Peering into the Brain through the Retrosplenial Cortex to Assess Cognitive Function of the Injured Brain.

The retrosplenial cortex (RSC) is a posterior cortical area that has been drawing increasing interest in recent years, with a growing number of studies studying its contribution to cognitive and sensory functions. From an anatomical perspective, it has been established that the RSC is extensively and often reciprocally connected with the hippocampus, neocortex, and many midbrain regions. Functionally, the RSC is an important hub of the default-mode network. This endowment, with vast anatomical and functional connections, positions the RSC to play an important role in episodic memory, spatial and contextual learning, sensory-cognitive activities, and multi-modal sensory information processing and integration. Additionally, RSC dysfunction has been reported in cases of cognitive decline, particularly in Alzheimer's disease and stroke. We review the literature to examine whether the RSC can act as a cortical marker of persistent cognitive dysfunction after traumatic brain injury (TBI). Because the RSC is easily accessible at the brain's surface using in vivo techniques, we argue that studying RSC network activity post-TBI can shed light into the mechanisms of less-accessible brain regions, such as the hippocampus. There is a fundamental gap in the TBI field about the microscale alterations occurring post-trauma, and by studying the RSC's neuronal activity at the cellular level we will be able to design better therapeutic tools. Understanding how neuronal activity and interactions produce normal and abnormal activity in the injured brain is crucial to understanding cognitive dysfunction. By using this approach, we expect to gain valuable insights to better understand brain disorders like TBI.

Differential Excitability of PV and SST Neurons Results in Distinct Functional Roles in Inhibition Stabilization of Up States.

Up states are the best studied example of an emergent neural dynamic regime. Computational models based on a single class of inhibitory neurons indicate that Up states reflect bistable dynamic systems in which positive feedback is stabilized by strong inhibition and predict a paradoxical effect in which increased drive to inhibitory neurons results in decreased inhibitory activity. To date, however, computational models have not incorporated empirically defined properties of parvalbumin (PV) and somatostatin (SST) neurons. Here we first experimentally characterized the frequency-current (F-I) curves of pyramidal (Pyr), PV, and SST neurons from mice of either sex, and confirmed a sharp difference between the threshold and slopes of PV and SST neurons. The empirically defined F-I curves were incorporated into a three-population computational model that simulated the empirically derived firing rates of pyramidal, PV, and SST neurons. Simulations revealed that the intrinsic properties were sufficient to predict that PV neurons are primarily responsible for generating the nontrivial fixed points representing Up states. Simulations and analytical methods demonstrated that while the paradoxical effect is not obligatory in a model with two classes of inhibitory neurons, it is present in most regimes. Finally, experimental tests validated predictions of the model that the Pyr ↔ PV inhibitory loop is stronger than the Pyr ↔ SST loop.SIGNIFICANCE STATEMENT Many cortical computations, such as working memory, rely on the local recurrent excitatory connections that define cortical circuit motifs. Up states are among the best studied examples of neural dynamic regimes that rely on recurrent excitatory excitation. However, this positive feedback must be held in check by inhibition. To address the relative contribution of PV and SST neurons, we characterized the intrinsic input-output differences between these classes of inhibitory neurons and, using experimental and theoretical methods, show that the higher threshold and gain of PV leads to a dominant role in network stabilization.

Decreased reproducibility and abnormal experience-dependent plasticity of network dynamics in Fragile X circuits.

Fragile X syndrome is a neurodevelopmental disorder associated with a broad range of neural phenotypes. Interpreting these findings has proven challenging because some phenotypes may reflect compensatory mechanisms or normal forms of plasticity differentially engaged by experiential differences. To help minimize compensatory and experiential influences, we used an ex vivo approach to study network dynamics and plasticity of cortical microcircuits. In Fmr1-/y circuits, the spatiotemporal structure of Up-states was less reproducible, suggesting alterations in the plasticity mechanisms governing network activity. Chronic optical stimulation revealed normal homeostatic plasticity of Up-states, however, Fmr1-/y circuits exhibited abnormal experience-dependent plasticity as they did not adapt to chronically presented temporal patterns in an interval-specific manner. These results, suggest that while homeostatic plasticity is normal, Fmr1-/y circuits exhibit deficits in the ability to orchestrate multiple forms of synaptic plasticity and to adapt to sensory patterns in an experience-dependent manner-which is likely to contribute to learning deficits.

