Morphine (Sigma; Cat No: 25-27-2) and 5-aza-2′-deoxycytidine (5-aza-dC; Sigma; Cat No: 2353-33-5) were dissolved in saline; saline was set as the control. Suberoylanilide hydroxamic acid (SAHA; Selleck Chemicals, Houston, TX, United States; Cat No: S1047) was dissolved in 5% dimethyl sulfoxide (DMSO; Sigma; Cat No: D2650-100 mL); 5% DMSO was set as the control. Morphine was subcutaneously given with a dose of 1.5 mg per kg of body weight. The use of 5-aza-dC was continued for three consecutive days at 5 μg/5 μL daily, 30 min before surgery. SAHA was continued for 7 consecutive days, at a dose of 50 mg/kg in a volume of 5 μL, 30 min before surgery. All drug doses are based on preliminary experiments (Zhou et al., 2014 (link); Cao et al., 2015 (link); Garriga et al., 2018 (link)), and did not alter the nociceptive response per se. Intrathecal injections were performed manually between L5 and L6 spinal segments, through the theca of the spinal cord into the subarachnoid space (Hylden and Wilcox, 1980 (link)). On entry of the needle, a quick flicking motion of the mouse’s tail or leg was noticed. For anesthesia, sodium pentobarbital and isoflurane (Sigma; Cat No: 26675-46-7) were used.
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