Regulation of intracellular free calcium by cholinergic receptors on chick ciliary ganglion neurons

Neurons are specialized in their multiple uses of calcium as a signalling molecule as well as in their many mechanisms that regulate the influx and removal of calcium from the cytoplasm. In this work, I have focused on neurons of the chick ciliary ganglion, where a single neurotransmitter, acetylcholine (ACh), can stimulate as many as three classes of cholinergic receptors: two with nicotinic pharmacology (one synaptic and the other extrasynaptic) and one with muscarinic pharmacology.; All three classes of acetylcholine receptors (AChRs) elevate intracellular free calcium ( (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar}) as measured using the calcium indicator, fluo-3. The synaptic-type nicotinic AChRs are highly permeable to calcium, as demonstrated using constant field analysis, and are more like other neuronal AChRs in this respect than like synaptic AChRs on muscle. Moreover, the receptors are as effective at elevating (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} in the neurons under physiological conditions as are the more abundant {dollar}alpha{dollar}Bgt-AChRs, indicating that the same neurons can express and maintain at least two classes of nicotinic receptors with this feature. Both classes of nicotinic AChRs rely heavily on activation of voltage-dependent calcium channels (VDCCs) for global elevation of cytoplasmic calcium. When the membrane potential is held near rest, however, the synaptic-type AChRs can promote substantial calcium influx without contributions from VDCCs. As a result, the receptors could, in principle, regulate calcium-dependent events without cell depolarization being a prerequisite. Calcium flux through nicotinic AChRs may have greatest impact on calcium levels in microdomains near the plasma membrane. In this case, the two classes of receptors could induce wholly different sets of calcium-dependent events by virtue of the different distributions the receptor subtypes have in the membrane.; Muscarinic receptors stimulate the production of inositol phosphates and evoke an oscillatory pattern of (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} elevation. This oscillatory pattern of (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} response is not seen upon nicotine stimulation. Also unlike the nicotinic AChRs, muscarinic receptors mobilize both intracellular and extracellular calcium sources. The endogenous ligand, acetylcholine, can induce oscillatory or non-oscillatory patterns of (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} elevation, depending upon the concentration of agonist used. The pattern depends on the combination of receptors activated. Muscarinic receptors are activated at low concentrations of ACh and induce an oscillatory (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} elevation. Both nicotinic and muscarinic receptors are active at higher ACh concentrations, together inducing a sustained (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} elevation. Each of the temporal patterns of (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm i{rcub}{dollar} elevation may engage a unique set of calcium-dependent cellular processes, including, for example, exocytosis, cytoskeletal restructuring, gene expression, or cell death.