Antidepressant treatment is associated with epigenetic alterations in the promoter of P11 in a genetic model of depression.

P11 (S100A10) has been associated with the pathophysiology of depression both in human and rodent models. Different types of antidepressants have been shown to increase P11 levels in distinct brain regions and P11 gene therapy was recently proven effective in reversing depressive-like behaviours in mice. However, the molecular mechanisms that govern P11 gene expression in response to antidepressants still remain elusive. In this study we report decreased levels of P11, associated with higher DNA methylation in the promoter region, in the prefrontal cortex of the Flinders Sensitive Line (FSL) genetic rodent model of depression. This hypermethylated pattern was reversed to normal, as indicated by the control line, after chronic administration of escitalopram (a selective serotonin reuptake inhibitor; SSRI). The escitalopram-induced hypomethylation was associated with both an increase in P11 gene expression and a reduction in mRNA levels of two DNA methyltransferases that have been shown to maintain DNA methylation in adult forebrain neurons (Dnmt1 and Dnmt3a). In conclusion, our data further support a role for P11 in depression-like states and suggest that this gene is controlled by epigenetic mechanisms that can be affected by antidepressant treatment.

Role of p11 in cellular and behavioral effects of 5-HT4 receptor stimulation.

p11 (S100A10), a member of a large family of S100 proteins, interacts with serotonin receptor 1B (5-HTR1B), modulates 5-HT1B receptor signal transduction, and is required for antidepressant responses to activation of this receptor. In the current study, we investigated the specificity of the interaction between 5-HTR1B and p11 by screening brain-expressed S100 proteins against serotonin and noradrenergic receptors. The data indicate that p11 is unique among its family members for its interactions with defined serotonin receptors. We identify a novel p11-interacting receptor (5-HTR4) and characterize the interaction between p11 and 5-HTR4, demonstrating that (1) p11 and 5-HTR4 mRNA and protein are coexpressed in brain regions that are relevant for major depression, (2) p11 increases 5-HTR4 surface expression and facilitates 5-HTR4 signaling, and (3) p11 is required for the behavioral antidepressant responses to 5-HTR4 stimulation in vivo. The essential role played by p11 in modulating signaling through 5-HT4 as well as 5-HT1B receptors supports the concept that this protein may be a key determinant of vulnerability to depression.

Antidepressants reverse the attenuation of the neurotrophic MEK/MAPK cascade in frontal cortex by elevated platform stress; reversal of effects on LTP is associated with GluA1 phosphorylation.

Exposure to stress causes dysfunctions in circuits connecting hippocampus and prefrontal cortex (H-PFC). Long term potentiation (LTP) induced in vivo in rats at H-PFC synapses is impaired by acute elevated platform stress in a manner that can be restored by treatment with certain antidepressants. To identify biochemical pathways in rat frontal cortex underlying this stress-mediated impairment of synaptic plasticity, we examined the phosphorylation state of receptors, signaling proteins and transcription factors implicated in neuronal plasticity. Transient changes in the phosphorylation states of Ser217/221-MEK, Thr183/Tyr185-p42MAPK, Thr202/Tyr204-p44MAPK, Thr180/Tyr182-p38MAPK, Thr218/Tyr220-ERK5, Thr308-Akt, Ser63-ATF-1, Ser1303-GluN2B, Tyr490/515-TrkA/B were found. BDNF was down-regulated after elevated platform stress suggesting that it could regulate the MEK/MAPK signaling cascade. Acute treatment with the antidepressants tianeptine and imipramine reversed the stress-induced down-regulation of P-Ser217/221-MEK. However, stress-induced impairment of H-PFC LTP was only restored by acute treatment with tianeptine and not by imipramine. Tianeptine, but not imipramine, increased the phosphorylation of Ser831-GluA1. Altogether, these results indicate that acute elevated platform stress down-regulates a putative BDNF/MEK/MAPK signaling cascade in the frontal cortex in a manner that is reversible by the antidepressants tianeptine and imipramine. Moreover, changes in LTP may be associated with phosphorylation of AMPA receptors and with some specificity for certain antidepressants. Indeed, stress-induced impairment of H-PFC LTP was only restored by acute treatment with tianeptine and not by imipramine. Tianeptine, but not imipramine, increased the phosphorylation of Ser831-GluA1, indicating a potential effect on AMPA receptor phosphorylation being involved in the reversal of LTP.

