Involvement of astrocytic endothelin-1 in neuropathic pain processing: a pain-behavioural and gene expressionprofiling study

Abstract

Neuropathic pain is becoming a disease of global burden with growing prevalence worldwide. It is difficult to treat and there is no assured cure for it so far. Patients have to live with it relying on combinations of drug treatments to ameliorate the effect. The cause of neuropathic pain is often considered as a dysfunction of the nervous system which involves both peripheral and central nervous system sensitization. However, the mechanism behind it remains poorly understood at the molecular level. In my thesis, I have investigated Endothelin-1 (ET-1), a potent vasoconstrictor and neuro-modulator, in the central pain-process in neuropathic pain. ET-1 induction has been reported in a variety of pathological states such as cancer, ischaemia, and neuropathic pain. The effect of central endogenous ET-1 in development of neuropathic pain has not been adequately studied. Therefore, in my first study, I evaluated the influence of endogenous ET-1 in neuropathic pain by antagonizing Endothelin Receptor type A (ETAR), which is documented as the receptor that ET-1 acts on to induce pain, in the CNS. BQ-123, an ETAR selective antagonist, was administered intrathecally to study the effect of central ET-1 in neuropathic pain induced by Peripheral Sciatic Ligation (PSL) in Sprague Dawley (SD) rat. I found that repeated administration of BQ-123 alleviated mechanical allodynia, which developed after PSL as a typical symptom of neuropathic pain. Thus, ET-1 and ETAR may involve in PSL- induced neuropathic pain.

To the best of our knowledge, exogenous administration of ET-1 in the CNS has been shown to exert a pain-inhibiting effect in thermal and inflammatory pain, but the mechanism and the cellular origin of such pain-inhibiting nature remains elusive. Hence, I further investigated the effect of central ET-1 in neuropathic pain. Here, transgenic mouse overexpressing ET-1 in astrocytes (GET-1) was used as a model. I found that GET-1 mice exhibited 15-fold of ET-1 mRNA induction in the spinal cord by real-time PCR. However, GET-1 transgenic mice did not show altered mechanical threshold compared to non-transgenic (Ntg) mice at basal level under physiological condition. After PSL, I found that GET-1 transgenic mice did not show signs of neuropathic pain while age-matched Ntg mice had mechanical allodynia. High expression of ETAR observed only in Ntg mice after PSL supports the pain-alleviating effect of ETAR antagonist shown in our first study. Moreover, I explored the relationship between ET-1 and glial glutamate transporter EAAT2 which is responsible for major clearance of glutamate in the CNS. Interestingly, high expression level of EAAT2 was observed distinctively in GET-1 transgenic mice which did not develop PSL-induced neuropathic pain. Thus, EAAT2 may be the key factor in neuron protection from central sensitization by enhanced glutamate release in induction of neuropathic pain.

These results suggest that endogenous central ET-1 may mediate neuropathic pain partially via ETAR. Taken together with the evidence that GET-1 did not develop neuropathic pain and showed high expression of EAAT2, I concluded that overexpressed astrocytic ET-1 in GET-1 mice exerted an analgesic effect in neuropathic pain by modulating expression of EAAT2.