This study in rats with DMT found that it reduced anxiety by extinguishing of cued fear memory and reduces immobility in the forced swim test (a proxy for depression).

Abstract

“Depression and anxiety disorders are debilitating diseases resulting in substantial economic costs to society. Traditional antidepressants often take weeks to months to positively affect mood and are ineffective for about 30% of the population. Alternatives, such as ketamine, a dissociative anesthetic capable of producing hallucinations, and the psychoactive tisane ayahuasca, have shown great promise due to their fast-acting nature and effectiveness in treatment-resistant populations. Here, we investigate the effects of N,N-dimethyltryptamine (DMT), the principle hallucinogenic component of ayahuasca, in rodent behavioral assays relevant to anxiety and depression using adult, male, Sprague–Dawley rats. We find that while DMT elicits initial anxiogenic responses in several of these paradigms, its long-lasting effects tend to reduce anxiety by facilitating the extinction of cued fear memory. Furthermore, DMT reduces immobility in the forced swim test, which is a characteristic behavioral response induced by many antidepressants. Our results demonstrate that DMT produces antidepressant and anxiolytic behavioral effects in rodents, warranting further investigation of ayahuasca and classical psychedelics as treatments for depression and post-traumatic stress disorder.”

Authors: Lindsay P. Cameron, Charlie J. Benson, Lee E. Dunlap & David E. Olson

Summary

We investigated the effects of DMT, the principal hallucinogenic component of ayahuasca, on anxiety and depression in adult male Sprague-Dawley rats. We found that DMT reduces anxiety by facilitating the extinction of cued fear memory and immobility in the forced swim test.

Introduction

Mood and anxiety disorders are among the leading causes of disability worldwide, yet we still lack effective treatments. Ketamine is an archetype for a new generation of neurotherapeutics that produce rapid clinical antidepressant effects.

Psilocybin and LSD are promising compounds that can elicit beneficial clinical effects similar to ketamine. Ayahuasca, a traditional botanical mixture, has also attracted considerable attention owing to its potent antidepressant and anxiolytic effects.

Ayahuasca is an Amazonian tisane that can be prepared by boiling the Banisteriopsis caapi vine and the leaves of the shrub Psychotria viridis. It has been shown to reduce various measures of psychopathology and improve cognitive performance on tests such as the Stroop and Wisconsin Card Sorting Tests.

Ayahuasca is a complex mixture of psychoactive chemicals including tryptamine and -carboline alkaloids. Harmine and harmaline have been shown to have different effects on rodent behaviors, including decreased immobility, anxiolytic effects, and exploratory behavior.

Ayahuasca contains a hallucinogenic component called DMT. We investigated the effects of DMT in animal models of anxiety, PTSD, and depression.

Results and Discussion

We chose a 10 mg/kg dose of DMT for all of our studies because it was expected to produce hallucinogenic effects based on human studies.

Rats treated with DMT displayed flat body posture and hind limb abduction in their home cages, but after 30 minutes began to engage in normal behaviors, and by 1 hour post-administration were qualitatively indistinguishable from vehicle-treated control animals.

Animals treated with DMT displayed reduced exploratory behavior and traveled a significantly shorter total distance than vehicle-treated controls, but did not affect thigmotaxis. Furthermore, the drug reduced the time spent engaging in stereotypies but not the total number.

We next assessed the effects of DMT on EPM behavior. The results indicate that DMT administration does not grossly impair locomotion 1 h after dosing, but does have anxiogenic affects.

We find that a single hallucinogenic dose of DMT administered to rats decreases exploratory behavior and promotes acute anxiety, and that this effect is consistent with the effects of ayahuasca reported by Pic-Taylor and coworkers.

Ayahuasca does not impair spatial memory, nor does it impact cued fear memory, but it does enhance both foreground and background contextual fear memory. Therefore, we tested the effects of DMT on fear memory in rats.

There are several possible explanations for the differing results between our study and that of Oliveira and co-workers.

DMT did not significantly impact fear conditioning, but did promote cued fear extinction. The DMT-treated group had a stronger extinction memory than the control group.

Patients with PTSD exhibit deficits in cued extinction recall, and DMT promotes fear extinction in rats. This finding is consistent with previous reports demonstrating that psilocybin and MDMA promote cued fear extinction in mice.

In a second experiment, animals were subjected to foreground contextual conditioning and then subjected to contextual extinction training. The DMT-treated animals were not statistically different from the vehicle control group.

Ayahuasca has been found to have antidepressant properties in rodents. The effects of DMT on the forced swim test were indistinguishable from those of ketamine.

Ayahuasca is known to produce anxiogenic effects in the open field and in the EPM, but it also displays robust antidepressant properties in the forced swim test. DMT and harmine both decrease immobility and increase swimming behavior in this behavioral paradigm.

