Miniature calomel electrode for recording DC potential changes accompanying spreading depression in the freely moving rat

Microfabricated reference electrodes and their biosensing applications

Over the past two decades, there has been an increasing trend towards miniaturization of both biological and chemical sensors and their integration with miniaturized sample pre-processing and analysis systems. These miniaturized lab-on-chip devices have several functional advantages including low cost, their ability to analyze smaller samples, faster analysis time, suitability for automation, and increased reliability and repeatability. Electrical based sensing methods that transduce biological or chemical signals into the electrical domain are a dominant part of the lab-on-chip devices. A vital part of any electrochemical sensing system is the reference electrode, which is a probe that is capable of measuring the potential on the solution side of an electrochemical interface. Research on miniaturization of this crucial component and analysis of the parameters that affect its performance, stability and lifetime, is sparse. In this paper, we present the basic electrochemistry and thermodynamics of these reference electrodes and illustrate the uses of reference electrodes in electrochemical and biological measurements. Different electrochemical systems that are used as reference electrodes will be presented, and an overview of some contemporary advances in electrode miniaturization and their performance will be provided.

Peri-infarct depolarizations reveal penumbra-like conditions in striatum

Spreading depression-like peri-infarct depolarizations not only characterize but also worsen penumbra conditions in cortical border zones of experimental focal ischemia. We intended to investigate the relevance of ischemic depolarization in subcortical regions of ischemic territories. Calomel electrodes measured DC potentials simultaneously in the lateral and medial portions of the caudate nucleus (CN) of 11 anesthetized cats after permanent occlusion of the middle cerebral artery. Additionally, platinum electrodes measured cerebral blood flow (CBF) in the CN, and laser Doppler probes CBF in the cortex. Depolarizations (negative DC shifts >10 mV) were obtained in 10 of 11 cats. Further differentiation revealed that short-lasting spreading depression-like depolarizations (SDs; 5 of 10 cats: 5.24 +/- 1.22 min total duration; 23.3 +/- 4.2 mV amplitude) were predominantly found in medial and longer depolarizations (LDs; 4 of 10 cats: 64.7 +/- 47.5 min; 25.0 +/- 11.3 mV) in the lateral CN. Terminal depolarizations (TDs; 6 of 10 cats; without repolarization) occurred immediately after occlusion or at later stages, being then accompanied by elevations of intracranial pressure presumably inducing secondary CBF reduction. CBF tended to be lower in regions with TDs (33.3 +/- 29.9% of control) and LDs (37.3 +/- 22.8%) than in regions with SDs (51.5 +/- 48.0%). We conclude that in focal ischemia, transient peri-infarct depolarizations emerge not only in cortical but also in striatal gray matter, thereby demonstrating the existence of subcortical zones of ischemic penumbra. The generation of these ischemic depolarizations is a multifocal process possibly linked to brain swelling and intracranial pressure rise in the later course of focal ischemia, and therefore a relevant correlate of progressively worsening conditions.

Calcium ion transients in peri-infarct depolarizations may deteriorate ion homeostasis and expand infarction in focal cerebral ischemia in cats

BACKGROUND AND PURPOSE

Harmful effects of peri-infarct depolarizations (PIDs) may depend on recurrent Ca(2+) influx. Thus far, few studies have documented the relevance of PIDs in gyrencephalic animals, and the progressive nature of this process has not been investigated over extended periods. We therefore studied in prolonged focal ischemia in cats spatial and temporal profiles of extracellular calcium ([Ca(2+)](o)) shifts in relation to direct current (DC) potential, nitric oxide (NO) concentration and regional cerebral blood flow alterations, and final pathological outcome.

METHODS

In halothane-anesthetized cats receiving either vehicle (n=12) or MK-801 treatment (5 mg/kg IV; n=10), the left middle cerebral artery was permanently occluded. Laser-Doppler probes, ion-selective microelectrodes, and NO electrodes measured simultaneously regional cerebral blood flow, DC potential, electrocorticogram, [Ca(2+)](o), and NO concentrations in ectosylvian and suprasylvian gyri of the left cerebral cortex.

RESULTS

Persistent depolarization immediately after middle cerebral artery occlusion occurred in 10 ectosylvian and 4 suprasylvian gyri of vehicle-treated animals and in 9 ectosylvian and 3 suprasylvian gyri of MK-801-treated animals. PIDs associated with transient decreases of [Ca(2+)](o) were detected in suprasylvian gyri of only 4 vehicle-treated animals, of which 3 developed recurrent PIDs. Electrocorticogram was suppressed during PIDs, and electrocorticogram recovery worsened in a stepwise manner with consecutive depolarizations. PID duration increased slightly with ongoing ischemia and evolved to persistent depolarization at a final stage. NO transients were not detected during PID, and regional cerebral blood flow transients were not pronounced. Infarction was larger with initial persistent depolarization than with PID and was smallest in MK-801-treated animals.

