Attenuation of the Infiltration of Angiotensin II Expressing CD3+ T-Cells and the Modulation of Nerve Growth Factor in Lumbar Dorsal Root Ganglia – A Possible Mechanism Underpinning Analgesia Produced by EMA300, An Angiotensin II Type 2 (AT2) Receptor Antagonist

Abstract

Recent preclinical and proof-of-concept clinical studies have shown promising analgesic efficacy of selective small molecule angiotensin II type 2 (AT2) receptor antagonists in the alleviation of peripheral neuropathic pain. However, their cellular and molecular mechanism of action requires further investigation. To address this issue, groups of adult male Sprague–Dawley rats with fully developed unilateral hindpaw hypersensitivity, following chronic constriction injury (CCI) of the sciatic nerve, received a single intraperitoneal bolus dose of the small molecule AT2 receptor antagonist, EMA300 (10 mg kg-1), or vehicle. At the time of peak EMA300-mediated analgesia (∼1 h post-dosing), groups of CCI-rats administered either EMA300 or vehicle were euthanized. A separate group of rats that underwent sham surgery were also included. The lumbar (L4–L6) dorsal root ganglia (DRGs) were obtained from all experimental cohorts and processed for immunohistochemistry and western blot studies. In vehicle treated CCI-rats, there was a significant increase in the expression levels of angiotensin II (Ang II), but not the AT2 receptor, in the ipsilateral lumbar DRGs. The elevated levels of Ang II in the ipsilateral lumbar DRGs of CCI-rats were at least in part contributed by CD3+ T-cells, satellite glial cells (SGCs) and subsets of neurons. Our findings suggest that the analgesic effect of EMA300 in CCI-rats involves multimodal actions that appear to be mediated at least in part by a significant reduction in the otherwise increased expression levels of Ang II as well as the number of Ang II-expressing CD3+ T-cells in the ipsilateral lumbar DRGs of CCI-rats. Additionally, the acute anti-allodynic effects of EMA300 in CCI-rats were accompanied by rescue of the otherwise decreased expression of mature nerve growth factor (NGF) in the ipsilateral lumbar DRGs of CCI-rats. In contrast, the increased expression levels of TrkA and glial fibrillary acidic protein in the ipsilateral lumbar DRGs of vehicle-treated CCI-rats were not attenuated by a single bolus dose of EMA300. Consistent with our previous findings, there was also a significant decrease in the augmented levels of the downstream mediators of Ang II/AT2 receptor signaling, i.e., phosphorylated-p38 mitogen-activated protein kinase (MAPK) and phosphorylated-p44/p42 MAPK, in the ipsilateral lumbar DRGs.

Angiotensin Type 2 Receptors: Painful, or Not?

“…A more recent study suggests a nociceptive action of AT 2 R. In that study, it was demonstrated that another AT 2 R antagonist, EMA300, inhibits peripheral neuropathic pain, again not via effects on neuronal AT 2 R function (Khan et al, 2017). EMA 300 and 401 are small compounds that have high selectivity for the AT 2 R (>1000-fold binding selectivity for the AT 2 R over FIGURE 2 | Molecular mechanisms AT 2 R-mediated nociception (A).…”

Lack of association between angiotensin-converting enzyme inhibitors and angiotensin receptor blockers and pain improvement in patients with oral cancer

“…Previous studies indicate that ACEis and ARBs modulate important signalling mechanisms involved in inflammatory and neuropathic pain [4][5][6][7]. Pain is one of the most common complaints of patients with oral malignancies and can be present even before any cancer treatment has been initiated [8].…”

“…Ye et al [ 13 ] demonstrated in mouse models that NGF is implicated in changes in the expression of the transient vanilloid receptor 1 (TRPV1) in trigeminal ganglion cells, suggesting an interaction between NGF and TRPV1. The expression of NGF in the neurons is regulated by the angiotensin II (AT2) receptor which, in turn, can modulate the function of TRPV1 [ 6 ]. Moreover, the antagonism of the AT2 receptor also decreases the infiltration of CD3+ T cells and macrophages in the dorsal root ganglion, which correlates with a reduction in allodynia in animals with neuropathic pain [ 6 , 14 , 15 ].…”

“…The expression of NGF in the neurons is regulated by the angiotensin II (AT2) receptor which, in turn, can modulate the function of TRPV1 [ 6 ]. Moreover, the antagonism of the AT2 receptor also decreases the infiltration of CD3+ T cells and macrophages in the dorsal root ganglion, which correlates with a reduction in allodynia in animals with neuropathic pain [ 6 , 14 , 15 ]. Therefore, it appears that the AT2/NGF/TRPV1 interaction is an important mechanism in mediating nociception and inflammation.…”

Lack of association between angiotensin converting enzyme inhibitors and angiotensin receptor blockers and pain improvement in patients with oral cancer

“…[13] The expression of NGF in the neurons is regulated by the angiotensin II (AT2) receptor which in turn can modulate the function of TRPV1. [6] Moreover, the antagonism of the AT2 receptor also decreases the infiltration of CD3+ T cells and macrophages in the dorsal root ganglion (DRG), which correlates with a reduction in allodynia in animals with neuropathic pain. [6, 14, 15] Therefore, it appears that the AT2/NGF/TRPV1 interaction is an important mechanism in mediating nociception and inflammation.…”