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Journal of Neuroscience, Vol 13, 5324-5333, Copyright © 1993 by Society for Neuroscience
Glutamate-activated currents in outside-out patches from spiny versus aspiny hilar neurons of rat hippocampal slices
CT Livsey, E Costa and S Vicini
FIDIA-Georgetown Institute for the Neurosciences, Georgetown University School of Medicine, Washington, D.C. 20007.
The desensitization rate of non-NMDA glutamate receptors was investigated
in outside-out membrane patches obtained from morphologically identified
spiny "mossy cells" (SMCs) and aspiny hilar interneurons (AHIs) in young
rat hippocampal slices. The fast application of a 1 mM step of L-glutamate
for 50-100 msec in the presence of TTX and dizolcipine (MK-801) onto
patches excised from these neurons produced large glutamate-activated
currents (GACs) that decayed with a single or double exponential time
course despite the continued presence of agonist. These desensitization
rates of alpha- amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
(AMPA)/kainate- sensitive receptors differed markedly between patches
obtained from the two cell types. The fast time constant of desensitization
in AHIs (n = 34) averaged 3.3 +/- 0.93 msec (mean +/- SD), while that of
SMCs (n = 57) averaged 6.8 +/- 2.0 msec. Current-voltage relationships of
the GACs did not differ between SMCs and AHIs, with comparable reversal
potentials and no evidence of inward rectification. We also failed to
observe significant Ca2+ permeability in either cell type. However, brief
(< 1 msec) pulses of 1 mM glutamate produced rapidly decaying GACs with
distinct kinetics in the two neuronal classes. Furthermore, analysis of the
single glutamate-activated channel currents in outside- out patches from
hilar neurons revealed a larger predominant single- channel current in AHIs
versus SMCs. Lastly, we observed a greater sensitivity to cyclothiazide in
SMCs versus AHIs, with half-maximal removal of desensitization being 90 mM
and 200 mM, respectively. Taken together, these differences in GACs between
SMCs and AHIs might indicate a functional correlate to the substantial
heterogeneity in the molecular structure of glutamate receptor subunits or
might be related to posttranslational modifications of these subunits,
perhaps provided by the unique microenvironment in the spines covering
SMCs.
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