Comparison of plasticity in vivo and in vitro in the developing visual cortex of normal and protein kinase A RIbeta-deficient mice

Howard P. Roffwarg: sleep pioneer, legend, and ontogenetic hypothesis author

Narrated in this article are accounts of the many contributions Howard P. Roffwarg, MD, made to the field of sleep research and sleep medicine across his entire professional career as a student, a mentor, a leader in the Sleep Research Society, a sleep medicine clinician, and a scientist who performed experimental investigations in humans and animals. Dr Roffwarg was the originator of what is known as the "Ontogenetic Hypothesis" of sleep. His research over many years on physiology has contributed greatly to much of the experimental support substantiating a role for rapid eye-movement sleep (REMS) in the early development of the brain. Though much is still unknown, the Ontogenetic Hypothesis, still to this day, inspires many neuroscientists in their investigations. These studies have demonstrated roles for both REMS and NREMS in development as well as on brain function throughout his life span. Dr Howard P. Roffwarg, is one of the legends in the field of sleep research.

Kv3.1 channels regulate the rate of critical period plasticity

Experience-dependent plasticity within visual cortex is controlled by postnatal maturation of inhibitory circuits, which are both morphologically diverse and precisely connected. Gene-targeted disruption of the voltage-dependent potassium channel K_v3.1 broadens action potentials and reduces net inhibitory function of parvalbumin (PV)-positive GABA subtypes within the neocortex. In mice lacking K_v3.1, the rate of input loss from an eye deprived of vision was slowed two-fold, despite otherwise normal critical period timecourse and receptive field properties. Rapid ocular dominance plasticity was restored by local or systemic enhancement of GABAergic transmission with acute benzodiazepine infusion. Diazepam instead exacerbated a global suppression of slow-wave oscillations during sleep described previously in these mutant mice, which therefore did not account for the rescued plasticity. Rapid ocular dominance shifts closely reflected K_v3.1 gene dosage that prevented prolonged spike discharge of their target pyramidal cells in vivo or the spike amplitude decrement of fast-spiking cells during bouts of high-frequency firing in vitro. Late postnatal expression of this unique channel in fast-spiking interneurons thus subtly regulates the speed of critical period plasticity with implications for mental illnesses.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

β-Catenin in the Adult Visual Cortex Regulates NMDA-Receptor Function and Visual Responses

The formation, plasticity and maintenance of synaptic connections is regulated by molecular and electrical signals. β-Catenin is an important protein in these events and regulates cadherin-mediated cell adhesion and the recruitment of pre- and postsynaptic proteins in an activity-dependent fashion. Mutations in the β-catenin gene can cause cognitive disability and autism, with life-long consequences. Understanding its synaptic function may thus be relevant for the treatment of these disorders. So far, β-catenin's function has been studied predominantly in cell culture and during development but knowledge on its function in adulthood is limited. Here, we show that ablating β-catenin in excitatory neurons of the adult visual cortex does not cause the same synaptic deficits previously observed during development. Instead, it reduces NMDA-receptor currents and impairs visual processing. We conclude that β-catenin remains important for adult cortical function but through different mechanisms than during development.

Core Differences in Synaptic Signaling Between Primary Visual and Dorsolateral Prefrontal Cortex

Neurons in primary visual cortex (V1) are more resilient than those in dorsolateral prefrontal cortex (dlPFC) in aging, schizophrenia and Alzheimer’s disease. The current study compared glutamate and neuromodulatory actions in macaque V1 to those in dlPFC, and found striking regional differences. V1 neuronal firing to visual stimuli depended on AMPA receptors, with subtle NMDA receptor contributions, while dlPFC depends primarily on NMDA receptors. Neuromodulatory actions also differed between regions. In V1, cAMP signaling increased neuronal firing, and the phosphodiesterase PDE4A was positioned to regulate cAMP effects on glutamate release from axons. HCN channels in V1 were classically located on distal dendrites, and enhanced cell firing. These data contrast with dlPFC, where PDE4A and HCN channels are concentrated in thin spines, and cAMP-HCN signaling gates inputs and weakens firing. These regional differences may explain why V1 neurons are more resilient than dlPFC neurons to the challenges of age and disease.

The spatial-temporal interaction in the LTP induction between layer IV to layer II/III and layer II/III to layer II/III connections in rats' visual cortex during the development

During the early developmental period, long-term potentiation (LTP) can be induced in both vertical and horizontal connections in the rat visual cortex. However, the temporal difference in LTP change between the two pathways during animal development remains unclear. In this study, LTP in vertical (from layer IV to layer II/III) and horizontal (from layer II/III to layer II/III) synaptic connections were recorded in brain slices from the same rats, and the developmental changes of LTP in both directions were compared within the animals' eye-opening period. The results showed that the LTP amplitudes declined to unobservable levels on P16 in the horizontal connections and on P20 in the vertical synaptic connections. Meanwhile, V-LTP (LTP induced in the vertical direction) was always stronger than H-LTP (LTP induced in the horizontal direction) under the same conditions of pairing stimulus (PS). Next, H-LTP and V-LTP were induced from the same neuron in layer II/III to determine the spatiotemporal interactions between layer II/III horizontal inputs and ascending synaptic inputs during the maturation of rat visual cortex. The data show that the weak PS, which failed to induce H-LTP alone, was able to induce H-LTP effectively while V-LTP was performed on P10. Our results suggest that V-LTP can strengthen H-LTP induction in the visual cortex during the early developmental period. In contrast, the regulatory effect of H-LTP on V-LTP was much weaker.

The use of in vivo, ex vivo, in vitro, computational models and volunteer studies in vision research and therapy, and their contribution to the Three Rs

Much is known about mammalian vision, and considerable progress has been achieved in treating many vision disorders, especially those due to changes in the eye, by using various therapeutic methods, including stem cell and gene therapy. While cells and tissues from the main parts of the eye and the visual cortex (VC) can be maintained in culture, and many computer models exist, the current non-animal approaches are severely limiting in the study of visual perception and retinotopic imaging. Some of the early studies with cats and non-human primates (NHPs) are controversial for animal welfare reasons and are of questionable clinical relevance, particularly with respect to the treatment of amblyopia. More recently, the UK Home Office records have shown that attention is now more focused on rodents, especially the mouse. This is likely to be due to the perceived need for genetically-altered animals, rather than to knowledge of the similarities and differences of vision in cats, NHPs and rodents, and the fact that the same techniques can be used for all of the species. We discuss the advantages and limitations of animal and non-animal methods for vision research, and assess their relative contributions to basic knowledge and clinical practice, as well as outlining the opportunities they offer for implementing the principles of the Three Rs (Replacement, Reduction and Refinement).

