Optical detection of dendritic spike initiation in hippocampal CA1 pyramidal neurons

Synchronous circadian voltage rhythms with asynchronous calcium rhythms in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), the master circadian clock, contains a network composed of multiple types of neurons which are thought to form a hierarchical and multioscillator system. The molecular clock machinery in SCN neurons drives membrane excitability and sends time cue signals to various brain regions and peripheral organs. However, how and at what time of the day these neurons transmit output signals remain largely unknown. Here, we successfully visualized circadian voltage rhythms optically for many days using a genetically encoded voltage sensor, ArcLightD. Unexpectedly, the voltage rhythms are synchronized across the entire SCN network of cultured slices, whereas simultaneously recorded Ca²⁺ rhythms are topologically specific to the dorsal and ventral regions. We further found that the temporal order of these two rhythms is cell-type specific: The Ca²⁺ rhythms phase-lead the voltage rhythms in AVP neurons but Ca²⁺ and voltage rhythms are nearly in phase in VIP neurons. We confirmed that circadian firing rhythms are also synchronous and are coupled with the voltage rhythms. These results indicate that SCN networks with asynchronous Ca²⁺ rhythms produce coherent voltage rhythms.

Intracellular long-wavelength voltage-sensitive dyes for studying the dynamics of action potentials in axons and thin dendrites

In CNS neurons most of synaptic integration takes place in thin dendritic branches that are difficult to study with conventional physiological recording techniques (electrodes). When cellular compartments are too small, or too many, for electrode recordings, optical methods bring considerable advantages. Here we focused our experimental effort on the development and utilization of new kinds of voltage-sensitive dyes (VSD). The new VSDs have bluish appearance in organic solvents, and hence are dubbed "blue dyes". They have preferred excitation windows for voltage recording that are shifted to longer wavelengths (approximately 660nm). Excitation in deep red light and emission in the near-infrared render "blue VSDs" potentially useful in measurements from fluorescent structures below the tissue surface because light scattering is minimized at longer wavelengths. Seven new molecules were systematically tested using intracellular injection. In comparison to the previously used red dye (JPW-3028) the blue dyes have better sensitivity (DeltaF/F) by approximately 40%. Blue dyes take little time to fill the dendritic tree, and in this aspect they are comparable with the fastest red dye JPW-3028. Based on our results, blue VSDs are well suited for experimental exploration of thin neuronal processes in semi intact preparations (brain slice). In some cases only six sweeps of temporal averaging were needed to acquire excellent records of individual action potentials in basal and oblique dendritic branches, or in axons and axon collaterals up to 200microm away from the cell body. Signal-to-noise ratio of these recordings was approximately 10. The combination of blue dyes and laser illumination approach imposed little photodynamic damage and allowed the total number of recording sweeps per cell to exceed 100. Using these dyes and a spot laser illumination technique, we demonstrate the first recording of action potentials in the oblique dendrite and distal axonal segment of the same pyramidal cell.

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.

Input-output relations in the entorhinal cortex-dentate-hippocampal system: evidence for a non-linear transfer of signals

In the current study we analyzed the input-output relations in the entorhinal-dentate-hippocampal system, a major network involved in long-term memory. In anesthetized guinea pigs, the system was driven by activation of perforant path neurons in the entorhinal cortex (ENT), via presubicular fibers directly stimulated in the dorsal psalterium. Perforant path neuron discharge activated in parallel the dentate gyrus (DG) and hippocampal field CA2. Whereas the output from the DG activated hippocampal field CA3, the output from the sole field CA2 was sufficient for activation of field CA1. Signals from field CA3 operated in concert with CA2, likely contributing to discharge field CA1. These findings indicate the existence of two in parallel disynaptic systems: an ENT-CA2-CA1 and an ENT-DG-CA3 system. The convergence of the latter with the former gives origin the classical trisynaptic circuit, the ENT-DG-CA3-CA1 system. The input-output relations between the population excitatory postsynaptic potentials (pEPSP) evoked in the DG, CA3, CA2 and CA1 and the population spike (PS) evoked in the structure upstream (the input) were described by smooth sigmoid curves. In contrast, the input-output relations of the PS versus the pEPSP within each structure were described by steep sigmoid curves. The net input-output functions of the DG (ENT-DG system), field CA2 (ENT-CA2 system), field CA3 (ENT-DG-CA3 system) and field CA1 (ENT-CA2-CA1&ENT-DG-CA3-CA1 system) were described by sigmoid curves. While the DG and field CA2 exhibited steep sigmoids, fields CA3 and CA1 had less steep sigmoid functions. The present study demonstrates that all structures downstream to the ENT operate according to sigmoid input-output functions, characterized by specific parameters. These different behaviors may contribute to different memory processes. We additionally demonstrate that field CA1 can be activated by field CA2, independently from field CA3. This functional dissociation between CA3 and CA1 may subserve specific roles of each field in memory encoding/retrieval.

