The piriform cortex of the opossum was studied by current source-density (CSD) analysis of field potentials to determine the laminar and temporal distribution of synaptic currents evoked by lateral olfactory tract (LOT) stimulation. 2. Extracellular conductivity was measured as a function of depth at high resolution and incorporated into CSD computations. Inclusion of the conductivity term resulted in relatively subtle changes in the shapes of CSD profiles. Resolution and accuracy of CSD computations was further improved by use of a new smoothing approach and averaging of multiple potential profiles obtained at the same site. 3. The CSD depth profile resulting from LOT stimulation revealed six major synaptic events that were consistently present at anterior, middle, and posterior sites: one during the first (A1) peak of the initial surface negative dichrotic field potential component, three during the second (B1) peak, one during the surface positive field potential component (period 2), and one during the second surface negative component (period 3). In addition, CSD profiles were computed for the population spike generated by synchronous discharge of action potentials. Depths of the net inward and outward membrane currents underlying these events were correlated with the cortical lamination as determined histologically by placement of small dye marks. 4. In agreement with previous reports it is concluded that the large inward membrane current in layer Ia during the A1 wave underlies a monosynaptic EPSP evoked in distal apical dendritic segments of pyramidal cells by afferent fibers. This EPSP displays a marked paired shock facilitation. 5. Based on anatomic and physiological considerations it is concluded that the three spatially and temporally distinct inward membrane currents (sinks) that were observed in layers III, superficial Ib, and mid- to deep-Ib during the B1 wave, underlie disynaptic EPSPs resulting from direct synaptic interactions between pyramidal cells. It is postulated that the layer III sink is generated in basal dendrites largely via local axon collaterals, the superficial layer Ib sink in intermediate apical dendritic segments by association fibers originating in the anterior piriform cortex, and the deep Ib sink in proximal apical segments by association fibers originating largely in the posterior piriform cortex. 6. The latencies of the layer Ia and superficial layer Ib sinks (presumed mono- and large disynaptic EPSPs, respectively) increased from anterior to posterior. Amplitude of the superficial Ib sink relative to the Ia sink increased from anterior to posterior.(ABSTRACT TRUNCATED AT 400 WORDS)