MeCP2 regulates the timing of critical period plasticity that shapes functional connectivity in primary visual cortex

Mutations in methyl-CpG-binding protein 2 (MeCP2) cause Rett syndrome, an autism spectrum-associated disorder with a host of neurological and sensory symptoms, but the pathogenic mechanisms remain elusive. Neuronal circuits are shaped by experience during critical periods of heightened plasticity. The maturation of cortical GABA inhibitory circuitry, the parvalbumin(+) (PV(+)) fast-spiking interneurons in particular, is a key component that regulates the initiation and termination of the critical period. Using MeCP2-null mice, we examined experience-dependent development of neural circuits in the primary visual cortex. The functional maturation of parvalbumin interneurons was accelerated upon vision onset, as indicated by elevated GABA synthetic enzymes, vesicular GABA transporter, perineuronal nets, and enhanced GABA transmission among PV interneurons. These changes correlated with a precocious onset and closure of critical period and deficient binocular visual function in mature animals. Reduction of GAD67 expression rescued the precocious opening of the critical period, suggesting its major role in MECP2-mediated regulation of experience-driven circuit development. Our results identify molecular changes in a defined cortical cell type and link aberrant developmental trajectory to functional deficits in a model of neuropsychiatric disorder.

PRKAR1B mutation associated with a new neurodegenerative disorder with unique pathology

Pathological accumulation of intermediate filaments can be observed in neurodegenerative disorders, such as Alzheimer's disease, frontotemporal dementia and Parkinson's disease, and is also characteristic of neuronal intermediate filament inclusion disease. Intermediate filaments type IV include three neurofilament proteins (light, medium and heavy molecular weight neurofilament subunits) and α-internexin. The phosphorylation of intermediate filament proteins contributes to axonal growth, and is regulated by protein kinase A. Here we describe a family with a novel late-onset neurodegenerative disorder presenting with dementia and/or parkinsonism in 12 affected individuals. The disorder is characterized by a unique neuropathological phenotype displaying abundant neuronal inclusions by haematoxylin and eosin staining throughout the brain with immunoreactivity for intermediate filaments. Combining linkage analysis, exome sequencing and proteomics analysis, we identified a heterozygous c.149T>G (p.Leu50Arg) missense mutation in the gene encoding the protein kinase A type I-beta regulatory subunit (PRKAR1B). The pathogenicity of the mutation is supported by segregation in the family, absence in variant databases, and the specific accumulation of PRKAR1B in the inclusions in our cases associated with a specific biochemical pattern of PRKAR1B. Screening of PRKAR1B in 138 patients with Parkinson's disease and 56 patients with frontotemporal dementia did not identify additional novel pathogenic mutations. Our findings link a pathogenic PRKAR1B mutation to a novel hereditary neurodegenerative disorder and suggest an altered protein kinase A function through a reduced binding of the regulatory subunit to the A-kinase anchoring protein and the catalytic subunit of protein kinase A, which might result in subcellular dislocalization of the catalytic subunit and hyperphosphorylation of intermediate filaments.

Experience-dependent regulation of functional maps and synaptic protein expression in the cat visual cortex

Although the basis of our knowledge of experience-dependent plasticity comes from studies on carnivores and primates, studies examining the physiological and molecular mechanisms that underlie development and plasticity have increasingly employed mice. We have used several common rearing paradigms, such as dark-rearing and monocular deprivation (MD), to examine the timing of the physiological and molecular changes to altered experience in the cat primary visual cortex. Dark-rearing from birth or for 1 week starting at 4 weeks of age produced a similar reduction in the amplitude of responses measured through intrinsic signal imaging and a reduction in orientation selectivity. One week of visual experience following dark-rearing until 4 weeks of age yielded normal responses in both amplitude and orientation selectivity. The depression of deprived-eye responses was similar in magnitude after 2 and 7 days of MD. In contrast, non-deprived-eye responses almost doubled in magnitude after 7 days compared with 2 days of MD. These changes in the functional properties of primary visual cortex neurons were mirrored by specific changes in synaptic protein expression. Changes in proteins such as the NR2A and NR2B subunits of the N-methyl-D-aspartate receptor, postsynaptic density protein 95, alpha-CA(2+) /calmodulin-dependent protein kinase II (αCaMKII), and GABA(A) α1a indicated that the levels of sensory activity regulated mechanisms associated with both excitatory (NR2A and NR2B) and inhibitory (GABA(A) α1a) transmission so as to maintain response homeostasis. Additionally, we found that MD regulated the AMPA receptor glutamate (GluR1) subunit as well as signalling molecules (αCaMKII and synaptic Ras GTPase activating protein, SynGAP) downstream of N-methyl-D-aspartate receptors. Proteins in a common signalling pathway appeared to have similar developmental expression profiles that were broadly similar between cats and rodents.

Activity-regulated genes as mediators of neural circuit plasticity

Modifications of neuronal circuits allow the brain to adapt and change with experience. This plasticity manifests during development and throughout life, and can be remarkably long lasting. Evidence has linked activity-regulated gene expression to the long-term structural and electrophysiological adaptations that take place during developmental critical periods, learning and memory, and alterations to sensory map representations in the adult. In all these cases, the cellular response to neuronal activity integrates multiple tightly coordinated mechanisms to precisely orchestrate long-lasting, functional and structural changes in brain circuits. Experience-dependent plasticity is triggered when neuronal excitation activates cellular signaling pathways from the synapse to the nucleus that initiate new programs of gene expression. The protein products of activity-regulated genes then work via a diverse array of cellular mechanisms to modify neuronal functional properties. Synaptic strengthening or weakening can reweight existing circuit connections, while structural changes including synapse addition and elimination create new connections. Posttranscriptional regulatory mechanisms, often also dependent on activity, further modulate activity-regulated gene transcript and protein function. Thus, activity-regulated genes implement varied forms of structural and functional plasticity to fine-tune brain circuit wiring.

