Figure 6. Dominant local inhibition in the superficial layers of the retrosplenial cortex. A. Table indicating the percentage of connectivity between all types of pairs tested. No FS-FS pairs were tested in this study. FS→LR
connections existed in 11 of the 21 pairs recorded (52%). FS→RS
connections existed in 1 of the 2 pairs recorded (50%). LR→FS
connections existed in 4 of the 23 pairs recorded (17%). There were no
RS→FS connections recorded and no E→E connections recorded (LR→LR,
LR→RS, RS→LR, RS→RS). The heat map to the right indicates the
probability of connections between the neuron types indicated in each
cell of the table. Deeper gold indicates connection probabilities of
near 50%, while lighter gold indicates lower probabilities and white
indicates a connection probability of 0. B. Bar graph
representing the total connectivity probability between all inhibitory
to excitatory directional pairs (52%) and all inhibitory to excitatory
directional pairs (16%). Bootstrap resampling followed by a t-test
revealed a significant difference in probability to observe I→E
connections versus E→I connections. C. Representative
trace of the connection between a layer 3 FS cell (held at -65 mV) and
layer 3 LR cell (held at -55 mV). The neurons were 27 um apart with the
LR cell located superficial to the FS cell. Schematic shows the patched
pair in which the FS cell is being stimulated to spike at 10 Hz and the
responses of the LR cell are being recorded. The purple trace is the
responses of the LR cell to each FS cell spike (indicated by the orange
arrows). D. Representative trace of the connection
between a layer 3 LR cell (held at -65 mV) and layer 3 FS cell (held at
-55 mV). The neurons were 80 um apart with the LR cell located
superficial to the FS cell. Schematic shows the patched pair in which
the LR cell is being stimulated to spike at 10 Hz and the responses of
the FS cell are being recorded. The orange trace is the responses of the
FS cell to each LR cell spike (indicated by the purple arrows).
E. Bar graph showing the average amplitude of the IPSPs
recorded from the FS→LR pairs (red) and the EPSPs recorded from the
LR→FS pairs (blue). Error bars are standard error. F.
Bar graph showing the average latency to onset of the IPSPs recorded
from the FS→LR pairs (red) and the EPSPs recorded from the LR→FS pairs
(blue). Error bars are standard error. Latency to onset was calculated
as the time from the peak of the presynaptic action potential to the
beginning of the postsynaptic IPSP/EPSP. G. Bar graph
showing the average latency to peak of the IPSPs recorded from the FS→LR
pairs (red) and the EPSPs recorded from the LR→FS pairs (blue). Error
bars are standard error. Latency to peak was calculated as the time from
the onset of the postsynaptic IPSP/EPSP to the peak of the postsynaptic
IPSP/EPSP. H. Group synaptic dynamics for FS→LR
connections (n=9). Inhibition onto LR cells exhibited strong short-term
depression. I. Group synaptic dynamics for LR→FS
connections (n=2). Excitation onto FS cells did not exhibit short-term
depression or facilitation.