Depression is a common cause of mortality and morbidity, but the biological bases of the deficits in emotional and cognitive processing remain incompletely understood. Current antidepressant therapies are effective in only some patients and act slowly. We propose an excitatory synapse hypothesis of depression in which chronic stress and genetic susceptibility cause changes in the strength of subsets of glutamatergic synapses at multiple locations, including the prefrontal cortex, hippocampus and nucleus accumbens, leading to a dysfunction of cortico-mesolimbic reward circuitry that underlies many of the symptoms of depression. This hypothesis accounts for current depression treatments and suggests an updated framework for the development of better therapeutic compounds.

Missense mutations in ubiquilin 2 (UBQLN2) cause ALS with frontotemporal dementia (ALS-FTD). Animal models of ALS are useful for understanding the mechanisms of pathogenesis and for preclinical investigations. However, previous rodent models carrying UBQLN2 mutations failed to manifest any sign of motor neuron disease. Here, we show that lines of mice expressing either the ALS-FTD-linked P497S or P506T UBQLN2 mutations have cognitive deficits, shortened lifespans, and develop motor neuron disease, mimicking the human disease. Neuropathologic analysis of the mice with end-stage disease revealed the accumulation of ubiquitinated inclusions in the brain and spinal cord, astrocytosis, a reduction in the number of hippocampal neurons, and reduced staining of TAR-DNA binding protein 43 in the nucleus, with concomitant formation of ubiquitin + inclusions in the cytoplasm of spinal motor neurons. Moreover, both lines displayed denervation muscle atrophy and age-dependent loss of motor neurons that correlated with a reduction in the number of large-caliber axons. By contrast, two mouse lines expressing WT UBQLN2 were mostly devoid of clinical and pathological signs of disease. These UBQLN2 mouse models provide valuable tools for identifying the mechanisms underlying ALS-FTD pathogenesis and for investigating therapeutic strategies to halt disease.A LS is a progressive neurodegenerative disorder associated with loss of upper and lower motor neurons (1, 2). The disease usually manifests in the fifth decade of life, but can occur as early as the late teens. Its hallmark symptoms are progressive muscle weakness, which usually leads to death between 3 and 5 y after first diagnosis. Some patients with ALS also develop frontotemporal dementia (FTD). Genetic findings have linked mutations in different genes to the range of symptoms seen in ALS (3, 4).A common pathologic feature in nearly all ALS cases (∼97%), including all sporadic and most familial cases, is a reduction in TAR-DNA binding protein 43 (TDP-43) in the nucleus and its accumulation in ubiquitin + inclusions in the cytoplasm of spinal motor neurons (5-8). The few exceptions where this pathology is not seen are in ALS cases linked to mutations in the SOD1 and FUS genes (7-10). This has led to speculation that pathogenesis in the vast majority of ALS cases may be mechanistically linked directly or indirectly to TDP-43 pathology (7, 11). Intriguingly, TDP-43 pathology is also a common hallmark of certain forms of FTD where the pathology is found in the brain (5,7,12).Missense mutations (P497H, P497S, P506T, P509S, or P525S) in ubiquilin 2 (UBQLN2) were identified as the cause of X-linked dominant ALS-FTD (13). The afflicted individuals had abnormal inclusions in neurons of the hippocampus and TDP-43 pathology in spinal motor neurons. Additional UBQLN2 mutations have now been identified, and interestingly, like the original mutations, encode missense mutations in the central domain of UBQLN2 protein (14-16). The function of the central domain of UBQLN2 is beginning to em...

SUMMARY Synaptic transmission is bioenergetically demanding, and the diverse processes underlying synaptic plasticity elevate these demands. Therefore, mitochondrial functions including ATP synthesis and Ca2+ handling, are likely essential for plasticity. Although axonal mitochondria have been extensively analyzed, LTP is predominantly induced postsynaptically, where mitochondria are understudied. Additionally, though mitochondrial fission is essential for their function, signaling pathways that regulate fission in neurons remain poorly understood. We found that NMDAR-dependent LTP induction prompted a rapid burst of dendritic mitochondrial fission, and elevations of mitochondrial matrix Ca2+. The fission burst was triggered by cytosolic Ca2+ elevation, and required CaMKII, actin, and Drp1, as well as dynamin 2. Preventing fission impaired mitochondrial matrix Ca2+ elevations, structural LTP in cultured neurons, and electrophysiological LTP in hippocampal slices. These data illustrate a novel pathway whereby synaptic activity controls mitochondrial fission, and show that dynamic control of fission regulates plasticity induction perhaps by modulating mitochondrial Ca2+ handling.

Selective serotonin reuptake inhibitors (SSRIs) are the primary pharmacological treatment for depression, but SSRIs are effective in only half of the patients and typically take several weeks to relieve symptoms. The NMDA receptor antagonist ketamine exerts a rapid antidepressant action, but has troubling side effects. We hypothesized that negative allosteric modulators of GABA A receptors would exert similar effects on brain activity as ketamine, but would not exert as many side effects if targeted only to GABA A receptors containing α5 subunits, which are enriched in the hippocampus and prefrontal cortex. Here, we show that the α5-selective negative modulator L-655,708 reversed the alterations in hedonic behavior in the sucrose preference and social interaction tests produced by two different chronic stress paradigms in rats within 24 h of systemic administration. Similar effects were observed with another α5-selective negative modulator, MRK-016. L-655,708 had no effect on hedonic or open-field behavior in unstressed animals. Within 24 h, L-655,708 injection also restored the strength of pathologically weakened excitatory synaptic transmission at the stress-sensitive temporoammonic-CA1 synapse, measured electrophysiologically, and increased levels of the GluA1 subunit of the AMPA receptor, measured with western blotting. We suggest that the ability of L-655,708 to restore excitatory synaptic strength rapidly may underlie its ability to restore stressinduced behavioral alterations rapidly, supporting evidence that dysfunction of multiple excitatory synapses in cortico-mesolimbic reward pathways contributes, in part, to the genesis of depression. Negative allosteric modulators of α5 subunit-containing GABA A receptors represent a promising novel class of fast-acting and clinically viable antidepressant compounds.

Depression is a leading cause of mortality and morbidity. Selective serotonin reuptake inhibitors, such as fluoxetine, are the most commonly prescribed antidepressant medication. SSRIs produce their therapeutic effects by elevating extracellular concentrations of serotonin. Although this elevation occurs rapidly, there is a paradoxical delay of weeks-to-months of continuous treatment before most patients experience meaningful relief of their depressive symptoms. Here, we address the effects of chronic fluoxetine treatment and prolonged elevation of serotonin in the rat hippocampus. Previous work has shown that acute administration of fluoxetine rapidly potentiates the excitatory temporoammonic synapse onto CA1 pyramidal cells in the hippocampus via activation of serotonin 1B receptor in brain slices. In contrast to observations in brain slices, we report here that prolonged administration of fluoxetine was required to produce significant changes in temporoammonic-CA1 synaptic strength in ex vivo brain slices. Evidence of potentiation included increases in the ratio of AMPA receptor-to NMDA receptor-mediated temporoammonic-CA1 synaptic responses, occlusion of electrically evoked long-term potentiation, enhanced long-term depression, impaired anpirtoline-mediated potentiation, and impaired memory recall in the Morris water maze task. These synaptic and behavioral changes coincided with the alleviation of anhedonic behavioral state. We conclude that the effects of elevated serotonin accumulate slowly in vivo and may account for the delay to relief of depressive 6