Interpretation and Optimization of Absorbance and Fluorescence Signals from Voltage-sensitive Dyes

Ventral hippocampal projections to the medial prefrontal cortex regulate social memory

Inputs from the ventral hippocampus (vHIP) to the medial prefrontal cortex (mPFC) are implicated in several neuropsychiatric disorders. Here, we show that the vHIP-mPFC projection is hyperactive in the Mecp2 knockout mouse model of the autism spectrum disorder Rett syndrome, which has deficits in social memory. Long-term excitation of mPFC-projecting vHIP neurons in wild-type mice impaired social memory, whereas their long-term inhibition in Rett mice rescued social memory deficits. The extent of social memory improvement was negatively correlated with vHIP-evoked responses in mPFC slices, on a mouse-per-mouse basis. Acute manipulations of the vHIP-mPFC projection affected social memory in a region and behavior selective manner, suggesting that proper vHIP-mPFC signaling is necessary to recall social memories. In addition, we identified an altered pattern of vHIP innervation of mPFC neurons, and increased synaptic strength of vHIP inputs onto layer five pyramidal neurons as contributing factors of aberrant vHIP-mPFC signaling in Rett mice.

Appearance of fast astrocytic component in voltage-sensitive dye imaging of neural activity

Background

Voltage-sensitive dye (VSD) imaging and intrinsic optical signals (IOS) are widely used methods for monitoring spatiotemporal neural activity in extensive networks. In spite of that, identification of their major cellular and molecular components has not been concluded so far.

Results

We addressed these issues by imaging spatiotemporal spreading of IOS and VSD transients initiated by Schaffer collateral stimulation in rat hippocampal slices with temporal resolution comparable to standard field potential recordings using a 464-element photodiode array. By exploring the potential neuronal and astroglial molecular players in VSD and IOS generation, we identified multiple astrocytic mechanisms that significantly contribute to the VSD signal, in addition to the expected neuronal targets. Glutamate clearance through the astroglial glutamate transporter EAAT2 has been shown to be a significant player in VSD generation within a very short (<5 ms) time-scale, indicating that astrocytes do contribute to the development of spatiotemporal VSD transients previously thought to be essentially neuronal. In addition, non-specific anion channels, astroglial K⁺ clearance through K_ir4.1 channel and astroglial Na⁺/K⁺ ATPase also contribute to IOS and VSD transients.

Conclusion

VSD imaging cannot be considered as a spatially extended field potential measurement with predominantly neuronal origin, instead it also reflects a fast communication between neurons and astrocytes.

Monitoring Spiking Activity of Many Individual Neurons in Invertebrate Ganglia

Optical recording with fast voltage sensitive dyes makes it possible, in suitable preparations, to simultaneously monitor the action potentials of large numbers of individual neurons. Here we describe methods for doing this, including considerations of different dyes and imaging systems, methods for correlating the optical signals with their source neurons, procedures for getting good signals, and the use of Independent Component Analysis for spike-sorting raw optical data into single neuron traces. These combined tools represent a powerful approach for large-scale recording of neural networks with high temporal and spatial resolution.

Long-term potentiation in hilar circuitry modulates gating by the dentate gyrus

The dentate gyrus serves as a gateway to the hippocampus, filtering and processing sensory inputs as an animal explores its environment. The hilus occupies a strategic position within the dentate gyrus from which it can play a pivotal role in these functions. Inputs from dentate granule cells converge on the hilus, and excitatory hilar mossy cells redistribute these signals back to granule cells to transform a pattern of cortical input into a new pattern of output to the hippocampal CA3 region. Using voltage-sensitive dye to image electrical activity in rat hippocampal slices, we explored how long-term potentiation (LTP) of different excitatory synapses modifies the flow of information. Theta burst stimulation of the perforant path potentiated responses throughout the molecular layer, but left responses in the CA3 region unchanged. By contrast, theta burst stimulation of the granule cell layer potentiated responses throughout the molecular layer, as well as in the CA3 region. Theta burst stimulation of the granule cell layer potentiated CA3 responses not only to granule cell layer stimulation but also to perforant path stimulation. Potentiation of responses in the CA3 region reflected NMDA receptor-dependent LTP of upstream synapses between granule cells and mossy cells, with no detectable contribution from NMDA receptor-independent LTP of local CA3 mossy fiber synapses. Potentiation of transmission to the CA3 region required LTP in both granule cell→mossy cell and mossy cell→granule cell synapses. This bidirectional plasticity enables hilar circuitry to regulate the flow of information through the dentate gyrus and on to the hippocampus.

