Voluntary Drinking Preference in Mice

To assess voluntary drinking, mice were singly housed with access to two 50 mL conical tubes fitted with sipper tops following established procedures^{40 (link),57 (link)}. Briefly, every morning fluid consumption was measured, and bottle placement was switched to account for individual side bias. Mice that displayed a > 30% side/bottle preference during morphine preference were excluded from analysis. Mice were allowed to habituate to both bottles containing only water for 4 days, then water bottles were replaced for sucrose or morphine preference assessment for an additional 4 days. For sucrose preference, water bottles were replaced with bottles containing either 1% sucrose or water. For morphine preference, water bottles were replaced with bottles containing a 0.2% sucrose solution with either 0.05 mg/mL morphine sulfate or 0.01 mg/mL quinine (Sigma, 22640, bitter taste control)^{60 (link),61 (link)}. Drinking was measured to determine solution preference over time. Results are reported as percent preference (solution consumed/total fluid consumed × 100), average preference (4-day average of percent preference), and fluid intake (volume normalized to mouse body weight, 4-day average).