The Antinociceptive and Anti-Inflammatory Properties of the α7 nAChR Weak Partial Agonist p-CF₃N,N-diethyl-N′-phenylpiperazine

Abstract

Chronic pain and inflammatory diseases can be regulated by complex mechanisms involving 7 nicotinic acetylcholine receptors (nAChRs), making this subtype a promising drug target for anti-inflammatory therapies. Recent evidence suggests that suchtreatment of inflammatory pain may rely on metabotropic-like rather than ionotropic activation of the7 receptor subtype in non-neuronal cells. We previously identified para-trifluoromethyl (-CF) ,-diethyl-'-phenylpiperazinium (diEPP) iodide to be among the compounds classified as silent agonists, which are very weak 7 partial agonists that are able to induce positive allosteric modulator (PAM)-sensitive desensitization. Such drugs have been shown to selectively promote7 ionotropic-independent functions. Therefore, we here further investigated the electrophysiological profile of -CF diEPP and its in vivo antinociceptive activity using oocytes expressing7, 42, or 34 nAChRs. The evoked currents confirmed -CF diEPP to be 7-selective with a maximal agonism 5% that of acetylcholine (ACh). Coapplication of-CF diEPP with the type II PAM 4-naphthalene-1-yl-3a,4,5,9b-tetrahydro-3--cyclopenta[c]quinoline-8-sulfonic acid amide (TQS) produced desensitization that could be converted to PAM-potentiated currents, which at a negative holding potential were up to 13-fold greater than ACh controls. Voltage-dependence experiments indicated that channel block may limit both control ACh and TQS-potentiated responses. Although no -CF diEPP agonist activity was detected for the heteromeric nAChRs, it was a noncompetitive antagonist of these receptors. The compound displayed remarkable antihyperalgesic and antiedema effects in in vivo assays. The antinociceptive activity was dose and time dependent. The anti-inflammatory components were sensitive to the 7-selective antagonist methyllycaconitine, which supports the idea that these effects are mediated by the7 nAChR.

A silent agonist of α7 nicotinic acetylcholine receptors modulates inflammation ex vivo and attenuates EAE

“…In addition, m-bromo PEP significantly reduced paw edema (, with mice treated with 10 mg/kg dose differing from the vehicle group. These results therefore corroborate previously published data demonstrating that p-CF 3 diEPP, a structurally similar α7 nAChR silent agonist, protects against inflammatory pain [ 30 ].…”

“…We then confirm our findings in vivo, by showing that both mecamylamine and m-bromo PEP significantly delay the onset and reduce the severity of EAE, while lowering the numbers of pro-inflammatory cells in the CNS in conjunction with altered chemokine, integrin and metalloprotease expression within the brain. Finally, we confirm previous studies showing that α7 nAChR silent agonism also reduces inflammatory pain [ 27 , 30 ]. Taken together, our results support the hypothesis that silent agonists of α7 nAChRs modulate pro-inflammatory responses, likely by inducing a non-conductive receptor state rather than via channel opening.…”

“…It was recently shown that p-CF 3 diEPP, another phenylpiperazine-based silent agonist, reduces inflammatory pain [ 30 ]. We thus confirmed whether m-bromo PEP would similarly inhibit inflammation in the same animal model for inflammatory pain.…”

“…Recently, attention has been drawn to a new class of drugs, termed silent agonists [ 25 – 29 ], which produce very little channel activation (have < 10% efficacy compared to ACh) but are profoundly desensitizing [ 27 ]. Indeed, two silent agonists have been shown to be very effective with in vivo models of neuropathic and inflammatory pain [ 27 , 30 ], supporting the idea that the desensitized state is responsible for the anti-inflammatory roles of the α7 nAChR.…”

“…These studies also demonstrated that nicotine inhibits leukocyte entry into the CNS of EAE mice and shifts the cytokine secretion profile away from the M1/Th1/Th17 response. In light of the recent publications demonstrating that α7 nAChR silent agonists reduce inflammatory pain [ 27 , 30 ], the goal of the present study was to test the hypothesis that a silent agonist of the α7 nAChR would reduce bone marrow-derived monocyte/macrophage (BMDM) numbers and protect against EAE. Using a variety of α7 nAChR-selective and non-selective ligands, such as agonists, antagonists, as well as a α7-selective silent agonist 1-ethyl-4-(3-(bromo)phenyl)piperazine (m-bromo PEP) [ 29 ], we first provide evidence that α7 nAChR-dependent effects on BMDM numbers, phenotype and cytokine production ex vivo are likely due to non-conducting channel conformations.…”