The interaction of sex and route of drug administration in cocaine-induced conditioned taste aversions

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.

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.

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.

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.

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.

Sex-dependent consequences of pre-pubertal gonadectomy: Social behavior, stress and ethanol responsivity

Alcohol consumption can be enhanced or moderated by sensitivity to its aversive and appetitive properties, including positive social outcomes. These differences emerge post-pubertally, suggesting a potential role of gonadal hormones. To determine the role of gonadal hormones in sensitivity to the social impairing and social context-related attenuations in the aversive effects of ethanol, prepubertal male and female rats were gonadectomized (GX) or sham (SH) operated on postnatal day (P) 25, or left non-manipulated (NM). In adulthood (P70), rats were restrained for 90 min prior to challenge with 0.0 or 1.0 g/kg ethanol and social interaction (SI) testing. At P77, groups of 4 same-sex littermates from the same surgical condition were given access to a supersaccharin (SS) solution (3% sucrose, 0.125% saccharin), followed by an intraperitoneal injection of ethanol (0.0, 0.50, 1.0, 1.5 g/kg). Intakes of SS were examined 24h later for expression of conditioned taste aversions. Acute stress prior to SI testing increased frequency of play fighting in both sexes, whereas there were no GX effects on this measure, social investigation nor contact. GX, however, decreased baseline social preference (a social anxiety-like effect) in males, while inducing anxiolytic-like increases in baseline social preference in females. The social drinking test revealed that females developed ethanol conditioned taste aversions at a lower dose relative to males, regardless of surgical condition. These findings suggest a potential role for gonadal hormones in moderating social-anxiety like behaviors but not sensitivity to the social impairing effects of ethanol or ethanol's aversive consequences in a social context.

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.

Sex differences in adult Wistar rats in the voluntary consumption of ethanol after pre-exposure to ethanol-induced flavor avoidance learning

Vulnerability to ethanol abuse may be a function of the balance between the opposing (aversive and rewarding) motivational effects of the drug. The study of these effects is particularly important for understanding alcohol addiction. Research in this field seems to point out that ethanol effects are determined by a set of internal factors (sex, ethanol intake history, etc.), as well as by environmental conditions surrounding the individual (i.e., stress) and, of course, the interactions between all these factors. This work explores sex differences in sensitivity to aversive effects of ethanol using the procedure of flavor avoidance learning (FAL), as well as the effect of this learning experience on subsequent voluntary ethanol consumption, in adult rats. The results obtained indicated a slight sex based difference in the amount of FAL acquired in that females acquisition was weaker (experiment 1), and a differing influence of previous experience with the aversive effects of ethanol on the voluntary consumption of the drug for each sex (experiment 2). In particular, it was observed that female ethanol-naive rats showed a higher intake level and preference for ethanol than both ethanol-experienced female rats and ethanol-naive male rats. In contrast, the ethanol-experienced male rats showed a greater consumption of and preference for ethanol than ethanol-naive male rats and ethanol-experienced female rats. These data are discussed noting a range of possible explicative factors (sex hormones, hedonic processing, etc.), but further studies are warranted to elucidate the mechanisms by which ethanol pre-exposure influences the subsequent intake of ethanol differently by sex.

Sex differences in animal models of psychiatric disorders

Psychiatric disorders are characterized by sex differences in their prevalence, symptomatology and treatment response. Animal models have been widely employed for the investigation of the neurobiology of such disorders and the discovery of new treatments. However, mostly male animals have been used in preclinical pharmacological studies. In this review, we highlight the need for the inclusion of both male and female animals in experimental studies aiming at gender-oriented prevention, diagnosis and treatment of psychiatric disorders. We present behavioural findings on sex differences from animal models of depression, anxiety, post-traumatic stress disorder, substance-related disorders, obsessive-compulsive disorder, schizophrenia, bipolar disorder and autism. Moreover, when available, we include studies conducted across different stages of the oestrous cycle. By inspection of the relevant literature, it is obvious that robust sex differences exist in models of all psychiatric disorders. However, many times results are conflicting, and no clear conclusion regarding the direction of sex differences and the effect of the oestrous cycle is drawn. Moreover, there is a lack of considerable amount of studies using psychiatric drugs in both male and female animals, in order to evaluate the differential response between the two sexes. Notably, while in most cases animal models successfully mimic drug response in both sexes, test parameters and treatment-sensitive behavioural indices are not always the same for male and female rodents. Thus, there is an increasing need to validate animal models for both sexes and use standard procedures across different laboratories.

