Endothelin-1 over-expression in astrocyte attenuates peripheral neuropathic pain by up-regulating the expression of astroglial glutamate transporter type-1 lumbar spinal cord

Abstract

OBJECTIVES: The astroglial glutamate transporter type 1 (EAAT2) is responsible for the rapid clearance of glutamate from synaptic cleft in the spinal cord. In spite of its importance in modulating both physiological and pathological pain, endogenous modulator has not been identified yet. Recently, a beneficial role of central endothelin-1 (ET-1) has been demonstrated in a transgenic mouse model targeting ET-1 over-expression in astrocytes (GET-1) in pathological pain. The present study aimed to investigate whether astrocytic ET-1 serves as an endogenous modulator of glutamate transporters in neuropathic pain. METHODS: Partial ligation of the sciatic nerve in male homozygous GET-1 and non-transgenic mice was employed as a neuropathic pain model. All animals were tested for thermal hyperalgesia/allodynia and mechanical allodynia before and after four days administration of EAAT2 agonist ceftriaxone (CEF) at 200mg/kg/day. C6 rat astrocyte cells with/without ET-1 over-expression were in-house derived and treated with ET-1 receptor antagonists BQ-123 and BQ-788 at 10ng/ml. RNA and protein were extracted from the L4-L6 spinal cord of mice and cells for real-time PCR and western blot analysis. RESULTS: GET-1 mice exhibited less neuropathic pain and CEF produced synergestic effect in pain suppression. Both mRNA and protein expression of EAAT2 were significantly up-regulated in the spinal L4-L6 regions in sham GET-1 mice. Furthermore, over-expressed ET-1 in C6 cells up-regulated both mRNA and protein expression of EAAT2 which could be partially reversed by ET-1 receptor antagonists. CONCLUSION: Taken together, these results indicate that the astrocytic ET-1 may exert pain suppression by modulating spinal EAAT2 expression and its glutamate clearance activity.