Limbic System Response to Psilocybin and Ketamine Administration in Rats: A Neurochemical and Behavioral Study

Pathophysiology of depression is related with reduced volume of the hippocampus and amygdala and hypertrophy of the nucleus accumbens. The mechanism of these changes is not well under-stood, but clinical studies have shown that administration of the fast-acting antidepressant keta-mine reversed the decrease in hippocampus and amygdala volume in depressed patients, and the magnitude of this effect correlated with the reduction of depressive symptoms. In the present study, we attempted to find out whether the psychedelic substance psilocybin affects neurotransmission in the limbic system in comparison to ketamine. Psilocybin and ketamine increased the release of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens of naive rats as demonstrated using microdialysis. Both drugs influenced glutamate and GABA release in the nucleus accumbens, hippocampus and amygdala and increased ACh levels in the hippocampus. The changes in D2, 5-HT1A and 5-HT2A receptor density in the nucleus accumbens and hippocampus was observed as a long-lasting effect. A marked anxiolytic effect of psilocybin in acute phase and 24 h post-treatment was shown in the open field test. These data provide the neurobiological background for psilocybin effect on stress, anxiety and structural changes in the limbic system and translate into antidepres-sant effect of psilocybin in depressed patients.