Differential coupling of alpha 7 and non-alpha 7 nicotinic acetylcholine receptors to calcium-induced calcium release and voltage-operated calcium channels in PC12 cells

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca2+ levels. Following nAChR stimulation Ca2+ can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca2+ channel (VOCC) activation; Ca2+ levels can subsequently be amplified via Ca2+-induced Ca2+ release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca2+ sources contributing to alpha 7 and non-alpha 7 nAChR-mediated increases in intracellular Ca2+ in PC12 cells. Application of the alpha 7 nAChR positive allosteric modulator PNU 120596 (10 mu M), in conjunction with the alpha 7 nAChR agonist, compound A [(R)-N-(1-azabicyclo [2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide), 10 nM], produces a rapid increase in fluo-3 fluorescence that is prevented by the selective alpha 7 nAChR antagonist alpha-bungarotoxin. The non-alpha 7 nAChR agonist 5-Iodo-A-85380 produces alpha-bungarotoxin-insensitive increases in intracellular Ca2+ (EC50 11.2 mu M). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca2+ entry following either non-alpha 7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the alpha 7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca2+ stores. These results indicate that alpha 7 and non-alpha 7 nAChRs are differentially coupled to Ca2+-induced Ca2+ release and VOCCs, respectively.