Psilocin and ketamine microdosing: effects of subchronic intermittent microdoses in the elevated plus-maze in male Wistar rats

This placebo-controlled animal study (n=40) investigated the effects of ketamine (0.5 – 3 mg/kg) and psilocin (0.05 – 0.075 mg/kg) microdosing on anxiety-related explorative behavior in rats and found that both substances caused mild anxiety as measured by a reduction of explorative behavior on an elevated open surface.

Abstract

“Introduction: Short-term moderate doses of serotonergic and dissociative hallucinogens can be useful in the treatment of anxiety. Recently, a trend has developed for long-term intermittent ‘microdosing’ (usually one-tenth of a ‘full’ active dose), with reports of long-lasting relief from anxiety and related disorders; however, there is no scientific evidence for the efficacy of therapeutic microdosing nor to show its lasting effects. The objective of this study was to test for lasting effects on anxiety in rats after microdosing with ketamine or psilocin.

Methods: Over 6 days, Wistar rats (N=40) were administered ketamine (0.5 or 3 mg/kg), psilocin (0.05 or 0.075 mg/kg), or saline on three occasions. A 5-min elevated plus-maze test was conducted 48 h after the final drug treatment (N=8). Dependent variables were entries (frequency), spent time (%), and distance traveled (cm) in each zone, as well as total frequency of rears, stretch-attend postures, and head dips. Statistical analyses of drug effects used separate independent one-way analysis of variance and pair-wise comparisons using independent t-tests.

Results: Statistical effects were modest or borderline and were most consistent with a mildly anxiogenic profile, which was significant at lower doses; however, this conclusion remains tentative. The lower doses of ketamine and psilocin produced comparable effects (to one another) across each variable, as did the higher doses.

Discussion: This pattern of effects may suggest a common (e.g. neurotransmitter/receptor) mechanism. We conclude that microdosing with hallucinogens for therapeutic purposes might be counter-productive; however, more research is needed to confirm our findings and to establish their translational relevance to clinical ‘psychedelic’ therapy.“

Authors: Rachel R. Horsley, Tomáš Páleníček, Jan Kolin & Karel Valeš

Summary

Introduction

Ketamine is a dissociative anesthetic with psychedelic effects at subanaesthetic doses, and has also substantial affinity for the serotonin transporter, as well as additional effects on dopamine and opioid neurotransmission. Psilocin and psilocybin act as competitive agonists at 5-HT receptors.

Although psychedelic hallucinogens and dissociative anaesthetics have distinct pharmacological properties, they exert some of their psychomimetic effects through a common pathway. This is why research has oriented to psychedelic hallucinogens and dissociative anaesthetics as deleterious to mental health.

Contemporary research suggests that psilocybin and ketamine can have therapeutic properties when administered at moderate psychomimetic doses. Two daily doses of intranasal ketamine (50 mg) produced minimal subjective psychedelic effects. Recent trends have developed for self-administering ‘microdoses’ of psychedelic drugs, such as psilocybin, lysergic acid, ketamine, and methoxetamine, on an intermittent regime, which is intended to be ‘sub-perceptual’, and to treat psychological ill-health, as well as enhancing creativity, spirituality, and social relationships.

Currently, there is no published scientific research on therapeutic microdosing with serotonergic or dissociative psychedelics. A preclinical model would allow for microdosing drug testing and development, as well as understanding the neural mechanisms that may be involved.

The present study tested whether subcutaneous ketamine and psilocin would have anxiolytic/genic effects when tested ‘drug-free’ on the elevated plus-maze.

Design

The ketamine and psilocin data were analyzed separately, with drug (low, high, or saline) as a between-subjects factor. The dependent variables were total arm entries and total distance traveled (cm) within the maze, frequency of entries and time (%) spent within each zone, and the distance traveled per visit to open and closed arms.

Subjects

Forty Wistar rats were caged in groups of four and were given EPM and saline treatments. The data were collected concurrently and the research received ethical approval.

Apparatus

Rats were placed in a maze with two open arms and two closed arms, which formed a plus (+) shape. Behavioral capture and pre-processing were performed using a Logitech HD Pro C920 web-camera.

