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. 2022 Mar 17;13:841873. doi: 10.3389/fphar.2022.841873

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Copyright © 2022 Towers, Setaro and Lynch.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Effect of sex, estrous cycle phase, and dose on responding during extinction testing in female and male rats. Mean (±SEM) number of responses made on the lever formerly associated with fentanyl during the first six 1-h extinction sessions for males (A; left) and females (right) within the 0.25 (n = 8 males, n = 9 females), 0.75 (n = 8 males, n = 9 females), 1.5 (n = 8 males, n = 8 females), and 3.0 (n = 8 males, n = 7 females) dose (μg/kg) conditions and across all doses and extinction sessions run (B) (n = 29 males, n = 29 females). (*) Significantly higher responding in females compared with males. (+) Significantly higher responding in session 1 compared with sessions 2–6. (^) Significantly higher responding in estrus females compared with non-estrus females and males.