Genistein’s Effects on Alpha7 nAChR Currents Depends upon Exposure Duration and Time Course of Expression in Xenopus Oocytes

Abstract

Several previous studies that explored tyrosine phosphorylation/dephosphorylation-involvement in functional upregulation of α7-nAChRs found that genistein, a broad-spectrum inhibitor of protein tyrosine kinases (PTKs), increased Ach-evoked α7-nAChR currents. Interpretation of genistein's effects as due to perturbation of tyrosine-phosphorylation processes is complicated, however, by subsequent findings that genistein is a strong positive allosteric modulator (PAM) of α7-nAChRs. Our study asked if genistein's enhancement of α7-nAChRs currents is entirely attributable to PAM activity, or if non-PAM effects contribute as well. We compared the effects of brief (1 min) vs. extended (24 h) exposure of oocytes expressing α7-nAChRs to genistein (100 μM), both 2 and 5 days following cRNA injections (2.0 ng). On day 2, the Ach-evoked (500 μM) peak currents for control cells, and cells exposed for 1 min, or 24 h, to genistein were (mean ± SEM, nA): 29.32 ± 12.10 (n=24), 130.95 ± 35.42 (n=22), and 374 ± 105.21 (n=8). On day 5, the comparable values were: 108.61 ± 14.85 (n=10), 310.81 ± 56.22 (n=9), and 605.00 ± 128.03 nA (n=11). On day 2, 1 min genistein caused a 4.5 fold-facilitation; 24 h exposure caused a 12.8 fold-facilitation. On day 5, 1 min genistein caused a 2.4 fold-facilitation; 24 h genistein caused a 5.6 fold-facilitation. Thus, genistein-enhancement of α7 currents varied inversely with the degree of functional α7-nAChR expression, being greater for day 2 than 5. Genistein's effect on desensitization of α7-nAChR currents was also separable from facilitation. On day 2, genistein increased peak current without affecting desensitization. On day 5, genistein increased peak current and slowed desensitization. Our results suggest that genistein's enhancement of α7-nAChR currents is not entirely due to PAM effects, but involves other mechanisms too, particularly on day 2.