Kinetically distinguishable AMPA receptors in rat hippocampus are associated with the loss of glutamate-sensitive conformational transitions

• Published in: Neurochemistry international 2000, Vol.36 (1), p.83-90
• Main Authors: Szárics, Éva, Nyitrai, Gabriella, Kovács, Ilona, Kardos, Julianna
• Format: Article
• Language: English
• Subjects: Animals; Biological and medical sciences; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; Dizocilpine; Dizocilpine Maleate - pharmacology; Fractal kinetics; Fundamental and applied biological sciences. Psychology; Glutamic Acid - pharmacology; Hippocampal AMPA-kainate receptors; Hippocampus - metabolism; Kinetics; Male; Ouabain; Ouabain - pharmacology; Protein Conformation - drug effects; Rats; Rats, Wistar; Receptors, AMPA - chemistry; Receptors, AMPA - metabolism; Receptors, Kainic Acid - chemistry; Receptors, Kainic Acid - metabolism; Sodium - metabolism; sodium transport; Spectrometry, Fluorescence; Stopped-flow method; Synaptosomes - metabolism; Transmembrane NA + ion flux; Vertebrates: nervous system and sense organs
• ISSN: 0197-0186; 1872-9754
• DOI: 10.1016/S0197-0186(99)00089-3

We describe a stopped-flow method to study α-amino-7-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-kainate receptor-mediated Na + ion flux through native membranes. Resealed plasmalemma vesicles and nerve endings from the rat hippocampus were mixed rapidly with a membrane impermeant form of the fluorescence indicator, sodium binding benzofurane oxazole and the changes in fluorescence intensity in response to various [Glu] on the time scale of 0.04 ms–10 s were monitored at a sampling rate of 6.55 kHz. Inhibitors like ouabain (1 mM) and 5-methyl-10,11-dihydro-5 H-dibenzo[a,d]cyclohepten-5,10-imine maleate (dizocilpine, 50 μM) enhanced Na + ion translocation under low-[Na +] and physiological conditions, respectively. Dependence of AMPA-kainate receptor kinetics on [Glu] was described in a model of channel activation by faster and slower desensitizing receptors. The model accounted for almost all of the Na + ion flux activity in the 30 μM–10 mM range of [Glu]. We found that the values of the initial rate constant for Na + ion influx, J A, and rate constant for desensitization, α, for the faster desensitizing receptor were dependent on data sampling rate, whereas the initial rate constant for Na + ion flux through the slower desensitizing receptor, J B, varied much less with the sampling rate. These phenomena can be described by (1) a fractal model of short-lived AMPA-kainate receptor channel with many closely spaced states (fractal dimension ∼1.8) and (2) a model of long-lived AMPA-kainate receptor channel with two discrete states.