Figure 4.
Glutamate uncaging evokes lateral IPSCs in ET cells but not MCs. A, Experimental design. IPSCs in MCs and ET cells were recorded in response to glutamate uncaging at a distant glomerulus (gray circle; recordings done in 100 μm MNI-glutamate, 35-μm-diameter laser pulse). The uncaged glutamate can excite SA cells in the conditioning glomerulus that project to the test glomerulus. MC recordings were done in slices with a cut through the EPL to isolate glomerular layer inhibition. B, Example current recordings (Vhold = −7 mV), showing an evoked IPSC in an ET cell (left) but not the MC (right). Three representative trials are shown at top, and averages are at bottom. The open bars below the averaged traces indicate the 100 ms laser pulse. The evoked IPSC in the ET cell occurred with a delay of ∼30 ms following the start of the laser pulse, presumably reflecting the time it takes for glutamate to accumulate and activate SA cells at the conditioning glomerulus, either directly or via a polysynaptic mechanism. C, Histogram summarizing baseline-subtracted current measurements made across 45 MC–laser spot pairs and 38 ET cell–laser spot pairs. D, The spatial spread of the laser-evoked glutamate transients was assessed by measuring excitatory currents in an ET cell directly evoked by the uncaged glutamate. The top image shows the ET cell, along with light spots applied at different locations. The bottom traces show evoked currents. E, Summary of evoked current amplitude versus distance between the light spot and the nearest dendritic process of the test cell. Recordings were obtained in six MCs and two ET cells.