Short-Term Synaptic Plasticity as a Mechanism for Sensory Timing.

The ability to detect time intervals and temporal patterns is critical to some of the most fundamental computations the brain performs, including the ability to communicate and appraise a dynamically changing environment. Many of these computations take place on the scale of tens to hundreds of milliseconds. Electrophysiological evidence shows that some neurons respond selectively to duration, interval, rate, or order. Because the time constants of many time-varying neural and synaptic properties, including short-term synaptic plasticity (STP), are also in the range of tens to hundreds of milliseconds, they are strong candidates to underlie the formation of temporally selective neurons. Neurophysiological studies indicate that STP is indeed one of the mechanisms that contributes to temporal selectivity, and computational models demonstrate that neurons embedded in local microcircuits exhibit temporal selectivity if their synapses undergo STP. Converging evidence suggests that some forms of temporal selectivity emerge from the dynamic changes in the balance of excitation and inhibition imposed by STP.

Delayed in vitro development of Up states but normal network plasticity in Fragile X circuits.

A broad range of neurophysiological phenotypes have been reported since the generation of the first mouse model of Fragile X syndrome (FXS). However, it remains unclear which phenotypes are causally related to the cognitive deficits associated with FXS. Indeed, because many of these phenotypes are known to be modulated by experience, a confounding factor in the interpretation of many studies is whether some phenotypes are an indirect consequence of abnormal development and experience. To help diminish this confound we first conducted an in vitro developmental study of spontaneous neural dynamics in cortical organotypic cultures. A significant developmental increase in network activity and Up states was observed in both wild-type and Fmr1(-/y) circuits, along with a specific developmental delay in the emergence of Up states in knockout circuits. To determine whether Up state regulation is generally impaired in FXS circuits, we examined Up state plasticity using chronic optogenetic stimulation. Wild-type and Fmr1(-/y) stimulated circuits exhibited a significant decrease in overall spontaneous activity including Up state frequency; however, no significant effect of genotype was observed. These results demonstrate that developmental delays characteristic of FXS are recapitulated during in vitro development, and that Up state abnormalities are probably a direct consequence of the disease, and not an indirect consequence of abnormal experience. However, the fact that Fmr1(-/y) circuits exhibited normal homeostatic modulation of Up states suggests that these plasticity mechanisms are largely intact, and that some of the previously reported plasticity deficits could reflect abnormal experience or the engagement of compensatory mechanisms.

Neural coding: time contraction and dilation in the striatum.

How the brain encodes time is poorly understood. New research on rats provides evidence that striatal neurons encode time, and that the code can dilate or contract to time different intervals.

Learning-induced bidirectional plasticity of intrinsic neuronal excitability reflects the valence of the outcome.

Long-term memory is supported not only by modulation of synaptic strength, but also by modifications in intrinsic neuronal properties. Learning-induced enhancement of neuronal excitability has been shown in the hippocampus and the piriform cortex, where it lasts for days and is involved in maintaining the learned skills. The basolateral amygdala (BLA) is suggested to encode positive and negative significance of information, thus forming a unique experimental setting to monitor bidirectional changes as a function of the valence change. In rodents, olfaction is a major modality that guides goal-directed behavior. Here, we show that intrinsic neuronal excitability in BLA pyramidal neurons is differentially modified by positive and negative olfactory learning and explore the cellular mechanisms of such bidirectional intrinsic neuronal plasticity. Learning of complex olfactory-discrimination task, in which success was rewarded with drinking water, resulted with enhanced intrinsic excitability. Such enhancement is mediated by reduction in the slow potassium current. In contrast, olfactory fear conditioning, in which the animal learned to associate the odor with an electric shock, resulted in decreased intrinsic excitability, mediated by activation of the µ-opioid-sensitive potassium current. We suggest that positive and negative changes in BLA excitability contribute to the encoding of opposite odor-value behaviors.

