Studies reporting the effects of psychedelic drugs on mouse behavior are rapidly emerging. However, the reproducibility and reliability of these effects remain uncertain. Here we implemented a multi-institutional collaborative approach to assess which effects of psilocybin on mouse behavior are robust and replicable. Five independent laboratories performed the same experiments to test the acute and persistent effects of psilocybin (2 mg kg[-1], intraperitoneally) on various behaviors. Psilocybin acutely increased anxiety- and avoidance-related behaviors and decreased fear expression in a robust and replicable manner. Surprisingly, however, psilocybin did not have replicable effects 24 h after administration in terms of reducing anxiety- and depression-like behaviors or facilitating fear extinction learning. Additionally, we did not observe psilocybin-induced alterations in social preference or social reward learning. Our characterization of psilocybin's behavioral effects, using ~200 male and female mice per experiment across five laboratories, demonstrates several robust acute effects and suggests that some of psilocybin's reported persistent effects in mice may be modest and/or inconsistent.

Brain myeloid cells, including infiltrating monocyte-derived macrophages (MDMs) and microglia, play critical roles in ischemic brain injury, clearing debris and shaping neuroinflammation and neurological outcomes. Yet the underlying mechanisms remain unclear. Here, using single-cell RNA sequencing and fate mapping with an MDM-specific Ms4a3[creERT2]-Ai14 mouse line, we identified serotonin receptor Htr2b[+] MDMs as a crucial cell population involved in resolving neuroinflammation within the ischemic penumbra. Htr2b[+] MDMs significantly accumulate in the peri-infarct region and exhibit enhanced phagocytosis and inflammation-resolving gene signatures. Mechanistically, Htr2b activation promotes nuclear translocation of Tfe3, a key lysosomal regulator, enhancing lysosomal biogenesis and supporting phagocytic activity. MDM-specific Htr2b ablation impairs debris clearance, exacerbates neuroinflammation and ischemic injury, whereas pharmacological agonism or Tfe3 activation restores lysosomal biogenesis in brain MDMs and improves post-stroke outcomes. Thus we demonstrate a previously unrecognized Htr2b[+] MDM subset that integrates serotonin signaling to license intrinsic phagocytic function, alleviating ischemic brain injury.

Controlling whether an experience is encoded into long-term memory and thus later remembered is a crucial aspect of memory. Here we examined single-neuron activity in the human brain while participants formed memories for novel visual stimuli and later retrieved them with confidence ratings. Category-selective amygdala neurons exhibited elevated firing during encoding of later remembered items relative to later forgotten items. This effect was predictive of the episodic aspect of the memory, was restricted to the preferred category of each cell, emerged earlier and was stronger in the amygdala compared to the hippocampus, and did not depend on stimulus valence and arousal. Successful memory formation was accompanied by enhanced spike-field coherence between the activity of amygdala category cells and hippocampal field potentials. These findings demonstrate that stimulus-specific recruitment of the amygdala predicts memory formation and suggest category cells in the right amygdala as a target for interventions to treat disorders affecting episodic memory in humans.

Understanding how the heterogeneous activity patterns of individual neurons give rise to coherent population dynamics and cognition is a central challenge in neuroscience. In the hippocampus, neurons are tuned to spatial location, yet their activity is also modulated by multiple external and internal variables and appears to drift randomly over time, making it difficult to link hippocampal computations to behavior. By analyzing hippocampal population dynamics in behaving mice, we reveal a structured organization underlying the diversity of single-neuron responses-the hippocampus encodes different aspects of experience as disentangled variables represented in the co-activity dynamics of cell assemblies. The temporal and contextual components of these representations are governed by local inhibitory interactions and input-driven cell-assembly dynamics, respectively. Our findings reveal the circuit basis of disentangled hippocampal representations, offering a unifying geometric principle of population coding that supports flexible memory representations and enables rapid generalization of prior knowledge to new situations.

The selective loss of dopaminergic neurons in the substantia nigra pars compacta is a defining feature of Parkinson's disease (PD), yet its underlying mechanisms remain unclear. NUS1, which encodes neurite outgrowth inhibitor-B receptor (NgBR), was recently identified as a PD risk gene. Here we show that NgBR is highly expressed in microglia, where its deficiency drives progressive PD-like motor deficits and dopaminergic neurodegeneration via the LCN2-SLC22A17 iron-transport axis in mice. NgBR deficiency in mouse and human microglia triggers endoplasmic reticulum stress and PERK-ATF4-NUPR1 signaling, which preferentially induces LCN2-mediated iron efflux over cytokine release. Subsequently, microglia-derived iron-loaded Holo-LCN2, but not iron-free Apo-LCN2, is internalized by SLC22A17, a receptor highly expressed in dopaminergic neurons, leading to iron overload and ferroptosis. Pharmacological inhibition of LCN2-SLC22A17 axis or ferroptosis alleviated dopaminergic neuron loss and PD-like symptoms caused by microglial NgBR deficiency in vitro and in vivo. Our findings identify disrupted intercellular iron homeostasis as a pathological driver of PD and highlight the LCN2-SLC22A17 axis as a potential therapeutic target.

