Activity-Dependent Layer-Specific Changes in the Extracellular Chloride Concentration and Chloride Driving Force in the Rat Hippocampus
ABSTRACT The transmembrane distribution of chloride anions (Cl⁻) determines the direction of the Cl⁻ flux through GABA(A) receptors; this establishes whether GABA(A) receptor-mediated responses are hyperpolarizing or depolarizing in neurons. Thus an activity-dependent reduction in the efficacy of inhibitory responses can be the result of an activity-induced reduction of the Cl⁻ driving force. Using Cl(-)-sensitive electrodes, we measured the extracellular Cl⁻ concentration ([Cl⁻](o)) in each layer of the hippocampus under control conditions and after stimulation. In the control condition, [Cl⁻](o) was lower within the CA1 region (112.9 ± 1.3 mM; mean ± SD) than the CA3/dentate gyrus areas (117.7 ± 1.2 mM). Stimulation of CA3 pyramidal cells led to an increase in the [Cl⁻](o). The maximum values were observed in the stratum lacunosum-moleculare (253.4 ± 51.1 mM) and in the hilus (261 ± 43.7 mM), whereas in the granular cell layer, it reached only 159.5 ± 41 mM. The stimulation-induced [Cl⁻](o) increase was followed by a period of decreasing [Cl⁻](o) that fell below the control values. The maximum undershoot (21.6 ± 0.7 mM) was observed in the s. radiatum. Systemic application of the gap junction blocker carbenoxolone significantly decreased the stimulation-induced Cl⁻ extrusion in the dentate gyrus but only slightly modified it in the CA1 area. Carbenoxolone also drastically reduced the Cl⁻ clearance. The time constant of the Cl⁻ clearance was similar between layers (83.4 ± 15.9 ms) but increased after carbenoxolone application (207.1 ± 44.4 ms). Stimulation-induced changes in the [Cl⁻](o) significantly decreased the Cl⁻ driving force and resulted in large fluctuations between layers (Δ = 9.4 mV). The lowest value was observed in the stratum radiatum of the CA1 and the hilar region (7.7 mV), whereas the highest value was calculated for the granule cell layer (16.3 mV). We suggest that a decrease of the extracellular space is mainly responsible for the rapid [Cl⁻](o) increase while the gap junction coupled astrocytic network plays a key role in the activity-dependent redistribution and clearance of Cl⁻ across layers of the hippocampus.
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ABSTRACT: It has been more than 100 years since Paul Ehrlich reported that various water-soluble dyes injected into the circulation did not enter the brain. Since Ehrlich's first experiments, only a small number of molecules, such as alcohol and caffeine have been found to cross the blood-brain barrier, and this selective permeability remains the major roadblock to treatment of many central nervous system diseases. At the same time, many central nervous system diseases are associated with disruption of the blood-brain barrier that can lead to changes in permeability, modulation of immune cell transport, and trafficking of pathogens into the brain. Therefore, advances in our understanding of the structure and function of the blood-brain barrier are key to developing effective treatments for a wide range of central nervous system diseases. Over the past 10 years it has become recognized that the blood-brain barrier is a complex, dynamic system that involves biomechanical and biochemical signaling between the vascular system and the brain. Here we reconstruct the structure, function, and transport properties of the blood-brain barrier from an engineering perspective. New insight into the physics of the blood-brain barrier could ultimately lead to clinical advances in the treatment of central nervous system diseases.Frontiers in Neuroengineering 01/2013; 6:7. DOI:10.3389/fneng.2013.00007
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