Abnormal 5-HT modulation of stress behaviors in the Kv4.2 knockout mouse

The signaling proteins GPR158 and RGS7 modulate excitability of L2/3 pyramidal neurons and control A-type potassium channel in the prelimbic cortex

Stress profoundly affects physiological properties of neurons across brain circuits and thereby increases the risk for depression. However, the molecular and cellular mechanisms mediating these effects are poorly understood. In this study, we report that chronic physical restraint stress in mice decreases excitability specifically in layer 2/3 of pyramidal neurons within the prelimbic subarea of the prefrontal cortex (PFC) accompanied by the induction of depressive-like behavioral states. We found that a complex between G protein-coupled receptor (GPCR) 158 (GPR158) and regulator of G protein signaling 7 (RGS7), a regulatory GPCR signaling node recently discovered to be a key modulator of affective behaviors, plays a key role in controlling stress-induced changes in excitability in this neuronal population. Deletion of GPR158 or RGS7 enhanced excitability of layer 2/3 PFC neurons and prevented the impact of stress. Investigation of the underlying molecular mechanisms revealed that the A-type potassium channel Kv4.2 subunit is a molecular target of the GPR158-RGS7 complex. We further report that GPR158 physically associates with Kv4.2 channel and promotes its function by suppressing inhibitory modulation by cAMP-protein kinase A (PKA)-mediated phosphorylation. Taken together, our observations reveal a critical mechanism that adjusts neuronal excitability in L2/3 pyramidal neurons of the PFC and may thereby modulate the effects of stress on depression.

Chemical Constituents and Antidepressant-Like Effects in Ovariectomized Mice of the Ethanol Extract of Alternanthera philoxeroides

The previously unreported flavone glycoside, demethyltorosaflavone B (2) and the E-propenoic acid substituted flavone, torosaflavone E (3a), were isolated together with nine previously reported metabolites, including indole-3-carbaldehyde, oleanonic acid, vanillic acid, p-hydroxybenzoic acid, altheranthin (1a), alternanthin B (1b), demethyltorosaflavone D (3b), luteolin 8-C-E-propenoic acid (4) and chrysoeriol 7-O-rhamnoside (5), from the ethanol extract of the aerial part of Althernanthera philoxeroides. The crude ethanol extract was evaluated for its in vitro estrogenic activity in MCF-7 breast cancer cell line. The crude ethanol extract was also investigated in vivo for its antidepressant-like effects on ovariectomized mice using tail suspension and forced swimming tests, while its effect on the locomotor activity was evaluated by a Y-maze test. The effect of the crude extract on the serum corticosterone level, size and volume of uterus of the ovariectomized mice were also investigated. The expression of the mouse cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB), brain-derived neurotrophic factor (BDNF) and β-actin mRNAs in hippocampus and frontal cortex was also evaluated, using semiquantitative reverse transcription-polymerase chain reaction. The crude extract and the isolated compounds 1a, 1b, 3a, 3b and 5, were evaluated for their inhibitory effects on monoamine oxidases (MAOs)-A and -B.

Effects of fluoxetine on protein expression of potassium ion channels in the brain of chronic mild stress rats

The purpose of this study is to investigate the expression of major potassium channel subtypes in the brain of chronical mild stress (CMS) rats and reveal the effects of fluoxetine on the expression of these channels. Rats were exposed to a variety of unpredictable stress for three weeks and induced anhedonia, lower sucrose preference, locomotor activity and lower body weight. The protein expressions were determined by Western blot. CMS significantly increased the expression of Kv2.1 channel in frontal cortex but not in hippocampus, and the expression level was normalized after fluoxetine treatment. The expression of TREK-1 channel was also obviously increased in frontal cortex in CMS rats. Fluoxetine treatment might prevent this increase. However, the expression of Kv3.1 and Kv4.2 channels was considerably decreased in hippocampus after CMS, and was not affected by fluoxetine. These results suggest that different subtypes of potassium channels are associated with the pathophysiology of depression and that the therapeutical effects of fluoxetine may relate to Kv2.1 and TREK-1 potassium channels.

