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. 2020 Sep 8;32(10):108123. doi: 10.1016/j.celrep.2020.108123

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© 2020 The Author(s)

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

PMC Copyright notice

Cells in HDB and MEC Respond to Optogenetic Stimulation of PPN

(A) ChR2-expressing rats participated in two consecutive sessions in the open field: one baseline exploration and one stimulation session.

(B) Top row: examples of speed tuning curves (mean firing rate as a function of running speed) for two positive speed cells (two left panels) and one negative speed cell (middle panel) in HDB (green), and for two positive speed cells in MEC (right panels in blue), all recorded during the baseline session. Bottom row: corresponding raster plots (top) and peri-stimulus time histograms (bottom; PSTH, bins, 1 ms) for the respective speed cells shown in the top row. Absolute changes in firing rate are aligned to the onset of optogenetic laser pulses in PPN (blue windows) during the stimulation session. The cells in the second, third, and fifth rows have some of the shortest response latencies that were recorded in the HDB and MEC cell populations (all <10th percentile for the region). With local activation latencies of ~1 ms (Rowland et al., 2018), a latency of 7 ms from PPN stimulation to excitation or inhibition in MEC is likely compatible with a polysynaptic activation pathway from PPN, even if the pathway includes unmyelinated fibers.

(C) Total number of unresponsive (white), excited (filled), and inhibited (dashed) cells in HDB (top, green) and MEC (bottom, blue) following PPN stimulation. The total number of recorded cells is displayed in the center.

(D) Left: proportion of speed cells among all responsive cells in HDB (top, green) and MEC (bottom, blue) (sum of excited and inhibited cells in [C]). Middle: summary of numbers of responsive speed cells recorded in HDB (green) and MEC (blue) following PPN optogenetic stimulation, broken down by type of response (excitation, filled bars; inhibition, dashed bars) and speed cell class (positive speed cells, left bars; negative speed cells, right bars). Right: same as middle, but for all non-speed cells that were recorded in HDB (left) and MEC (right).

(E) Color-coded summary of the changes in firing activity of all responsive cells in HDB (left panel) and MEC (right panel), ordered by response direction (excitation/inhibition) and response latency. Z-scored firing rate is color-coded (scale bar).

(F) Distributions showing response latencies in all responsive units (top) and all responsive speed cells (bottom) recorded in HDB (green) and MEC (blue). ^∗∗∗p < 0.001, Wilcoxon rank-sum test.