge-like ethanol exposure produced differential effects on body weight between aged and adolescent and adult rats. In addition, aged rats had a significantly longer loss of righting reflex and demonstrated a trend toward tolerance following the 2.0-g/kg exposure. No significant effects on anxiety-like behavior as measured by open arm entries, depressive-like symptoms as measured by immobility in the forced swim test, or cognitive performance as measured by latency and path length in the Morris water maze were found. These results demonstrate that aged animals are differentially sensitive to the impact of chronic intermittent ethanol exposure in some, but not all behaviors. Future research is needed to understand the mechanisms of these differential effects.

Amphetamine-induced activation of neurons within the rat nucleus of the solitary tract

Despite generally being a reinforcing drug of abuse, amphetamine (amph) also produces effects such as hypophagia and conditioned taste avoidance (CTA), which may indicate that amph acts as an aversive homeostatic stressor. Stress-responsive prolactin-releasing peptide (PrRP)-positive noradrenergic and glucagon-like peptide-1 (GLP-1)-positive neurons in the caudal nucleus of the solitary tract (cNTS) are modulated by metabolic state, and are prime candidates for mediating amph-induced hypophagia and CTA. The present study used dual immunolabeling and fluorescent in situ hybridization (RNAscope) to examine acute amph-induced activation of cFos expression in phenotypically-identified cNTS neurons in ad lib-fed vs. overnight-fasted male Sprague Dawley rats. We also examined the impact of food deprivation on amph-induced CTA. Compared to control saline treatment, amph activated significantly more cNTS neurons, including PrRP-negative noradrenergic (NA) neurons, GABAergic neurons, and glutamatergic neurons, but not PrRP or GLP-1 neurons. Amph also increased neural activation within a subset of central cNTS projection targets, including the lateral parabrachial nucleus and central amygdala, but not the paraventricular hypothalamus. Food deprivation did not alter amph-induced neural activation or impact the ability of amph to support CTA. These findings indicate that PrRP-negative NA and other cNTS neurons are recruited by acute amph treatment regardless of metabolic state, and may participate in amph-induced hypophagia and CTA.

Stereoselective effects of the second-generation synthetic cathinone α-pyrrolidinopentiophenone (α-PVP): assessments of conditioned taste avoidance in rats

RATIONALE

Work with α-pyrrolidinopentiophenone (α-PVP), a second-generation synthetic cathinone, has been generally limited to the racemate. Given that with other synthetic cathinones, there are behavioral and neurochemical differences between their enantiomers, differences may also be seen with α-PVP.

OBJECTIVES

The present study assessed the relative contribution of each enantiomer to the aversive effects of racemic-α-PVP by comparing their ability to induce a conditioned taste avoidance.

METHODS

Adult male Sprague-Dawley rats were exposed every other day for four exposures to a novel saccharin solution followed immediately by an injection of 0 (saline vehicle) or 1.5, 3, or 6 mg/kg of S-, R-, or racemic-α-PVP (IP). On alternating days, all subjects were given access to water to assess any unconditioned effects of α-PVP on general fluid consumption.

RESULTS

Rats injected with the racemate and S-isomer of α-PVP displayed avoidance of the drug-associated saccharin solution, although this avoidance was dose-dependent only for the subjects injected with the racemate. There was no evidence of taste avoidance in animals injected with the R-enantiomer at any dose tested. Animals injected with 3 mg/kg racemic-α-PVP did not differ in avoidance from those treated with 1.5 mg/kg of the S-enantiomer, but subjects treated with 6 mg/kg racemic-α-PVP displayed a significantly stronger avoidance than those treated with 3 mg/kg S-α-PVP.

CONCLUSIONS

The present work suggests that the aversive effects of racemic α-PVP are mediated primarily by its S-isomer. The fact that at the highest dose tested (6 mg/kg), the racemate induces an avoidance greater than the simple additive effects of the S- and R-isomers (at 3 mg/kg) suggests that while the R-isomer may not induce taste avoidance at this dose, it may interact synergistically with the S-isomer in mediating the effects of the racemic mixture. These results were discussed in terms of similar effects with other behavioral and physiological endpoints reported with a number of psychostimulants and suggest that the enantiomers of α-PVP are an important variable in characterizing its behavioral effects.

