Quantification of optical signals with electrophysiological signals in neural activities of Di-4-ANEPPS stained rat hippocampal slices

Biomolecular condensates can function as inherent catalysts

We report the discovery that chemical reactions such as ATP hydrolysis can be catalyzed by condensates formed by intrinsically disordered proteins (IDPs), which themselves lack any intrinsic ability to function as enzymes. This inherent catalytic feature of condensates derives from the electrochemical environments and the electric fields at interfaces that are direct consequences of phase separation. The condensates we studied were capable of catalyzing diverse hydrolysis reactions, including hydrolysis and radical-dependent breakdown of ATP whereby ATP fully decomposes to adenine and multiple carbohydrates. This distinguishes condensates from naturally occurring ATPases, which can only catalyze the dephosphorylation of ATP. Interphase and interfacial properties of condensates can be tuned via sequence design, thus enabling control over catalysis through sequence-dependent electrochemical features of condensates. Incorporation of hydrolase-like synthetic condensates into live cells enables activation of transcriptional circuits that depend on products of hydrolysis reactions. Inherent catalytic functions of condensates, which are emergent consequences of phase separation, are likely to affect metabolic regulation in cells.

Action of GABAB receptor on local network oscillation in somatosensory cortex of oral part: focusing on NMDA receptor

The balance of activity between glutamatergic and GABAergic networks is particularly important for oscillatory neural activities in the brain. Here, we investigated the roles of GABAB receptors in network oscillation in the oral somatosensory cortex (OSC), focusing on NMDA receptors. Neural oscillation at the frequency of 8-10 Hz was elicited in rat brain slices after caffeine application. Oscillations comprised a non-NMDA receptor-dependent initial phase and a later NMDA receptor-dependent oscillatory phase, with the oscillator located in the upper layer of the OSC. Baclofen was applied to investigate the actions of GABAB receptors. The later NMDA receptor-dependent oscillatory phase completely disappeared, but the initial phase did not. These results suggest that GABAB receptors mainly act on NMDA receptor, in which metabotropic actions of GABAB receptors may contribute to the attenuation of NMDA receptor activities. A regulatory system for network oscillation involving GABAB receptors may be present in the OSC.

Functional Dissection of Ipsilateral and Contralateral Neural Activity Propagation Using Voltage-Sensitive Dye Imaging in Mouse Prefrontal Cortex

Prefrontal cortex (PFC) intrahemispheric activity and the interhemispheric connection have a significant impact on neuropsychiatric disorder pathology. This study aimed to generate a functional map of FC intrahemispheric and interhemispheric connections. Functional dissection of mouse PFCs was performed using the voltage-sensitive dye (VSD) imaging method with high speed (1 ms/frame), high resolution (256 × 256 pixels), and a large field of view (∼10 mm). Acute serial 350 μm slices were prepared from the bregma covering the PFC and numbered 1-5 based on their distance from the bregma (i.e., 1.70, 1.34, 0.98, 0.62, and 0.26 mm) with reference to the Mouse Brain Atlas (Paxinos and Franklin, 2008). The neural response to electrical stimulation was measured at nine sites and then averaged, and a functional map of the propagation patterns was created. Intracortical propagation was observed in slices 3-5, encompassing the anterior cingulate cortex (ACC) and corpus callosum (CC). The activity reached area 33 of the ACC. Direct white matter stimulation activated area 33 in both hemispheres. Similar findings were obtained via DiI staining of the CC. Imaging analysis revealed directional biases in neural signals traveling within the ACC, whereby the signal transmission speed and probability varied based on the signal direction. Specifically, the spread of neural signals from cg2 to cg1 was stronger than that from cingulate cortex area 1(cg1) to cingulate cortex area 2(cg2), which has implications for interhemispheric functional connections. These findings highlight the importance of understanding the PFC functional anatomy in evaluating neuromodulators like serotonin and dopamine, as well as other factors related to neuropsychiatric diseases.

Assessing seizure liability in vitro with voltage-sensitive dye imaging in mouse hippocampal slices

Non-clinical toxicology is a major cause of drug candidate attrition during development. In particular, drug-induced seizures are the most common finding in central nervous system (CNS) toxicity. Current safety pharmacology tests for assessing CNS functions are often inadequate in detecting seizure-inducing compounds early in drug development, leading to significant delays. This paper presents an in vitro seizure liability assay using voltage-sensitive dye (VSD) imaging techniques in hippocampal brain slices, offering a powerful alternative to traditional electrophysiological methods. Hippocampal slices were isolated from mice, and VSD optical responses evoked by stimulating the Schaffer collateral pathway were recorded and analyzed in the stratum radiatum (SR) and stratum pyramidale (SP). VSDs allow for the comprehensive visualization of neuronal action potentials and postsynaptic potentials on a millisecond timescale. By employing this approach, we investigated the in vitro drug-induced seizure liability of representative pro-convulsant compounds. Picrotoxin (PiTX; 1-100 μM), gabazine (GZ; 0.1-10 μM), and 4-aminopyridine (4AP; 10-100 μM) exhibited seizure-like responses in the hippocampus, but pilocarpine hydrochloride (Pilo; 10-100 μM) did not. Our findings demonstrate the potential of VSD-based assays in identifying seizurogenic compounds during early drug discovery, thereby reducing delays in drug development and providing insights into the mechanisms underlying seizure induction and the associated risks of pro-convulsant compounds.

A CCR5 antagonist, maraviroc, alleviates neural circuit dysfunction and behavioral disorders induced by prenatal valproate exposure

BACKGROUND

Valproic acid (VPA) is a clinically used antiepileptic drug, but it is associated with a significant risk of a low verbal intelligence quotient (IQ) score, attention-deficit hyperactivity disorder and autism spectrum disorder in children when it is administered during pregnancy. Prenatal VPA exposure has been reported to affect neurogenesis and neuronal migration and differentiation. In addition, growing evidence has shown that microglia and brain immune cells are activated by VPA treatment. However, the role of VPA-activated microglia remains unclear.

METHODS

Pregnant female mice received sodium valproate on E11.5. A microglial activation inhibitor, minocycline or a CCR5 antagonist, maraviroc was dissolved in drinking water and administered to dams from P1 to P21. Measurement of microglial activity, evaluation of neural circuit function and expression analysis were performed on P10. Behavioral tests were performed in the order of open field test, Y-maze test, social affiliation test and marble burying test from the age of 6 weeks.

RESULTS

Prenatal exposure of mice to VPA induced microglial activation and neural circuit dysfunction in the CA1 region of the hippocampus during the early postnatal periods and post-developmental defects in working memory and social interaction and repetitive behaviors. Minocycline, a microglial activation inhibitor, clearly suppressed the above effects, suggesting that microglia elicit neural dysfunction and behavioral disorders. Next-generation sequencing analysis revealed that the expression of a chemokine, C-C motif chemokine ligand 3 (CCL3), was upregulated in the hippocampi of VPA-treated mice. CCL3 expression increased in microglia during the early postnatal periods via an epigenetic mechanism. The CCR5 antagonist maraviroc significantly suppressed neural circuit dysfunction and post-developmental behavioral disorders induced by prenatal VPA exposure.

CONCLUSION

These findings suggest that microglial CCL3 might act during development to contribute to VPA-induced post-developmental behavioral abnormalities. CCR5-targeting compounds such as maraviroc might alleviate behavioral disorders when administered early.