Enhanced extinction of aversive memories by high-frequency stimulation of the rat infralimbic cortex.

Electrical stimulation of the rodent medial prefrontal cortex (mPFC), including the infralimbic cortex (IL), immediately prior to or during fear extinction training facilitates extinction memory. Here we examined the effects of high-frequency stimulation (HFS) of the rat IL either prior to conditioning or following retrieval of the conditioned memory, on extinction of Pavlovian fear and conditioned taste aversion (CTA). IL-HFS applied immediately after fear memory retrieval, but not three hours after retrieval or prior to conditioning, subsequently reduced freezing during fear extinction. Similarly, IL-HFS given immediately, but not three hours after, retrieval of a CTA memory reduced aversion during extinction. These data indicate that HFS of the IL may be an effective method for reducing both learned fear and learned aversion.

Differential involvement of protein synthesis and actin rearrangement in the reacquisition of contextual fear conditioning.

Extinction learning is associated with a decline of the conditioned fear response (CR). However, re-exposure to the unconditioned stimulus (US, shock) is associated with the return of the fear response. This study aimed to study the role of protein synthesis and actin rearrangement in the CA1 hippocampal subregion and the basolateral amygdala (BLA) in acquisition and reacquisition of contextual fear conditioning. To that end, we trained rats on contextual fear conditioning and extinction, and on the last extinction training session we reconditioned the animals by re-exposure to the US. Immediately after, rats were microinfused with the protein synthesis inhibitor anisomycin or the actin rearrangement inhibitor cytochalasin D into either the BLA or the CA1. The results of this study show differential involvement of anisomycin and cytochalasin D in the acquisition and reacquisition of contextual fear conditioning. Specifically, while the microinfusion of anisomycin into the BLA or the CA1 immediately after reconditioning of fear did not inhibit the return of fear, the microinfusion of cytochalsin D into either the BLA or the CA1 attenuated fear responses. Interestingly, the initial acquisition of contextual fear memory is dependent on intra-BLA and CA1 protein synthesis and cytoskeletal rearrangement, since the microinfusion of these drugs blocked the formation of long-term fear memory. The results suggest that the two processes of acquisition and reacquisition of fear are not identical and they engage different mechanisms.

Re-emergence of extinguished auditory-cued conditioned fear following a sub-conditioning procedure: effects of hippocampal and prefrontal tetanic stimulations.

Post-extinction exposure of rats to a sub-conditioning procedure can evoke conditioned fear, which may correspond to fear return and/or fear learning potentiation. The aim of the present study was to clarify this issue and examine the effects of tetanic stimulation of the hippocampus (HPC) and medial prefrontal cortex (mPFC), two brain regions implicated in post-extinction modulation of conditioned fear. Rats were initially submitted to five tone-shock pairings with either a 0.7-mA or 0.1-mA shock. Tone-evoked freezing was observed only with the higher shock intensity, indicating that the 0.1-mA shock corresponded to a sub-conditioning procedure. All conditioned rats underwent fear extinction with 20 tone-alone trials. When retrained with the sub-conditioning procedure, they displayed again tone-evoked freezing, except when the initial tone was unpaired or a new tone was paired with the 0.1-mA shock, demonstrating fear return rather than fear learning potentiation. We also found that HPC and mPFC tetanic stimulations, applied 24h after the sub-conditioning procedure, similarly reduced this fear return. However, mPFC inactivation abolished temporary HPC tetanus effect, whereas HPC inactivation did not interfere with mPFC tetanus effect. These data confirm our previous findings and reveal the nature of HPC-mPFC interactions in post-extinction modulation of conditioned fear.

Exposure to a novel context following contextual fear conditioning enhances the induction of hippocampal long-term potentiation.

