Cocaine-induced conditioned taste aversions in male and female Wistar rats

Male and female C57BL/6 mice display drug-induced aversion and reward in the combined conditioned taste avoidance/conditioned place preference procedure

BACKGROUND

Drugs of abuse have rewarding and aversive effects that, in balance, impact abuse potential. Although such effects are generally examined in independent assays (e.g., CPP and CTA, respectively), a number of studies have examined these effects concurrently in rats in a combined CTA/CPP design. The present study assessed if similar effects can be produced in mice which would allow for determining how each is affected by subject and experiential factors relevant to drug use and abuse and the relationship between these affective properties.

METHODS

Male and female C57BL/6 mice were exposed to a novel saccharin solution, injected (IP) with saline or 5.6, 10 or 18 mg/kg of the synthetic cathinone, methylone, and placed on one side of the place conditioning apparatus. The following day, they were injected with saline, given access to water and placed on the other side of the apparatus. After four conditioning cycles, saccharin avoidance and place preferences were assessed in a final two-bottle CTA test and a CPP Post-Test, respectively.

RESULTS

In the combined CTA/CPP design, mice acquired a significant dose-dependent CTA (p = 0.003) and a significant CPP (p = 0.002). These effects were independent of sex (all p's > 0.05). Further, there was no significant relationship between the degree of taste avoidance and place preference (p > 0.05).

CONCLUSIONS

Similar to rats, mice displayed significant CTA and CPP in the combined design. It will be important to extend this design in mice to other drugs and to examine the impact of different subject and experiential factors on these effects to facilitate predictions of abuse liability.

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 Cue Competition Effects With a Conditioned Taste Aversion Preparation

This study aimed to test whether male and female rats might show differences in cue competition effects in a conditioned taste aversion (CTA) model. Experiment 1 tested for sex differences in overshadowing. After conditioning of a flavored compound AB or only one simple flavor A (being A and B a solution of sugar 10% and salt 1%, counterbalanced), consumption of the A solution at test was larger in the former than in the latter case only in males. Thus, the usual effect of overshadowing was observed in males but not in females. Experiment 2 examined sex differences in blocking with the same stimuli used in Experiment 1. After conditioning of AB, the consumption of B was larger for the animals that previously received a single conditioning trial with A than for those that received unpaired presentations of A and the illness. As observed in Experiment 1, the typical blocking effect appeared only in males but not in females. The present findings thus support the hypothesis that sex dimorphism might be expressed in classical conditioning, or at least, in cue competition effects such as overshadowing and blocking with a taste aversion model.

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.

Female rats exhibit less avoidance than male rats of a cocaine-, but not a morphine-paired, saccharin cue

Rats avoid intake of an otherwise palatable taste cue when paired with drugs of abuse (Grigson and Twining, 2002). In male rats, avoidance of drug-paired taste cues is associated with conditioned blunting of dopamine in the nucleus accumbens (Grigson and Hajnal, 2007), conditioned elevation in circulating corticosterone (Gomez et al., 2000), and greater avoidance of the drug-paired cue predicts greater drug-taking (Grigson and Twining, 2002). While female rats generally are more responsive to drug than male rats, in this self-administration model, female rats consume more of a cocaine-paired saccharin cue and take less drug than males (Cason and Grigson, 2013). What is not known, however, is whether the same is true when a saccharin cue predicts availability of an opiate, particularly when the amount of drug experienced is held constant via passive administration by the experimenter. Here, avoidance of a saccharin cue was evaluated following pairings with experimenter delivered cocaine or morphine in male and female rats. Results showed that males and females avoided intake of a taste cue when paired with experimenter administered morphine or cocaine, and individual differences emerged whereby some male and female rats exhibited greater avoidance of the drug-paired cue than others. Female rats did not drink more of the saccharin cue than males when paired with morphine in Experiment 1, however, they did drink more of the saccharin cue than male rats when paired with cocaine in Experiment 2. While no pattern with estrous cycle emerged, avoidance of the cocaine-paired cue, like avoidance of a morphine-paired cue (Gomez et al., 2000), was associated with a conditioned elevation in corticosterone in both male and female rats.

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.

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.

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.

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.

