Michal R. Zochowski

Imaging membrane potential with voltage-sensitive dyes

Membrane potential can be measured optically using a variety of molecular probes. These measurements can be useful in studying function at the level of an individual cell, for determining how groups of neurons generate a behavior, and for studying the correlated behavior of populations of neurons. Examples of the three kinds of measurements are presented. The signals obtained from these measurements are generally small. Methodological considerations necessary to optimize the resulting signal-to-noise ratio are discussed.

Distinct Intracellular Calcium Transients in Neurites and Somata Integrate Neuronal Signals

Intracellular calcium signals have distinct temporal and spatial patterns in neurons in which signal initiation and repetitive spiking occurs predominantly in the neurite. We investigated the functional implications of the coexpression of different isoforms of ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (InsP3Rs) using immunocytochemistry, Western blotting, and calcium imaging in neuronally differentiated PC12 cells. InsP3R type III, an isoform that has been shown to be upregulated in neuronal apoptosis, is exclusively expressed in the soma, serving as a gatekeeper for high-magnitude calcium surges. InsP3R type I is expressed throughout the cell and can be related to signal initiation and repetitive spiking in the neurite. RyR types 2 and 3 are distributed throughout the cell. In the soma, they serve as amplifying molecular switches, facilitating recruitment of the InsP3R type III-dependent pool. In the neurite, they decrease the probability of repetitive spiking. Use of a cell-permeant analog of InsP3 suggested that regional specificity in InsP3 production and surface-to-volume effects play minor roles in determining temporal and spatial calcium signaling patterns in neurons. Our findings suggest that additional modulatory processes acting on the intracellular channels are necessary to generate spatially specific calcium signaling.

Synthesis, spectra, delivery and potentiometric responses of new styryl dyes with extended spectral ranges

Styryl dyes have been among the most widely used probes for mapping membrane potential changes in excitable cells. However, their utility has been somewhat limited because their excitation wavelengths have been restricted to the 450-550 nm range. Longer wavelength probes can minimize interference from endogenous chromophores and, because of decreased light scattering, improve recording from deep within tissue. In this paper we report on our efforts to develop new potentiometric styryl dyes that have excitation wavelengths ranging above 700 nm and emission spectra out to 900 nm. We have prepared and characterized dyes based on 47 variants of the styryl chromophores. Voltage-dependent spectral changes have been recorded for these dyes in a model lipid bilayer and from lobster nerves. The voltage sensitivities of the fluorescence of many of these new potentiometric indicators are as good as those of the widely used ANEP series of probes. In addition, because some of the dyes are often poorly water soluble, we have developed cyclodextrin complexes of the dyes to serve as efficient delivery vehicles. These dyes promise to enable new experimental paradigms for in vivo imaging of membrane potential.

Odorant specificity of three oscillations and the DC signal in the turtle olfactory bulb

The odour‐induced population response in the in vivo turtle (Terepene sp.) olfactory bulb consists of three oscillatory components (rostral, middle and caudal) that ride on top of a DC signal. In an initial step to determine the functional role of these four signals, we compared the signals elicited by different odorants. Most experiments compared isoamyl acetate and cineole, odorants which have very different maps of input to olfactory bulb glomeruli in the turtle and a different perceptual quality for humans. We found substantial differences in the response to the two odours in the rise‐time of the DC signal and in the latency of the middle oscillation. The rate of rise for cineole was twice as fast as that for isoamyl acetate. Similarly, the latency for the middle oscillation was about twice as long for isoamyl acetate as it was for cineole. On the other hand, a number of characteristics of the signals were not substantially different for the two odorants. These included the latency of the rostral and caudal oscillation, the frequency and envelope of all three oscillations and their locations and spatial extents. A smaller number of experiments were carried out with hexanone and hexanal; the oscillations elicited by these odorants did not appear to be different from those elicited by isoamyl acetate and cineole. Qualitative differences between the oscillations in the turtle and those in two invertebrate phyla suggest that different odour processing strategies may be used.

