Figure 6. Activation of transplanted interneurons introduce inhibitory gain control to native pyramidal cells. *A*, Representative traces recorded from a native pyramidal cell at various levels of current injection in the absence and presence of photostimulation. In the absence of photostimulation, 40-pA current injection (red trace) brought the cell over threshold and generated two APs, giving a frequency of 5 Hz; 100-pA (brown trace) and 200-pA (black trace) current injection induced higher spiking frequency at 18.7 and 29.2 Hz, respectively. With simultaneous blue-light photostimulation, 40 pA was no longer sufficient to evoke AP, and the spike frequencies induced by 100 and 200 pA were reduced to 15.6 and 28 Hz, respectively. *B*, *C*, I-F curves of the cell showed in *A* under various conditions. Curves were fit linearly with a data range picked manually. The slope factor of the fit line for control condition is 0.136 (open squares, *r*
^{2} = 0.955), whereas the slope factor with photostimulation is 0.138 (open circles, *r*
^{2} = 0.990). After the addition of 100 μM gabazine, the slope factor is 0.136 (closed squares, *r*
^{2} = 0.988), and that with photostimulation is 0.135 (closed circles, *r*
^{2} = 0.970). By comparing the fit lines, we found that photostimulation right shifts the curve by 23.7 pA, which can be reversed by the addition of gabazine. The shift in the presence of gabazine is 3.6 pA. *D*, The slope factors from fit results were normalized, averaged, and compared. In the absence of gabazine, the normalized slope factor with photostimulation is 1.013 ± 0.026, which is not different from the control group (paired *t* test, *n* = 9, *p* = 0.630). In the presence of 100 μM gabazine, the normalized slope factor with photostimulation is 1.125 ± 0.066 and is not significantly different from that without photostimulation (paired *t* test, *n* = 5, *p* = 0.334). *E*, Parallel shifts in I-F curves were compared with and without gabazine. On average, activation of transplanted interneurons, tGAD2, right shifts the I-F curve by 21.1 ± 5.5 pA, which can be rectified by 100 μM gabazine to -2.2 ± 6.8 pA (two-sample *t* test, *n* = 9 and 5, *p* = 0.023). The minor shift in the presence of gabazine is not different from 0 (one-sample *t* test, *p* = 0.661). Similarly, activation of native interneurons, nGAD2, shifts the I-F curves to the right (342.8 ± 30.3 pA, *n* = 5), and subsequent addition of 100 μM gabazine completely eliminated the effect (-5.4 ± 7.5 pA after gabazine, *n* = 4; two-sample *t* test, *p* = 0.00002). The shift recorded in the presence of gabazine is not different from 0 (one-sample *t* test, *p* = 0.460).