Corticotropin-releasing factor receptor blockade enhances conditioned aversive properties of cocaine in rats

Hypothalamic CRF1 receptor mechanisms are not sufficient to account for binge-like palatable food consumption in female rats

OBJECTIVE

The present study evaluated the effect of systemic injection of the CRF1 receptor antagonist R121919, the corticosterone synthesis inhibitor metyrapone and central amygdala (CeA) injections of the nonselective CRF antagonist D-Phe-CRF_(12-41) in rats in which binge eating was evoked by stress and cycles of food restriction.

METHOD

Female rats were subjected or not to repeated cycles of regular chow food restriction/ad libitum feeding during which they were also given limited access (2 h) to palatable food. On the test day, rats were either exposed or not to the sight of the palatable food for 15 min without allowing access, before assessing food consumption.

RESULTS

Systemic injections of R121919, but not of the metyrapone, blocked binge-like eating behavior. Restricted and stressed rats showed up-regulation of crh1 receptor mRNA signal in the bed nucleus of the stria terminalis and CeA but not in basolateral amygdala (BLA) or in the paraventricular nucleus. Injection D-Phe-CRF_(12-41) in CeA but not in the BLA-blocked binge-like eating behavior.

DISCUSSION

These findings demonstrate that extra-hypothalamic CRF1 receptors, rather than those involved in endocrine functions, are involved in binge eating and the crucial role of CRF receptors in CeA. CRF1 receptor antagonism may represent a novel pharmacological treatment for binge-related eating disorders.

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.

Gene Network Analysis in Amygdala following Taste Aversion Learning in Rats

Conditioned taste aversion (CTA) is an adaptive behavior that benefits survival of animals including humans and also serves as a powerful model to study the neural mechanisms of learning. Memory formation is a necessary component of CTA learning and involves neural processing and regulation of gene expression in the amygdala. Many studies have been focused on the identification of intracellular signaling cascades involved in CTA, but not late responsive genes underlying the long-lasting behavioral plasticity. In this study, we explored in silico experiments to identify persistent changes in gene expression associated with CTA in rats. We used oligonucleotide microarrays to identify 248 genes in the amygdala regulated by CTA. Pathway Studio and IPA software analyses showed that the differentially expressed genes in the amygdala fall in diverse functional categories such as behavior, psychological disorders, nervous system development and function, and cell-to-cell signaling. Conditioned taste aversion is a complex behavioral trait which involves association of visceral and taste inputs, consolidation of taste and visceral information, memory formation, retrieval of stored information, and extinction phase. In silico analysis of differentially expressed genes is therefore necessary to manipulate specific phase/stage of CTA to understand the molecular insight.

Adiponectin and energy homeostasis: consensus and controversy

Adiponectin regulates energy homeostasis through the modulation of glucose and fatty acid metabolism in peripheral tissues. However, its central effect on energy balance remains unclear and controversial. Despite the disparate data, recent advances in our understanding of the signal transduction mechanisms used by adiponectin in the periphery and in the hypothalamus suggest that intracellular cross-talk between adiponectin, leptin and insulin may occur at several levels. The present review will summarize recent reports describing the peripheral and central effects of adiponectin and discuss progress concerning its molecular mechanisms. We will also particularly focus on apparent controversies and related mechanisms associated with the central effects of adiponectin on energy homeostasis.

The runway model of drug self-administration

Behavioral scientists have employed operant runways as a means of investigating the motivational impact of incentive stimuli for the better part of the past 100 years. In this task, the speed with which a trained animal traverses a long straight alley for positive incentive stimuli, like food or water, provides a reliable index of the subject's motivation to seek those stimuli. The runway is therefore a particularly appropriate tool for investigating the drug-seeking behavior of animals working for drugs of abuse. The current review describes our laboratory's work over the past twenty years developing and implementing an operant runway model of drug self-administration. Procedures are described that methodologically dissociate the antecedent motivational processes that induce an animal to seek a drug, from the positive reinforcing consequences of actually earning the drug. Additional work is reviewed on the use of the runway method as a means of modeling the factors that often result in a "relapse" of drug self-administration after a period of abstinence (i.e., a response reinstatement test), as are runway studies that revealed the presence of opposing positive and negative consequences of self-administered cocaine. This body of work suggests that the runway method has served as a powerful behavioral tool for the study of the behavioral and neurobiological basis of drug self-administration.