CONCLUSIONS

PID is not a common finding in peri-infarct zones in cats, and it is suppressed by the N:-methyl-D-aspartate antagonist MK-801. However, if repeated PIDs are generated, they result in a stepwise, progressive breakdown of neuronal function and ion homeostasis, probably contributing to the growth of infarction in focal cerebral ischemia. Recurrent Ca(2+) influx is a mechanism that presumably contributes to this process.

Profiles of cortical tissue depolarization in cat focal cerebral ischemia in relation to calcium ion homeostasis and nitric oxide production

Cortical depolarization was investigated in a topographic gradient of ischemic density after 1-hour transient middle cerebral artery occlusion in halothane-anesthetized cats. A laser Doppler flow probe, an ion-selective microelectrode, and a nitric oxide (NO) electrode measured regional CBF (rCBF), direct current (DC) potential, extracellular Ca2+ concentration ([Ca2+]o), and NO concentration in ectosylvian and suprasylvian gyri of nine animals. Recordings revealed 12 of 18 sites with persistent negative shifts of the DC potential, severe rCBF reduction, and a drop of [Ca2+]o characteristic for core regions of focal ischemia. Among these sites, two types were distinguished by further analysis. In Type 1 (n = 5), rapid, negative DC shifts resembled anoxic depolarization as described for complete global ischemia. In this type, ischemia was most severe (8.9 +/- 2.5% of control rCBF), [Ca2+]o dropped fast and deepest (0.48 +/- 0.20 mmol/L), and NO concentration increased transiently (36.1 +/- 24.0 nmol/L at 2.5 minutes), and decreased thereafter. In Type 2 (n = 7), the DC potential fell gradually over the first half of the ischemic episode, rCBF and [Ca2+]o reductions were smaller than in Type 1 (16.2 +/- 8.2%; 0.77 +/- 0.41 mmol/L), and NO increased continuously during ischemia (53.1 +/- 60.4 nmol/L at 60 minutes) suggesting that in this type NO most likely exerts its diverse actions on ischemia-threatened tissue. In the remaining six recording sites, a third type (Type 3) attributable to the ischemic periphery was characterized by minimal DC shifts, mild ischemia (37.2 +/- 13.3%), nonsignificant alterations of [Ca2+]o, but decreased NO concentrations during middle cerebral artery occlusion. Reperfusion returned the various parameters to baseline levels within 1 hour, the recovery of [Ca2+]o and NO concentration being delayed in Type 1. An NO synthase inhibitor (N(G)-nitro-L-arginine, 50 mg/kg intravenously; four animals) abolished NO elevation during ischemia. In conclusion, even in the core of focal cerebral ischemia and reperfusion, different ischemic densities produce different types of cortical tissue manifesting distinctive chronological profiles of depolarization, Ca2+ influx, and NO synthesis.

Reduction of infarct volume by halothane: effect on cerebral blood flow or perifocal spreading depression-like depolarizations

Halothane is a strong inhibitor of potassium evoked spreading depression (SD) in cats. In the current study, we investigate halothane effects on induction of perifocal SD-like depolarizations, CBF, and infarct evolution in focal ischemia. Calomel and platinum electrodes measured cortical direct current potential and CBF in ectosylvian, suprasylvian, and marginal gyri. Left middle cerebral artery occlusion (MCAO) induced permanent focal ischemia for 16 hours in artificially ventilated cats (30% oxygen, 70% nitrous oxide) under halothane (0.75%, n = 8) or alpha-chloralose anesthesia (60 mg/kg intravenously, n = 7). Under alpha-chloralose, MCAO induced severe ischemia in ectosylvian and suprasylvian gyri(mean CBF < 10 mL/100 g/min), and direct current potentials turned immediately into terminal depolarization. In marginal gyri, CBF reduction was mild (more than 20 mL/100 g/min), and in six of seven animals, frequent SD-like depolarizations turned into terminal depolarization at a later stage of the experiments. Under halothane, MCAO induced severe ischemia (less than 10 mL/100 g/min) and immediate terminal depolarization only in ectosylvian gyrus. In suprasylvian gyrus, residual CBF remained significantly higher (more than 10 mL/100 g/min) than under alpha-chloralose, whereas in marginal gyri, CBF did not differ between groups. Compared with chloralose, the number of transient depolarizations was significantly reduced in marginal gyrus, and in suprasylvian gyrus transient but significantly longer depolarizations than in marginal gyrus were recorded. Except for one animal, transient depolarizations did not turn into terminal depolarization under halothane, and infarct volume reduction was particularly seen in suprasylvian gyrus. We conclude that halothane, the most commonly used anesthetic in studies of experimental brain ischemia, has protective properties, which may depend on both cerebrovascular and electrophysiologic influences.

