A quantitative study of the actions of excitatory amino acids and antagonists in rat hippocampal slices

Abstract

1 A quantitative pharmacological investigation of the actions of excitatory amino acids on hippocampal CA1 neurones has been made using a new slice preparation developed for grease gap recording; d.c. potential was measured across a grease barrier placed between alvear fibres and the bathing medium. 2 In Mg²⁺-free perfusate, N-methyl-D-aspartate (NMDA, 1–100 μM), quisqualate (1–500 μM), kainate (1–200 μM) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1–100 μM) caused dose-dependent depolarizations. 3 The dose-response relationships were fitted to logistic expressions. The maximum responses to AMPA, NMDA and kainate were similar; their respective EC₅₀ values were 5, 13 and 23 μM. Quisqualate had a smaller maximum; its EC₅₀ value was 10 μM. The slopes of the dose-response relationships were different for the 4 agonists; the order of steepness of the slopes was NMDA > AMPA > kainate > quisqualate. 4 Similar amino acid-induced depolarizations were observed in slices of just the CA1 region or in whole slices bathed in tetrodotoxin. Isolated alvear fibres, however, were insensitive to the excitatory amino acids. 5 D-2-Amino-5-phosphonovalerate (APV, 50 μM) selectively and reversibly antagonized responses induced by NMDA (apparent pA₂ = 5.21). 6 Kynurenic acid (1 mM) reversibly depressed responses to the three agonists tested. The doseratios for antagonism of AMPA, kainate and quisqualate were 6.9, 5.6 and 4.6 respectively. 7 This preparation has a different sensitivity profile to agonists from those of previously reported preparations of spinal cord, neocortex and cerebellum. The greater sensitivity to NMDA may be due to the higher density of NMDA receptors in the hippocampus. The effects of the antagonists, APV and kynurenate, are similar to those found in other brain areas.