Nicotine affects ethanol-conditioned taste, but not place, aversion in a simultaneous conditioning procedure

The conditioned taste aversion (CTA) induced by ethanol is a key factor limiting ethanol intake. Nicotine, a drug co-consumed with ethanol, may decrease this aversion by modulating the unconditioned effects of ethanol or by disrupting the association between ethanol and its associated cues. This study analyzed ethanol-induced CTA and conditioned place aversion (CPA) in Long-Evans rats with subchronic exposure to nicotine. The rats were treated with nicotine (0.0 or 0.4 mg/kg) three times before conditioning (on lickometer training sessions 3, 4, and 5) and across conditioning days. During the conditioning the rats were given ethanol (1.3 g/kg) preceded and followed by presentation of a taste (NaCl) and tactile (rod or hole floors) conditioned stimulus (CS+), respectively. On CS- conditioning days, the rats were given vehicle and exposed to alternative stimuli. Three CTA and CPA testing sessions were then conducted. It was found that nicotine reduced ethanol-induced CTA and enhanced locomotor activity, but did not significantly modify the magnitude of ethanol-induced CPA. The effects of nicotine on CTA were observed during both conditioning and testing sessions, and were specific to the NaCl CS+, having no effect on reactivity to water. The dissociation between the effect of nicotine on ethanol-induced CTA and CPA suggests that nicotine does not alter ethanol's motivational properties by generally increasing its positive rewarding effects, nor does it blunt all aversive-like responses to this drug. Instead, nicotine may impede ethanol-induced CTA induced by ethanol by disrupting the neural underpinnings of this specific form of associative learning.

A genetic reduction in the serotonin transporter differentially influences MDMA and heroin induced behaviours

BACKGROUND

Despite ongoing study and research to better understand drug addiction, it continues to be a heavy burden. Only a small percentage of individuals who take drugs of abuse go on to develop addiction. However, there is growing evidence to suggest that a reduction in the serotonin transporter may play an important role for those that transition to compulsive drug taking. Studies have demonstrated that reduced serotonin transporter function potentiates self-administration of psychostimulant drugs ("ecstasy," MDMA; cocaine); however, additional research revealed no differences between genotypes when the opioid heroin was self-administered. These results suggest that a reduction in the serotonin transporter may confer susceptibility to the development of addiction to some classes of drugs but not others. Importantly, the mechanism underlying facilitated psychostimulant self-administration is currently unknown.

METHODS

Therefore, to continue investigating the relationship between compromised serotonergic function and different classes of drugs, a series of experiments was conducted investigating locomotor activity (LMA) and conditioned taste aversion (CTA) in the serotonin transporter knockout (SERT KO) rat model.

RESULTS

MDMA-induced hyperactivity was reduced, while MDMA-induced CTA was enhanced, in SERT KO rats. However, there were no genotype differences in heroin-induced behaviours.

CONCLUSIONS

These results reinforce the idea that a reduction in the serotonin transporter drives differential effects between disparate classes of drugs of abuse.

Evaluation of reinforcing and aversive effects of voluntary Δ9-tetrahydrocannabinol ingestion in rats

Edible cannabis-infused products are an increasingly popular method of using cannabis in the United States. Yet, preclinical research to determine mechanisms underlying abuse of Δ⁹-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis, has focused primarily on the effects of parenteral administration. The purpose of this study was to examine the rewarding and aversive effects of oral THC in a novel rodent voluntary ingestion model. Adult male and female Sprague Dawley rats were given access to sucrose-sweetened solutions during daily sessions. A range of THC concentrations, each paired with a unique flavor previously tested alone, was introduced into these solutions for four-session exposure periods and drinking volumes were measured. Injected (i.p.) THC doses were also paired with unique flavors to compare the effects of route of THC administration on drinking. Introduction of THC into sucrose solutions dose-dependently decreased drinking upon initial exposure, though drinking generally increased in subsequent sessions. By contrast, i.p. THC produced sustained dose-dependent decreases in drinking in rats of both sexes. Subsequent exposure to paired flavors in the absence of THC resulted in further decreases in drinking, suggesting route-specific aversion. Additional testing using saccharin-sweetened solutions in a two-bottle choice paradigm was also conducted, with THC producing sustained dose-dependent decreases in drinking after initial exposure in rats of both sexes. Though self-administration of ingested THC was not demonstrated, evidence of route-specific THC aversion was observed, which suggests that certain routes and/or rates of THC administration may mitigate some of its aversive effects.

