Scientists discovered that both serotonin and psilocybin activate 5-HT1B receptors to dampen signaling through the claustrum, a little-known brain hub tied to consciousness and network integration. In mice, psilocybin (1 mg/kg) suppressed input from the anterior cingulate cortex to claustrum neurons for up to 24 hours, an effect blocked by a 5-HT1B antagonist. This reveals a new circuit-level mechanism—beyond the well-known 5-HT2A pathway—showing how psilocybin reshapes brain activity.