Basolateral amygdala oscillations enable fear learning in a biophysical model

The basolateral amygdala (BLA) is a key site where fear learning takes place through synaptic plasticity. Rodent research shows prominent low theta (~3-6 Hz), high theta (~6-12 Hz), and gamma (>30 Hz) rhythms in the BLA local field potential recordings. However, it is not understood what role these rhythms play in supporting the plasticity. Here, we create a biophysically detailed model of the BLA circuit to show that several classes of interneurons (PV, SOM, and VIP) in the BLA can be critically involved in producing the rhythms; these rhythms promote the formation of a dedicated fear circuit shaped through rhythmic gating of spike-timing-dependent plasticity. Each class of interneurons is necessary for the plasticity. We find that the low theta rhythm is a biomarker of successful fear conditioning. Finally, we discuss how the peptide released by the VIP cell may alter the dynamics of plasticity to support the necessary fine timing.

ATLAS: A rationally designed anterograde transsynaptic tracer

Neural circuits, which constitute the substrate for brain processing, can be traced in the retrograde direction, from postsynaptic to presynaptic cells, using methods based on introducing modified rabies virus into genetically marked cell types. These methods have revolutionized the field of neuroscience. However, similarly reliable, transsynaptic, and non-toxic methods to trace circuits in the anterograde direction are not available. Here, we describe such a method based on an antibody-like protein selected against the extracellular N-terminus of the AMPA receptor subunit GluA1 (AMPA.FingR). ATLAS (Anterograde Transsynaptic Label based on Antibody-like Sensors) is engineered to release the AMPA.FingR and its payload, which can include Cre recombinase, from presynaptic sites into the synaptic cleft, after which it binds to GluA1, enters postsynaptic cells through endocytosis and subsequently carries its payload to the nucleus. Testing in vivo and in dissociated cultures shows that ATLAS mediates monosynaptic tracing from genetically determined cells that is strictly anterograde, synaptic, and non-toxic. Moreover, ATLAS shows activity dependence, which may make tracing active circuits that underlie specific behaviors possible.

Molecular layer disinhibition unlocks climbing-fiber-instructed motor learning in the cerebellum

Climbing fibers supervise cerebellar learning by providing signals to Purkinje cells (PCs) that instruct adaptive changes to mistakenly performed movements. Yet, climbing fibers are regularly active, even during well performed movements, suggesting that a mechanism dynamically regulates the ability of climbing fibers to induce corrective plasticity in response to motor errors. We found that molecular layer interneurons (MLIs), whose inhibition of PCs powerfully opposes climbing-fiber-mediated excitation, serve this function. Optogenetically suppressing the activity of floccular MLIs in mice during the vestibulo-ocular reflex (VOR) induces a learned increase in gain despite the absence of performance errors. Suppressing MLIs when the VOR is mistakenly underperformed reveled that their inhibitory output is necessary to orchestrate gain-increase learning by conditionally permitting climbing fibers to instruct plasticity induction during ipsiversive head turns. Ablation of an MLI circuit for PC disinhibition prevents gain-increase learning during VOR performance errors which was rescued by re-imposing PC disinhibition through MLI activity suppression. Our findings point to a decisive role for MLIs in gating climbing-fiber-mediated learning through their context-dependent inhibition of PCs.

Regional synapse gain and loss accompany memory formation in larval zebrafish

Defining the structural and functional changes in the nervous system underlying learning and memory represents a major challenge for modern neuroscience. Although changes in neuronal activity following memory formation have been studied [B. F. Grewe et al., Nature 543, 670-675 (2017); M. T. Rogan, U. V. Stäubli, J. E. LeDoux, Nature 390, 604-607 (1997)], the underlying structural changes at the synapse level remain poorly understood. Here, we capture synaptic changes in the midlarval zebrafish brain that occur during associative memory formation by imaging excitatory synapses labeled with recombinant probes using selective plane illumination microscopy. Imaging the same subjects before and after classical conditioning at single-synapse resolution provides an unbiased mapping of synaptic changes accompanying memory formation. In control animals and animals that failed to learn the task, there were no significant changes in the spatial patterns of synapses in the pallium, which contains the equivalent of the mammalian amygdala and is essential for associative learning in teleost fish [M. Portavella, J. P. Vargas, B. Torres, C. Salas, Brain Res. Bull 57, 397-399 (2002)]. In zebrafish that formed memories, we saw a dramatic increase in the number of synapses in the ventrolateral pallium, which contains neurons active during memory formation and retrieval. Concurrently, synapse loss predominated in the dorsomedial pallium. Surprisingly, we did not observe significant changes in the intensity of synaptic labeling, a proxy for synaptic strength, with memory formation in any region of the pallium. Our results suggest that memory formation due to classical conditioning is associated with reciprocal changes in synapse numbers in the pallium.

