Involvement of the NMDA Receptor in a Non-cholinergic Action of Acetylcholinesterase in Guinea-pig Substantia Nigra Pars Compacta Neurons

Evidence is accumulating that a soluble, secretory form of acetylcholinesterase may have novel, non–cholinergic functions in certain brain regions, such as the substantia nigra. In this study, application of human recombinant acetylcholinesterase (rhAChE) to pars compacta neurons in the rostral substantia nigra resulted in a sustained hyperpolarization that was not only mimicked by application of N‐methyl–D–aSpartate (NMDA) but also blocked by the NMDA receptor antagonists MK801 and 2–amino–5phosphonopentanoic acid. Neither the rhAChE–nor the NMDA–induced hyperpolarization was seen when the calcium chelator BAPTA was injected into the neuron; hence the effect is mediated by accumulation of intracellular calcium. This intracellular calcium appears sufficient to compromise neuronal metabolism since the rhAChE–induced hyperpolarization was reversed by application of the K–ATP channel antagonist tolbutamide. Butyrylcholinesterase, a protein of similar molecular weight to acetylcholinesterase, which also hydrolyses acetylcholine, had no effect whatsoever. The results suggest that, independent of its normal catalytic function, acetylcholinesterase can act via the NMDA receptor complex to enhance calcium entry into nigral neurons and jeopardize cell metabolism. This non–classical action of acetylcholinesterase might thus be an important factor in the mechanisms underlying parkinsonian neurodegeneration.