Effect of sex on ethanol consumption and conditioned taste aversion in adolescent and adult rats

RATIONALE

Vulnerability to alcoholism is determined by many factors, including the balance of pleasurable vs. aversive alcohol-induced sensations: pleasurable sensations increase intake, while aversive sensations decrease it. Female sex and adolescent age are associated with lower sensitivity to intake-reducing effects and more rapid development of alcohol abuse.

OBJECTIVES

This study assessed voluntary drinking and the aversive effects of alcohol to determine whether these measures are inversely related across the sexes and development.

METHODS

Voluntary drinking of 20 % ethanol in an every-other-day (EOD) availability pattern and the dose-response relationship of ethanol conditioned taste aversion (CTA) were assessed in male and female adolescent and adult rats.

RESULTS

CTA was sex specific in adult but not adolescent rats, with adult females exhibiting less aversion. Voluntary ethanol consumption varied according to age and individual differences but was not sex specific. Adolescents initially drank more than adults, exhibited greater day-to-day variation in consumption, were more susceptible to the alcohol deprivation effect, and took longer to establish individual differences in consumption patterns.

CONCLUSIONS

These results show that the emergence of intake patterns differs between adolescents and adults. Adolescents as a group initiate drinking at high levels but decrease intake as they mature. A subset of adolescents maintained high drinking levels into adulthood. In contrast, most adults consumed at steady, low levels, but a small subset quickly established and maintained high-consumption patterns. Adolescents also showed marked deprivation-induced increases. Sex differences were not observed in EOD drinking during either adolescence or adulthood.

Differences in sensitivity to ethanol-induced conditioned taste aversions emerge after pre- or post-pubertal gonadectomy in male and female rats

We have previously demonstrated that gonadectomy either prior to (early) or after (late) puberty elevated ethanol consumption in males to levels similar to intact adult females-effects that were attenuated by testosterone replacement. To assess whether alterations in the aversive effects of ethanol might contribute to gonadectomy-associated increases in ethanol intake in males, the present study examined the impact of gonadectomy on conditioned taste aversions (CTA) to ethanol in male and female Sprague-Dawley rats. Animals were gonadectomized, received sham surgery (SH) or non-manipulated (NM) on postnatal (P) day 23 (early) or 67 (late) and tested for CTA to ethanol in adulthood. Water-deprived rats were given 1 hr access every-other-day to 10% sucrose followed by an injection of ethanol (0, 1g/kg) for 5 test sessions. Test data were analyzed to determine the first day significant aversions emerged in each ethanol group (i.e., sucrose intakes significantly less than their saline-injected counterparts). Early gonadectomized males acquired the CTA more rapidly than did early SH and NM males (day 1 vs 3 and 4 respectively), whereas a gonadectomy-associated enhancement in ethanol CTA was not evident in late males. Among females, gonadectomy had little impact on ethanol-induced CTA, with females in all groups showing an aversion by the first or second day, regardless of surgery age. These data suggest that previously observed elevations in ethanol intake induced by either pre- or post-pubertal gonadectomy in males are not related simply to gonadectomy-induced alterations in the aversive effects of ethanol indexed via CTA.

Sex differences in the effects of ethanol pre-exposure during adolescence on ethanol-induced conditioned taste aversion in adult rats

Alcohol use, which typically begins during adolescence and differs between males and females, is influenced by both the rewarding and aversive properties of the drug. One way adolescent alcohol use may modulate later consumption is by reducing alcohol's aversive properties. Here, we used a conditioned taste aversion (CTA) paradigm to determine if pre-exposure to alcohol (ethanol) during adolescence would attenuate ethanol-induced CTA assessed in adulthood in a sex-dependent manner. Male and female Long-Evans rats were given intraperitoneal (i.p.) injections of saline or 3.0g/kg ethanol in a binge-like pattern during postnatal days (PD) 35-45. In adulthood (>PD 100), rats were given access to 0.1% saccharin, followed by saline or ethanol (1.0 or 1.5g/kg, i.p.), over four conditioning sessions. We found sex differences in ethanol-induced CTA, with males developing a more robust aversion earlier in conditioning. Sex differences in the effects of pre-exposure were also evident: males, but not females, showed an attenuated CTA in adulthood following ethanol pre-exposure, which occurred approximately nine weeks earlier. Taken together, these findings indicate that males are more sensitive to the aversive properties of ethanol than females. In addition, the ability of pre-exposure to the ethanol US to attenuate CTA is enhanced in males compared to females.