A biomarker-authenticated model of schizophrenia implicating NPTX2 loss of function

Schizophrenia is a polygenetic disorder whose clinical onset is often associated with behavioral stress. Here, we present a model of disease pathogenesis that builds on our observation that the synaptic immediate early gene NPTX2 is reduced in cerebrospinal fluid of individuals with recent onset schizophrenia. NPTX2 plays an essential role in maintaining excitatory homeostasis by adaptively enhancing circuit inhibition. NPTX2 function requires activity-dependent exocytosis and dynamic shedding at synapses and is coupled to circadian behavior. Behavior-linked NPTX2 trafficking is abolished by mutations that disrupt select activity-dependent plasticity mechanisms of excitatory neurons. Modeling NPTX2 loss of function results in failure of parvalbumin interneurons in their adaptive contribution to behavioral stress, and animals exhibit multiple neuropsychiatric domains. Because the genetics of schizophrenia encompasses diverse proteins that contribute to excitatory synapse plasticity, the identified vulnerability of NPTX2 function can provide a framework for assessing the impact of genetics and the intersection with stress.

Relocation of an Extrasynaptic GABAA Receptor to Inhibitory Synapses Freezes Excitatory Synaptic Strength and Preserves Memory

The excitatory synapse between hippocampal CA3 and CA1 pyramidal neurons exhibits long-term potentiation (LTP), a positive feedback process implicated in learning and memory in which postsynaptic depolarization strengthens synapses, promoting further depolarization. Without mechanisms for interrupting positive feedback, excitatory synapses could strengthen inexorably, corrupting memory storage. Here, we reveal a hidden form of inhibitory synaptic plasticity that prevents accumulation of excitatory LTP. We developed a knockin mouse that allows optical control of endogenous α5-subunit-containing γ-aminobutyric acid (GABA)A receptors (α5-GABARs). Induction of excitatory LTP relocates α5-GABARs, which are ordinarily extrasynaptic, to inhibitory synapses, quashing further NMDA receptor activation necessary for inducing more excitatory LTP. Blockade of α5-GABARs accelerates reversal learning, a behavioral test for cognitive flexibility dependent on repeated LTP. Hence, inhibitory synaptic plasticity occurs in parallel with excitatory synaptic plasticity, with the ensuing interruption of the positive feedback cycle of LTP serving as a possible critical early step in preserving memory.

Comparison of RNA Editing Activity of APOBEC1-A1CF and APOBEC1-RBM47 Complexes Reconstituted in HEK293T Cells

RNA editing is an important form of regulating gene expression and activity. APOBEC1 cytosine deaminase was initially characterized as pairing with a cofactor, A1CF, to form an active RNA editing complex that specifically targets APOB RNA in regulating lipid metabolism. Recent studies revealed that APOBEC1 may be involved in editing other potential RNA targets in a tissue-specific manner, and another protein, RBM47, appears to instead be the main cofactor of APOBEC1 for editing APOB RNA. In this report, by expressing APOBEC1 with either A1CF or RBM47 from human or mouse in an HEK293T cell line with no intrinsic APOBEC1/A1CF/RBM47 expression, we have compared direct RNA editing activity on several known cellular target RNAs. By using a sensitive cell-based fluorescence assay that enables comparative quantification of RNA editing through subcellular localization changes of eGFP, the two APOBEC1 cofactors, A1CF and RBM47, showed clear differences for editing activity on APOB and several other tested RNAs, and clear differences were observed when mouse versus human genes were tested. In addition, we have determined the minimal domain requirement of RBM47 needed for activity. These results provide useful functional characterization of RBM47 and direct biochemical evidence for the differential editing selectivity on a number of RNA targets.