The One-Stop Gyrification Station - Challenges and New Technologies

The evolution of the folded cortical surface is an iconic feature of the human brain shared by a subset of mammals and considered pivotal for the emergence of higher-order cognitive functions. While our understanding of the neurodevelopmental processes involved in corticogenesis has greatly advanced over the past 70 years of brain research, the fundamental mechanisms that result in gyrification, along with its originating cytoarchitectural location, remain largely unknown. This review brings together numerous approaches to this basic neurodevelopmental problem, constructing a narrative of how various models, techniques and tools have been applied to the study of gyrification thus far. After a brief discussion of core concepts and challenges within the field, we provide an analysis of the significant discoveries derived from the parallel use of model organisms such as the mouse, ferret, sheep and non-human primates, particularly with regard to how they have shaped our understanding of cortical folding. We then focus on the latest developments in the field and the complementary application of newly emerging technologies, such as cerebral organoids, advanced neuroimaging techniques, and atomic force microscopy. Particular emphasis is placed upon the use of novel computational and physical models in regard to the interplay of biological and physical forces in cortical folding.

Long-term effects of early postnatal nicotine exposure on cholinergic function in the mouse hippocampal CA1 region

In rodent models of smoking during pregnancy, early postnatal nicotine exposure results in impaired hippocampus-dependent memory, but the underlying mechanism remains elusive. Given that hippocampal cholinergic systems modulate memory and rapid development of hippocampal cholinergic systems occurs during nicotine exposure, here we investigated its impacts on cholinergic function. Both nicotinic and muscarinic activation produce transient or long-lasting depression of excitatory synaptic transmission in the hippocampal CA1 region. We found that postnatal nicotine exposure impairs both the induction and nicotinic modulation of NMDAR-dependent long-term depression (LTD). Activation of muscarinic receptors decreases excitatory synaptic transmission and CA1 network activity in both wild-type and α2 knockout mice. These muscarinic effects are still observed in nicotine-exposed mice. M1 muscarinic receptor activity is required for mGluR-dependent LTD. Early postnatal nicotine exposure has no effect on mGluR-dependent LTD induction, suggesting that it has no effect on the function of m1 muscarinic receptors involved in this form of LTD. Our results demonstrate that early postnatal nicotine exposure has more pronounced effects on nicotinic function than muscarinic function in the hippocampal CA1 region. Thus, impaired hippocampus-dependent memory may arise from the developmental disruption of nicotinic cholinergic systems in the hippocampal CA1 region.

Chronic Treatment of Ascorbic Acid Leads to Age-Dependent Neuroprotection against Oxidative Injury in Hippocampal Slice Cultures

Increased oxidative damage in the brain, which increases with age, is the cause of abnormal brain function and various diseases. Ascorbic acid (AA) is known as an endogenous antioxidant that provides neuronal protection against oxidative damage. However, with aging, its extracellular concentrations and uptake decrease in the brain. Few studies have dealt with age-related functional changes in the brain to sustained ascorbate supplementation. This study aimed to investigate the susceptibility of hippocampal neurons to oxidative injury following acute and chronic AA administration. Oxidative stress was induced by kainic acid (KA, 5 µM) for 18 h in hippocampal slice cultures. After KA exposure, less neuronal cell death was observed in the 3 w cultured slice compared to the 9 w cultured slice. In the chronic AA treatment (6 w), the 9 w-daily group showed reduced neuronal cell death and increased superoxide dismutase (SOD) and Nrf2 expressions compared to the 9 w. In addition, the 9 w group showed delayed latencies and reduced signal activity compared to the 3 w, while the 9 w-daily group showed shorter latencies and increased signal activity than the 9 w. These results suggest that the maintenance of the antioxidant system by chronic AA treatment during aging could preserve redox capacity to protect hippocampal neurons from age-related oxidative stress.

Cellular-resolution mapping uncovers spatial adaptive filtering at the rat cerebellum input stage

Long-term synaptic plasticity is thought to provide the substrate for adaptive computation in brain circuits but very little is known about its spatiotemporal organization. Here, we combined multi-spot two-photon laser microscopy in rat cerebellar slices with realistic modeling to map the distribution of plasticity in multi-neuronal units of the cerebellar granular layer. The units, composed by ~300 neurons activated by ~50 mossy fiber glomeruli, showed long-term potentiation concentrated in the core and long-term depression in the periphery. This plasticity was effectively accounted for by an NMDA receptor and calcium-dependent induction rule and was regulated by the inhibitory Golgi cell loops. Long-term synaptic plasticity created effective spatial filters tuning the time-delay and gain of spike retransmission at the cerebellum input stage and provided a plausible basis for the spatiotemporal recoding of input spike patterns anticipated by the motor learning theory.

Casali, Tognolina et al. use two-photon laser microscopy to spatially map long-term synaptic plasticity in rat cerebellar granular cells following stimulation of mossy fibers. Their data allow them to apply realistic modeling to test hypotheses about the synaptic spiking dynamics and reveal the importance of synaptic inhibition to defining these microcircuits.

Optogenetic Manipulation of Postsynaptic cAMP Using a Novel Transgenic Mouse Line Enables Synaptic Plasticity and Enhances Depolarization Following Tetanic Stimulation in the Hippocampal Dentate Gyrus

cAMP is a positive regulator tightly involved in certain types of synaptic plasticity and related memory functions. However, its spatiotemporal roles at the synaptic and neural circuit levels remain elusive. Using a combination of a cAMP optogenetics approach and voltage-sensitive dye (VSD) imaging with electrophysiological recording, we define a novel capacity of postsynaptic cAMP in enabling dentate gyrus long-term potentiation (LTP) and depolarization in acutely prepared murine hippocampal slices. To manipulate cAMP levels at medial perforant path to granule neuron (MPP-DG) synapses by light, we generated transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC) in DG granule neurons. Using these Tg(CMV-Camk2a-RFP/bPAC)3Koka mice, we recorded field excitatory postsynaptic potentials (fEPSPs) from MPP-DG synapses and found that photoactivation of PAC during tetanic stimulation enabled synaptic potentiation that persisted for at least 30 min. This form of LTP was induced without the need for GABA receptor blockade that is typically required for inducing DG plasticity. The paired-pulse ratio (PPR) remained unchanged, indicating the cAMP-dependent LTP was likely postsynaptic. By employing fast fluorescent voltage-sensitive dye (VSD: di-4-ANEPPS) and fluorescence imaging, we found that photoactivation of the PAC actuator enhanced the intensity and extent of dentate gyrus depolarization triggered following tetanic stimulation. These results demonstrate that the elevation of cAMP in granule neurons is capable of rapidly enhancing synaptic strength and neuronal depolarization. The powerful actions of cAMP are consistent with this second messenger having a critical role in the regulation of synaptic function.

Postnatal Development of Functional Projections from Parasubiculum and Presubiculum to Medial Entorhinal Cortex in the Rat

Neurons in parasubiculum (PaS), presubiculum (PrS), and medial entorhinal cortex (MEC) code for place (grid cells) and head direction. Directional input has been shown to be important for stable grid cell properties in MEC, and PaS and PrS have been postulated to provide this information to MEC. In line with this, head direction cells in those brain areas are present at postnatal day 11 (P11), having directional tuning that stabilizes shortly after eye opening, which is before premature grid cells emerge in MEC at P16. Whether functional connectivity between these structures exists at those early postnatal stages is unclear. Using anatomical tracing, voltage-sensitive dye imaging and single-cell patch recordings in female and male rat brain slices between P2 and P61, we determined when the pathways from PaS and PrS to MEC emerge, become functional, and how they develop. Anatomical connections from PaS and PrS to superficial MEC emerge between P4 and P6. Monosynaptic connectivity from PaS and PrS to superficial MEC was measurable from P9 to P10 onward, whereas connectivity with deep MEC was measurable from P11 to P12. From P14/P15 on, reactivity of MEC neurons to parasubicular and presubicular inputs becomes adult-like and continues to develop until P28-P30. The maturation of the efficacy of both inputs between P9 and P21 is paralleled by maturation of morphological properties, changes in intrinsic properties of MEC principal neurons, and changes in the GABAergic network of MEC. In conclusion, synaptic projections from PaS and PrS to MEC become functional and adult-like before the emergence of grid cells in MEC.SIGNIFICANCE STATEMENT Head direction information, crucial for grid cells in medial entorhinal cortex (MEC), is thought to enter MEC via parasubiculum (PaS) and presubiculum (PrS). Unraveling the development of functional connections between PaS, PrS, and MEC is key to understanding how spatial navigation, an important cognitive function, may evolve. To gain insight into the development, we used anatomical tracing techniques, voltage-sensitive dye imaging, and single-cell recordings. The combined data led us to conclude that synaptic projections from PaS and PrS to MEC become functional and adult-like before eye opening, allowing crucial head direction information to influence place encoding before the emergence of grid cells in rat MEC.