The prior behavioral experience of an animal can influence the direction and the probability of long-term plasticity induced at the activated synapses. In the present study, we compared alterations in long-term potentiation in the rat CA1 of the hippocampus following post-fear conditioning exposure to the conditioning context vs. a novel context. Furthermore, we examined whether the alterations in long-term potentiation are dependent on the prior formation of context-shock fear memory association. Whereas retrieval of fear memory 1 h after conditioning in the conditioning context was associated with impairment in the magnitude of long-term potentiation, exposure to a novel context at the same time point was associated with a robust increase in long-term potentiation. This effect was time-dependent, as exposure to a novel context 24 h after conditioning resulted in impaired long-term potentiation. Furthermore, preventing the formation of a fear context-shock association resulted in different modifications to long-term potentiation levels, regardless of whether association formation was prevented behaviorally (i.e. using a minimal context-shock association) or pharmacologically (using the N-methyl-d-aspartic acid receptor antagonist MK801). Our findings suggest that exposure to a novel environment following fear conditioning induces a form of metaplasticity that enhances the acquisition of novel information and could prevent acute stress-associated impairments in long-term potentiation.

A novel role for protein synthesis in long-term neuronal plasticity: maintaining reduced postburst afterhyperpolarization.

Memory consolidation, the process of transformation of short-term to long-term memory, has been shown to be protein synthesis dependent in a variety of different learning paradigms, brain structures, and species. At the cellular level, protein synthesis was shown to be crucial for induction of long-term synaptic plasticity; application of protein synthesis inhibitors prevents the transformation of early long-term potentiation (LTP) to late LTP. Thus, protein synthesis has been traditionally thought to affect long-term memory consolidation by stabilizing synaptic transmission. However, long-term memory is not supported only by modulation of synaptic strength; modifications in intrinsic neuronal properties also subserve learning-related behavioral changes. Learning-induced reduction in the postburst afterhyperpolarization (AHP), which results with enhanced neuronal excitability and decreased spike frequency adaptation, is apparent in hippocampal and cortical pyramidal neurons. Such postburst AHP reduction lasts for days after training completion and is implicated in maintaining learned skills. Short-term modulation of intrinsic neuronal excitability can be also induced in vitro. Intense synaptic activation induces AHP reduction and enhanced neuronal excitability in hippocampal pyramidal neurons. Here, we show that synaptic activation-induced short-term postburst AHP reduction can be transformed to long-term AHP reduction, such that persists for prolonged time periods. This long-lasting AHP reduction is protein synthesis dependent for up to 1 h after induction. We suggest that, much like synaptic plasticity, activity-induced long-lasting modulation of intrinsic neuronal excitability requires molecular consolidation. It would appear that both synaptic and intrinsic modifications and maintenance are activated jointly to enable long-lasting memories.

D-cycloserine into the BLA reverses the impairing effects of exposure to stress on the extinction of contextual fear, but not conditioned taste aversion.

We investigated whether the N-methyl-D-aspartate (NMDA) receptor partial agonist D-cycloserine (DCS, 20 microg/side) microinfused into the basolateral amygdala (BLA) would reverse stress-induced impairment of extinction in two aversive learning paradigms: contextual fear conditioning and conditioned taste aversion (CTA). We found that DCS in the BLA show differential involvement in the extinction of these two paradigms and in its modulation of stress-induced impairment of extinction. This may suggest that the dysfunctional extinction of fear and taste aversion following exposure to a stressful experience may be modulated by different mechanisms.

Hepatotoxicity of anti-inflammatory and analgesic drugs: ultrastructural aspects.

With the increasing incidence of drug-induced liver disease, attempts are being made to better understand the mechanisms behind these frequently life-endangering reactions. Analgesics and anti-inflammatory drugs are a major group exhibiting hepatotoxicity. We review research relating to these reactions, focusing on ultrastructural findings, which may contribute to the comprehension and possible avoidance of drug-induced liver disease. We also present some original observations on clinical material and cultured cells exposed to acetaminophen alone or in combination with the antioxidant N-acetylcysteine or the P-glycoprotein inhibitor verapamil.