Fig. 4. Systemic captopril reduces excitatory input to D1-MSNs, counteracts fentanyl reward, and increases sociability.

(A) Proposed mechanism by which captopril regulates glutamate release onto D1-MSNs via MERF. (B) Schematic showing viral injection of ChrimsonR-tdTomato in mPFC and Cre-dependent GCaMP8m in NAc, separated by fluorescent image showing viral expression (left), and setup for simultaneous optogenetic stimulation (594 nm) and fiber photometry recording (405/470 nm, right). (C) Traces showing average response to 2, 10, and 40 pulses of red light at 20 Hz after injection of saline (left), and average change in response following injection of captopril (30 mg/kg, i.p.; right). (D) Percent change in slope of the input-output curve following injection of captopril versus saline (n=6). (E-G) Schematic of unbiased place conditioning procedure (E), with percent time on fentanyl side (F) and CPP score (G) for groups receiving fentanyl (0.04 mg/kg, s.c.) preceded by vehicle (n=11, dark grey) or captopril (30 mg/kg, i.p.; n=11, dark blue). (H-J) Schematic of unbiased place conditioning procedure (H), with percent time on fentanyl side (I) and CPP score (J) for groups receiving saline preceded by vehicle (n=11, grey) or captopril (30 mg/kg, i.p.; n=11, blue). (K) Left, schematic of reciprocal social interaction test following injection of vehicle or captopril (30 mg/kg, i.p.). Right, total social interaction time after captopril (n=18, blue) or vehicle (n=18, grey). (L-O) Time spent huddling (L), interacting nose-to-nose (M), socially exploring (N), or following (O) the partner mouse throughout the assay. Data are mean ± s.e.m. for all panels; open and closed circles indicate female and male mice, respectively. *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001, one-sample t-test (D), simple effect of session/treatment (F), and treatment main effect (G, K-N); see Data S1 for complete statistics.