Effects of β-Funaltrexamine on lipopolysaccharide-induced chemokine expression and sickness behavior in mice

Background: Neurological conditions including infection, neurodegeneration, and psychiatric disorders, involve neuroinflammation. Yet, most of the medications used to treat these disorders are not considered potently anti-inflammatory. We previously discovered that beta-funaltrexamine (β-FNA), a selective mu-opioid receptor (MOR) antagonist, inhibits inflammatory signaling in vitro in human astroglial cells. Notably, the mechanism of action does not seem to involve actions at the MOR. We also determined that β-FNA inhibited bacterial lipopolysaccharide (LPS)-induced sickness behavior and neuroinflammation in mice. In these initial in vivo experiments, mice were administered β-FNA, immediately followed by LPS administration, then assessed 24 h later. In the present study, we examined the extent to which β-FNA is protective when treatment occurs several hours after LPS administration. Also, we investigated the protective effects at an earlier time point (8 h) after LPS administration, when sickness behavior is more pronounced. Lastly, previous in vitro findings indicated that naltrexone (a non-selective opioid receptor antagonist from which β-FNA is derived) does not inhibit inflammatory signaling in astrocytes; thus, we questioned if it would also be ineffective in vivo.

Methods: In the current study, male C57BL/6J mice were administered LPS (0.83 mg/kg, i.p.) followed by treatment with β-FNA (50 mg/kg, i.p.) at 4 and/or 10 h post-LPS, depending on the experiment. At 8 or 24 h post-LPS, sickness behavior was assessed using a 10-min open-field test, followed by termination and collection of plasma and brain. Levels of inflammatory chemokines (interferon γ-induced protein, CXCL10; and monocyte chemotactic protein 1, CCL2) in tissues were measured using an enzyme-linked immunosorbent assay.

Results: Two-way analysis of variance revealed that at 24 and 8 hours, LPS increased CCL2 and CXCL10 in the brain and plasma. β-FNA treatment was protective depending on the dosing schedule, and Naltrexone did not significantly reduce inflammation.

Conclusions: β-FNA treatment 4 or 10 h after LPS administration inhibited CCL2 levels in plasma and brain (but did not significantly affect CXCL10 levels), and it failed to reduce sickness behavior. Conversely, β-FNA treatment within minutes of LPS treatment resulted in attenuation of both sickness behavior and chemokine expression. Delayed β-FNA treatment was not protective against LPS-induced behavioral deficits or inflammation 8 h post-LPS. Interestingly, naltrexone did not significantly reduce LPS-induced CCL2 and CXCL10 levels or sickness behavior. These results suggest that the timing of β-FNA treatment is critical for neuroprotection and supports earlier findings that the protective effects of βFNA likely involve mechanisms beyond actions at MOR. However, further examination of the anti-inflammatory and neuroprotective effects of β-FNA is necessary.