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Major depression is an established risk factor for subsequent dementia, and depression in late life may also represent a prodromal state of dementia. Considering current challenges in the clinical development of disease modifying therapies for dementia, the focus of research is shifting towards prevention and modification of risk factors to alter the neurodegenerative disease trajectory. Understanding mechanistic commonalities underlying affective symptoms and cognitive decline may reveal biomarkers to aid early identification of those at risk of progressing to dementia during the preclinical phase of disease, thus allowing for timely intervention. Adult hippocampal neurogenesis (AHN) is a phenomenon that describes the birth of new neurons in the dentate gyrus throughout life and it is associated with spatial learning, memory and mood regulation. Microglia are innate immune system macrophages in the central nervous system that carefully regulate AHN via multiple mechanisms. Disruption in AHN is associated with both dementia and major depression and microgliosis is a hallmark of several neurodegenerative diseases. Emerging evidence suggests that psychedelics promote neuroplasticity, including neurogenesis, and may also be immunomodulatory. In this context, psilocybin, a serotonergic agonist with rapid-acting antidepressant properties has the potential to ameliorate intersecting pathophysiological processes relevant for both major depression and neurodegenerative diseases. In this narrative review, we focus on the evidence base for the effects of psilocybin on adult hippocampal neurogenesis and microglial form and function; which may suggest that psilocybin has the potential to modulate multiple mechanisms of action, and may have implications in altering the progression from major depression to dementia in those at risk., Competing Interests: Declaration of competing interest DA, MB, TM, ST, LV and ACV declare no conflict of interest relevant to the content of this manuscript. ZRH: Investigational material received from COMPASS Pathways. DMT: Shareholder of Myogenes and 428 Pharma. JJR: Paid advisory boards for Clerkenwell Health (Past), Beckley PsyTech (Past), Delica Therapeutics (Past). Paid articles for Janssen. Assistance for attendance at conferences from Compass Pathways (past) and Janssen. Grant funding (received and managed by King's College London) from Compass Pathfinder, Beckley PsyTech, Multidisciplinary Association for Psychedelic Studies, National Institute for Health Research, Wellcome Trust, Biomedical Research Centre at the South London and Maudsley NHS Foundation Trust. No shareholdings in pharmaceutical companies. No shareholdings in companies developing psychedelics. AHY: Employed by King's College London; Honorary Consultant South London and Maudsley NHS Foundation Trust (NHS UK). Editor of Journal of Psychopharmacology and Deputy Editor, BJPsych Open. Paid lectures and advisory boards for the following companies with drugs used in affective and related disorders: FlowNeuroscience, Novartis, Roche, Janssen, Takeda, Noema pharma, Compass, Astrazenaca, Boehringer Ingelheim, Eli Lilly, LivaNova, Lundbeck, Sunovion, Servier, Livanova, Janssen, Allegan, Bionomics, Sumitomo Dainippon Pharma, Sage, Novartis, Neurocentrx. Principal Investigator in the Restore-Life VNS registry study funded by LivaNova. Principal Investigator on ESKETINTRD3004: “An Open-label, Long-term, Safety and Efficacy Study of Intranasal Esketamine in Treatment-resistant Depression.” Principal Investigator on “The Effects of Psilocybin on Cognitive Function in Healthy Participants”. Principal Investigator on “The Safety and Efficacy of Psilocybin in Participants with Treatment-Resistant Depression (P-TRD)”. Principal Investigator on “A Double-Blind, Randomized, Parallel-Group Study with Quetiapine Extended Release as Comparator to