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

Lewis (LEW) and Fischer (F344) rat strains differ on a variety of physiological and behavioral endpoints, including reactivity to drugs of abuse. Although they differ in drug reactivity, such assessments are generally limited to morphine and cocaine. To determine if these differences generalize to other drugs, the present study examined these strains for their reactivity to the affective properties of nicotine, specifically their sensitivity to nicotine in the conditioned taste aversion preparation. For four or five conditioning cycles given every other day, rats from both strains were allowed access to saccharin and injected with nicotine (0.1, 0.4, 0.8 mg/kg) or vehicle. On intervening days, all rats were given access to water and injected with vehicle. Under this one-bottle training and testing procedure, neither strain displayed aversions at the lowest dose of nicotine (0.1 mg/kg). Aversions were evident for both strains at 0.4 and 0.8 mg/kg, although the F344 rats acquired the aversions at 0.4 mg/kg faster and displayed a significantly greater aversion at 0.8 mg/kg than subjects from the LEW strain. For both strains, aversions were evident at all doses (and in a dose-dependent manner) when subjects were given access to saccharin and water in a two-bottle test. There were, however, no strain differences on this test. Differences between the two strains in their acquisition of nicotine-induced taste aversions were discussed in the context of aversion assessments with other compounds as well as in relation to differences in the self-administration of nicotine in the two strains.

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

Although taste aversion learning has been reported to be a function of a variety of factors, one that has received considerable attention is the subject's sex, wherein males generally display stronger taste aversions than females. An exception to these findings is with cocaine for which females have been shown to display greater aversions than males. Although suggestive of a Sex x Drug interaction, cocaine was administered subcutaneously (s.c.) in this report while others administered drug intraperitoneally (i.p.). Thus, there may be a Sex x Route interaction. To address the contributions of sex and route in cocaine aversions, the present study examined aversions in male and female Sprague-Dawley rats administered a range of doses of cocaine either s.c. or i.p. At the two higher doses of cocaine tested (20 and 32 mg/kg), aversions were a function of route with animals injected s.c. with cocaine displaying greater aversions than those injected i.p. Although there was no main effect of sex at either dose there was an interaction between sex and route at the 20 mg/kg dose. Specifically, s.c.-injected males displayed stronger aversions than i.p.-injected males. There were no differences between the two routes for females. Further, males displayed stronger aversions than females when injected s.c. There was no sex difference when both groups were injected i.p. This interaction was no longer evident at the highest does of cocaine (32 mg/kg). These data indicate that sex differences in aversion learning with cocaine are a function of the route of cocaine administration (and are dose specific).

Assessment of the contributions of Na+ channel inhibition and general peripheral action in cocaine-induced conditioned taste aversion

While the rewarding properties of cocaine appear to be mediated by its blockade of central monoamine uptake, the mechanisms and sites of action for cocaine's aversive effects have yet to be determined. Using the conditioned taste aversion (CTA) preparation, the present study examined the role of Na(+) channel blockade in cocaine's aversive effects by comparing cocaine to the local anesthetic procaine at three doses (18, 32 and 50 mg/kg). Furthermore, the role of cocaine's peripheral actions in its aversive effects was examined by comparing cocaine to the quaternary analog cocaine methiodide (equimolar to the three doses of cocaine) in establishing CTAs. Procaine and cocaine methiodide each dose-dependently suppressed saccharin consumption, indicating that the aversive effects of cocaine are, in part, mediated by its inhibition of Na(+) channels and via its activity in the PNS. However, the fact that the aversions induced by procaine and cocaine methiodide were weaker than those induced by cocaine at each dose tested suggests other factors are involved in its aversive effects. Possible reasons for the weaker aversions induced by procaine and cocaine methiodide relative to cocaine were discussed.

Morphine-induced conditioned taste aversions: assessment of sexual dimorphism

Although sex differences in taste aversions have been reported with emetics such as lithium chloride (LiCl), little is known whether such findings generalize to other aversion-inducing drugs, including recreational compounds. One particular class of recreational compounds that induces taste aversions but that has not been examined for sex differences in its aversive properties is the opioids. To assess sex differences in the aversive properties of the opioids, Experiment 1 examined the acquisition and extinction of morphine-induced taste aversions in male and female rats. To determine whether the specific parametric conditions used in Experiment 1 would support sex differences in general, Experiment 2 examined possible sex differences in the acquisition and extinction of LiCl-induced taste aversions, a compound for which sex differences have been previously reported. During acquisition, male and female rats were given 20-min access to a novel saccharin solution and injected with either morphine (0, 10, 18 and 32 mg/kg s.c.; Experiment 1) or LiCl (0, 0.3, 0.6 and 1.2 mEq s.c.; Experiment 2) every fourth day for a total of four conditioning trials. During extinction, subjects were allowed access to saccharin but were not injected (for a total of eight trials). There were no sex differences in acquisition with either morphine or LiCl. There were also no sex differences in extinction with morphine; however, sex differences were found with LiCl, an effect consistent with prior assessments with this drug. The basis for and implications of the differences in the effects of sex on morphine- and LiCl-induced taste aversions were discussed.