Network Plasticity Involved in the Spread of Neural Activity Within the Rhinal Cortices as Revealed by Voltage-Sensitive Dye Imaging in Mouse Brain Slices

The rhinal cortices, such as the perirhinal cortex (PC) and the entorhinal cortex (EC), are located within the bidirectional pathway between the neocortex and the hippocampus. Physiological studies indicate that the perirhinal transmission of neocortical inputs to the EC occurs at an extremely low probability, though many anatomical studies indicated strong connections exist in the pathway. Our previous study in rat brain slices indicated that an increase in excitability in deep layers of the PC/EC border initiated the neural activity transfer from the PC to the EC. In the present study, we hypothesized that such changes in network dynamics are not incidental observations but rather due to the plastic features of the perirhinal network, which links with the EC. To confirm this idea, we analyzed the network properties of neural transmission throughout the rhinal cortices and the plastic behavior of the network by performing a single-photon wide-field optical recording technique with a voltage-sensitive dye (VSD) in mouse brain slices of the PC, the EC, and the hippocampus. The low concentration of 4-aminopyridine (4-AP; 40 μM) enhanced neural activity in the PC, which eventually propagated to the EC via the deep layers of the PC/EC border. Interestingly, washout of 4-AP was unable to reverse entorhinal activation to the previous state. This change in the network property persisted for more than 1 h. This observation was not limited to the application of 4-AP. Burst stimulation to neurons in the perirhinal deep layers also induced the same change of network property. These results indicate the long-lasting modification of physiological connection between the PC and the EC, suggesting the existence of plasticity in the perirhinal-entorhinal network.

Overall Assay of Neuronal Signal Propagation Pattern With Long-Term Potentiation (LTP) in Hippocampal Slices From the CA1 Area With Fast Voltage-Sensitive Dye Imaging

Activity-dependent changes in the input-output (I-O) relationship of a neural circuit are central in the learning and memory function of the brain. To understand circuit-wide adjustments, optical imaging techniques to probe the membrane potential at every component of neurons, such as dendrites, axons and somas, in the circuit are essential. We have been developing fast voltage-sensitive dye (VSD) imaging methods for quantitative measurements, especially for single-photon wide-field optical imaging. The long-term continuous measurements needed to evaluate circuit-wide modifications require stable and quantitative long-term recordings. Here, we show that VSD imaging (VSDI) can be used to record changes in circuit activity in association with theta-burst stimulation (TBS)-induced long-term potentiation (LTP) of synaptic strength in the CA1 area. Our optics, together with the fast imaging system, enabled us to measure neuronal signals from the entire CA1 area at a maximum frame speed of 0.1 ms/frame every 60 s for over 12 h. We also introduced a method to evaluate circuit activity changes by mapping the variation in recordings from the CA1 area to coordinates defined by the morphology of CA1 pyramidal cells. The results clearly showed two types of spatial heterogeneity in LTP induction. The first heterogeneity is that LTP increased with distance from the stimulation site. The second heterogeneity is that LTP is higher in the stratum pyramidale (SP)-oriens region than in the stratum radiatum (SR). We also showed that the pattern of the heterogeneity changed according to the induction protocol, such as induction by TBS or high-frequency stimulation (HFS). We further demonstrated that part of the heterogeneity depends on the I-O response of the circuit elements. The results show the usefulness of VSDI in probing the function of hippocampal circuits.

A Multidisciplinary Approach Reveals an Age-Dependent Expression of a Novel Bioactive Peptide, Already Involved in Neurodegeneration, in the Postnatal Rat Forebrain

The basal forebrain has received much attention due to its involvement in multiple cognitive functions, but little is known about the basic neuronal mechanisms underlying its development, nor those mediating its primary role in Alzheimer’s disease. We have previously suggested that a novel 14-mer peptide, ‘T14’, could play a pivotal role in Alzheimer’s disease, via reactivation of a developmental signaling pathway. In this study, we have characterized T14 in the context of post-natal rat brain development, using a combination of different techniques. Ex-vivo rat brain slices containing the basal forebrain, at different stages of development, were used to investigate large-scale neuronal network activity in real time with voltage-sensitive dye imaging. Subsequent Western blot analysis revealed the expression profile of endogenous T14, its target alpha7 nicotinic receptor and the familiar markers of Alzheimer’s: amyloid beta and phosphorylated Tau. Results indicated maximal neuronal activity at the earliest ages during development, reflected in a concomitant profile of T14 peptide levels and related proteins. In conclusion, these findings show that the peptide, already implicated in neurodegenerative events, has an age-dependent expression, suggesting a possible contribution to the physiological mechanisms underlying brain maturation.

Fast voltage-sensitive dye imaging of excitatory and inhibitory synaptic transmission in the rat granular retrosplenial cortex

Rodent granular retrosplenial cortex (GRS) has dense connections between the anterior thalamic nuclei (ATN) and hippocampal formation. GRS superficial pyramidal neurons exhibit distinctive late spiking (LS) firing property and form patchy clusters with prominent apical dendritic bundles. The aim of this study was to investigate spatiotemporal dynamics of signal transduction in the GRS induced by ATN afferent stimulation by using fast voltage-sensitive dye imaging in rat brain slices. In coronal slices, layer 1a stimulation, which presumably activated thalamic fibers, evoked propagation of excitatory synaptic signals from layers 2-4 to layers 5-6 in a direction perpendicular to the layer axis, followed by transverse signal propagation within each layer. In the presence of ionotropic glutamate receptor antagonists, inhibitory responses were observed in superficial layers, induced by direct activation of inhibitory interneurons in layer 1. In horizontal slices, excitatory signals in deep layers propagated transversely mainly from posterior to anterior via superficial layers. Cortical inhibitory responses upon layer 1a stimulation in horizontal slices were weaker than those in the coronal slices. Observed differences between coronal and horizontal planes suggest anisotropy of the intracortical circuitry. In conclusion, ATN inputs are processed differently in coronal and horizontal planes of the GRS and then conveyed to other cortical areas. In both planes, GRS superficial layers play an important role in signal propagation, which suggests that superficial neuronal cascade is crucial in the integration of multiple information sources.NEW & NOTEWORTHY Superficial neurons in the rat granular retrosplenial cortex (GRS) show distinctive late-spiking (LS) firing property. However, little is known about spatiotemporal dynamics of signal transduction in the GRS. We demonstrated LS neuron network relaying thalamic inputs to deep layers and anisotropic distribution of inhibition between coronal and horizontal planes. Since deep layers of the GRS receive inputs from the subiculum, GRS circuits may work as an integrator of multiple sources such as sensory and memory information.

