Effects of mescaline and psilocin on acquisition, consolidation, and performance of light-dark discrimination in two inbred strains of mice

How to account for hallucinations in the interpretation of the antidepressant effects of psychedelics: a translational framework

RATIONALE

Recent trials with psychedelics in major depressive disorder and treatment-resistant depression showed remarkable improvements in depressive symptoms that can last for up to several months after even a single administration. The lack of an appropriate placebo control group-as patients are often able to discriminate the subjective effects of the drug-and an incomplete understanding of the role of the hallucinogenic and mystical experience, hampers the interpretation of these therapeutic effects.

OBJECTIVES

To control for these factors, we developed a translational framework based on establishing pharmacokinetic/pharmacodynamic (PK/PD) relationships in rodents and humans for hallucinogenic (i.e., discriminative stimulus effects in rodents and humans; head twitch responses in rodents; questionnaires in humans) and therapeutic effects. For the latter, we selected the pattern separation and attentional set-shifting tasks as measures for cognitive flexibility because of their high translational value. We predict that these PK/PD analyses will lead to a more objective evaluation of improvements in patients compared to relying only on the currently used self-reported questionnaires. We hypothesize that-if the role of the hallucinogenic experience is not central in the antidepressant effects of psychedelics-the ED₅₀'s for the therapeutic effects will be significantly lower than for the hallucinogenic and mystical effects.

CONCLUSION

Our framework will help to inform future studies that aim at the elucidation of the mechanism(s) of action of psychedelics in depression, and the role of the acute subjective and/or hallucinogenic experience in their effects.

The effect of psilocin on memory acquisition, retrieval, and consolidation in the rat

The involvement of the serotonin system in the pathophysiology of schizophrenia has been elucidated by experiments with hallucinogens. Application of a hallucinogen to humans leads to changes in perception, cognition, emotions, and induction of psychotic-like symptoms that resemble symptoms of schizophrenia. In rodent studies, their acute administration affects sensorimotor gating, locomotor activity, social behavior, and cognition including working memory, the phenotypes are considered as an animal model of schizophrenia. The complexity and singularity of human cognition raises questions about the validity of animal models utilizing agonists of 5-HT_2A receptors. The present study thus investigated the effect of psilocin on memory acquisition, reinforced retrieval, and memory consolidation in rats. Psilocin is a main metabolite of psilocybin acting as an agonist at 5-HT_2A receptors with a contribution of 5-HT_2C and 5-HT_1A receptors. First, we tested the effect of psilocin on the acquisition of a Carousel maze, a spatial task requiring navigation using distal cues, attention, and cognitive coordination. Psilocin significantly impaired the acquisition of the Carousel maze at both doses (1 and 4 mg/kg). The higher dose of psilocin blocked the learning processes even in an additional session when the rats received only saline. Next, we examined the effect of psilocin on reinforced retrieval and consolidation in the Morris water maze (MWM). The dose of 4 mg/kg disrupted reinforced retrieval in the MWM. However, the application of a lower dose was without any significant effect. Finally, neither the low nor high dose of psilocin injected post-training caused a deficit in memory consolidation in the MWM. Taken together, the psilocin dose dependently impaired the acquisition of the Carousel maze and reinforced retrieval in MWM; however, it had no effect on memory consolidation.

Strain-dependent effects of naloxone on discrimination learning in mice

The effects of post-trial naloxone administration were studied in two inbred strains of mice. C57BL/6 (C57) and DBA/2 (DBA), tested in two different experimental conditions. These were a Y-water maze (requiring escape from water) and the Yerkes--Thompson--Bovet-Nitti apparatus for pattern discrimination (requiring escape from electric shock). Two series of experiments were carried out. In the first series, naloxone improved acquisition in DBA and impaired acquisition in C57 mice either trained to swim towards tha dark or to discriminate between oblique bars. Light-oriented behavior, perhaps due to non-conditioned tendencies, was improved in the Y-water maze following naloxone administration on both strains.

Strain dependent effects of ketamine on locomotor activity and antinociception in mice

The effects of ketamine (12.5, 25 and 50 mg/kg) on locomotor activity and response to nociceptive stimuli were investigated in the inbred strains of mice: BALB/c (BALB), C57BL/6 (C57) and DBA/2 (DBA). In the BALB and in the C57 mice ketamine exerted activity stimulating effects, which were already present at doses lower than those inducing antinociception. Locomotor depressent effects were evident in the DBA mice following the administration of doses higher than those necessary to induce analgesia. It is suggested that: (1) ketamine affects locomotor activity and response to painful stimuli through different mechanisms, (2) the brain regional and biochemical differences reported for the strains considered may account for their different responses to ketamine administtation.

Effects of LSD-25 on avoidance behavior and locomotor activity in mice

C57BL/6(C57) mice were tested in a shuttle box under the influence of LSD-25 in two sets of experiments. In the first set, the pretrial administrations of LSD (0.05, 0.1, and 0.2 mg/kg) were followed by dose-related performance improvements. In the second set, the posttrial administration of LSD (0.2 mg/kg) immediately after each experimental session improved the performances of the mice. No effect was evident when the hallucinogen was injected 2 h after training, thus showing that LSD improved the memory processes of the C57 mice in these experimental conditions. Dose-related activity depressant effects were evident in a third set of experiments, carried out in a toggle-floor box.

Effects of mescaline and amphetamine on simultaneous visual discrimination in two inbred strains of mice

The effects of mescaline and amphetamine were investigated in BALB/cJ (BALB) and C57BL/6J (C57) mice using the five-choice Yerkes--Thompson Bryant--Bovet-Nitti apparatus for patterns discrimination. Two sets of experiments were carried out. In the first set, pretrial administration of mescaline (5, 10, and 20 mg/kg) was followed by performance improvements in the C57 mice, while performances of the BALB strain were impaired by the treatment, as compared with those of the saline-injected (4 ml/kg) controls. The pretrial administration of amphetamine (0.1, 0.25, and 0.5 mg/kg) improved performances of both strains. In a second set of experiments, the same effects as in the pretrial experiments were observed in both strains following administration of mescaline (20 mg/kg) and amphetamine (0.5 mg/kg) immediately after each experimental session. No effect was evident when the drugs were injected 2 h after training, suggesting that effects of the pretrial treatments were due to influences of mescaline and amphetamine on the consolidation processes of the two strains of mice tested.