Drug discrimination

Hallucinogens in Drug Discrimination

Hallucinogens comprise a diverse collection of chemicals with multifarious receptor actions in the central nervous system. Preclinical drug screening methods have proven invaluable in the evaluation and characterization of hallucinogen psychopharmacology. Used in concert with structural chemistry and receptor pharmacology methods, preclinical drug discrimination research has informed our current understanding of hallucinogens and the neurochemical receptor mechanisms responsible for their interoceptive stimulus effects. This chapter summarizes the strengths and limitations of drug discrimination as an in vivo drug detection method and offers a brief review of historical and contemporary drug discrimination research with classical hallucinogens.

Comparison of the discriminative stimulus effects of dimethyltryptamine with different classes of psychoactive compounds in rats

RATIONALE

There has been increased recreational use of dimethyltryptamine (DMT), but little is known of its discriminative stimulus effects.

OBJECTIVES

The present study assessed the similarity of the discriminative stimulus effects of DMT to other types of hallucinogens and to psychostimulants.

METHODS

Rats were trained to discriminate DMT from saline. To test the similarity of DMT to known hallucinogens, the ability of (+)-lysergic acid diethylamide (LSD), (-)-2,5-dimethoxy-4-methylamphetamine (DOM), (+)-methamphetamine, or (+/-)3,4-methylenedioxymethyl amphetamine (MDMA) to substitute in DMT-trained rats was tested. The ability of DMT to substitute in rats trained to discriminate each of these compounds was also tested. To assess the degree of similarity in discriminative stimulus effects, each of the compounds was tested for substitution in all of the other training groups.

RESULTS

LSD, DOM, and MDMA all fully substituted in DMT-trained rats, whereas DMT fully substituted only in DOM-trained rats. Full cross-substitution occurred between DMT and DOM, LSD and DOM, and (+)-methamphetamine and MDMA. MDMA fully substituted for (+)-methamphetamine, DOM, and DMT, but only partially for LSD. In MDMA-trained rats, LSD and (+)-methamphetamine fully substituted, whereas DMT and DOM did not fully substitute. No cross-substitution was evident between (+)-methamphetamine and DMT, LSD, or DOM.

CONCLUSIONS

DMT produces discriminative stimulus effects most similar to those of DOM, with some similarity to the discriminative stimulus effects of LSD and MDMA. Like DOM and LSD, DMT seems to produce predominately hallucinogenic-like discriminative stimulus effects and minimal psychostimulant effects, in contrast to MDMA which produced hallucinogen- and psychostimulant-like effects.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Antagonism of discriminative stimulus effects of delta(9)-THC and (R)-methanandamide in rats

RATIONALE

In previous drug discrimination studies we observed surmountable antagonism by Delta(9)-tetrahydrocannabinol (THC) in the presence of constant doses of SR-141716 [N-(piperidin-1-yl)-5-(4-chloro-phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (0.3 and 1 mg/kg), but there was only marginal evidence for surmountable antagonism with combinations of SR-141716 and (R)-methanandamide, a chiral analog of the endocannabioid anandamide.

OBJECTIVE

Here we examine antagonism where the cannabinoid CB1 receptor agonist [Delta(9)-THC and (R)-methanandamide] dose is held constant (i.e., the training dose) and the antagonist {i.e., SR-141716 and AM-251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; 2 ml/kg]} dose varied. We also tested the cannabinoid CB2 receptor antagonist SR-144528 {N-[(1S)-endo-1,3,3-trimethylbicyclo(2.2.1)heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide}.

METHODS

Different groups of rats were trained to discriminate between vehicle and three different doses of Delta(9)-THC (1.8, 3, and 5.6 mg/kg, presumably reflecting different efficacy demands) as well as 10 mg/kg (R)-methanandamide. Dose-generalization tests involved different doses of the cannabinoid CB1 receptor agonists. Antagonist tests varied the dose of the antagonist (range: 0.1 and 3 mg/kg for SR-141716 and AM-251, and 1 to 10 mg/kg for SR-144528).

RESULTS

SR-141716 and AM-251 doses dependently blocked the agonist-induced discriminative stimulus effects. SR-141716 tended to be slightly more potent than AM-251. The effective dose 50 (ED(50)) of SR-141716 was higher in the 5.6 mg/kg Delta(9)-THC-trained group relative to the two other Delta(9)-THC-trained groups. The cannabinoid CB2 receptor antagonist SR-144528 combined with the training dose of 1.8 mg/kg Delta(9)-THC, as well as when combined with the training dose of 10 mg/kg (R)-methanandamide, did not markedly change drug-appropriate (agonist) responses.

CONCLUSION

Data support that the discriminative stimulus effects of (R)-methanandamide and its overlap with the Delta(9)-THC cue are, indeed, CB1 receptor mediated events as revealed in antagonism tests with the selective central CB1 receptor antagonists SR-141716 and AM-251. The activation of cannabinoid CB2 receptors appears to be insignificant for these discriminations.