Minocycline suppresses morphine-induced respiratory depression, suppresses morphine-induced reward, and enhances systemic morphine-induced analgesia

Abstract

Recent data suggest that opioids can activate immune-like cells of the central nervous system (glia). This opioid-induced glial activation is associated with decreased analgesia, owing to the release of proinflammatory mediators. Here, we examine in rats whether the putative microglial inhibitor, minocycline, may affect morphine-induced respiratory depression and/or morphine-induced reward (conditioned place preference). Systemic co-administration of minocycline significantly attenuated morphine-induced reductions in tidal volume, minute volume, inspiratory force, and expiratory force, but did not affect morphine-induced reductions in respiratory rate. Minocycline attenuation of respiratory depression was also paralleled with significant attenuation by minocycline of morphine-induced reductions in blood oxygen saturation. Minocycline also attenuated morphine conditioned place preference. Minocycline did not simply reduce all actions of morphine, as morphine analgesia was significantly potentiated by minocycline co-administration. Lastly, morphine dose-dependently increased cyclooxygenase-1 gene expression in a rat microglial cell line, an effect that was dose-dependently blocked by minocycline. Together, these data support that morphine can directly activate microglia in a minocycline-suppressible manner and suggest a pivotal role for minocycline-sensitive processes in the mechanisms of morphine-induced respiration depression, reward, and pain modulation.

Figure 1. Minocycline attenuates morphine induced respiratory depression without effecting respiratory rate

Morphine induced respiratory depression and its attenuation by minocycline were quantified using full body plesthysmography allowing the quantification of tidal volume (A), respiratory rate (B), minute volume (E), inspiratory force (C) and expiratory force (D). The data are displayed as area under the curve (AUC) for each parameter. (F) Analysis of the distribution of minute volume (MV) values reveals that morphine significantly increases very low minute volume values, consistent with significant respiratory depression. As distributions for minocycline+morphine and minocycline+vehicle groups were superimposable, these groups were pooled for simplicity of data presentation in this figure. (G) Morphine induced decreases in blood oxygen saturation (PulseOx), an effect significantly attenuated by minocycline. (H) Analysis of the distribution of PulseOx values reveals that morphine significantly increases low PulseOx values, consistent with significant respiratory depression which is attenuated by minocycline. A one-way ANOVA with Bonferroni posthoc test comparing morphine + water with each other data group. * = P < 0.05

Figure 2. Minocycline attenuates morphine induced respiratory depression without effecting respiratory rate

Morphine induced respiratory depression and its attenuation by minocycline were quantified using full body plesthysmography allowing the quantification of tidal volume (A), respiratory rate (B), minute volume (E), inspiratory force (C) and expiratory force (D).). Minocycline (50 mg/kg) + morphine ( ), minocycline (25 mg/kg) + morphine ( ), water + morphine (—), minocycline (50 mg/kg) + saline ( ), water + saline ( ). The mean data for each group are displayed as the time course of respiratory changes at baseline and following morphine administration (10 data point moving average to smooth data). A one-way ANOVA with Bonferroni posthoc test comparing morphine + water with each other data group. * = P < 0.05

Figure 3. Minocycline attenuates morphine conditioned place preference

Morphine (7.5 mg/kg) produces significant conditioned place preference (morphine + water (vehicle): black fill). Minocycline (50 mg/kg per day; gavage) significantly attenuated this morphine induced conditioned place preference (morphine + minocycline: checkerboard fill). In the absence of morphine, neither water (saline + vehicle; white fill) nor minocycline (saline + minocycline: dotted fill) significantly altered place preference. A 2-way ANOVA with Bonferroni was conducted. * = P < 0.05 compared to morphine + water (vehicle).

Figure 4. Minocycline potentiates systemic morphine analgesia

Minocycline (25 mg/kg and 50 mg/kg ◆ gavages) significantly potentiated morphine (4 mg/kg, ■) analgesia, whilst saline treatment had no effect (○). Minocycline potentiated the onset, peak and duration of analgesia (A) as well as the total area under the analgesia curve (B). A one-way ANOVA with Bonferroni posthoc test comparing morphine + water with each other data group. ** = P < 0.01; *** = P < 0.001.

Figure 5. Morphine enhances COX-1 mRNA expression in a rat microglial cell line, an effect blocked by minocycline

HAPI microglia cells response in a proinflammatory fashion in vitro to acute (4 hr) morphine exposure under CNS conditions by significantly up regulating COX-1 mRNA expression. Minocycline dose dependently attenuates this proinflammatory activation by significantly attenuating COX-1 mRNA upregulation. n=3 replicates per condition. *** P < 0.001 media controls vs, morphine alone treatments; # P < 0.05 1000 μM morphine alone vs. morphine+minocycline; × P < 0.05 100 μM morphine alone vs morphine+minocycline.