BDNF variant Val66Met interacts with estrous cycle in the control of hippocampal function

Fear extinction and BDNF: translating animal models of PTSD to the clinic

Brain-derived neurotrophic factor (BDNF) is the most studied neurotrophin involved in synaptic plasticity processes that are required for long-term learning and memory. Specifically, BDNF gene expression and activation of its high-affinity tropomyosin-related kinase B (TrkB) receptor are necessary in the amygdala, hippocampus and prefrontal cortex for the formation of emotional memories, including fear memories. Among the psychiatric disorders with altered fear processing, there is post-traumatic stress disorder (PTSD) which is characterized by an inability to extinguish fear memories. Since BDNF appears to enhance extinction of fear, targeting impaired extinction in anxiety disorders such as PTSD via BDNF signalling may be an important and novel way to enhance treatment efficacy. The aim of this review is to provide a translational point of view that stems from findings in the BDNF regulation of synaptic plasticity and fear extinction. In addition, there are different systems that seem to alter fear extinction through BDNF modulation like the endocannabinoid system and the hypothalamic-pituitary adrenal axis. Recent work also finds that the pituitary adenylate cyclase-activating polypeptide and PAC1 receptor, which are upstream of BDNF activation, may be implicated in PTSD. Especially interesting are data that exogenous fear extinction enhancers such as antidepressants, histone deacetylases inhibitors and D-cycloserine, a partial N-methyl d-aspartate agonist, may act through or in concert with the BDNF-TrkB system. Finally, we review studies where recombinant BDNF and a putative TrkB agonist, 7,8-dihydroxyflavone, may enhance extinction of fear. These approaches may lead to novel agents that improve extinction in animal models and eventually humans.

Reduced aging defects in estrogen receptive brainstem nuclei in the female hamster

The nucleus pararetroambiguus (NPRA) and the commissural nucleus of the solitary tract (NTScom) show estrogen nuclear receptor-α immunoreactivity (nuclear ER-α-IR). Both cell groups are involved in estrous cycle related adaptations. We examined in normally cycling aged hamsters the occurrence/amount/frequency of age-related degenerative changes in NPRA and NTScom during estrus and diestrus. In 2640 electron microscopy photomicrographs plasticity reflected in the ratio of axon terminal surface/dendrite surface (t/d) was morphometrically analyzed. Medial tegmental field (mtf, nuclear ER-α-IR poor), served as control. In aged animals, irrespective of nuclear ER-α-IR+ or nuclear ER-α-IR- related cell groups, extensive diffuse degenerative structural aberrations were observed. The hormonal state had a strong influence on t/d ratios in NPRA and NTScom, but not in mtf. In NPRA and NTScom, diestrous hamsters had significantly smaller t/d ratios (NPRA, 0.750 ± 0.050; NTScom, 0.900 ± 0.039) than the estrous hamsters (NPRA, 1.083 ± 0.075; NTScom, 1.204 ± 0.076). Aging affected axodendritic ratios only in mtf (p < 0.001).

IN CONCLUSION

in the female hamster brain, estrous cycle-induced structural plasticity is preserved in NPRA and NTScom during aging despite the presence of diffuse age-related neurodegenerative changes.

Signaling pathways underlying the pathophysiology and treatment of depression: novel mechanisms for rapid-acting agents

Basic and clinical studies demonstrate that stress and depression are associated with atrophy and loss of neurons and glia, which contribute to the decreased size and function of limbic brain regions that control mood and depression, including the prefrontal cortex and hippocampus. Here, we review findings that suggest that opposing effects of stress and/or depression and antidepressants on neurotrophic factor expression and signaling partly explain these effects. We also discuss recent reports that suggest a possible role for glycogen synthase kinase 3 and upstream wingless (Wnt)-frizzled (Fz) signaling pathways in mood disorders. New studies also demonstrate that the rapid antidepressant actions of NMDA receptor antagonists are associated with activation of glutamate transmission and induction of synaptogenesis, providing novel targets for a new generation of fast-acting, more efficacious therapeutic agents.

Postpartum depressive symptoms and the BDNF Val66Met functional polymorphism: effect of season of delivery

Postpartum depression (PPD) is an often underdiagnosed and undertreated mood disorder, with negative impact on the mother's and infant's health. Seasonal variation has been discussed as a risk factor for PPD. Candidate genes, such as those encoding for the brain-derived neurotrophic factor (BDNF), serotonin transporter (5-HTT), and Period2 (PER2), have been associated with depression and seasonal disorders. The present study is aimed to examine whether functional polymorphic variants, BDNF Val66Met, 5-HTTLPR, or PER2 SNP 10870, are associated with PPD symptoms and whether these genetic polymorphisms interact with season in predicting PPD symptoms. This case-control study comprised of 275 women from a population-based cohort of delivering women in Sweden, who completed a questionnaire containing the Edinburgh postnatal depression scale (EPDS) at 6 weeks and 6 months postpartum. Stressful life events (SLEs) and maternity stressors were also assessed. The results did not reveal any statistically significant overall association between the studied genetic polymorphisms and PPD symptoms. However, a significant association between BDNF Met66 carrier status and development of PPD symptoms at 6 weeks postpartum, even when controlling for prepartum and postpartum environmental risk factors, was evident among mothers delivering during autumn/winter. No gene-gene interactions were found but a cumulative effect was detected with carriers of a greater number of 5-HTTLPR S and BDNFVal66Met Met alleles reporting higher EPDS scores, if delivered during autumn/winter. Our findings propose a role of the BDNF gene in the development of PPD symptoms, potentially mediated by season of delivery.

