Marble-burying Test
Mice administered psilocybin buried 32.84% fewer marbles over 30 minutes than vehicle treated mice (p=0.001, Fig 1a). The effect of psilocybin was not statistically different from the effect exerted by a positive control, the SSRI escitalopram (48.43% reduction in marble-burying relative to vehicle; p<0.0001, Fig 1a). Using a two-way-ANOVA design, we examined the effects of acute treatment with psilocybin and pretreatment with the 5-HT2A antagonist, M100907 (volinanserin) (Fig 1b). A strong main effect of psilocybin was noted (F1,40=24.8, p<0.0001). We also observed a main effect of M100907 (F1,40=7.7, p=0.008, Fig. 1b). However, there was no interaction between psilocybin and M100907 and the post-hoc comparison of M100907 and vehicle was not significant (p>0.10) while the post-hoc comparison of M100907+psilocybin and vehicle was (p<0.0001). These findings indicate that the mechanism whereby psilocybin reduces marble-burying behavior is likely independent of 5-HT2A receptor signaling.
Could stimulation of 5-HT1A receptors underlie psilocybin’s effect on marble-burying? Psilocybin and the 5-HT1A agonist, 8-OH-DPAT, both exerted significant main effects to reduce marble-burying (F1,37=10.4, p=0.0026 and F1,37=74.3, p<0.0001, respectively, Fig. 2a). However, psilocybin did not interact with 8-OH-DPAT (F1,37=0.9, p=0.34 for psilocybin-DPAT interaction term) indicating that 5-HT1A stimulation was unlikely to account for psilocybin’s effect in the marble-burying paradigm. Moreover, the combined effect of psilocybin and 8-OH-DPAT was significantly greater than that of vehicle or psilocybin alone (p<0.0001 and p<0.0001 respectively). To consolidate this observation, we tested the effects of treatment with psilocybin preceded by the 5-HT1A receptor antagonist WAY100635. Using a two-way-ANOVA design, we noted a strong main effect of psilocybin (F1,61=42.47, p<0.0001, Fig. 2b), while the main effect of WAY100635 and the psilocybin-WAY100635 interaction were both not significant (F1,61=0.4, p=0.521 and F1,61=0.0003, p=0.985, respectively, Fig. 2b). Post-hoc testing confirmed that pretreatment with WAY100635 does not block the effect of psilocybin to reduce marble-burying (p>0.10, Fig 2b).
Buspirone is a 5-HT1A receptor partial agonist and a weak dopamine D2 receptor antagonist [23, 24]. Unlike 8-OH-DPAT, we found an interaction of buspirone with psilocybin (F1,75=5.805, p=0.018, Fig. 3a). Thus, although psilocybin and buspirone alone both significantly reduced marble-burying compared to vehicle (F1,75=6.53, p=0.015; F1,75=68.53, p=0.005; respectively, Fig. 3a), their co-administration did not yield a further reduction in marble-burying. We also tested the effect of pretreatment with WAY100635 on the reduction in marble-burying induced by buspirone (buspirone F1,59=19.45, p<0.0001, WAY100635 F1,59=3.07, p=0.08, WAY100635 X buspirone F1,59=1.2, p=0.274, Fig. 3b). Contrasting with the lack of effect of WAY100635 on the psilocybin-induced reduction in marble-burying (Fig 2b), we found a significant effect of WAY100635 to attenuate the effect of buspirone on marble-burying (buspirone vs. vehicle, p<0.001; WAY100635 +buspirone vs. vehicle, p>0.10).
Open Field Test
Mice were placed in an open field on completion of the MBT and were monitored for 30 minutes using the Ethovision XT-12 Video Tracking System (Noldus Information Technology BV). As shown in Supplemental Fig. 1a there was no significant difference in distance travelled between vehicle treated mice and those administered psilocybin or buspirone. Similarly, there was no difference in time spent by the mice in the center of the open field (center duration) (Supplemental Fig 1b) or in the periphery of the open field (periphery duration) (Supplemental Fig 1c) under treatment with psilocybin or buspirone compared to treatment with vehicle.
Persistence of effect
A key question not addressed in studies thus far is whether the effect of psilocybin and other psychedelic compounds on marble-burying is transient or persistent. We examined marble-burying in a subset of mice treated with vehicle or psilocybin 7 days following the initial MBT. No significant effect of psilocybin was observed (vehicle 18.6 ± 1.6, n=5; psilocybin 17.75± 2.21 n=4; p =0.53) (Supplemental Fig. 2).
Requirement for bolus administration
We further examined whether the effect of psilocybin on marble-burying requires a bolus injection of the full dose of the drug (at 4.4 mg/kg) or whether the same effect can be achieved by administering the same quantity of drug in staggered fashion over a period of 3.0 hours i.e. i.p. injections of 1.1 mg.kg every 60 minutes (4 in total) with the MBT performed 30 minutes after the last injection. When administered in this fashion, no significant effect of psilocybin on marble-burying was observed (vehicle 19±0.89 n=6; psilocybin 19±1.32 n=9; p>0.10) (Supplemental Fig. 3).
Evaluation of HTR
To determine whether the dose of psilocybin that inhibited marble-burying in this study would induce a significant increase in the number of head twitches observed in ICR mice as compared to vehicle, HTR was measured in a magnetometer device as described above. We further examined the effect of co-administration of buspirone 5 mg/kg with psilocybin 4.4 mg/kg to determine whether co-administration of buspirone would attenuate the HTR-enhancing effect of psilocybin. Fig 4a shows the time course of the effect of psilocybin and buspirone on HTR. Three way ANOVA with repeated measures showed a significant within-subjects effect of time (F9,288 = 5.001, p <0.0032), reflecting the changes in HTR rate during the course of the test; a time by psilocybin interaction (F9,288 = 3.224, p = 0.001), reflecting differential, psilocybin-dependent changes in the HTR rate during the course of the test and a triple, time by psilocybin by buspirone interaction (F9,288 = 2.687, p=0.0072), reflecting differential psilocybin- and buspirone-dependent, changes in the HTR rate over time. Significant between-subject effects of psilocybin (F1,32 = 19.22, p=0.0001) and buspirone were observed (F1,32 = 7.483, p=0.0101) and a significant psilocybin by buspirone interaction (F1,32 = 5.237, p=0.0289) indicating effects over time. When evaluating the total number of HTRs during the 20-minute measurement period (Fig, 4b) there were significant effects of psilocybin (F1,32 = 19.22, p=0.0001) and buspirone (F1,32 = 7.483, p=0.0101) and a significant psilocybin x buspirone interaction (F1,32 = 5.237, p=0.0289). In post-hoc tests, there was a significant effect of psilocybin to increase HTR compared to vehicle (p<0.001) while buspirone and buspirone + psilocybin effects were significantly lower than the effect of psilocybin (p=0.0002 and p=0.0009, respectively)