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. Author manuscript; available in PMC: 2016 Mar 1.
Published in final edited form as: Addict Biol. 2013 Dec 13;20(2):275–284. doi: 10.1111/adb.12120

Figure 2.

Figure 2

Gradual development of mechanical hyperalgesia across heroin self-administration history. Paw withdrawal thresholds were measured in heroin self-administering rats over the escalation period just prior to subsequent self-administration sessions. As long-access (Heroin LgA 12h, n = 8) rats increase their heroin intake across subsequent twelve-hour sessions (A), thresholds continue to decrease over time (B). Heroin intake for LgA rats increased significantly over the entire session (but not in the first hour, Heroin LgA 1h) compared to baseline on days 11–14 (* p < 0.05, ** p < 0.01), and paw withdrawal thresholds were different from short-access (Heroin ShA, n = 5) rats on day 14 (** p < 0.01). Paw withdrawal thresholds of ShA rats do not decrease significantly compared to baseline, and their heroin intake remains relatively constant (averaging 0.37 mg/kg/day).