Long term potentiation is accompanied by a reduction in dendritic responsiveness to glutamic acid

Abstract

HIPPOCAMPAL synapses show an unusual degree of physiological plasticity. Relatively modest levels of repetitive stimulation cause considerable enhancement in subsequent responses to single pulse stimulation, and this potentiation lasts for hours and even days^1–6. These findings, which have now been replicated in several laboratories, have aroused much interest, first because they seem to represent an excellent starting point for the analysis of synaptic plasticity in mammalian brain, and second, because their rapid development and persistence suggest that they may be related to processes involved in behavioural plasticity. The studies reported here represent an attempt to analyse the mechanisms underlying physiological plasticity in the hippocampus, in particular the locus and anatomical specificity of the effect. The change in magnitude of response which occurs after repetitive stimulation could reflect either presynaptic or postsynaptic adjustment. If the latter were the case, the change might involve a particular dendritic region innervated by the stimulated input, or alteration in the status of the entire postsynaptic neurone. Seeking data relevant to these questions, we have measured the effects of glutamic acid applied iontophoretically to different levels of the dendritic trees of the pyramidal cells before and after potentiation of the extracellular postsynaptic response.