Extinction of avoidance behavior by safety learning depends on endocannabinoid signaling in the hippocampus

The development of exaggerated avoidance behavior is largely responsible for the decreased quality of life in patients suffering from anxiety disorders. Studies using animal models have contributed to the understanding of the neural mechanisms underlying the acquisition of avoidance responses. However, much less is known about its extinction. Here we provide evidence in mice that learning about the safety of an environment (i.e., safety learning) rather than repeated execution of the avoided response in absence of negative consequences (i.e., response extinction) allowed the animals to overcome their avoidance behavior in a step-down avoidance task. This process was context-dependent and could be blocked by pharmacological (3 mg/kg, s.c.; SR141716) or genetic (lack of cannabinoid CB1 receptors in neurons expressing dopamine D1 receptors) inactivation of CB1 receptors. In turn, the endocannabinoid reuptake inhibitor AM404 (3 mg/kg, i.p.) facilitated safety learning in a CB1-dependent manner and attenuated the relapse of avoidance behavior 28 days after conditioning. Safety learning crucially depended on endocannabinoid signaling at level of the hippocampus, since intrahippocampal SR141716 treatment impaired, whereas AM404 facilitated safety learning. Other than AM404, treatment with diazepam (1 mg/kg, i.p.) impaired safety learning. Drug effects on behavior were directly mirrored by drug effects on evoked activity propagation through the hippocampal trisynaptic circuit in brain slices: As revealed by voltage-sensitive dye imaging, diazepam impaired whereas AM404 facilitated activity propagation to CA1 in a CB1-dependent manner. In line with this, systemic AM404 enhanced safety learning-induced expression of Egr1 at level of CA1. Together, our data render it likely that AM404 promotes safety learning by enhancing information flow through the trisynaptic circuit to CA1.

Optical imaging of the rat brain suggests a previously missing link between top-down and bottom-up nervous system function

Optical imaging with voltage-sensitive dyes enables the visualization of extensive yet highly transient coalitions of neurons (assemblies) operating throughout the brain on a subsecond time scale. We suggest that operating at the mesoscale level of brain organization, neuronal assemblies may provide a functional link between "bottom-up" cellular mechanisms and "top-down" cognitive ones within anatomically defined regions. We demonstrate in ex vivo rat brain slices how varying spatiotemporal dynamics of assemblies reveal differences not previously appreciated between: different stages of development in cortical versus subcortical brain areas, different sensory modalities (hearing versus vision), different classes of psychoactive drugs (anesthetics versus analgesics), different effects of anesthesia linked to hyperbaric conditions and, in vivo, depths of anesthesia. The strategy of voltage-sensitive dye imaging is therefore as powerful as it is versatile and as such can now be applied to the evaluation of neurochemical signaling systems and the screening of related new drugs, as well as to mathematical modeling and, eventually, even theories of consciousness.

The Features and Functions of Neuronal Assemblies: Possible Dependency on Mechanisms beyond Synaptic Transmission

"Neuronal assemblies" are defined here as coalitions within the brain of millions of neurons extending in space up to 1-2 mm, and lasting for hundreds of milliseconds: as such they could potentially link bottom-up, micro-scale with top-down, macro-scale events. The perspective first compares the features in vitro versus in vivo of this underappreciated "meso-scale" level of brain processing, secondly considers the various diverse functions in which assemblies may play a pivotal part, and thirdly analyses whether the surprisingly spatially extensive and prolonged temporal properties of assemblies can be described exclusively in terms of classic synaptic transmission or whether additional, different types of signaling systems are likely to operate. Based on our own voltage-sensitive dye imaging (VSDI) data acquired in vitro we show how restriction to only one signaling process, i.e., synaptic transmission, is unlikely to be adequate for modeling the full profile of assemblies. Based on observations from VSDI with its protracted spatio-temporal scales, we suggest that two other, distinct processes are likely to play a significant role in assembly dynamics: "volume" transmission (the passive diffusion of diverse bioactive transmitters, hormones, and modulators), as well as electrotonic spread via gap junctions. We hypothesize that a combination of all three processes has the greatest potential for deriving a realistic model of assemblies and hence elucidating the various complex brain functions that they may mediate.

Impaired function of α2-containing nicotinic acetylcholine receptors on oriens-lacunosum moleculare cells causes hippocampus-dependent memory impairments

Children of mothers who smoked during pregnancy are at significantly greater risk for cognitive impairments including memory deficits, but the mechanisms underlying this effect remain to be understood. In rodent models of smoking during pregnancy, early postnatal nicotine exposure results in impaired long-term hippocampus-dependent memory, functional loss of α2-containing nicotinic acetylcholine receptors (α2^∗ nAChRs) in oriens-lacunosum moleculare (OLM) cells, increased CA1 network excitation, and unexpected facilitation of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses. Here we demonstrate that α2 knockout mice show the same pattern of memory impairment as previously observed in wild-type mice exposed to early postnatal nicotine. However, α2 knockout mice and α2 knockout mice exposed to early postnatal nicotine did not share all of the anomalies in hippocampal function observed in wild-type mice treated with nicotine during development. Unlike nicotine-treated wild-type mice, α2 knockout mice and nicotine-exposed α2 knockout mice did not demonstrate increased CA1 network excitation following Schaffer collateral stimulation and facilitated LTP, indicating that the effects are likely adaptive changes caused by activation of α2^∗ nAChRs during nicotine exposure and are unlikely related to the associated memory impairment. Thus, the functional loss of α2^∗ nAChRs in OLM cells likely plays a critical role in mediating this developmental-nicotine-induced hippocampal memory deficit.

Layer-specific potentiation of network GABAergic inhibition in the CA1 area of the hippocampus

One of the most important functions of GABAergic inhibition in cortical regions is the tight control of spatiotemporal activity of principal neuronal ensembles. However, electrophysiological recordings do not provide sufficient spatial information to determine the spatiotemporal properties of inhibitory plasticity. Using Voltage Sensitive Dye Imaging (VSDI) in mouse hippocampal slices, we demonstrate that GABA_A-mediated field inhibitory postsynaptic potentials undergo layer-specific potentiation upon activation of metabotropic glutamate receptors (mGlu). VSDI recordings allowed detection of pharmacologically isolated GABA_A-dependent hyperpolarization signals. Bath-application of the selective group-I mGlu receptor agonist, (S)-3,5-Dihydroxyphenylglycine (DHPG), induces an enhancement of the GABAergic VSDI-recorded signal, which is more or less pronounced in different hippocampal layers. This potentiation is mediated by mGlu₅ and downstream activation of IP₃ receptors. Our results depict network GABAergic activity in the hippocampal CA1 region and its sub-layers, showing also a novel form of inhibitory synaptic plasticity tightly coupled to glutamatergic activity.

Excitatory Postrhinal Projections to Principal Cells in the Medial Entorhinal Cortex

The postrhinal cortex (POR) provides substantial input to the entorhinal cortex, mainly targeting superficial layers of the medial entorhinal cortex (MEC). Major inputs to POR originate in the visual and parietal cortex, thus providing neurons in MEC with a subset of cortical information relevant to their spatial firing properties. The POR takes a position that is comparable with that of the perirhinal cortex (PER) with regard to the lateral entorhinal cortex (LEC). Neurons in LEC and MEC show different functional properties likely reflecting differences in their respective inputs. Projections from PER to LEC exert a main inhibitory influence, which may relate to the sparse object-selective firing in LEC. In view of the continuous, spatially modulated firing properties of principal neurons in MEC, we tested in rats the hypothesis that projections from POR to MEC are functionally different from the PER-to-LEC counterpart in providing an excitatory drive to MEC. Our combined confocal and quantitative electron-microscopic observations indicated that POR projections target mainly principal cells in MEC, including neurons that project to the hippocampus. The ultrastructure of the majority of the synapses indicated that they are excitatory. Voltage-sensitive dye imaging in sagittal slices confirmed this morphologically derived conclusion, showing that the MEC network always responded with an overall depolarization, indicative for net excitatory transmission. In vitro single-cell recordings from principal cells showed only excitatory responses upon POR stimulation. These results show that POR provides an excitatory projection to MEC, differing fundamentally from the inhibitory projection of PER to LEC.

