Behavioral and pharmacokinetic interactions between monoamine oxidase inhibitors and the hallucinogen 5-methoxy-NN-dimethyltryptamine

Earlier research shows that tryptamine hallucinogens are frequently consumed together with monoamine oxidase inhibitors (MAOIs) in preparations such as ayahuasca and in recreational use, but the consequences of combined use are incompletely understood. Hallucinogens classically exert their effects via 5-HT2A receptor activation, yet indoleamines such as 5-MeO-DMT also interact with other serotonin receptor subtypes, notably 5-HT1A. Prior studies from this laboratory demonstrated that MAO-A inhibitors change the locomotor effects of 5-MeO-DMT in rats and enhance its engagement with 5-HT2A receptors; one mechanistic hypothesis is that MAO-A inhibition alters 5-MeO-DMT pharmacokinetics and/or increases its conversion to bufotenine, a potent 5-HT2A agonist. This study set out to clarify the mechanism underlying interactions between 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) and MAOIs. Specifically, Halberstadt examined whether MAOI pretreatment alters the relative contributions of 5-HT1A and 5-HT2A receptors to 5-MeO-DMT-induced disruption of prepulse inhibition (PPI) and whether MAO-A inhibition changes the pharmacokinetics of 5-MeO-DMT and its O-demethylation to bufotenine in rats. The work combined behavioural pharmacology (acoustic startle/PPI and locomotor monitoring) with quantitative LC-ESI-SRM-MS/MS measurement of drug levels in plasma and whole brain.

Adult male Sprague-Dawley rats (initial weight 250–275 g) were used. Animals were pair-housed on a reverse light–dark cycle and acclimatised for ~1 week; all procedures conformed to institutional animal care guidelines. Drugs were administered subcutaneously in a volume of 1 mL/kg. Doses reported in the paper include 5-MeO-DMT (freebase) at 0, 0.1, or 1.0 mg/kg, the non-selective MAOI pargyline at 0 or 10 mg/kg, the MAO-A inhibitor clorgyline at 0 or 0.3 mg/kg, the 5-HT1A antagonist WAY-100,635 at 1.0 mg/kg, and the 5-HT2A antagonist MDL 11,939 at 0.3 mg/kg. MDL 11,939 was prepared in acidified saline with Tween 80; other drugs were dissolved in isotonic saline. Antagonists were given 20 min prior to 5-MeO-DMT in antagonist experiments, and MAOI pretreatments were given 20 min before 5-MeO-DMT. Behavioural assessments comprised two main assays. Acoustic startle and PPI were measured in SR-LAB startle chambers using standard PULSE-ALONE (40-ms 120-dB white noise) and PREPULSE + PULSE trials with prepulses at 68, 71, and 77 dB (3, 6, and 12 dB above 65-dB background). Rats were tested 45 min after 5-MeO-DMT administration to target the time window when MAOI + 5-MeO-DMT produce delayed locomotor hyperactivity (approx. 40–70 min post-injection). For locomotor/pharmacokinetic correlation, activity was recorded in a Behavioural Pattern Monitor (BPM) and animals were removed at specified time points (10–60 min post-dose) for collection of trunk blood and whole brain for drug analysis. Experimental group sizes were provided: experiment 1 (pargyline interaction) n = 10/group (60 total), experiment 2 (clorgyline) n = 12/group (48 total), and experiment 3 (antagonist blockade) n = 10/group (60 total) with pargyline co-administration for antagonist tests. For pharmacokinetic and metabolite analysis, plasma and whole-brain homogenates were prepared and extracted using solid phase extraction. Quantification was performed by LC-ESI-SRM-MS/MS (Agilent 1100 LC coupled to Thermo TSQ Quantum Access triple quadrupole) with N-methylserotonin as internal standard. Monitored transitions were 5-MeO-DMT 219 → 174 and bufotenine 205 → 166. Calibration, recovery, accuracy, and precision metrics were reported and used to derive concentrations; Helsel's Robust Method was used to handle values below the limit of detection in some groups. Pharmacokinetic metrics included Cmax and AUC20→70 min, and statistical comparisons used ANOVA for continuous outcomes and nonparametric Tarone–Ware tests for equivalence of censored analyte levels where appropriate. Behavioural data were analysed primarily with two- or three-factor ANOVA (factors: pretreatment, treatment, and prepulse intensity as a repeated measure), with post hoc Tukey tests or Bonferroni corrections as indicated.