Teratogenic effects of maternal antidepressant exposure on neural substrates of drug-seeking behavior in offspring

If neurotransmitter balance is upset in the developing nervous system by exposure to antidepressant drugs, structural and functional hedonic phenotypes of offspring may be affected. In order to test this hypothesis, two groups of pregnant Wistar dams were exposed to vehicle or fluoxetine by implantation on gestational day 14 of osmotic minipumps delivering 0 or 10 mg/kg/day fluoxetine for 14 days. The consequences of perinatal fluoxetine exposure on offspring conflict-exploratory behavior were quantified using the elevated plus-maze on postnatal day (PND) 30. Beginning on PND 60, the reinforcing properties of acutely administered cocaine were examined using a place conditioning procedure. Beginning on PND 90, a subset of rats were implanted with jugular catheters and allowed to acquire self-administration of cocaine in an operant environment. In support of the hedonic modulation hypothesis, perinatal fluoxetine produced a significant decline in both nucleus accumbens cell count (-9%) and serotonin transporter-like immunoreactivity in the raphe nucleus (-35%) on PND 120. In the elevated plus-maze, perinatal fluoxetine exposure decreased (-21%) overall activity. In the place conditioning trial, only the fluoxetine-treated group exhibited a significant place preference for the compartment paired previously with cocaine. In a cocaine self-administration extinction trial, there was a statistically significant increase (350%) in extinction response rate among fluoxetine-exposed offspring. These findings suggest that perinatal exposure to fluoxetine perturbs adult serotonergic neurotransmission and produces a positive hedonic shift for conditioned reinforcing effects of cocaine.

Differential anxiogenic, aversive, and locomotor effects of THC in adolescent and adult rats

RATIONALE

Unpleasant side effects of drugs of abuse often limit their repeated use; however, such effects may be attenuated in adolescents compared to adults.

OBJECTIVES

We investigated whether the anxiogenic, aversive, or locomotor effects of delta-9-tetrahydrocannabinol (THC) differ between adolescent and adult rats.

METHODS

We used the elevated plus maze (EPM) and light-dark tests of anxiety, the conditioned taste aversion and conditioned place aversion (CPA) tests of generalized aversion, and measures of stress hormone levels in serum to examine effects of THC in adolescent and adult rats. Locomotor activity was also recorded in the EPM, light-dark task, and CPA association sessions.

RESULTS

In the EPM and light-dark tasks, THC was anxiogenic in both age groups, but the drug was more anxiogenic in adults than in adolescents. In the place and taste aversion tasks, THC was aversive in both ages, and at 1.25 and 5 mg/kg, was more aversive in adults than in adolescents. The locomotor response to THC, as measured in the anxiety tasks and CPA, affected adults more than adolescents. Multiple measures revealed a locomotor-decreasing effect in adults, whereas some measures suggested a small locomotor-increasing effect in adolescent rats.

CONCLUSIONS

These results suggest that THC can have greater anxiogenic, aversive, and locomotor-reducing effects in adult rats than in adolescent rats. These findings suggest an explanation for reduced marijuana use in adult humans compared to teenagers.