Amphetamine modulation of long-term potentiation in the prefrontal cortex: dose dependency, monoaminergic contributions, and paradoxical rescue in hyperdopaminergic mutant

Amphetamine can improve cognition in healthy subjects and patients with schizophrenia, attention-deficit hyperactivity disorder, and other neuropsychiatric diseases; higher doses, however, can impair cognitive function, especially those mediated by the prefrontal cortex. We investigated how amphetamine affects prefrontal cortex long-term potentiation (LTP), a cellular correlate of learning and memory, in normal and hyperdopaminergic mice lacking the dopamine transporter. Acute amphetamine treatment in wild-type mice produced a biphasic dose-response modulation of LTP, with a low dose enhancing LTP and a high dose impairing it. Amphetamine-induced LTP enhancement was prevented by pharmacological blockade of D(1) - (but not D(2)-) class dopamine receptors, by blockade of β-adrenergic receptors, or by inhibition of cAMP-PKA signaling. In contrast, amphetamine-induced LTP impairment was prevented by inhibition of post-synaptic protein phosphatase-1, a downstream target of PKA signaling, or by blockade of either D(1) - or D(2)-class dopamine, but not noradrenergic, receptors. Thus, amphetamine biphasically modulates LTP via cAMP-PKA signaling orchestrated mainly through dopamine receptors. Unexpectedly, amphetamine restored the loss of LTP in dopamine transporter-knockout mice primarily by activation of the noradrenergic system. Our results mirror the biphasic effectiveness of amphetamine in humans and provide new mechanistic insights into its effects on cognition under normal and hyperdopaminergic conditions.

Sequential steps underlying neuronal plasticity induced by a transient exposure to gabazine

Periods of intense electrical activity can initiate neuronal plasticity leading to long lasting changes of network properties. By combining multielectrode extracellular recordings with DNA microarrays, we have investigated in rat hippocampal cultures the temporal sequence of events of neuronal plasticity triggered by a transient exposure to the GABA(A) receptor antagonist gabazine (GabT). GabT induced a synchronous bursting pattern of activity. The analysis of electrical activity identified three main phases during neuronal plasticity induced by GabT: (i) immediately after termination of GabT, an early synchronization (E-Sync) of the spontaneous electrical activity appears that progressively decay after 3-6 h. E-Sync is abolished by inhibitors of the ERK1/2 pathway but not by inhibitors of gene transcription; (ii) the evoked response (induced by a single pulse of extracellular electrical stimulation) was maximally potentiated 3-10 h after GabT (M-LTP); and (iii) at 24 h the spontaneous electrical activity became more synchronous (L-Sync). The genome-wide analysis identified three clusters of genes: (i) an early rise of transcription factors (Cluster 1), primarily composed by members of the EGR and Nr4a families, maximally up-regulated 1.5 h after GabT; (ii) a successive up-regulation of some hundred genes, many of which known to be involved in LTP (Cluster 2), 3 h after GabT likely underlying M-LTP. Moreover, in Cluster 2 several genes coding for K(+) channels are down-regulated at 24 h. (iii) Genes in Cluster 3 are up-regulated at 24 h and are involved in cellular homeostasis. This approach allows relating different steps of neuronal plasticity to specific transcriptional profiles.

Regulation of NMDA receptor Ca2+ signalling and synaptic plasticity

NMDARs (N-methyl-D-aspartate receptors) are critical for synaptic function throughout the CNS (central nervous system). NMDAR-mediated Ca(2+) influx is implicated in neuronal differentiation, neuronal migration, synaptogenesis, structural remodelling, long-lasting forms of synaptic plasticity and higher cognitive functions. NMDAR-mediated Ca(2+) signalling in dendritic spines is not static, but can be remodelled in a cell- and synapse-specific manner by NMDAR subunit composition, protein kinases and neuronal activity during development and in response to sensory experience. Recent evidence indicates that Ca(2+) permeability of neuronal NMDARs, NMDAR-mediated Ca(2+) signalling in spines and induction of NMDAR-dependent LTP (long-term potentiation) at hippocampal Schaffer collateral-CA1 synapses are under control of the cAMP/PKA (protein kinase A) signalling cascade. Thus, by enhancing Ca(2+) influx through NMDARs in spines, PKA can regulate the induction of LTP. An emerging concept is that activity-dependent regulation of NMDAR-mediated Ca(2+) signalling by PKA and by extracellular signals that modulate cAMP or protein phosphatases at synaptic sites provides a dynamic and potentially powerful mechanism for bi-directional regulation of synaptic efficacy and remodelling.

AKAP-independent localization of type-II protein kinase A to dynamic actin microspikes

Regulation of the cyclic AMP-dependent protein kinase (PKA) in subcellular space is required for cytoskeletal dynamics and chemotaxis. Currently, spatial regulation of PKA is thought to require the association of PKA regulatory (R) subunits with A-kinase anchoring proteins (AKAPs). Here, we show that the regulatory RIIalpha subunit of PKA associates with dynamic actin microspikes in an AKAP-independent manner. Both endogenous RIIalpha and a GFP-RIIalpha fusion protein co-localize with F-actin in microspikes within hippocampal neuron growth cones and the leading edge lamellae of NG108-15 cells. Live-cell imaging demonstrates that RIIalpha-associated microspikes are highly dynamic and that the coupling of RIIalpha to actin is tight, as the movement of both actin and RIIalpha are immediately and coincidently stopped by low-dose cytochalasin D. Importantly, co-localization of RIIalpha and actin in these structures is resistant to displacement by a cell-permeable disrupter of PKA-AKAP interactions. Biochemical fractionation confirms that a substantial pool of PKA RIIalpha is associated with the detergent-insoluble cytoskeleton and is resistant to extraction by a peptide inhibitor of AKAP interactions. Finally, mutation of the AKAP-binding domain of RIIalpha fails to disrupt its association with actin microspikes. These data provide the first demonstration of the physical association of a kinase with such dynamic actin structures, as well as the first demonstration of the ability of type-II PKA to localize to discrete subcellular structures independently of canonical AKAP function. This association is likely to be important for microfilament dynamics and cell migration and may prime the investigation of novel mechanisms for localizing PKA activity.

cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies

3'-5'-Cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger generated in response to activation of G(s) protein-coupled receptors. In the heart, cAMP mediates the catecholaminergic control on heart rate and contractility but, at the same time, it is responsible for the functional response to a wide variety of other hormones and neurotransmitters, raising the question of how the myocyte can decode the cAMP signal and generate the appropriate functional output to each individual extracellular stimulus. A growing body of evidence points to the spatial organization of the components of the cAMP signalling pathway in distinct, spatially segregated signalling domains as the key feature underpinning specificity of response and data is emerging, indicating that alteration of spatial control of the cAMP signal cascade associates with heart pathology. Most of the details of the molecular organization and regulation of individual cAMP signalling compartments are still to be elucidated but future research should provide the knowledge necessary to develop and test new therapeutic strategies that, by acting on a limited subset of downstream targets, would improve efficacy and minimize off-target effects.