Persistent expression of methamphetamine-induced CTA in periadolescent rats

It is well documented that the transition from periadolescence to adulthood produces profound changes in motivated behavior, and furthermore, attenuates the aversive experience of abused drugs. Little is known, however, about adolescent memory for the conditioned aversive effects of abused drugs following retention intervals that span this developmental transition. The present experiment investigated methamphetamine-induced conditioned taste aversion (CTA) in periadolescent rats to determine if the magnitude of conditioning was altered following retention intervals that extend to adulthood. Rats consumed saccharin (0.1%, w/v) and were immediately injected with saline or methamphetamine (3.0mg/kg) either once (PND 40) or three times (PND 38-40), and memory was assessed one or 50 days later on post natal days 41 or 90, respectively. Rats exhibited robust methamphetamine-induced CTA one and 50 days after conditioning, and the strength of responding did not change as a function of retention interval, regardless if animals were trained with one or three saccharin-methamphetamine pairings. These findings indicate that the expression of memory for the aversive effects of methamphetamine was resistant to degradation throughout the developmental period of periadolescence to adulthood.

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.

Dopamine, norepinephrine and serotonin transporter gene deletions differentially alter cocaine-induced taste aversion

Although cocaine is primarily known for its powerful hedonic effects, there is evidence that its affective experience has a notable aversive component that is less well understood. A variety of pharmacological and molecular approaches have implicated enhanced monoamine (MA) neurotransmission in the aversive effects of cocaine. Although numerous studies have yielded data supportive of the role of the monoamines (indirectly and directly), the specific system suggested to be involved differs across studies and paradigms (Freeman et al., 2005b; Grupp, 1997; Roberts and Fibiger, 1997). Monoamine transporter knockout mice have been useful in the study of many different aspects of cocaine effects relevant to human drug use and addiction, yet an assessment of the effects of deletion of the genes for the dopamine, norepinephrine and serotonin transporters (DAT, NET, and SERT, respectively) on cocaine's aversive properties has yet to be performed (Uhl et al., 2002). In the current investigation, the strength of cocaine-induced aversions was compared among three groups of transgenic mice with deletions of the genes responsible for the production of one of the monoamine transporters. When compared to their respective WT controls, dopamine transporter deletion slightly attenuated cocaine-induced aversion while deletion of SERT or NET resulted in a more significant delay in the onset and strength of cocaine-induced taste aversions. The data lead us to conclude that the action of cocaine to inhibit NET contributes most substantially to its aversive effects, with some involvement of SERT and minimal contribution of DAT.

Sex differences in learning processes of classical and operant conditioning

Males and females learn and remember differently at different times in their lives. These differences occur in most species, from invertebrates to humans. We review here sex differences as they occur in laboratory rodent species. We focus on classical and operant conditioning paradigms, including classical eyeblink conditioning, fear-conditioning, active avoidance and conditioned taste aversion. Sex differences have been reported during acquisition, retention and extinction in most of these paradigms. In general, females perform better than males in the classical eyeblink conditioning, in fear-potentiated startle and in most operant conditioning tasks, such as the active avoidance test. However, in the classical fear-conditioning paradigm, in certain lever-pressing paradigms and in the conditioned taste aversion, males outperform females or are more resistant to extinction. Most sex differences in conditioning are dependent on organizational effects of gonadal hormones during early development of the brain, in addition to modulation by activational effects during puberty and adulthood. Critically, sex differences in performance account for some of the reported effects on learning and these are discussed throughout the review. Because so many mental disorders are more prevalent in one sex than the other, it is important to consider sex differences in learning when applying animal models of learning for these disorders. Finally, we discuss how sex differences in learning continue to alter the brain throughout the lifespan. Thus, sex differences in learning are not only mediated by sex differences in the brain, but also contribute to them.

