Figure 1.

Olfactory forebrain circuitry and evoked responses. A, Simplified diagram of OB and PC circuitry depicting the stimulating sites used in the present experiment. The dotted ellipse around each electrode tip indicates the axons targeted for activation by stimulation through that electrode. Cell layers shown include the external plexiform layer (EPL), MCL, and GCL in the OB, and layers Ia, Ib, II, and III in the PC. Cell types are indicated as follows: M, mitral cell; G, granule cell; P, pyramidal cell; I, inhibitory interneuron; D, deep multipolar cells in layer III and the endopiriform nucleus. B, Location of stimulating and recording electrodes. Fine wire electrodes used for stimulation and for recording of local field potentials were chronically implanted at the beginning of the study. Microelectrodes used for recording extracellular unit activity were advanced into the OB or PC during the recording session using trajectories that avoided the chronic electrode assembly on the dorsal surface of the skull. Diagrams of coronal sections are derived from Paxinos and Watson (1997), with permission. C, Example of an M/T cell identified by antidromic activation of the LOT (four consecutive sweeps). D, Examples of local field potentials evoked under urethane anesthesia in the GCL of the OB by AF and LOT test stimulation in a previously potentiated animal. Solid traces were evoked before and dashed traces 5 min after a potentiation treatment. Each trace is an average of 12 evoked responses. E, Example of a depth profile used to determine the location of the MCL. Local field potentials evoked by LOT stimulation were recorded as a microelectrode was advanced in 50 μm steps along the tract shown in B. Each trace is the average of 24 evoked responses.