Recall of spatial patterns stored in a hippocampal slice by long-term potentiation

Nervous systems are thought to encode information as patterns of electrical activity distributed sparsely through networks of neurons. These networks then process information by transforming one pattern of electrical activity into another. To store information as a pattern, a neural network must strengthen synapses between designated neurons so that activation of some of these neurons corresponding to some features of an object can spread to activate the larger group representing the complete object. This operation of pattern completion endows a neural network with autoassociative memory. Pattern completion by neural networks has been modeled extensively with computers and invoked in behavioral studies, but experiments have yet to demonstrate pattern completion in an intact neural circuit. In the present study, imaging with voltage-sensitive dye in the CA3 region of a hippocampal slice revealed different spatial patterns of activity elicited by electrical stimulation of different sites. Stimulation of two separate sites individually, or both sites simultaneously, evoked "partial" or "complete" patterns, respectively. A complete pattern was then stored by applying theta burst stimulation to both sites simultaneously to induce long-term potentiation (LTP) of synapses between CA3 pyramidal cells. Subsequent stimulation of only one site then activated an extended pattern. Quantitative comparisons between response maps showed that the post-LTP single-site patterns more closely resembled the complete dual-site pattern. Thus, LTP induction enabled the CA3 region to complete a dual-site pattern upon stimulation of only one site. This experiment demonstrated that LTP induction can store information in the CA3 region of the hippocampus for subsequent retrieval.

Neuronal and astroglial correlates underlying spatiotemporal intrinsic optical signal in the rat hippocampal slice

Widely used for mapping afferent activated brain areas in vivo, the label-free intrinsic optical signal (IOS) is mainly ascribed to blood volume changes subsequent to glial glutamate uptake. By contrast, IOS imaged in vitro is generally attributed to neuronal and glial cell swelling, however the relative contribution of different cell types and molecular players remained largely unknown. We characterized IOS to Schaffer collateral stimulation in the rat hippocampal slice using a 464-element photodiode-array device that enables IOS monitoring at 0.6 ms time-resolution in combination with simultaneous field potential recordings. We used brief half-maximal stimuli by applying a medium intensity 50 Volt-stimulus train within 50 ms (20 Hz). IOS was primarily observed in the str. pyramidale and proximal region of the str. radiatum of the hippocampus. It was eliminated by tetrodotoxin blockade of voltage-gated Na(+) channels and was significantly enhanced by suppressing inhibitory signaling with gamma-aminobutyric acid(A) receptor antagonist picrotoxin. We found that IOS was predominantly initiated by postsynaptic Glu receptor activation and progressed by the activation of astroglial Glu transporters and Mg(2+)-independent astroglial N-methyl-D-aspartate receptors. Under control conditions, role for neuronal K(+)/Cl(-) cotransporter KCC2, but not for glial Na(+)/K(+)/Cl(-) cotransporter NKCC1 was observed. Slight enhancement and inhibition of IOS through non-specific Cl(-) and volume-regulated anion channels, respectively, were also depicted. High-frequency IOS imaging, evoked by brief afferent stimulation in brain slices provide a new paradigm for studying mechanisms underlying IOS genesis. Major players disclosed this way imply that spatiotemporal IOS reflects glutamatergic neuronal activation and astroglial response, as observed within the hippocampus. Our model may help to better interpret in vivo IOS and support diagnosis in the future.

Network hyperexcitability in hippocampal slices from Mecp2 mutant mice revealed by voltage-sensitive dye imaging

Dysfunctions of neuronal and network excitability have emerged as common features in disorders associated with intellectual disabilities, autism, and seizure activity, all common clinical manifestations of Rett syndrome (RTT), a neurodevelopmental disorder caused by loss-of-function mutations in the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2). Here, we evaluated the consequences of Mecp2 mutation on hippocampal network excitability, as well as synapse structure and function using a combination of imaging and electrophysiological approaches in acute slices. Imaging the amplitude and spatiotemporal spread of neuronal depolarizations with voltage-sensitive dyes (VSD) revealed that the CA1 and CA3 regions of hippocampal slices from symptomatic male Mecp2 mutant mice are highly hyperexcitable. However, only the density of docked synaptic vesicles and the rate of release from the readily releasable pool are impaired in Mecp2 mutant mice, while synapse density and morphology are unaffected. The differences in network excitability were not observed in surgically isolated CA1 minislices, and blockade of GABAergic inhibition enhanced VSD signals to the same extent in Mecp2 mutant and wild-type mice, suggesting that network excitability originates in area CA3. Indeed, extracellular multiunit recordings revealed a higher level of spontaneous firing of CA3 pyramidal neurons in slices from symptomatic Mecp2 mutant mice. The neuromodulator adenosine reduced the amplitude and spatiotemporal spread of VSD signals evoked in CA1 of Mecp2 mutant slices to wild-type levels, suggesting its potential use as an anticonvulsant in RTT individuals. The present results suggest that hyperactive CA3 pyramidal neurons contribute to hippocampal dysfunction and possibly to limbic seizures observed in Mecp2 mutant mice and RTT individuals.