Incentive salience of cocaine across the postpartum period of the female rat

RATIONALE-OBJECTIVES: Our prior conditioned place preference (CPP) work demonstrates that late (day16) postpartum female rats consistently prefer cocaine- over pup-associated chambers, whereas far fewer early postpartum (day8) females prefer the cocaine-associated chamber. The present study examines early and late postpartum females' preference for a cocaine-associated chamber when contrasted with a chamber associated with saline (rather than pups).

MATERIALS AND METHODS

Postpartum females were tested for conditioned preference for chambers associated with cocaine (10 mg/kg subcutaneous (SC) or 0.5, 5, 10, or 20 mg/kg intraperitoneal (IP) injections) versus saline; preferences of virgin female and male rats for select cocaine stimuli (10mg/kg SC or IP) were also tested. Locomotion was recorded during CPP conditioning and testing.

RESULTS

Early and late postpartum females expressed strikingly similar preference for the cocaine-associated chamber across all administration routes and doses. IP cocaine produced an orderly, inverted U-shaped dose-preference curve, with preference peaking at the 5 mg/kg dose (83% of females). While many postpartum females preferred 10mg/kg cocaine administered either SC or IP, both virgin females and males expressed strong aversion to SC cocaine and, while virgin females strongly preferred IP cocaine, males remained relatively indifferent. Across 10mg/kg IP cocaine-conditioning sessions, locomotor sensitization occurred exclusively in cocaine- but not saline-preferring postpartum females. Locomotor rate was lower in preferred versus nonpreferred chambers at CPP test.

CONCLUSIONS

Early and late postpartum females may be equally and uniquely susceptible to sampling and/or abuse of modestly salient doses of cocaine (10mg/kg SC; 5mg/kg IP) compared to virgin females and/or males.

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.

Early maternal separation and sex differences in the aversive effects of amphetamine in adult rats

Early life stress increases vulnerability to psychostimulant abuse, sometimes in a sex-dependent manner. These effects are presumably produced by modulation of neurobiological reward mechanisms; however, drug intake is also influenced by sensitivity to the drug's aversive properties, and little is known about the effects of early life stress on stimulant aversion. To assess this possibility, Sprague-Dawley rat litters experienced daily separation from the dam for 3 h (MS) or 15 min (H) on post-natal days 2-14; control litters (AFR) experienced twice-weekly routine animal facility care only. As adults, the animals were tested for conditioned taste aversions (CTA) induced by 1.5, 2.0 or 3.0 mg/kg d-amphetamine administered intraperitoneally over three acquisition trials followed by six drug-free extinction trials (n=7-8/sex/dose/rearing group). All groups acquired significant aversions compared to their vehicle-treated controls, but differential rearing had no effect on CTA acquisition or extinction. Collapsed across rearing groups, females exhibited significantly stronger aversions and slower extinction than their male counterparts at the low 1.5 mg/kg dose, and unlike the males, failed to fully extinguish relative to their vehicle controls at 1.5 and 2.0 mg/kg. These data underscore the importance of sex differences in assays of stimulant abuse liability, and further support the hypothesis that the effects of post-natal stress on the reinforcing efficacy of psychostimulants may be predominantly due to modulation of reward mechanisms.

Schedule-induced polydipsia alters cocaine- but not ethanol-induced suppression of saccharin consumption

Under traditional water-deprived conditions, both LiCl and morphine produce comparable suppression of saccharin consumption after repeated pairings. However, under conditions of spaced food deliveries (i.e., schedule-induced polydipsia; SIP), morphine produces a significantly weaker suppression than LiCl. The differential responses have been attributed to an increase in the rewarding effects of drugs of abuse (such as morphine) that masked the expression of the aversive effects, a masking not evident with LiCl which has no reported rewarding effects. The present study extended this characterization to two additional drugs of abuse; cocaine and ethanol. Following schedule-induced saccharin consumption, female Sprague-Dawley rats were given injections of LiCl, cocaine, ethanol (at doses comparably effective in conditioning taste aversions under water deprivation) or distilled water vehicle. Although cocaine and ethanol both suppressed SIP, only cocaine produced a significantly delayed suppression relative to LiCl. The differential effects of cocaine (and morphine), but not ethanol, may be a function of the different reward profiles of these drugs. Given the differential ability of drugs of abuse to suppress consumption under conditions of spaced feedings, the SIP procedure may be a useful baseline to assess the rewarding effects of such drugs. Further, given the differential results with cocaine and ethanol, the relative rewarding effects of drugs may be differentially indexed in this preparation, as well.