Diminished amygdala activation and behavioral threat response following traumatic brain injury

Each year, approximately 3.8 million people suffer mild to moderate traumatic brain injuries (mTBI) that result in an array of neuropsychological symptoms and disorders. Despite these alarming statistics, the neurological bases of these persistent, debilitating neuropsychological symptoms are currently poorly understood. In this study we examined the effects of mTBI on the amygdala, a brain structure known to be critically involved in the processing of emotional stimuli. Seven days after lateral fluid percussion injury (LFPI), mice underwent a series of physiological and behavioral experiments to assess amygdala function. Brain-injured mice exhibited a decreased threat response in a cued fear conditioning paradigm, congruent with a decrease in amygdala excitability determined with basolateral amygdala (BLA) field excitatory post-synaptic potentials together with voltage-sensitive dye imaging (VSD). Furthermore, beyond exposing a general decrease in the excitability of the primary input of the amygdala, the lateral amygdala (LA), VSD also revealed a decrease in the relative strength or activation of internuclear amygdala circuit projections after LFPI. Thus, not only does activation of the LA require increased stimulation, but the proportion of this activation that is propagated to the primary output of the amygdala, the central amygdala, is also diminished following LFPI. Intracellular recordings revealed no changes in the intrinsic properties of BLA pyramidal neurons after LFPI. This data suggests that mild to moderate TBI has prominent effects on amygdala function and provides a potential neurological substrate for many of the neuropsychological symptoms suffered by TBI patients.

Paired Burst Stimulation Causes GABAA Receptor-Dependent Spike Firing Facilitation in CA1 of Rat Hippocampal Slices

The theta oscillation (4–8 Hz) is a pivotal form of oscillatory activity in the hippocampus that is intermittently concurrent with gamma (25–100 Hz) burst events. In in vitro preparation, a stimulation protocol that mimics the theta oscillation, theta burst stimulation (TBS), is used to induce long-term potentiation. Thus, TBS is thought to have a distinct role in the neural network of the hippocampal slice preparation. However, the specific mechanisms that make TBS induce such neural circuit modifications are still unknown. Using electrophysiology and voltage-sensitive dye imaging (VSDI), we have found that TBS induces augmentation of spike firing. The augmentation was apparent in the first couple of brief burst stimulation (100 Hz four pulses) on a TBS-train in a presence of NMDA receptor blocker (APV 50 μM). In this study, we focused on the characterizes of the NMDA independent augmentation caused by a pair of the brief burst stimulation (the first pair of the TBS; paired burst stimulation-PBS). We found that PBS enhanced membrane potential responses on VSDI signal and intracellular recordings while it was absent in the current recording under whole-cell clamp condition. The enhancement of the response accompanied the augmentation of excitatory postsynaptic potential (EPSP) to spike firing (E-S) coupling. The paired burst facilitation (PBF) reached a plateau when the number of the first burst stimulation (priming burst) exceeds three. The interval between the bursts of 150 ms resulted in the maximum PBF. Gabazine (a GABA_A receptor antagonist) abolished PBF. The threshold for spike generation of the postsynaptic cells measured with a current injection to cells was not lowered by the priming burst of PBS. These results indicate that PBS activates the GABAergic system to cause short-term E-S augmentation without raising postsynaptic excitability. We propose that a GABAergic system of area CA1 of the hippocampus produce the short-term E-S plasticity that could cause exaggerated spike-firing upon a theta-gamma activity distinctively, thus making the neural circuit of the CA1 act as a specific amplifier of the oscillation signal.

(II) Physiological profiling of an endogenous peptide in the basal forebrain: Age-related bioactivity and blockade with a novel modulator

Previous studies have suggested that neurodegeneration is an aberrant form of development, mediated by a novel peptide from the C-terminus of acetylcholinesterase (AChE). Using voltage-sensitive dye imaging we have investigated the effects of a synthetic version of this peptide in the in vitro rat basal forebrain, a key site of degeneration in Alzheimer's disease. The brain slice preparation enables direct visualisation in real-time of sub-second meso-scale neuronal coalitions ('Neuronal Assemblies') that serve as a powerful index of brain functional activity. Here we show that (1) assemblies are site-specific in their activity profile with the cortex displaying a significantly more extensive network activity than the sub-cortical basal forebrain; (2) there is an age-dependency, in both cortical and sub-cortical sites, with the younger brain (p14 rats) exhibiting more conspicuous assemblies over space and time compared to their older counterparts (p35-40 rats). (3) AChE-derived peptide significantly modulates the dynamics of neuronal assemblies in the basal forebrain of the p14 rat with the degree of modulation negatively correlated with age, (4) the differential in assembly size with age parallels the level of endogenous peptide in the brain, which also declines with maturity, and (5) this effect is completely reversed by a cyclised variant of AChE-peptide, 'NBP14'. These observations are attributed to an enhanced calcium entry that, according to developmental stage, could be either trophic or toxic, and as such may provide insight into the basic neurodegenerative process as well as an eventual therapeutic intervention.

Corticospinal axons make direct synaptic connections with spinal motoneurons innervating forearm muscles early during postnatal development in the rat

KEY POINTS

Direct connections between corticospinal (CS) axons and motoneurons (MNs) appear to be present only in higher primates, where they are essential for discrete movement of the digits. Their presence in adult rodents was once claimed but is now questioned. We report that MNs innervating forearm muscles in infant rats receive monosynaptic input from CS axons, but MNs innervating proximal muscles do not, which is a pattern similar to that in primates. Our experiments were carefully designed to show monosynaptic connections. This entailed selective electrical and optogenetic stimulation of CS axons and recording from MNs identified by retrograde labelling from innervated muscles. Morphological evidence was also obtained for rigorous identification of CS axons and MNs. These connections would be transient and would regress later during development. These results shed light on the development and evolution of direct CS-MN connections, which serve as the basis for dexterity in humans. Recent evidence suggests there is no direct connection between corticospinal (CS) axons and spinal motoneurons (MNs) in adult rodents. We previously showed that CS synapses are present throughout the spinal cord for a time, but are eliminated from the ventral horn during development in rodents. This raises the possibility that CS axons transiently make direct connections with MNs located in the ventral horn of the spinal cord. This was tested in the present study. Using cervical cord slices prepared from rats on postnatal days (P) 7-9, CS axons were stimulated and whole cell recordings were made from MNs retrogradely labelled with fluorescent cholera toxin B subunit (CTB) injected into selected groups of muscles. To selectively activate CS axons, electrical stimulation was carefully limited to the CS tract. In addition we employed optogenetic stimulation after injecting an adeno-associated virus vector encoding channelrhodopsin-2 (ChR2) into the sensorimotor cortex on P0. We were then able to record monosynaptic excitatory postsynaptic currents from MNs innervating forearm muscles, but not from those innervating proximal muscles. We also showed close contacts between CTB-labelled MNs and CS axons labelled through introduction of fluorescent protein-conjugated synaptophysin or the ChR2 expression system. We confirmed that some of these contacts colocalized with postsynaptic density protein 95 in their partner dendrites. It is intriguing from both phylogenetic and ontogenetic viewpoints that direct and putatively transient CS-MN connections were found only on MNs innervating the forearm muscles in infant rats, as this is analogous to the connection pattern seen in adult primates.

