The real-time iontophoretic method using tetramethylammonium-selective microelectrodes and diffusion-weighted magnetic resonance imaging were used to measure the extracellular space volume fraction alpha, tortuosity lambda and apparent diffusion coefficient of water (ADC(W)) 240 min after the administration of pilocarpine in urethane-anaesthetized rats. The obtained data were correlated with extracellular lactate, glucose, and glutamate concentrations and the lactate/pyruvate-ratio, determined by intracerebral microdialysis. The control values of alpha and lambda were 0.19+/-0.004 and 1.58+/-0.01, respectively. Following pilocarpine application, alpha decreased to 0.134+/-0.012 100 min later. Thereafter alpha increased, reaching 0.176+/-0.009 140 min later. No significant changes in lambda were observed during the entire time course of the experiment. ADC(W) was significantly decreased 100 min after pilocarpine application (549+/-8 microm(2) s(-1)) compared to controls (603+/-11 microm(2) s(-1)); by the end of the experiments, ADC(W) had returned to control values. The basal cortical levels of lactate, the lactate/pyruvate ratio, glucose and glutamate were 0.61+/-0.05 mmol/l, 33.16+/-4.26, 2.42+/-0.13 mmol/l and 6.55+/-1.31 micromol/l. Pilocarpine application led to a rise in lactate, the lactate/pyruvate ratio and glutamate levels, reaching 2.92+/-0.60 mmol/l, 84.80+/-11.72 and 22.39+/-5.85 micromol/l within about 100 min, with a subsequent decrease to control values 140 min later. The time course of changes in glucose levels was different, with maximal levels of 3.49+/-0.24 mmol/l reached 40 min after pilocarpine injection and a subsequent decrease to 1.25+/-0.40 mmol/l observed 200 min later. Pathologically increased neuronal activity induced by pilocarpine causes cell swelling followed by a reduction in the ECS volume fraction, which can contribute to the accumulation of toxic metabolites and lead to the start of epileptic discharges.

Institute of Experimental Medicine Academy of Sciences of the Czech Republic Prague Czech Republic

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Diffusion in brain extracellular space