Rui M. Costa

Chronic Stress Causes Frontostriatal Reorganization and Affects Decision-Making

Brain Rewiring After Stress Chronic stress, mainly through the release of corticosteroids, affects executive behavior through sequential structural modulation of brain networks. Stress-induced deficits in spatial reference, working memory, and behavioral flexibility are associated with synaptic and dendritic reorganization in both the hippocampus and the medial prefrontal cortex. However, the effects of chronic stress on action selection strategies are unclear. Dias-Ferreira et al. (p. 621) examined whether chronic stress affects the ability of animals to select the appropriate actions based on the consequences of their choice, and found that rats exposed to chronic unpredictable stress rapidly shift toward using habitual strategies. The shift in behavioral strategies observed in chronically stressed animals corresponded to dramatic and divergent changes in connectivity in the associative and sensorimotor corticostriatal circuits underlying these behaviors. Chronic stress alters brain neural circuits and affects the ability of animals to perform actions based on their consequences. The ability to shift between different behavioral strategies is necessary for appropriate decision-making. Here, we show that chronic stress biases decision-making strategies, affecting the ability of stressed animals to perform actions on the basis of their consequences. Using two different operant tasks, we revealed that, in making choices, rats subjected to chronic stress became insensitive to changes in outcome value and resistant to changes in action-outcome contingency. Furthermore, chronic stress caused opposing structural changes in the associative and sensorimotor corticostriatal circuits underlying these different behavioral strategies, with atrophy of medial prefrontal cortex and the associative striatum and hypertrophy of the sensorimotor striatum. These data suggest that the relative advantage of circuits coursing through sensorimotor striatum observed after chronic stress leads to a bias in behavioral strategies toward habit.

Concurrent activation of striatal direct and indirect pathways during action initiation

The basal ganglia are subcortical nuclei that control voluntary actions, and they are affected by a number of debilitating neurological disorders. The prevailing model of basal ganglia function proposes that two orthogonal projection circuits originating from distinct populations of spiny projection neurons (SPNs) in the striatum—the so-called direct and indirect pathways—have opposing effects on movement: activity of direct-pathway SPNs is thought to facilitate movement, whereas activity of indirect-pathway SPNs is presumed to inhibit movement. This model has been difficult to test owing to the lack of methods to selectively measure the activity of direct- and indirect-pathway SPNs in freely moving animals. Here we develop a novel in vivo method to specifically measure direct- and indirect-pathway SPN activity, using Cre-dependent viral expression of the genetically encoded calcium indicator (GECI) GCaMP3 in the dorsal striatum of D1-Cre (direct-pathway-specific) and A2A-Cre (indirect-pathway-specific) mice. Using fibre optics and time-correlated single-photon counting (TCSPC) in mice performing an operant task, we observed transient increases in neural activity in both direct- and indirect-pathway SPNs when animals initiated actions, but not when they were inactive. Concurrent activation of SPNs from both pathways in one hemisphere preceded the initiation of contraversive movements and predicted the occurrence of specific movements within 500 ms. These observations challenge the classical view of basal ganglia function and may have implications for understanding the origin of motor symptoms in basal ganglia disorders.

Mechanism for the learning deficits in a mouse model of neurofibromatosis type 1

Neurofibromatosis type I (NF1) is one of the most common single-gene disorders that causes learning deficits in humans. Mice carrying a heterozygous null mutation of the Nf1 gene (Nf1+/−) show important features of the learning deficits associated with NF1 (ref. 2). Although neurofibromin has several known properties and functions, including Ras GTPase-activating protein activity, adenylyl cyclase modulation and microtubule binding, it is unclear which of these are essential for learning in mice and humans. Here we show that the learning deficits of Nf1+/− mice can be rescued by genetic and pharmacological manipulations that decrease Ras function. We also show that the Nf1+/− mice have increased GABA (γ-amino butyric acid)-mediated inhibition and specific deficits in long-term potentiation, both of which can be reversed by decreasing Ras function. Our results indicate that the learning deficits associated with NF1 may be caused by excessive Ras activity, which leads to impairments in long-term potentiation caused by increased GABA-mediated inhibition. Our findings have implications for the development of treatments for learning deficits associated with NF1.

