HT modulates cortical and hippocampal pyramidal cell functio

5-HT modulates cortical and hippocampal pyramidal cell function in a very complex manner, since most 5-HT receptor subtypes (excitatory as well as inhibitory) are expressed on pyramidal hydroxylase inhibitor as well as GABAergic interneurons that exert inhibitory control over pyramidal cells (Leiser et al., 2015, Dale et al., 2016). The implication of these opposite directed actions of 5-HT is that drugs that overall increase 5-HT tone (e.g. SSRIs) in general shows a neutral net effect on pyramidal cell function. However, the 5-HT3 receptor may be an exception, as it is almost exclusively expressed on a subpopulation of interneurons (Morales et al., 1996, Morales and Bloom, 1997, Bloom and Morales, 1998). Thus, activation of 5-HT3 receptors will increase GABAergic transmission in a subset of interneurons. Therefore, drugs that are 5-HT3 receptor antagonists, such as vortioxetine, may potentially attenuate the inhibitory control that this subset of interneurons exert on pyramidal cell output and thereby enhance glutamatergic signaling. In support of this notion, vortioxetine was found to be superior to an SSRI in promoting glutamate-dependent neuronal plasticity including measures such as long-term potentiation (Dale et al., 2014), cell proliferation in the hippocampal dentate gyrus (Betry et al., 2015), hippocampal CA1 dendritic remodeling (spine density, dendritic length and branching) (Chen et al., 2016) and maturation of dendritic spines (Waller et al., 2016). Interneurons, which exert the main inhibitory control of the brain, comprise an enormously diverse group of cells in terms of morphology and connectivity, molecular and physiological properties and different classification criteria have been applied by different investigators (Ascoli et al., 2008, Kepecs and Fishell, 2014). A significant subset of interneurons is characterized by expression of 5-HT3 receptors, but presently the knowledge about morphology, connectivity, molecular and physiological properties of 5-HT3 receptor expressing interneurons is limited. Cortical interneurons expressing 5-HT3 receptor mRNA are thought to express the calcium-binding proteins calbindin and calretinin, but not parvalbumin (Morales and Bloom, 1997, Puig et al., 2004). Furthermore, 5-HT3 receptor-expressing neurons in the rat cortex, hippocampus, and amygdala are reported to often show cholecystokinin immunoreactivity (Bloom and Morales, 1998). The present study is the first attempt to directly test the hypothesis that vortioxetine reduces GABAergic transmission in a subset of 5-HT3 receptor expressing interneurons. To that end, we investigated the effects of vortioxetine and serotonergic tool compounds on the activity of CA1 Stratum Radiatum (SR) interneurons in rat hippocampus slices using current- and voltage-clamping methods and subsequently phenotyped the biocytin filled patched neurons. Furthermore, since there is not enough information to infer which amino acid residues that underlie putative differential responses between species, we assessed vortioxetine’s functional effects across several species to get a notion of translatability of our rat data (i.e. canine, mice, rat, guinea pig and human) in cellular assays with heterologous expression of 5-HT3A receptors (i.e. in Xenopus laevis oocytes and in mammalian HEK-293 cells) and in mouse neuroblastoma N1E-115 cells with endogenous expression of 5-HT3A receptors.
Results
Discussion The data presented here serve to further elucidate vortioxetine’s key 5-HT3 receptor-dependent effects. First, in oocytes expressing 5-HT3 receptors we demonstrated that vortioxetine and the selective 5-HT3 receptor antagonist ondansetron functionally antagonize 5-HT-induced currents, and further that vortioxetine had a slow apparent off rate in this assay by comparison to ondansetron. Additionally, we found that vortioxetine’s potency as a functional antagonist at 5-HT3 receptors varied considerably with the species variant expressed in this cellular assay, while ondansetron’s potency in this assay was similar across all tested 5-HT3 receptor variants. In 5-HT3 receptor-expressing GABAergic interneurons from the CA1 SR, we found that vortioxetine and ondansetron blocked depolarizations induced by superfusion of either 5-HT or the 5-HT3 receptor agonist mCPBG. Taken together, these data add to a growing literature (see introduction) supporting the idea that vortioxetine can decrease GABAergic neurotransmission in some brain regions via a 5-HT3 receptor antagonism-dependent mechanism.