HPA axis function and drug addictive behaviors: insights from studies with Lewis and Fischer 344 inbred rats

Much research supports a link between stress and its concomitant hypothalamic-pituitary-adrenal (HPA) axis responses with behavioral sensitivity to psychoactive drugs. Our research demonstrates that Lewis inbred rats more readily acquire drug self-administration than Fischer 344 (F344) inbred rats and, compared to this strain, Lewis rats have hyporesponsive HPA axis responses to stress exposure. This association appears to conflict with investigations using outbred rats and suggests that the relationship between drug sensitivity and HPA axis responsiveness is more complicated than originally thought. It is essential to better understand this relationship because of its relevance to vulnerability and relapse to drug abuse. Thus, this paper reviews the literature in which these two inbred strains have been compared. We discuss strain differences in HPA axis function, in characteristics of the mesolimbic dopamine system, and in behaviors thought to reflect emotionality. Strain differences in unconditioned and conditioned effects of psychoactive drugs are then reviewed. Next, we discuss the possible role of sex and gonadal hormones on responsiveness to psychoactive drugs in these strains. Finally, a comparison of results obtained from these strains to three other comparator groups (e.g., high and low responders) suggests that a non-monotonic relationship between behavioral sensitivity to drugs and HPA axis responsiveness can explain much of the discrepancies in the literature.

The suppressive effects of intraperitoneal cocaine are augmented when evaluated in nondeprived rats

Rats suppress intake of a saccharin conditioned stimulus (CS) when paired with all drugs of abuse tested including morphine, cocaine, heroin, amphetamine, and ethanol. Although most of these drugs suppress intake when administered via a range of routes, the efficacy of cocaine is an exception. Specifically, cocaine-induced suppression of saccharin intake is much greater when administered subcutaneously than when administered intraperitoneally. The subcutaneous route of administration of cocaine, however, is somewhat problematic because, unless diluted, can cause stark necrosis. The present study, then, reexamined the effectiveness of intraperitoneal cocaine using less restrictive deprivation regimens that are known to facilitate the expression of the phenomenon. The results showed that, while only a 10- and 20-mg/kg dose of cocaine suppressed intake of the saccharin CS when evaluated in moderately water-deprived rats, all doses tested (i.e., 5, 10, and 20 mg/kg) significantly reduced CS intake when saccharin-cocaine pairings were evaluated in rats maintained on food and water ad libitum. Taken together, these data show that rats will readily avoid intake of a saccharin cue when paired with the intraperitoneal administration of cocaine and that the magnitude of the effect is augmented when examined in a need-free state.

Effects of SCH-23390 and raclopride on cocaine discrimination in male and female Wistar rats

Male and female rats were trained to discriminate 10.0 mg/kg cocaine from saline in a two-lever discrimination task. Injection-appropriate responding was reinforced by food pellet presentation on a tandem random-interval 30-s fixed-ratio 10 schedule. Generalization testing was conducted in extinction 10 min following an injection of saline, 1.0, 3.0, 5.6, or 10.0 mg/kg cocaine. No differences in the generalization gradients and ED(50)s were observed between male and female rats. Following the determination of the cocaine generalization gradient, the dopamine D(1) antagonist SCH-23390 (0.01-0.10 mg/kg) and the dopamine D(2) antagonist raclopride (0.1-1.6 mg/kg) were administered (independently) prior to the injection of the training dose of cocaine (10.0 mg/kg). Cocaine-antagonism tests were conducted in extinction. It was found, for each dopamine antagonist, that as the dose increased, the percentage of cocaine-appropriate responding decreased. No sex differences were observed between these generalization gradients.

