A large number of studies have demonstrated that the nucleus accumbens (NAC) is a critical site in the neuronal circuits controlling reward responses, motivation, and mood, but the neuronal cell type(s) underlying these processes are not yet known. Identification of the neuronal cell types that regulate depression-like states will guide us in understanding the biological basis of mood and its regulation by diseases like major depressive disorder. Taking advantage of recent findings demonstrating that the serotonin receptor chaperone, p11, is an important molecular regulator of depression-like states, here we identify cholinergic interneurons (CINs) as a primary site of action for p11 in the NAC. Depression-like behavior is observed in mice after decrease of p11 levels in NAC CINs. This phenotype is recapitulated by silencing neuronal transmission in these cells, demonstrating that accumbal cholinergic neuronal activity regulates depression-like behaviors and suggesting that accumbal CIN activity is crucial for the regulation of mood and motivation.he nucleus accumbens (NAC) has been identified as a critical site in the neuronal circuits controlling reward responses, motivation, and mood (1-3). For example, in major depressive disorder (MDD): (i) NAC activity is reduced in response to positive stimuli (4) and (ii) MDD symptoms can be reversed by deep brain stimulation in the NAC (5, 6). Although these regional studies implicate the NAC, the cell type(s) responsible for the pathophysiology remain to be elucidated. In the case of MDD, it is crucial that we determine which neuronal cell types regulate depressive-like behaviors to eventually develop highly targeted antidepressants that could be both faster acting and have fewer off-site effects. MDD is a debilitating psychiatric condition characterized by anhedonia (defined as a loss of interest in pleasurable things), loss of motivation, negative affect, behavioral despair, and changes in cognition and basic drives such as eating and sleeping. Despite its complexity, central features of the disease, such as anhedonia and behavioral despair, can be modeled in rodents to probe the underlying neuronal circuitry.The serotonin receptor (5HTR) interacting protein p11 is a small intracellular protein that regulates the localization of certain 5HTRs, including 5HTR1B and 5HTR4 (7,8), at the cellular surface. Constitutive p11 knockout (KO) mice show both anhedonia (e.g., reduced sucrose preference) and increased behavioral despair (e.g., increased immobility) (7-9). Recently, we identified the NAC as a key brain region in which loss of p11 induces depression-like behaviors (9). The NAC is composed of several different cell types (10). Approximately 95% of the neurons in this region are medium spiny neurons (MSNs), the projection neurons of the NAC. The cholinergic interneurons have a characteristically large soma and make up less than 1% of the neurons in the striatum (10). Cholinergic interneurons primarily target MSNs via muscarinic and nicotinic acetylcholine receptors, a...

The etiology of major depression remains unknown, but dysfunction of serotonergic signaling has long been implicated in the pathophysiology of this disorder. p11 is an S100 family member recently identified as a serotonin 1B (5-HT 1B ) and serotonin 4 (5-HT 4 ) receptor binding protein. Mutant mice in which p11 is deleted show depression-like behaviors, suggesting that p11 may be a mediator of affective disorder pathophysiology. Using somatic gene transfer, we have now identified the nucleus accumbens (NAcc) as a key site of p11 action. Reduction of p11 with adeno-associated virus (AAV)-mediated RNA interference (RNAi) in the NAcc, but not in the anterior cingulate, of normal adult mice resulted in depression-like behaviors nearly identical to those seen in p11 knockout mice. Restoration of p11 expression specifically in the NAcc of p11 knockout mice normalized depression-like behaviors. Human NAcc tissue shows a significant reduction of p11 protein in depressed patients when compared to matched healthy controls. These results suggest that p11 loss in rodent and human NAcc may contribute to the pathophysiology of depression. Normalization of p11 expression within this brain region with AAV-mediated gene therapy may be of therapeutic value.