Early postnatal nicotine exposure disrupts the α2* nicotinic acetylcholine receptor-mediated control of oriens-lacunosum moleculare cells during adolescence in rats

Maternal cigarette smoking during pregnancy and maternal nicotine exposure in animal models are associated with cognitive impairments in offspring. However, the underlying mechanism remains unknown. Oriens-lacunosum moleculare (OLM) cells expressing α2* nicotinic acetylcholine receptors (nAChRs) are an important component of hippocampal circuitry, gating information flow and long-term potentiation (LTP) in the CA1 region. Here we investigated whether early postnatal nicotine exposure alters the normal role of α2*-nAChR-expressing OLM cells during adolescence in rats. We found that early postnatal nicotine exposure significantly decreased not only the number of α2-mRNA-expressing interneurons in the stratum oriens/alveus, but also α2*-nAChR-mediated responses in OLM cells. These effects of nicotine were prevented by co-administration with the nonselective nAChR antagonist mecamylamine, suggesting that nicotine-induced activation, but not desensitization, of nAChRs mediates the effects. α2*-nAChR-mediated depolarization of OLM cells normally triggers action potentials, causing an increase in spontaneous inhibitory postsynaptic currents in synaptically connected pyramidal cells. However, these α2*-nAChR-mediated effects were profoundly reduced after early postnatal nicotine exposure, suggesting altered control of CA1 circuits by α2*-nAChR-expressing OLM cells. Furthermore, these effects were associated with altered excitatory neural activity and LTP as well as the loss of normal α2*-nAChR-mediated control of excitatory neural activity and LTP. These findings suggest the altered function of α2*-nAChR-expressing OLM cells as an important target of further study for identifying the mechanisms underlying the cognitive impairment induced by maternal smoking during pregnancy.

Initial photophysical characterization of the proteorhodopsin optical proton sensor (PROPS)

Fluorescence is not frequently used as a tool for investigating the photocycles of rhodopsins, largely because of the low quantum yield of the retinal chromophore. However, a new class of genetically encoded voltage sensors is based upon rhodopsins and their fluorescence. The first such sensor reported in the literature was the proteorhodopsin optical proton sensor (PROPS), which is capable of indicating membrane voltage changes in bacteria by means of changes in fluorescence. However, the properties of this fluorescence, such as its lifetime decay components and its origin in the protein photocycle, remain unknown. This paper reports steady-state and nanosecond time-resolved emission of this protein expressed in two strains of Escherichia coli, before and after membrane depolarization. The voltage-dependence of a particularly long lifetime component is established. Additional work to improve quantum yields and improve the general utility of PROPS is suggested.

Functional optical probing of the hippocampal trisynaptic circuit in vitro: network dynamics, filter properties, and polysynaptic induction of CA1 LTP

Decades of brain research have identified various parallel loops linking the hippocampus with neocortical areas, enabling the acquisition of spatial and episodic memories. Especially the hippocampal trisynaptic circuit [entorhinal cortex layer II → dentate gyrus (DG) → cornu ammonis (CA)-3 → CA1] was studied in great detail because of its seemingly simple connectivity and characteristic structures that are experimentally well accessible. While numerous researchers focused on functional aspects, obtained from a limited number of cells in distinct hippocampal subregions, little is known about the neuronal network dynamics which drive information across multiple synapses for subsequent long-term storage. Fast voltage-sensitive dye imaging in vitro allows real-time recording of activity patterns in large/meso-scale neuronal networks with high spatial resolution. In this way, we recently found that entorhinal theta-frequency input to the DG most effectively passes filter mechanisms of the trisynaptic circuit network, generating activity waves which propagate across the entire DG-CA axis. These "trisynaptic circuit waves" involve high-frequency firing of CA3 pyramidal neurons, leading to a rapid induction of classical NMDA receptor-dependent long-term potentiation (LTP) at CA3-CA1 synapses (CA1 LTP). CA1 LTP has been substantially evidenced to be essential for some forms of explicit learning in mammals. Here, we review data with particular reference to whole network-level approaches, illustrating how activity propagation can take place within the trisynaptic circuit to drive formation of CA1 LTP.

Early postnatal nicotine exposure causes hippocampus-dependent memory impairments in adolescent mice: Association with altered nicotinic cholinergic modulation of LTP, but not impaired LTP

Fetal nicotine exposure from smoking during pregnancy causes long-lasting cognitive impairments in offspring, yet little is known about the mechanisms that underlie this effect. Here we demonstrate that early postnatal exposure of mouse pups to nicotine via maternal milk impairs long-term, but not short-term, hippocampus-dependent memory during adolescence. At the Schaffer collateral (SC) pathway, the most widely studied synapses for a cellular correlate of hippocampus-dependent memory, the induction of N-methyl-D-aspartate receptor-dependent transient long-term potentiation (LTP) and protein synthesis-dependent long-lasting LTP are not diminished by nicotine exposure, but rather unexpectedly the threshold for LTP induction becomes lower after nicotine treatment. Using voltage sensitive dye to visualize hippocampal activity, we found that early postnatal nicotine exposure also results in enhanced CA1 depolarization and hyperpolarization after SC stimulation. Furthermore, we show that postnatal nicotine exposure induces pervasive changes to the nicotinic modulation of CA1 activity: activation of nicotinic receptors no longer increases CA1 network depolarization, acute nicotine inhibits rather than facilitates the induction of LTP at the SC pathway by recruiting an additional nicotinic receptor subtype, and acute nicotine no longer blocks LTP induction at the temporoammonic pathway. These findings reflect the pervasive impact of nicotine exposure during hippocampal development, and demonstrate an association of hippocampal memory impairments with altered nicotinic cholinergic modulation of LTP, but not impaired LTP. The implication of our results is that nicotinic cholinergic-dependent plasticity is required for long-term memory formation and that postnatal nicotine exposure disrupts this form of plasticity.

An aerator for brain slice experiments in individual cell culture plate wells

BACKGROUND

Ex vivo acute living brain slices are a broadly employed and powerful experimental preparation. Most new technology regarding this tissue has involved the chamber used when performing electrophysiological experiments. Alternatively we instead focus on the creation of a simple, versatile aerator designed to allow maintenance and manipulation of acute brain slices and potentially other tissue in a multi-well cell culture plate.

NEW METHOD

Here we present an easily manufactured aerator designed to fit into a 24-well cell culture plate. It features a nylon mesh and a single microhole to enable gas delivery without compromising tissue stability. The aerator is designed to be individually controlled, allowing both high throughput and single well experiments.

RESULTS

The aerator was validated by testing material leach, dissolved oxygen delivery, brain slice viability and neuronal electrophysiology. Example experiments are also presented, including a test of whether β1-adrenergic receptor activation regulates gene expression in ex vivo dorsal striatum using qPCR.

COMPARISON WITH EXISTING METHODS

Key differences include enhanced control over gas delivery to individual wells containing brain slices, decreased necessary volume, a sample restraint to reduce movement artifacts, the potential to be sterilized, the avoidance of materials that absorb water and small biological molecules, minimal production costs, and increased experimental throughput.

CONCLUSION

This new aerator is of high utility and will be useful for experiments involving brain slices and other potentially tissue samples in 24-well cell culture plates.

