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. 2021 Sep 20;8(5):ENEURO.0143-21.2021. doi: 10.1523/ENEURO.0143-21.2021

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Copyright © 2021 Borrelli et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

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Figure 8. — Thermal hyperalgesia following repeated neonatal morphine exposure. A, In examining normalized thermal nociceptive sensitivity on the 52.5°C hot plate, RM ANOVA (treatment and sex as factors; postnatal day as the RM) revealed a main effect of treatment (F_(1,72) = 65.01, *p < 0.001) but no effect of sex (F_(1,72) = 0.52; p = 0.47) or postnatal day (F_(1,72) = 0.04; p = 0.85), and no interactions (all p values ≥ 0.24). Morphine-treated pups displayed a nociceptive response of 8.91 ± 1.11 s (mean ± SEM) sooner (p < 0.001) than saline-treated pups on average, supporting the presence of hyperalgesia. B, In examining normalized thermal nociceptive sensitivity in the 48.5°C tail withdrawal assay, RM ANOVA (treatment and sex as factors, postnatal day as a repeated measure) indicated a main effect of treatment (F_(1,70) = 60.98, p < 0.001), postnatal day (F_(1,70) = 4.96, p = 0.029), and a treatment × sex interaction (F_(1,70) = 4.83, p = 0.03). However, there were no interactions with day (p values > 0.19), and, thus, tail withdrawal latencies were averaged across P7 and P14 to more clearly and specifically illustrate the treatment × sex interaction. Post hoc analysis revealed lower latencies in both morphine-treated males and females relative to all saline groups (Tukey’s test: SAL males vs MOR males, p = 0.002; SAL males vs MOR females, p < 0.001; SAL females vs MOR females, p < 0.001; SAL females vs MOR males, p < 0.001). While there was no difference in latencies between saline-treated males and females (p = 0.99), there was a nonsignificant lower latency in morphine-treated females compared with morphine-treated males (p = 0.072), hinting at greater hyperalgesia. Data are presented as the mean ± SEM of raw (non-normalized) data, averaged across P7 and P14.