All-optical synaptic electrophysiology probes mechanism of ketamine-induced disinhibition

Optical assays of synaptic strength would greatly facilitate studies of neuronal transmission and its dysregulation in disease. Here we introduce a genetic toolbox for all-optical interrogation of synaptic electrophysiology (‘synOptopatch’) via mutually exclusive expression of a channelrhodopsin actuator and an archaerhodopsin-derived voltage indicator. Optically induced activity in the channelrhodopsin-expressing neurons generated excitatory and inhibitory post-synaptic potentials which were optically resolved in the reporter-expressing neurons. We further developed a yellow spine-targeted Ca²⁺ indicator to localize optogenetically triggered synaptic inputs. We demonstrated synOptopatch recordings in cultured rodent neurons and in acute rodent brain slice. In synOptopatch measurements of primary rodent cultures, acute ketamine administration suppressed disynaptic inhibitory feedbacks, mimicking the effect of this drug on network function in both rodents and humans. We localized this action of ketamine to excitatory synapses onto interneurons. These results establish an in vitro all-optical model of disynaptic disinhibition, a synaptic defect hypothesized in schizophrenia-associated psychosis.

Structure and Function of an Actin-Based Filter in the Proximal Axon

The essential organization of microtubules within neurons has been described; however, less is known about how neuronal actin is arranged and the functional implications of its arrangement. Here, we describe, in live cells, an actin-based structure in the proximal axon that selectively prevents some proteins from entering the axon while allowing the passage of others. Concentrated patches of actin in proximal axons are present shortly after axonal specification in rat and zebrafish neurons imaged live, and they mark positions where anterogradely traveling vesicles carrying dendritic proteins halt and reverse. Patches colocalize with the ARP2/3 complex, and when ARP2/3-mediated nucleation is blocked, a dendritic protein mislocalizes to the axon. Patches are highly dynamic, with few persisting longer than 30 min. In neurons in culture and in vivo, actin appears to form a contiguous, semipermeable barrier, despite its apparently sparse distribution, preventing axonal localization of constitutively active myosin Va but not myosin VI.

Adaptive optics improves multiphoton super-resolution imaging

We improve multiphoton structured illumination microscopy using a nonlinear guide star to determine optical aberrations and a deformable mirror to correct them. We demonstrate our method on bead phantoms, cells in collagen gels, nematode larvae and embryos, Drosophila brain, and zebrafish embryos. Peak intensity is increased (up to 40-fold) and resolution recovered (up to 176 ± 10 nm laterally, 729 ± 39 nm axially) at depths ∼250 μm from the coverslip surface.

Techniques for studying protein trafficking and molecular motors in neurons

This review focused on techniques that facilitated the visualization of protein trafficking. In the mid-1990s the cloning of GFP allowed fluorescently tagged proteins to be expressed in cells and then visualized in real time. This advance allowed a glimpse, for the first time, of the complex system within cells for distributing proteins. It quickly became apparent, however, that time-lapse sequences of exogenously expressed GFP-labeled proteins can be difficult to interpret. Reasons for this include the relatively low signal that comes from moving proteins and high background rates from stationary proteins and other sources, as well as the difficulty of identifying the origins and destinations of specific vesicular carriers. In this review a range of techniques that have overcome these issues to varying degrees was reviewed and the insights into protein trafficking that they have enabled were discussed. Concentration will be on neurons, as they are highly polarized and, thus, their trafficking systems tend to be accessible for study. © 2016 Wiley Periodicals, Inc.

Live imaging of endogenous Ca²⁺/calmodulin-dependent protein kinase II in neurons reveals that ischemia-related aggregation does not require kinase activity

The Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) forms 12meric holoenzymes. These holoenzymes cluster into larger aggregates within neurons under ischemic conditions and in vitro when ischemic conditions are mimicked. This aggregation is thought to be mediated by interaction between the regulatory domain of one kinase subunit with the T-site of another kinase subunit in a different holoenzyme, an interaction that requires stimulation by Ca(2+) /CaM and nucleotide for its induction. This model makes several predictions that were verified here: Aggregation in vitro was reduced by the CaMKII inhibitors tatCN21 and tatCN19o (which block the T-site) as well as by KN93 (which is CaM-competitive). Notably, these and previously tested manipulations that block CaMKII activation all reduced aggregation, suggesting an alternative mechanism that instead requires kinase activity. However, experiments with the nucleotide-competitive broad-spectrum kinase inhibitors staurosporin and H7 showed that this is not the case. In vitro, staurosporine and H7 enabled CaMKII aggregation even in the absence of nucleotide. Within rat hippocampal neurons, an intra-body enabled live monitoring of endogenous CaMKII aggregation. This aggregation was blocked by tatCN21, but not by staurosporine, even though both effectively inhibit CaMKII activity. These results support the mechanistic model for CaMKII aggregation and show that kinase activity is not required. CaMKII aggregation is prevented by inhibiting kinase activity with mutations (red italics; shown previously) or inhibitors (red bold; shown here), indicating requirement of kinase activity. However, we show here that nucleotide-competitive inhibitors (green) allow CaMKII aggregation (including endogenous CaMKII within neurons), demonstrating that kinase activity is not required and supporting the current mechanistic model for CaMKII aggregation.