Halothane, but not alpha-chloralose, blocks potassium-evoked cortical spreading depression in cats

The effects of two anesthetics, halothane and alpha-chloralose, on induction of spreading depression and on extracellular glutamate elevation after intracortical potassium administration were investigated in artificially ventilated (30% oxygen/70% nitrous oxide) cats. High potassium concentrations were achieved using either direct KCl injections (7 microliters, 150 mM via a micropipette) or microdialysis by supplementing 100, 300 or 500 mM KCl, respectively, for 10 min to the perfusion solution (Ringer's). Changes of the cortical DC potential were recorded adjacent (1-2 mm: electrode DC1) and distant (6-7 mm: electrode DC2) to the injection site. Either under halothane (0.75% in the respiratory gas mixture) or under alpha-chloralose (60 mg/kg i.v.) anesthesia, prolonged negative shifts of the DC potential reflecting the elevated potassium levels after KCl injection were measured near the injection site (electrode DC1). In contrast, spreading depressions (transient short DC deflections) were almost exclusively observed under alpha-chloralose. Spreading depressions recorded with electrode DC1 were superimposed on the prolonged negative DC shifts and they propagated frequently to the more distant site (DC2). Upon KCl administration, dose dependent elevations of extracellular glutamate were measured. These elevations were not significantly altered by the type of anesthesia. Our results suggest that in cats, spreading depression induction is affected by anesthesia, i.e., spreading depression induction is inhibited by halothane as compared to alpha-chloralose. Furthermore, factors other than glutamate or high potassium seem to contribute to spreading depression induction.

Effects of single cortical spreading depression on metabolic heat production in the rat

The effect of a single cortical spreading depression (CSD), elicited unilaterally by 10% KCI injection at the occipital cortex, was observed on the metabolic heat production in the rat with unilateral lesions in the preoptic and anterior hypothalamus (PO/AH). The metabolic rate increased by maximally 20% above its pre-CSD level for 7 min with a rise in rectal temperature when CSD entered the frontal cortex contralateral to the PO/AH lesion. By contrast, with the same onset, the increase in metabolic rate was lower (maximum 12%) and of shorter duration (3 min) in the ipsilateral CSD trial. Non-CSD trial with 0.9% NaCl injection had no effect on metabolic rate. The results provide further evidence to support the view that the frontal cortex of the rat is involved in the central thermoregulation.

Impairment of thermoregulatory cooling behavior by single cortical spreading depression in the rat

Effects of single waves of unilateral cortical spreading depression (CSD) on skin-cooling operant behavior were studied in the rat with and without unilateral lesion in the preoptic and anterior hypothalamus (PO/AH). The skin-cooling behavior was severely suppressed for 6 min, with a resulting rise in rectal temperature (Tre), when CSD entered the frontal cortex contralateral to the PO/AH lesion. However, CSD ipsilateral to the PO/AH lesion slightly suppressed the skin-cooling behavior for 4 min with small rise in Tre. In the PO/AH intact rat, while 0.9% NaCl did not affect the skin-cooling behavior, unilateral CSD slightly suppressed it for 3 min almost in the same way as that of the ipsilateral CSD trial in the PO/AH lesioned rat. The results add further evidence to support the involvement of the frontal cortex in the central control of thermoregulation.

Unilateral cortical spreading depression in the rat: effects on feeding, drinking and other behaviors

The effects of unilateral cortical spreading depression (CSD) on feeding, drinking and other behaviors were studied in rats. CSD waves were elicited by a microinjection of KC1 solution through an implanted cannula. The animal's behavior and slow potential changes (SPC) accompanying CSD were then simultaneously monitored for 20 min. Intensive face grooming was observed while the CSD wave-front propagated across the sensorimotor cortex. Animals commenced their first feeding or drinking shortly after the invasion of the negative SPC into the frontal cortex of Krieg's area 6 and 10 [14] had reached its maximum height. A two-way switching effect between the first feeding or drinking and quiescence (freezing) and a close relationship between their onsets were observed. The results are discussed specifically in relation to cortico-hypothalamic pathways.

Circling and consumatory behavior induced by striatal and neocortical spreading depression

Single waves of spreading depression (SD) were induced in freely moving Sprague Dawley rats by microinjection of KCl solution into either neocortex or caudate nucleus. DC recordings of slow potential changes (SPC) revealed that a wave of SD propagates from the neocortex to the caudate nucleus in only a very few cases (12.5% of trials, involving 4.3% of the SPC waves). This result contrasts with earlier reported higher incidences of neocortical-striatal transmission of SD, and suggests that such transmission may vary across rat strains. Invasion of the caudate nucleus by a wave of neocortical SD was accompanied by contralateral circling in 50% of the cases. Similarly, SD induced by injection of KCl into the striatum was accompanied by circling in 43% of trials. It was also found that neocortical SD-induced feeding generally occurred only upon recovery of neocortex from the slow potential negativity, and that invasion of the caudate nucleus by SD is not involved in the elicited feeding phenomenon.