Improved probes for hybrid voltage sensor imaging

Hybrid voltage sensors (hVoS) probe membrane potential by coupling the fluorescence of membrane-anchored proteins to the movement of a membrane-embedded hydrophobic anion dipicrylamine. Fluorescence resonance energy transfer between these two components transduces voltage changes into fluorescence changes, providing a signal for imaging electrical activity in genetically targeted cells. To improve hVoS signals, we systematically varied the optical properties, membrane targeting motifs, and linkages of fluorescent proteins to optimize the normalized fluorescence change (ΔF/F) and signal/noise ratio. The best results were obtained with cerulean fluorescent protein tagged N-terminally with a GAP43 motif and C-terminally with a truncated h-ras motif. With 100 mV steps in PC12 cells, this probe produced ΔF/F = 26% (4 μM dipicrylamine), which was threefold greater than that obtained with the original farnesylated EGFP construct. We also obtained a fivefold greater signal/noise ratio, which was 70% of a theoretical optimum. We designate this GAP43-CerFP-t-h-ras construct as hVoS 2.0. With the aid of a theoretical analysis, we estimated that hVoS 2.0 places its fluorophore ∼40 Å from the bilayer midplane. hVoS 2.0 performed well in cultured hippocampal neurons, where single action potentials produced clear fluorescence changes in a single trial. This improved probe should help investigators image voltage in genetically targeted neurons.

Spatial control of the energy metabolism of yeast cells through electrolytic generation of oxygen

The metabolic dynamics of yeast cells is controlled by electric pulses delivered through a spatially extended yeast cell/Au electrode interface. Concomitant with voltage pulses, oxygen is generated electrolytically at the electrode surface and delivered to the cells. The generation of oxygen was investigated in dependence of the applied voltage, width of the voltage pulses and temperature of the electrolytic solution. The local oxygen pulses at the electrodes lead to a transient activation of the aerobic energy metabolism of the yeast cells causing a perturbation in their energy balance. The effect of these local perturbations on the temporal dynamics of glycolysis in yeast cells is quantified in dependence of the energy state of cells.

Membrane potential response profiles of CA1 pyramidal cells probed with voltage-sensitive dye optical imaging in rat hippocampal slices reveal the impact of GABA(A)-mediated feed-forward inhibition in signal propagation

The spatial and temporal distribution of excitatory and inhibitory membrane potential responses on a cell plays an important role in neuronal calculations in local neuronal circuits in the brain. The electrical dynamics of excitatory and inhibitory inputs along the somatodendritic extent of CA1 pyramidal cells during circuit activation were examined by stimulating strata radiatum (SR), oriens (SO), and lacunosum-moleculare (SLM) and measuring laminar responses with voltage-sensitive dye (VSD) optical recording methods. We first confirmed the linearity of the optical signal by comparing fluorescence changes in CA1 to global membrane potential changes when slices were bathed in high-potassium ([K+](O)=25 mM) solution. Except for a TTX-sensitive component in stratum pyramidale, fluorescence changes were equal in all strata, indicating that VSD sensitivity had reasonable linearity across layers. We then compared membrane potential profiles in slices exposed to picrotoxin, a GABA(A) receptor antagonist. We attributed the picrotoxin-induced changes in the first peak of the excitatory membrane potential to feed-forward inhibition and the later response (appearing 30 ms after stimulation) to feedback inhibition. A difference in feed-forward components was observed in perisomatic and distal apical dendritic regions after SR stimulation. SLM stimulation produced large differences in perisomatic and apical dendritic regions. SO stimulation, however, produced no feed-forward inhibition at the perisomatic region, but produces feed-forward inhibition in distal dendritic regions. These results suggest that actual inhibition of membrane potential response by feed-forward inhibition is greater at perisomatic regions after SR or SLM stimulation but is smaller at distal dendritic regions after SR, SO, and SLM stimulation.

