Role of corticotropin releasing factor in anxiety disorders: a translational research perspective

Anxiety disorders are a group of mental disorders that include generalized anxiety disorder (GAD), panic disorder, phobic disorders (e.g., specific phobias, agoraphobia, social phobia) and posttraumatic stress disorder (PTSD). Anxiety disorders are among the most common of all mental disorders and, when coupled with an awareness of the disability and reduced quality of life they convey, they must be recognized as a serious public health problem. Over 20 years of preclinical studies point to a role for the CRF system in anxiety and stress responses. Clinical studies have supported a model of CRF dysfunction in depression and more recently a potential contribution to specific anxiety disorders (i.e., panic disorder and PTSD). Much work remains in both the clinical and preclinical fields to inform models of CRF function and its contribution to anxiety. First, we will review the current findings of CRF and HPA axis abnormalities in anxiety disorders. Second, we will discuss startle reflex measures as a tool for translational research to determine the role of the CRF system in development and maintenance of clinical anxiety.     

Group I mGluRs and Long-Term Depression: Potential Roles in Addiction?

Addiction is an enormous societal problem. A number of recent studies have focused on adaptations at glutamatergic synapses that may play a role in the behavioral responses to drugs of abuse. These studies have largely focused on NMDA receptor-dependent forms of synaptic plasticity such as NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD). A growing body of evidence, however, suggests that metabotropic glutamate receptors (mGluRs) also play important roles in the behavioral responses to drugs of abuse and participate in producing synaptic plasticity at glutamate synapses. In this review, we focus first on the evidence supporting a role for mGluRs in addiction and then on the properties of mGluR-dependent forms of synaptic plasticity, focusing in particular on Gq-linked receptor-induced LTD.     

Impaired cognitive function and reduced anxiety‐related behavior in a promyelocytic leukemia (PML) tumor suppressor protein‐deficient mouse

The promyelocytic leukemia (PML) protein is a tumor suppressor factor mostly known by its involvement in acute promyelocytic leukemia (APL). Interestingly, recent studies have provided evidence that, in the central nervous system, PML is involved in neurogenesis. However, prospective studies of PML in brain are lacking. To further understand the role of PML in the mammalian brain, we studied plasticity and behavioral changes in PML knockout mice. If PML is involved in neurogenesis, and neurogenesis is an important process for proper brain development as well as learning and memory functions, we hypothesized that PML might have a role in plasticity and cognition. Behavioral studies demonstrated that PML knockout mice present abnormalities in conditioned learning and spatial memory, as determined by fear conditioning and Morris water maze tasks. Experiments to determine normal exploratory behavior interestingly revealed that PML knockout mice present reduced anxiety‐related responses as compared to control animals. This was confirmed when PML knockout mice spent more time in the open arms of an elevated plus‐maze, which is an indication of decreased anxiety. Additionally, impairments in hippocampus‐dependent learning were mirrored by altered long‐term plasticity at Schaffer collateral‐CA1 synapses. We now provide the first evidence for an important role of PML in the brain, indicating that PML might have a role in synaptic plasticity and associated behavioral processes.     

Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

Extinction describes the process of attenuating behavioral responses to neutral stimuli when they no longer provide the reinforcement that has been maintaining the behavior. There is close correspondence between fear and human anxiety, and therefore studies of extinction learning might provide insight into the biological nature of anxiety-related disorders such as post-traumatic stress disorder, and they might help to develop strategies to treat them. Preclinical research aims to aid extinction learning and to induce targeted plasticity in extinction circuits to consolidate the newly formed memory. Vagus nerve stimulation (VNS) is a powerful approach that provides tight temporal and circuit-specific release of neurotransmitters, resulting in modulation of neuronal networks engaged in an ongoing task. VNS enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here, we provide a detailed protocol for the preparation of custom-made parts and the surgical procedures required for VNS in rats. Using this protocol we show how VNS can facilitate the extinction of conditioned fear responses in an auditory fear conditioning task. In addition, we provide evidence that VNS modulates synaptic plasticity in the pathway between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA), which is involved in the expression and modulation of extinction memory.     

