Glutamate-activated currents in outside-out patches from spiny versus aspiny hilar neurons of rat hippocampal slices

Abstract

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.