Dopamine requires unique residues to signal via the serotonin 2A receptor

Psychotomimetic compensation versus sensitization

It is a paradox that psychotomimetic drugs can relieve symptoms that increase risk of and cooccur with psychosis, such as attention and motivational deficits (e.g., amphetamines), pain (e.g., cannabis) and symptoms of depression (e.g., psychedelics, dissociatives). We introduce the ideas of psychotomimetic compensation and psychotomimetic sensitization to explain this paradox. Psychotomimetic compensation refers to a short-term stressor or drug-induced compensation against stress that is facilitated by engagement of neurotransmitter/modulator systems (endocannabinoid, serotonergic, glutamatergic and dopaminergic) that mediate the effects of common psychotomimetic drugs. Psychotomimetic sensitization occurs after repeated exposure to stress and/or drugs and is evidenced by the gradual intensification and increase of psychotic-like experiences over time. Theoretical and practical implications of this model are discussed.

Serotonin receptors contribute to dopamine depression of lateral inhibition in the nucleus accumbens

Dopamine modulation of nucleus accumbens (NAc) circuitry is central to theories of reward seeking and reinforcement learning. Despite decades of effort, the acute dopamine actions on the NAc microcircuitry remain puzzling. Here, we dissect out the direct actions of dopamine on lateral inhibition between medium spiny neurons (MSNs) in mouse brain slices and find that they are pathway specific. Dopamine potently depresses GABAergic transmission from presynaptic dopamine D2 receptor-expressing MSNs (D2-MSNs), whereas it potentiates transmission from presynaptic dopamine D1 receptor-expressing MSNs (D1-MSNs) onto other D1-MSNs. To our surprise, presynaptic D2 receptors mediate only half of the depression induced by endogenous and exogenous dopamine. Presynaptic serotonin 5-HT1B receptors are responsible for a significant component of dopamine-induced synaptic depression. This study clarifies the mechanistic understanding of dopamine actions in the NAc by showing pathway-specific modulation of lateral inhibition and involvement of D2 and 5-HT1B receptors in dopamine depression of D2-MSN synapses.

Burke and Alvarez find that, in the nucleus accumbens, dopamine depresses or potentiates lateral inhibition between projection neurons, depending on the specific synapses isolated. Dopamine depression of D2-MSN GABA transmission involves activation of 5-HT1B receptors, suggesting that cross-talk between monoamines and receptors plays a role in accumbens circuit function.

Pivotal mental states

This paper introduces a new construct, the 'pivotal mental state', which is defined as a hyper-plastic state aiding rapid and deep learning that can mediate psychological transformation. We believe this new construct bears relevance to a broad range of psychological and psychiatric phenomena. We argue that pivotal mental states serve an important evolutionary function, that is, to aid psychological transformation when actual or perceived environmental pressures demand this. We cite evidence that chronic stress and neurotic traits are primers for a pivotal mental state, whereas acute stress can be a trigger. Inspired by research with serotonin 2A receptor agonist psychedelics, we highlight how activity at this particular receptor can robustly and reliably induce pivotal mental states, but we argue that the capacity for pivotal mental states is an inherent property of the human brain itself. Moreover, we hypothesize that serotonergic psychedelics hijack a system that has evolved to mediate rapid and deep learning when its need is sensed. We cite a breadth of evidences linking stress via a variety of inducers, with an upregulated serotonin 2A receptor system (e.g. upregulated availability of and/or binding to the receptor) and acute stress with 5-HT release, which we argue can activate this primed system to induce a pivotal mental state. The pivotal mental state model is multi-level, linking a specific molecular gateway (increased serotonin 2A receptor signaling) with the inception of a hyper-plastic brain and mind state, enhanced rate of associative learning and the potential mediation of a psychological transformation.

Aberrant alternative splicing of HTR2A exon II in peripheral blood lymphocytes of drug-naïve schizophrenic patients

The aim of the study was to characterize the pattern of transcript isoforms of HTR2A exon II in lymphocytes of the blood as peripheral biomarkers of schizophrenia development and the effectiveness of antipsychotic therapy. We primarily observed an increase in mRNA levels and elevation of alternative variants in a sample of drug-naïve schizophrenic patients compared to the control group. There was no association of the expression level of HTR2A transcript isoforms with the effectiveness of the antipsychotic therapy. Antipsychotic-induced akathisia was associated with a significant reduction in the mRNA levels of the studied isoforms. In summary, our results suggest that levels of HTR2A exon II transcript isoforms are upregulated in patients with schizophrenia, but at the same time, elevated expression level of the studied HTR2A transcripts is associated with fewer side effects of the therapy.

Serotonin 2A (5-HT2A) receptor affects cell-matrix adhesion and the formation and maintenance of stress fibers in HEK293 cells

5-HT_2A, a G-protein coupled receptor, is widely expressed in the human body, including in the gastrointestinal tract, platelets and the nervous system. It mediates various functions, for e.g. learning, memory, mood regulation, platelet aggregation and vasoconstriction, but its involvement in cell-adhesion remains largely unknown. Here we report a novel role for 5-HT_2A in cell–matrix adhesion.

In HEK293 cells, which are loosely adherent, expression and stimulation of human or rat 5-HT_2A receptor by agonists such as serotonin or 2,5-dimethoxy-4-iodoamphetamine (DOI) led to a significant increase in adhesion, while inhibition of 5-HT_2A by antipsychotics, such as risperidone, olanzapine or chlorpromazine prevented it. 5-HT_2A activation gave rise to stress fibers in these cells and was also required for their maintenance. Mechanistically, the 5-HT_2A-mediated adhesion was mediated by downstream PKC and Rho signaling. Since 5-HT_2A is associated with many disorders such as dementia, depression and schizophrenia, its role in cell–matrix adhesion could have implications for neural circuits.