Increasing the incentive salience of cocaine challenges preference for pup- over cocaine-associated stimuli during early postpartum: place preference and locomotor analyses in the lactating female rat

RATIONALE/OBJECTIVES

Prior studies using a dual-choice conditioned place preference (CPP) procedure revealed that postpartum female rats (dams) strongly prefer chambers associated with pups over those associated with subcutaneously administered cocaine almost exclusively during early but not late postpartum (Mattson et al. 2001). The present study examines whether early postpartum dams retain strong pup-associated chamber preference when contrasted with a cocaine stimulus of greater incentive salience (intraperitoneal [IP] injections with brief conditioning sessions). Locomotor rate was measured during conditioning (stimuli-present) and test (stimulus-absent) sessions.

MATERIALS AND METHODS

A three-chambered CPP apparatus was used to compare preferences for chambers associated with IP cocaine vs age-matched pups. Unconditioned stimuli were systematically assigned to the least-preferred chamber of separate groups of dams before conditioning. Control dams verified that unconditioned stimuli were necessary for CPP and stimulus-associated locomotion.

RESULTS

Compared with most late postpartum dams (60%), only 31% of early postpartum dams preferred the cocaine-associated chamber (P < 0.05). Substantially more dams preferred the pup-associated chamber during early postpartum (27%) than late postpartum (5%; P < 0.05). Locomotor sensitization emerged across cocaine-conditioning sessions in cocaine-preferring but not pup-preferring dams (P < 0.05). Locomotor rates were consistently lower in preferred vs nonpreferred chambers during test.

CONCLUSIONS

After increasing cocaine's incentive salience, more early postpartum dams prefer the cocaine-associated chamber than previously reported (Mattson et al. 2001). However, pup-associated chamber preference was still higher in early vs late postpartum. Pup- and cocaine-preferring dams expressed differences in the induction phase of locomotor sensitization across cocaine conditioning but expressed similar motoric patterns in their preferred chambers at test.

Effects of cross-fostering on cocaine-induced conditioned taste aversions in Fischer and Lewis rats

The systematic comparison between Fischer and Lewis rats is a popular animal model of genetic factors in drug abuse. Although genetic and environmental factors interact to affect drug abuse in humans, analogous effects have not yet been reported within the Fischer-Lewis model. In order to assess the contributions and interaction of genotype and early maternal environment on responses to a drug of abuse, the present study employed a cross-fostering design, where male and female Fischer and Lewis pups were reared by unrelated dams of their own strain (in-fostered) or of the other strain (cross-fostered). As adults, rats from both strains were tested for their ability to acquire a conditioned taste aversion to a novel saccharin solution that had been repeatedly paired with an injection of cocaine (32 mg/kg, subcutaneous). In-fostered Fischer females acquired significantly weaker aversions than in-fostered Lewis females across the multiple saccharin-cocaine pairings. However, cross-fostered Fischer females exhibited aversions that were not only significantly stronger than their in-fostered Fischer counterparts, but identical to all groups of the Lewis genotype. No strain differences or cross-fostering effects were observed in the males. The data with the female subjects cannot be accounted for simply by the genetic strain of the subjects and demonstrate a clear gene-environment interaction effect on responses to the aversive effects of cocaine in Fischer and Lewis rats. Implications for studying maternal behavior as a source of epigenetic modulation of drug abuse vulnerability were discussed.

Glucagon challenge in the rat: A robust method for the in vivo assessment of Glycogen phosphorlyase inhibitor efficacy

INTRODUCTION

Glycogen phosphorlyase inhibitors (GPi) act on the glycogenolytic pathway decreasing hepatic glucose output, making them potential candidates for Type 2 diabetes treatment. We established a robust in vivo method to assess GPis efficacy utilising glucagon-stimulated glycogenolysis.

METHODS

Blood glucose was monitored in both male AP Wistar and AP Zucker rats using tail prick samples pre- and post intraperitoneal or subcutaneous glucagon administration. The effect of glycogen phosphorylase inhibitors GPi296 (6-60 mg kg(-1) po) and DAB (5 mg kg(-1) po) upon glucose response to subcutaneous glucagon were examined in both strains.

