Dominant inhibition and rare local excitation in the superficial layers of RSG
Using paired whole-cell recordings, we sought to quantify the connectivity between these three major cell types in the superficial layers of RSG: LR and RS (both excitatory; E) and FS (the major inhibitory neurons in these layers; I). To our surprise, LR to FS connectivity was extremely rare (17%), suggesting a lack of locally driven excitation of FS cells. On the other hand, FS cells were frequently connected to, and inhibited, neighboring LR cells (52%) (Figure 6A). When all pairs were considered, the E→I connectivity was only 16%, whereas the I→E connectivity reached 53% (Figure 6B). The difference in probability to observe I→E connections versus E→I connections was significant (p<0.01), suggesting the superficial layers of the RSG represents an inhibition-dominated network. Additionally, we observed no LR→LR connections (0/36), nor any connectivity between LR and RS cells (0/6), indicating a complete lack of E→E connectivity. I→I pairs were not sampled.
The amplitudes of the evoked responses from a holding potential of -55 mV were similar between inhibition and excitation (Figure 6E). However, the latency to peak was smaller in LR→FS connections compared to FS→LR connections. IPSPs from FS to LR cells exhibited clear short-term depression. This was seen in paired recordings (Figure 6C&H) and also when recording from LR neurons during optogenetic stimulation of FS cells (data not shown). EPSPs from LR to FS cells did not exhibit either depression or facilitation (Figure 6D&I).