Biochemical and behavioral evidence for antidepressant-like effects of 5-HT6 receptor stimulation.

The primary action of several antidepressant treatments used in the clinic raises extracellular concentrations of serotonin (5-HT), which subsequently act on multiple 5-HT receptors. The present study examined whether 5-HT6 receptors might be involved in the antidepressant-like effects mediated by enhanced neurotransmission at 5-HT synapses. A selective 5-HT6 receptor antagonist, SB271046, was evaluated for its ability to counteract fluoxetine-induced biochemical and behavioral responses in mice. In addition, biochemical and behavioral effects of the 5-HT6 receptor agonist, 2-ethyl-5-methoxy-N,N-dimethyltryptamine (EMDT), were assessed in mice to ascertain whether enhancement of 5-HT6 receptor-mediated neurotransmission engenders antidepressant-like effects. SB271046 significantly counteracted the stimulatory actions of fluoxetine on cortical c-fos mRNA, phospho-Ser845-GluR1, and in the tail suspension antidepressant assay, whereas it had no effect on these parameters by itself. EMDT increased the phosphorylation states of Thr34-DARPP-32 and Ser845-GluR1, both in brain slices and in the intact brain, which were effects also seen with the antidepressant fluoxetine; as with fluoxetine, these effects were demonstrated to be independent of D1 receptor stimulation. Systemic administration of EMDT increased c-fos mRNA expression in the striatum and cerebral cortex and reduced immobility in the tail suspension test. The antidepressant-like effects of EMDT in the tail suspension test were prevented by SB271046. Our results indicate that 5-HT6 receptor stimulation may be a mechanism initiating some of the biochemical and behavioral outcomes of 5-HT reuptake inhibitors, such as fluoxetine. These findings also indicate that selective 5-HT6 receptor agonists may represent a novel antidepressant drug class.

Involvement of AMPA receptor phosphorylation in antidepressant actions with special reference to tianeptine.

Depression is associated with abnormal neuronal plasticity. AMPA receptors mediate transmission and plasticity at excitatory synapses in a manner which is positively regulated by phosphorylation at Ser831-GluR1, a CaMKII/PKC site, and Ser845-GluR1, a PKA site. Treatment with the selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor fluoxetine increases P-Ser845-GluR1 but not P-Ser831-GluR1. Here, it was found that treatment with another antidepressant, tianeptine, increased P-Ser831-GluR1 in the frontal cortex and the CA3 region of hippocampus and P-Ser845-GluR1 in the CA3 region of hippocampus. A receptorome profile detected no affinity for tianeptine at any monaminergic receptors or transporters, confirming an atypical profile for this compound. Behavioural analyses showed that mice bearing point mutations at both Ser831- and Ser845-GluR1, treated with saline, exhibited increased latency to enter the centre of an open field and increased immobility in the tail-suspension test compared to their wild-type counterparts. Chronic tianeptine treatment increased open-field locomotion and reduced immobility in wild-type mice but not in phosphomutant GluR1 mice. P-Ser133-CREB was reduced in the CA3 region of hippocampus in phosphomutant mice, and tianeptine decreased P-Ser133-CREB in this region in wild-type, but not in phosphomutant, mice. Tianeptine increased P-Ser133-CREB in the CA1 region in wild-type mice but not in phosphomutant GluR1 mice. There were higher basal P-Ser133-CREB and c-fos levels in frontal and cingulate cortex in phosphomutant GluR1 mice; these changes in level were counteracted by tianeptine in a GluR1-independent manner. Using phosphorylation assays and phosphomutant GluR1 mice, this study provides evidence that AMPA receptor phosphorylation mediates certain explorative and antidepressant-like actions under basal conditions and following tianeptine treatment.