Building spike representation in tetrodes

This paper presents a new technique for analyzing the recorded information from tetrodes in freely behaving rats, based on independent component analysis (ICA). The ion-specific pumps and channels allow fast transfer of charges such as Na+, K+, Cl- and eventually Ca2+ during each action potential (AP). These groups of charges under an electrical field have distinct spatial trajectories. Therefore, the generated signals within a tetrode are considered to be composed mainly by statistically independent signal sources that can be obtained by performing ICA. In order to compute the position of independent sources during AP generation, the triangulation method uses an iterative Newton-Raphson algorithm. The representation of the independent signal sources in three-dimensional tetrode space is then obtained. Since the charge movements are extensively spread on the neuron's surface, the representation in tetrode space reveals electrical spatial patterns of activation during each AP. The analysis of several spikes coming from the same neuron reveals small changes from spike to spike in the 3D shape. Since information within spikes is highly transferred by ionic fluxes these electrical patterns of activation reflect neuronal computation occurring during each AP.

Input source and strength influences overall firing phase of model hippocampal CA1 pyramidal cells during theta: relevance to REM sleep reactivation and memory consolidation

In simulation studies using a realistic model CA1 pyramidal cell, we accounted for the shift in mean firing phase from theta cycle peaks to theta cycle troughs during rapid-eye movement (REM) sleep reactivation of hippocampal CA1 place cells over several days of growing familiarization with an environment (Brain Res 855:176-180). Changes in the theta drive phase and amplitude between proximal and distal dendritic regions of the cell modulated the theta phase of firing when stimuli were presented at proximal and distal dendritic locations. Stimuli at proximal dendritic sites (proximal to 100 microm from the soma) invoked firing with a significant phase preference at the depolarizing theta peaks, while distal stimuli (>290 microm from the soma) invoked firing at hyperpolarizing theta troughs. The input location-related phase preference depended on active dendritic conductances, a sufficient electrotonic separation between input sites and theta-induced subthreshold membrane potential oscillations in the cell. The simulation results predict that the shift in mean theta phase during REM sleep cellular reactivation could occur through potentiation of distal dendritic (temporo-ammonic) synapses and depotentiation of proximal dendritic (Schaffer collateral) synapses over the course of familiarization.

Signal propagation in oblique dendrites of CA1 pyramidal cells

The electrophysiological properties of the oblique branches of CA1 pyramidal neurons are largely unknown and very difficult to investigate experimentally. These relatively thin dendrites make up the majority of the apical tree surface area and constitute the main target of Schaffer collateral axons from CA3. Their electrogenic properties might have an important role in defining the computational functions of CA1 neurons. It is thus important to determine if and to what extent the back- and forward propagation of action potentials (AP) in these dendrites could be modulated by local properties such as morphology or active conductances. In the first detailed study of signal propagation in the full extent of CA1 oblique dendrites, we used 27 reconstructed three-dimensional morphologies and different distributions of the A-type K(+) conductance (K(A)), to investigate their electrophysiological properties by computational modeling. We found that the local K(A) distribution had a major role in modulating action potential back propagation, whereas the forward propagation of dendritic spikes originating in the obliques was mainly affected by local morphological properties. In both cases, signal processing in any given oblique was effectively independent of the rest of the neuron and, by and large, of the distance from the soma. Moreover, the density of K(A) in oblique dendrites affected spike conduction in the main shaft. Thus the anatomical variability of CA1 pyramidal cells and their local distribution of voltage-gated channels may suit a powerful functional compartmentalization of the apical tree.

Direction-selective dendritic action potentials in rabbit retina

Dendritic spikes that propagate toward the soma are well documented, but their physiological role remains uncertain. Our in vitro patch-clamp recordings and two-photon calcium imaging show that direction-selective retinal ganglion cells (DSGCs) utilize orthograde dendritic spikes during physiological activity. DSGCs signal the direction of image motion. Excitatory subthreshold postsynaptic potentials are observed in DSGCs for motion in all directions and provide a weakly tuned directional signal. However, spikes are generated over only a narrow range of motion angles, indicating that spike generation greatly enhances directional tuning. Our results indicate that spikes are initiated at multiple sites within the dendritic arbors of DSGCs and that each dendritic spike initiates a somatic spike. We propose that dendritic spike failure, produced by local inhibitory inputs, might be a critical factor that enhances directional tuning of somatic spikes.