Estradiol acts via estrogen receptors alpha and beta on pathways important for synaptic plasticity in the mouse hippocampal formation

Estradiol affects hippocampal-dependent spatial memory and underlying structural and electrical synaptic plasticity in female mice and rats. Using estrogen receptor (ER) alpha and beta knockout mice and wild-type littermates, we investigated the role of ERs in estradiol effects on multiple pathways important for hippocampal plasticity and learning. Six hours of estradiol administration increased immunoreactivity for phosphorylated Akt throughout the hippocampal formation, whereas 48 h of estradiol increased immunoreactivity for phosphorylated TrkB receptor. Estradiol effects on phosphorylated Akt and TrkB immunoreactivities were abolished in ER alpha and ER beta knockout mice. Estradiol also had distinct effects on immunoreactivity for post-synaptic density 95 (PSD-95) and brain derived-neurotrophic factor (BDNF) mRNA in ER alpha and beta knockout mice. Thus, estradiol acts through both ERs alpha and beta in several subregions of the hippocampal formation. The different effects of estradiol at 6 and 48 h indicate that several mechanisms of estrogen receptor signaling contribute to this female hormone's influence on hippocampal synaptic plasticity. By further delineating these mechanisms, we will better understand and predict the effects of endogenous and exogenous ovarian steroids on mood, cognition, and other hippocampal-dependent behaviors.

Distribution of phosphorylated TrkB receptor in the mouse hippocampal formation depends on sex and estrous cycle stage

Tropomyosin-related kinase B receptor (TrkB) is a neurotrophin receptor important for the synaptic plasticity underlying hippocampal-dependent learning and memory. Because this receptor is widely expressed in hippocampal neurons, the precise location of TrkB activation is likely important for its specific actions. The goal of this study was to identify the precise sites of TrkB activation in the mouse hippocampal formation and to determine any changes in the distribution of activated TrkB under conditions of enhanced brain-derived neurotrophic factor (BDNF) expression and hippocampal excitability. Using electron microscopy, we localized TrkB phosphorylated at tyrosine 816 (pTrkB) in the hippocampal formation of male and female mice under conditions of naturally low circulating estradiol and naturally high circulating estradiol, when BDNF expression, TrkB signaling, and synaptic plasticity are enhanced. To compare relative amounts of pTrkB in each group, we counted profiles containing pTrkB-immunoreactivity (pTrkB-ir) in all hippocampal subregions. pTrkB-ir was in axons, axon terminals, dendrites, and dendritic spines of neurons in the hippocampal formation, but the majority of pTrkB-ir localized to presynaptic profiles. pTrkB-ir also was abundant in glial profiles, which were further identified as microglia using immunofluorescence and confocal microscopy. Axonal and glial pTrkB-ir and pTrkB-ir in the CA1 stratum radiatum were more abundant in high-estradiol states (proestrus females) than low-estradiol states (estrus and diestrus females and males). These findings suggest that presynaptic TrkB is positioned to modulate estradiol-mediated and BDNF-dependent synaptic plasticity. Furthermore, they suggest a novel role for TrkB in microglial function in the neuroimmune system.

5-HTTLPR and BDNF Val66Met polymorphisms moderate effects of stress on rumination

This study examined whether polymorphisms in the serotonin transporter (SLC6A4, 5-HTTLPR) and brain-derived neurotropic factor (BDNF Val66Met, rs6265) genes moderate the relationship between life stress and rumination. Participants were a large homogenous group of healthy, unmedicated, never depressed individuals with few current symptoms of depression (N = 273). Results indicate that individuals with two short (S) alleles of the 5-HTTLPR polymorphism or two Met alleles of the BDNF Val66Met polymorphism ruminate more under conditions of life stress, compared to the other genotypes. Moreover, the accumulation of risk alleles (i.e. S and Met alleles) across genes is associated with significantly greater rumination in the context of life stress. These results suggest that both 5-HTTLPR and BDNF Val66Met moderate the relationship between life stress and rumination. These findings support the notion that variation in these genes is associated with biological sensitivity to the negative effects of stress.

Variant brain-derived neurotrophic factor Val66Met endophenotypes: implications for posttraumatic stress disorder

Recently, a common single nucleotide polymorphism (SNP) has been identified in the gene encoding brain-derived neurotrophic factor (BDNF). The variant BDNF(Met) has been shown to have decreased activity-dependent BDNF secretion from neurons and to lead to impairments in specific forms of learning and altered susceptibility to stress. A mouse model containing BDNF(Met) has also been linked to increased anxiety-like behavior. In a translational study, mice and human carriers of the BDNF(Met) allele were compared in their ability to extinguish a learned fear memory. Both showed slower suppression of the learned fear response. In humans, the neural correlates of this behavior were validated using fMRI. As anxiety and fear extinction lie at the core of symptoms and therapeutic approaches to posttraumatic stress disorder (PTSD), we propose that BDNF genotype and neuroimaging may be useful as biomarkers to provide guidance for more customized therapeutic directions. The aim of this paper is to review the available knowledge on the BDNF Val66Met SNP, with emphasis on anxiety- and fear-related endophenotypes and its potential implications for PTSD.