Actin and myosin-dependent localization of mRNA to dendrites

The localization of mRNAs within axons and dendrites allows neurons to manipulate protein levels in a time and location dependent manner and is essential for processes such as synaptic plasticity and axon guidance. However, an essential step in the process of mRNA localization, the decision to traffic to dendrites and/or axons, remains poorly understood. Here we show that Myosin Va and actin filaments are necessary for the dendritic localization of the mRNA binding protein Staufen 1 and of mRNA encoding the microtubule binding protein Map2. Blocking the function or expression of Myosin Va or depolymerizing actin filaments leads to localization of Staufen 1 and of Map2 mRNA in both axons and dendrites. Furthermore, interaction with Myosin Va plays an instructive role in the dendritic localization of Hermes 1, an RNA binding protein. Wild-type Hermes 1 localizes to both axons and dendrites, whereas Hermes 1 fused with a Myosin Va binding peptide, localizes specifically to dendrites. Thus, our results suggest that targeting of mRNAs to the dendrites is mediated by a mechanism that is dependent on actin and Myosin Va.

Structure meets function: actin filaments and myosin motors in the axon

This review focuses on recent advances in the understanding of the organization and roles of actin filaments, and associated myosin motor proteins, in regulating the structure and function of the axon shaft. 'Patches' of actin filaments have emerged as a major type of actin filament organization in axons. In the distal axon, patches function as precursors to the formation of filopodia and branches. At the axon initial segment, patches locally capture membranous organelles and contribute to polarized trafficking. The trapping function of patches at the initial segment can be ascribed to interactions with myosin motors, and likely also applies to patches in the more distal axon. Finally, submembranous rings of actin filaments were recently described in axons, which form an actin-spectrin cytoskeleton, likely contributing to the maintenance of axon integrity. Continued investigation into the roles of axonal actin filaments and myosins will shed light on fundamental aspects of the development, adult function and the repair of axons in the nervous system.

Recombinant probes for visualizing endogenous synaptic proteins in living neurons

The ability to visualize endogenous proteins in living neurons provides a powerful means to interrogate neuronal structure and function. Here we generate recombinant antibody-like proteins, termed Fibronectin intrabodies generated with mRNA display (FingRs), that bind endogenous neuronal proteins PSD-95 and Gephyrin with high affinity and that, when fused to GFP, allow excitatory and inhibitory synapses to be visualized in living neurons. Design of the FingR incorporates a transcriptional regulation system that ties FingR expression to the level of the target and reduces background fluorescence. In dissociated neurons and brain slices, FingRs generated against PSD-95 and Gephyrin did not affect the expression patterns of their endogenous target proteins or the number or strength of synapses. Together, our data indicate that PSD-95 and Gephyrin FingRs can report the localization and amount of endogenous synaptic proteins in living neurons and thus may be used to study changes in synaptic strength in vivo.

Actin and microtubule-based cytoskeletal cues direct polarized targeting of proteins in neurons

Neuronal proteins are transported to either the axon or dendrites through the action of kinesin motors; however, understanding of how cytoskeletal elements steer these cargo-motor complexes to one compartment or the other has remained elusive. Three recent developments-the discovery of an actin-based filter within the axon initial segment, the identification of the pivotal role played by myosin motors in dendritic targeting, and the determination of the properties of a kinesin motor that cause it to prefer axonal to dendritic microtubules-have now provided a structural framework for understanding polarized targeting in neurons.