Glutamate release increases during mossy-CA3 LTP but not during Schaffer-CA1 LTP

Abstract It is still a matter of dispute whether the expression of hippocampal long-term potentiation (LTP) is due to enhanced transmitter release or enhanced postsynaptic sensitivity. Recently we developed a novel method to monitor synaptically released glutamate. In this method, brain slice preparations are stained with a voltage-sensitive dye RH155 which preferentially stains glial cells, and synaptically induced glial depolarization (SIGD) are optically detected in the presence of the blockers for ionotropic glutamate receptors. We have previously shown that SIGD is due to uptake of synaptically released glutamate by glial glutamate transporters. Here we applied this method to examine change in glutamate release during hippocampal LTP. To examine mossy-CA3 LTP, stimulating electrodes were placed in dentate gyrus and tetanic stimulation was delivered in the presence of 50 micro m APV. The amplitude of SIGD after inducing LTP was significantly greater than that in control experiments in which tetanus was not delivered. The amplitude of SIGD after inducing LTP by a brief (3-5 min) application of 50 micro m forskolin was also significantly greater than that in control experiments. At the Schaffer-CA1 synapse, the change in the amplitude of SIGD during LTP induced either by 100 Hz tetanus LTP or 200 Hz tetanus was not significantly greater than that of control experiments. These results provide evidence for increased glutamate release from the presynaptic terminals as the expression mechanism for both tetanus-induced and forskolin-induced LTP at mossy-CA3 synapses, and evidence supporting a postsynaptic expression mechanism at Schaffer-CA1 synapses.

Effects of uniform extracellular DC electric fields on excitability in rat hippocampal slices in vitro

The effects of uniform steady state (DC) extracellular electric fields on neuronal excitability were characterized in rat hippocampal slices using field, intracellular and voltage-sensitive dye recordings. Small electric fields (</40/ mV mm(-1)), applied parallel to the somato-dendritic axis, induced polarization of CA1 pyramidal cells; the relationship between applied field and induced polarization was linear (0.12 +/- 0.05 mV per mV mm(-1) average sensitivity at the soma). The peak amplitude and time constant (15-70 ms) of membrane polarization varied along the axis of neurons with the maximal polarization observed at the tips of basal and apical dendrites. The polarization was biphasic in the mid-apical dendrites; there was a time-dependent shift in the polarity reversal site. DC fields altered the thresholds of action potentials evoked by orthodromic stimulation, and shifted their initiation site along the apical dendrites. Large electric fields could trigger neuronal firing and epileptiform activity, and induce long-term (>1 s) changes in neuronal excitability. Electric fields perpendicular to the apical-dendritic axis did not induce somatic polarization, but did modulate orthodromic responses, indicating an effect on afferents. These results demonstrate that DC fields can modulate neuronal excitability in a time-dependent manner, with no clear threshold, as a result of interactions between neuronal compartments, the non-linear properties of the cell membrane, and effects on afferents.

Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys

Previous studies showed that dorsal psalterium (PSD) volleys to the entorhinal cortex (ENT) activated in layer II perforant path neurons projecting to the dentate gyrus. The discharge of layer II neurons was followed by the sequential activation of the dentate gyrus (DG), field CA3, field CA1. The aim of the present study was to ascertain whether in this experimental model field, CA2, a largely ignored sector, is activated either directly by perforant path volleys and/or indirectly by recurrent hippocampal projections. Field potentials evoked by single-shock PSD stimulation were recorded in anesthetized guinea pigs from ENT, DG, fields CA2, CA1, and CA3. Current source-density (CSD) analysis was used to localize the input/s to field CA2. The results showed the presence in field CA2 of an early population spike superimposed on a slow wave (early response) and of a late and smaller population spike, superimposed on a slow wave (late response). CSD analysis during the early CA2 response showed a current sink in stratum lacunosum-moleculare, followed by a sink moving from stratum radiatum to stratum pyramidale, suggesting that this response represented the activation and discharge of CA2 pyramidal neurons, mediated by perforant path fibers to this field. CSD analysis during the late response showed a current sink in middle stratum radiatum of CA2 followed by a sink moving from inner stratum radiatum to stratum pyramidale, suggesting that this response was mediated by Schaffer collaterals from field CA3. No early population spike was evoked in CA3. However, an early current sink of small magnitude was evoked in stratum lacunosum-moleculare of CA3, suggesting the presence of synaptic currents mediated by perforant path fibers to this field. The results provide novel information about the perforant path system, by showing that dorsal psalterium volleys to the entorhinal cortex activate perforant path neurons that evoke the parallel discharge of granule cells and CA2 pyramidal neurons and depolarization, but no discharge of CA3 pyramidal neurons. Consequently, field CA2 may mediate the direct transfer of ENT signals to hippocampal and extrahippocampal structures in parallel with the DG-CA3-CA1 system and may provide a security factor in situations in which the latter is disrupted.