Identification and localization of functional subdivisions in the visual cortex of the adult mouse

We investigated the anatomical characteristics of the mouse visual system through in situ hybridization for the neuronal activity marker zif268. Our main goal was to delineate the full extent of the cortical region processing visual information and additionally to identify the monocularly and binocularly driven subregions therein. We therefore analyzed the neocortex of monocularly and binocularly enucleated mice versus visually stimulated control mice. These visual manipulations revealed eye-specific parcellations at the neocortical level. In binocularly enucleated mice we detected an unambiguous lateral border between visually driven and nonvisual cortex based on the clear deprivation-induced reduction in zif268 expression in the first. However, medially a transition zone of intermediate intensity was found between primarily visual, that is V1 and multimodal retrosplenial cortex. Also in monocularly enucleated mice, the visual cortex contralateral to the deprived eye clearly displayed distinct regions of lower signal than the ipsilateral cortex. Yet interspersed between these regions of basal activity we could clearly identify a zone of high activity spanning the V1-V2L border. A second zone of higher activity was noticeable near the medial border of visual cortex. Comparison with binocularly enucleated mice indicates the presence of both binocular input as well as nonvisual input in this medial cortical region and thus confirms the transitional nature of the recently described rostromedial areas.

Bidirectional synaptic mechanisms of ocular dominance plasticity in visual cortex

As in other mammals with binocular vision, monocular lid suture in mice induces bidirectional plasticity: rapid weakening of responses evoked through the deprived eye followed by delayed strengthening of responses through the open eye. It has been proposed that these bidirectional changes occur through three distinct processes: first, deprived-eye responses rapidly weaken through homosynaptic long-term depression (LTD); second, as the period of deprivation progresses, the modification threshold determining the boundary between synaptic depression and synaptic potentiation becomes lower, favouring potentiation; and third, facilitated by the decreased modification threshold, open-eye responses are strengthened via homosynaptic long-term potentiation (LTP). Of these processes, deprived-eye depression has received the greatest attention, and although several alternative hypotheses are also supported by current research, evidence suggests that α-amino-3- hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor endocytosis through LTD is a key mechanism. The change in modification threshold appears to occur partly through changes in N-methyl-d-aspartate (NMDA) receptor subunit composition, with decreases in the ratio of NR2A to NR2B facilitating potentiation. Although limited research has directly addressed the question of open-eye potentiation, several studies suggest that LTP could account for observed changes in vivo. This review will discuss evidence supporting this three-stage model, along with outstanding issues in the field.

Sensory deprivation unmasks a PKA-dependent synaptic plasticity mechanism that operates in parallel with CaMKII

Calcium/calmodulin kinase II (CaMKII) is required for LTP and experience-dependent potentiation in the barrel cortex. Here, we find that whisker deprivation increases LTP in the layer IV to II/III pathway and that PKA antagonists block the additional LTP. No LTP was seen in undeprived CaMKII-T286A mice, but whisker deprivation again unmasked PKA-sensitive LTP. Infusion of a PKA agonist potentiated EPSPs in deprived wild-types and deprived CaMKII-T286A point mutants but not in undeprived animals of either genotype. The PKA-dependent potentiation mechanism was not present in GluR1 knockouts. Infusion of a PKA antagonist caused depression of EPSPs in undeprived but not deprived cortex. LTD was occluded by whisker deprivation and blocked by PKA manipulation, but not blocked by cannabinoid antagonists. NMDA receptor currents were unaffected by sensory deprivation. These results suggest that sensory deprivation causes synaptic depression by reversing a PKA-dependent process that may act via GluR1.

Age-dependent decline in supragranular long-term synaptic plasticity by increased inhibition during the critical period in the rat primary visual cortex

Supragranular long-term potentiation (LTP) and depression (LTD) are continuously induced in the pathway from layer 4 during the critical period in the rodent primary visual cortex, which limits the use of supragranular long-term synaptic plasticity as a synaptic model for the mechanism of ocular dominance (OD) plasticity. The results of the present study demonstrate that the pulse duration of extracellular stimulation to evoke a field potential (FP) is critical to induction of LTP and LTD in this pathway. LTP and LTD were induced in the pathway from layer 4 to layer 2/3 in slices from 3-wk-old rats when FPs were evoked by 0.1- and 0.2-ms pulses. LTP and LTD were induced in slices from 5-wk-old rats when evoked by stimulation with a 0.2-ms pulse but not by stimulation with a 0.1-ms pulse. Both the inhibitory component of FP and the inhibitory/excitatory postsynaptic potential amplitude ratio evoked by stimulation with a 0.1-ms pulse were greater than the values elicited by a 0.2-ms pulse. Stimulation with a 0.1-ms pulse at various intensities that showed the similar inhibitory FP component with the 0.2-ms pulse induced both LTD and LTP in 5-wk-old rats. Thus extracellular stimulation with shorter-duration pulses at higher intensity resulted in greater inhibition than that observed with longer-duration pulses at low intensity. This increased inhibition might be involved in the age-dependent decline of synaptic plasticity during the critical period. These results provide an alternative synaptic model for the mechanism of OD plasticity.

Developmental switch in requirement for PKA RIIbeta in NMDA-receptor-dependent synaptic plasticity at Schaffer collateral to CA1 pyramidal cell synapses

The cAMP/protein kinase A (PKA) signaling cascade is crucial for synaptic plasticity in a wide variety of species. PKA regulates Ca2+ permeation through NMDA receptors (NMDARs) and induction of NMDAR-dependent synaptic plasticity at the Schaffer collateral to CA1 pyramidal cell synapse. Whereas the role of PKA in induction of NMDAR-dependent LTP at CA1 synapses is established, the identity of PKA isoforms involved in this phenomenon is less clear. Here we report that protein synthesis-independent NMDAR-dependent LTP at the Schaffer collateral-CA1 synapse in the hippocampus is deficient, but NMDAR-dependent LTD is normal, in young (postnatal day 10 (P10)-P14) mice lacking PKA RIIbeta, the PKA regulatory protein that links PKA to NMDARs at synaptic sites. In contrast, in young adult (P21-P28) mice lacking PKA RIIbeta, LTP is normal and LTD is abolished. These findings indicate that distinct PKA isoforms may subserve distinct forms of synaptic plasticity and are consistent with a developmental switch in the signaling cascades required for LTP induction.

