The Use of In Vivo Fluorescence Image Sequences to Indicate the Occurrence and Propagation of Transient Focal Depolarizations in Cerebral Ischemia

Abstract

A method for the detection and tracking of propagated fluorescence transients as indicators of depolarizations in focal cerebral ischemia is described, together with initial results indicating the potential of the method. The cortex of the right cerebral hemisphere was exposed for nonrecovery experiments in five cats anesthetized with chloralose and subjected to permanent middle cerebral artery (MCA) occlusion. Fluorescence with 370-nm excitation (attributed to the degree of reduction of the NAD/H couple) was imaged with an intensified charge-coupled device camera and digitized. Sequences of images representing changes in gray level from a baseline image were examined, together with the time courses of mean gray levels in specified regions of interest. Spontaneous increases in fluorescence occurred, starting most commonly at the edge of areas of core ischemia; they propagated usually throughout the periinfarct zone and resolved to varying degrees and at varying rates, depending on proximity of the locus to the MCA input. When a fluorescence transient reached the anterior cerebral artery territory, its initial polarity reversed from an increase to a decrease in fluorescence. An initial increase in fluorescence in response to the arrival of a transient may characterize cortex that will become infarcted, if pathophysiological changes in the periinfarct zone are allowed to evolve naturally.

Transient Changes in Cortical Glucose and Lactate Levels Associated with Peri-Infarct Depolarisations, Studied with Rapid-Sampling Microdialysis

“…The fluorescence time series ( Figure 2C) was recorded from an ROI immediately beside the microdialysis probe (Figures 2A, 2B). As in previous experiments (Strong et al, 1996), a sustained increase in fluorescence suggestive of 'terminal' ischaemic depolarisation sometimes occurred on the SG soon after MCAO, but on other occasions this was delayed for approximately 1 h. Such events on the SG often propagated into the MG as increases in fluorescence, but usually resolved there within some 2 mins, thus behaving on the MG as PIDs. During the remainder of the MCAO period, if there were four or more transient fluorescence increases seen in a given experiment, these showed a clear trend to originate on a single gyrus-SG or MG.…”

“…In cases where PID(s) propagated to the region of the microdialysis probe, a small 'region of interest' (ROI) contour was drawn proximal to probe and the time course of fluorescence in this ROI reviewed: the time at which the fluorescence level began to increase sharply was taken as the time of onset of the PID at the probe (Figure 2). In a few instances, a decrease in fluorescence was observed on the medial MG, and was interpreted as indicating propagation of a depolarisation that had elicited a response more characteristic of the hyperaemic response to cortical spreading depression (CSD) than of the PID response (Strong et al, 1996).…”

“…We therefore opted to locate the probe in the MG, which is known to lie within penumbra (Strong et al, 1983). There is evidence for diminished vasoreactivity on this gyrus (Strong et al, 1988), and for an appreciable incidence of PIDs there (Strong et al, 1996).…”

“…Recurrent, spontaneous depolarisations occur and propagate in the penumbra in a number of species including non-human primates (Branston et al, 1977;Strong et al, 2000), cat (Strong et al, 1983;Saito et al, 1995), and the rat (Nedergaard and Astrup, 1986;Gill et al, 1992;Iijima et al, 1992). These events-periinfarct depolarisations (PIDs)-appear to arise principally at the edge of the core area and spread into the peri-infarct zone (Strong et al, 1996). A role for PIDs as a cause rather than simply as a marker of the progression of infarct growth has been indicated by the positive correlation between number of PID events and infarct size (Gill et al, 1992;Chen et al, 1993;Mies et al, 1993) and, critically, by the demonstration that induction of depolarisations within or adjacent to the penumbra can enlarge the infarct (Back et al, 1996;Busch et al, 1996;Takano et al, 1996).…”

“…Individual PIDs were detected by monitoring for transient changes in 450 nm cortical fluorescence, with particular reference to the area of cortex adjacent to the microdialysis probe (Strong et al, 1996). In the accompanying paper , we describe initial results of the application of rapid sampling microdialysis, combined for the first time with electrophysiological monitoring to detect PID-like events, in patients with acute brain injury.…”