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We examined the hypotheses that aging alters the capacity of brain tissue to regulate extracellular K+ activity (K+o), and that age-related decreases in glucose metabolism may underlie these alterations. Hippocampal slices from young adult (6-9 months old), middle-aged (16-19 months old), and aged (26-29 months old) Fischer 344 rats were exposed to physiological solutions containing 5-20 mM glucose maintained at 36-37 degrees C. Schaffer collaterals in each slice were stimulated at 40 Hz for 2 s, and the resulting changes in K+o were recorded with K+o-sensitive microelectrodes placed in stratum pyramidale of hippocampal subfield CA1. We found that K+ clearance from the extracellular space of hippocampal slices was significantly slowed in the middle-aged group compared with the young adult group in physiological solutions containing 5 and 10 mM glucose. Age-related differences in K+o clearance disappeared in 20 mM glucose. Also, the rate of K+o clearance was modified by glucose concentration. These results suggest that K+ transport rates are modified by age, and that age-related alterations in glucose metabolism may be involved.

The present study was undertaken to determine whether glycolytic energy production was critical to the survival of brain tissue subjected to metabolic stress. Specifically, the contributions of glycolysis (1) to recovery of ion homeostasis after anoxia or high frequency electrical stimulation, and (2) to recovery of synaptic transmission after anoxia, were examined. Energy metabolism in rat hippocampal slices was manipulated by varying glucose concentrations, and by substituting lactate for glucose. Ion transport was slower and recovery of synaptic transmission after anoxia was greatly impeded in the absence of glycolysis. These results support the hypothesis that glycolytic ATP production is tied directly or indirectly to ion transport. The results also suggest that recovery of synaptic transmission following anoxia requires glycolytic ATP.

Hippocampal slices exposed to brief anoxia combined with elevated glucose exhibit greater postanoxic recovery of synaptic transmission. Glucose may have improved recovery of synaptic transmission by enhancing the production of metabolic energy during and after anoxia. This enhancement should provide more ATP for energy-requiring ion transport processes, and lead (1) to a delayed onset of complete depolarization of CA1 pyramidal cells during anoxia (anoxic depolarization) and (2) to greater ion transport activity following anoxia. A delay in anoxic depolarization would protect neurons from damage if the duration of anoxic depolarization was shortened. Greater postanoxic ion transport would allow the re-establishment of ion gradients supportive of neuronal and synaptic excitability. The effects of glucose and anoxia on ion homeostasis and synaptic transmission were examined in rat hippocampal slices exposed to different glucose concentrations (5-20 mM). The duration of anoxic depolarization was held constant so that postanoxic damage related to this duration was controlled. We found that K+ transport and recovery of synaptic transmission after anoxia in hippocampal slices improved as glucose concentration increased. Also, anoxic depolarization was delayed as glucose concentration increased. Thus, added glucose may improve postanoxic recovery of synaptic transmission by better supporting ion transport.

Age-related changes in the capacity of the brain to survive short anoxic episodes were studied in stratum pyramidale (region CA1) of hippocampal slices from control (6-7 months) and aged (26-27 months) rats. Our primary interest was in how aging affected the ability of slices to maintain or to recover extracellular potassium ion (K+o) homeostasis and orthodromically-stimulated field potentials during and after anoxia. During anoxia, K+o homeostasis was lost faster in slices from aged rats. Following anoxia, K+o homeostasis recovered more slowly, and synaptic transmission recovered less completely, in aged slices. These studies provide what is believed to be the first demonstration that aging diminishes the capacity of brain tissue to maintain K+o during anoxia and to recover K+o homeostasis and synaptic transmission following anoxia, and support suggestions that the aged brain is more vulnerable to anoxia.

In rat hippocampal slices exposed to short duration anoxia, recovery of synaptic transmission was dependent upon the magnitude of the K+o undershoot (K+o drop below resting level) seen following return to normoxic conditions. Because the K+o undershoot is likely to be related to ion pump activity, our results imply that recovery of brain from stroke depends upon how well ion transport and energy production recover.

Previous reports showed that cytochrome alpha,alpha 3 responded to heightened brain activity with shifts toward oxidation in adult rats but toward reduction in aged animals. To determine whether this change indicates an age-associated limitation in mitochondrial respiratory capacity, the present study compared oxygen consumption in hippocampal slices of young adult (6 month, control) and aged (26 month) rats. Slices were used to insure that results were independent of cerebrovascular factors. Age was without effect on oxygen consumption under 'resting' conditions (i.e. with slices bathed at 3.5 mM K+), but oxygen consumption was not increased as much in hippocampal slices from aged rats under heightened energy demands (produced by raising the extracellular potassium ion activity [( K+]o to 50 mM). This lesser oxygen consumption response to enhanced metabolic demand suggests that there are age-associated limits to the brain's ability to increase its metabolic rate.

We examined the possibility that Ca2+-sensitive inhibition of synaptic transmission following anoxia involves compromise of ion transport activity. Rat hippocampal slices were superfused with artificial cerebrospinal fluids containing different concentrations of CaCl2, and subjected to short anoxia. Durations of anoxia were sufficient to provoke anoxic depolarization, indicated by a sudden rise in extracellular K+ (K+o). Following anoxia, apparent K+ transport was assessed by measuring the magnitude of subnormal K+o (the K+o undershoot) in hippocampal region CA1. Recovery of synaptic transmission 1 h after anoxia was determined by evaluation of the magnitudes of the orthodromically stimulated population spike recorded from CA1 pyramidal cells. K+o undershoots and recovery of synaptic transmission decreased as CaCl2 or the duration of anoxic depolarization increased. These data suggest: (1) that increased artificial cerebrospinal fluid CaCl2 compromised K+ reaccumulation after anoxia; and (2) that ion transport dysfunction may inhibit recovery of synaptic transmission.

The relationship between extracellular potassium ion activity and neuronal excitability during anoxia was investigated in hippocampal slices in vitro. Extracellular field potentials and K+ activity were measured with double-barreled ion-selective microelectrodes placed either in the stratum pyramidale or stratum radiatum of field CA1. Orthodromic spike activity of CA1 pyramidal cells and field excitatory postsynaptic potentials (f-EPSPs) failed rapidly after anoxia with little change in potassium ion activity and without failure of the Schaffer collateral prevolley or antidromic responses of pyramidal cells. As [K+]o approached 8-10 mM, f-EPSPs and orthodromic spike activity recovered spontaneously. Continued anoxia resulted in massive release of K+ into the extracellular space and complete electrical silence. Presynaptic activity and antidromically elicited spike activity recovered promptly upon reoxygenation after anoxia, but synaptic transmission remained blocked for many minutes. Spontaneous recovery of f-EPSPs and spike activity suggests that a simple mechanism involving depolarization or hyperpolarization of neuronal elements cannot account for failure of synaptic transmission observed during anoxia. However, continued elevation of [K+]o and the associated loss of pre- and postsynaptic excitability with more prolonged anoxia indicated that depolarization was responsible for the eventual electrical silence as anoxia progressed.