Myosin-dependent targeting of transmembrane proteins to neuronal dendrites

The distinct electrical properties of axonal and dendritic membranes are largely a result of specific transport of vesicle-bound membrane proteins to each compartment. How this specificity arises is unclear because kinesin motors that transport vesicles cannot autonomously distinguish dendritically projecting microtubules from those projecting axonally. We hypothesized that interaction with a second motor might enable vesicles containing dendritic proteins to preferentially associate with dendritically projecting microtubules and avoid those that project to the axon. Here we show that in rat cortical neurons, localization of several distinct transmembrane proteins to dendrites is dependent on specific myosin motors and an intact actin network. Moreover, fusion with a myosin-binding domain from Melanophilin targeted Channelrhodopsin-2 specifically to the somatodendritic compartment of neurons in mice in vivo. Together, our results suggest that dendritic transmembrane proteins direct the vesicles in which they are transported to avoid the axonal compartment through interaction with myosin motors.

The role of Kif5B in axonal localization of Kv1 K(+) channels

Here we present evidence that the kinesin, Kif5B, is involved in the transportation and axonal targeting of Kv1 channels. We show that a dominant negative variant of Kif5B specifically blocks localization to the axon of expressed, tagged versions of Kv1.3 in cultured cortical slices. In addition, the dominant negative variant of Kif5B blocks axonal localization of endogenous Kv1.1, Kv1.2, and Kv1.4 in cortical neurons in dissociated cultures. We also found evidence that Kif5B interacts with Kv1 channels. Endogenous Kv1.2 colocalized with Kif5B in cortical neurons and coimmunoprecipitated with Kif5B from brain lysate. The T1 domain of Shaker K(+) channels has been shown to play a critical role in targeting the channel to the axon. We have three pieces of evidence to suggest that the T1 domain also mediates interaction between Kv1 channels and Kif5B: Addition of the T1 domain to a heterologous protein, TfR, is sufficient to cause the resulting fusion protein, TfRT1, to colocalize with Kif5B. Also, the T1 domain is necessary for interaction of Kv1.3 with Kif5B in a coimmunoprecipitation assay. Finally, dominant negative variants of Kif5B block axonal targeting of TfRT1, but have no effect on dendritic localization of TfR. Together these data suggest a model where Kif5B interacts with Kv1 channels either directly or indirectly via the T1 domain, causing the channels to be transported to axons.

Polarized targeting of ion channels in neurons

Since the time of Cajal it has been understood that axons and dendrites perform distinct electrophysiological functions that require unique sets of proteins [Cajal SR Histology of the nervous system, Oxford University Press, New York, (1995)]. To establish and maintain functional polarity, neurons localize many proteins specifically to either the axonal or the somatodendritic compartment. In particular, ion channels, which are the major regulators of electrical activity in neurons, are often distributed in a polarized fashion. Recently, the ability to introduce tagged proteins into neurons in culture has allowed the molecular mechanisms underlying axon- and dendrite-specific targeting of ion channels to be explored. These investigations have identified peptide signals from voltage-gated Na(+) and K(+) channels that direct trafficking to either axonal or dendritic compartments. In this article we will discuss the molecular mechanisms underlying polarized targeting of voltage-gated ion channels from the Kv4, Kv1, and Na(v)1 families.

A role for Kif17 in transport of Kv4.2

Although kinesins are known to transport neuronal proteins, it is not known what role they play in the targeting of their cargos to specific subcellular compartments in neurons. Here we present evidence that the K+ channel Kv4.2, which is a major regulator of dendritic excitability, is transported to dendrites by the kinesin isoform Kif17. We show that a dominant negative construct against Kif17 dramatically inhibits localization to dendrites of both introduced and endogenous Kv4.2, but those against other kinesins found in dendrites do not. Kv4.2 colocalizes with Kif17 but not with other kinesin isoforms in dendrites of cortical neurons. Native Kv4.2 and Kif17 coimmunoprecipitate from brain lysate, and introduced, tagged versions of the two proteins coimmunoprecipitate from COS cell lysate, indicating that the two proteins interact, either directly or indirectly. The interaction between Kif17 and Kv4.2 appears to occur through the extreme C terminus of Kv4.2 and not through the dileucine motif. Thus, the dileucine motif does not determine the localization of Kv4.2 by causing the channel to interact with a specific motor protein. In support of this conclusion, we found that the dileucine motif mediates dendritic targeting of CD8 independent of Kif17. Together our data show that Kif17 is probably the motor that transports Kv4.2 to dendrites but suggest that this motor does not, by itself, specify dendritic localization of the channel.