Barrels by the sea: Barrels XX meeting report

The 20th annual Barrels meeting brought together researchers who utilize behavioral, physiological, anatomical, and molecular techniques to understand the structure and function of the barrel system. Barrels XX featured talks on the role inhibition has in shaping cortical responses within the barrel system, the molecular cues that influence the development of the whisker-to-barrel system, and the synaptic plasticity that can shape responses within the system. The meeting highlighted why the whisker-to-barrel system is an ideal model to investigate the development of cortical circuitry and how its functioning can influence behavioral responses.     

Actin Out: Regulation of the Synaptic Cytoskeleton

The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during spinogenesis and experience-dependent plasticity. Multiple mutations associated with human neurodevelopmental and psychiatric disorders involve genes that encode regulators of the synaptic cytoskeleton. A major, unresolved question is how the disruption of specific actin filament structures leads to the onset and progression of complex synaptic and behavioral phenotypes. This review will cover established and emerging mechanisms of actin cytoskeletal remodeling and how this influences specific aspects of spine biology that are implicated in disease.     

Barrels by the sea: Barrels XX meeting report

The 20th annual Barrels meeting brought together researchers who utilize behavioral, physiological, anatomical, and molecular techniques to understand the structure and function of the barrel system. Barrels XX featured talks on the role inhibition has in shaping cortical responses within the barrel system, the molecular cues that influence the development of the whisker-to-barrel system, and the synaptic plasticity that can shape responses within the system. The meeting highlighted why the whisker-to-barrel system is an ideal model to investigate the development of cortical circuitry and how its functioning can influence behavioral responses.     

Anandamide and 2-arachidonoylglycerol: Pharmacological Properties, Functional Features, and Emerging Specificities of the Two Major Endocannabinoids

Since the discovery of endocannabinoids and their receptors, two major members of the endocannabinoid family, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), have been regarded almost as twin brothers. Pharmacological properties were initially considered to be similar, as these molecules were believed mutually exchangeable and almost indistinguishable in the regulation of synaptic functions, such as long- and short-term synaptic plasticity, and in behavioral aspects, such as learning and memory, reward and addiction, antinociception, and anxiety. In recent years, however, endocannabinoid signaling specificity began to emerge, in particular, due to the production of genetically engineered mice lacking key enzymes in endocannabinoid synthesis or degradation, together with the development of selective inhibitors of AEA or 2-AG catabolic enzymes. Evidence now suggests that AEA and 2-AG possess specific pharmacological properties, are engaged in different forms of synaptic plasticity, and take part in different behavioral functions. In this review, we provide an overview on similarities and specificities of the two endocannabinoids in the CNS and on the unresolved questions concerning their role in synaptic signaling.     

14-3-3 proteins in neuronal development and function

The 14-3-3 proteins are small, cytosolic, evolutionaritly conserved proteins expressed abundantly in the nervous system. Although they were discovered more than 30 yr ago, their function in the nervous system has remained enigmatic. Several recent studies have helped to clarify their biological function. Crystallographic investigations have revealed that 14-3-3 proteins exist as dimers and that they contain a specific region for binding to other proteins. The interacting proteins, in turn, contain a 14-3-3 binding motif; proteins that interact with 14-3-3 dimers include PKC and Raf, protein kinases with critical roles in neuronal signaling. These proteins are capable of activating Raf in vitro, and this role has been verified by in vivo studies inDrosophila. Most interestingly, mutations in theDrosophila 14-3-3 genes disrupt neuronal differentiation, synaptic plasticity, and behavioral plasticity, establishing a role for these proteins in the development and function of the nervous system.     

