544. MOLECULAR AND CELLULAR MECHANISMS OF THE LONG-LASTING EFFECTS OF PSYCHEDELICS AND THEIR NON-HALLUCINOGENIC DERIVATIVES

Abstract Background Major depressive disorder (MDD) is a multifactorial and heterogeneous disorder that affects around 280 million individuals worldwide, with approximately 80 million experiencing treatment-resistant depression (TRD). Neuronal hypotrophy and axon hypomyelination in the prefrontal cortex (PFC) are hallmark features of MDD, highlighting the need for treatments that address these deficits to treat MDD and TRD. Psychedelics, such as psilocybin, have shown rapid and robust therapeutic effects by targeting the 5-HT2A receptor. However, it is unclear whether the hallucinatory effects (which pose a challenge to the widespread application of these drugs) of psychedelics can be dissociated from the therapeutic benefits. Aims & Objectives This study aims to determine how 2-Bromo-LSD, a non-hallucinogenic compound, regulates neuronal morphology, myelination and gene expression and how these effects compare to classic serotonergic psychedelic drugs. Method Adult male and female mice (P60-90; C57BL6/J) were treated with saline or 2-Br-LSD (IP, 0.3, 1.0 or 3.0 mg/kg). Mice were tested 24 hours after treatment in the open field and forced swim test. In other experiments, mice were subjected to chronic unpredictable stress (2 daily stressors for 5 weeks). At the end of this period, stressed mice were treated with saline, one (3 mg/kg) or 4 doses of 2-Br-LSD (1 mg/kg) or psilocybin (1 mg/kg), and tested in the open field, splash test, Y-maze or sucrose consumption. Embryonic cortical neurons from rats (E18) were cultured for 3 or 18 days in vitro (DIV) for dendritogenesis or dendritic spine quantifications, respectively. Following treatment in stress-naïve mice, brains were harvested 24 hrs after treatment. Brains were split at the midline, and one of the hemispheres was processed for Golgi staining while the other was flash-frozen. Following PFC dissection, RNA was extracted and processed for bulk RNA sequencing in the flash-frozen hemispheres. A differential expression analysis assessed bulk transcriptional regulation following 2-Br-LSD treatment. Then, in silico, cell-type-specific deconvolution was performed to parse cell-type contributions to transcriptomic changes on the normalized data using the CIBERSORTx software suite and a mouse single-cell RNAseq dataset from the Allen Institute for reference matrix construction. Results 2-Br-LSD induced dendritogenesis and spinogenesis in cultured rat cortical neurons and increased active coping behaviour in mice in the forced swim test, an effect blocked by the 5-HT2A-selective antagonist volinanserin (M100907). 2-Br-LSD also reverses the behavioural effects of chronic stress, similar to psilocybin. Differential expression analysis of bulk RNAseq resulted in 1520 differentially expressed genes (DEGs) in females and 710 in males (only 88 overlap), related to cell signalling, axon guidance, and neuroplasticity, which were localized mainly at the synapse. Interestingly, several DEGs are involved in the modulation of PFC myelination, further supported by the deconvolution analysis, which showed differential expression of myelination-related genes in PFC oligodendrocytes. Discussion & Conclusions Our results indicate that 2-Br-LSD has effects comparable to those of classical psychedelic drugs, which have been shown to produce lasting antidepressant-like effects. This suggests that 2-Br-LSD has the potential to be an effective treatment for MDD.