Assessment of the stimulus properties of anxiolytic drugs by means of the conditioned taste aversion procedure

Conditioned Taste Avoidance Drug Discrimination Procedure: Assessments and Applications

In the present chapter, we summarize much of the work on the taste avoidance drug discrimination procedure, presenting the logic for its initial introduction and the extension of the procedure in the investigation of the discriminative properties of various drugs. Results from these assessments parallel those from more traditional operant and maze designs in classifying and characterizing the discriminative properties of drug. At the same time, this design reveals a procedure that is sensitive in such assessments by indexing these stimulus properties more rapidly and at lower doses than in the more traditional procedures (in some cases for drugs heretofore resistant in their detection). Importantly, much remains to be learned about the taste avoidance procedure in that the nature of such learning remains unknown and the specific parameters under which it can be established and generalized and its neurochemical and neuroanatomical bases are largely unexplored. The application of drug discrimination learning to human drug abuse continues to be an important consideration for this specific design (as well as that of drug discrimination procedures in general), and recent parallels between drug use and food intake in terms of its regulation by interoceptive stimuli suggests a possible role of the loss of stimulus control in drug escalation and addiction (with possible therapeutic implications via the modulation of these interoceptive cues).

Dopamine mediates cocaine-induced conditioned taste aversions as demonstrated with cross-drug preexposure to GBR 12909

Although cocaine readily induces taste aversions, little is known about the mechanisms underlying this effect. It has been suggested that its inhibitory effects at one of the monoamine transporters may be mediating this suppression. Using the cross-drug preexposure preparation, the present series of studies examined a possible role of dopamine (DA) in this effect. Male Sprague-Dawley rats were exposed to cocaine (18 mg/kg; Experiment 1) or the selective DA transporter (DAT) inhibitor GBR 12909 (50 mg/kg; Experiment 2) prior to the pairing of a novel saccharin solution with injections of GBR 12909 (32 mg/kg), cocaine (18 mg/kg) or vehicle in a conditioned taste aversion (CTA) procedure. Preexposure to cocaine attenuated aversions induced by itself but not aversions induced by GBR 12909 (Experiment 1). Conversely, preexposure to GBR 12909 attenuated aversions induced by itself and cocaine (Experiment 2). This asymmetry suggests that cocaine and GBR 12909 induce CTAs via similar, but non-identical, mechanisms. These data are discussed in the context of previous work demonstrating roles for dopamine, norepinephrine and serotonin in cocaine-induced CTAs.

Application of the conditioned taste aversion paradigm to assess discriminative stimulus properties of psychostimulants in rats

BACKGROUND

The conditioned taste aversion (CTA) paradigm is one of the reliable methods to evaluate the discriminative stimulus properties of drugs and is characterized by a short conditioning period and no need for special equipment. This method, however, has not yet been fully investigated for psychostimulants such as cocaine and methamphetamine.

METHODS

In the present study, rats were trained to discriminate between cocaine and a vehicle using CTA and substitution tests with various psychostimulants were conducted to evaluate the usefulness of the method for assessing the discriminative stimulus properties of this pharmacological class. Male rats received an intraperitoneal (i.p.) injection of cocaine (10mg/kg) 10 min prior to access saccharin for 20-min, and immediately after the saccharin access they received an i.p. dose of LiCl (1.8 mEq; n=8, Group CL) or the vehicle (n=8, Group CW) on the day of conditioning; on the other days (2 or 3 days between the cocaine conditioning days), they were injected with saline prior to access to saccharin without the LiCl or vehicle injection after the access.

RESULTS

By the fifteenth cocaine conditioning trial, all animals acquired discrimination. In the substitution test, cocaine dose dependently decreased saccharin consumption. The psychostimulants, methamphetamine, methylphenidate, bupropion and sibutramine, substituted for cocaine, whereas the opioid μ agonist morphine and the cannabinoid agonist, Δ9-tetrahydrocannabinol, did not substitute for cocaine. Mazindol did not substitute for cocaine although it has CNS stimulant activities.

CONCLUSION

These results suggest that discriminative stimulus properties of psychostimulants can be evaluated using the CTA paradigm.

The inability of CCK to block (or CCK antagonists to substitute for) the stimulus effects of chlordiazepoxide

To further examine the relationship between cholecystokinin (CCK) and GABA, the present study assessed the ability of the CCK-A antagonist devazepide and the CCK-B antagonist L-365,260 to substitute for the stimulus effects of chlordiazepoxide (CDP), as well as the ability of CCK-8s to block these effects, in female Long-Evans rats within the conditioned taste aversion baseline of drug discrimination learning. Both devazepide and L-365,260 failed to substitute for the discriminative stimulus properties of CDP, and CCK-8s failed to block its stimulus effects. The benzodiazepine diazepam did substitute for, and the benzodiazepine antagonist flumazenil did block, the stimulus effects of CDP. This suggests that the lack of substitution for, or antagonism of, CDP by the CCK antagonists and CCK-8s, respectively, was not due to the inability of the present design to assess such effects. Possible bases for the current findings, e.g., necessity of an anxiogenic baseline, drug and receptor specificity, as well as the dose-response nature of the interaction, were discussed. Given that a relationship between CCK and GABA has been reported in other designs, the present results suggest that such a relationship may be preparation specific.

Drug discrimination using a Pavlovian conditional discrimination paradigm in pigeons

Three pigeons were studied using a discriminated autoshaping procedure in which the presence or absence of methadone served as a conditional stimulus signalling which of two key light CSs would be followed by grain access. Drug sessions alternated randomly with no-drug sessions. Methadone (2.0 mg/kg) was administered prior to drug sessions in which a black vertical line on a white background served as CS+ and a diffuse white keylight served as CS- (reversed for bird 681). Saline or no injection was administered prior to no-drug sessions and the CS+/CS- contingencies were reversed. Discriminated performances emerged in which over 80% of the responding occurred to the appropriate stimulus. Stimulus control by methadone was assessed by presenting a range of methadone doses during 10-trial extinction sessions. A graded dose-effect curve was produced with low doses of methadone controlling saline-appropriate responding and higher doses controlling drug-appropriate responding. A range of doses of morphine, cocaine, and pentobarbital were also tested. Morphine produced methadone-appropriate responding while cocaine and pentobarbital did not.