Nicotine and other nicotinic acetylcholine receptor agonists have been shown to exert neuroprotective actions in vivo and in vitro by an as yet unknown mechanism. Even the identification of the subtype of nicotinic receptor(s) mediating this action has not been determined. In neural cell lines, the induction of cytoprotection often requires exposure to nicotine for up to 24 hr to produce a full protective effect. One phenomenon associated with chronic exposure of neural cells to nAChR agonists is the increased expression of nAChRs (upregulation), possibly as a response to desensitization. Because nicotinic receptors desensitize rapidly in the continuous presence of agonist, we investigated whether the neuroprotective actions produced by different nicotinic receptor agonists was related to their ability to induce nicotinic receptor upregulation. Differentiated PC12 cells were preincubated for 24 hr with various nAChR ligands, and the cells were subsequently deprived of both NGF and serum to induce cytotoxicity. Under control conditions cell viability was reduced to 66.5 +/- 5.4% of control by trophic factor withdrawal. For those cells pretreated with nicotine (1 nM-100 microM) cell viability increased from 74.2 +/- 1.5 to 97.3 +/- 4%. The neuroprotective action of nicotine was blocked by co-treatment with either 5 microM mecamylamine or 10 nM methyllycaconitine (MLA). The high potency blockade by MLA suggested that neuroprotection was mediated through the alpha7 nicotinic receptor subtype. For the seven agonists examined for neuroprotective activity, only nicotine was capable of evoking a near maximal (near 100% cell viability) neuroprotective action. The next most effective group included epibatidine, 4OHGTS-21, methycarbamylcholine, and 1,1-dimethyl-4-phenyl-piperazinium iodide. These least effective group included cytisine and tetraethylammonium. Incubation of differentiated PC12 cells with 10 microM nicotine increased the number of [(125)I]alpha bungarotoxin ([(125)I]alphaBGTbinding sites by 41% from 82.6 +/- 3.67 to 117 +/- 10.3 fmol/mg protein). Under similar conditions of incubation, the nicotinic receptor agonist cytisine (that was least effective in terms of neuroprotection) failed to increase the number of [(125)I]alphaBGT binding sites. Cells expressing increased levels of cell surface [(125)I]alphaBGT binding sites received added neuroprotective benefit from nicotine. Thus the induced upregulation of the alpha7 subtype of nicotinic receptors during chronic exposure to nicotine may be responsible for the drug's neuroprotective action.
Copyright 2001 Wiley-Liss, Inc.