Neuropeptidomics of mouse hypothalamus after imipramine treatment reveal somatostatin as a potential mediator of antidepressant effects.

Excessive activation of the hypothalamic-pituitary-adrenal (HPA) axis has been associated with numerous diseases, including depression, and the tricyclic antidepressant imipramine has been shown to suppress activity of the HPA axis. Central hypothalamic control of the HPA axis is complex and involves a number of neuropeptides released from multiple hypothalamic subnuclei. The present study was therefore designed to determine the effects of imipramine administration on the mouse hypothalamus using a peptidomics approach. Among the factors found to be downregulated after acute (one day) or chronic (21 days) imipramine administration were peptides derived from secretogranin 1 (chromogranin B) as well as peptides derived from cerebellin precursors. In contrast, peptides SRIF-14 and SRIF-28 (1-11) derived from somatostatin (SRIF, somatotropin release inhibiting factor) were significantly upregulated by imipramine in the hypothalamus. Because diminished SRIF levels have long been known to occur in depression, a second part of the study investigated the roles of individual SRIF receptors in mediating potential antidepressant effects. SRA880, an antagonist of the somatostatin-1 autoreceptor (sst1) which positively modulates release of endogenous SRIF, was found to synergize with imipramine in causing antidepressant-like effects in the tail suspension test. Furthermore, chronic co-administration of SRA880 and imipramine synergistically increased BDNF mRNA expression in the cerebral cortex. Application of SRIF or L054264, an sst2 receptor agonist, but not L803807, an sst4 receptor agonist, increased phosphorylation of CaMKII and GluR1 in cerebrocortical slices. Our present experiments thus provide evidence for antidepressant-induced upregulation of SRIF in the brain, and strengthen the notion that augmented SRIF expression and signaling may counter depressive-like symptoms. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Tianeptine potentiates AMPA receptors by activating CaMKII and PKA via the p38, p42/44 MAPK and JNK pathways.

Impairments of cellular plasticity appear to underlie the pathophysiology of major depression. Recently, elevated levels of phosphorylated AMPA receptor were implicated in the antidepressant effect of various drugs. Here, we investigated the effects of an antidepressant, Tianeptine, on synaptic function and GluA1 phosphorylation using murine hippocampal slices and in vivo single-unit recordings. Tianeptine, but not imipramine, increased AMPA receptor-mediated neuronal responses both in vitro and in vivo, in a staurosporine-sensitive manner. Paired-pulse ratio was unaltered by Tianeptine, suggesting a postsynaptic site of action. Tianeptine, 10 µM, enhanced the GluA1-dependent initial phase of LTP, whereas 100 µM impaired the latter phases, indicating a critical role of GluA1 subunit phosphorylation in the excitation. Tianeptine rapidly increased the phosphorylation level of Ser(831)-GluA1 and Ser(845)-GluA1. Using H-89 and KN-93, we show that the activation of both PKA and CaMKII is critical in the effect of Tianeptine on AMPA responses. Moreover, the phosphorylation states of Ser(217/221)-MEK and Thr(183)/Tyr(185)-p42MAPK were increased by Tianeptine and specific kinase blockers of the MAPK pathways (PD 98095, SB 203580 and SP600125) prevented the effects of Tianeptine. Overall these data suggest that Tianeptine potentiates several signaling cascades associated with synaptic plasticity and provide further evidence that a major mechanism of action for Tianeptine is to act as an enhancer of glutamate neurotransmission via AMPA receptors.

Monoamines, BDNF, IL-6 and corticosterone in CSF in patients with Parkinson's disease and major depression.

