Fig. 1

Chronic morphine treatment promotes cold hypersensitivity by enhancing the excitability of menthol-sensitive neurons. a Top, illustration representing the experimental paradigm of morphine treatment in mice. Saline or morphine injections were performed twice a day for 5 days. Cold sensitivity was measured using the cold plate test. Bottom graph shows the latency of the first nociceptive response (paw withdrawal, licking, shaking, or jumping to try to escape) to a cold stimulus. Each circle represents the latency of an individual mouse treated with saline (black) or morphine (red) and recorded on the first and last day of injection. Statistical analysis was performed using Two-Way ANOVA followed by Sidak’s post hoc test (**p < 0.001; D1 compared to D5, n = 7 mice per group). b Representative current-clamp recordings showing the menthol-induced depolarization (menthol: 100 μM) and AP firing of DRG neurons from saline or morphine-treated mice (MS). c The percentage of menthol-responsive small diameter neurons exhibiting spontaneous activity was significantly increased between saline (black, 30%; n = 14) and morphine (red, 80%; n = 16) groups (from 5 and 6 mice respectively). d Spontaneous AP frequency in menthol-responsive neurons from saline (black: 0.03 ± 0.02 Hz, n = 10) versus morphine-treated mice (red: 0.34 ± 0.12 Hz, n = 6). e Resting membrane potential of menthol-responsive neurons from vehicle and morphine-treated mice (59.6 ± 1.4 mV vs. 55.9 ± 0.98 mV, n = 15 and 17, respectively). f Action potential threshold of menthol-responsive neurons from vehicle and morphine-treated mice (28.4 ± 1.77 mV vs. 44.75 ± 2.73 mV, n = 13 and 6, respectively). Error bars indicate ±SEM. Statistical analysis was performed using unpaired t-test and the Mann-Whitney test in (d) (**p < 0.05)