Distribution and functional expression of Kv4 family α subunits and associated KChIP β subunits in the bed nucleus of the stria terminalis

Regulation of BNSTALG neuronal firing activity is tightly regulated by the opposing actions of the fast outward potassium current, IA , mediated by α subunits of the Kv4 family of ion channels, and the transient inward calcium current, IT . Together, these channels play a critical role in regulating the latency to action potential onset, duration, and frequency, as well as dendritic back-propagation and synaptic plasticity. Previously we have shown that Type I-III BNSTALG neurons express mRNA transcripts for each of the Kv4 α subunits. However, the biophysical properties of native IA channels are critically dependent on the formation of macromolecular complexes of Kv4 channels with a family of chaperone proteins, the potassium channel-interacting proteins (KChIP1-4). Here we used a multidisciplinary approach to investigate the expression and function of Kv4 channels and KChIPs in neurons of the rat BNSTALG . Using immunofluorescence we demonstrated the pattern of localization of Kv4.2, Kv4.3, and KChIP1-4 proteins in the BNSTALG . Moreover, our single-cell reverse-transcription polymerase chain reaction (scRT-PCR) studies revealed that mRNA transcripts for Kv4.2, Kv4.3, and all four KChIPs were differentially expressed in Type I-III BNSTALG neurons. Furthermore, immunoelectron microscopy revealed that Kv4.2 and Kv4.3 channels were primarily localized to the dendrites and spines of BNSTALG neurons, and are thus ideally situated to modulate synaptic transmission. Consistent with this observation, in vitro patch clamp recordings showed that reducing postsynaptic IA in these neurons lowered the threshold for long-term potentiation (LTP) induction. These results are discussed in relation to potential modulation of IA channels by chronic stress.

Reduced phosphorylation of GluA1 subunits relates to anxiety-like behaviours in mice

Anxiety and depression are highly prevalent and frequently co-morbid conditions. The ionotropic glutamate receptors N-methyl-D-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) mediate actions of monoaminergic antidepressants and have been directly targeted by novel fast-acting antidepressants. Less is known about the role of these receptors in anxiety-like states. Here we investigate how two distinct anxiolytic agents, buspirone, a partial 5-HT(1A) agonist, and diazepam, a benzodiazepine, influence phosphorylation of GluA1 subunits of AMPA receptors at the potentiating residue Ser(845) and Ser(831) in corticolimbic regions. To test the functional relevance of these changes, phosphomutant GluA1 mice lacking phosphorylatable Ser(845) and Ser(831) were examined in relevant behavioural paradigms. These mutant mice exhibited a reduced anxiety-like phenotype in the light/dark exploration task and elevated plus maze, but not in the novelty induced hypophagia paradigm. These data indicate that reduced potentiation of the AMPA receptor signalling, via decreased GluA1 phoshorylation, is specifically involved in approach-avoidance based paradigms relevant for anxiety-like behaviours.

Head movement: a novel serotonin-sensitive behavioral endpoint for tail suspension test analysis

The tail suspension test (TST) as an antidepressant and depression-related behavior screen, has many advantages over the forced swim test (FST) in terms of procedural simplicity and consistent SSRI response. However, the FST has traditionally offered more specific neuromodulatory information by differentiating between serotonin (5-HT) and norepinephrine sensitive behavior categories. Head movement is a newly characterized behavior endpoint in the FST and TST with a selective 5-HT sensitivity. In this investigation, we show that the baseline and drug response profile of head movement previously found in the 129S6 strain of mice (Lockridge et al., 2010) is reproducible in the C57 strain. Head movement is inversely correlated to FST swimming and elevated in the TST by SSRI administration. The use of a weighted bin sample analysis method differentiates TST behaviors into fluoxetine-responsive head movement and desipramine-responsive struggling. The use of 5-HT subtype receptor agonists, after depleting endogenous 5-HT with pCPA, shows the head movement suppressing effect of 5-HT2A and 5-HT2C postsynaptic receptor activation. 5-HT1A and 5-HT1B agonists were ineffective. We propose that a head movement focused analysis can add sensitive and reliable 5-HT detection capability to mouse TST testing with minimal effort but significant reward.

