Psilocybin (O-phosphoryl-4-hydroxy-N,N-dimethyltryptamine) and its active metabolite psilocin (4-hydroxy-N,N-dimethyltrypt amine) are the main psychoactive indolealkylamines contained in hallucinogenic mushrooms (Hasler et al., 1997; Passie et al., 2002; Tyls et al., 2013). They act as agonists at serotonin receptors, mainly 5-HT_2A/C and 5-HT_1A subtypes (for a review, see Tyls et al., 2013). Recently, affinities to other than serotonergic receptors have also been reported (e.g., D1 and D3 receptors); however, their role in the neurobiology of psilocin’s and psilocybin’s action is disputable (Ray, 2010). Nevertheless, most of the evidence agrees that hallucinogenic effects are mediated mainly via agonism at postsynaptic 5-HT_2A receptors with a contribution of 5-HT_2C and 5-HT_1A receptors (Nichols, 2004). A secondary role of the activated limbic dopaminergic system also seems to be responsible for some of its effects (Vollenweider et al., 1999).

The acute effects of psilocybin in humans are characterized by changes in perception, cognition, emotions, and induction of psychotic-like symptoms that resemble early stages of schizophrenia (Vollenweider et al., 1998). Psilocybin also attenuates neurocognitive parameters in humans (e.g., disrupted sustained attention and altered visual information processing) and disrupts sensorimotor processing (Tyls et al., 2013), effects that are also typically disrupted in psychotic patients (Park and Holzman, 1992; Vollenweider et al., 1997, 1998; Geyer, 1998). Due to this phenomenological resemblance of acute intoxication with psychosis and its long history of safe clinical use (including psychiatric treatment) it is, nowadays, the most frequently used serotonergic model of psychosis in humans (Tyls et al., 2013). Furthermore, the therapeutic potential of psilocin/psilocybin has been recently investigated in several psychiatric disorders such as obsessive–compulsive disorder, anxiety, and addiction (Moreno et al., 2006; Grob et al., 2011; Bogenschutz and Pommy, 2012; Stebelska, 2013; dos Santos, 2014). Despite the evidence of how psilocybin changes cognitive performance in humans, little is known about its effects on cognition in animals.

To further investigate the effect of psilocin on cognition in animals we decided to test spatial memory using two spatial paradigms: the Morris water maze (MWM) and the Carousel maze. Spatial navigation as a behavioral manifestation of knowledge of the environment (i.e., cognitive map; Tolman, 1948) is considered as a useful and experimentally well-accessible animal analogy of declarative memory (O’Keefe and Nadel, 1978; Eichenbaum, 2001; Morris, 2013). Testing of spatial navigation requires sensitive spatial paradigms, such as the MWM (Morris, 1981; Morris et al., 1982; Stuchlik et al., 2007). This test can be used in many configurations, allowing testing of reference and working memory acquisition, consolidation, and retrieval. The water maze typically requires so-called allocentric spatial memory, i.e., guidance of navigation by spatial relationships of the distal visual cues located in the experimental room. Another powerful and relatively novel spatial paradigm is active place avoidance in a Carousel maze, previously also referred to as active allothetic place avoidance (AAPA) (Cimadevilla et al., 2000, 2001; Stuchlik et al., 2004; Wesierska et al., 2005). In this test, animals walking on a slowly rotating arena are required to avoid a sector located in a stable position of the room. It was demonstrated that rats avoiding places remember the arena- and room-based frames of reference (Fenton et al., 1998), which are continuously dissociated by the rotation of the arena. In its active place avoidance version, animals must, besides allocentric mapping, segregate spatial information into coherent representations of the room and arena frames and to select the room frame as the only one relevant for navigation. This ability was shown to strictly require hippocampus and termed cognitive coordination (Wesierska et al., 2005). Importantly, this task was demonstrated to require the presence of visual extra-arena information in the room frame (Fajnerova et al., 2014) as well as vestibular inertial stimulation caused by continuous arena rotation, which likely enhances the attention of the rats to distal visual cues (Blahna et al., 2011).

The aim of the present study is to test the hypothesis that psilocin attenuates acquisition of the Carousel maze (demanding mainly cognitive coordination) and reinforced retrieval and memory consolidation in the MWM (demanding allocentric memory). The effect of psilocin on the locomotor activity was assessed in the Carousel maze in order to dissociate effects upon cognition and/or motor and motivational functions.

