Large and Small Dendritic Spines Serve Different Interacting Functions in Hippocampal Synaptic Plasticity and Homeostasis.

The laying down of memory requires strong stimulation resulting in specific changes in synaptic strength and corresponding changes in size of dendritic spines. Strong stimuli can also be pathological, causing a homeostatic response, depressing and shrinking the synapse to prevent damage from too much Ca(2+) influx. But do all types of dendritic spines serve both of these apparently opposite functions? Using confocal microscopy in organotypic slices from mice expressing green fluorescent protein in hippocampal neurones, the size of individual spines along sections of dendrite has been tracked in response to application of tetraethylammonium. This strong stimulus would be expected to cause both a protective homeostatic response and long-term potentiation. We report separation of these functions, with spines of different sizes reacting differently to the same strong stimulus. The immediate shrinkage of large spines suggests a homeostatic protective response during the period of potential danger. In CA1, long-lasting growth of small spines subsequently occurs consolidating long-term potentiation but only after the large spines return to their original size. In contrast, small spines do not change in dentate gyrus where potentiation does not occur. The separation in time of these changes allows clear functional differentiation of spines of different sizes.

Discovery and In Vitro Characterization of SPL028: Deuterated N,N-Dimethyltryptamine.

The psychedelic N,N- dimethyltryptamine (DMT) is in clinical development for the treatment of major depressive disorder. However, when administered via intravenous infusion, its effects are short-lived due to rapid clearance. Here we describe the synthesis of deuterated analogues of DMT with the aim of prolonging the half-life and decreasing the clearance rate while maintaining similar pharmacological effects. The molecule with the greatest degree of deuteration at the α-carbon (N,N-D2-dimethyltryptamine, D2-DMT) demonstrated the longest half-life and intrinsic clearance in hepatocyte mitochondrial fractions when compared with DMT. The in vitro receptor binding profile of D2-DMT was comparable to that of DMT, with the highest affinity at the 5-HT1A, 5-HT2A, and 5-HT2C receptors. D2-DMT was therefore the preferred candidate to consider for further evaluation.

Pharmacokinetics of N,N-dimethyltryptamine in Humans.

BACKGROUND AND OBJECTIVE: N,N-dimethyltryptamine (DMT) is a psychedelic compound under development for the treatment of major depressive disorder (MDD). This study evaluated the preclinical and clinical pharmacokinetics and metabolism of DMT in healthy subjects. METHODS: The physiochemical properties of DMT were determined using a series of in vitro experiments and its metabolic profile was assessed using monoamine oxidase (MAO) and cytochrome P450 (CYP) inhibitors in hepatocyte and mitochondrial fractions. Clinical pharmacokinetics results are from the phase I component of a phase I/IIa randomised, double-blind, placebo-controlled, parallel-group, dose-escalation trial (NCT04673383). Healthy adults received single escalating doses of DMT fumarate (SPL026) via a two-phase intravenous (IV) infusion. Dosing regimens were calculated based on pharmacokinetic modelling and predictions with progression to each subsequent dose level contingent upon safety and tolerability. RESULTS: In vitro clearance of DMT was reduced through the inhibition of MAO-A, CYP2D6 and to a lesser extent CYP2C19. Determination of lipophilicity and plasma protein binding was low, indicating that a high proportion of DMT is available for distribution and metabolism, consistent with the very rapid clinical pharmacokinetics. Twenty-four healthy subjects received escalating doses of DMT administered as a 10-min infusion over the dose range of 9-21.5 mg (DMT freebase). DMT was rapidly cleared for all doses: mean elimination half-life was 9-12 min. All doses were safe and well tolerated and there was no relationship between peak DMT plasma concentrations and body mass index (BMI) or weight. CONCLUSION: This is the first study to determine, in detail, the full pharmacokinetics profile of DMT following a slow IV infusion in humans, confirming rapid attainment of peak plasma concentrations followed by rapid clearance. These findings provide evidence which supports the development of novel DMT infusion regimens for the treatment of MDD. CLINICAL TRIAL REGISTRATION: Registered on ClinicalTrials.gov (NCT04673383).

Safety, tolerability, pharmacodynamic and wellbeing effects of SPL026 (dimethyltryptamine fumarate) in healthy participants: a randomized, placebo-controlled phase 1 trial.

Background: Due to their potential impact on mood and wellbeing there has been increasing interest in the potential of serotonergic psychedelics such as N,N-dimethyltryptamine (DMT) in the treatment of major depressive disorder (MDD). Aim: The aim of Part A of this study was to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamic (PD) profile of escalating doses of SPL026 (DMT fumarate) in psychedelic-naïve healthy participants to determine a dose for administration to patients with MDD in the subsequent Phase 2a part of the trial (Part B: not presented in this manuscript). Methods: In the Phase 1, randomized, double-blind, placebo-controlled, parallel-group, single dose-escalation trial, psychedelic-naïve participants were randomized to placebo (n = 8) or four different escalating doses [9, 12, 17 and 21.5 mg intravenously (IV)] of SPL026 (n = 6 for each dose) together with psychological support from 2 therapy team members. PK and acute (immediately following dosing experience) psychometric measures [including mystical experience questionnaire (MEQ), ego dissolution inventory (EDI), and intensity rating visual analogue scale (IRVAS)] were determined. Additional endpoints were measured as longer-term change from baseline to days 8, 15, 30 and 90. These measures included the Warwick and Edinburgh mental wellbeing scale and Spielberger's state-trait anxiety inventory. Results: SPL026 was well tolerated, with an acceptable safety profile, with no serious adverse events. There was some evidence of a correlation between maximum plasma concentration and increased IRVAS, MEQ, and EDI scores. These trends are likely to require confirmation in a larger sample size. Using the analysis of the safety, tolerability, PD, PK results, doses of 21.5 mg SPL026 were the most likely to provide an intense, tolerated experience. Conclusion: Based on the data obtained from this part of the trial, a dose of 21.5 mg SPL026 given as a 2-phase IV infusion over 10 min (6 mg/5 min and 15.5 mg/5 min) was selected as the dose to be taken into patients in Part B (to be presented in a future manuscript).Clinical trial registration:www.clinicaltrials.gov, identifier NCT04673383; https://www.clinicaltrialsregister.eu, identifier 2020-000251-13; https://www.isrctn.com/, identifier ISRCTN63465876.

Improving Mouse Models for Dementia. Are All the Effects in Tau Mouse Models Due to Overexpression?

Mouse models of Alzheimer's disease have commonly used transgenic overexpression of genes involved in production of amyloid ß (APP and/or PSEN1/2) or Tau (MAPT) with mutations that result in familial forms of dementia. We discuss possible improvements that may create full models while avoiding the problems of overexpression and report synaptic results in APPKI models. We stress use of inappropriate controls without overexpression of the normal human protein and the mismatch between the learning deficits reported in mice with plaques but no tangles and the human condition. We focus on Tau overexpression, including new data that support previous reports of the grossly nonlinear relationship between Tau overexpression and neurofibrillary tangle load, with a twofold increase in Tau protein, resulting in a 100-fold increase in tangle density. These data also support the hypothesis that a high concentration of soluble Tau, in overexpression models, plays an important direct role in neurodegeneration, rather than only via aggregation. Finally, we hypothesize that there is an optimal concentration range over which Tau can bind to microtubules and a threshold beyond which much of the overexpressed protein is unable to bind. The excess thus causes toxicity in ways not necessarily related to the process in human dementias.