Type I and II positive allosteric modulators differentially modulate agonist-induced up-regulation of a7 nicotinic acetylcholine receptors

Long-term treatment with nicotine or selective a7 nicotinic acetylcholine receptor (nAChR) agonists increases the number of a7 nAChRs and this up-regulation may be involved in the mechanism underlying the sustained procognitive effect of these compounds. Here, we investigate the influence of type I and II a7 nAChR positive allosteric modulators (PAMs) on agonist-induced a7 nAChR up-regulation. We show that the type II PAMs, PNU-120596 (10 mu M) or TQS (1 and 10 mu M), inhibit up-regulation, as measured by protein levels, induced by the a7 nAChR agonist A-582941 (10 nM or 10 mu M), in SH-EP1 cells stably expressing human a7 nAChR, whereas the type I PAMs AVL-3288 or NS1738 do not. Contrarily, neither type I nor II PAMs affect 10 mu M nicotine-induced receptor up-regulation, suggesting that nicotine and A-582941 induce up-regulation through different mechanisms. We further show in vivo that 3 mg/kg PNU-120596 inhibits up-regulation of the a7 nAChR induced by 10 mg/kg A-582941, as measured by [125I]-bungarotoxin autoradiography, whereas 1 mg/kg AVL-3288 does not. Given that type II PAMs decrease desensitization of the receptor, whereas type I PAMs do not, these results suggest that receptor desensitization is involved in A-582941-induced up-regulation. Our results are the first to show an in vivo difference between type I and II a7 nAChR PAMs, and demonstrate an agonist-dependent effect of type II PAMs occurring on a much longer time scale than previously appreciated. Furthermore, our data suggest that nicotine and A-582941 induce up-regulation through different mechanisms, and that this confers differential sensitivity to the effects of a7 nAChR PAMs. These results may have implications for the clinical development of a7 nAChR PAMs.