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Heightened aggression is characteristic of multiple neuropsychiatric disorders and can have various negative effects on patients, their families and the public. Recent studies in humans and animals have implicated brain reward circuits in aggression and suggest that, in subsets of aggressive individuals, domination of subordinate social targets is reinforcing. In this study, we showed that, in male mice, orexin neurons in the lateral hypothalamus activated a small population of glutamic acid decarboxylase 2 (GAD2)-expressing neurons in the lateral habenula (LHb) via orexin receptor 2 (OxR2) and that activation of these GAD2 neurons promoted male–male aggression and conditioned place preference for aggression-paired contexts. Moreover, LHb GAD2 neurons were inhibitory within the LHb and dampened the activity of the LHb as a whole. These results suggest that the orexin system is important for the regulation of inter-male aggressive behavior and provide the first functional evidence of a local inhibitory circuit within the LHb.Flanigan et al. show that activation of inhibitory neurons in the lateral habenula by the neuropeptide orexin (hypocretin) promotes both inter-male aggression and conditioned place preference for contexts associated with winning aggressive contests.

Key Points Matrix metalloproteinases (MMPs) are a large family of mostly secreted, extracellularly acting proteolytic enzymes. In the brain, they have well-described roles in slowly emerging, but long-lasting pathophysiological processes of cell loss and synaptic dysfunction associated with acute injury, ischaemia, neurodegeneration and demyelination. Remodelling of synapse structure and function also underlies normal cognitive processes, such as learning and memory. This Review focuses on recent studies that indicate that MMPs have important roles in driving such synapse plasticity under non-pathological conditions that are distinct from their roles in neuropathophysiology. MMPs are secreted as inactive pro-enzymes (zymogens). Under basal conditions, a large pool of mostly pro-MMPs is situated perisynaptically, poised for activation by plasticity-inducing stimuli, such as long-term potentiation (LTP). Upon induction of LTP, but not other forms of short- or long-lasting plasticity, pro-MMPs are rapidly (within ∼15 min) converted to proteolytically active MMPs through an NMDA receptor-dependent mechanism. Such proteolytically active MMPs then signal through β1-containing integrins to promote dendritic spine enlargement and synaptic potentiation concurrently. Intercellular adhesion molecule 5, which binds to and activates integrins, may be a direct target of perisynaptic MMP proteolysis during LTP. LTP-associated MMP proteolysis is probably then terminated by an increase in the activity of endogenous inhibitors called tissue inhibitors of metalloproteinases. When MMP activity is blocked pharmacologically or genetically, LTP, spine enlargement and behavioural measures of cognitive function are all impaired. Several psychiatric and neurological disorders, including drug addiction, neuropathic pain syndromes and fragile X syndrome, are associated with abnormal or deficient synaptic plasticity. Recent studies indicate that aberrant MMP expression, localization and function may contribute to synaptic plasticity deficits associated with such disorders. A key area for future research is to elucidate how MMP activity transitions from normal, adaptive roles in local synaptic remodelling to deleterious roles that have important pathophysiological cellular and synaptic consequences. This transition probably involves abnormal regulatory mechanisms, leading to excessive, prolonged and widespread MMP activity. Aberrant matrix metalloproteinase (MMP) activity is a well-known contributor to synaptic dysfunction and neuronal loss in CNS injury and disease. In this Review, George W. Huntley discusses how MMPs also make an important contribution to synaptic functional and structural remodelling under nonpathophysiological conditions. Matrix metalloproteinases (MMPs) are extracellularly acting enzymes that have long been known to have deleterious roles in brain injury and disease. In particular, widespread and protracted MMP activity can contribute to neuronal loss and synaptic dysfunction. However, recent studies show that rapid and focal MMP-mediated proteolysis proactively drives synaptic structural and functional remodelling that is crucial for ongoing cognitive processes. Deficits in synaptic remodelling are associated with psychiatric and neurological disorders, and aberrant MMP expression or function may contribute to the molecular mechanisms underlying these deficits. This Review explores the paradigm shift in our understanding of the contribution of MMPs to normal and abnormal synaptic plasticity and function.