Emergence of Functional Specificity in Balanced Networks with Synaptic Plasticity

In rodent visual cortex, synaptic connections between orientation-selective neurons are unspecific at the time of eye opening, and become to some degree functionally specific only later during development. An explanation for this two-stage process was proposed in terms of Hebbian plasticity based on visual experience that would eventually enhance connections between neurons with similar response features. For this to work, however, two conditions must be satisfied: First, orientation selective neuronal responses must exist before specific recurrent synaptic connections can be established. Second, Hebbian learning must be compatible with the recurrent network dynamics contributing to orientation selectivity, and the resulting specific connectivity must remain stable for unspecific background activity. Previous studies have mainly focused on very simple models, where the receptive fields of neurons were essentially determined by feedforward mechanisms, and where the recurrent network was small, lacking the complex recurrent dynamics of large-scale networks of excitatory and inhibitory neurons. Here we studied the emergence of functionally specific connectivity in large-scale recurrent networks with synaptic plasticity. Our results show that balanced random networks, which already exhibit highly selective responses at eye opening, can develop feature-specific connectivity if appropriate rules of synaptic plasticity are invoked within and between excitatory and inhibitory populations. If these conditions are met, the initial orientation selectivity guides the process of Hebbian learning and, as a result, functionally specific and a surplus of bidirectional connections emerge. Our results thus demonstrate the cooperation of synaptic plasticity and recurrent dynamics in large-scale functional networks with realistic receptive fields, highlight the role of inhibition as a critical element in this process, and paves the road for further computational studies of sensory processing in neocortical network models equipped with synaptic plasticity.     

Cortical network reorganization guided by sensory input features

 Sensory experience alters the functional organization of cortical networks. Previous studies using behavioral training motivated by aversive or rewarding stimuli have demonstrated that cortical plasticity is specific to salient inputs in the sensory environment. Sensory experience associated with electrical activation of the basal forebrain (BasF) generates similar input specific plasticity. By directly engaging plasticity mechanisms and avoiding extensive behavioral training, BasF stimulation makes it possible to efficiently explore how specific sensory features contribute to cortical plasticity. This review summarizes our observations that cortical networks employ a variety of strategies to improve the representation of the sensory environment. Different combinations of receptive-field, temporal, and spectrotemporal plasticity were generated in primary auditory cortex neurons depending on the pitch, modulation rate, and order of sounds paired with BasF stimulation. Simple tones led to map expansion, while modulated tones altered the maximum cortical following rate. Exposure to complex acoustic sequences led to the development of combination-sensitive responses. This remodeling of cortical response characteristics may reflect changes in intrinsic cellular mechanisms, synaptic efficacy, and local neuronal connectivity. The intricate relationship between the pattern of sensory activation and cortical plasticity suggests that network-level rules alter the functional organization of the cortex to generate the most behaviorally useful representation of the sensory environment. Received: 14 January 2002 / Accepted: 15 March 2002 Correspondence to: M.P. Kilgard (e-mail: kilgard@utdallas.edu, Tel.: +1-972-8832345, Fax: +1-972-8832491)     

Genetic influences on behavioral inhibition and anxiety in juvenile rhesus macaques

In humans and other animals, behavioral responses to threatening stimuli are an important component of temperament. Among children, extreme behavioral inhibition elicited by novel situations or strangers predicts the subsequent development of anxiety disorders and depression. Genetic differences among children are known to affect risk of developing behavioral inhibition and anxiety, but a more detailed understanding of genetic influences on susceptibility is needed. Nonhuman primates provide valuable models for studying the mechanisms underlying human behavior. Individual differences in threat-induced behavioral inhibition (freezing behavior) in young rhesus monkeys are stable over time and reflect individual levels of anxiety. This study used the well-established human intruder paradigm to elicit threat-induced freezing behavior and other behavioral responses in 285 young pedigreed rhesus monkeys. We examined the overall influence of quantitative genetic variation and tested the specific effect of the serotonin transporter promoter repeat polymorphism. Quantitative genetic analyses indicated that the residual heritability of freezing duration (behavioral inhibition) is h ² = 0.384 ( P  = 0.012) and of 'orienting to the intruder' (vigilance) is h ² = 0.908 ( P  = 0.00001). Duration of locomotion and hostility and frequency of cooing were not significantly heritable. The serotonin transporter polymorphism showed no significant effect on either freezing or orienting to the intruder. Our results suggest that this species could be used for detailed studies of genetic mechanisms influencing extreme behavioral inhibition, including the identification of specific genes that are involved in predisposing individuals to such behavior.     