Real-time imaging of amygdalar network dynamics in vitro reveals a neurophysiological link to behavior in a mouse model of extremes in trait anxiety

In humans and numerous other mammalian species, individuals considerably vary in their level of trait anxiety. This well known phenomenon is closely related to the etiology of several psychiatric disorders, but its neurophysiological basis remains poorly understood. Here, we applied voltage-sensitive dye imaging to brain slices from animals of the high (HAB), normal (NAB), and low (LAB) trait anxiety mouse model and investigated whether evoked neuronal activity propagations from the lateral (LA) to the central (CeA) amygdala differ in their relative strength among HAB, NAB, and LAB mice. For this purpose, we divided a real-time measure of neuronal population activity in the CeA by a respective measure obtained for the LA. This calculation yielded the metric "CeA/LA activity." Our data clearly demonstrate a positive correlation between trait anxiety levels evaluated by the elevated plus-maze test and CeA/LA activity. Moreover, we found reduced CeA/LA activity in HAB mice, which responded with decreased anxiety levels to an environmental enrichment and, inversely, detected increased anxiety levels and CeA/LA activity in LAB mice that experienced chronic mild stress. We did not observe differences in the spread of neuronal activity in the motor and visual cortex among HAB, NAB, and LAB animals. Collectively, these findings provide evidence that, in mammals, interindividual variability in trait anxiety is causally linked to individual variations in the physiological constitution of the LA-to-CeA circuitry that give rise to a differential regulation of neuronal signal flow through this fundamental input-output network of the amygdala.

Inhibitory neuron and hippocampal circuit dysfunction in an aged mouse model of Alzheimer's disease

In Alzheimer's disease (AD), a decline in explicit memory is one of the earliest signs of disease and is associated with hippocampal dysfunction. Amyloid protein exerts a disruptive impact on neuronal function, but the specific effects on hippocampal network activity are not well known. In this study, fast voltage-sensitive dye imaging and extracellular and whole-cell electrophysiology were used on entorhinal cortical-hippocampal slice preparations to characterize hippocampal network activity in 12–16 month old female APPswe/PSEN1DeltaE9 (APdE9 mice) mice. Aged APdE9 mice exhibited profound disruptions in dentate gyrus circuit activation. High frequency stimulation of the perforant pathway in the dentate gyrus (DG) area of APdE9 mouse tissue evoked abnormally large field potential responses corresponding to the wider neural activation maps. Whole-cell patch clamp recordings of the identified inhibitory interneurons in the molecular layer of DG revealed that they fail to reliably fire action potentials. Taken together, abnormal DG excitability and an inhibitory neuron failure to generate action potentials are suggested to be important contributors to the underlying cellular mechanisms of early-stage Alzheimer's disease pathophysiology.

A new nonscanning confocal microscopy module for functional voltage-sensitive dye and Ca2+ imaging of neuronal circuit activity

Recent advances in fluorescent confocal microscopy and voltage-sensitive and Ca(2+) dyes have vastly improved our ability to image neuronal circuits. However, existing confocal systems are not fast enough or too noisy for many live-cell functional imaging studies. Here, we describe and demonstrate the function of a novel, nonscanning confocal microscopy module. The optics, which are designed to fit the standard camera port of the Olympus BX51WI epifluorescent microscope, achieve a high signal-to-noise ratio (SNR) at high temporal resolution, making this configuration ideal for functional imaging of neuronal activities such as the voltage-sensitive dye (VSD) imaging. The optics employ fixed 100- × 100-pinhole arrays at the back focal plane (optical conjugation plane), above the tube lens of a usual upright microscope. The excitation light travels through these pinholes, and the fluorescence signal, emitted from subject, passes through corresponding pinholes before exciting the photodiodes of the imager: a 100- × 100-pixel metal-oxide semiconductor (MOS)-type pixel imager with each pixel corresponding to a single 100- × 100-μm photodiode. This design eliminated the need for a scanning device; therefore, acquisition rate of the imager (maximum rate of 10 kHz) is the only factor limiting acquisition speed. We tested the application of the system for VSD and Ca(2+) imaging of evoked neuronal responses on electrical stimuli in rat hippocampal slices. The results indicate that, at least for these applications, the new microscope maintains a high SNR at image acquisition rates of ≤0.3 ms per frame.

θ-Frequency resonance at the cerebellum input stage improves spike timing on the millisecond time-scale

The neuronal circuits of the brain are thought to use resonance and oscillations to improve communication over specific frequency bands (Llinas, 1988; Buzsaki, 2006). However, the properties and mechanism of these phenomena in brain circuits remain largely unknown. Here we show that, at the cerebellum input stage, the granular layer (GRL) generates its maximum response at 5-7 Hz both in vivo following tactile sensory stimulation of the whisker pad and in acute slices following mossy fiber bundle stimulation. The spatial analysis of GRL activity performed using voltage-sensitive dye (VSD) imaging revealed 5-7 Hz resonance covering large GRL areas. In single granule cells, resonance appeared as a reorganization of output spike bursts on the millisecond time-scale, such that the first spike occurred earlier and with higher temporal precision and the probability of spike generation increased. Resonance was independent from circuit inhibition, as it persisted with little variation in the presence of the GABAA receptor blocker, gabazine. However, circuit inhibition reduced the resonance area more markedly at 7 Hz. Simulations with detailed computational models suggested that resonance depended on intrinsic granule cells ionic mechanisms: specifically, K slow (M-like) and KA currents acted as resonators and the persistent Na current and NMDA current acted as amplifiers. This form of resonance may play an important role for enhancing coherent spike emission from the GRL when theta-frequency bursts are transmitted by the cerebral cortex and peripheral sensory structures during sensory-motor processing, cognition, and learning.

Developmental regulation of spatio-temporal patterns of cortical circuit activation

Neural circuits are refined in an experience-dependent manner during early postnatal development. How development modulates the spatio-temporal propagation of activity through cortical circuits is poorly understood. Here we use voltage-sensitive dye imaging (VSD) to show that there are significant changes in the spatio-temporal patterns of intracortical signals in primary visual cortex (V1) from postnatal day 13 (P13), eye opening, to P28, the peak of the critical period for rodent visual cortical plasticity. Upon direct stimulation of layer 4 (L4), activity spreads to L2/3 and to L5 at all ages. However, while from eye opening to the peak of the critical period, the amplitude and persistence of the voltage signal decrease, peak activation is reached more quickly and the interlaminar gain increases with age. The lateral spread of activation within layers remains unchanged throughout the time window under analysis. These developmental changes in spatio-temporal patterns of intracortical circuit activation are mediated by differences in the contributions of excitatory and inhibitory synaptic components. Our results demonstrate that after eye opening the circuit in V1 is refined through a progression of changes that shape the spatio-temporal patterns of circuit activation. Signals become more efficiently propagated across layers through developmentally regulated changes in interlaminar gain.

An evaluation of in vivo voltage-sensitive dyes: pharmacological side effects and signal-to-noise ratios after effective removal of brain-pulsation artifacts

In the current study, we investigated pharmacological side effects and signal-to-noise ratios (SNRs) of two commonly used voltage-sensitive dyes (VSDs): the blue dye RH-1691 (1 mg/ml) and the red dye di-4-ANEPPS (0.1 mg/ml), applied in vivo to the rat barrel cortex. Blue dyes are often favored over red dyes in in vivo studies due to their apparent superior SNR, partly because their fluorescence spectrum is farther away from the hemoglobin absorption spectrum, making them less prone to heartbeat-associated brain-pulsation artifacts (BPA). We implemented a previously reported template-based BPA removal algorithm and evaluated its applicability to di-4-ANEPPS before comparing characteristics of the two dyes. Somatosensory-evoked potentials (SEPs) were also recorded. Whereas SEPs recorded before and after application of di-4-ANEPPS failed to exhibit demonstrable differences, RH-1691 caused a significant and prolonged increase in SEP amplitude for several hours. In contrast, neither dye influenced the spontaneous cortical activity as assessed by the spectral content of the EEG. Both dyes turned out to be strikingly similar with respect to changes in fractional fluorescence as a function of SEP response amplitude, as well as regarding shot noise characteristics after removal of the BPA. Thus there is strong evidence that the increased SNR for RH-1691 is a consequence of an artificially increased signal. When applying an appropriate BPA removal algorithm, di-4-ANEPPS has proven to be suitable for single-trial in vivo VSD imaging (VSDI) and produces no detectable neurophysiological changes in the system under investigation. Taken together, our data argue for a careful re-evaluation of pharmacological side effects of RH-1691 and support the applicability of di-4-ANEPPS for stable single-trial in vivo VSDI recordings.