Adolescent rats are protected from the conditioned aversive properties of cocaine and lithium chloride

In humans, most drug use is initiated during adolescence and adolescent users are more likely to become drug-dependent than adult users. Repeated, high levels of use are required for the transition from use to addiction. Individual levels of drug use are thought to result from a balance between the pleasant or rewarding and the unpleasant or aversive properties of the drug. Repeated high levels of drug use are required for the transition from drug use to dependence. We hypothesized that diminished aversive effects of drugs of abuse during adolescence might be one reason for higher rates of use and addiction during this phase. We therefore tested adolescent and adult CD rats in single-dose cocaine conditioned taste aversion (CTA) at a range of doses (10-40 mg/kg), and examined whether various behavioral markers of addiction vulnerability were correlated to outcome in cocaine CTA. We found that adolescents are indeed less susceptible to cocaine CTA. In fact, age was the predominant predictor of CTA outcome, predominating over measures of novelty-seeking, anxiety, and stress hormone levels, which are all known to be related to drug intake in other models. Furthermore, we found that adolescent rats are also less susceptible to conditioned taste aversion to a low dose of a non-addictive substance, lithium chloride. These results suggest that one explanation for elevated drug use and addiction among adolescents is reduced aversive or use-limiting effects of the drugs. This contributes to our understanding of why adolescence is a particularly vulnerable period for development of drug abuse.

Effects of CP 154,526, a CRF1 receptor antagonist, on behavioral responses to cocaine in rats

We examined the influence of CP 154,526, a selective antagonist of corticotropin-releasing factor (CRF)1 receptors, in the locomotor, sensitizing, discriminative stimulus and rewarding effects of cocaine, as well as on the cocaine-induced reinstatement of cocaine-seeking behavior in male Wistar rats. CP 154,526 in doses of 5, 10 and 20 mg/kg, which did not affect basal locomotor activity, dose-dependently reduced the hyperactivation evoked by cocaine. To assess the effects of CP 154,526 on the expression of cocaine sensitization, the rats were injected with either saline or cocaine (10 mg/kg) for 5 days, and were then challenged with cocaine (10 mg/kg) after pretreatment with saline or CP 154,526 on day 5 of withdrawal. The cocaine-induced hyperactivity in sensitized rats was reduced by CP 154,526 (10 and 20 mg/kg). In rats trained to discriminate cocaine (10 mg/kg) from saline, pretreatment with CP 154,526 (5-20 mg/kg) did not affect the cocaine (1.25-10 mg/kg)-induced discriminative stimulus effects. In a self-administration model, the rats were trained to self-administer cocaine (0.5 mg/kg/infusion) in the FR 5 schedule of reinforcement. Administration of CP 154,526 (10-20 mg/kg) did not alter the rewarding effects of cocaine, assessed as the number of active-lever presses and infusions; however, following a 10-day extinction phase, CP 154,526 (5-20 mg/kg) significantly decreased in a dose-dependent manner the cocaine (10 mg/kg) priming-induced reinstatement of cocaine-seeking behavior. The present study implies that CRF1 receptors control the expression of cocaine hyperactivation and sensitization as well as the cocaine-induced relapse behavior, but do not play any role in cocaine discrimination and self-administration. These findings may suggest that CRF1 receptor antagonists should be considered as possible medications in the treatment of cocaine addiction.

The therapeutic potential of CRF1 antagonists for anxiety

Preclinical studies suggest that the brain corticotropin-releasing factor (CRF) systems mediate anxiety-like behavioural and somatic responses through actions at the CRF1 receptor. CRF1 antagonists block the anxiogenic-like effects of CRF and stress in animal models. Cerebrospinal fluid levels of CRF are elevated in some anxiety disorders and normalise with effective treatment, further implicating CRF systems as a therapeutic target. Prototypical CRF1 antagonists are highly lipophilic, non-competitive antagonists of peptide ligands. Modification of the chemotype and the identification of novel pharmacophores are yielding more drug-like structures with increased hydrophilicity at physiological pHs. Newer compounds exhibit improved solubility, pharmacokinetic properties, potency and efficacy. Several clinical candidates have entered Phase I/II trials. However, unmet challenges await resolution during further discovery, clinical development and therapeutic application of CRF1 antagonists.

