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).