Alpha7 nicotinic acetylcholine receptor (nAChR) silent agonists show little or no ionotropic activity in the absence of a positive allosteric modulator (PAM). However, they can induce metabotropic-like signal transduction, showing effectiveness in suppressing acute inflammation in mouse models. This activity is associated with desensitized states of the receptor. Our laboratories seek to define structure function relationships for these compounds. We express the receptor in Xenopus oocytes and use two-electrode voltage clamping to detect silent agonism by application of compounds at 30 mM (agonism ≤ 0.1 times ACh controls) and co-application of 30 mM compound with 10 mM PNU-120596, a tool to reveal the PAM-sensitive desensitized (Ds) state of the receptor by rendering it conductive. The lead silent compound diEPP (N,N-diethyl-N’-phenylpiperazine) showed virtually no agonism but PNU co-application enhanced responses (0.002 ± 0.003 vs. 1.3 ± 0.3, respectively relative to responses evoked by control applications of 60 µM ACh). It was successfully modified to generate more effective silent derivatives. The p-CF3-diEPP displayed one of the best silent profiles (0.032 ± 0.003; 61.8 ± 7.7, relative to ACh controls), by strongly inducing the Ds state. A 3-fold decrease in silent activity observed for p-F-diEPP (0.010 ± 0.002; 22.9 ± 5.8) was ascribed to greater fluorine bond contribution for the CF3 group in the ligand binding pocket. However, p-SF5-diEPP retained the silent profile (0.055 ± 0.013; 1.94 ± 0.52), preferentially favoring the alpha7 PAM insensitive desensitized state (Di) over Ds. Replacement of the N,N-diethyl group of diEPP with a spirocycle derived from 1,5-diiodopentane produced a potent alpha7 silent compound (0.00 ± 0.06; 23.9 ± 2.7). However the N-spirocycle substituent and p-CF3 group were not synergetic, as including these two groups in the same phenyl piperazine moiety resulted in conversion to a partial agonist, with loss of the silent profile (0.14 ± 0.01; 6.27 ± 1.66), suggesting the possibility of different binding modes for the disubstituted compound relative to its parents.

Structure function studies of silent agonists of the alpha 7 nicotinic acetylcholine receptor

253rd National Meeting (2017)

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