SYNAPTIC EVENTS THAT GENERATE FAST OSCILLATIONS IN PIRIFORM CORTEX

Prominent, odor-evoked, fast (40-60 Hz) oscillations have been reporte d in the olfactory bulb and piriform (primary olfactory) cortex of bot h awake-behaving and anesthetized animals. The present study used curr ent source-density analysis to examine the origin of the fast oscillat ions evoked by single weak shocks to afferent fibers. These shock-evok ed oscillations closely resemble those evoked by odor. The results rev ealed that each cycle of the oscillatory field potential was generated by a stereotyped series of membrane currents similar to those previou sly characterized in the non-oscillatory response to strong afferent f iber shocks. Each cycle began with a strong inward current in layer Ia identified as an EPSC mediated by afferent fibers in distal apical de ndrites of pyramidal cells. This afferent input was followed by a stro ng inward current in layer lb identified as an EPSC mediated by intrin sic association fibers in middle apical dendritic segments. These exci tatory events were followed by a smaller inward current at the depth o f pyramidal cell somata (layers II and superficial III) that may be th e depolarizing Cl -mediated IPSC previously identified in the strong-s hock response. Based on an analysis of the timing of the EPSCs it was concluded that the weak shock-evoked oscillation is generated in the o lfactory bulb and that the resulting periodic activity in afferent fib ers drives the oscillation in the piriform cortex. It was further conc luded that, as previously demonstrated for strong shock responses, mon o- and disynaptic EPSCs within each cycle of the fast oscillation cons ist of successive waves that propagate from rostral to caudal within t he piriform cortex, following the course of the afferent and associati onal 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 affere nt and association fiber EPSCs in dendrites, it is proposed that the o rdering of synaptic events during fast oscillations may be, in part, f or the purpose of resetting synaptic efficacies during the learning of olfactory discriminations.