Imaging Nervous System Activity with Voltage‐Sensitive Dyes

Optical recording with a voltage‐sensitive dye is advantageous where membrane potential must be recorded in many sites at once. This unit describes methods for making voltage‐sensitive dye measurements on different preparations to study (1) how a neuron integrates its synaptic input into its action potential output by measuring membrane potential everywhere synaptic input occurs and where spikes are initiated; (2) how a nervous system generates a behavior in Aplysia abdominal ganglion; and (3) responses to sensory stimuli and generation of motor output in the vertebrate brain by simultaneous measurement of population signals from many areas. The approach is three‐pronged: (1) find the dye with the largest signal‐to‐noise ratio; (2) reduce extraneous sources of noise; and (3) maximize the number of photons measured to reduce the relative shot noise. A discussion of optical recording methods including the choice of dyes, light sources, optics, cameras, and minimizing noise is also provided.

QUALITATIVE AND QUANTITATIVE ANALYSIS OF MONOCYTE TRANSENDOTHELIAL MIGRATION BY CONFOCAL MICROSCOPY AND THREE-DIMENSIONAL IMAGE RECONSTRUCTION

SummaryA novel method for qualitative and quantitative analysis of monocyte transendothelial migration is described. By labeling monocytes and endothelial cells with different fluorophores, and utilizing confocal microscopy and three-dimensional image reconstruction, transmigrating monocytes were resolved and quantified within a subendothelial collagen gel. Comparison of monocyte migration across endothelial monolayers derived from human brain microvessels versus umbilical veins revealed diapedesis across brain endothelium to be significantly delayed. Inclusion of astrocytes within the subendothelial collagen gel resulted in the formation of an array of astrocytic processes that simulated the glia limitans surrounding brain microvessels in situ, thus yielding a more physiologic paradigm of the blood-brain barrier. By virtue of its unique capacity to provide information on the total number of migrating cells, this analytic approach overcomes significant caveats associated with sampling only aspects of the migration process. The potential adaptability of this method to computer-assisted analysis further enhances its prospective use in high-throughput screening.

Memory recall and spike-frequency adaptation

The brain can reproduce memories from partial data; this ability is critical for memory recall. The process of memory recall has been studied using autoassociative networks such as the Hopfield model. This kind of model reliably converges to stored patterns that contain the memory. However, it is unclear how the behavior is controlled by the brain so that after convergence to one configuration, it can proceed with recognition of another one. In the Hopfield model, this happens only through unrealistic changes of an effective global temperature that destabilizes all stored configurations. Here we show that spike-frequency adaptation (SFA), a common mechanism affecting neuron activation in the brain, can provide state-dependent control of pattern retrieval. We demonstrate this in a Hopfield network modified to include SFA, and also in a model network of biophysical neurons. In both cases, SFA allows for selective stabilization of attractors with different basins of attraction, and also for temporal dynamics of attractor switching that is not possible in standard autoassociative schemes. The dynamics of our models give a plausible account of different sorts of memory retrieval.

4 – Voltage and Calcium Imaging of Brain Activity: Examples from the Turtle and the Mouse

This chapter discusses measurements of population signals with examples from two in vivo preparations: the olfactory bulb of the turtle and of the mouse. It begins with a general discussion of optical recording methods, including the choice of dyes, light sources, optics, cameras, and minimizing noise; and is followed by a more detailed description of voltage-sensitive dye measurements in the turtle and calcium dye measurements in the mouse. Because the light-measuring apparatus is already reasonably optimized, any improvement in the sensitivity of the optical measurements of neuron activity would need to come from the development of better dyes and/or investigating signals from additional optical properties of the dyes. However, because one of the chromophores must be hydrophobic and does not penetrate into brain tissue, it has not been possible to measure signals with a fast pair of dyes in intact tissues. An important new direction is the development of methods for neuron-type-specific staining. Three quite different approaches have been tried. First, the use of retrograde staining procedures has recently been investigated in the embryonic chick and lamprey spinal cords. The second approach is based on the use of cell-type-specific staining. Third approach is to construct a genetically encoded combination of a potassium channel and green fluorescent protein.

Significance of Updating Schemes in Computational Models: Dynamics of Neutral Networks

A Hopfield neural network was constructed with relevance to protein dynamics. The dynamics of this network was analyzed by determining the distribution of first passage times between memories and its dependence on temperature. The distribution depended on the updating scheme. This illustrates the importance of choosing an updating scheme that leads to physically meaningful results in computational models of dynamic processes, such as in neural networks or molecular dynamics.