Protein kinase signaling in synaptic plasticity and memory

The relay of extracellular signals into changes in cellular physiology involves a Byzantine array of intracellular signaling pathways, of which cytoplasmic protein kinases are a crucial component. In the nervous system, a great deal of effort has focused on understanding the conversion of patterns of synaptic activity into long-lasting changes in synaptic efficacy that are thought to underlie memory. The goal is both to understand synaptic plasticity mechanisms, such as long-term potentiation, at a molecular level and to understand the relationship of these synaptic mechanisms to behavioral memory. Although both involve the activation of multiple signaling pathways, recent studies are beginning to define discrete roles and mechanisms for individual kinases in the different temporal phases of both synaptic and behavioral plasticity.     

GPR55, a G-Protein Coupled Receptor for Lysophosphatidylinositol,Plays a Role in Motor Coordination

The G-protein coupled receptor 55 (GPR55) is activated by lysophosphatidylinositols and some cannabinoids. Recent studies found prominent roles for GPR55 in neuropathic/inflammatory pain, cancer and bone physiology. However, little is known about the role of GPR55 in CNS development and function. To address this question, we performed a detailed characterization of GPR55 knockout mice using molecular, anatomical, electrophysiological, and behavioral assays. Quantitative PCR studies found that GPR55 mRNA was expressed (in order of decreasing abundance) in the striatum, hippocampus, forebrain, cortex, and cerebellum. GPR55 deficiency did not affect the concentrations of endocannabinoids and related lipids or mRNA levels for several components of the endocannabinoid system in the hippocampus. Normal synaptic transmission and short-term as well as long-term synaptic plasticity were found in GPR55 knockout CA1 pyramidal neurons. Deleting GPR55 function did not affect behavioral assays assessing muscle strength, gross motor skills, sensory-motor integration, motor learning, anxiety or depressive behaviors. In addition, GPR55 null mutant mice exhibited normal contextual and auditory-cue conditioned fear learning and memory in a Pavlovian conditioned fear test. In contrast, when presented with tasks requiring more challenging motor responses, GPR55 knockout mice showed impaired movement coordination. Taken together, these results suggest that GPR55 plays a role in motor coordination, but does not strongly regulate CNS development, gross motor movement or several types of learned behavior.     

Functional roles of synapsin: lessons from invertebrates

Data collected from the invertebrate models have allowed to establish several of the basic mechanisms of neuronal function and pioneered the studies on the molecular and cellular mechanisms involved in behavioral responses. In the 1970s, the first synaptic proteins – including synapsin – being identified, the first attempts to evaluate their synaptic function were done using available invertebrate preparations. Forty years later, it appears that deductions made from invertebrate synapsin were largely validated in vertebrates, probably reflecting the phylogenic conservation of some specific synapsin sub-domains. In this review, in light of insights got from invertebrate preparations, we discuss the role of synapsin in synaptogenesis and synaptic function, especially on short term plasticity.     

CPEB4 Knockout Mice Exhibit Normal Hippocampus-Related Synaptic Plasticity and Memory

Regulated RNA translation is critical to provide proteins needed to maintain persistent modification of synaptic strength, which underlies the molecular basis of long-term memory (LTM). Cytoplasmic polyadenylation element-binding proteins (CPEBs) are sequence-specific RNA-binding proteins and regulate translation in various tissues. All four CPEBs in vertebrates are expressed in the brain, including the hippocampal neurons, suggesting their potential roles in translation-dependent plasticity and memory. Although CPEB1 and CPEB3 have been shown to control specific kinds of hippocampus-related LTM, the role of CPEB2 and CPEB4 in learning and memory remains elusive. Thus, we generated CPEB4 knockout (KO) mice and analyzed them using several behavioral tests. No difference was found in the anxiety level, motor coordination, hippocampus-dependent learning and memory between the KO mice and their wild-type (WT) littermates. Electrophysiological recordings of multiple forms of synaptic plasticity in the Schaffer collateral pathway-CA1 neurons also showed normal responses in the KO hippocampal slices. Morphological analyses revealed that the CPEB4-lacking pyramidal neurons possessed slightly elongated dendritic spines. Unlike its related family members, CPEB1 and CPEB3, CPEB4 seems to be dispensable for hippocampus-dependent plasticity, learning and memory.