Regulation of hippocampus-dependent memory by cyclic AMP-dependent protein kinase

The hippocampus is crucial for the consolidation of new declarative long-term memories. Genetic and behavioral experimentation have revealed that several protein kinases are critical for the formation of hippocampus-dependent long-term memories. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of hippocampus-dependent memory. We review evidence that PKA is required for hippocampus-dependent memory in mammals, and we highlight some of the proteins that have been implicated as targets of PKA. Future directions and open questions regarding the role of PKA in memory storage are also described.

Exposure to enriched environments increases brain nitric oxide synthase and improves cognitive performance in prepubertal but not in young rats

Rats were randomly assigned to enriched (EE) or standard environments (SE) at 21 or 73 days of age, for 17 days. Half of the rats of each rearing condition were trained in a radial maze (RM). At 38 days (pre-pubertal) or 90 days (young), rats were sacrificed and brain cytosolic and mitochondrial nitric oxide synthase (mtNOS) activity was assayed. Western blot analysis of brain mtNOS was conducted. In the pre-pubertal group, EE rats improved their performance in the RM while SE rats did not. In the young group, SE and EE rats showed a random performance in the RM. In SE pre-pubertal rats, training increased brain cytosolic NOS and mtNOS activity by 68% and 82%. In EE non-trained pre-pubertal rats, brain cytosolic NOS and mtNOS activity increased by 80% and 60%, as compared with SE non-trained pre-pubertal rats. In EE pre-pubertal rats that were trained, brain cytosolic NOS and mtNOS activity increased by 70% and 90%, as compared with SE pre-pubertal rats that were not trained. A higher protein expression of brain mtNOS was found in EE rats, as compared with SE animals. Mitochondrial complex I activity was higher in EE than in SE rats. Training had no effect on complex I activity neither in SE nor in EE rats. In young rats, no significant differences in enzyme activities were found between EE and SE rats. These results support the hypothesis that brief exposure to EE and training produce effects on behavioral performance and on biochemical parameters in an age-dependent manner.

Reduced presynaptic efficiency of excitatory synaptic transmission impairs LTP in the visual cortex of BDNF-heterozygous mice

The neurotrophin brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival, axonal and dendritic growth and synapse formation. BDNF has also been reported to mediate visual cortex plasticity. Here we studied the cellular mechanisms of BDNF-mediated changes in synaptic plasticity, excitatory synaptic transmission and long-term potentiation (LTP) in the visual cortex of heterozygous BDNF-knockout mice (BDNF(+/-)). Patch-clamp recordings in slices showed an approximately 50% reduction in the frequency of miniature excitatory postsynaptic currents (mEPSCs) compared to wild-type animals, in the absence of changes in mEPSC amplitudes. A presynaptic impairment of excitatory synapses from BDNF(+/-) mice was further indicated by decreased paired-pulse ratio and faster synaptic fatigue upon prolonged repetitive stimulation at 40 Hz. In accordance, presynaptic theta-burst stimulation (TBS) failed to induce LTP at layer IV to layers II-III synapses during extracellular field-potential recordings in BDNF(+/-) animals. Changes in postsynaptic function could not be detected, as no changes were observed in either the amplitudes of evoked EPSCs, the ratios of AMPA : NMDA currents or the kinetics of evoked AMPA and NMDA EPSCs. In line with this observation, an LTP pairing paradigm that relies on direct postsynaptic depolarization under patch-clamp conditions could be induced successfully in BDNF(+/-) animals. These data suggest that a chronic reduction in the expression of BDNF to nearly 50% attenuates the efficiency of presynaptic glutamate release in response to repetitive stimulation, thereby impairing presynaptically evoked LTP in the visual cortex.

Developmental synaptic plasticity at the thalamocortical input to barrel cortex: mechanisms and roles

The thalamocortical (TC) input to layer IV provides the major pathway for ascending sensory information to the mammalian sensory cortex. During development there is a dramatic refinement of this input that underlies the maturation of the topographical map in layer IV. Over the last 10 years our understanding of the mechanisms of the developmental and experience-driven changes in synaptic function at TC synapses has been greatly advanced. Here we describe these studies that point to a key role for NMDA receptor-dependent synaptic plasticity, a role for kainate receptors and for a rapid maturation in GABAergic inhibition. The expression mechanisms of some of the forms of neonatal synaptic plasticity are novel and, in combination with other mechanisms, produce a layer IV circuit that exhibits functional properties necessary for mature sensory processing.

Experience-Dependent Plasticity in Adult Visual Cortex

Experience-dependent plasticity is a prominent feature of the mammalian visual cortex. Although such neural changes are most evident during development, adult cortical circuits can be modified by a variety of manipulations, such as perceptual learning and visual deprivation. Elucidating the underlying mechanisms at the cellular and synaptic levels is an essential step in understanding neural plasticity in the mature animal. Although developmental and adult plasticity share many common features, notable differences may be attributed to developmental cortical changes at multiple levels. These range from shifts in the molecular profiles of cortical neurons to changes in the spatiotemporal dynamics of network activity. In this review, we will discuss recent progress and remaining challenges in understanding adult visual plasticity, focusing on the primary visual cortex.

Monocular cells without ocular dominance columns

In many regions of the mammalian cerebral cortex, cells that share a common receptive field property are grouped into columns. Despite intensive study, the function of the cortical column remains unknown. In the squirrel monkey, the expression of ocular dominance columns is variable, with columns present in some animals and not in others. By searching for differences between animals with and without columns, it should be possible to infer how columns contribute to visual processing. Single-cell recordings outside layer 4C were made in nine squirrel monkeys, followed by labeling of ocular dominance columns in layer 4C. In the squirrel monkey, compared with the macaque, cells outside layer 4C were more likely to respond to stimulation of either eye whether ocular dominance columns were present or not. In three animals lacking ocular dominance columns, single cells were recorded from layer 4C. Remarkably, 20% of cells in layer 4C were monocular despite the absence of columns. This observation means that ocular dominance columns are not necessary for monocular cells to occur in striate cortex. In macaques each row of cytochrome oxidase (CO) patches is aligned with an ocular dominance column and receives koniocellular input serving one eye only. In squirrel monkeys this was not true: CO patches and ocular dominance columns had no spatial correlation and the koniocellular input to CO patches was binocular. Thus even when ocular dominance columns occur in the squirrel monkey, they do not transform the functional architecture to resemble that of the macaque.

