Heterosynaptic changes accompany long‐term but not short‐term potentiation of the perforant path in the anaesthetized rat.

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Abstract
Brief high-frequency trains of electrical stimulation delivered to the preforant path result in long-term potentiation (l.t.p.) of field potentials recorded extracellularly from granule cells of the dentate gyrus. L.t.p. of the population spike is often disproportionately greater than l.t.p. of the population excitatory post-synaptic potential (e.p.s.p.). The basis of this effect in rats anesthetized with sodium pentobarbitone was investigated. A series of graded stimuli were given before and after tetanization of the perforant path. From data obtained in this way, stimulus-response curves were plotted and the relation (E-S curve) between the slope of the population e.p.s.p. (E) and the amplitude of the population spike (S). Curves relating spike onset latency to the slope of the e.p.s.p. were also constructed. Tetranization of the combined medial and lateral components of th perforant path led to long-term changes in the relation between the e.p.s.p. and the population spike. For a given e.p.s.p., the corresponding population spike was of greater amplitude and earlier onset. This E-S potentiation was marked by a shift to the left of the E-S amplitude curve and a downward displacement of the E-S latency curve. Tetranization of the lateral component of the perforant path had 2 long-term effects on responses evoked by test stimuli to the untetanized medial component: long-term depression of the medial e.p.s.p. and long-term E-S potentiation. The net result of these 2 heterosynaptically induced effects was to leave unaltered information transfer across medial perforant path-granule cell synapses: for a given test volley the e.p.s.p. was smaller, but because of E-S potentiation the population spike remained relatively unaffected. Short-term potentiation, which has a time course of only a few minutes and is presumed to be mediated by presynaptic mechanisms, was not accompanied by E-S potentiation or by corresponding changes in spike latency. Possible mechanisms of long-term heterosynaptic depression of the e.p.s.p. and of homo- and heterosynaptic E-S potentiation, are discussed. Although these effects probably reflect a generalized post-synaptic change, this change is unlikely to be a prolonged reduction in the membrane potential of granule cells.