The genetics of selective serotonin reuptake inhibitors

Selective serotonin reuptake inhibitors (SSRIs) are among the most widely prescribed drugs in psychiatry. Based on the fact that SSRIs increase extracellular monoamine levels in the brain, the monoamine hypothesis of depression was introduced, postulating that depression is associated with too low serotonin, dopamine and noradrenaline levels. However, several lines of evidence indicate that this hypothesis is too simplistic and that depression and the efficacy of SSRIs are dependent on neuroplastic changes mediated by changes in gene expression. Because a coherent view on global gene expression is lacking, we aim to provide an overview of the effects of SSRI treatment on the final targets of 5-HT receptor signal transduction pathways, namely the transcriptional regulation of genes. We address gene polymorphisms in humans that affect SSRI efficacy, as well as in vitro studies employing human-derived cells. We also discuss the molecular targets affected by SSRIs in animal models, both in vivo and in vitro. We conclude that serotonin transporter gene variation in humans affects the efficacy and side-effects of SSRIs, whereas SSRIs generally do not affect serotonin transporter gene expression in animals. Instead, SSRIs alter mRNA levels of genes encoding serotonin receptors, components of non-serotonergic neurotransmitter systems, neurotrophic factors, hypothalamic hormones and inflammatory factors. So far little is known about the epigenetic and age-dependent molecular effects of SSRIs, which might give more insights in the working mechanism(s) of SSRIs.

Rhythm and blues: animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

Kv4 potassium channels modulate hippocampal EPSP-spike potentiation and spatial memory in rats

Kv4 channels regulate the backpropagation of action potentials (b-AP) and have been implicated in the modulation of long-term potentiation (LTP). Here we showed that blockade of Kv4 channels by the scorpion toxin AmmTX3 impaired reference memory in a radial maze task. In vivo, AmmTX3 intracerebroventricular (i.c.v.) infusion increased and stabilized the EPSP-spike (E-S) component of LTP in the dentate gyrus (DG), with no effect on basal transmission or short-term plasticity. This increase in E-S potentiation duration could result from the combination of an increase in excitability of DG granular cells with a reduction of GABAergic inhibition, leading to a strong reduction of input specificity. Radioactive in situ hybridization (ISH) was used to evaluate the amounts of Kv4.2 and Kv4.3 mRNA in brain structures at different stages of a spatial learning task in naive, pseudoconditioned, and conditioned rats. Significant differences in Kv4.2 and Kv4.3 mRNA levels were observed between conditioned and pseudoconditioned rats. Kv4.2 and Kv4.3 mRNA levels were transiently up-regulated in the striatum, nucleus accumbens, retrosplenial, and cingulate cortices during early stages of learning, suggesting an involvement in the switch from egocentric to allocentric strategies. Spatial learning performance was positively correlated with the levels of Kv4.2 and Kv4.3 mRNAs in several of these brain structures. Altogether our findings suggest that Kv4 channels could increase the signal-to-noise ratio during information acquisition, thereby allowing a better encoding of the memory trace.

Effects of genetic deletion of the Kv4.2 voltage-gated potassium channel on murine anxiety-, fear- and stress-related behaviors

Background

Potassium channels have been proposed to play a role in mechanisms of neural plasticity, and the Kv4.2 subunit has been implicated in the regulation of action-potential back-propagation to the dendrites. Alterations in mechanisms of plasticity have been further proposed to underlie various psychiatric disorders, but the role of Kv4.2 in anxiety or depression is not well understood.

Methods

In this paper, we analyzed the phenotype Kv4.2 knockout mice based on their neurological function, on a battery of behaviors including those related to anxiety and depression, and on plasticity-related learning tasks.

Results

We found a novelty-induced hyperactive phenotype in knockout mice, and these mice also displayed increased reactivity to novel stimulus such as an auditory tone. No clear anxiety- or depression-related phenotype was observed, nor any alterations in learning/plasticity-based paradigms.

Conclusions

We did not find clear evidence for an involvement of Kv4.2 in neuropsychiatric or plasticity-related phenotypes, but there was support for a role in Kv4.2 in dampening excitatory responses to novel stimuli.