Regulation of markers of synaptic function in mouse models of depression: chronic mild stress and decreased expression of VGLUT1.

Depression has been linked to failure in synaptic plasticity originating from environmental and/or genetic risk factors. The chronic mild stress model regulates the expression of synaptic markers of neurotransmitter function and associated depressive-like behaviour. Moreover, mice heterozygous for the synaptic vesicle protein vesicular glutamate transporter 1 (VGLUT1), have been proposed as a genetic model of deficient glutamate function linked to depressive-like behaviour. Here, we aimed to identify, in these two experimental models, mechanisms of failure in synaptic plasticity, common to stress and impaired glutamate function. First, we show that chronic mild stress induced a transient decrease of different plasticity markers (VGLUT1, synapsin 1, sinaptophysin, rab3A and activity regulated cytoskeletal protein - Arc) but a long-lasting decrease of the brain derived neurotrophic factor as well as depressive-like behaviour. The immediate early gene Arc was also down-regulated in VGLUT1+/- heterozygous mice. In contrast, an opposite regulation of synapsin 1 was observed. Finally, both models showed a marked increase of cortical Arc response to novelty. Increased Arc response to novelty could be suggested as a molecular mechanism underlying failure to adapt to environmental changes, common to chronic stress and altered glutamate function. Further studies should investigate whether these changes are associated to depressive-like behaviour both in animal models and in depressed patients.

Effects of neonatal stress on markers of synaptic plasticity in the hippocampus: implications for spatial memory.

Early stressful adverse situations may increase the vulnerability to cognitive deficits and psychiatric disorders, such as depression. Maternal separation (MS) has been used as an animal model to study changes in neurochemistry and behavior associated with exposure to early-life stress. This study investigated the effects of neonatal stress (MS) on the expression of synaptic plasticity markers in the hippocampus and a purported relationship to cognitive processes. Spatial learning (Morris water maze) significantly increased the expression of total levels of the neural cell adhesion molecule (NCAM), as well as its three major isoforms (NCAM-120, -140, and -180) both in the control and MS groups. Interestingly, these increases in NCAM expression after learning were lower in MS animals when compared with control rats. MS induced a significant decrease in total levels of NCAM, and specifically, in the NCAM-140 isoform expression. In the hippocampus of MS rats there was a significant decrease in brain-derived neurotrophic factor and synaptophysin mRNA densities. Cell proliferation, measured as BrdU-positive cells, was also decreased in the dentate gyrus of MS rats. Altogether these results suggest that MS can alter normal brain development, providing a potential mechanism by which early environmental stressors may influence vulnerability to show cognitive impairments later in life.

Regulation of serotonin (5-HT) function by a VGLUT1 dependent glutamate pathway.

Unraveling the mechanisms of 5-HT neuron control might provide new insights into depression pathophysiology. In addition to the inhibitory 5-HT1A autoreceptors, cortico-raphe glutamatergic descending pathways are suggested to modulate 5-HT activity in the DRN. Here we studied how decreased VGLUT1 levels in the brain stem affect glutamate regulation of 5-HT function. VGLUT1+/- mice (C57BL/6) and wild type (WT) littermates were used. VGLUT1 expression in the DRN, 5-HT turnover and immuno histochemical analysis of neuronal activity in different areas was studied. Moreover, the functionality of the inhibitory 5-HT1A autoreceptor was assessed using electrophysiological, biochemical and pharmacological approaches. VGLUT1 immunoreactivity was markedly lower in the DRN of the VGLUT1+/- mice and specifically, in the surroundings of GABA and 5-HT cell bodies. These mice showed decreased induced neuronal activity in 5-HT cells bodies and in different forebrain areas, as well as decreased hippocampal cell proliferation and 5-HT turnover. Further, 5-HT1A autoreceptor desensitization was evidenced by electrophysiological studies, GTP-γ-S coupling to 5-HT1A autoreceptor and a lower hypothermic response to 5-HT1A activation. This study shows first time that VGLUT1 dependent glutamate innervation of the DRN could modulate 5-HT function.

Sustained stress-induced changes in mice as a model for chronic depression.

RATIONALE: Major depression is a chronic disabling disorder, often preceded by stress. Despite emerging clinical interest in mechanisms perpetuating episodes of depression and/or establishing increased vulnerability for relapse, little attention has been paid to address these aspects in experimental models. Here, we studied the long-term neuroadaptive effects of chronic mild stress (CMS) as well as the effectiveness of a course of an antidepressant treatment. METHODS: CMS was applied for 6 weeks, and paroxetine was administered from the third week and continued for 2 weeks thereafter. In order to validate our CMS procedure, we first studied short-term (24 h after CMS) hippocampal cell proliferation and neurogenesis, along with anhedonic-like behaviour. Subsequently, we examined the long-term (one month after CMS) anhedonia, hippocampal neurogenesis, the regulation of c-Fos immunoreactivity and neurotransmitter levels in different areas as well as cortical spine density and hippocampal expression of synaptic proteins. RESULTS: CMS induced a decrease in short-term neurogenesis that was fully recovered in the long term. In addition, CMS-induced lasting anhedonia and region-specific changes in neuronal activity (c-Fos immunoreactivity) and neurotransmitter (glutamate and GABA) levels. Repeated paroxetine reverted these effects with the exception of decreased neuronal activity in the dentate gyrus (DG) and GABA levels in the ventral hippocampus. Moreover, CMS downregulated the GAD65 and VGLUT1 expressions. CONCLUSION: This study shows region-specific long-term neurobiological adaptations induced by CMS and residual hippocampal signs after paroxetine treatment. We propose the use of this model to study molecular mechanisms involved in chronic depression and vulnerability for relapse.

Increased vulnerability to depressive-like behavior of mice with decreased expression of VGLUT1.

BACKGROUND: Many studies link depression to an increase in the excitatory-inhibitory ratio in the forebrain. Presynaptic alterations in a shared pathway of the glutamate/gamma-aminobutyric acid (GABA) cycle may account for this imbalance. Evidence suggests that decreased vesicular glutamate transporter 1 (VGLUT1) levels in the forebrain affect the glutamate/GABA cycle and induce helpless behavior. We studied decreased VGLUT1 as a potential factor enhancing a depressive-like phenotype in an animal model. METHODS: Glutamate and GABA synthesis as well as oxidative metabolism were studied in heterozygous mice for the VGLUT1+/- and wildtype. The regulation of neurotransmitter levels, proteins involved in the glutamate/GABA cycle, and behavior by both genotype and chronic mild stress (CMS) were studied. Finally, the effect of chronic imipramine on VGLUT1 control and CMS mice was studied. RESULTS: VGLUT1+/- mice showed increased neuronal synthesis of glutamate; decreased cortical and hippocampal GABA, VGLUT1, and excitatory amino acid transporter 1 (EAAT1) as well as helplessness and anhedonia. CMS induced an increase of glutamate and a decrease of GABA, the vesicular GABA transporter (VGAT), and glutamic acid decarboxylase 65 (GAD65) in both areas and led to upregulation of EAAT1 in the hippocampus. Moreover, CMS induced anhedonia, helplessness, anxiety, and impaired recognition memory. VGLUT1+/- CMS mice showed a combined phenotype (genotype plus stress) and specific alterations, such as an upregulation of VGLUT2 and hyperlocomotion. Moreover, an increased vulnerability to anhedonia and helplessness reversible by chronic imipramine was shown. CONCLUSIONS: These studies highlight a crucial role for decreased VGLUT1 in the forebrain as a biological mediator of increased vulnerability to chronic mild stress.