Nicotine inhibits microglial proliferation and is neuroprotective in global ischemia rats

Ischemic injury in rodent models reliably leads to the activation of microglia, which might play a detrimental role in neuronal survival. Our preliminary studies suggest that nicotine plays a potential role in decreasing the numbers of cultured microglia in vitro. In the present study, we found treatment with nicotine 2, 6, and 12 h after ischemia for 7 days significantly increased the survival of CA1 pyramidal neurons in ischemia/reperfusion rats. This effect was accompanied by a significant reduction in the increase of microglia rather than astrocytes, as well as a significant reduction of enhanced expression of tumor necrosis factor-alpha (TNF-α) and interleukin-1beta (IL-1β) in CA1 induced by ischemia/reperfusion. Nicotine inhibits microglial proliferation in primary cultures with and without the stimulation of granulocyte-macrophage colony-stimulating factor (GM-CSF). Pre-treatment with α-bungarotoxin, a selective α7 nicotinic acetylcholine receptor (α7 nAChR) antagonist, could prevent the inhibitory effects of nicotine on cultured microglial proliferation suggesting that nicotine inhibits the microglial proliferation in an α7 nAChR-dependent fashion. Our results suggest that nicotine inhibits the inflammation mediated by microglia via α7 nAChR and is neuroprotective against ischemic stroke, even when administered 12 h after the insult. α7 nAChR agonists may have uses as anti-ischemic compounds in humans.