Negative cognitive biases—where mood colors learning and memory—are a core feature of major depressive disorder, and reversing them may be key to how rapid-acting antidepressants work. In rats, a single dose of ketamine, scopolamine, or psilocybin selectively weakened a negative affective bias induced in an associative learning task. Low doses of ketamine and psilocybin, but not high doses, reversed the valence of the bias 24 hours later. Only psilocybin produced a lasting positive bias that depended on new learning. Ketamine's relearning effects required protein synthesis in the medial prefrontal cortex and could be altered by cue reactivation, pointing to experience-dependent neural plasticity as a shared mechanism for both the rapid and sustained effects of these drugs.
Antidepressant doses used in conventional rodent models of depression often exceed those used in clinical practice by 1.5 to 25 times, potentially engaging mechanisms irrelevant to human therapeutic effects. A review of forced swim test studies found median doses of 10 mg/kg across antidepressants, while the more recently developed affective bias test showed doses closer to clinical levels. In a separate analysis of 232 ketamine and 202 fluoxetine rodent studies, median doses were also 10 mg/kg, exceeding animal equivalent doses by 1.6–6.5 times. This mismatch may explain why positive preclinical results often fail to translate into clinical efficacy.
The way NMDA antagonists alter negative affective biases in male rats could predict how well these drugs will work in humans with major depressive disorder.
Psilocybin, the active compound in magic mushrooms, produces rapid and lasting antidepressant effects in people with major depressive disorder, but the underlying brain mechanisms are not fully understood. In rats, psilocin (the active metabolite of psilocybin) alters negative affective biases—a key feature of depression—by acting on a specific circuit in the medial prefrontal cortex. It suppresses excitatory signals to cortico-amygdala projection neurons while enhancing excitatory transmission to other targets, effects dependent on 5HT1A and 5HT2A receptors. These changes persist for at least 24 hours and shift from suppressed excitation to enhanced inhibition in those same cells. Chemogenetically inhibiting these neurons reproduced psilocybin's effects on affective biases and reward memories, identifying this circuit as a key substrate for its antidepressant actions.
Psilocybin, a serotonergic psychedelic, can rapidly and lastingly reverse impaired reward processing in a rat model of depression. In rats with chronic interferon-alpha-induced depression, a single dose of psilocybin (0.3 mg/kg) restored reward-induced behavioral biases within 24 hours, and the effect persisted for at least 7 days. This suggests that restoring blunted reward processing may contribute to psilocybin's sustained antidepressant effects.