Stimulus control by 5-methoxy-N,N-dimethyltryptamine in wild-type and CYP2D6-humanized mice
Research highlights
► 5-Methoxy-N,N-dimethyltryptamine is a naturally-occurring hallucinogen. ► CYP2D6-humanized mice have increased levels of bufotenine following 5-MeO-DMT. ► Wild type and Tg-CYP2D6 mice did not differ in rate of acquisition of stimulus control. ► 5-MeO-DMT did not generalize to bufotenine and only partially to acetylbufotenine. ► Harmaline, a mono amine oxidase inhibitor, enhanced the stimulus effects of 5-MeO-DMT.
Introduction
Man's knowledge of plant sources of hallucinogens predates written history [Schultes and Hofmann, 1980]. However, it is only recently that chemical identification of the active principles of these botanicals became possible and pharmacological studies undertaken. The object of the present investigation, 5-methoxy-N,N-dimethyltryptamine [5-MeO-DMT], together with its close relative, N,N-dimethyltryptamine [DMT], accounts in large measure for the hallucinogenic effects of a variety of South American snuffs. However, 5-MeO-DMT is inactive when taken orally, presumably due to rapid first-pass deamination. Thus, the oral efficacy of preparations such as Ayahuasca is thought to be due to the presence of harmaline, a naturally-occurring inhibitor of mono amine oxidase [Gambelunghe et al., 2008, Shulgin and Shulgin, 1997]. Although Ayahuasca enjoys legal status both in the United States and South America for use by members of the O Centro Espirita Beneficenti Uniao do Vegetal [Gonzales, 2005], 5-MeO-DMT is included in schedule I of the Controlled Substances Act of the USA [Federal Register, 2010].
Our previous studies [Yu et al., 2003, Yu et al., 2004] using recombinant human cytochrome 450 isozymes demonstrated that the formation of bufotenine from 5-MeO-DMT is primarily catalyzed by the CYP2D6 enzyme. In addition, we observed [Shen et al., 2010a, Shen et al., 2010b] that CYP2D6-humanized [Tg-CYP2D6] mice have 60% higher bufotenine levels in serum than wild-type mice following the administration of 5-MeO-DMT. Furthermore, pretreatment with harmaline led to higher levels of both 5-MeO-DMT and bufotenine in wild-type and Tg-CYP2D6 mice.
The study of psychoactive drugs in animals gives rise to questions of interpretation and extrapolation to the human condition. However, with the demonstration that the hallucinogens, LSD and mescaline, can function as discriminative stimuli in rats [Hirschhorn and Winter, 1971], it was suggested that drug-induced stimulus control in non-verbal species might provide insight into their mechanisms of action [Winter, 1974; for recent reviews, see Nichols, 2004, Fantegrossi et al., 2008, Winter, 2009]. It has previously been reported that 5-MeO-DMT can function as a discriminative stimulus in the rat [Glennon et al., 1981, Spencer et al., 1987, Winter et al., 2000] but we are aware of no previous studies of stimulus control by 5-MeO-DMT in mice.
The present investigation examined the acquisition of stimulus control by 5-MeO-DMT in both wild-type and Tg-CYP2D6 mice and the potentiation of the stimulus effects of 5-MeO-DMT by harmaline. In addition, the hypothesis that 5-MeO-DMT acts via bufotenine as its active principle was tested.
Section snippets
Methods
The behavioral methods employed in the present investigation are essentially as described previously [Winter et al., 2005, Krall et al., 2008].
Acquisition of stimulus control in wild-type and Tg-CYP2D6 mice
Preliminary experiments determined that a dose of 5-MeO-DMT of 0.3 mg/kg did not alter response rates relative to control values. All mice then began training at that dose. Of the 6 WT mice trained, 5 reached criterion performance in a mean of 59 sessions [SE 4.3; range = 45–70]. The WT mouse which failed to reach criterion after 85 sessions was removed from the study. Response rates expressed as responses per minute in the mice reaching criterion were not significantly different following
Discussion
The data obtained in training sessions indicate that 5-MeO-DMT at a dose of 0.3 mg/kg administered subcutaneously 15 min before training will establish stimulus control in both wild type and Tg-CYP2D6 mice. The fact that the two groups appear to be equally sensitive to 5-MeO-DMT in terms of rate of responding is in keeping with our previous observation [Shen et al., 2010] that wild-type and Tg-CYP2D6 mice do not differ in serum level of 5-MeO-DMT following IP administration of the drug.
The
Conclusions
The present investigation has demonstrated stimulus control by 5-MeO-DMT in both wild-type and Tg-CYP2D6 mice. The groups did not differ in sensitivity to the rate depressant effects of 5-MeO-DMT. Furthermore, the groups did not differ in their responses to the administration of either bufotenine or acetylbufotenine in that the former was without 5-MeO-DMT-like stimulus effects while the latter yielded intermediate effects. In both groups of mice, harmaline was found to increase the stimulus
Acknowledgments
This study was supported in part by award number DA03385 [J.C. Winter] and DA021172 [A-M Yu] from the National Institute on Drug Abuse, and National Institutes of Health.
This report is dedicated to the memory of Stephen G. Holtzman [1943–2011], an esteemed colleague and a major contributor to the study of the stimulus effects of drugs.
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