Discriminative Stimulus Effects of Substituted Tryptamines in Rats

Classic serotonin-mediated hallucinogens such as psilocybin have a long history of use and have recently been investigated in clinical trials for conditions including depression, anxiety related to cancer, persistent pain, and as adjuncts in substance-use cessation. At the same time, novel synthetic tryptamines continue to appear on recreational markets, many described in TiKHAL, and several have been identified as compounds of interest by the US Drug Enforcement Administration. These include a range of 4-acetoxy and 4-hydroxy substituted tryptamines and variants with different N-substituents; some have been detected in biological samples and others are readily available online. Gatch and colleagues set out to determine whether a set of eight substituted tryptamines produce hallucinogen-like discriminative stimulus effects in rats trained to discriminate the 5-HT2A-preferring hallucinogen DOM (0.5 mg/kg, i.p.) from saline. The rationale is that drug discrimination provides a useful preclinical model of subjective drug effects and can flag compounds with potential abuse liability when self-administration models are not informative for hallucinogens. The study therefore tested whether these DEA-noted tryptamines substitute for DOM and examined their relative potencies and effects on operant response rate.

Male Sprague-Dawley rats were used, housed individually on a 12/12 light/dark cycle with ad libitum water and food restricted to maintain body weight at 320–350 g. All procedures were performed under institutional animal care guidelines. Twenty-seven rats were trained in standard operant chambers to discriminate (-)-2,5-dimethoxy-4-methylamphetamine (DOM) 0.5 mg/kg, i.p., from saline using a two-lever choice procedure. The pretreatment interval for the training drug was 30 minutes. Reinforcement consisted of 45 mg food pellets delivered after 10 consecutive responses on the drug-appropriate lever (fixed ratio 10); training sessions lasted up to 10 minutes with a maximum of 20 pellets. Rats reached criteria for testing after approximately 60 training sessions and when they obtained 9 of 10 sessions at ≥85% injection-appropriate responding for both the first reinforcer and total session. Substitution tests examined eight test compounds: 4-AcO-DET, 4-OH-MET, 4-OH-DET, 4-AcO-MiPT, 5-MeO-MiPT, 4-AcO-DMT, 4-AcO-DiPT, and 4-OH-DiPT. Group sizes varied by compound (ranging from n = 6 to n = 9). Vehicles and pretreatment times differed by compound (e.g. saline or deionized water; pretreatment 15–60 min as specified). During substitution tests both levers were active and rats could earn reinforcers from either lever; test sessions lasted up to 20 minutes or until 20 reinforcers were obtained. A within-subjects, repeated-measures design was used so each rat received all doses of the assigned test drug, including vehicle and training-drug controls. Drugs were supplied by commercial sources or NIDA; optically active test compounds were tested as racemates. Compounds were dissolved in either deionized water or 0.9% saline and administered intraperitoneally in a volume of 1 mL/kg. Primary outcome measures were percent drug-appropriate responding and response rate (responses per second). Full substitution was defined as ≥80% drug-appropriate responding and not statistically different from the training drug. Response-rate data were analysed by one-way repeated-measures ANOVA with planned contrasts versus vehicle, potencies (ED50 with SEM) were estimated by fitting straight lines to the linear portion of dose-response data, and a two-way ANOVA was conducted on ED50 values.

All eight substituted tryptamines tested produced full substitution for DOM in DOM-trained rats, defined as ≥80% drug-appropriate responding, indicating DOM-like discriminative stimulus effects. The potencies of the test compounds were reported as less than or equal to that of DOM, with an approximately 10-fold range in potency across the compounds. Specific rank ordering of all compounds was not clearly reported in the extracted text, but comparisons between congeners were presented. Effects on response rate varied by compound. No change in response rate was observed following any dose of 5-MeO-MiPT, 4-AcO-MiPT, or 4-OH-DiPT. 4-AcO-DET and 4-AcO-DiPT did not produce statistically significant decreases in response rate, although two rats failed to earn a food pellet at the highest dose tested for each of those compounds. Several 4-hydroxy and some 4-acetoxy compounds decreased response rate at doses that produced full substitution: 4-OH-MET reduced response rate [F(5,40) = 8.741, p < 0.001], with 5 of 9 rats failing to earn a pellet at 2.5 mg/kg; 4-OH-DET reduced rate [F(5,35) = 12.699, p < 0.001], with 3 of 8 rats failing to earn a pellet at 3.2 mg/kg; 4-OH-DMT (psilocin) decreased rate at 1 and 3.2 mg/kg [F(5,35) = 12.825, p < 0.001], with 2 of 8 rats failing at 3.2 mg/kg; and 4-AcO-DMT decreased rate [F(4,32) = 12.347, p < 0.001], with 2 of 9 rats failing to earn a pellet at 2.5 mg/kg. Comparative potency findings included that 4-AcO-DET was approximately 10-fold less potent than its 4-hydroxy analogue 4-OH-DET, and that 4-AcO-DMT and 4-AcO-DiPT were about 2-fold less potent than their respective 4-hydroxy congeners. For the N-methyl-N-isopropyltryptamine series, a 5-MeO substitution did not differ significantly in potency compared with a 4-AcO substitution. The extracted text states that none of the compounds produced adverse effects (such as paralysis, tremors, convulsions, or lethality) at the doses tested, unlike some other compounds reported in earlier work.

Gatch and colleagues interpret the full substitution of the eight test tryptamines for DOM as evidence that these compounds produce hallucinogen-like subjective effects in rats, consistent with action at serotonin receptors known to mediate hallucinogenic effects. The authors note that several of the tested compounds have documented activity at 5-HT2A and 5-HT1A receptors, receptor actions commonly implicated in the subjective effects of hallucinogens, and that cross-substitution between classic hallucinogens is well established. Direct administration of 4-OH-DMT (psilocin) fully substituted for DOM, which the researchers present as supporting evidence that psilocin is the behaviourally active metabolite of psilocybin. The pattern of relative potencies led the authors to conclude that 4-acetoxy substituted compounds were generally less potent than the corresponding 4-hydroxy congeners, and that N,N-diisopropyl substitution conferred lower potency compared with dimethyl, diethyl, or mixed N-substituents. In terms of implications, the investigators suggest that because the test compounds produced DOM-like discriminative stimulus effects, they may have similar subjective effects in humans and thereby similar potential for recreational use or abuse liability. They also emphasise the usefulness of drug discrimination as a preclinical tool for detecting hallucinogen-like effects, given the lack of reliable self-administration models for these drugs. The authors report no acute adverse effects at the doses tested and contrast this with previous studies in which other compounds produced severe toxic effects. Limitations and uncertainties are noted implicitly: behavioural substitution in animals is a model for subjective effects and is not a direct measure of human abuse or clinical effects, and some potency details rely on comparisons for which full data (e.g. a complete rank order) were not clearly reported in the extracted text.