Synaptic events that generate fast oscillations in piriform cortex.

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 nonoscillatory 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 Ib 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.(ABSTRACT TRUNCATED AT 250 WORDS)