RESULTS

In the Wistar rat glucagon induced dose related increases in blood glucose, with the maximum increase occurring 20 min post dose (4.0+/-0.88 mmol l(-1), intraperitoneal; and 2.8+/-0.72 mmol l(-1), subcutaneous, ns). Intraperitoneal glucagon administration produced shorter duration blood glucose elevation than observed with the subcutaneous route of administration. In the Zucker rat, no differences were observed between the 10 and 13 week old rats in response to glucagon (3-200 microg kg(-1) subcutaneous). The maximum blood glucose increase was lower in the Wistar rat compared to the Zucker rats (2.9+/-0.20 vs 7.7+/-1.22 mmol l(-1), P<0.0000018). GPi296 and DAB both produced similar inhibition in each strain.

DISCUSSION

Subcutaneous glucagon administration induced more sustained increases in blood glucose than intraperitoneal administration. Blood glucose response to glucagon was higher in the Zucker rat compared to the Wistar rat; there was no difference in inhibition mediated by either GPi296 or DAB between the two strains. We believe that subcutaneous glucagon administration produces a robust model for the assessment of GPis in either rat strain.

The effects of cocaine, alcohol and cocaine/alcohol combinations in conditioned taste aversion learning

We have recently reported that alcohol attenuates cocaine place preferences. Although the basis for this effect is unknown, alcohol may attenuate cocaine reward by potentiating its aversive effects. To examine this possibility, these experiments assessed the effects of alcohol on cocaine-induced taste aversions under conditions similar to those that resulted in attenuated place preferences. Specifically, Experiments 1 and 2 assessed the effects of alcohol (0.5 g/kg) on taste aversions induced by 20, 30 and 40 mg/kg cocaine. Experiment 3 examined the role of intertrial interval in the effects of alcohol (0.5 g/kg) on cocaine (30 mg/kg) taste aversions. In Experiments 1 and 2, cocaine was effective at conditioning aversions. Alcohol produced no measurable effect. Combining cocaine and alcohol produced no greater aversion than cocaine alone (and, in fact, weakened aversions at the lowest dose of cocaine). In Experiment 3, varying the intertrial interval from 3 days (as in the case of Experiments 1 and 2) to 1 day (a procedure identical to that in which alcohol attenuated cocaine place preferences) resulted in significant alcohol- and cocaine-induced taste aversions. Nonetheless, alcohol remained ineffective in potentiating cocaine aversions. Thus, under these conditions alcohol does not potentiate cocaine's aversiveness. These results were discussed in terms of their implication for the effects of alcohol on cocaine-induced place preferences. Further, the effects of alcohol on place preferences conditioned by cocaine were discussed in relation to other assessments of the effects of alcohol on the affective properties of cocaine and the implications of these interactions for alcohol and cocaine co-use.

Assessment of monoamine transporter inhibition in the mediation of cocaine-induced conditioned taste aversion

Although the mechanisms of cocaine reward have been well characterized, the pharmacological basis of cocaine's aversive effects is less understood. Using the conditioned taste aversion (CTA) preparation, the present study examined the role of monoamine uptake inhibition in cocaine's aversive effects by comparing cocaine to three reuptake inhibitors with relative specificity for the transporters of dopamine (DAT; GBR 12909), norepinephrine (NET; desipramine) and serotonin (SERT; clomipramine). Specifically, 104 male Sprague-Dawley rats were given 20-min access to a novel saccharin solution followed immediately by a subcutaneous injection of cocaine, GBR 12909, desipramine, clomipramine (each at 18, 32 or 50 mg/kg; 12 groups) or drug vehicle (equivolume to the highest cocaine dose). Over trials, cocaine and desipramine each dose-dependently suppressed saccharin consumption and did so in an equivalent manner when matched by dose. However, both GBR 12909 and clomipramine conditioned weaker aversions than cocaine at the two lowest doses (18 and 32 mg/kg). At the highest dose (50 mg/kg), GBR 12909 produced equivalent suppression of saccharin consumption to cocaine while clomipramine's conditioned suppression remained relatively weak at this dose. These results suggest that cocaine's adrenergic actions resulting from NET inhibition may play a more significant role in the mediation of its aversive effects than its actions at DAT and SERT.