Persistent dendritic spine enlargement is associated with stable long-term potentiation (LTP), and the latter is thought to underlie long-lasting memories. Extracellular proteolytic remodeling of the synaptic microenvironment could be important for such plasticity, but whether or how proteolytic remodeling contributes to persistent modifications in synapse structure and function is unknown. Matrix metalloproteinase-9 (MMP-9) is an extracellular protease that is activated perisynaptically after LTP induction and required for LTP maintenance. Here, by monitoring spine size and excitatory postsynaptic potentials (EPSPs) simultaneously with combined 2-photon time-lapse imaging and whole-cell recordings from hippocampal neurons, we find that MMP-9 is both necessary and sufficient to drive spine enlargement and synaptic potentiation concomitantly. Both structural and functional MMP-driven forms of plasticity are mediated through β1-containing integrin receptors, are associated with integrin-dependent cofilin inactivation within spines, and require actin polymerization. In contrast, postsynaptic exocytosis and protein synthesis are both required for MMP-9-induced potentiation, but not for initial MMP-9-induced spine expansion. However, spine expansion becomes unstable when postsynaptic exocytosis or protein synthesis is blocked, indicating that the 2 forms of plasticity are expressed independently but require interactions between them for persistence. When MMP activity is eliminated during theta-stimulation-induced LTP, both spine enlargement and synaptic potentiation are transient. Thus, MMP-mediated extracellular remodeling during LTP has an instructive role in establishing persistent modifications in both synapse structure and function of the kind critical for learning and memory.

Late-onset Parkinson's disease (PD) is dominated clinically and experimentally by a focus on dopamine neuron degeneration and ensuing motor system abnormalities. There are, additionally, a number of non-motor symptoms - including cognitive and psychiatric - that can appear much earlier in the course of the disease and also significantly impair quality of life. The neurobiology of such cognitive and psychiatric non-motor symptoms is poorly understood. The recognition of genetic forms of late-onset PD, which are clinically similar to idiopathic forms in both motor and non-motor symptoms, raises the perspective that brain cells and circuits - and the behaviors they support - differ in significant ways from normal by virtue of the fact that these mutations are carried throughout life, including especially early developmental critical periods where circuit structure and function is particularly susceptible to the influence of experience-dependent activity. In this focused review, we support this central thesis by highlighting studies of LRRK2-G2019S mouse models. We describe work that shows that in G2019S mutants, corticostriatal activity and plasticity are abnormal by P21, the end of a period of excitatory synaptogenesis in striatum. Moreover, by young adulthood, impaired striatal synaptic and non-synaptic forms of plasticity likely underlie altered and variable performance by mutant mice in validated tasks that test for depression-like and anhedonia-like behaviors. Mechanistically, deficits in cellular, synaptic and behavioral plasticity may be unified by mutation-linked defects in trafficking of AMPAR subunits and other membrane channels, which in turn may reflect impairment in the function of the Rab family of GTPases, a major target of LRRK2 phosphorylation. These findings underscore the need to better understand how PD-related mutant proteins influence brain structure and function during an extended period of brain development, and offer new clues for future therapeutic strategies to target non-motor cognitive or psychiatric symptoms of PD.

Axon growth potential is highest in young neurons but diminishes with age, thus becoming a significant obstacle to axonal regeneration after injury in maturity. The mechanism for the decline is incompletely understood, and no effective clinical treatment is available to rekindle innate growth capability. Here, we show that Smad1-dependent bone morphogenetic protein (BMP) signaling is developmentally regulated and governs axonal growth in dorsal root ganglion (DRG) neurons. Down-regulation of the pathway contributes to the age-related decline of the axon growth potential. Reactivating Smad1 selectively in adult DRG neurons results in sensory axon regeneration in a mouse model of spinal cord injury (SCI). Smad1 signaling can be effectively manipulated by an adeno-associated virus (AAV) vector encoding BMP4 delivered by a clinically applicable and minimally invasive technique, an approach devoid of unwanted abnormalities in mechanosensation or pain perception. Importantly, transected axons are able to regenerate even when the AAV treatment is delivered after SCI, thus mimicking a clinically relevant scenario. Together, our results identify a therapeutic target to promote axonal regeneration after SCI.

