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  • The observation that vortioxetine blocks HT induced

    2024-05-14

    The observation that vortioxetine blocks 5-HT-induced currents in 5-HT3 receptor-expressing oocytes suggests that vortioxetine acts to functionally antagonize these receptors, and is in line with previously published data (Bang-Andersen et al., 2011). For example, Mørk et al. (2012) demonstrated that vortioxetine and ondansetron were capable of blocking the Bezold-Jarisch reflex in rats, a 5-HT-induced bradycardia response that is thought to be mediated via peripheral 5-HT3 receptors expressed on the Vagus nerve. Using rat 5-HT3 receptors, the potency ratio between ondansetron and vortioxetine across the in vitro radioligand binding, the in vivo Bezold-Jarisch reflex assay, and the oocyte functional data are comparable, in that they are within less than an order of magnitude (Table 1). Interestingly, in spite of the fact that vortioxetine and ondansetron have similar affinities in receptor binding assays (1.1 nM and 0.36, respectively; Table 1), the apparent off-rate of these two compounds according to the oocyte functional data are markedly different. Specifically, we observed that ondansetron-treated SB742457 took approximately 30 min to recover from treatment, while vortioxetine-treated cells took at least 3 h to recover. The mechanism underlying these apparent differences in off-rates cannot be understood on the basis of these data alone and are outside the intended scope of this paper. However, it is likely that there are differences in the manner with which ondansetron and vortioxetine bind to the 5-HT3 receptor at the molecular level. In a future study it would be interesting to study the binding kinetics of vortioxetine and ondansetron at the A and B subunits of the 5-HT3 receptor. As originally reported in (Bang-Andersen et al., 2011), this assay suggests that vortioxetine was approximately 55-fold more potent at r5-HT3A receptors by comparison with the h5-HT3A receptor variant, suggesting a substantial difference in the concentrations required to functionally antagonize 5-HT3 receptor from these species. Given that ondansetron has a similar IC50 at both of these 5-HT3 receptor variants, this observation may suggest that ondansetron and vortioxetine bind to separate portions of the 5-HT3A receptor. But it is important to note that radioligand binding assays have found only a three-fold difference in vortioxetine’s Ki for these two 5-HT3 receptor variants, i.e. r5-HT3 = 1.1 nM; h5-HT3 receptor = 3.7 nM (Leiser et al., 2014). Thus, it is unclear to what degree this difference in functional potencies should alter our assessments of the relevance of vortioxetine’s 5-HT3 receptor-mediated mechanisms in the clinic. Several technological advances will be necessary in order to develop our understanding of this issue. First, it will be necessary to develop a radioligand capable of directly estimating 5-HT3 receptor occupancy in humans using positron emission tomography. Additionally, it will be necessary to develop a non-invasive method capable of differentiating GABAergic and glutamatergic neurotransmission in humans. Current options for noninvasively assessing GABAergic and glutamatergic neurotransmission are incapable of clearly discriminating changes in glutamatergic and GABAergic neurotransmission. For example, glutamate/cycling, which can be detected using magnetic resonance spectroscopy, cannot differentiate changes in these two transmitter systems, since they are closely metabolically linked (see Pehrson and Sanchez, 2015 for review). Moreover, this is a limitation that is shared by other imaging-focused functional models, such as BOLD measurements in fMRI. Thus, it is not clear that these translational issues can be addressed experimentally at this time. Based on reported cellular and anatomical patterns of 5-HT receptor expression on GABAergic and glutamatergic neurons, this laboratory previously hypothesized that vortioxetine treatment would increase the activity of glutamatergic pyramidal neurons via a 5-HT3 receptor-dependent reduction in activity in a subset of GABAergic interneurons in the cortex and hippocampus (Pehrson and Sanchez, 2014, Dale et al., 2016, Pehrson et al., 2016b). The slice electrophysiology data reported here represent the first attempt to directly test this hypothesis by measuring vortioxetine’s effects at 5-HT3 receptor expressing GABAergic interneurons in the CA1 SR. CA1 interneurons responded to the application of 5-HT with either a direct depolarization leading to action potential firing or with a triphasic response consisting of an initial hyperpolarization which was most likely mediated by the activation of 5-HT1A receptors, followed by two temporally district depolarizations and action potential firing. Different responses to 5-HT were most likely due to the heterogeneity of recorded cells since no transgenic markers were used in our study to identify patched neurons. At least 16 subtypes of interneurons have been identified in the hippocampus with different firing properties and functions that express different subsets of 5-HT receptors and are likely to respond to 5-HT in varying ways (Freund and Buzsáki, 1996, Parra et al., 1998, Dale et al., 2016). The key finding presented here is the observation that vortioxetine (20 µM) and the selective 5-HT3 receptor antagonist ondansetron (10 µM) blocked depolarizations induced by either 5-HT (100 µM) or the 5-HT3 receptor agonist mCPBG (20 µM). Importantly, we confirmed the expression of 5-HT3 receptors in these cells by filling each with biocytin and later co-labeling for 5-HT3 receptor mRNA via in situ hybridization.