Cocaine discrimination and time-course effects in male and female Wistar rats

Previously, sex differences have been observed in the behavioral effects of acute and chronic cocaine administration. In the present experiment, male and female rats were trained to discriminate intraperitoneal injections of 10.0 mg/kg cocaine from its vehicle. It was hypothesized that the subjective effects of cocaine might differ between male and female rats. It was further hypothesized that generalization gradients between male and female rats might differ as a function of the time since cocaine administration. In addition, we were interested to see whether multiple generalization gradients could be determined within the same experimental session. For that purpose, two different types of generalization tests were conducted in extinction, one in which subjects were tested both 10 min and 30 min following cocaine administration (vehicle, 1.0, 3.0, 5.6, 10 or 17 mg/kg) and one in which subjects were only tested 30 min after cocaine administration. The generalization gradients obtained 30 min following drug administration were shifted to the right of the gradient obtained 10 min following drug administration. The two 30-min gradients were not different from one another, showing that multiple generalization gradients can be obtained within the same experimental session.

Smoked cocaine self-administration in females

Although approximately 32% of all smoked cocaine ("crack") users are women, most studies investigating cocaine use have recruited only male subjects. Therefore, this study examined the smoked cocaine self-administration patterns of female crack abusers. A work requirement, where up to five tokens were earned by subjects, was followed by the administration of a sample delivery of one of three dose sizes [5.0 mg (placebo), 0.2 mg/kg, 0.4 mg/kg] of cocaine. The three dose sizes of cocaine were administered in counterbalanced order across subjects, with each subject receiving one dose size within a session and all dose sizes across the three experimental sessions. A self-administration phase followed the sample delivery, during which a token could be exchanged every 30 min for monetary reimbursement or a delivery of cocaine in the same dose size as the sample delivery. The results show that females' patterns of behavioral and subjective responding to smoked cocaine exhibit clear dose-related effects, thus affirming this self-administration model as safe and appropriate for use with women.

Effect of 6-hydroxydopamine lesions of the amygdala on intravenous cocaine self-administration under a progressive ratio schedule of reinforcement

Bilateral six-hydroxydopamine (6-OHDA) lesions were placed in the amygdala of rats self-administering cocaine (1.5 mg/kg per injection i.v.) under a progressive ratio schedule of reinforcement. Post-lesion access to three doses of cocaine (1.5, 0.75 and 0.37 mg/kg per injection i.v.) revealed a lesion effect only at the highest dose. At this dose, the lesion caused a significant increase in breaking point. No change in the breaking point was produced at the lower two doses. The biochemical results show a significant reduction in dopamine and DOPAC levels within the amygdala and an increase in dopamine within the NACC. In contrast, noradrenaline and serotonin (5-HT) levels were unaffected by the lesion in any of the dissected areas. These results demonstrate that no specific effect on cocaine reinforcement was produced by 6-OHDA lesions of the amygdala. The possibility that the lesion may have attenuated the anxiogenic qualities of the high dose of cocaine is discussed.

The effects of acute and chronic cocaine administration on paced responding in intact and gonadectomized male and female Wistar rats

Intact and gonadectomized male and female Wistar rats pressed a lever on a multiple (tandem Random-Interval 30-s, Differential Reinforcement of High Rate 0.5-s) (tandem Random-Interval 30-s, Differential Reinforcement of Low-Rate 5.0-s) schedule of reinforcement. The pacing requirements maintained high and low response rates under equal reinforcement frequencies. Low doses of cocaine (1 and 3 mg/kg) either did not affect or slightly increased high and low response rates of intact and gonadectomized female rats, while they did not affect or decrease high and low response rates of intact and castrated male rats. Higher doses of cocaine (up to 30 mg/kg) dose-dependently decreased both high and low response rates for all subjects. Intact male rats were less sensitive to the rate-decreasing effects of these doses of cocaine than castrated male rats or intact and ovariectomized female rats. Chronic cocaine administration consistently resulted in behavioral sensitization only in intact male subjects. The results of this experiment provide further support for the notion that the behavioral effects of psychomotor stimulant drugs are not necessarily rate dependent but may depend upon the extent to which schedule contingencies allow for behavioral variability without negatively affecting behavioral outcomes.