Neurotrophins and glucocorticoids are robust synaptic modifiers, and deregulation of their activities is a risk factor for developing stress-related disorders. Low levels of brain-derived neurotrophic factor (BDNF) increase the desensitization of glucocorticoid receptors (GR) and vulnerability to stress, whereas higher levels of BDNF facilitate GR-mediated signaling and the response to antidepressants. However, the molecular mechanism underlying neurotrophic-priming of GR function is poorly understood. Here we provide evidence that activation of a TrkB-MAPK pathway, when paired with the deactivation of a GR-protein phosphatase 5 pathway, resulted in sustained GR phosphorylation at BDNF-sensitive sites that is essential for the transcription of neuronal plasticity genes. Genetic strategies that disrupted GR phosphorylation or TrkB signaling in vivo impaired the neuroplasticity to chronic stress and the effects of the antidepressant fluoxetine. Our findings reveal that the coordinated actions of BDNF and glucocorticoids promote neuronal plasticity and that disruption in either pathway could set the stage for the development of stress-induced psychiatric diseases.BDNF | glucocorticoid | coincidence detection | stress | antidepressant G lucocorticoids can either facilitate or deteriorate the structure and function of brain circuits involved in perception, cognition, and mood by modulating neurotransmission and remodeling dendritic spines as a function of time at exposure, dose, and duration (1, 2). Neuronal growth defects and cognitive impairment manifest upon disruption of circadian oscillations and of stress-mediated peaks of glucocorticoids, common features of major depression (3). Paradoxically, despite the often high glucocorticoid concentrations in patients with depression, signaling through glucocorticoid receptors (GR) appears defective, as evidenced by the inability of the endocrine stress response to be suppressed by dexamethasone, and reduction in expression of GR-sensitive genes and an inability of GR to suppress inflammation (4, 5). This so called "glucocorticoid-resistant" state is not solely the result of GR down-regulation (6, 7). What accounts for this lack of GR responsiveness during chronic stress is not understood.One hypothesis to account for glucocorticoid resistance is that additional stress-sensitive pathways influence GR function (8). For example, brain-derived neurotrophic factor (BDNF), is essential for the enhancement of contextual fear memories by glucocorticoids (9, 10). In contrast, BDNF-deficiency diminished the complexity of hippocampal dendritic arborization without further atrophy by stress (11). This raises the interesting question of whether BDNF initiates a cellular pathway that could modulate GR activity during stress. Using mass spectrometry, we previously found that BDNF promotes the phosphorylation of GR at two conserved phosphorylation sites (Fig. 1A) involved in the expression of select glucocorticoid-regulated genes when BDNF and glucocorticoid stimulation were paired in ...

Clinical and experimental evidence point to a possible role of cerebrovascular dysfunction in Alzheimer's disease (AD). The 5xFAD mouse model of AD expresses human amyloid precursor protein and presenilin genes with mutations found in AD patients. It remains unknown whether amyloid deposition driven by these mutations is associated with cerebrovascular changes. 5xFAD and wild type mice (2 to 12months old; M2 to M12) were used. Thinned skull in vivo 2-photon microscopy was used to determine Aβ accumulation on leptomeningeal or superficial cortical vessels over time. Parenchymal microvascular damage was assessed using FITC-microangiography. Collagen-IV and CD31 were used to stain basal lamina and endothelial cells. Methoxy-XO4, Thioflavin-S or 6E10 were used to visualize Aβ accumulation in living mice or in fixed brain tissues. Positioning of reactive IBA1 microglia and GFAP astrocytes at the vasculature was rendered using confocal microscopy. Platelet-derived growth factor receptor beta (PDGFRβ) staining was used to visualize perivascular pericytes. In vivo 2-photon microscopy revealed Methoxy-XO4(+) amyloid perivascular deposits on leptomeningeal and penetrating cortical vessels in 5xFAD mice, typical of cerebral amyloid angiopathy (CAA). Amyloid deposits were visible in vivo at M3 and aggravated over time. Progressive microvascular damage was concomitant to parenchymal Aβ plaque accumulation in 5xFAD mice. Microvascular inflammation in 5xFAD mice presented with sporadic FITC-albumin leakages at M4 becoming more prevalent at M9 and M12. 3D colocalization showed inflammatory IBA1(+) microglia proximal to microvascular FITC-albumin leaks. The number of perivascular PDGFRβ(+) pericytes was significantly decreased at M4 in the fronto-parietal cortices, with a trend decrease observed in the other structures. At M9-M12, PDGFRβ(+) pericytes displayed hypertrophic perivascular ramifications contiguous to reactive microglia. Cerebral amyloid angiopathy and microvascular inflammation occur in 5xFAD mice concomitantly to parenchymal plaque deposition. The prospect of cerebrovascular pharmacology in AD is discussed.

