Potentiation of monosynaptic EPSPs in the perforant path-dentate granule cell synapse

Abstract

1. In rabbits, initially anaesthetized by urethane/chloralose and maintained on urethane alone, the perforant path, contacting the apical dendrites of the dentate granule cells by way of boutons en-passage, was activated by paired stimuli. The effect of the first conditioning stimulus was studied by recording the extracellular field response and the extra- and intracellular responses of single granule cells to a second test stimulus. 2. The monosynaptic test EPSP recorded extra- as well as intracellularly was potentiated by a preceding volley in the perforant path. The rate of rise and the amplitude of the extracellular test EPSP increased by as much as 100% at an optimal conditioning-test interval of about 25 msec. The total duration of the potentiation was 200–300 msec. Sometimes the potentiation was followed by a slight subnormal phase. 3. The EPSP potentiation was not due to a larger presynaptic test volley since (1) the size of the presynaptic fibre potential was not effected, and (2) removal of the entorhinal area did not reduce the effect. 4. The potentiation was not due to the recurrent inhibitory hyperpolarization of the postsynaptic membrane because (1) weak conditioning volleys potentiated the EPSPs without discharging the granule cells or producing any inhibition. (2) Conditioning antidromic volleys produced marked inhibition and IPSPs but had no effect on the test perforant path EPSP. 5. Different mechanisms that may be responsible for the EPSP potentiation are discussed. The potentiation is compared with that observed in other types of synapses. Potentiation of EPSPs may be an important mechanism behind the frequency potentiation of mass responses characteristic of the hippocampal formation.