Reduction of opioid withdrawal and potentiation of acute opioid analgesia by systemic AV411 (ibudilast)

Abstract

Morphine-induced glial proinflammatory responses have been documented to contribute to tolerance to opioid analgesia. Here, we examined whether drugs previously shown to suppress glial proinflammatory responses can alter other clinically relevant opioid effects; namely, withdrawal or acute analgesia. AV411 (ibudilast) and minocycline, drugs with distinct mechanisms of action that result in attenuation of glial proinflammatory responses, each reduced naloxone-precipitated withdrawal. Analysis of brain nuclei associated with opioid withdrawal revealed that morphine altered expression of glial activation markers, cytokines, chemokines, and a neurotrophic factor. AV411 attenuated many of these morphine-induced effects. AV411 also protected against spontaneous withdrawal-induced hyperactivity and weight loss recorded across a 12-day timecourse. Notably, in the spontaneous withdrawal study, AV411 treatment was delayed relative to the start of the morphine regimen so to also test whether AV411 could still be effective in the face of established morphine dependence, which it was. AV411 did not simply attenuate all opioid effects, as co-administering AV411 with morphine or oxycodone caused three-to-five-fold increases in acute analgesic potency, as revealed by leftward shifts in the analgesic dose response curves. Timecourse analyses revealed that plasma morphine levels were not altered by AV411, suggestive that potentiated analgesia was not simply due to prolongation of morphine exposure or increased plasma concentrations. These data support and extend similar potentiation of acute opioid analgesia by minocycline, again providing converging lines of evidence of glial involvement. Hence, suppression of glial proinflammatory responses can significantly reduce opioid withdrawal, while improving analgesia.

Figure 1. AV411 and minocycline significantly reduced naloxone precipitated morphine withdrawal

A: Co-administration of AV411 (7.5mg/kg twice daily) with morphine significantly reduced the area under the naloxone precipitated opioid withdrawal behaviors across a 60min post-naloxone timecourse, compared to vehicle+morphine treated rats. To enable of comparison of data, data are expressed as percent of maximal observed withdrawal score as the vehicle (PEG) caused a substantial modification in the behaviors observed (P<0.001, ###). Behaviors included in this analysis included: jumping, rearing, exploration (movement greater than one body length), teeth chattering, wet dog shakes, abnormal posture, ptosis, diarrhea, penis licking, pica (oral stimulation by filling of mouth with bedding), paw chewing, cleaning, salivation, vocalization, chewing (large jaw movements including masseter muscle contraction) and fidgeting (a writhing type of behavior involving small shifts in body position). Counts of each of these behaviors were made upon their presentation. In cases where the response was prolonged, for example ptosis, counts were made every 30s. As seen in the figure, some behaviors still persisted compared to AV411+saline treated rats (P<0.05, *). Vehicle+morphine rats displayed significant withdrawal behaviors compared to AV411+vehicle controls (P<0.001; ×××), n=10-11/group. B: Co-administration of minocycline (25mg/kg gavage twice daily) with morphine significantly reduced the sum of all scored naloxone precipitated opioid withdrawal behaviors compared to vehicle+morphine treated rats (P<0.01, **). n=6/group. Scored behavior and presentation are as in A, above.

Figure 2. Morphine-induced increases in microglial (CD11b expression) and astrocytic (GFAP expression) activation were significantly reduced by AV411 treatment

For simplicity, representative examples of the drug-induced changes in immunohistochemically-detected glial activation markers are illustrated from one brain region, chosen at random. Microglial CD11b expression from the dorsal periaqueductal gray of representative rats (A, B and C) following naloxone precipitated withdrawal in rats that received AV411+saline (A), AV411+morphine (B) and vehicle+morphine (C). Astrocytic GFAP expression from the dorsal periaqueductal gray of representative rats (D, E and F) following naloxone precipitated withdrawal in rats that received AV411+saline (D), AV411+morphine (E) and vehicle+morphine (F). Image acquisition was done using an Olympus XB61 microscope and NIH image. Scale bar=100μm.

Figure 3. AV411 prevents spontaneous morphine and oxycodone withdrawal induced weight loss

AV411 (7.5mg/kg twice daily) protects against morphine (A; ■) and oxycodone (B; ◆) induced weight loss when compared to morphine (■) or oxycodone (◆) plus vehicle treated rats. It appears that these weight loss effects are likely due to the opioid withdrawal, since AV411+saline rats do not show such weight loss (○). Data are expressed as the change in weight after the start of spontaneous withdrawal, compared to weight just prior to the initiation of spontaneous withdrawal. A two-way ANOVA with Bonferroni post hoc analysis revealed significant differences between AV411 and vehicle treated rats just one day after withdrawal that continued thought the rest of the observation (day 15 to day 26; P<0.05). n=6-9/group.

Figure 4. AV411 potentiates both morphine and oxycodone analgesia

AV411 (7.5mg/kg; ■) significantly potentiated morphine (A and C: 4mg/kg) and oxycodone (B and D: 4mg/kg) analgesia compared to vehicle treated rats (□). The EC50 of morphine+vehicle dose response (30min after opioid administration) was 4.1mg/kg whilst AV411 co-administration reduced this significantly to 0.77mg/kg (P<0.0001). The EC50 of oxycodone+vehicle dose response was 2.6mg/kg whilst AV411 co-administration reduced this significantly to 0.78mg/kg (P<0.001). n=6/group.