Synaptic sIPSCs sensitive to strychnine (i.e., mediated by GlyRs) were found in only 4 of 29 cells. Two cells were found in the ganglion cell layer of
geph +/+ cultures and two further were recorded in the amacrine cell layer of
geph +/− cultures. The sIPSCs sensitive to strychnine showed a mean peak amplitude of 47.1 ± 24.9 pA (4 cells) and a mean τ
w of 18.5 ± 4.9 ms (4 cells). Two cells expressed both GABAergic and glycinergic sIPSCs simultaneously and one of them, an amacrine cell is analyzed in detail in
Figure 5 . The amplitude–time plot is shown in
Figure 5A . Before the drug application, sIPSCs occurred at a frequency of 0.9 Hz with a mean amplitude of 74.2 ± 41.9 pA (592 events). Strychnine (0.5 μM) strongly reduced the mean amplitude, but also induced an increase in sIPSC frequency to 3.8 Hz. which was probably due to disinhibition of the local network. By the application of 10 μM bicuculline in addition to 0.5 mM strychnine the remaining sIPSCs nearly completely disappeared, indicating that they were mediated by GABA
ARs. The withdrawal of strychnine evoked the reappearance of a subpopulation of sIPSCs. These sIPSCs occurred at a frequency of 0.24 Hz and persisted in the presence of bicuculline, indicating they were mediated by GlyRs
(Fig. 5A) . Thus, this amacrine cell expressed two types of sIPSCs, one mediated by GABA
ARs and another by GlyRs. Indeed, we found two types of sIPSCs that differed in their decay kinetics
(Fig. 5B) . In the absence of the antagonists, fast- and slow-decaying sIPSCs were found, the first persisted in the presence of strychnine, whereas the latter remained in the presence of bicuculline, suggesting that the fast-decaying sIPSCs were mediated by GABA
ARs and the slow-decaying by GlyRs. For a detailed analysis, several sIPSCs traces were selected from each group, superimposed, and averaged
(Fig. 5C) . Single sIPSCs were fitted with two exponentials, and the τ
ws were calculated (see the Methods section), resulting in a mean τ
w for GABAergic sIPSCs of 5.6 ± 3.9 ms (1019 events) and of 10.1 ± 10.8 ms (223 events) for glycinergic sIPSCs, which was significantly different (
P < 0.01). The superimposed and normalized averaged traces and the cumulative fraction plot of the τ
ws
(Fig. 5D) indicate that GABA
ARs expressed in this cell decayed significantly faster than GlyRs. In addition, we found that the GABAergic sIPSCs were characterized by a significantly larger amplitude (GABAergic sIPSCs: 24.6 ± 14.7 pA, 1019 events; glycinergic sIPSCs: 17.1 ± 10.4 pA, 223 events;
P < 0.01). The amplitude distribution histograms and corresponding cumulative fraction plots are displayed in
Figure 5E . Because the differences in the decay kinetics between GABAergic and glycinergic sIPSCs could have arisen from different locations of the two types of synapses in a different electrotonic distance from the recording electrode, we plotted the τ
ws of both groups of sIPSCs against the corresponding increase times (T10/90s; see the Methods section;
Figs. 5F 5G ). We found no correlation between τ
ws and T10/90s, indicating that the differences in the decay kinetics of GABA
ARs and GlyRs is not due to dendritic filtering. The second cell that showed glycinergic and GABAergic sIPSCs simultaneously was found in the ganglion cell layer. In this cell glycinergic sIPSCs were characterized by a mean peak amplitude of 41.6 ± 20.4 pA (545 events) and a mean τ
w of 25.4 ± 17.5 ms (545 events) and they occurred at a frequency of 1.3 Hz. GABAergic sIPSCs showed a mean peak amplitude of 17.6 ± 9.6 pA (2880 events) and a mean τ
w of 22.7 ± 15.7 ms (2880 events) and they occurred at a frequency of 1.7 Hz.