The results presented here suggest that DMT has similar effects to ketamine, a fast-acting antidepressant, including anxiogenic effects, facilitation of fear extinction learning, and reduced immobility in the fear susceptibility test.

Ketamine is believed to promote structural and functional plasticity in the prefrontal cortex, which is involved in extinction learning and the top-down control of mood. DMT might produce similar effects, but is rapidly metabolized, suggesting long-lasting changes contributing to its behavioral effects.

Researchers believe that DMT, a hallucinogenic compound, is produced by a variety of animals including humans. This suggests that DMT might play a natural role in the regulation of anxiety and mood.

We report the effects of DMT, a principal hallucinogenic component of ayahuasca, on rodent behaviors related to anxiety and depression. DMT produces behavioral responses in several paradigms relevant to anxiety and depression.

Sprague-Dawley rats were housed two per cage and given ad libitum access to food and water. Experiments were performed on 8- to 14-week-old male rats during the light-on phase.

DMT was synthesized in our laboratory using the following procedure: tryptamine, sodium cyanoborohydride, glacial acetic acid, 37% formaldehyde(aq), CH2Cl2, and 1M NaOH(aq). The resulting white solid was dried under reduced pressure to yield the pure compound as the fumarate salt (2:1 DMT:fumaric acid). DMT-fumaric acid has 1H NMR, 13C NMR, and IR values of 10.8, 7.5, 7.1, 6.9, 2.9, 2.8, 2.4, 59.2, 43.4, 21.9, and 749cm1. We used DMT-fumarate (2:1) in 0.9% sterile saline to elicit robust effects in the forced swim test72 (Brown and Lucki, 2013) without impairing motor function. Ketamine-HCl was administered at 10 mg/kg via intraperitoneal injection using an injection volume of 1 mL/kg.

Novelty-Induced Locomotion (NIL)

Rats were administered DMT or vehicle 1 hr before behavioral testing and allowed to freely explore an AccuScan Instruments open field chamber for 45 mins. Horizontal motion, rotations, and stereotypies were recorded in 1-min intervals and analyzed using the program Integra.

Elevated Plus Maze (EPM)

A plus-shaped black plastic platform was used to measure anxiety in rats. Rats were administered DMT or vehicle 1 hr before being placed into the center of the maze facing an open arm and allowed to explore freely for 5 min.

Fear Conditioning (FC)

On day 1, drug-nave animals were placed in a fear conditioning apparatus for 3.5 min prior to three presentations of auditory cues. The apparatus consisted of a 30.5 cm x 24.1 cm x 21 cm internal soundproof chamber. The animals were exposed to a novel context for 2 min prior to 8 presentations of auditory cues (80 dB white noise, 30 s) spaced 30 s apart. Freezing responses were scored using Med Associates Video Freeze software v2.25.

Cued Fear Extinction

On day 1, drug-nave animals were fear conditioned in the absence of drug, and on day 3, they were administered DMT or vehicle 1 hr prior to extinction training. Extinction training consisted of exposure to a novel context for 2 mins prior to 8 presentations of auditory cues.

Contextual Fear Extinction

Drug-nave animals were subjected to an optimal foreground contextual fear conditioning protocol consisting of six foot shocks spaced 58 s apart. The animals were then placed in the fear conditioning apparatus for 10 min and their freezing behavior was scored using Med Associates Video Freeze software.

Forced Swim Test

Rats were placed in a Plexiglas cylinder filled with water and their activity was video recorded. They were given three administrations of DMT, ketamine, or vehicle at 23.5, 6, and 1 hr before the test phase.

Data Analysis

Statistical analyses were performed using GraphPad Prism (version 7.0a), and comparisons between DMT- and vehicle-treated groups were accomplished using two-tailed Student’s t-tests or two-way ANOVAs with a Sidak Post-Hoc test.

Figure 2.

DMT (10 mg/kg) impacts exploratory behavior and anxiety in rats. There is no difference between the treatment groups with respect to the total distance moved (k) or average velocity (l) as well as the number of open arm entries (i) and the number of rearing (f) in the elevated plus maze.

Figure 3.

Acute dose of DMT (10 mg/kg) did not affect contextual or cued fear memory, but increased immediate freezing following foot shocks, but had no effect on contextual or cued fear memory.

Figure 4.

DMT (10 mg/kg) facilitates cued but not contextual fear extinction. The DMT-treated group demonstrated significantly lower freezing responses in the absence of drug on day 4 than the VEH-treated group. Animals were fear conditioned in context A, dosed prior to contextual extinction training, and tested for contextual fear memory in the absence of drug on day 6. Both treatment groups effectively extinguish contextual fear memories over time.

Figure 5.

Acute doses of DMT and ketamine elicit similar antidepressant responses in the forced swim test.