The reversal potential of excitatory amino acid action on granule cells of the rat dentate gyrus

Abstract

The responses of granule cells to glutamate, aspartate, N-methyl-D-aspartate (NMDA), quisqualate and kainate applied by ionophoresis on to their dendrites in the middle molecular layer of the dentate gyrus were studied with intracellular electrodes using an in vitro hippocampal slice preparation. On passive depolarization 75% of the granule cells displayed anomalous rectification, which persisted in the presence of TTX and TEA but was eliminated by Co2+ or the intracellular injection of Cs+. Short ionophoretic applications of all the excitatory amino acids evoked dose-dependent depolarizations that were highly localized: movement of the ionophoretic electrode by as little as 10 microns could substantially change the size of the response. The depolarizations evoked by glutamate, asparatate, quisqualate and kainate were unaffected by TTX and Co2+. The depolarization evoked by NMDA was unaffected by TTX but markedly reduced by Co2+. Following intracellular injection of Cs+, neurones could be depolarized to +30 mV and the depolarizations produced by glutamate, quisqualate, NMDA and kainate reversed. The reversal potentials (E) were Eglutamate: -5.6 +/- 0.4 mV; ENMDA: 1.8 +/- 1.9 mV; Equisqualate: -3.9 +/- 1.9 mV; Ekainate: -4.6 +/- 2.0 mV. The excitatory post-synaptic potential (e.p.s.p.) evoked by stimulation of the medial perforant path could also be reversed and Ee.p.s.p. was -5.5 +/- 1.1 mV. The 6 mV difference between ENMDA and the equilibrium potential for the other exogenously applied excitatory amino acids and the statistically significant difference between ENMDA and Ee.p.s.p. (P less than 0.005; d.f.: 7) is consistent with our earlier hypothesis that both the transmitter released by the medial perforant path and exogenously applied glutamate are unlikely to interact with NMDA receptors.