The T1 domain of Kv1.3 mediates intracellular targeting to axons

Shaker K+ channels play an important role in modulating electrical excitability of axons. Recent work has demonstrated that the T1 tetramerization domain of Kv1.2 is both necessary and sufficient for targeting of the channel to the axonal surface [Gu, C., Jan, Y.N. & Jan, L.Y. (2003) Science,301, 646-649]. Here we use a related channel, Kv1.3, as a model to investigate cellular mechanisms that mediate axonal targeting. We show that the T1 domain of Kv1.3 is necessary and sufficient to mediate targeting of the channel to the axonal surface in pyramidal neurons in slices of cortex from neonatal rat. The T1 domain is also sufficient to cause preferential axonal localization of intracellular protein, which indicates that the domain probably does not work through compartment-specific endocytosis or compartment-specific vesicle docking. To determine whether the T1 domain mediates axonal trafficking of transport vesicles, we compared the trafficking of vesicles containing green fluorescent protein-labelled transferrin receptor with those containing the same protein fused with the T1 domain in living cortical neurons. Vesicles containing the wild-type transferrin receptor did not traffic to the axon, in accord with previously published results; however, those containing the transferrin receptor fused to T1 did traffic to the axon. These results are consistent with the T1 domain of Kv1.3 mediating axonal targeting by causing transport vesicles to traffic to axons and they represent the first evidence that such a mechanism might underlie axonal targeting.

An evolutionarily conserved dileucine motif in Shal K+ channels mediates dendritic targeting

The molecular mechanisms underlying polarized sorting of proteins in neurons are poorly understood. Here we report the identification of a 16 amino-acid, dileucine-containing motif that mediates dendritic targeting in a variety of neuronal cell types in slices of rat brain. This motif is present in the carboxy (C) termini of Shal-family K+ channels and is highly conserved from C. elegans to humans. It is necessary for dendritic targeting of potassium channel Kv4.2 and is sufficient to target the axonally localized channels Kv1.3 and Kv1.4 to the dendrites. It can also mediate dendritic targeting of a non-channel protein, CD8.

Nystagmus induced by pharmacological inactivation of the brainstem ocular motor integrator in monkey

A common cause of pathological nystagmus is malfunction of the mechanism by which the brain integrates eye velocity signals to produce eye position commands. For horizontal gaze, neurons in the nucleus prepositus hypoglossi-medial vestibular nucleus region (NPH-MVN) play a vital role in this neural integrator function. We studied the effects on gaze stability of pharmacological intervention in the NPH-MVN of monkeys by microinjections of eight drugs. Agents with agonist or antagonist actions at gamma-aminobutyric acid (GABA), glutamate, and kainate receptors all caused gaze-evoked nystagmus with centripetal eye drifts; glycine and strychnine had no effect. When the GABAA-agonist muscimol was injected near the center of MVN, the eyes drifted away from the central position with increasing-velocity waveforms, implying an unstable neural integrator. The observed effects of these drugs on gaze stability may be related to inactivation either of neurons within NPH-MVN or the cerebellar projections to them that control the fidelity of neural integration. Drugs that influence GABA or glutamine transmission may have a role in the treatment of nystagmus due to an abnormal neural integrator.

Molecular determinants for subcellular localization of PSD-95 with an interacting K+ channel

Ion channels and PSD-95 are colocalized in specific neuronal subcellular locations by an unknown mechanism. To investigate mechanisms of localization, we used biolistic techniques to express GFP-tagged PSD-95 (PSD-95:GFP) and the K(+)-selective channel Kv1.4 in slices of rat cortex. In pyramidal cells, PSD-95:GFP required a single PDZ domain and a region including the SH3 domain for localization to postsynaptic sites. When transfected alone, PSD-95:GFP was present in dendrites but absent from axons. When cotransfected with Kv1.4, PSD-95:GFP appeared in both axons and dendrites, while Kv1.4 was restricted to axons. When domains that mediate the interaction of Kv1.4 and PSD-95 were disrupted, Kv1.4 localized nonspecifically. Our results provide evidence that Kv1.4 itself may determine its subcellular location, while an associated MAGUK protein is a necessary but not sufficient cofactor.