Protein synthesis-independent plasticity mediates rapid and precise recovery of deprived eye responses

Monocular deprivation (MD) for a few days during a critical period of development leads to loss of cortical responses to stimulation of the deprived eye. Despite the profound effects of MD on cortical function, optical imaging of intrinsic signals and single-unit recordings revealed that deprived eye responses and orientation selectivity recovered a few hours after restoration of normal binocular vision. Moreover, recovery of deprived eye responses was not dependent upon mRNA translation, but required cortical activity. Interestingly, this fast recovery and protein synthesis independence was restricted to the hemisphere contralateral to the previously deprived eye. Collectively, these results implicate a relatively simple mechanistic process in the reactivation of a latent set of connections following restoration of binocular vision and provide new insight into how recovery of cortical function can rapidly occur in response to changes in sensory experience.

Cyclic nucleotides modulate genioglossus and hypoglossal responses to excitatory inputs in rats

RATIONALE

Previous studies modulating pharyngeal muscle activity with pharmacologic approaches have targeted membrane receptors on pharyngeal motoneurons. Whether modulation of intracellular pathways can increase pharyngeal muscle activity, however, has not been investigated but is relevant to pharmacologic treatments of obstructive sleep apnea.

OBJECTIVES

To determine if modulating the second messenger cyclic adenosine-3'-5'-monophosphate (cAMP) at the hypoglossal motor nucleus (HMN) will increase genioglossus activity across sleep- wake states.

METHODS

Forty-eight rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states and genioglossus and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the HMN to perfuse artificial cerebrospinal fluid and (1) forskolin (500 microM, adenylyl cyclase activator to increase cAMP), (2) a cAMP analog (500 microM), (3) iso-butyl-methylxanthine (IBMX; 300 microM, phosphodiesterase inhibitor), or (4) a cyclic guanosine-3'-5'-monophosphate (cGMP) analog (500 microM, 8-Br-cGMP).

MEASUREMENTS AND MAIN RESULTS

Forskolin and the cAMP analog at the HMN increased respiratory-related and tonic genioglossus activities in wakefulness and non-REM sleep but not REM sleep. IBMX did not affect genioglossus activity in awake or sleeping rats. However, IBMX abolished the robust excitatory responses to serotonin and phenylephrine at the HMN, but responses to non-N-methyl-D-aspartate receptor activation remained. These effects of IBMX were mimicked by 8-Br-cGMP.

CONCLUSIONS

Genioglossus responses to manipulation of cAMP at the HMN are differentially modulated by sleep-wake state. Selective abolition of serotonin and phenylephrine responses after IBMX suggests that under conditions of nonspecific phosphodiesterase inhibition the HMN is unresponsive to certain, otherwise potent, excitatory inputs. Similar responses with 8-Br-cGMP suggest this effect is likely mediated by cGMP pathways.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

Beta1-adrenergic receptor mediation in the lichen glucan PB-2-induced enhancement of long-term potentiation in the rat dentate gyrus in vivo

The mechanism underlying the enhancement of long-term potentiation (LTP) induced by the systemic administration of PB-2, an alpha(1-3)(1-4)glucan-containing fraction extracted from the lichen Flavoparmelia baltimorensis and a putative LTP-enhancing agent, was investigated in the rat dentate gyrus in vivo. Particular attention was paid to the role of adrenaline beta-receptors. An intravenous (i.v.) injection of PB-2 enhanced the induction of LTP, which was in turn inhibited by an i.v. injection of the adrenaline beta1-receptor antagonist atenolol. An intracerebroventricular injection of atenolol did not affect the induction of LTP, but completely suppressed the PB-2-induced enhancement of LTP. The infusion of atenolol into the recording site attenuated the PB-2-induced facilitation of LTP. These results suggest that the adrenaline beta1-receptors contribute to the enhancement of LTP induced by the systemic administration of PB-2, and that the functional beta1-receptors are located both centrally and peripherally.

Reversible blockade of experience-dependent plasticity by calcineurin in mouse visual cortex

Numerous protein kinases have been implicated in visual cortex plasticity, but the role of serine/threonine protein phosphatases has not yet been established. Calcineurin, the only known Ca2+/calmodulin-activated protein phosphatase in the brain, has been identified as a molecular constraint on synaptic plasticity in the hippocampus and on memory. Using transgenic mice overexpressing calcineurin inducibly in forebrain neurons, we now provide evidence that calcineurin is also involved in ocular dominance plasticity. A transient increase in calcineurin activity is found to prevent the shift of responsiveness in the visual cortex following monocular deprivation, and this effect is reversible. These results imply that the balance between protein kinases and phosphatases is critical for visual cortex plasticity.

Molecular substrates of plasticity in the developing visual cortex

Ocular dominance plasticity may be the paradigmatic in vivo model of activity-dependent plasticity. More than four decades of intense research has delineated the network-level rules that govern synaptic change in this model. The recent characterization of a murine model for ocular dominance plasticity has facilitated rapid progress on a new front, extending our understanding of the molecular mechanisms underlying ocular dominance plasticity. In this review, we highlight recent advances in this research effort, focusing in particular on signaling pathways mediating shifts in ocular dominance, and mechanisms underlying the timing of the critical period.

LTP and LTD vary with layer in rodent visual cortex

Mechanisms of plasticity in the visual cortex have been studied with long-term potentiation (LTP), long-term depression (LTD) and ocular dominance plasticity (ODP). It is now possible to compare results from these three forms of plasticity using knockout mice, and also by pharmacological manipulations. A review of the literature shows that if both LTP and LTD are completely abolished, then ODP will also be abolished. In other situations, there is little correlation. We hypothesize that this lack of correlation occurs because the mechanisms for LTP and LTD vary with layer in the visual cortex, and results show that they do.

Reduced ocular dominance plasticity and long-term potentiation in the developing visual cortex of protein kinase A RII alpha mutant mice

The cAMP-dependent protein kinase (PKA) signalling pathway has been shown to play an important role in long-term potentiation (LTP) and depression (LTD), and ocular dominance plasticity in the visual cortex. In order to investigate further the involvement of individual PKA subunits in visual cortical plasticity, LTP and LTD in vitro and ocular dominance plasticity in vivo in the developing visual cortex were examined in mice lacking the RII alpha subunit of PKA. Here we show that LTP in layers II/III was decreased in RII alpha knockout mice, but LTD was almost unaffected, and the ocular dominance shift induced by monocular deprivation was also partially blocked. These data provide evidence that RII alpha is involved in LTP and ocular dominance plasticity, and further suggest that different afferent inputs could selectively activate particular subunits of PKA and thereby direct specific aspects of visual cortical plasticity.

