Activation of α7 nicotinic acetylcholine receptor by nicotine selectively up-regulates cyclooxygenase-2 and prostaglandin E2 in rat microglial cultures

Background: Nicotinic acetylcholine (Ach) receptors are ligand-gated pentameric ion channels whose main function is to transmit signals for the neurotransmitter Ach in peripheral and central nervous system. However, the alpha 7 nicotinic receptor has been recently found in several non-neuronal cells and described as an important regulator of cellular function. Nicotine and ACh have been recently reported to inhibit tumor necrosis factor-alpha (TNF-alpha) production in human macrophages as well as in mouse microglial cultures. In the present study, we investigated whether the stimulation of alpha 7 nicotinic receptor by the specific agonist nicotine could affect the functional state of activated microglia by promoting and/or inhibiting the release of other important pro-inflammatory and lipid mediator such as prostaglandin E-2. Methods: Expression of alpha 7 nicotinic receptor in rat microglial cell was examined by RT-PCR, immunofluorescence staining and Western blot. The functional effects of alpha 7 receptor activation were analyzed in resting or lipopolysaccharide (LPS) stimulated microglial cells pre-treated with nicotine. Culture media were assayed for the levels of tumor necrosis factor, interleukin-1 beta, nitric oxide, interleukin-10 and prostaglandin E-2. Total RNA was assayed by RT-PCR for the expression of COX-2 mRNA. Results: Rat microglial cells express alpha 7 nicotinic receptor, and its activation by nicotine dose-dependently reduces the LPS-induced release of TNF-alpha, but has little or no effect on nitric oxide, interleukin-10 and interleukin-1 beta. By contrast, nicotine enhances the expression of cyclooxygenase-2 and the synthesis of one of its major products, prostaglandin E-2. Conclusions: Since prostaglandin E-2 modulates several macrophage and lymphocyte functions, which are instrumental for inflammatory resolution, our study further supports the existence of a brain cholinergic anti-inflammatory pathway mediated by alpha 7 nicotinic receptor that could be potentially exploited for novel treatments of several neuropathologies in which local inflammation, sustained by activated microglia, plays a crucial role.