Essential values of cocaine and non-drug alternatives predict the choice between them

This study investigated the relationship between reinforcer value and choice between cocaine and two non-drug alternative reinforcers in rats. The essential value (EV, a behavioral economic measure based on elasticity of demand) of intravenous cocaine and food (Experiment 1) or saccharin (Experiment 2) was determined in the first phase of each experiment. Food had higher EV than cocaine, whereas the EVs of cocaine and saccharin did not differ. In the second phase of each experiment, rats were allowed to make mutually exclusive choices between cocaine and the non-drug alternative reinforcer. The main findings were that the EV of cocaine was a positive predictor of cocaine preference and the EV of food or saccharin was a negative predictor of cocaine preference. An analysis of within-session patterns of choice behavior revealed sequential dependencies, whereby rats were more likely to choose cocaine on a particular trial after having chosen the non-drug alternative on previous trials. When the time between choices was increased, these sequential dependencies disappeared. The results of these experiments are consistent with the suggestion that addiction-like behavior involves both overvaluation of drug reinforcers and undervaluation of non-drug reinforcers.

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.

Sex as a biological variable: Drug use and abuse

The study of sex as a biological variable is a necessary emphasis across a wide array of endpoints, including basic neuroscience, medicine, mental health, physiology and behavior. The present review summarizes work from clinical and preclinical populations on sex differences in drug use and abuse, ranging from initiation to escalation/dysregulation and from drug cessation/abstinence to relapse. These differences are analyzed in the context of the addiction cycle conceptualization of Koob and his colleagues and address patterns of drug use (binge/intoxication), motivation underlying its use (withdrawal/negative affect) and likelihood and causes of craving and relapse of drug taking (preoccupation/anticipation). Following this overview, an assessment of the basis for the reported sex differences is discussed in the context of the affective (rewarding and aversive) properties of drugs of abuse and how such properties and their balance vary with sex and contribute to drug intake. Finally, the interaction of sex with several experiential (drug history) and subject (age) factors and how these interactions affect reward and aversion are discussed to highlight the importance of understanding such interactions in predicting drug use and abuse. We note that sex as a biological variable remains one of critical evaluation and that such investigations of sex differences in drug use and abuse continue and be expanded to assess all facets of their mediation, including these affective properties, how their balance may be impacted by the multiple conditions under which drugs are taken and how this overall balance affects drug use and addiction vulnerability.

Re-evaluation of the reward comparison hypothesis for alcohol abuse

This study examined whether various doses of ethanol induced reward or aversion and then evaluated Grigson's reward comparison hypothesis (1997). Rats were given a 0.1% saccharin solution (conditioned stimulus 1 [CS1]) 15min prior to administration of a 0, 0.05, 0.125, 0.20, 0.35, or 0.50g/kg dose of ethanol (unconditioned stimulus [US]). The rats were then exposed to a paired compartment (CS2) for 30min. The low dose of 0.05g/kg ethanol did not induce conditioned suppression (i.e., conditioned taste aversion [CTA]) or conditioned place preference (CPP). The dose of 0.125g/kg ethanol induced CPP but not CTA. High doses of ethanol, including 0.35g/kg and 0.50g/kg, produced CTA but not CPP. The middle dose of 0.20g/kg ethanol simultaneously induced CTA and CPP. As a result, the reward comparison hypothesis cannot explain the present finding that the middle dose of ethanol induced CTA and CPP. Meanwhile, the high doses of ethanol induced motivationally aversive CTA but not rewarding CPP. The reward comparison hypothesis should be updated further.

Conditioned taste avoidance, conditioned place preference and hyperthermia induced by the second generation 'bath salt' α-pyrrolidinopentiophenone (α-PVP)

BACKGROUND

α-Pyrrolidinopentiophenone (α-PVP) has been reported to be rewarding in a variety of pre-clinical models. Given that a number of drugs of abuse have both rewarding and aversive effects, the balance of which influences addiction potential, the present study examined the aversive properties of α-PVP by assessing its ability to induce taste avoidance. This assessment was made in a combined taste avoidance/place conditioning design that also allowed an evaluation of the relationship between α-PVP's aversive and rewarding effects.

