Over-expression of astrocytic ET-1 attenuates neuropathic pain by inhibition of ERK1/2 and Akt(s) via activation of ETA receptor.

A differential role of endothelin-1 (ET-1) in pain processing has recently been suggested. However, the function of central ET-1 in neuropathic pain (NP) has not been fully elucidated to date. We report here the action of endogenous central ET-1 in sciatic nerve ligation-induced NP (SNL-NP) in a transgenic animal model that over-expresses ET-1 in the astrocytes (GET-1 mice). We hypothesized that the over-expression of astrocytic ET-1 would exert anti-allodynic and anti-hyperalgesic effects in NP, as demonstrated by mechanical threshold and plantar withdrawal latency using the von Frey filament and heat stimuli. In our animal model, GET-1 mice showed an increase in the withdrawal threshold and latency in response to the mechanical and thermal stimuli, respectively, in pain behavior tests after SNL. ET-1 and endothelin type A receptor (ETA-R) levels were increased significantly in L4-L6 segments of the spinal cord (ipsilateral to SNL) of GET-1 mice at 7 and 21days after surgery. Moreover, intrathecal administration of a specific ETA-R antagonist, BQ-123, attenuated the anti-allodynic and anti-hyperalgesic phenotype in GET-1 mice. The effects of BQ-123 on the mRNA expression of extracellular signal-regulated protein kinase 1/2 (ERK1/2) and protein kinase B/serine protein kinase (Akt(s)) were assessed in the ipsilateral L4-L6 segments harvested 30min after BQ-123 administration on day 7 after surgery. Phosphorylation of ERK1/2 and Akt(s) in the ipsilateral spinal cord of GET-1 mice was reduced following SNL, whereas no reduction was observed after intrathecal injection of BQ-123. In conclusion, our results showed that the xover-expression of astrocytic ET-1 reduced SNL-induced allodynia and hyperalgesia by inhibiting the activation of ERK1/2 and Akt(s) via the ETA-R-mediated pathway.

Targeted Overexpression of Astrocytic Endothelin-1 Attenuates Neuropathic Pain by Upregulating Spinal Excitatory Amino Acid Transporter-2.

We previously demonstrated that endogenous endothelin-1 (ET-1) inhibits pathological pain in a transgenic mouse model with astrocyte-specific ET-1 overexpression (GET-1 mice); however, the underlying mechanism is unclear. ET-1 regulates excitatory amino acid transporter-2 (EAAT-2), a predominant subtype of glutamate transporters that plays critical role in pain modulation in spinal astrocytes. We hypothesized that astrocytic ET-1 overexpression alleviates neuropathic pain through modulating EAAT-2. GET-1 or nontransgenic (NTg) mice either received sham operation or sciatic nerve ligation (SNL) with or without ceftriaxone (CEF, an EAAT-2 inducer, for 4 days before termination). In GET-1 mice, mRNA and protein expressions of EAAT-2, but not EAAT-1, were upregulated associated with reduced SNL-induced neuropathic pain. Despite that SNL induced a significant reduction of EAAT-2 mRNA expression in both genotypes of mice, post-SNL EAAT-2 mRNA expression was higher in GET-1 mice than that in NTg mice. EAAT-2 induction by CEF reduced SNL-induced neuropathic pain in both NTg and GET-1 mice. In cultured rat astrocytic cell line, overexpression of ET-1 mRNA expression also elevated EAAT-2 mRNA expression, which was reversed by ET receptor antagonists. In conclusion, overexpressed astrocytic ET-1 suppressed neuropathic pain by upregulating spinal EAAT-2 expression via ET receptors.

Effects of repeated central administration of endothelin type A receptor antagonist on the development of neuropathic pain in rats.

Endothelin-1 (ET-1) predominates in the endothelin family effectively in vascular tone control, mitogenesis, and neuromodulation. Its receptors are widespread in the central nervous system (CNS) associated with endogenous pain control, suggesting an important role of ET-1 in central pain processing. This study aimed to evaluate the effect of central ET-1 on the development of neuropathic pain behaviour by repeated intrathecal administration of endothelin type A receptor (ETAR) antagonist (BQ-123) in a sciatic nerve ligation (SNL) animal model. BQ-123 was administered intrathecally to rats at dosages 15 µg, 20 µg, 25 µg, and 30 µg, daily for 3 days. Mechanical allodynia was assessed daily 30 minutes before/after injection, 1 hour after injection of BQ-123 from post-SNL day 4 to day 6, and once on day 7 (without BQ-123 administration) before rats were sacrificed. Increasing trends of mechanical threshold were observed, and they reached significance at all dosages on post-SNL day 7 (P < 0.05 at dosage 15 µg and P < 0.001 at dosages 20 µg, 25 µg, and 30 µg) in comparison to control group. BQ-123 at dosage 30 µg showed the most stable and significant mechanical threshold rise. Repeated central administration of BQ-123 alleviated mechanical allodynia after SNL. Our results provide insight into the therapeutic strategies, including timing, against neuropathic pain development with ETAR antagonist.

Over-expression of endothelin-1 in astrocytes, but not endothelial cells, ameliorates inflammatory pain response after formalin injection.

AIMS: Endothelin-1 (ET-1) has been suggested to be involved in different types of pain due to its neuromodulatory nature. However, its role in inflammatory pain processing, specifically the origin-specific effect, has not yet been clearly defined. Therefore, the aim of this study is to determine the role of cell-type specific ET-1 induction in the modulation of inflammatory pain processing. MAIN METHODS: The current study assesses the effects of ET-1 over-expression specifically targeted to astrocytes (GET-1) or endothelial cells (TET-1) on the expression of pain-like behaviors induced by a model of inflammatory pain, consisting of a formalin injection into the hind paw. KEY FINDINGS: The baseline sensitivity thresholds of GET-1 and TET-1 mice to the response elicited by tactile and radiant heat stimulation were similar to those observed in age-matched non-transgenic (NTg) controls. Relative to the NTg controls, GET-1 mice displayed a marked decrease in pain-like behavioral responses during the second phase of formalin-induced pain (i.e., 15-20 min after injection), whereas the responses elicited in TET-1 mice were unaltered. The levels of mRNA encoding adrenomedullin, calcitonin gene-related peptide and calcitonin-like receptor were elevated in the spinal cord of saline-injected GET-1 mice compared to those of NTg mice. SIGNIFICANCE: The current results support a suppressor role for astrocyte-derived ET-1 in inflammatory pain and suggest that the study of GET-1 mice might provide mechanistic insights for improving the treatment of inflammatory pain.