Concurrent reflectance imaging and microdialysis in the freely behaving cat

Rats were implanted with 0.3-mm-diameter dialysis tubing through the hippocampus and subsequently perfused with Ringer's solution at a flow rate of 2 microliter/min. Samples of the perfusate representing the extracellular fluid were collected over 5-min periods and subsequently analyzed for contents of the amino acids glutamate, aspartate, glutamine, taurine, alanine, and serine. Samples were collected before, during, and after a 10-min period of transient complete cerebral ischemia. The extracellular contents of glutamate and aspartate were increased, respectively, eight- and threefold during the ischemic period; the taurine concentration also was increased 2.6-fold. During the same period the extracellular content of glutamine was significantly decreased (to 68% of the control value), whereas the concentrations of alanine and serine did not change significantly during the ischemic period. The concentrations of gamma-aminobutyric acid (GABA) were too low to be measured reliably. It is suggested that the large increase in the content of extracellular glutamate and aspartate in the hippocampus induced by the ischemia may be one of the causal factors in the damage to certain neurons observed after ischemia.

An increase in extracellular K+ concentration ([K+]c) of the rat hippocampus following fluid-percussion concussive brain injury was demonstrated with microdialysis. The role of neuronal discharge was examined with in situ administration of 0.1 mM tetrodotoxin, a potent depressant of neuronal discharges, and of 0.5 to 20 mM cobalt, a blocker of Ca++ channels. While a small short-lasting [K+]c increase (1.40- to 2.15-fold) was observed after a mild insult, a more pronounced longer-lasting increase (4.28- to 5.90-fold) was induced without overt morphological damage as the severity of injury rose above a certain threshold (unconscious for 200 to 250 seconds). The small short-lasting increase was reduced with prior administration of tetrodotoxin but not with cobalt, indicating that neuronal discharges are the source of this increase. In contrast, the larger longer-lasting increase was resistant to tetrodotoxin and partially dependent on Ca++, suggesting that neurotransmitter release is involved. In order to test the hypothesis that the release of the excitatory amino acid neurotransmitter glutamate mediates this increase in [K+]c, the extracellular concentration of glutamate ([Glu]c) was measured along with [K+]c. The results indicate that a relatively specific increase in [Glu]c (as compared with other amino acids) was induced concomitantly with the increase in [K+]c. Furthermore, the in situ administration of 1 to 25 mM kynurenic acid, an excitatory amino acid antagonist, effectively attenuated the increase in [K+]c. A dose-response curve suggested that a maximum effect of kynurenic acid is obtained at a concentration that substantially blocks all receptor subtypes of excitatory amino acids. These data suggest that concussive brain injury causes a massive K+ flux which is likely to be related to an indiscriminate release of excitatory amino acids occurring immediately after brain injury.

The factors responsible for the unusual susceptibility of the hippocampus to seizures and ischemic cell damage are not well understood. The CA1 pyramidal subfield of the hippocampus is particularly vulnerable to seizure activity and damage after ischemia. The possibility was examined that regional differences exist in extracellular volume, which might influence neuronal excitability and response to injury in the hippocampus. CA1 stratum pyramidale exhibited an exceptionally low extracellular volume fraction (EVF) of 0.12, whereas the EVFs of CA3 and dentate were considerably higher--0.18 and 0.15, respectively. The EVF of CA1 stratum pyramidale was reversibly reduced by 30 percent when the extracellular potassium concentration was raised from 3.5 to 8.5 mM, a procedure that induced spontaneous electrographic seizures in CA1. Thus there are regional variations in the properties of the extracellular space in the hippocampus that might underlie the propensity of the CA1 region to develop seizures and to suffer damage after ischemia.