Microglial α7 nicotinic acetylcholine receptors drive a phospholipase C/IP₃ pathway and modulate the cell activation toward a neuroprotective role

Abstract

Microglia perform both neuroprotective and neurotoxic functions in the brain, with this depending on their state of activation and their release of mediators. Upon P2X(7) receptor stimulation, for example, microglia release small amounts of TNF, which protect neurons, whereas LPS causes massive TNF release leading to neuroinflammation. Here we report that, in rat primary cultured microglia, nicotine enhances P2X(7) receptor-mediated TNF release, whilst suppressing LPS-induced TNF release but without affecting TNF mRNA expression via activation of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs). In microglia, nicotine elicited a transient increase in intracellular Ca(2+) levels, which was abolished by specific blockers of alpha7 nAChRs. However, this response was independent of extracellular Ca(2+) and blocked by U73122, an inhibitor of phospholipase C (PLC), and xestospongin C, a blocker of the IP(3) receptor. Repeated experiments showed that currents were not detected in nicotine-stimulated microglia. Moreover, nicotine modulation of LPS-induced TNF release was also blocked by xestospongin C. Upon LPS stimulation, inhibition of TNF release by nicotine was associated with the suppression of JNK and p38 MAP kinase activation, which regulate the post-transcriptional steps of TNF synthesis. In contrast, nicotine did not alter any MAP kinase activation, but enhanced Ca(2+) response in P2X(7) receptor-activated microglia. In conclusion, microglial alpha7 nAChRs might drive a signaling process involving the activation of PLC and Ca(2+) release from intracellular Ca(2+) stores, rather than function as conventional ion channels. This novel alpha7 nAChR signal may be involved in the nicotine modification of microglia activation towards a neuroprotective role by suppressing the inflammatory state and strengthening the protective function.

Alpha7 nicotinic acetylcholine receptor: A link between inflammation and neurodegeneration

“…This anti-inflammatory potential of acetylcholine and nicotine is based on the inhibition of the NF-κB pathway through a specific nicotinic anti-inflammatory pathway' dependent on the alpha7nAChR (Wang et al, 2004). The inhibition of TNF production in LPS-stimulated microglial cultures is also associated with a reduction in the activation of ERK and p38MAPK (Shytle et al, 2004;Suzuki et al, 2006) (Fig. 2).…”

“…Nicotine induces anti-inflammatory mechanisms that diminish local inflammatory responses (Hellstrom-Lindahl et al, 2004a;Streit, 2002;Wang et al, 2000a, b). Among other, nicotine abrogates the production of TNF in culture of microglia through a mechanism dependent on ERK and p38 MAPK (Shytle et al, 2004;Suzuki et al, 2006). Experimental models of AD indicate that alpha7nAChR is the central core of the nicotine-mediated neuroprotection.…”

“…However, microglia can also be neurotoxic by producing free radicals, inflammatory cytokines and other toxic factors. Recent studies indicate that neurons and astrocytes can regulate innate immune responses in microglia via both alpha7nAChRs and purinergic P2X 7 receptors (Shytle et al, 2004;Suzuki et al, 2006). Primary cultures of both resting and actívate microglia and astrocytes show choline acetyltransferase (ChAT) activity and synthesize acetylcholine (De Rosa et al, 2005) suggesting that this neurotransmitter act as a local immune regulator and contribute to the regulation of microglial functions.…”

“…On the other hand, LPSstimulated microglia causes massive TNF production leading to inflammation (Suzuki et al, 2006). Acetylcholine and nicotine inhibit the production of TNF in LPS-stimulated mouse microglial cultures (De Rosa et al, 2005;Shytle et al, 2004).…”

“…2). Nicotine also acts as an anti-inflammatory agent on microglia by increasing the expression of COX-2 and the synthesis of prostaglandin PGE 2 , known to downregulate microglial activation and expression of proinflammatory genes including TNF (De Rosa et al, 2005;Suzuki et al, 2006). However, nicotine has no effect on the release of IL-1β or IL-10, but it down-regulates nitric oxide (NO) production .…”