Evaluate the Efficacy and Safety of Seltorexant 20 mg as Adjunctive Therapy to Antidepressants in Adult and Elderly Patients with Major Depressive Disorder with Insomnia Symptoms Who Have Responded Inadequately to Antidepressant Therapy.” (Janssen). Principal Investigator on “ An Open-label, Long-term, Safety and Efficacy Study of Aticaprant as Adjunctive Therapy in Adult and Elderly Participants with Major Depressive Disorder (MDD).” (Janssen). Principal Investigator on “A Randomized, Double-blind, Multicentre, Parallel-group, Placebo-controlled Study to Evaluate the Efficacy, Safety, and Tolerability of Aticaprant 10 mg as Adjunctive Therapy in Adult Participants with Major Depressive Disorder (MDD) with Moderate-to-severe Anhedonia and Inadequate Response to Current Antidepressant Therapy”. Principal Investigator on “ A Study of Disease Characteristics and Real-life Standard of Care Effectiveness in Patients with Major Depressive Disorder (MDD) With Anhedonia and Inadequate Response to Current Antidepressant Therapy Including an SSRI or SNR.” (Janssen). UK Chief Investigator for Compass; COMP006 & COMP007 studies. UK Chief Investigator for Novartis MDD study MIJ821A12201. Grant funding (past and present): NIMH (USA); CIHR (Canada); NARSAD (USA); Stanley Medical Research Institute (USA); MRC (UK); Wellcome Trust (UK); Royal College of Physicians (Edin); BMA (UK); UBC-VGH Foundation (Canada); WEDC (Canada); CCS Depression Research Fund (Canada); MSFHR (Canada); NIHR (UK). Janssen (UK) EU Horizon 2020. No shareholdings in pharmaceutical companies., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Background and Aims: Long-term harms of cannabis may be exacerbated in adolescence, but little is known about the acute effects of cannabis in adolescents. We aimed to (i) compare the acute effects of cannabis in adolescent and adult cannabis users and (ii) determine if cannabidiol (CBD) acutely modulates the effects of delta-9-tetrahydocannabinol (THC)., Design: Randomised, double-blind, placebo-controlled, crossover experiment. The experiment was registered on ClinicalTrials.gov (NCT04851392)., Setting: Laboratory in London, United Kingdom., Participants: Twenty-four adolescents (12 women, 16- to 17-year-olds) and 24 adults (12 women, 26- to 29-year-olds) who used cannabis 0.5-3 days/week and were matched on cannabis use frequency (mean = 1.5 days/week)., Intervention: We administered three weight-adjusted vaporised cannabis flower preparations: 'THC' (8 mg THC for 75 kg person); 'THC + CBD' (8 mg THC and 24 mg CBD for 75 kg person); and 'PLA' (matched placebo)., Measurements: Primary outcomes were (i) subjective 'feel drug effect'; (ii) verbal episodic memory (delayed prose recall); and (iii) psychotomimetic effect (Psychotomimetic States Inventory)., Findings: Compared with 'PLA', 'THC' and 'THC + CBD' significantly (P < 0.001) increased 'feel drug effect' (mean difference [MD] = 6.3, 95% CI = 5.3-7.2; MD = 6.8, 95% CI = 6.0-7.7), impaired verbal episodic memory (MD = -2.7, 95% CI = -4.1 to -1.4; MD = -2.9, 95% CI = -4.1 to -1.7) and increased psychotomimetic effects (MD = 7.8, 95% CI = 2.8-12.7; MD = 10.8, 95% CI = 6.2-15.4). There was no evidence that adolescents differed from adults in their responses to cannabis (interaction P ≥ 0.4). Bayesian analyses supported equivalent effects of cannabis in adolescents and adults (Bayes factor [BF 01 ] >3). There was no evidence that CBD significantly modulated the acute effects of THC., Conclusions: Adolescent cannabis users are neither more resilient nor more vulnerable than adult cannabis users to the acute psychotomimetic, verbal memory-impairing or subjective effects of cannabis. Furthermore, in adolescents and adults, vaporised cannabidiol does not mitigate the acute harms caused by delta-9-tetrahydocannabinol., (© 2023 The Authors. Addiction published by John Wiley & Sons Ltd on behalf of Society for the Study of Addiction.)