Anxiety is a psychiatric non-motor symptom of Parkinson’s that can appear in the prodromal period, prior to significant loss of midbrain dopamine neurons and motor symptoms. Parkinson’s-related anxiety affects females more than males, despite the greater prevalence of Parkinson’s in males. How stress, anxiety and Parkinson’s are related and the basis for a sex-specific impact of stress in Parkinson’s are not clear. We addressed this using young adult male and female mice carrying a G2019S knockin mutation of leucine-rich repeat kinase 2 ( Lrrk2 G2019S ) and Lrrk2 WT control mice. In humans, LRRK2 G2019S significantly elevates the risk of late-onset Parkinson’s. To assess within-sex differences between Lrrk2 G2019S and control mice in stress-induced anxiety-like behaviors in young adulthood, we used a within-subject design whereby Lrrk2 G2019S and Lrrk2 WT control mice underwent tests of anxiety-like behaviors before (baseline) and following a 28 day (d) variable stress paradigm. There were no differences in behavioral measures between genotypes in males or females at baseline, indicating that the mutation alone does not produce anxiety-like responses. Following chronic stress, male Lrrk2 G2019S mice were affected similarly to male wildtypes except for novelty-suppressed feeding, where stress had no impact on Lrrk2 G2019S mice while significantly increasing latency to feed in Lrrk2 WT control mice. Female Lrrk2 G2019S mice were impacted by chronic stress similarly to wildtype females across all behavioral measures. Subsequent post-stress analyses compared cFos immunolabeling-based cellular activity patterns across several stress-relevant brain regions. The density of cFos-activated neurons across brain regions in both male and female Lrrk2 G2019S mice was generally lower compared to stressed Lrrk2 WT mice, except for the nucleus accumbens of male Lrrk2 G2019S mice, where cFos-labeled cell density was significantly higher than all other groups. Together, these data suggest that the Lrrk2 G2019S mutation differentially impacts anxiety-like behavioral responses to chronic stress in males and females that may reflect sex-specific adaptations observed in circuit activation patterns in some, but not all stress-related brain regions.

Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular signals that generate normal cortical networks remain poorly understood. Semaphorin 7A (Sema7A) is an atypical member of the semaphorin family that is GPI-linked, expressed principally postnatally, and enriched in sensory cortex. Significantly, SEMA7A is deleted in individuals with 15q24 microdeletion syndrome, characterized by developmental delay, autism, and sensory perceptual deficits. We studied the role that Sema7A plays in establishing functional cortical circuitry in mouse somatosensory barrel cortex. We found that Sema7A is expressed in spiny stellate cells and GABAergic interneurons and that its absence disrupts barrel cytoarchitecture, reduces asymmetrical orientation of spiny stellate cell dendrites, and functionally impairs thalamocortically evoked synaptic responses, with reduced feed-forward GABAergic inhibition. These data identify Sema7A as a regulator of thalamocortical and local circuit development in layer 4 and provide a molecular handle that can be used to explore the coordinated generation of excitatory and inhibitory cortical circuits. Significance Sensory experience exerts profound control over the structure and function of developing cortical circuits during an early postnatal critical period. Abnormalities in this process contribute to perceptual and cognitive deficits, but molecular mechanisms generating excitatory and inhibitory cortical networks during this period remain poorly understood. We show here that Semaphorin 7A (Sema7A) is highly expressed in mouse somatosensory cortex when tactile information conveyed by the thalamus shapes development of somatosensory cortical networks. In mice lacking Sema7A, the anatomical layout of the somatosensory cortex is disrupted, dendritic arbors are misoriented, inhibitory connections develop abnormally, and thalamocortical activity fails to elicit a normal balance of excitation and inhibition. Taken together, our data indicate that maturation of thalamocortical and local circuits in cortex requires Sema7A.

Antipsychotic treatment in patients with schizophrenia often reduces hallucinations and delusions, but cognitive deficits that impair performance of everyday activities may persist or worsen. Our findings reveal a mechanism by which increased NF-κB activity leads to increased HDAC2 levels, impairing synaptic plasticity and memory during prolonged antipsychotic treatment. Antipsychotic drugs remain the standard for schizophrenia treatment. Despite their effectiveness in treating hallucinations and delusions, prolonged exposure to antipsychotic medications leads to cognitive deficits in both schizophrenia patients and animal models. The molecular mechanisms underlying these negative effects on cognition remain to be elucidated. Here we demonstrate that chronic antipsychotic drug exposure increases nuclear translocation of NF-κB in both mouse and human frontal cortex, a trafficking event triggered via 5-HT 2A -receptor-dependent downregulation of the NF-κB repressor IκBα. This upregulation of NF-κB activity led to its increased binding at the Hdac2 promoter, thereby augmenting Hdac2 transcription. Deletion of HDAC2 in forebrain pyramidal neurons prevented the negative effects of antipsychotic treatment on synaptic remodeling and cognition. Conversely, virally mediated activation of NF-κB signaling decreased cortical synaptic plasticity via HDAC2. Together, these observations may aid in developing therapeutic strategies to improve the outcome of schizophrenia treatment.