The biochemical basis of major depression (MD) in Parkinson's disease (PD) is largely unknown. To increase our understanding of MD in PD patients, the levels of monoamine metabolites (HVA, 5-HIAA and MHPG), BDNF, orexin-A, IL-6 and corticosterone were examined in cerebrospinal fluid. The analyses were performed in MD patients with (n = 11) and without (n = 12) PD at baseline and after 12 weeks' of treatment with the antidepressant citalopram, and in patients with solely PD (n = 14) at baseline and after 12 weeks. The major findings were that PD patients with MD had significantly lower baseline levels of MHPG, corticosterone and IL-6 when compared to patients with solely MD. In response to citalopram treatment, patients with solely MD exhibited an expected decrease in 5-HIAA and MHPG levels which was not found in PD patients with MD. Moreover, the levels of BDNF and IL-6 were lower in PD patients with MD compared with patients with solely MD after treatment with citalopram. Thus, the biochemical basis and the response to citalopram differ between PD patients with MD and patients with solely MD.

Protection of stress-induced impairment of hippocampal/prefrontal LTP through blockade of glucocorticoid receptors: implication of MEK signaling.

We previously reported that exposure to acute and chronic stress impairs long-term potentiation (LTP) in the hippocampal-prefrontal cortex pathway and showed evidence for a fundamental role of the prefrontal cortex in maladaptive responses to stress. The goal of the current studies was to examine whether blockade of glucocorticosteroid receptors (GR), by mifepristone (a Type II glucocorticoid receptor antagonist), just after exposure to acute stress could prevent stress-induced impairment of prefrontal LTP. We further examine the effects of mifepristone on mitogen-activated protein/ERK kinase (MEK) signaling pathway in the prefrontal cortex. The data show that an acute injection of mifepristone after stress restored the stress-induced blockade of prefrontal LTP and reduction of phospho-Ser217/221-MEK. These findings have significance for the treatment of memory deficits in hypercortisolemic states, such as stress and depression.

Behavioral and biochemical responses to d-amphetamine in MCH1 receptor knockout mice.

The melanin-concentrating hormone (MCH) system is anatomically and functionally interlaced with the mesocorticolimbic dopamine system. Therefore, we investigated whether MCH(1) receptor knockout (KO) mice are more susceptible than wild-type (WT) mice to psychostimulant-induced locomotor stimulation and sensitization, dopamine receptor-mediated phosphorylation events and c-fos expression within the frontal cortex and ventral striatum. MCH(1) receptor KO mice have 20% higher basal locomotor activity, are hypersensitive to the locomotor activating effects of d-amphetamine (1 mg/kg), and develop behavioral sensitization to a regimen of repeated d-amphetamine administration that does not induce sensitization in WT mice. In addition, d-amphetamine-mediated regulation of p44-mitogen activated protein kinase (MAPK) phosphorylation within the frontal cortex was significantly enhanced in MCH(1) receptor KO mice, when compared with WT mice. No significant genotype difference in the effects of d-amphetamine on MAPK phosphorylation events within the ventral striatum, phosphorylation at Ser(897) of the NR1 subunit of the NMDA receptor or Ca(2+) and cyclic AMP response-element binding-protein (CREB) at Ser(133) in the frontal cortex was detected. d-Amphetamine (3 mg/kg) increased c-fos expression within the frontal cortex in MCH(1) receptor KO mice, but not WT mice. There were no d-amphetamine-induced changes in c-fos expression within the ventromedial striatum in KO or WT mice. Overall, MCH(1) receptor KO mice are hypersensitive to the behavioral and molecular effects of the dopaminergic psychostimulant d-amphetamine. Increased frontal cortical MAPK phosphorylation and c-fos expression in MCH(1) receptor KO mice indicates that the MCH(1) receptor may be an important target for treating neuropsychiatric disorders characterized by frontal cortex dysfunction, including depression, attention deficit hyperactivity disorder (ADHD) and schizophrenia.