Dynamic reorganization of striatal circuits during the acquisition and consolidation of a skill

The learning of new skills is characterized by an initial phase of rapid improvement in performance and a phase of more gradual improvements as skills are automatized and performance asymptotes. Using in vivo striatal recordings, we observed region-specific changes in neural activity during the different phases of skill learning, with the associative or dorsomedial striatum being preferentially engaged early in training and the sensorimotor or dorsolateral striatum being engaged later in training. Ex vivo recordings from medium spiny striatal neurons in brain slices of trained mice revealed that the changes observed in vivo corresponded to regional- and training-specific changes in excitatory synaptic transmission in the striatum. Furthermore, the potentiation of glutamatergic transmission observed in dorsolateral striatum after extensive training was preferentially expressed in striatopallidal neurons, rather than striatonigral neurons. These findings demonstrate that region- and pathway-specific plasticity sculpts the circuits involved in the performance of the skill as it becomes automatized.

Matrix Metalloproteinase-9 Is Required for Hippocampal Late-Phase Long-Term Potentiation and Memory

Matrix metalloproteinases (MMPs) are extracellular proteases that have well recognized roles in cell signaling and remodeling in many tissues. In the brain, their activation and function are customarily associated with injury or pathology. Here, we demonstrate a novel role for MMP-9 in hippocampal synaptic physiology, plasticity, and memory. MMP-9 protein levels and proteolytic activity are rapidly increased by stimuli that induce late-phase long-term potentiation (L-LTP) in area CA1. Such regulation requires NMDA receptors and protein synthesis. Blockade of MMP-9 pharmacologically prevents induction of L-LTP selectively; MMP-9 plays no role in, nor is regulated during, other forms of short-term synaptic potentiation or long-lasting synaptic depression. Similarly, in slices from MMP-9 null-mutant mice, hippocampal LTP, but not long-term depression, is impaired in magnitude and duration; adding recombinant active MMP-9 to null-mutant slices restores the magnitude and duration of LTP to wild-type levels. Activated MMP-9 localizes in part to synapses and modulates hippocampal synaptic physiology through integrin receptors, because integrin function-blocking reagents prevent an MMP-9-mediated potentiation of synaptic signal strength. The fundamental importance of MMP-9 function in modulating hippocampal synaptic physiology and plasticity is underscored by behavioral impairments in hippocampal-dependent memory displayed by MMP-9 null-mutant mice. Together, these data reveal new functions for MMPs in synaptic and behavioral plasticity.

Start/stop signals emerge in nigrostriatal circuits during sequence learning

Learning new action sequences subserves a plethora of different abilities such as escaping a predator, playing the piano, or producing fluent speech. Proper initiation and termination of each action sequence is critical for the organization of behaviour, and is compromised in nigrostriatal disorders like Parkinson’s and Huntington’s diseases. Using a self-paced operant task in which mice learn to perform a particular sequence of actions to obtain an outcome, we found neural activity in nigrostriatal circuits specifically signalling the initiation or the termination of each action sequence. This start/stop activity emerged during sequence learning, was specific for particular actions, and did not reflect interval timing, movement speed or action value. Furthermore, genetically altering the function of striatal circuits disrupted the development of start/stop activity and selectively impaired sequence learning. These results have important implications for understanding the functional organization of actions and the sequence initiation and termination impairments observed in basal ganglia disorders.