Effects of cocaine on fixed-interval behavior and schedule-induced alcohol consumption in male and female rats

Three male and three female Wistar rats pressed a lever on a fixed-interval 60-s schedule of food reinforcement while they had simultaneous access to an alcohol solution. They were challenged with different doses of cocaine hydrochloride (vehicle, 1, 3, 10, and 30 mg/kg) once lever press rates and lick rates had stabilized. Low doses of cocaine (1 and 3 mg/kg) did not systematically affect lever press rates or lick rates. The administration of 10 and 30 mg/kg cocaine dose-dependently decreased lever press rates and schedule-induced licking to a greater extent in female than in male rats. Lick rates decreased even when cocaine administration did not affect the number of pellets obtained during an experimental session. Lever press rates accelerated throughout the interreinforcement interval during control sessions. Licking was mostly limited to the first 10 s (males) or 20 s (females) after pellet presentation. Cocaine administration did not affect the distribution of lever presses and licks during the interreinforcement interval. The results of the present experiment extend previous observations that cocaine's rate-dependent effects on lever press rates may be limited to situations in which changes in lever press frequency and/or distribution negatively affect reinforcement frequency and/or the physiological consequences of schedule-induced behavior.

L-tryptophan decreases the breaking point under a progressive ratio schedule of intravenous cocaine reinforcement in the rat

L-Tryptophan (100 mg/kg, IP), the serotonin [5-hydroxytryptamine (5-HT)] amino acid precursor, significantly reduced the mean breaking point maintained under a self-administration progressive ratio schedule of IV cocaine reinforcement (0.6 mg/injection). This effect was produced over the 5 days of self-administration following treatment. Responding maintained under the same progressive ratio schedule for food reinforcement was not affected by L-tryptophan (100 mg/kg, IP). Rats administered L-tryptophan (100 mg/kg, IP) and denied access to cocaine on the day of treatment resumed normal self-administration patterns under a progressive ratio schedule on following test days. This indicates that L-tryptophan treatment alone did not induce long-term effects on cocaine self-administration. Thus, it would appear that the combination of this 5-HT manipulation and cocaine administration altered the reinforcing efficacy of the drug and induced a long-term decrement in breaking point under a progressive ratio schedule. This may have been due to an associative aversion to cocaine self-administration behaviour learned on the day of treatment and carried over to the subsequent 5 days of self-administration access.

Rewarding and aversive properties of IP and SC cocaine: assessment by place and taste conditioning

Three experiments were conducted to compare the effectiveness of intraperitoneally (IP) administered or subcutaneously (SC) administered cocaine to produce place and/or taste conditioning after four conditioning trials. In each experiment, IP (5-20 mg/kg) cocaine produced a place preference, but SC (0.5-20 mg/kg) cocaine at concentrations that prevented necrosis, did not produce a place preference. The failure of SC cocaine to produce a place preference was not a function of conditioning trial duration. On the other hand, SC cocaine (20 mg/kg) produced conditioned taste avoidance, but IP cocaine (20 mg/kg) did not produce conditioned taste avoidance. The results suggest that IP cocaine, but not SC cocaine, is rewarding.

Sex differences in locomotor activity after acute and chronic cocaine administration

Adult, intact and gonadectomized male and female Wistar rats (n = 9) were exposed to an automated open field to assess the behavioral effects of acute cocaine administration (saline, 1.0 and 10.0 mg/kg subcutaneous). The subjects were exposed to the open field for 10 min, removed to be injected and returned to the open field for another 30 min. Three saline and two drug sessions were run in counterbalanced order. Locomotor activity in intact and castrated male rats and ovariectomized female rats decreased following injection, irrespective of the dose of cocaine. The locomotor activity of intact female rats was higher than that of any other group of subjects. It decreased during the session after saline and 1.0 mg/kg cocaine, but increased towards the end of the 30 min session after 10.0 mg/kg. Rearing measures paralleled the observations on locomotor activity. To determine the effects of chronic, home-cage, cocaine administration, five of the subjects in each group were injected with 10.0 mg/kg cocaine for 9 consecutive days. The remaining four subjects received saline injections. On day 10, all subjects were re-exposed to the open-field for 10 min, removed, injected with 10.0 mg/kg cocaine and returned to the open field for another 30 min. Chronic home cage cocaine administration produced an increase in cocaine's effects on locomotor activity and rearing in intact female rats only. However, behavioral sensitization was also observed in intact female rats who had been treated with saline for 9 consecutive days, suggesting that behavioral sensitization to cocaine in intact female rats may develop very rapidly and independent of environmental context.