Nicotinic acetylcholine receptors in mouse and rat optic nerves

Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by selectively staining the axonal population with a calcium indicator. Nicotine (1-50 muM) induced an axonal calcium elevation that was eliminated when calcium was removed from the bath, suggesting that nicotine induces calcium influx into axons. The nicotine response was blocked by d-tubocurarine and mecamylamine but not alpha-bungarotoxin, indicating the presence of calcium permeable, non-alpha7 nicotinic acetylcholine receptor ( nAChR) subtype. Agonist efficacy order for eliciting the axonal nAChR calcium response was cytisine similar to nicotine >> acetylcholine. The nicotine-mediated calcium response was attenuated during the process of normal myelination, decreasing by approximately 10-fold from P1 (premyelinated) to P30 (myelinated). Nicotine also caused a rapid reduction in the compound action potential in neonatal optic nerves, consistent with a shunting of the membrane after opening of the nonspecific cationic nicotinic channels. Voltage-gated calcium channels contributed little to the axonal calcium elevation during nAChR activation. During repetitive stimulations, the compound action potential in neonatal mouse optic nerves underwent a gradual reduction in amplitude that could be partially prevented by d-tubocurarine, suggesting an activity-dependent release of acetylcholine that activates axonal AChRs. We conclude that mammalian optic nerve axons express nAChRs and suggest that these receptors are activated in an activity-dependent fashion during optic nerve development to modulate axon excitability and biology.