Synaptic events that generate fast oscillations in piriform cortex

Abstract

Prominent, odor-evoked, fast (40-60 Hz) oscillations have been reported in the olfactory bulb and piriform (primary olfactory) cortex of both awake-behaving and anesthetized animals. The present study used current source-density analysis to examine the origin of the fast oscillations evoked by single weak shocks to afferent fibers. These shock-evoked oscillations closely resemble those evoked by odor. The results revealed that each cycle of the oscillatory field potential was generated by a stereotyped series of membrane currents similar to those previously characterized in the non-oscillatory response to strong afferent fiber shocks. Each cycle began with a strong inward current in layer la identified as an EPSC mediated by afferent fibers in distal apical dendrites of pyramidal cells. This afferent input was followed by a strong inward current in layer lb identified as an EPSC mediated by intrinsic association fibers in middle apical dendritic segments. These excitatory events were followed by a smaller inward current at the depth of pyramidal cell somata (layers II and superficial III) that may be the depolarizing Cl-·mediated IPSC previously identified in the strong-shock response. Based on an analysis of the timing of the EPSCs it was concluded that the weak shock-evoked oscillation is generated in the olfactory bulb and that the resulting periodic activity in afferent fibers drives the oscillation in the piriform cortex. It was further concluded that, as previously demonstrated for strong shock responses, mono- and disynaptic EPSCs within each cycle of the fast oscillation consist of successive waves that propagate from rostral to caudal within the piriform cortex, following the course of the afferent and associational fiber systems. Based on the resemblance of the temporal ordering of EPSCs to the "theta burst" paradigm that effectively induces long-term potentiation, and the repetitive spatial juxtaposition of afferent and association fiber EPSCs in dendrites, it is proposed that the ordering of synaptic events during fast oscillations may be, in part, for the purpose of resetting synaptic efficacies during the learning of olfactory discriminations. Copyright © 1993 society for neuroscience.