Nerve growth factor rapidly induces prolonged acetylcholine release from cultured basal forebrain neurons: Differentiation between neuromodulatory and neurotrophic influences

Abstract

Long-term exposure to nerve growth factor (NGF) is well established to have neurotrophic effects on basal forebrain cholinergic neurons, but its potential actions as a fast-acting neuromodulator are not as well understood. We report that NGF (0.1-100 ng/ml) rapidly (<60 min) and robustly enhanced constitutive acetylcholine (ACh) release (148-384% of control) from basal forebrain cultures without immediate persistent increases in choline acetyltransferase activity. More ACh was released in response to NGF when exposure was coupled with a higher depolarization level, suggesting activity dependence. In a long-term potentiation-like manner, brief NGF exposure (10 ng/ml; 60 min) induced robust and prolonged increases in ACh release, a capacity that was shared with the other neurotrophins. K252a (10-100 nM), BAPTA-AM (25 μM), and Cd2+ (200 μM) prevented NGF enhancement of ACh release, suggesting the involvement of TrkA receptors, Ca2+, and voltage-gated Ca2+ channels, respectively. Forskolin (10 μM), a cAMP generator, enhanced constitutive ACh release but did not interact synergistically with NGF. Tetrodotoxin (1 μM) and cycloheximide (2 μM) did not prevent NGF-induced ACh release, indicative of action at the level of the cholinergic nerve terminal and that new protein synthesis is not required for this neurotransmitter-like effect, respectively. In contrast, after a 24 hr NGF treatment, distinct protein synthesis-dependent and independent effects on choline acetyltransferase activity and ACh release were observed. These results indicate that neuromodulator/neurotransmitter-like (protein synthesis-independent) and neurotrophic (translation-dependent) actions likely make distinct contributions to the enhancement of cholinergic activity by NGF.