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Front Pharmacol. 2022; 13: 925879.
Published online 2022 Jun 16. doi: 10.3389/fphar.2022.925879
PMCID: PMC9243434
PMID: 35784746
Published online 2022 Jun 16. doi: 10.3389/fphar.2022.925879
PMCID: PMC9243434
PMID: 35784746
"In contrast, administration of the selective agonist SA4503 or fluvoxamine (with a high affinity for Sigma-1 receptor) for 2 weeks caused a decrease in the immobilization time of CaMKIV−/− mice in the FST and the tail suspension test (TST), while those effects were abolished by preliminary treatment with the sigma-1 receptor antagonist NE-100 (Moriguchi et al., 2015). The abovementioned evidence indicates that a significant role of sigma-1 receptors in regulating Ca2+-dependent mechanisms of antidepressant action may not only be related to extracellular Ca2+ influx but also to intracellular Ca2+ homeostasis (Urani et al., 2002; Choi et al., 2022).
Sigma-1 Receptors Regulate Excitatory and Inhibitory Balance
The classic “monoamine hypothesis” underlies the development of most clinical antidepressants that primarily exert their effects by enhancing the function of monoamine transmitters (Li, 2020). The translocation property of sigma-1 receptors allows them to modulate proteins directly not only at the ER-mitochondrion interface but also at the membrane where many ion channels, receptors and kinases are found, rendering sigma-1 receptors a unique inter-organelle signaling modulator in living tissues, including the CNS that we focused on (Su et al., 2010). In recent years, an increasing body of evidence suggests that sigma-1 receptors directly interact with proteins at neuronal cell membranes (more information is reviewed in (Ryskamp et al., 2019)) and enhance neurotransmission (Bermack and Debonnel, 2001; Bermack and Debonnel, 2005; Sambo et al., 2017; Sambo et al., 2018). The active roles of sigma-1 receptors in the regulation of neurotransmission, including the glutamatergic, GABAergic, serotonergic, dopaminergic (Sambo et al., 2018) and noradrenergic systems (Dhir A. and Kulkarni SK., 2008), is well documented. Those neurotransmission systems may form the basis for the dynamic balance of E/I neural networks in the brain. Especially, sigma-1 receptors have been reported to regulate presynaptic glutamate release and modulate NMDA receptor activity via direct PPI associations (Kourrich, 2017). Furthermore, sigma-1 receptor activation can also lead to alterations in NMDA receptors that upregulate and traffic to the plasma membrane thus further modulating neuronal intrinsic excitability (Pabba et al., 2014). In addition, Mtchedlishvil et al. found that pregnenolone and a selective sigma-1 receptor agonist (SKF-10047) inhibit the GABA-dependent inhibitory postsynaptic currents in rat hippocampal cell cultures (Mtchedlishvili and Kapur, 2003). We therefore suggest that sigma-1 receptors may regulate the E/I balance in direct or indirect manners. Importantly, there is abundant evidence that the medial prefrontal cortex (mPFC) or the hippocampus rely on the dynamic balance between E/I neurotransmitters for various advanced functions, such as emotion regulation and expression, as well as cognitive functions. Those two neurotransmitters achieve a dynamic balance to maintain normal physiological function under normal circumstances (Ferguson and Gao, 2018). Preclinical studies have shown that chronic stress can lead to decreased E/I neurotransmitter transmission in the mPFC (Fee et al., 2017). Consistent with this, a recent study found that chronic stress can lead to decreased glutamate and GABA neurotransmitter transmission in rat mPFC (Fee et al., 2017; Duman et al., 2019). In this context, GABAergic and glutamatergic neurotransmission may be almost rebalanced but the synapses remain impaired, thus the E/I rebalance may be at a low level. However, we recently found in our laboratory that regulation of the E/I rebalance in the mPFC may be an important mechanism and rate-limiting step in the efficacy of antidepressant effects (Yin et al., 2021).
The glutamatergic and GABAergic as well as serotonergic systems are heavily implicated in antidepressant actions. In this section, we mainly focused on the possible mechanisms by which sigma-1 receptors exert their antidepressant effects through the regulation of E/I balance by theseneurotransmitter systems.