Role of α7- and α4β2-nAChRs in the neuroprotective effect of nicotine in stress-induced impairment of hippocampus-dependent memory

We have previously shown that nicotine prevents stress-induced memory impairment. In this study, we have investigated the role of a7- and alpha 4 beta 2-nicotinic acetylcholine receptors (nAChRs) in the protective effect of nicotine during chronic stress conditions. Chronic psychosocial stress was induced using a form of rat intruder model. During stress, specific antagonist for either a7- nAChRs [ methyllycaconitine (MLA)] or alpha 4 beta 2-nAChRs [dihydro-beta-erythroidine (DHbE)] was infused into the hippocampus using a 4-wk osmotic pump at a rate of 82 mu g/side.d and 41 mu g/side.d, respectively. Three weeks after the start of infusion, all rats were subjected to a series of cognitive tests in the radial arm water maze (RAWM) for six consecutive days or until the animal reached days to criterion (DTC) in the fourth acquisition trial and in all memory tests. DTC is defined as the number of days the animal takes to make no more than one error in three consecutive days. In the short-term memory test, MLA-infused stressed/nicotine-treated rats made similar errors to those of stress and significantly more errors compared to those of stress/nicotine, nicotine or control groups. This finding was supported by the DTC values for the short memory tests. Thus, MLA treatment blocked the neuroprotective effect of nicotine during chronic stress. In contrast, DHbE infusion did not affect the RAWM performance of stress/nicotine animals. These results strongly suggest the involvement of alpha 7-nAChRs, but not alpha 4 beta 2-nAChRs, in the neuroprotective effect of chronic nicotine treatment during chronic stress conditions.