Ca2+ influx regulates BDNF transcription by a CREB family transcription factor-dependent mechanism

CREB is a transcription factor implicated in the control of adaptive neuronal responses. Although one function of CREB in neurons is believed to be the regulation of genes whose products control synaptic function, the targets of CREB that mediate synaptic function have not yet been identified. This report describes experiments demonstrating that CREB or a closely related protein mediates Ca2+-dependent regulation of BDNF, a neurotrophin that modulates synaptic activity. In cortical neurons, Ca2+ influx triggers phosphorylation of CREB, which by binding to a critical Ca2+ response element (CRE) within the BDNF gene activates BDNF transcription. Mutation of the BDNF CRE or an adjacent novel regulatory element as well as a blockade of CREB function resulted in a dramatic loss of BDNF transcription. These findings suggest that a CREB family member acts cooperatively with an additional transcription factor(s) to regulate BDNF transcription. We conclude that the BDNF gene is a CREB family target whose protein product functions at synapses to control adaptive neuronal responses.

A calcium responsive element that regulates expression of two calcium binding proteins in Purkinje cells

Calbindin D28 encodes a calcium binding protein that is expressed in the cerebellum exclusively in Purkinje cells. We have used biolistic transfection of organotypic slices of P12 cerebellum to identify a 40-bp element from the calbindin promoter that is necessary and sufficient for Purkinje cell specific expression in this transient in situ assay. This element (PCE1) is also present in the calmodulin II promoter, which regulates expression of a second Purkinje cell Ca2+ binding protein. Expression of high levels of exogenous calbindin or calretinin decreased transcription mediated by PCE1 in Purkinje cells 2.5- to 3-fold, whereas the presence of 1 microM ionomycin in the extracellular medium increased expression. These results demonstrate that PCE1 is a component of a cell-specific and Ca2+-sensitive transcriptional regulatory mechanism that may play a key role in setting the Ca2+ buffering capacity of Purkinje cells.

The oculomotor integrator: testing of a neural network model

An important part of the vestibulo-ocular reflex is a group of cells in the caudal pons, known as the neural integrator, that converts eye-velocity commands, from the semicircular canals for example, to eye-position commands for the motoneurons of the extraocular muscles. Previously, a recurrently connected neural network model was developed by us that learns to simulate the signal processing done by the neural integrator, but it uses an unphysiological learning algorithm. We describe here a new network model that can learn the same task by using a local, Hebbian-like learning algorithm that is physiologically plausible. Through the minimization of a retinal slip error signal the model learns, given randomly selected initial synaptic weights, to both integrate simulated push-pull semicircular canal afferent signals and compensate for orbital mechanics as well. Approximately half of the model's 14 neurons are inhibitory, half excitatory. After learning, inhibitory cells tend to project contralaterally, thus forming an inhibitory commissure. The network can, of course, recover from lesions. The mature network is also able to change its gain by simulating abnormal visual-vestibular interactions. When trained with a sine wave at a single frequency, the network changed its gain at and near the training frequency but not at significantly higher or lower frequencies, in agreement with previous experimental observations. Commissural connections are essential to the functioning of this model, as was the case with our previous model. In order to determine whether a commissure plays a similar role in the real neural integrator, a series of electrical perturbations were performed on the midlines of awake, behaving juvenile rhesus monkeys and the effects on the monkeys' eye movements were examined. Eye movements were recorded using the coil system before, during, and after electrical stimulation in the midline of the pons just caudal to the abducens nuclei, which reversibly made the integrator leaky. Eye movements were also recorded from two of the monkeys before and after a midline electrolytic lesion was made at the location where stimulation produced a leaky integrator. This lesion disabled the integrator irreversibly. The eye movements that were produced by the monkeys as a result of these perturbations were then compared with eye movements produced by the model after analogous perturbations. The results are compatible with the hypothesis that integration comes about by positive feedback through lateral inhibition effected by an inhibitory commissure.

A neural network model of the vestibulo-ocular reflex using a local synaptic learning rule

Vertebrates use the vestibulo-ocular reflex to maintain clear vision during head movements. This reflex requires eye-velocity com m ands from the semicircular canals to be integrated (mathematically) to produce eye-position com m ands for the extraocular muscles. This is accomplished by a neural network in the caudal pons. A model of this network is proposed using positive feedback via lateral inhibition. The model has been adapted to a learning network. We have developed a synaptic learning rule using only local information to make the model more physiological.