Figure 4. Remifentanil IVSA under increased response requirement. Reinforcers earned (A, C) and total active responses (B, D) per session in saline-treated and Ex4-treated WT mice and in saline-treated and Ex4-treated KO mice. A, B, With 10-µg/kg/infusion remifentanil available as the reinforcer, response requirement was increased successively from FR 1 to FR 3, FR 5 schedule of reinforcement, resulting in decreasing remifentanil intake in all groups. C, D, Under a PR schedule of reinforcement, saline (S), 10- and 32-µg/kg/infusion remifentanil were made available. In A, B, KO, n = 3; WT, n = 4–5; in C, D, KO, n = 3–4; WT, n = 4–6.Figure Contributions: Annika Billefeld Bornebusch and Saiy Kiasari collected the data. Morgane Thomsen designed the experiments. Annika Billefeld Bornebusch and Morgane Thomsen analyzed the data.