Dysfunctional immune responses impair remyelination and contribute to neurodegeneration. While immune and metabolic responses to acute myelin injury are well characterized, myeloid cell adaptations to chronic myelin damage remain poorly understood. Here we performed a comparative analysis of genetic and toxic mouse models of acute and chronic demyelination, which identified a distinct microglial state associated with chronic demyelination, characterized by sustained pro-inflammatory activation, lysosomal lipid sequestration and reduced lipid droplet formation. Transcriptomics profiling further revealed a metabolic program marked by upregulated cholesterol synthesis and diminished sterol efflux. Treatment with a desmosterol mimetic possessing 24-dehydrocholesterol reductase inhibitor activity reversed this microglial state, attenuated pro-inflammatory signaling (including C-C Motif Chemokine Ligand 4 expression), reduced myelin damage and improved motor performance in a mouse model of leukodystrophy. Our findings uncover a pro-inflammatory microglial state characterized by lysosomal dysfunction and increased myelin reactivity, highlighting a novel therapeutic target for chronic demyelinating disorders.

Psychedelics produce long-lasting antidepressant effects, but their hallucinogenic properties limit clinical use. Using structure-guided design of 5-HT2AR-β-arrestin-biased agonists, we identified β-arrestin signaling as a critical driver of prolonged antidepressant-like effects in mice. Here we show that the slow-dissociation kinetics of lysergic acid diethylamide (LSD) enable progressive accumulation of 5-HT2AR engagement during repeated low-dose administration, approaching the receptor occupancy achieved by an acute high dose while avoiding its high peak concentration. This kinetic separation sustains β-arrestin signaling while limiting robust Gαq-mediated Ca[2+] responses. Consistent with this framework, repeated low-dose LSD and repeated low-dose psilocybin pretreatment followed by a high-dose psychedelic challenge produced sustained antidepressant-like behavioral effects in mice. In parallel, these regimens produced no detectable psychedelic-associated behavioral disruption in mice and cynomolgus macaques. Together, these findings uncover the signaling basis of sustained psychedelic efficacy and provide a rational framework for developing nonhallucinogenic psychedelic therapies for psychiatric disorders.

Extracellular vesicles (EVs) are present in biofluids and may be useful biomarkers to non-invasively monitor brain cells, but isolating cell type-specific EVs remains challenging. Here we introduce an approach to immuno-isolate EVs from neurons and glia based on protein surface markers. We first selected transmembrane proteins suitable as cell type-specific EV markers based on gene expression and EV proteomic data. We developed a high-purity EV isolation method and cataloged the proteins present on EVs in human cerebrospinal fluid and plasma. We then computationally identified cell type-specific candidate markers for isolating EVs from neurons, astrocytes, oligodendrocytes and microglia. Using ultrasensitive immunoassays against several predicted neuron-specific proteins, we confirmed one marker, NRXN3, as present on EVs in cerebrospinal fluid and plasma. Finally, we developed efficient EV immuno-isolation methods and applied them to isolate NRXN3[+] EVs. Our work provides a general methodology for the isolation of cell type-specific EVs, advancing the goal of using EVs to study brain diseases.

Flexible cognition requires the brain to generalize across tasks while preserving task-specific information. Previous work showed that task representations are compressed in the association cortex and expanded in sensory and motor systems, but the mechanisms underlying these transformations are unclear. Here we test whether the geometry of intrinsic brain connectivity constrains how representations change between cortical regions. We analyzed functional MRI acquired during 16 diverse cognitive tasks together with activity flow modeling, finding that connectivity dimensionality predicts representational changes across the cortex. Low-dimensional connectivity compressed task representations and increased similarities across tasks, whereas high-dimensional connectivity expanded representations and supported conjunctive coding of task features. Activity flow modeling reproduced the observed compression-to-expansion pattern along the sensory-association-motor hierarchy and generated representational geometries that more closely matched targets than sources, consistent with transformation rather than transfer of information. These findings identify network geometry as a systems-level principle that shapes cortical representations and supports flexible cognition.