METHODS

Male Sprague-Dawley rats were exposed to a novel saccharin solution, injected with one of four doses of α-PVP (0, 0.3, 1.0 and 3.0mg/kg) (IP) and placed on one side of a place conditioning apparatus. The next day, they were injected with vehicle, given access to water and placed on the other side. Following four conditioning cycles, saccharin avoidance and place preferences were then assessed. The effects of α-PVP on body temperature were also examined.

RESULTS

α-PVP induced dose-dependent taste avoidance as well as significant increases in time spent on the drug-paired side (although this effect was not dependent on dose). α-PVP also induced dose- and time-dependent hyperthermia.

CONCLUSIONS

α-PVP induced significant taste avoidance whose strength relative to the psychostimulants methylenedioxypyrovalerone (MDPV) and cocaine paralleled their relative binding to the dopamine transporter. Similar to other drugs of abuse, α-PVP has both aversive and rewarding effects. It will be important to assess how various experiential and subject variables impact these effects and their balance to predict abuse liability.

Conditioned taste aversions: From poisons to pain to drugs of abuse

Learning what to eat and what not to eat is fundamental to our well-being, quality of life, and survival. In particular, the acquisition of conditioned taste aversions (CTAs) protects all animals (including humans) against ingesting foods that contain poisons or toxins. Counterintuitively, CTAs can also develop in situations in which we know with absolute certainty that the food did not cause the subsequent aversive systemic effect. Recent nonhuman animal research, analyzing palatability shifts, has indicated that a wider range of stimuli than has been traditionally acknowledged can induce CTAs. This article integrates these new findings with a reappraisal of some known characteristics of CTA and presents a novel conceptual analysis that is broader and more comprehensive than previous accounts of CTA learning.

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.

Withdrawal from Chronic Nicotine Exposure Produces Region-Specific Tolerance to Alcohol-Stimulated GluA1 Phosphorylation

BACKGROUND

Nicotine use increases alcohol drinking, suggesting that the combination of these drugs may produce synergistic effects in activating reward circuitry. Alternatively, use of either of these drugs may facilitate the development of cross-tolerance to the other to promote intake escalation.

METHODS

In this study, adult male Wistar rats were chronically exposed to room air or chronic, intermittent nicotine vapor, which has been shown to produce symptoms of nicotine dependence as evidenced by elevated nicotine self-administration and a host of somatic and motivational withdrawal symptoms. We examined regional neuroadaptations in nicotine-experienced versus nonexperienced animals, focusing on changes in phosphorylation of the AMPA glutamate channel subunit GluA1 in reward-related brain regions as excitatory neuroadaptations are heavily implicated in both alcohol and nicotine addiction.

RESULTS

During withdrawal, nicotine exposure and alcohol challenge (1 g/kg) interactively produced neuroadaptations in GluA1 phosphorylation in a brain region-dependent manner. Alcohol robustly increased protein kinase A-mediated phosphorylation of GluA1 at serine 845 in multiple regions. However, this neuroadaptation was largely absent in 3 areas (dorsomedial prefrontal cortex, dorsal striatum, and central amygdala) in nicotine-experienced animals. This interactive effect suggests a molecular tolerance to alcohol-stimulated phosphorylation of GluA1 in the context of nicotine dependence.

CONCLUSIONS

Nicotine may modify the rewarding or reinforcing effects of alcohol by altering glutamate signaling in a region-specific manner, thereby leading to increased drinking in heavy smokers.

Effects of 3,4-methylenedioxypyrovalerone (MDPV) pre-exposure on the aversive effects of MDPV, cocaine and lithium chloride: Implications for abuse vulnerability

BACKGROUND

Drug use is thought to be a balance of the rewarding and aversive effects of drugs. Understanding how various factors impact these properties and their relative balance may provide insight into their abuse potential. In this context, the present study attempted to evaluate the effects of drug history on the aversive effects of 3,4-methylenedioxypyrovalerone (MDPV), one of a variety of synthetic cathinones (collectively known as "bath salts").