PPI and startle: When 5-MeO-DMT (1 mg/kg) was administered 45 min before testing, it did not alter PPI in vehicle-pretreated rats. However, rats pretreated with the non-selective MAOI pargyline (10 mg/kg) showed a significant reduction in PPI after 5-MeO-DMT, yielding a pretreatment × treatment interaction (F(2,54) = 17.59, p < 0.0001) and a main effect of 5-MeO-DMT (F(2,54) = 18.87, p < 0.0001). Pairwise tests indicated that 1 mg/kg 5-MeO-DMT significantly reduced PPI at all three prepulse intensities in pargyline-pretreated animals (p < 0.01). Pargyline alone reduced the startle amplitude (significant interaction between pargyline and 5-MeO-DMT on startle: F(2,54) = 5.07, p < 0.01), but subgroup analyses matched for startle magnitude demonstrated that the PPI disruption produced by pargyline + 5-MeO-DMT persisted independently of startle changes (pretreatment × treatment: F(2,36) = 10.44, p = 0.0003). A similar pattern was observed with selective MAO-A inhibition: clorgyline (0.3 mg/kg) did not alter PPI by itself, but clorgyline-pretreated rats exhibited significant PPI reduction after 1 mg/kg 5-MeO-DMT (pretreatment × treatment: F(1,44) = 24.74, p < 0.0001), with 5-MeO-DMT lowering PPI at all three prepulse intensities (p < 0.01). Startle magnitude was not significantly affected by clorgyline or 5-MeO-DMT in this experiment. Receptor antagonism: Antagonist studies tested whether 5-HT1A and 5-HT2A receptors mediated the PPI disruption produced by pargyline + 5-MeO-DMT. Pretreatment with the 5-HT1A antagonist WAY-100,635 (1 mg/kg) significantly antagonised the PPI reduction (pretreatment × treatment: F(1,36) = 73.11, p < 0.0001; post hoc p < 0.01). Similarly, the 5-HT2A antagonist MDL 11,939 (0.3 mg/kg) substantially attenuated the PPI disruption (pretreatment × treatment: F(1,36) = 23.96, p < 0.0001; post hoc p < 0.01). Startle magnitude was not meaningfully altered by MDL 11,939 and only trends were noted with WAY-100,635. Pharmacokinetics and bufotenine: Method validation indicated acceptable recovery (62.7–88.0%), reproducible calibration (mean r2 ≈ 0.99), and accuracy/precision within acceptable bounds for both analytes. After 1 mg/kg 5-MeO-DMT, plasma and brain concentrations peaked within 20 min and then declined. Clorgyline did not significantly increase plasma Cmax (vehicle 52.7 ± 11.8 ng/mL vs clorgyline 78.1 ± 9.5 ng/mL), but clorgyline significantly slowed plasma clearance, producing elevated plasma concentrations from 30–70 min and a marked increase in plasma AUC20→70 min (vehicle 12.2 ± 1.7 ng•h/mL vs clorgyline 31.4 ± 2.3 ng•h/mL; F(1,59) = 45.6, p < 0.0001). The central exposure change was striking: whole-brain Cmax increased from 30.3 ± 11.2 ng/g (vehicle) to 633.8 ± 146.5 ng/g with clorgyline (F(1,9) = 16.8, p < 0.004), and 5-MeO-DMT remained detectable for longer in brain tissue when MAO-A was inhibited. By contrast, bufotenine concentrations in plasma and brain were mostly below the limit of detection (plasma LLOQ 1–2 ng/mL; brain LLOQ 10 ng/g), and clorgyline did not produce a clear increase in bufotenine levels (plasma Cmax ~1.3 ng/mL vehicle vs 1.8 ng/mL clorgyline). Locomotor correlation: BPM monitoring confirmed the expected behavioural interaction: clorgyline pretreatment produced a significant interaction with time block on locomotor crossings (F(5,48) = 3.64, p < 0.008) and increased crossings in later time blocks compared with vehicle, consistent with delayed hyperactivity coincident with elevated central 5-MeO-DMT.

Halberstadt and colleagues interpret these results as showing that MAOI pretreatment markedly prolongs and alters the behavioural profile of 5-MeO-DMT by increasing its systemic and, especially, central accumulation. Whereas 5-MeO-DMT alone produces brief PPI and locomotor effects primarily mediated by 5-HT1A receptors, MAO-A inhibition allowed 5-MeO-DMT to disrupt PPI at 45 min post-dose and to recruit 5-HT2A receptor mechanisms; this was supported by antagonist blockade with both WAY-100,635 and MDL 11,939. Pharmacokinetic data support a mechanism of inhibited deamination: clorgyline greatly increased brain 5-MeO-DMT concentrations (whole-brain Cmax rising into the low micromolar range) and prolonged its presence in the central nervous system, while conversion to bufotenine in rats was negligible and therefore unlikely to account for the altered behavioural profile. The authors place their findings in the context of prior animal studies reporting MAOI-driven increases in blood and brain 5-MeO-DMT and note that a deuterated 5-MeO-DMT isotopologue resistant to MAO-A metabolism produces behavioural effects similar to 5-MeO-DMT combined with an MAOI, supporting a pharmacokinetic explanation. They further propose that the delayed appearance of 5-HT2A-mediated behaviours could involve downstream interactions with dopaminergic systems, since 5-HT2A activation can facilitate phasic dopamine release and produce delayed dopaminergic sensitisation; future studies are proposed to test this possibility. Regarding translational relevance, the discussion highlights toxicological concerns because MAOIs and tryptamine hallucinogens are used together in humans (for example in ayahuasca preparations). The authors point out case reports of severe toxicity and death following combined use, and suggest that MAOI-driven increases in central concentrations of 5-MeO-DMT could underlie such adverse outcomes. They also caution that while these findings for 5-MeO-DMT are likely relevant to other tryptamines that are MAO substrates (for example DMT), species differences exist: mice have shown increased O-demethylation to bufotenine in some studies, whereas this rat study found negligible bufotenine formation. The authors therefore call for further research to characterise the influence of MAOIs on DMT pharmacokinetics and on the mechanisms of action of ayahuasca. Limitations and uncertainties acknowledged in the text include species differences in metabolism (rats vs mice), the low detectability of bufotenine in rat samples which limits conclusions about its role, and the need for additional work to establish whether dopaminergic mechanisms contribute to the delayed 5-HT2A-mediated behavioural responses. The authors refrain from extending causal claims beyond the data, instead framing implications in terms of probable mechanistic pathways and public health relevance given contemporary patterns of polypharmacy with MAOIs and tryptamines.