EFFECT OF ACIDIC AMINO-ACID ANTAGONISTS ON SYNAPTIC TRANSMISSION IN THE HIPPOCAMPAL-FORMATION INVITRO

The effects on synaptic efficacy of the putative acidic amino acid antagonists, 2-amino-4-phosphonobutyric acid (APB), 2-amino-3-phosphonopropionic acid (APP), 1-hydroxy-3-amino-pyrrolidone-2 (HA-966) and glutamic acid diethyl ester (GDEE), were tested by bath application to the hippocampal slice preparation. On the basis of previous work, we hypothesized that APB, HA-966 and GDEE might antagonize synaptic responses to either glutamate or aspartate, but APP should antagonize only synaptic responses to aspartate. APB and HA-966 reduced the amplitude of the extracellular EPSP recorded during stimulation of the perforant path fibers, but APP and GDEE were without effect. APB, APP and HA-966, but not GDEE, consistently inhibited transmission at Schaffer collateral and commissural synapses. The mossy fiber evoked extracellular EPSP was unaffected by these agents. At the concentrations used in this study (usually 2.5 mM) none of these drugs affected the amplitude of presynaptic fiber potentials or antidromic responses, indicating that they probably acted at synapses. The spontaneous activity of hippocampal pyramidal cells, but not of dentate granule cells, increased in the presence of 2.5 mM APB. The amplitude of the population spike generated by Schaffer commissural stimulation initially increased following introduction of APB into the medium and then declined in parallel with the extracellular EPSP. In addition, APB reduced the duration of recurrent inhibition during the period when pyramidal cell firing was enhanced. These results can be explained by an antagonism at the synapse between pyramidal cell and inhibitory interneuron. The actions of these antagonists support previous suggestions that glutamate serves as transmitter of the hippocampal perforant path fibers, that asparate and possibly also glutamate mediate transmission at Schaffer commissural synapses and that neither amino acid serves as transmitter of the mossy fibers. © 1979.