Activation of muscarinic cholinergic receptors blocks apoptosis of cultured cerebellar granule neurons

We have recently reported that the majority of cultured rat cerebellar granule neurons undergo apoptosis when maintained in the presence of physiological concentrations of K⁺ (nondepolarizing conditions). We now report that exposure of cultured cerebellar granule neurons, maintained under nondepolarizing conditions, to the muscarinic cholinergic receptor (mAchR) agonists carbachol and muscarine results in a concentration- and time- dependent inhibition of apoptosis. The nicotinic cholinergic receptor agonist (-)-nicotine fails to mimic, and the nicotinic cholinergic receptor antagonist dihydro-β-erythroidine fails to antagonize, the survival- promoting effects of carbachol. In contrast, relatively low concentrations of atropine completely prevent the effects of carbachol in blocking apoptotic death of cultured granule neurons. Although the m1- and m2-preferring mAchR antagonists pirenzepine and gallamine, respectively, fail to reverse the effects of carbachol, the m3-preferring antagonist 4-diphenylacetoxyl-N- methylpiperidine methiodide completely blocks the survival-promoting effects of carbachol. These data demonstrate that activation of the mAchR (possibly of the m3 subtype) blocks apoptosis of cultured cerebellar granule neurons. The antiapoptotic effects of mAchR agonists are not indirectly mediated via glutamate release from granule neurons, because antagonists of either N- methyI-D-aspartate or non-N-methyl-D-aspartate glutamate receptors fail to affect the anti-apoptotic effects of carbachol or muscarine. Moreover, exposure of cultured cerebellar granule neurons to antiapoptotic concentrations of carbachol, in contrast to high concentrations of K⁺ or glutamate receptor agonists, results in only a small and transient elevation of the intracellular Ca²⁺ concentration, as measured by fura-2 microfluorimetry. Slow neurotransmitters such as acetylcholine, acting via their cognate G protein-coupled receptors, may prevent neuronal apoptosis in the developing (and perhaps adult) central nervous system.