Our findings indicate that psilocin affects learning and memory acquisition necessary for spatial navigation and that it also has a disruptive effect on memory retrieval in MWM. Previous reports on the effects of psilocin/psilocybin on cognition in animals are very sparse, use different paradigms and did not bring uniform results, which make them difficult to compare. One study in rats reported impaired acquisition of a conditioned avoidance response (Sugrue, 1969) while another one in mice did not find any effect on the acquisition of an escape avoidance conditioning task (Collins et al., 1966). However, both studies were conducted with very high doses of psilocin/psilocybin. Furthermore, opposing effects of low psilocin doses on memory consolidation were found in two different mice strains when psilocin was injected immediately after each training session in a light–dark discrimination task (Castellano, 1978). On the other hand, most of the studies with other hallucinogens congruently found a disruption of cognitive performance. Similar findings to ours were shown for 2,5-dimethoxy-4-iodoamphetamine (DOI) in a special water maze, where this compound worsened the performance during acute intoxication as well as when subsequently tested without the influence of the drug (Kant et al., 1998). Early studies with LSD and mescaline congruently found disruption of spatial navigation, exploration, and discrimination within different frameworks in T-maze/Y-maze (Castellano, 1971; Davies and Redfern, 1973; Molinengo et al., 1986; Koupilova et al., 1989, 1999). LSD and other hallucinogens also induced errors in swimming through an underwater maze and increased starting latency of swimming through the maze (Uyeno, 1969, 1986). Recently, the use of a translational touch-screen based approach also showed alterations in visuo-spatial learning after acute LSD treatment in rats (Talpos et al., 2014). Finally, chronic mescaline treatment completely blocked the ability to switch between learned trials during operant conditioning (food reinforcement) (Fundaro et al., 1986).

There are two main aspects that have a crucial role in psilocin induced changes. Firstly, psilocybin, like other hallucinogens, had an inhibitory effect on locomotor and exploratory activity with the higher dose used (Collins et al., 1966; Halberstadt et al., 2011; Palenicek et al., 2012, 2013). However, we believe that the main effect of locomotor inhibition can be excluded, since the lower psilocin dose in the Carousel maze did not significantly differ from the controls during any of the sessions even though it induced the deficit in this task. Furthermore, the higher dose during session 4, when the animals showed peak performance, did not induce any significant locomotor inhibition. The psilocin-treated animals also showed a trend toward an increase in their locomotion within every session similarly to the control group, suggesting either developing tolerance to the drug or task sensitization. On the other hand, such an increase might also be attributable to the gradual acquisition of an avoidance strategy, which requires a highly coordinated spatiotemporal locomotion pattern in order to avoid a sector.

Secondly, since active avoidance in the Carousel maze as well as the MWM are spatial tasks requiring extra-maze cue orientation for successful acquisition, altered spatiotemporal perception (hallucinatory effects) can definitely contribute/underlie these changes. Unlike human studies on psilocybin (for a review, see Tyls et al., 2013); unfortunately, we are unable to directly confirm hallucinatory effects in rats in our setting. However, previous studies in rodents found an attenuation of time perception after hallucinogens (Hanks and González-Maeso, 2013) and studies in primates also support hallucinogenic effects in these species (Uyeno, 1969; Fantegrossi et al., 2004). Therefore, we might expect temporal and visual perceptual alterations to be present also in our setting. Indeed, it is well known that psilocin induces attention distraction (Hasler et al., 2004; Carter et al., 2005), which can also underlie the learning deficits. Interestingly, learning in the Carousal maze was also disrupted in the after-session, with no exposition to psilocin, indicating that psilocin blocked the learning process until the last session. Also, the higher dose of psilocin disrupted memory retrieval in the MWM, an effect that persisted for another 5 days. In other words, the rats that received psilocin during the retrieval trial did not improve their performance during this task, contrary to the control group, suggesting that not only retrieval but also re-acquisition was corrupted. This supports the idea that psilocin also had amnestic effects.

It is well known from the literature that psilocin effects are mainly mediated through 5-HT_2A/C and 5-HT_1A receptors (Tyls et al., 2013). These receptors are widely distributed in the neocortex, basal ganglia, limbic system, and hippocampus and in the case of 5-HT_1A also in rapheal nuclei (Barnes and Sharp, 1999). All of these areas are known to be involved in cognitive processes and memory. It is well known that 5-HT_2A/C receptors in particular are rapidly downregulated/desensitized after stimulation (Roth et al., 1990, 1995, 1998). This can explain the loss of the locomotor inhibitory effect observed in the Carousel maze. Since systemic administration of 5-HT_2A receptor antagonists induces cognitive deficit in rats (Ma and Yu, 1993; Fedotova and Ordyan, 2010) the down-regulation/desensitization of 5-HT_2A/C receptors after repeated administration of psilocin might also yield some of the effects observed in our setting. It is of interest that stimulation of 5-HT_1A and 5-HT_1B receptors is also known to impair acquisition of spatial memory tasks (Buhot et al., 1995; Herremans et al., 1995; Koenig et al., 2008). Nevertheless, further experiments are needed to explain the receptor mechanisms underlying these changes.

In conclusion, our study demonstrated that 5-HT_2A/C and 5-HT_1A agonist psilocin impaired spatial learning in the Carousel maze and it also had an amnestic effect in the MWM. On the other hand, it had no effect on memory consolidation.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.