Regional variations in the localization of insoluble kinase A regulatory isoforms during rodent brain development

In eukaryothes, the second messenger cAMP regulates many cellular functions by binding to the regulatory subunits of cAMP-dependent protein kinases, and releasing the catalytic subunits. In the mammalian brain all four regulatory isoforms (RIalpha and beta, RIIalpha and beta) are present. Apparently, they are simple inhibitors of the catalytic subunits. It is still unclear why four isoforms are needed, but possibly they can target kinase activity at precise intracellular locations. Therefore, we examined the distribution of the insoluble regulatory isoforms in rat and mouse brains during prenatal (from embryonic day 8) and postnatal development up to senescence (13 months), via immunohistochemistry. RIIalpha labelling is always restricted to the ventricular ependyma. Punctated RIIbeta labelling is observed in the embryo from early stages of development, and is mainly localized in the cortical plate. After birth, punctate RIIbeta labelling is present throughout almost the whole brain, often observed in proximity of neurofilaments. It shows different characteristics and relationships to the other isoforms: for example in the CA1 hippocampal field, RIIbeta is substituted by RIalpha 2 weeks after birth, while in CA2 it persists for life. In other regions, as in the reticular formation, RIIbeta and RIalpha aggregates are found in the same cell, although clearly segregated. The different regulatory isoforms show distinct patterns of distribution that change consistently during development. A careful characterization of second messenger systems may be as useful as the study of neurotransmitters to understand neuronal properties and their modifications during development, so as to relate biochemical to functional properties.

Layer variations of long-term depression in rat visual cortex

In vitro long-term depression (LTD) is thought to be a model for the loss of cortical responsiveness to an eye deprived of vision during the critical period. Using whole cell recording, the present study investigates the mechanisms of LTD in vitro across layers in developing rat visual cortex. LTD was induced in layers II/III, V, and VI but not layer IV with 10-min 1-Hz stimulation paired with postsynaptic depolarization. LTD in layers II/III and V could be blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-aminophosphonovaleric acid (D-AP5) but not by 100 microM (2S)-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), a metabotropic glutamate receptor inhibitor. In contrast, LTD in layer VI was blocked by 100 microM LY341495 and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) but not D-AP5 and partially blocked by application of guanosine 5'-O-(2-thiodiphosphate) thilothium salt (GDP-beta-S) in patch pipette, suggesting an involvement of postsynaptic group I metabotropic glutamate receptors (mGluRs). These results indicate that LTD in developing rat visual cortex varies with layer: LTD was absent in layer IV, suggesting a unique plasticity mechanism at geniculocortical synapses; LTD in layers II/III and V depends on NMDA receptors but not mGluRs, and LTD in layer VI requires mGluRs but not NMDA receptors.

GABAA receptor maturation in relation to eye opening in the rat visual cortex

Changes in subunit composition of N-methyl-D-aspartate (NMDA) receptors have been reported to be affected by visual experience and may therefore form a major aspect of neuronal plasticity in the CNS during development. In contrast, putative alterations in the expression and functioning of the inhibitory GABAA receptor around eye opening have not been well defined yet. Here we describe the timing of changes in GABAA receptor subunit expression and the related synaptic functioning in the neonatal rat visual cortex and the influence of visual experience on this process. Quantitative analysis of all GABAA receptor subunit transcripts revealed a marked alpha3 to alpha1 subunit switch, in addition to a change in alpha4 and alpha5 expression. The changes were correlated with an acceleration of the decay of spontaneous inhibitory postsynaptic currents (sIPSCs). Both changes in receptor expression and synaptic functioning were initiated well before eye opening. Moreover, dark rearing could not prevent the robust upregulation of alpha1 or the change in sIPSC kinetics, indicating that this is not dependent of sensory (visual) input. Upon eye opening a positive correlation was observed between a faster decay of the sIPSCs and an increase in sIPSC frequency, which was absent in dark-reared animals. Thus, lack of extrinsic input to the cortex does not affect overall developmental regulation of synaptic functioning of GABAA receptors. However, we cannot exclude the possibility that visual experience is involved in proper shaping of the inhibitory network of the primary visual cortex.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Substrates for coincidence detection and calcium signaling for induction of synaptic potentiation in the neonatal visual cortex

Regulation of the efficacy of synaptic transmission by activity-dependent processes has been implicated in learning and memory as well as in developmental processes. We previously described transient potentiation of excitatory synapses onto layer 2/3 pyramidal neurons in the visual cortex that is induced by coincident presynaptic stimulation and postsynaptic depolarization. In the adult visual cortex, activation of N-methyl-d-aspartate (NMDA) glutamate receptors is necessary to induce this plasticity. These receptors act as coincidence detectors, sensing presynaptic glutamate release and postsynaptic depolarization, and cause an influx of Ca(2+) that is necessary for the potentiation. In the neurons of the neonatal visual cortex, on the other hand, coincident presynaptic stimulation and postsynaptic depolarization induce stable long-term potentiation (LTP). In addition, reduced but significant LTP can be induced in many neurons in the presence of the NMDA receptor (NMDAR) antagonist, 2-amino-5-phosphonovaleric acid despite the Ca(2+) requirement. Therefore there must be an alternative postsynaptic Ca(2+) source and coincidence detection mechanism linked to the LTP induction mechanism in the neonatal cortex operating in addition to NMDARs. In this study, we find that in layer 2/3 pyramidal neurons, release of Ca(2+) from inositol trisphosphate (InsP(3)) receptor-mediated intracellular stores and influx through voltage-gated Ca(2+) channels (VGCCs) provide alternative postsynaptic Ca(2+) sources. We hypothesize that InsP(3)Rs are coincidence detectors, sensing presynaptic glutamate release through linkage with group I metabotropic glutamate receptors (mGluRs), and depolarization, through VGCCs. We also find that the downstream protein kinases, PKA and PKC, have a role in potentiation in layer 2/3 pyramidal neurons of the neonatal visual cortex.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Long term potentiation varies with layer in rat visual cortex