The corticotropin-releasing factor (CRF)-system and monoaminergic afferents in the central amygdala: investigations in different mouse strains and comparison with the rat

Corticotropin-releasing-factor (CRF) containing systems and monoaminergic afferents of the central amygdaloid nucleus (Ce) are crucial players in central nervous stress responses. For functional analyses of specific roles of these systems, numerous mouse models have been generated which lack or overexpress individual signal transduction components. Since data concerning system morphologies in murine brain are rarely available, mouse studies are usually designed and interpreted based on previous findings in rats, although interspecies differences are frequent. In the present study, in situ hybridization for CRF mRNA and correlative immunocytochemistry for CRF and monoaminergic afferents revealed numerous CRF mRNA-reactive neurons in the lateral Ce subnucleus (CeL) codistributed with dense dopaminergic fiber plexus in mice as has been demonstrated in rats. However, while in rats the lateral capsular Ce (CeLc) displays only scarce CRF immunoreactive (CRF-ir) innervation, particularly dense CRF-ir fiber plexus were observed in the CeLc in mice, with differences in labeling densities between different strains. CRF-ir terminal fibers overlap with the moderate serotonergic innervation of this subnucleus in mice. Additionally, CRF mRNA-reactive neurons were found immediately dorsal to the amygdala in the region of the interstitial nucleus of the posterior limb of the anterior commissure/amygdalostriatal transition area in both species. In mice, this region displayed dense CRF-ir fiber plexus, with variations between the strains. The results indicate that in mice and rats dopaminergic afferents represent the primary monoaminergic input to the CRF neurons in the CeL. In mice only, CRF-ir afferents provide dense innervation of CeLc neurons. Since the CeLc lacks dopaminergic input in both species but possesses moderate serotonergic afferents, CRF/serotonin interactions may occur selectively in mouse CeLc. The observed interspecies and interstrain differences in CRF input and CRF/monoaminergic interactions may influence the interpretation of findings concerning Ce functions in stress and fear in mouse models.

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.

Asymmetric serial interactions between ethanol and cocaine in taste aversion learning

Although the interaction between ethanol and cocaine is well documented, it has generally been limited to situations in which the two drugs are given concurrently. Little exists on the interaction between ethanol and cocaine when one drug is given prior to the other. In Experiment 1, female Long-Evans rats were given five exposures to ethanol (2 g/kg ip) or vehicle prior to taste aversion conditioning with cocaine (32 mg/kg sc) for a total of five conditioning trials. In Experiment 2, rats were given five exposures to cocaine (32 mg/kg sc) or vehicle prior to taste aversion conditioning with ethanol (2 g/kg ip) for a total of five conditioning trials. Ethanol-preexposed, cocaine-conditioned animals (Experiment 1) displayed attenuated aversions to the cocaine-associated solution, drinking significantly greater amounts of saccharin than vehicle-preexposed, conditioned subjects. Conversely, cocaine-preexposed, ethanol-conditioned animals (Experiment 2) displayed robust aversions to the ethanol-associated solution, drinking levels comparable to those consumed by vehicle-preexposed, conditioned subjects and drinking significantly less than controls. Although the basis for these asymmetric effects is not known, they may have implications for abuse vulnerability in that drug history may impact subsequent drug toxicity that, in turn, may alter drug acceptability.

Ethanol preexposure attenuates the interaction of ethanol and cocaine in taste aversion learning

Although the potentiating effects of ethanol and cocaine have been well documented, little has been reported regarding the effects of ethanol or cocaine history on this interaction. In the present study, female Long-Evans rats received five exposures to ethanol (3.5 g/kg ip) or vehicle prior to taste aversion conditioning in which a novel saccharin solution was paired with either ethanol (0.56 g/kg ip), cocaine (25 mg/kg sc) or the combination (or the drugs' vehicle) for a total of five conditioning trials. Nonpreexposed subjects conditioned with the ethanol/cocaine combination displayed aversions, drinking levels significantly less than nonpreexposed subjects conditioned with either drug alone. Further, the aversions produced by the combination were greater than the sum of the aversions produced by ethanol and cocaine, alone. Ethanol-preexposed animals conditioned with the combination displayed an attenuated aversion, drinking significantly greater amounts of saccharin than nonpreexposed conditioned subjects and not differing from controls. Although the basis for the attenuation by ethanol of the aversions induced by the drug combination is not known, the present findings may have implications for the use and abuse of the combination in that alcohol history may reduce the subsequent toxicity of the combination that in turn may affect its acceptability.

