MUSCARINIC M3 RECEPTORS AND DYNAMICS OF INTRACELLULAR CA2+ CEREBELLAR GRANULE NEURONS

Activation of muscarinic receptors with carbachol has no effect on intracellular Ca2+ concentration in cerebellar granule cell cultures. Only after elevating intracellular Ca2+ concentrations using either 40 mM KCl or activating glutamatergic receptors was carbachol able to increase intracellular Ca2+. The response lasted about 10 s, and the median increase in intracellular Ca2+ with either 100 muM or 300 muM carbachol was about 85 nM. Carbachol at 30 muM elicited an increase in intracellular calcium that was half maximal. After a 16 min or 32 min delay following cell depolarization, responses to carbachol were only found in about 20% of the cells studied and the median increase in intracellular Ca2+ was about 14 nM. (-)-Quinuclidinylxanthene-9-carboxylate hemioxalate (QNX) at 10 nM (a muscarinic m3 antagonist), but not methoctramine at 5 muM (a muscarinic m2 antagonist), attenuated the action of 100 muM carbachol. This carbachol-mediated response was elicited in the absence of extracellular Ca2+ and in the presence of 10 muM dantrolene, but was blocked by 1 muM thapsigargin. Lastly, treatments of cerebellar granule cell cultures with carbachol (100 muM) for up to 12 h results in a desensitization of the carbachol-mediated release of stored Ca2+. Thus, carbachol acts on muscarinic m3 receptors to induce the release of Ca2+ from IP3-sensitive Ca2+ stores that must be filled by a prior period of high intracellular Ca2+.