Enkephalin Treatment Reduces Elevated Brain Glia in Mice With Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is a chronic, autoimmune disorder that culminates in severe motor and cognitive deficits. The progression of the disease is associated with inflammation and demyelination of the brain and spinal cord. Activation and proliferation of glia, including astrocytes and oligodendrocytes, in the brain are hallmarks. Persons with MS (PwMS) have significantly low levels of the neuropeptide [Met⁵]-enkephalin (Met⁵-Enk), also termed Opioid Growth Factor (OGF) that is an inhibitory growth factor and modulates cellular homeostasis. Treatment with low doses of naltrexone, an opioid antagonist that blocks the receptor OGFr, is reported to decrease inflammation, neutralize pro-inflammatory cytokine secretion, and stabilize disease progression. Intermittent blockade of OGFr with low doses of naltrexone stimulates serum enkephalin levels. In this investigation, brains of mice with experimental autoimmune encephalomyelitis (EAE) were treated with low dose naltrexone (LDN) or OGF/Met-Enk and examined for reactive astrocytes and oligodendrocytes.

Female C57BL/6J mice were inoculated with myelin oligodendrocyte glycoprotein (MOG_35-55) to induce EAE. At the initial appearance of clinical behavioral symptoms, mice were randomized to receive 10 mg/kg [Met⁵]-enkephalin (OGF/Met-Enk), 0.1 mg/kg naltrexone (LDN), or 0.1 mL phosphate-buffered saline (PBS) daily. At 35 days post EAE induction, mice were euthanized, and the brains were removed and prepared for immunohistochemistry. The corpus callosum, subventricular zone, and dentate gyrus were stained with glial fibrillary acidic protein (GFAP) and adenomatous polyposis coli protein (APC) antibodies to detect astrocytes and oligodendrocytes, respectively.

The number of reactive astroglia in EAE brains was elevated relative to the number in normal mouse brains. Daily treatment with OGF/Met-Enk reduced GFAP-positive cells in all 3 brain regions, and LDN reduced astrocyte number in the corpus callosum and subventricular zone. Astrocytes in treated EAE mice had process lengths that were comparable with those in normal mice, suggesting less reactivity. The numbers of oligodendrocytes in the corpus callosum of EAE+LDN mice were significantly reduced relative to EAE+PBS tissues.

These results demonstrate quantitative changes in reactive glia in EAE mouse brains following treatment with OGF/Met-Enk and LDN and corroborate other studies showing that OGF/Met-Enk and LDN treatment increase plasma enkephalins, reduce inflammation and decrease pro-inflammatory cytokines.