METHODS

Different groups of male Sprague-Dawley rats were exposed to either vehicle or MDPV (1.8mg/kg) once every fourth day for five total injections prior to taste avoidance conditioning in which a novel saccharin solution was repeatedly paired with either vehicle, MDPV (1.8mg/kg), the related psychostimulant cocaine (18mg/kg) or the emetic lithium chloride (LiCl) (13.65mg/kg).

RESULTS

In animals pre-exposed to vehicle, all three drugs induced significant and comparable taste avoidance relative to animals injected with vehicle during conditioning. MDPV pre-exposure attenuated the avoidance induced by both MDPV and cocaine (greater attenuation for MDPV than cocaine), but had no effect on that induced by LiCl.

CONCLUSIONS

These findings suggest that a history of MDPV use may reduce or attenuate MDPV and cocaine's (but not LiCl's) aversive effects. The implications for such changes in MDPV's aversive effects to its potential use and abuse were discussed.

Role for the Rostromedial Tegmental Nucleus in Signaling the Aversive Properties of Alcohol

BACKGROUND

While the rewarding effects of alcohol contribute significantly to its addictive potential, it is becoming increasingly appreciated that alcohol's aversive properties also play an important role in the propensity to drink. Despite this, the neurobiological mechanism for alcohol's aversive actions is not well understood. The rostromedial tegmental nucleus (RMTg) was recently characterized for its involvement in aversive signaling and has been shown to encode the aversive properties of cocaine, yet its involvement in alcohol's aversive actions have not been elucidated.

METHODS

Adult male and female Long-Evans rats underwent conditioned taste aversion (CTA) procedures where exposure to a novel saccharin solution was paired with intraperitoneal administration of saline, lithium chloride (LiCl), or ethanol (EtOH). Control rats underwent the same paradigm except that drug and saccharin exposure were explicitly unpaired. Saccharin consumption was measured on test day in the absence of drug administration, and rats were sacrificed 90 to 105 minutes following access to saccharin. Brains were subsequently harvested and processed for cFos immunohistochemistry. The number of cFos-labeled neurons was counted in the RMTg and the lateral habenula (LHb)-a region that sends prominent glutamatergic input to the RMTg.

RESULTS

In rats that received paired drug and saccharin exposure, EtOH and LiCl induced significant CTA compared to saline to a similar degree in males and females. Both EtOH- and LiCl-induced CTA significantly enhanced cFos expression in the RMTg and LHb but not the hippocampus. Similar to behavioral measures, no significant effect of sex on CTA-induced cFos expression was observed. cFos expression in both the RMTg and LHb was significantly correlated with CTA magnitude with greater cFos being associated with more pronounced CTA. In addition, cFos expression in the RMTg was positively correlated with LHb cFos.

CONCLUSIONS

These data suggest that the RMTg and LHb are involved in the expression of CTA and are consistent with previous work implicating the RMTg in aversive signaling. Furthermore, increased cFos expression in the RMTg following EtOH-induced CTA suggests that this region plays a role in signaling alcohol's aversive properties.

The ontogeny of ethanol aversion

Recent work has suggested separate developmental periods within the broader framework of adolescence, with data suggesting distinct alterations and vulnerabilities within these intervals. While previous research has suggested reduced sensitivity to the aversive effects of alcohol in adolescence relative to adults, a more detailed ontogeny of this effect has yet to be conducted. The adolescent brain undergoes significant transitions throughout adolescence, including in regions linked with drug reward and aversion. The current study aimed to determine the ontogeny of ethanol aversion by utilizing a conditioned taste aversion procedure at six different ages to test the hypothesis that the transitions into, through, and out of adolescence are associated with ontogenetic alterations in sensitivity to the aversive properties of ethanol. Non-deprived animals given Boost® as the conditioned stimulus (CS) were used in Experiment 1, whereas Experiment 2 used water-restricted animals provided with a saccharin/sucrose solution as the CS. In both experiments, an attenuated sensitivity to the aversive properties of ethanol was evident in adolescents compared to adults, although more age differences were apparent in water deprived animals than when a highly palatable CS was given to ad libitum animals. Overall, the data suggest an attenuated sensitivity to the aversive properties of ethanol that is most pronounced during pre- and early adolescence, declining thereafter to reach the enhanced aversive sensitivity of adults.