Spatiotemporal analyses of interactions between entorhinal and CA1 projections to the subiculum in rat brain slices

The subiculum and the entorhinal cortex (EC) are important structures in processing and transmitting information between the neocortex and the hippocampus. The subiculum potentially receives information from the EC through two routes. In addition to a direct projection from EC to the subiculum, there is an indirect polysynaptic connection. The latter uses a number of possible pathways, which all converge onto the final projection from the hippocampal field CA1 to the subiculum. In this series of experiments we investigated to what extent activity in both pathways influences population activity of subicular neurons. We used voltage sensitive dyes in combined hippocampal-EC slices of the rat to measure the spatio-temporal activity patterns. To activate the two inputs to the subiculum, stimulation electrodes were placed in the stratum oriens/alveus of CA1 and in layer III of the medial EC. The response patterns evoked in the subiculum after electrical stimulation of each of these input pathways separately were compared with the response patterns after simultaneous stimulation of both areas (medial EC + CA1). A comparison of the computed added responses of the two individual stimulations with the measured responses after simultaneous stimulation suggests that both inputs are linearly added in the subiculum with very little nonlinear interactions. This strongly suggests that in the subiculum interaction at a single cell level of the direct and the indirect pathways from the EC is an unlikely scenario.

Voltage imaging reveals the CA1 region at the CA2 border as a focus for epileptiform discharges and long-term potentiation in hippocampal slices

Voltage-sensitive-dye imaging was used to study the initiation and propagation of epileptiform activity in transverse hippocampal slices. A portion of the slices tested generated epileptiform discharges in response to electrical shocks under normal physiological conditions. The fraction of slices showing epileptiform responses increased from 44 to 86% when bathing [K+] increased from 3.2 to 4 mM. Regardless of stimulation site in the dentate gyrus and hippocampus, discharges generally initiated in the CA3 region. After onset, discharges abruptly appeared in the CA1 region, right at the CA2 border. This spread from the CA3 region to the CA1 region was saltatory, occurring before detectable activity in the intervening CA2 and CA3 regions. Discharges did eventually propagate smoothly through the intervening CA3 region into the CA2 region, but on a slower timescale. The surge in the CA1 region did not spread back into the CA2 region, but spread through the CA1 region toward the subiculum. Tetanic stimulation, theta bursts, and GABA(A) receptor antagonists failed to alter this characteristic pattern, but did reduce the latency of discharge onset. The part of the CA1 region at the CA2 border, where epileptic responses emerged with relatively short latency, also expressed stronger long-term potentiation (LTP) than the rest of the CA1 region. The CA2 region, where discharges had long latencies and low amplitudes, expressed weaker LTP. Thus the CA1 region at the CA2 border has unique properties, which make this part of the hippocampus an important locus for both epileptiform activity and plasticity.

Heterogeneous spatial patterns of long-term potentiation in rat hippocampal slices

Although LTP (long-term potentiation) of synaptic transmission has received much attention as a model for learning and memory, its function within a neural circuit context remains poorly understood. To monitor LTP over an extensive circuit, we imaged responses in hippocampal slices using a voltage-sensitive dye. Following theta-burst stimulation, evoked optical signals showed an increase that lasted 40 min or more. Weak stimuli only potentiated the local area around the stimulating electrode, but stronger stimuli induced LTP over a wide area with a complex and non-uniform spatial pattern. The expression of LTP showed distinct peaks and valleys that depended on which axons were activated. Interestingly, the spatial distribution of LTP bore no relation to the spatial distribution of single-shock responses, but closely resembled the distribution of postsynaptic spikes evoked by theta bursts. Thus, postsynaptic spikes during induction constitute a critical determinant for the expression of LTP in intact circuits.

Spatiotemporal patterns of dorsal root-evoked network activity in the neonatal rat spinal cord: optical and intracellular recordings

Spatiotemporal patterns of dorsal root-evoked potentials were studied in transverse slices of the rat spinal cord by monitoring optical signals from a voltage-sensitive dye with multiple-photodiode optic camera. Typically, dorsal root stimulation generated two basic waveforms of voltage images: dual-component images consisting of fast, spike-like signal followed by a slow signal in the dorsal horn, and small, slow signals in the ventral horn. To qualitatively relate the optical signals to membrane potentials, whole cell recordings were combined with measurements of light absorption in the area around the soma. The slow optical signals correlated closely with subthreshold postsynaptic potentials in all regions of the cord. The spike-like component was not associated with postsynaptic action potentials, suggesting that the fast signal was generated by presynaptic action potentials. Firing in a single neuron could not be detected optically, implying that local voltage images originated from synchronously activated neuronal ensembles. Blocking glutamatergic synaptic transmission inhibited excitatory postsynaptic potentials (EPSPs) and significantly reduced the slow optical signals, indicating that they were mediated by glutamatergic synapses. Suppressing glycine-mediated inhibition increased the amplitude of both optical signals and EPSPs, while blocking GABA(A) receptor-mediated synapses, increased the amplitude and time course of EPSPs and prolonged the duration of voltage images in larger areas of the slice. The close correlation between evoked EPSPs and their respective local voltage images shows the advantage of the high temporal resolution optical system in measuring both the spatiotemporal dynamics of segmental network excitation and integrated potentials of neuronal ensembles at identified sites.