Neural Coding by Two Classes of Principal Cells in the Mouse Piriform Cortex

SP and SL cells perform different frequency transformations on a naturalistic stimulus. A, A train of five stimuli at 40 Hz was applied to the LOT, mimicking typical in vivo output of the olfactory bulb during a sniff. The mean firing frequency of the resultant postsynaptic APs was measured over a range of stimulus strengths. This panel shows the mean firing frequency for SP cells (red; n = 9) and SL cells (black; n = 6) plotted against stimulus strength, normalized to the strength that first generates one AP on average. The dashed lines are the data for individual cells; the circles show the mean of all cells of that type (±SEM). B, Illustrative postsynaptic responses measured in an SP cell (top; red) and an SL cell (bottom; black) at either twice (left) or four times (right) the stimulus threshold for firing an AP. With the weaker stimulus, the SP cell reliably follows the 40 Hz input train, whereas the SL cell fires at 20 Hz because of the large afterhyperpolarization after each AP. With the stronger stimulus, the SP cell fires an initial burst of APs, whereas the SL cell now fires a single AP on each EPSP. C, Summary data for the same experiment done with layer Ib (Assn) stimulation at 40 Hz (n = 10 for SP and SL). This resembles the response to LOT stimulation (A), except that SP cells tend to burst more strongly after Ib stimulation. D, Illustrative postsynaptic responses to layer Ib stimulation, displayed as in B. E, Simplified canonical circuit of the piriform cortex. Two circuits are shown, differing by the identity of the neuron that first receives LOT input (to layer Ia) from the olfactory bulb (SP cell, top; SL cell, bottom). The response to LOT stimulation then passes to SP and SL cells in the second layer of synaptic processing, the Assn inputs in layer Ib. F, Averaged output of the model in E after two layers of synaptic processing (LOT, and then Assn). The shaded pink band represents the mean AP firing frequency (±1SD) at the output of SP cells; the shaded gray band represents the output of SL cells. The two cell types generate broad, yet distinctive, ranges of AP firing frequencies that depend on the strength of synaptic input.