Procedure

The EPM is sensitive to changes in anxiety and locomotion, as well as exploratory activity. Rats were placed on the central square of a dimly lit maze and allowed to move freely for 5 min.

Drugs and doses

Commercially available racemic ketamine solution was diluted in physiological saline to a volume of 0.5 or 3 mg/ml for subcutaneous injection at 0.5 or 3 mg/kg in a volume of 1 ml/kg. Psilocin was added to 200-l physiological saline and acidified with 10 l of glacial acetic acid (30%) and shaken vigorously until dissolved.

Psilocin is behaviorally effective at doses from 0.25 to 4 mg/kg in the open field, PPI, carrousel maze and Morris water maze. Ketamine is behaviorally effective at doses from 5 to 50 mg/kg in the open field, PPI, forced swim test, and the EPM.

Statistical analysis

Each drug was analyzed using a one-way analysis of variance. Significant main effects were explored using independent t-tests based on the individual error terms.

Ketamine

Drug treatment reduced the frequency of entries into the open arms by 0.5 mg/kg compared with saline, but did not affect total arm entries, closed arms, or center zone.

Ketamine reduced time in the open arms and center of the maze and increased time in the closed arms compared with saline, but these differences were not statistically significant.

Drug treatment affected the mean distance traveled per visit to the open arms, but not the mean distance traveled per visit to the closed arms, nor did it affect total distance traveled.

Ketamine had no effect on stretch-attend postures or head dips, but increased rears at the low dose. The high dose was not significantly different from saline.

Psilocin

Psilocin marginally reduced open arm entries, but the main effect was marginal. Psilocin did not significantly affect total arm entries, nor entries into the closed arms or central square.

Psilocin treatment reduced open arm time and increased closed arm time, and was significant for the 0.075 mg/kg dose, but marginal for the 0.05 mg/kg dose.

Psilocin had no effect on total distance traveled in the maze, mean distance traveled per visit to open or closed arms, or on rears.

Discussion

Subchronic intermittent microdosing with ketamine and psilocin appeared to produce a mildly anxiogenic behavioral profile, as shown on the classic EPM criterion: reduced open arm entries.

Ketamine

Ketamine at 0.5 mg/kg resulted in anxiogenesis, as indicated by the significant reduction in open arm entries. However, there was also evidence of increased distance traveled per visit to the open arms and increased rears, which might also reflect antidepressant action.

Ketamine showed no clear effect on the EPM, with only a marginal reduction in percentage time in the center zone. Ketamine is mildly anxiogenic or unclear in this test, with an increase in distance traveled per visit to the open arms.

Psilocin

Psilocin at 0.05 mg/kg reduced open arm entries and time spent in the open arms and increased time spent in the closed arms, suggesting anxiogenesis. However, psilocin did not affect head dips or stretch-attend postures.

Data for 0.075 mg/kg were consistent with anxiogenesis, although nonsignificant on the key measure of open arm entries.

Ketamine and psilocin

Ketamine and psilocin act by a final common pathway, and both result in similar pharmacological effects. 5-HT2A receptors may underlie the similarity of behavior effects observed across psilocin and ketamine.

Behavioral effects were observed for both compounds 48 h after the final drug administration, when acute pharmacological effects of the drugs had subsided. It would seem unlikely that biologically active metabolites of ketamine explain behavioral effects, as levels of hydroxynorketamines, norketamine, and dehydronorketamine had declined to zero by 480 min.

The elimination half-life of psilocin in rats is 117.3 40.3 min, but some behavioral effects can persist for days beyond the acute pharmacological phase. This suggests that events involving 5-HT synapses, in particular 5-HT2A receptors, may be involved.

Conclusion

Intermittent microdoses of ketamine or psilocin can affect anxiety, but the effects are modest. It would be useful to examine effects in other animal models of anxiety, as well as in models of depression.

Ketamine and psilocybin/psilocin can have psychoactive effects in humans that can damage double-blinding in clinical trials. Our intermittent microdosing paradigm limits these effects in preclinical studies and further reduces their effects by screening for effects drug free.