Key points: 5‐HT increases the excitability of brainstem and spinal motoneurons, including the jaw‐closing motoneurons, by depolarizing the membrane potential and decreasing the medium‐duration afterhyperpolarization. In this study, we focused on how 5‐HT enhances postsynaptic glutamatergic responses in the dendrites of the jaw‐closing motoneurons. We demonstrate that 5‐HT augments glutamatergic signalling by enhancing the function of the GluN2A‐containing NMDA receptor (NMDAR) through the activation of 5‐HT_2A receptors (5‐HT_2ARs) and Src kinase. To enhance glutamatergic responses, activation of the 5‐HT_2ARs must occur within ∼60 μm of the location of the glutamate responses. 5‐HT inputs to the jaw‐closing motoneurons can significantly vary their input–output relationship, which may contribute to wide‐range regulation of contractile forces of the jaw‐closing muscles. Various motor behaviours are modulated by 5‐HT. Although the masseter (jaw‐closing) motoneurons receive both glutamatergic and serotonergic inputs, it remains unclear how 5‐HT affects the glutamatergic inputs to the motoneuronal dendrites. We examined the effects of 5‐HT on postsynaptic responses evoked by single‐ or two‐photon uncaging of caged glutamate (glutamate responses) to the dendrites of masseter motoneurons in postnatal day 2–5 rats of either sex. Application of 5‐HT induced membrane depolarization and enhanced the glutamate‐response amplitude. This enhancement was mimicked by the 5‐HT_2A receptor (5‐HT_2AR) agonist and was blocked by the 5‐HT_2A/2CR antagonist. However, neither the 5‐HT_2BR nor the 5‐HT_2CR agonists altered glutamate responses. Blockade of the NMDA receptors (NMDARs), but not AMPA receptors, abolished the 5‐HT‐induced enhancement. Furthermore, the selective antagonist for the GluN2A subunit abolished the 5‐HT‐induced enhancement. 5‐HT increased GluN2A phosphorylation, while the Src kinase inhibitor reduced the 5‐HT‐induced enhancement and GluN2A phosphorylation. When exposure to the 5‐HT_2AR agonist was targeted to the dendrites, the enhancement of glutamate responses was restricted to the loci of the dendrites near the puff loci. Electron microscopic immunohistochemistry revealed that both the NMDARs and the 5‐HT_2ARs were close to each other in the same dendrite. These results suggest that activation of dendritic 5‐HT_2ARs enhances the function of local GluN2A‐containing NMDARs through Src kinase. Such enhancement of the glutamate responses by 5‐HT may contribute to wide‐range regulation of contractile forces of the jaw‐closing muscles.

Sleep bruxism (SB) is a sleep-related movement disorder characterized by grinding and clenching of the teeth during sleep. We previously found a significant association between SB and a single nucleotide polymorphism (SNP), rs6313, in the neuronal serotonin 2A receptor gene (HTR2A), and established human induced pluripotent stem cell (iPSC)-derived neurons from SB patients with a genetic variant. To elucidate the electrophysiological characteristics of SB iPSC-derived neural cells bearing an SB-related genetic variant, we generated ventral hindbrain neurons from SB patients and unaffected controls, and explored the intrinsic membrane properties of these neurons using the patch-clamp technique. We found that the electrophysiological properties of iPSC-derived neurons mature in a time-dependent manner in long-term control cultures. SB neurons exhibited higher action potential firing frequency, higher gain, and shorter action potential half duration. This is the first in vitro modeling of SB using patient-specific iPSCs. The revealed electrophysiological characteristics may serve as a benchmark for further investigation of pathogenic mechanisms underlying SB. Moreover, our results on long-term cultures provide a strategy to define the functional maturity of human neurons in vitro, which can be implemented for stem cell research of neurogenesis, and neurodevelopmental disorders.

In Alzheimer’s disease (AD), accumulation of amyloid β-protein (Aβ) is one of the major mechanisms causing neuronal cell damage. Disruption of cell membranes by Aβ has been hypothesized to be the important event associated with neurotoxicity in AD. Curcumin has been shown to reduce Aβ-induced toxicity; however, due to its low bioavailability, clinical trials showed no remarkable effect on cognitive function. As a result, GT863, a derivative of curcumin with higher bioavailability, was synthesized. The purpose of this study is to clarify the mechanism of the protective action of GT863 against the neurotoxicity of highly toxic Aβ oligomers (Aβo), which include high-molecular-weight (HMW) Aβo, mainly composed of protofibrils in human neuroblastoma SH-SY5Y cells, focusing on the cell membrane. The effect of GT863 (1 μM) on Aβo-induced membrane damage was assessed by phospholipid peroxidation of the membrane, membrane fluidity, membrane phase state, membrane potential, membrane resistance, and changes in intracellular Ca²⁺ ([Ca²⁺]i). GT863 inhibited the Aβo-induced increase in plasma-membrane phospholipid peroxidation, decreased membrane fluidity and resistance, and decreased excessive [Ca²⁺]i influx, showing cytoprotective effects. The effects of GT863 on cell membranes may contribute in part to its neuroprotective effects against Aβo-induced toxicity. GT863 may be developed as a prophylactic agent for AD by targeting inhibition of membrane disruption caused by Aβo exposure.