Effects of cocaine context on NAcc dopamine and behavioral activity after repeated intravenous cocaine administration

In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kgx6) or saline (0.2 mlx6) on alternating days. One group in each of these experiments ('Cocaine Cues') occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group ('Novel'), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.

Daily melatonin administration to middle-aged male rats suppresses body weight, intraabdominal adiposity, and plasma leptin and insulin independent of food intake and total body fat

Pineal melatonin secretion declines with aging, whereas visceral fat, plasma insulin, and plasma leptin tend to increase. We have previously demonstrated that daily melatonin administration at middle age suppressed male rat intraabdominal visceral fat, plasma leptin, and plasma insulin to youthful levels; the current study was designed to begin investigating mechanisms that mediate these responses. Melatonin (0.4 microg/ml) or vehicle was administered in the drinking water of 10-month-old male Sprague Dawley rats (18/treatment) for 12 weeks. Half (9/treatment) were then killed, and the other half were submitted to cross-over treatment for an additional 12 weeks. Twelve weeks of melatonin treatment decreased (P<0.05) body weight (BW; by 7% relative to controls), relative intraabdominal adiposity (by 16%), plasma leptin (by 33%), and plasma insulin (by 25%) while increasing (P<0.05) locomotor activity (by 19%), core body temperature (by 0.5 C), and morning plasma corticosterone (by 154%), restoring each of these parameters toward more youthful levels. Food intake and total body fat were not changed by melatonin treatment. Melatonin-treated rats that were then crossed over to control treatment for a further 12 weeks gained BW, whereas control rats that were crossed to melatonin treatment lost BW, but food intake did not change in either group. Feed efficiency (grams of BW change per g cumulative food intake), a measure of metabolic function, was negative in melatonin-treated rats and positive in control rats before cross-over (P<0.001); this relationship was reversed after cross-over (P<0.001). Thus, melatonin treatment in middle age decreased BW, intraabdominal adiposity, plasma insulin, and plasma leptin, without altering food intake or total adiposity. These results suggest that the decrease in endogenous melatonin with aging may alter metabolism and physical activity, resulting in increased BW, visceral adiposity, and associated detrimental metabolic consequences.

Conditioned suppression with cocaine as the unconditioned stimulus

A conditioned-suppression procedure was used to study drug conditioning using cocaine as the unconditioned stimulus (UCS). Rats were first trained to nose poke for food-reinforcement during daily 60-min sessions. At least 1 week following jugular vein catheterization, a 5-min tone-light compound stimulus was presented 30 min into the food-reinforcement session. Two minutes after the onset of the stimulus, either 0 (saline), 1.0, 3.0 or 5.6 mg/kg cocaine, was administered i.v. to separate groups of rats. For another group, the stimulus was presented, and the 5.6 mg/kg dose of cocaine was injected in an unpaired fashion (i.e., at different times). After 5 days of training a test was given with the tone-light stimulus presented alone. No disruption of responding during the tone-light stimulus was observed in the saline and 1.0 mg/kg cocaine groups or for the unpaired group. When the tone-light stimulus was paired with 5.6 mg/kg cocaine; however, it produced nearly a 50% reduction in responding, which then gradually extinguished when the stimulus was presented alone for 5 days. The 3.0 mg/kg cocaine group produced intermediate suppression. When the tone-light compound stimulus was shortened to 70 s and the interstimulus interval (ISI) was 0, 30, or 60 s in three separate groups of rats, the most robust conditioned suppression was observed at the 60 s ISI. Therefore, the conditioned suppression procedure, using 3.0 or 5.6 mg/kg i.v. cocaine doses as the UCS, produced robust conditioning effects comparable to other drugs and more conventional reinforcers. The conditioned suppression procedure may be a useful model for studying the classically conditioned effects of cocaine.

