Thesis Defense Seminar: Hannah P. Yarbrough

Over 280 million people worldwide live with major depressive disorder, and roughly one-third of patients do not respond adequately to first-line treatments like selective serotonin reuptake inhibitors, leaving many with treatment-resistant depression (Al-Harbi, 2012; World Health Organization, 2020). Psilocybin, a naturally occurring compound found in Psilocybe mushrooms, has received FDA recognition as a breakthrough therapy for treatment-resistant depression and produces lasting antidepressant effects after only one or two administrations (Wang et al., 2024; Griffiths et al., 2016). In the gut, liver, and brain, psilocybin undergoes dephosphorylation to its pharmacologically active metabolite psilocin, a serotonin (5-HT) receptor agonist with psychoplastogenic properties known to produce structural changes in neurons, including dendritic spine growth, increased synaptic density, and elevated brain-derived neurotrophic factor expression in animal models (Braun et al., 2024; Ly et al., 2018; Shao et al., 2021). However, whether these morphological changes translate into measurable alterations in synaptic strength at the cellular level has remained an open question. To address this gap in knowledge, this study examined psilocin’s effects on neuronal excitability and synaptic strength in the invertebrate Aplysia, a model system tractable for detailed neurophysiological analyses.

The excitatory monosynaptic connection between sensory neurons (SNs) in the pleural ganglia and motor neurons (MNs) in the pedal ganglia in Aplysia is an extensively studied preparation in which a thorough neurophysiological analysis of psilocin's effects can be conducted. The SN-MN synapse shares serotonergic signaling cascades and ion channels with mammalian neurons. Furthermore, 5-HT is known to produce long-term facilitation of the excitatory postsynaptic potential (EPSP) evoked in MNs by SN activation (Kandel, 2001; Hawkins et al., 2017). The present study used isolated in-vitro pleural-pedal ganglia preparations from adult Aplysia to test whether psilocin produces lasting changes in neuronal excitability and synaptic strength. Preparations were assigned to three treatment groups: 1) psilocin (10 μM in 0.2% acetonitrile used as vehicle, 2-hour bath application), 2) 5-HT (50 μM, approximately 2-hour application), and 3) vehicle (0.2% acetonitrile, 2-hour application). Three parameters were recorded before and 24 hours after treatment: (1) SN excitability, (2) MN excitability, and (3) the strength of the SN-MN synaptic connection, measured as EPSP amplitude.

Psilocin induced a significant increase in EPSP amplitude at the SN-MN synapse 24 hours after drug washout, at a level comparable to that produced by 5-HT. No changes were detected in SN or MN excitability across groups. The persistence of synaptic facilitation in the absence of changes in neuronal excitability points to a synaptic rather than somatic site of action. In Aplysia, EPSP amplitude correlates proportionally with the number of physical SN-MN contact sites, connecting the functional finding here to the structural synaptic remodeling that psilocin produces in animal preparations (Zhang et al., 2003; Ly et al., 2018). A single brief psilocin exposure produced a lasting increase in synaptic strength that mirrors the temporal dissociation seen in mammalian preparations, where structural synaptic changes persist well after psilocin has cleared from the body (Cameron et al., 2021; Ly et al., 2021). These results suggest cellular-level evidence that psilocin's psychoplastogenic effects on neuronal structure are accompanied by lasting physiological changes in synaptic strength, which are consistent with the broader hypothesis that psychedelics can restore synaptic connectivity lost or diminished in depression.