UC Davis

Major depressive disorder (MDD) affects more than 264 million people worldwide. A wide range of antidepressants are available to treat the disease including, but are not limited to, Prozac™, Elavil™, and Nardil™. Unfortunately, 1 in 3 patients prescribed these traditional antidepressants do not show significant mood improvement in response to the first regimen, and those who do require upwards of 2–4 weeks of treatment before observing any therapeutic effects. This delay of therapeutic onset with currently prescribed medications further highlights the necessity of novel faster-acting antidepressants. Recently, the Food and Drug Administration (FDA) approved esketamine—a ketamine enantiomer with rapid therapeutic effects (hours to days)—for treatment-resistant depression (TRD). It is hypothesized that ketamine produces its therapeutic effects by rectifying the atrophy of neurites, the reduction of the number of dendritic spines, and the loss of synapses in the prefrontal cortex (PFC) observed in postmortem brains of patients who suffered from MDD and rodent models of depression. However, ketamine has serious side effects, including the potential for abuse. Therefore, we are interested in developing, identifying, and optimizing novel chemical scaffolds with unique mechanisms of action that are better tolerated. One series of scaffolds that we are interested in is the classic psychedelics. Recently, the classic psychedelics have undergone a renaissance and have begun to be explored as next-generation therapeutics for the treatment of depression with psilocybin, lysergic acid diethylamide (LSD), and 5-methoxy-N, N-dimethyltryptamine (5-MeO-DMT) leading the way in clinical trials. Thus, we postulate that the scaffolds of the classic psychedelics would prove to be promising targets for next-generation antidepressants. Here, I discuss efforts to develop next-generation therapeutics starting from classic psychedelics. First, I explored the similarities between ketamine and classic psychedelics with respect to their effects on structural neural plasticity. We categorized molecules that promoted plasticity as psychoplastogens, small molecules that can induce structural changes in the brain. Next, I investigated the mechanism of actions of the classic psychedelics and discovered the importance of tropomyosin receptor kinase B (TrkB), brain-derived neurotrophic factor (BDNF), the mammalian target of rapamycin (mTOR), serotonin 2A (5-HT2A), and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors in psychoplastogen-induced plasticity. Furthermore, I examined transient stimulation versus chronic stimulation with the psychoplastogens and observed their effects in promoting long-lasting structural plasticity. Finally, to better facilitate drug discovery of fast-acting antidepressants, I developed two orthogonal assays. First, I established phenotypic assays to examine the efficacy of small molecules in promoting neuritogenesis, spinogenesis, and synaptogenesis. I clustered known fast-acting antidepressants, traditional antidepressants, and non-antidepressant compounds by combining these structural plasticity phenotypes using principal component analysis (PCA). Second, we developed and characterized the first cellular assay to determine the hallucinogenic potential of small molecules using an engineered 5-HT2AR fluorescent biosensor. We identified two novel hallucinogens and several non-hallucinogens through this platform. We then validated their effects in the head-twitch response—a behavioral assay that correlates well with the hallucinogenic potential of small molecules in mice. Lastly, we combined the structural plasticity assays with the 5-HT2AR hallucinogenic potential counter-screen to identify a novel non-hallucinogenic small molecule, AAZ-A-154 (AAZ), with therapeutic potential. We then confirmed these findings using behavioral assays. Overall, this work describes mechanistic studies aimed at understanding the actions of psychedelics, the identification of new potential antidepressants through phenotypic assays, and a novel counter-screen for hallucinogenic potential. These studies will facilitate drug discovery efforts aimed at better-tolerated, fast-acting, next-generation antidepressants.