Long term potentiation (LTP) in various layers of rat visual cortex was studied in 90 cells with visually identified, whole-cell recordings. LTP was induced in layer II/III, layer V or layer VI with theta burst stimulation (TBS), but was not observed in layer IV. In the presence of a NMDA antagonist, D-AP5, in the bath solution, potentiation was blocked in layer II/III, some depression was seen in layer V, and potentiation still remained in layer VI. After addition of a specific mGluR1 antagonist, LY367385, to the bath solution, LTP was reduced in layer II/III and layer V, and was blocked in layer VI. After a specific mGluR5 antagonist, MPEP was applied in the bath solution, LTP was enhanced in layer VI, and blocked in layer V. We conclude that: (1) LTP in layer VI is different from other layers, depending on mGluR1, but not NMDA receptors. (2) In layer II/III, LTP is NMDA-dependent and is not blocked by group I mGluR antagonists. (3) LTP in layer V is both NMDA receptor and mGluR5 receptor-dependent. (4) LTP was not induced in layer IV with TBS.

Mouse visual cortex

Neurons in mouse visual cortex have diverse receptive field properties and they respond selectively to specific features of visual stimuli. Owing to the lateral position of the eyes, only about a third of the visual cortex receives input from both eyes, but many cells in this region are binocular. Similar to higher mammals, closing one eye during a critical period shifts the responses of cells, such that they are better driven by the non-deprived eye. In this review I illustrate how the combination of transgenic mouse technology with single cell recording and modern imaging techniques might lead to a further understanding of the mechanisms that underlie the development, plasticity, and function of the mammalian visual cortex.

Two forms of synaptic plasticity with distinct dependence on age, experience, and NMDA receptor subtype in rat visual cortex

In visual cortex, NMDA receptor (NMDAR) properties depend primarily on NR2A and NR2B subunits, and NR2 subunit composition changes with age and visual experience. We examined the roles of these NR2 subunits in activity-dependent long-term modification of synaptic responses, which were evoked in layer 2/3 cells by stimulation of layer 4 in rat visual cortical slices. We used theta-burst stimulation (TBS) of presynaptic fibers or low-frequency stimulation paired with postsynaptic depolarization, which has been commonly used to induce NMDAR-dependent long-term potentiation (LTP) in visual cortex. In pyramidal cells, however, TBS produced long-term depression (LTD) at inhibitory synapses rather than LTP at excitatory synapses. This was observed in association with LTP of extracellular field potentials that reflect postsynaptic potentials in a population of cells (field-LTP). This result is inconsistent with the previous view that field-LTP reflects LTP of excitatory connections. However, pairing stimulation produced LTP at excitatory synapses of pyramidal cells frequently during development but rarely in adulthood. In contrast, inhibitory LTD and field-LTP occurred similarly in both developing and mature cortex. Experiments using NR2B selective and NR2 subunit nonselective NMDAR antagonists demonstrated that NR2A- and NR2B-containing NMDARs contribute selectively to inhibitory LTD-field-LTP and excitatory LTP, respectively. In addition, we found that the developmental decline in the NR2B component was paralleled by a decline in the incidence of excitatory LTP, and these declines were both prevented by dark rearing. These results implicate NR2 subunit composition in the regulation of neocortical plasticity and demonstrate differential subunit regulation at inhibitory and excitatory connections.

Protein kinase A-anchoring (AKAP) domains in brefeldin A-inhibited guanine nucleotide-exchange protein 2 (BIG2)

Like other guanine nucleotide-exchange proteins (GEPs) that activate ADP-ribosylation factor (ARF) GTPases, brefeldin A-inhibited GEP2, BIG2, contains an approximately 200-aa Sec7 domain that is responsible for this catalytic activity and its inhibition by brefeldin A. The Sec7 domain is located near the center of the molecule and serves to accelerate replacement of GDP bound to ARF with GTP. To explore possible functions of the N-terminal region of BIG2 (1-832), we used three coding-region constructs as bait to screen a human heart cDNA library in a yeast two-hybrid system, retrieving two unique clones that encode a type I protein kinase A (PKA) regulatory subunit, RI alpha. Coimmunoprecipitation experiments confirmed interaction of in vitro translated BIG2 and RI alpha, as well as of the endogenous proteins in cytosol of cultured HepG2 cells. Using 28 deletion mutants, we found three regions of BIG2 that interacted with R subunits of PKA. Residues 27-48 (domain A) interacted with RI alpha and RI beta, 284-301 (domain B) interacted with RII alpha and RII beta, and 517-538 (domain C) interacted with RI alpha, RII alpha, and RII beta. Sequence analysis and helical wheel projection of amino acids in the three domains revealed potential amphipathic wheel structures characteristic for binding of PKA R subunits. Western blot analysis of subcellular fractions demonstrated translocation of BIG2 (and BIG1) from cytosol to the Golgi and other membrane structures after incubation of cells with 8-Br-cAMP or forskolin. All findings are consistent with a role for BIG2 as an A kinase-anchoring protein (or AKAP) that could coordinate cAMP and ARF regulatory pathways.

Developmental plasticity of mouse visual acuity

Monocular deprivation in mice between postnatal days 19 and 32 has been reported to significantly shift ocular dominance within the binocular region of primary visual cortex; however, it is not known whether visual deprivation in mice during this physiologically defined critical period also results in amblyopia, as it does in other mammals. We addressed this uncertainty by psychophysically assessing in adulthood (postnatal day 70 or older) the grating acuity of normal and monocularly deprived mice, using the Visual Water Task. The visual acuity of mice tested with their nondeprived eyes was equivalent to that of normal mice ( approximately 0.5 cycles/degree); however, acuity measured with eyes monocularly deprived of vision transiently between postnatal days 19 and 32 was reduced by over 30% ( approximately 0.31 cycles/degree). Identical binocular deprivation produced a significant, but smaller, decrease in acuity ( approximately 0.38 cycles/degree). The effects of monocular and binocular deprivation were long lasting and occurred only if visual deprivation occurred between postnatal days 19 and 32. These data indicate that the deleterious effects of early visual deprivation on visual acuity in mice are similar to those reported in other mammals, and together with electrophysiological evidence of ocular dominance plasticity, suggest that the mechanisms of mouse visual plasticity are fundamentally the same as that in other mammals. Therefore, the mouse is probably a good model for investigating the basic cellular and molecular mechanisms underlying visual developmental plasticity and amblyopia.