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. Author manuscript; available in PMC: 2022 Aug 2.
Published in final edited form as: Neuropharmacology. 2019 Mar 14;151:189–194. doi: 10.1016/j.neuropharm.2019.03.019

Figure 1.

Figure 1.

Timeline of the study. Adult male and female C57BL/6J mice underwent surgery and were implanted with a catheter in the right jugular vein and a back-mounted port. The mice were allowed 1 week to recover before the first acquisition session. During the acquisition phase, the mice were trained to nosepoke on an FR1 schedule to obtain intravenous heroin (30 μg/kg/infusion in sessions 1–5; 60 μg/kg/infusion in session 6 onward). The mice then underwent the escalation phase and were divided into short access (ShA; 1 h) or long access (LgA; 6 h) groups. During the escalation phase, the mice were allowed to self-administer heroin in 10 sessions on a Monday-Wednesday-Friday schedule. Immediately after the 10th escalation session, somatic signs of opioid withdrawal (e.g., paw tremors, jumps, etc.) were recorded for 15 min following administration of the preferential μ-opioid receptor antagonist naloxone (1 mg/kg) to precipitate withdrawal.