Evidence for opponent-process actions of intravenous cocaine

The present experiment was devised to test a prediction of the Opponent-Process Theory of drug action. This theory presumes that the initial affective experience of a subject treated with cocaine would be diametrically different immediately after administration compared to some point later in time when the positive impact of the drug had subsided. A conditioned place-preference procedure was employed in which a novel environment was paired with the effects of cocaine either immediately after, 5 min after, or 15 min after an intravenous injection of 0.75 mg/kg cocaine. It was hypothesized that animals would come to prefer environments associated with the immediate positive effects of cocaine and avoid environments associated with the drug's subsequent negative effects. The results confirmed this hypothesis. While the 0-min delay and 5-min delay groups exhibited conditioned preferences for the cocaine-paired environment, the 15-min delay group came to avoid the side of the preference apparatus paired with cocaine. These data, therefore, serve as additional support for an Opponent-Process account of cocaine's actions.

Intravenous cocaine precipitates panic-like flight responses and lasting hyperdefensiveness in laboratory rats

There is an emerging body of clinical evidence that cocaine use in humans can result in serious fear or panic-related emotional disturbances. The present study evaluated the effects of intravenous cocaine administration upon defensive responses of rats to a threatening conspecific in a test situation, an oval runaway, permitting the display of the full range of the rat defensive repertoire. A battery of tests was employed to evaluate avoidance/escape, flight, freezing, defensive upright and defensive attack behaviors. In the first experiment male Long-Evans rats implanted with a chronic indwelling jugular catheter were placed in the runway and tested immediately after administration of either 0, 1, or 4 mg/kg of cocaine hydrochloride. The 4-mg/kg dose produced a dramatic flight response, the direction of which depended upon the direction of the approaching threat source. The same dose produced increased defensive upright postures during forced contact with the stimulus animal. Experiment 2 examined the time course for cocaine-induced hyperdefensiveness. Rats were administered either saline or 4 mg/kg cocaine intravenously and were tested following a delay of either 0, 5, 15, or 30 min following infusion. Cocaine-treated rats again displayed high levels of flight, which declined with increased time between infusion and testing. However, increased defensiveness persisted even at the 30 min delay for several defensive measures including avoidance, freezing, and defensive upright posture. Thus, following an initial period of rapid flight with intravenous cocaine administration, there was a lasting hyperdefensiveness in cocaine-treated rats. The present results suggest that cocaine may exert its panic-producing effects by acting upon neurobehavioral systems subserving defensive behavior, and that understanding of these systems is critical for understanding the neurobiology of panic disorder.

Cocaine preexposure fails to sensitize the acquisition of cocaine-induced taste aversions

In two separate experiments, rats were given either an intraperitoneal (IP) injection of 10 mg/kg cocaine once a day for 10 consecutive days (Experiment 1) or a single IP injection of 40 mg/kg of cocaine (Experiment 2) prior to receiving repeated pairings of a novel saccharin solution with cocaine (32 mg/ kg; subcutaneous; SC). Although vehicle-preexposed subjects given saccharin-cocaine pairings readily acquired an aversion to the cocaine-associated saccharin solution, subjects preexposed to cocaine (whether 10 times or only once) displayed a retarded acquisition of the aversion. That is, cocaine preexposure attenuated the acquisition of cocaine-induced taste aversions. There was no difference in the degree of attenuation between the two preexposure conditions. Thus, under conditions that are effective in inducing sensitization within other behavioral preparations there was no evidence of sensitized cocaine-induced taste aversions. The results from the present investigation are similar to reports from this laboratory and others demonstrating that preexposure to cocaine, as with a range of other psychoactive drugs, results in weaker taste aversions. The basis for the attenuating effects of cocaine preexposure was discussed in terms of an adaptation to the aversive effects of cocaine.