Adolescent delta-9-tetrahydrocannabinol (THC) exposure fails to affect THC-induced place and taste conditioning in adult male rats

BACKGROUND

Adolescent initiation of drug use has been linked to problematic drug taking later in life and may represent an important variable that changes the balance of the rewarding and/or aversive effects of abused drugs which may contribute to abuse vulnerability. The current study examined the effects of adolescent THC exposure on THC-induced place preference (rewarding effects) and taste avoidance (aversive effects) conditioning in adulthood.

METHODS

Forty-six male Sprague-Dawley adolescent rats received eight injections of an intermediate dose of THC (3.2mg/kg) or vehicle. After these injections, animals were allowed to mature and then trained in a combined CTA/CPP procedure in adulthood (PND ~90). Animals were given four trials of conditioning with intervening water-recovery days, a final CPP test and then a one-bottle taste avoidance test.

RESULTS

THC induced dose-dependent taste avoidance but did not produce place conditioning. None of these effects was impacted by adolescent THC exposure.

CONCLUSIONS

Adolescent exposure to THC had no effect on THC taste and place conditioning in adulthood. The failure to see an effect of adolescent exposure was addressed in the context of other research that has assessed exposure of drugs of abuse during adolescence on drug reactivity in adulthood.

Effect of norbinaltorphimine on ∆⁹-tetrahydrocannabinol (THC)-induced taste avoidance in adolescent and adult Sprague-Dawley rats

RATIONALE

The aversive effects of ∆(9)-tetrahydrocannabinol (THC) are mediated by activity at the kappa opioid receptor (KOR) as assessed in adult animals; however, no studies have assessed KOR involvement in the aversive effects of THC in adolescents. Given that adolescents have been reported to be insensitive to the aversive effects induced by KOR agonists, a different mechanism might mediate the aversive effects of THC in this age group.

OBJECTIVES

The present study was designed to assess the impact of KOR antagonism on the aversive effects of THC in adolescent and adult rats using the conditioned taste avoidance (CTA) procedure.

METHODS

Following a single pretreatment injection of norbinaltorphimine (norBNI; 15 mg/kg), CTAs induced by THC (0, 0.56, 1.0, 1.8, and 3.2 mg/kg) were assessed in adolescent (n = 84) and adult (n = 83) Sprague-Dawley rats.

RESULTS

The KOR antagonist, norBNI, had weak and inconsistent effects on THC-induced taste avoidance in adolescent rats in that norBNI both attenuated and strengthened taste avoidance dependent on dose and trial. norBNI had limited impact on the final one-bottle avoidance and no effects on the two-bottle preference test. Interestingly, norBNI had no effect on THC-induced taste avoidance in adult rats as well.

CONCLUSIONS

That norBNI had no significant effect on THC-induced avoidance in adults, and a minor and inconsistent effect in adolescents demonstrates that the aversive effects of THC are not mediated by KOR activity as assessed by the CTA design in Sprague-Dawley rats.

Sex differences in the long-lasting consequences of adolescent ethanol exposure for the rewarding effects of cocaine in mice

RATIONALE

The practice of binge drinking is very common among adolescents of both sexes. It can have long-term consequences with respect to drug consumption during adulthood, but knowledge on these effects in females is limited.

OBJECTIVES

The long-lasting effects of intermittent exposure to ethanol (EtOH) during adolescence on different cocaine-elicited behaviours, including locomotor reactivity, conditioned place preference (CPP) and intravenous self-administration, were evaluated in male and female adult mice. It was hypothesized that an EtOH binge during adolescence would increase sensitivity to the effects of a sub-threshold dose of cocaine and has a differential impact on the drug's effects in males and females.

METHODS

Adolescent OF1 mice (postnatal day (PND) 26) underwent a 2-week pre-treatment schedule consisting of 16 doses of EtOH (2.5 g/kg) or saline (twice daily administrations separated by a 4-h interval i.p.) administered on two consecutive days separated by an interval of 2 days. Three weeks later (PND > 60), we assessed locomotor activity responses induced by an acute injection of different doses of cocaine in experiment 1 and the rewarding effects of cocaine on the CPP (1 mg/kg) and intravenous self-administration (1 mg/kg/infusion) paradigms in experiment 2.

