Field potentials in rat hippocampus: Monosynaptic nature and heterosynaptic post-tetanic potentiation

Abstract

Subicular, commisural, and fornical stimulation evoked large field responses in the dentate gyrus and dorsal hippocampus of rats. Hippocampal responses were negative above and positive below the upper third of apical dendrites in CA1, CA2, and CA3; the application of a pressing foot on the alveus resulted in a distortion of this polarity pattern. Since a compound action potential was found in the fornix during the rising phase of hippocampal field responses, it was concluded that the latter correlated with excitation of pyramidal cells. The following frequency of dentate and hippocampal potentials was high (up to 125/sec) and similar for both; thus, it was concluded that both were monosynaptic responses. On the basis of excluding several impossible mechanisms, hippocampal field potentials were attributed to a flow of cations, through the extracellular clefts, from the stratum pyramidale down to the stratum radiale. Hippocampal- and dentate-evoked potentials suffered post-tetanic potentiation; in the case of hippocampal responses, this was both homo- and heterosynaptic. The possible relevance of the latter to mechanisms of learning, and the possible genesis of both by (K +) 0 accumulation were discussed.