Key Points Synaptogenesis is the culmination of a continuous process, which can be divided into the following stages: (1) axon guidance or pathfinding; (2) gross target recognition; (3) fine target recognition; and (4) elaboration of synaptic contacts onto appropriate cellular domains. Furthermore, synaptic connections are organized topographically, an essential anatomical substrate for orderly 'maps' of sensory surfaces, such as the retina. Sperry proposed that the topographically ordered distribution of synapses was established by “highly specific cytochemical affinities” between an axon and the environment through which it grows, and ultimately its target neuron. He proposed an orderly mapping of two or more standing gradients that are orthogonal to one another, so that an incoming axon is guided by signals encoding both latitude and longitude. Subsequent models have addressed the nature of standing gradients, and how a growth cone might sense and respond to the subtle differences in the molecular environment generated by such gradients. Haydon and Drapeau proposed two general modes of synapse specification. 'Selective' neurons send their neurites only to their appropriate target; 'promiscuous' neurons form synapses with a number of targets, and final specificity is achieved by pruning away the incorrect terminal sites in an activity-mediated process. Neuronal differentiation is the first step in synapse specification. Neurons, and the position they hold within a larger group, impart information. Group identification might be encoded, at least in part, by differential adhesion, and neighbour relationships within groups might be established by gap-junction-mediated communication, or by regulated patterns of calcium waves. The final topographic order of axons within a target might reflect an ordered distribution of axons within a fibre tract. However, retinal axon ordering alone does not seem to be sufficient for dorsoventral patterning in the optic tectum. In the dorsal thalamus, collections of neurons born contemporaneously parse into distinct nuclei. It is remarkable that targeting is precise from the earliest stages of innervation, because thalamic axons from different nuclei travel together through a similar environment, and are presented with an array of possible areal targets. Presynaptic assembly cannot be entirely nonspecific, or all potential partners brought into close proximity would form synapses with each other. Evidence indicates that a particular recognition threshold must be passed in order for synapse-initiation molecules to link. In vitro studies indicate that an interaction between β-neurexin and neuroligins can trigger synapse initiation. Several other molecules have been suggested to be involved in the early stages of synapse recognition/initiation, including EphB and Narp. Stabilizing a synapse is likely to require various molecules, but activity seems to be essential; strong evidence indicates that neurotrophins are involved, and recent work indicates that local synthesis of synaptic proteins might also be important. In Drosophila , homophilic binding between pre- and postsynaptically localized Fasciclin II is required to maintain a neuromuscular synapse, and members of the cadherin superfamily might have a similar role in vertebrates. Synaptogenesis should be viewed as an ongoing process that includes the modification and elimination of existing synapses and the generation of new synapses. Consistent with this, several guidance and recognition molecules continue to be expressed in adult nervous systems, and many have been implicated in the generation of synapse plasticity. A striking feature of the mature central nervous system is the precision of the synaptic circuitry. In contemplating the mature circuitry, it is impossible to imagine how more than 20 billion neurons in the human brain become precisely connected through trillions of synapses. Remarkably, much of the final wiring can be established in the absence of neural activity or experience; so the algorithms that allow precise connectivity must be encoded largely by the genetic programme. This programme, honed over nearly one billion years of evolution, generates networks with the flexibility to respond to a wide range of physiological challenges. There are several contemporary models of how synapse specificity is achieved, many of them proposed before the identification of guidance or recognition molecules. Here we review a selection of models as frameworks for defining the nature and complexity of synaptogenesis, and evaluate their validity in view of progress made in identifying the molecular underpinnings of axon guidance, targeting and synapse formation.

Amyotrophic lateral sclerosis is a progressive neurodegenerative disorder primarily involving motoneurons. A subset of individuals with familial autosomal dominant forms of the disease have mutations of the copper/zinc superoxide dismutase (Cu/Zn SOD, SOD-1) gene, which encodes a ubiquitously expressed enzyme that plays a key role in oxygen free radical scavenging. This observation suggests that altered or reduced SOD-1 activity may play a role in the neurodegenerative process. To explore this possibility further, we have introduced a mutation into the mouse SOD-1 gene that corresponds to one of the changes found in the human gene in familial amyotrophic lateral sclerosis. Integration and expression of this mouse gene in transgenic mice was identified by the presence of a unique restriction enzyme site in the transgene coding sequence generated by introduction of the mutation. We report here that high expression of this altered gene in the central nervous systems of transgenic mice is associated with an age-related rapidly progressive decline of motor function accompanied by degenerative changes of motoneurons within the spinal cord, brain stem, and neocortex. These findings indicate a causative relationship between altered SOD activity and motoneuron degeneration. Moreover, biochemical studies indicate normal levels of total SOD activity in transgenic mouse tissues, results that indicate that the neurodegenerative disorder does not result from a diminution of activity and, as such, represents a dominant \"gain of function\" mutation.