RESULTS

Pre-exposure to EtOH during adolescence altered motor reactivity to cocaine in a dose- and sex-dependent manner, increased sensitivity to cocaine in CPP and enhanced self-administration in adult mice.

CONCLUSIONS

The effects of intermittent exposure to ethanol during adolescence are evident in adulthood, during which greater sensitivity and intake of cocaine is observed and differ in each sex.

Sex differences in 3,4-methylenedioxypyrovalerone (MDPV)-induced taste avoidance and place preferences

Synthetic cathinones, otherwise known as "bath salts", have gained significant attention in the last few years as a result of increased use and abuse. One such compound, 3,4-methylenedioxypyrovalerone (MDPV), is pharmacologically and behaviorally similar to cocaine and has been shown to possess both aversive and rewarding effects. For a host of other drugs, each of these effects (and their relative balance) can be influenced by a variety of factors, including sex, which in turn impacts drug taking behavior. In this context, the present assessment sought to determine whether males and females differed in MDPV-induced CTA and CPP. Both male and female Sprague-Dawley rats underwent a combined CTA/CPP procedure, in which an injection of one of three doses of MDPV (1.0, 1.8 or 3.2mg/kg) was paired with both a novel saccharin solution and a novel environment and changes in preferences for these stimuli were examined. Taste avoidance was evident in both sexes, although this avoidance was weaker in females compared to males. MDPV also produced place preferences in all drug-treated animals, but these preferences did not vary as a function of sex. The fact that females showed a weaker avoidance response compared to males (despite comparable preferences) suggests that females may have a heightened susceptibility to use and abuse of MDPV, paralleling results seen with cocaine and other stimulants. The present findings extend the behavioral characterization of MDPV and the factors that may alter its aversive and rewarding effects.

On the positive and negative affective responses to cocaine and their relation to drug self-administration in rats

RATIONALE

Acute cocaine administration produces an initial rewarding state followed by a dysphoric/anxiogenic "crash."

OBJECTIVE

The objective of this study was to determine whether individual differences in the relative value of cocaine's positive and negative effects would account for variations in subsequent drug self-administration.

METHODS

The dual actions of cocaine were assessed using a conditioned place test (where animals formed preferences for environments paired with the immediate rewarding effects of 1.0mg/kg i.v. cocaine or aversions of environments associated with the anxiogenic effects present 15-min postinjection) and a runway test (where animals developed approach-avoidance "retreat" behaviors about entering a goal box associated with cocaine delivery). Ranked scores from these two tests were then correlated with each other and with the escalation in the operant responding of the same subjects observed over 10 days of 1- or 6-h/day access to i.v. (0.4 mg/inj) cocaine self-administration.

RESULTS

Larger place preferences were associated with faster runway start latencies (r s = -0.64), but not with retreat frequency or run times; larger place aversions predicted slower runway start times (r s = 0.62), increased run times (r s = 0.65), and increased retreats (r s = 0.62); response escalation was observed in both the 1- and 6-h self-administration groups and was associated with increased CPPs (r s = 0.58) but not CPAs, as well as with faster run times (r s = -0.60).

CONCLUSIONS

Together, these data suggest that animals exhibiting a greater positive than negative response to acute (single daily injections of) cocaine are at the greatest risk for subsequent escalated cocaine self-administration, a presumed indicator of cocaine addiction.

A proposed resolution to the paradox of drug reward: Dopamine's evolution from an aversive signal to a facilitator of drug reward via negative reinforcement

The mystery surrounding how plant neurotoxins came to possess reinforcing properties is termed the paradox of drug reward. Here we propose a resolution to this paradox whereby dopamine - which has traditionally been viewed as a signal of reward - initially signaled aversion and encouraged escape. We suggest that after being consumed, plant neurotoxins such as nicotine activated an aversive dopaminergic pathway, thereby deterring predatory herbivores. Later evolutionary events - including the development of a GABAergic system capable of modulating dopaminergic activity - led to the ability to down-regulate and 'control' this dopamine-based aversion. We speculate that this negative reinforcement system evolved so that animals could suppress aversive states such as hunger in order to attend to other internal drives (such as mating and shelter) that would result in improved organismal fitness.