Enhancement of dorsal hippocampal activity by knockdown of HCN1 channels leads to anxiolytic- and antidepressant-like behaviors

The hippocampus is an integral brain region for affective disorders. TRIP8b knockout mice lacking functional HCN channels as well as both HCN1 and HCN2 knockout mice have been shown to display antidepressant-like behaviors. The mechanisms or brain regions involved in these alterations in behavior, however, are not clear. We developed a lentiviral shRNA system to examine whether knockdown of HCN1 protein in the dorsal hippocampal CA1 region is sufficient to produce antidepressant-like effects. We found that knockdown of HCN1 channels increased cellular excitability and resulted in physiological changes consistent with a reduction of I(h). Rats infused with lentiviral shRNA-HCN1 in the dorsal hippocampal CA1 region displayed antidepressant- and anxiolytic-like behaviors associated with widespread enhancement of hippocampal activity and upregulation of BDNF-mTOR signaling pathways. Our results suggest that HCN1 protein could be a potential target for treatment of anxiety and depression disorders.

Entorhinal theta-frequency input to the dentate gyrus trisynaptically evokes hippocampal CA1 LTP

There exists substantial evidence that some forms of explicit learning in mammals require long-term potentiation (LTP) at hippocampal CA3-CA1 synapses. While CA1 LTP has been well characterized at the monosynaptic level, it still remains unclear how the afferent systems to the hippocampus can initiate formation of this neuroplastic phenomenon. Using voltage-sensitive dye imaging (VSDI) in a mouse brain slice preparation, we show that evoked entorhinal cortical (EC) theta-frequency input to the dentate gyrus highly effectively generates waves of neuronal activity which propagate through the entire trisynaptic circuit of the hippocampus (“HTC-Waves”). This flow of activity, which we also demonstrate in vivo, critically depends on frequency facilitation of mossy fiber to CA3 synaptic transmission. The HTC-Waves are rapidly boosted by the cognitive enhancer caffeine (5 μM) and the stress hormone corticosterone (100 nM). They precisely follow the rhythm of the EC input, involve high-frequency firing (>100 Hz) of CA3 pyramidal neurons, and induce NMDA receptor-dependent CA1 LTP within a few seconds. Our study provides the first experimental evidence that synchronous theta-rhythmical spiking of EC stellate cells, as occurring during EC theta oscillations, has the capacity to drive induction of CA1 LTP via the hippocampal trisynaptic pathway. Moreover, we present data pointing to a basic filter mechanism of the hippocampus regarding EC inputs and describe a methodology to reveal alterations in the “input–output relationship” of the hippocampal trisynaptic circuit.

β-Adrenergic modulation of spontaneous spatiotemporal activity patterns and synchrony in hyperexcitable hippocampal circuits

A description of healthy and pathological brain dynamics requires an understanding of spatiotemporal patterns of neural activity and characteristics of its propagation between interconnected circuits. However, the structure and modulation of the neural activation maps underlying these patterns and their propagation remain elusive. We investigated effects of β-adrenergic receptor (β-AR) stimulation on the spatiotemporal characteristics of emergent activity in rat hippocampal circuits. Synchronized epileptiform-like activity, such as interictal bursts (IBs) and ictal-like events (ILEs), were evoked by 4-aminopyridine (4-AP), and their dynamics were studied using a combination of electrophysiology and fast voltage-sensitive dye imaging. Dynamic characterization of the spontaneous IBs showed that they originated in dentate gyrus/CA3 border and propagated toward CA1. To determine how β-AR modulates spatiotemporal characteristics of the emergent IBs, we used the β-AR agonist isoproterenol (ISO). ISO significantly reduced the spatiotemporal extent and propagation velocity of the IBs and significantly altered network activity in the 1- to 20-Hz range. Dual whole cell recordings of the IBs in CA3/CA1 pyramidal cells and optical analysis of those regions showed that ISO application reduced interpyramidal and interregional synchrony during the IBs. In addition, ISO significantly reduced duration not only of the shorter duration IBs but also the prolonged ILEs in 4-AP. To test whether the decrease in ILE duration was model dependent, we used a different hyperexcitability model, zero magnesium (0 Mg(2+)). Prolonged ILEs were readily formed in 0 Mg(2+), and addition of ISO significantly reduced their durations. Taken together, these novel results provide evidence that β-AR activation dynamically reshapes the spatiotemporal activity patterns in hyperexcitable circuits by altering network rhythmogenesis, propagation velocity, and intercellular/regional synchronization.

Endogenously released ACh and exogenous nicotine differentially facilitate long-term potentiation induction in the hippocampal CA1 region of mice

We examined the role of α7- and β2-containing nicotinic acetylcholine receptors (nAChRs) in the induction of long-term potentiation (LTP). Theta-burst stimulation (TBS), mimicking the brain's naturally occurring theta rhythm, induced robust LTP in hippocampal slices from α7 and β2 knockout mice. This suggests TBS is capable of inducing LTP without activation of α7- or β2-containing nAChRs. However, when weak TBS was applied, the modulatory effects of nicotinic receptors on LTP induction became visible. We showed that during weak TBS, activation of α7 nAChRs occurs by the release of ACh, contributing to LTP induction. Additionally, bath-application of nicotine activated β2-containing nAChRs to promote LTP induction. Despite predicted nicotine-induced desensitization, synaptically mediated activation of α7 nAChRs still occurs in the presence of nicotine and contributed to LTP induction. Optical recording of single-stimulation-evoked excitatory activity with a voltage-sensitive dye revealed enhanced excitatory activity in the presence of nicotine. This effect of nicotine was robust during high-frequency stimulation, and was accompanied by enhanced burst excitatory postsynaptic potentials. Nicotine-induced enhancement of excitatory activity was observed in slices from α7 knockout mice, but was absent in β2 knockout mice. These results suggest that the nicotine-induced enhancement of excitatory activity is mediated by β2-containing nAChRs, and is related to the nicotine-induced facilitation of LTP induction. Thus, our study demonstrates that the activation of α7- and β2-containing nAChRs differentially facilitates LTP induction via endogenously released ACh and exogenous nicotine, respectively, in the hippocampal CA1 region of mice.

A series of case studies: practical methodology for identifying antinociceptive multi-target drugs

Since the introduction of drug discovery based on single targets, the number of newly developed drugs has steadily declined, and the reliablility of the current drug-discovery paradigm has been unceasingly questioned. As an alternative, an emerging approach pursuing multi-targeting drugs has arisen to reflect multifactorial diseases caused by the complex networks of various mechanisms. The purpose of this paper is to review multi-target drugs and introduce our progress in establishing a practical methodology for identifying antinociceptive multi-target drugs. We have adopted a system of ex vivo efficacy screening using long-term potentiation in rat spinal cord as a surrogate biomarker for neuropathic pain. A bait-target approach is also adopted to lure an unknown target combination that induces synergistic mechanisms.