The energetic costs of behavioral chronic stress are unlikely to be sustainable without neuronal plasticity. Mitochondria have the capacity to handle synaptic activity up to a limit before energetic depletion occurs. Protective mechanisms driven by the induction of neuronal genes likely evolved to buffer the consequences of chronic stress on excitatory neurons in prefrontal cortex (PFC), as this circuitry is vulnerable to excitotoxic insults. Little is known about the genes involved in mitochondrial adaptation to the buildup of chronic stress. Using combinations of genetic manipulations and stress for analyzing structural, transcriptional, mitochondrial, and behavioral outcomes, we characterized NR4A1 as a stress-inducible modifier of mitochondrial energetic competence and dendritic spine number in PFC. NR4A1 acted as a transcription factor for changing the expression of target genes previously involved in mitochondrial uncoupling, AMP-activated protein kinase activation, and synaptic growth. Maintenance of NR4A1 activity by chronic stress played a critical role in the regressive synaptic organization in PFC of mouse models of stress (male only). Knockdown, dominant-negative approach, and knockout of Nr4a1 in mice and rats (male only) protected pyramidal neurons against the adverse effects of chronic stress. In human PFC tissues of men and women, high levels of the transcriptionally active NR4A1 correlated with measures of synaptic loss and cognitive impairment. In the context of chronic stress, prolonged expression and activity of NR4A1 may lead to responses of mitochondria and synaptic connectivity that do not match environmental demand, resulting in circuit malfunction between PFC and other brain regions, constituting a pathological feature across disorders.SIGNIFICANCE STATEMENT The bioenergetic cost of chronic stress is too high to be sustainable by pyramidal prefrontal neurons. Cellular checkpoints have evolved to adjust the responses of mitochondria and synapses to the buildup of chronic stress. NR4A1 plays such a role by controlling the energetic competence of mitochondria with respect to synapse number. As an immediate-early gene, Nr4a1 promotes neuronal plasticity, but sustained expression or activity can be detrimental. NR4A1 expression and activity is sustained by chronic stress in animal models and in human studies of neuropathologies sensitive to the buildup of chronic stress. Therefore, antagonism of NR4A1 is a promising avenue for preventing the regressive synaptic reorganization in cortical systems in the context of chronic stress.

Reorganization of the neurovascular unit has been suggested in the epileptic brain, although the dynamics and functional significance remain unclear. Here, we tracked the in vivo dynamics of perivascular mural cells as a function of electroencephalogram (EEG) activity following status epilepticus. We segmented the cortical vascular bed to provide a size- and type-specific analysis of mural cell plasticity topologically. We find that mural cells are added and removed from veins, arterioles, and capillaries after seizure induction. Loss of mural cells is proportional to seizure severity and vascular pathology (e.g., rigidity, perfusion, and permeability). Treatment with platelet-derived growth factor subunits BB (PDGF-BB) reduced mural cell loss, vascular pathology, and epileptiform EEG activity. We propose that perivascular mural cells play a pivotal role in seizures and are potential targets for reducing pathophysiology.

The brain evolved cellular mechanisms for adapting synaptic function to energy supply. This is particularly evident when homeostasis is challenged by stress. Signaling loops between the mitochondria and synapses scale neuronal connectivity with bioenergetics capacity. A biphasic “inverted U shape” response to the stress hormone glucocorticoids is demonstrated in mitochondria and at synapses, modulating neural plasticity and physiological responses. Low dose enhances neurotransmission, synaptic growth, mitochondrial functions, learning, and memory whereas chronic, higher doses produce inhibition of these functions. The range of physiological effects by stress and glucocorticoid depends on the dose, duration, and context at exposure. These criteria are met by neuronal activity and the circadian, stress-sensitive and ultradian, stress-insensitive modes of glucocorticoid secretion. A major hallmark of stress-related neuropsychiatric disorders is the disrupted glucocorticoid rhythms and tissue resistance to signaling with the glucocorticoid receptor (GR). GR resistance could result from the loss of context-dependent glucocorticoid signaling mediated by the downregulation of the activity-dependent neurotrophin BDNF. The coincidence of BDNF and GR signaling changes glucocorticoid signaling output with consequences on mitochondrial respiration efficiency, synaptic plasticity, and adaptive trajectories.

Stress can either promote or impair learning and memory. Such opposing effects depend on whether synapses persist or decay after learning. Maintenance of new synapses formed at the time of learning upon neuronal network activation depends on the stress hormone-activated glucocorticoid receptor (GR) and neurotrophic factor release. Whether and how concurrent GR and neurotrophin signaling integrate to modulate synaptic plasticity and learning is not fully understood. Here, we show that deletion of the neurotrophin brain-derived neurotrophic factor (BDNF)–dependent GR-phosphorylation (PO4) sites impairs long-term memory retention and maintenance of newly formed postsynaptic dendritic spines in the mouse cortex after motor skills training. Chronic stress and the BDNF polymorphism Val66Met disrupt the BDNF-dependent GR-PO4pathway necessary for preserving training-induced spines and previously acquired memories. Conversely, enrichment living promotes spine formation but fails to salvage training-related spines in mice lacking BDNF-dependent GR-PO4sites, suggesting it is essential for spine consolidation and memory retention. Mechanistically, spine maturation and persistence in the motor cortex depend on synaptic mobilization of the glutamate receptor subunit A1 (GluA1) mediated by GR-PO4. Together, these findings indicate that regulation of GR-PO4via activity-dependent BDNF signaling is important for the formation and maintenance of learning-dependent synapses. They also define a signaling mechanism underlying these effects.