Fig. 1.

Optogenetically driven induction of plasticity in the IL–NAcSh circuit augments an ensemble-level metric of synaptic strength. (A) Surgical schematic, parasagittal view. Mice were bilaterally transfected in the IL with an AAV8 construct containing the gene for Chronos driven by a Syn promoter with a fusion GFP reporter. (B) Pharmacological characterization of bimodal light-evoked field potentials recorded ex vivo in the NAcSh. The vertical dashed blue line indicates a 1-ms light pulse. (Top) The zero-calcium bath reversibly abolished the N2 component. (Middle) The AMPAR antagonist abolished the N2 component dose dependently. (Bottom) TTX abolished the N2 component, reducing the N1 component. (Scale bars: x axis, 10 ms; y axis, 0.2 mV.) (C) An example of the IO regimen assayed on a single slice. Stimulus duration was varied from 0.1 to 1.0 ms. (D) N1 and N2 peak amplitudes are plotted from examples of traces in C, and the linear regression slope was calculated. To test if the IO slope can serve as an independent metric of circuit-specific synaptic strength, IO ramps were assayed before and after ex vivo bath application of plasticity-inducing optogenetic stimulation protocols. (E) Experimental timeline. Mice were killed and slices were prepared at time 0. After allowing 1 h for tissue incubation, the first IO slope (pink PRE in timeline) was collected. From this, the stimulus duration that elicited 50% of the maximum N2 amplitude was determined and was used as the static stimulus parameter during regular samplings for the bath-application plasticity assay [12.5 min baseline sampling (gold pre in timeline), followed by one of three protocols and then 37.5 min of additional sampling; the final 12.5 min served as the gold post in timeline]. A second postplasticity IO slope assay was collected (pink POST in timeline). (F) The percent of N2 change normalized to baseline sampling (gold pre and post time points in timeline). Representative waveform traces were plotted from gold pre and post time points in timeline for each protocol. (Scale bars: x axis, 10 ms; y axis, 0.2 mV.) (G) IO slopes from pink PRE and POST time points in timeline. (H) The percentage of N2 change in F plotted against the change in IO slope in G, with Pearson correlation, r. Dots in F and H and thin lines in G represent individual slices. Sample size in F and G is noted below the x axis as the number of slices followed by number of mice in parentheses. Larger dots represent mean ± 1 SE. *P = value shown on plot; n.s., not significant.