Neurofibromin Regulation of ERK Signaling Modulates GABA Release and Learning

We uncovered a role for ERK signaling in GABA release, long-term potentiation (LTP), and learning, and show that disruption of this mechanism accounts for the learning deficits in a mouse model for learning disabilities in neurofibromatosis type I (NF1). Our results demonstrate that neurofibromin modulates ERK/synapsin I-dependent GABA release, which in turn modulates hippocampal LTP and learning. An Nf1 heterozygous null mutation, which results in enhanced ERK and synapsin I phosphorylation, increased GABA release in the hippocampus, and this was reversed by pharmacological downregulation of ERK signaling. Importantly, the learning deficits associated with the Nf1 mutation were rescued by a subthreshold dose of a GABA(A) antagonist. Accordingly, Cre deletions of Nf1 showed that only those deletions involving inhibitory neurons caused hippocampal inhibition, LTP, and learning abnormalities. Importantly, our results also revealed lasting increases in GABA release triggered by learning, indicating that the mechanisms uncovered here are of general importance for learning.

Rapid alterations in corticostriatal ensemble coordination during acute dopamine-dependent motor dysfunction

Dopaminergic dysregulation can cause motor dysfunction, but the mechanisms underlying dopamine-related motor disorders remain under debate. We used an inducible and reversible pharmacogenetic approach in dopamine transporter knockout mice to investigate the simultaneous activity of neuronal ensembles in the dorsolateral striatum and primary motor cortex during hyperdopaminergia ( approximately 500% of controls) with hyperkinesia, and after rapid and profound dopamine depletion (<0.2%) with akinesia in the same animal. Surprisingly, although most cortical and striatal neurons ( approximately 70%) changed firing rate during the transition between dopamine-related hyperkinesia and akinesia, the overall cortical firing rate remained unchanged. Conversely, neuronal oscillations and ensemble activity coordination within and between cortex and striatum did change rapidly between these periods. During hyperkinesia, corticostriatal activity became largely asynchronous, while during dopamine-depletion the synchronicity increased. Thus, dopamine-related disorders like Parkinsons disease may not stem from changes in the overall levels of cortical activity, but from dysfunctional activity coordination in corticostriatal circuits.

Dopaminergic Control of Sleep–Wake States

Dopamine depletion is involved in the pathophysiology of Parkinsons disease, whereas hyperdopaminergia may play a fundamental role in generating endophenotypes associated with schizophrenia. Sleep disturbances are known to occur in both schizophrenia and Parkinsons disease, suggesting that dopamine plays a role in regulating the sleep–wake cycle. Here, we show that novelty-exposed hyperdopaminergic mice enter a novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during rapid-eye-movement (REM) sleep. Treatment with haloperidol, a D2 dopamine receptor antagonist, reduces this abnormal intrusion of REM-like activity during wakefulness. Conversely, mice acutely depleted of dopamine enter a different novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during slow-wave sleep (SWS). This dopamine-depleted state is marked by an apparent suppression of SWS and a complete suppression of REM sleep. Treatment with D2 (but not D1) dopamine receptor agonists recovers REM sleep in these mice. Altogether, these results indicate that dopamine regulates the generation of sleep–wake states. We propose that psychosis and the sleep disturbances experienced by Parkinsonian patients result from dopamine-mediated disturbances of REM sleep.

Impaired Synaptic Plasticity and Motor Learning in Mice with a Point Mutation Implicated in Human Speech Deficits

Summary The most well-described example of an inherited speech and language disorder is that observed in the multigenerational KE family, caused by a heterozygous missense mutation in the FOXP2 gene [1]. Affected individuals are characterized by deficits in the learning and production of complex orofacial motor sequences underlying fluent speech and display impaired linguistic processing for both spoken and written language [2]. The FOXP2 transcription factor is highly similar in many vertebrate species, with conserved expression in neural circuits related to sensorimotor integration and motor learning [3, 4]. In this study, we generated mice carrying an identical point mutation to that of the KE family, yielding the equivalent arginine-to-histidine substitution in the Foxp2 DNA-binding domain. Homozygous R552H mice show severe reductions in cerebellar growth and postnatal weight gain but are able to produce complex innate ultrasonic vocalizations. Heterozygous R552H mice are overtly normal in brain structure and development. Crucially, although their baseline motor abilities appear to be identical to wild-type littermates, R552H heterozygotes display significant deficits in species-typical motor-skill learning, accompanied by abnormal synaptic plasticity in striatal and cerebellar neural circuits.