Differential coupling of α7 and non‐α7 nicotinic acetylcholine receptors to calcium‐induced calcium release and voltage‐operated calcium channels in PC12 cells

Abstract

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca 2+ levels. Following nAChR stimulation Ca 2+ can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca 2+ channel (VOCC) activation; Ca 2+ levels can subsequently be amplified via Ca 2+ -induced Ca 2+ release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca 2+ sources contributing to a7 and non-a7 nAChR-mediated increases in intracellular Ca 2+ in PC12 cells. Application of the a7 nAChR positive allosteric modulator PNU 120596 (10 lM), in conjunction with the a7 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 a7 nAChR antagonist a-bungarotoxin. The non-a7 nAChR agonist 5-Iodo-A-85380 produces a-bungarotoxin-insensitive increases in intracellular Ca 2+ (EC 50 ¼ 11.2 lM). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca 2+ entry following either non-a7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the a7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca 2+ stores. These results indicate that a7 and non-a7 nAChRs are differentially coupled to Ca 2+ -induced Ca 2+ release and VOCCs, respectively.

Presynaptic α7- and β2-Containing Nicotinic Acetylcholine Receptors Modulate Excitatory Amino Acid Release from Rat Prefrontal Cortex Nerve Terminals via Distinct Cellular Mechanisms

“…1, B and C). This concentration of compound A was maximally effective, consistent with our previous functional study (Dickinson et al, 2007) and was used for all subsequent experiments. KCl- 3 H]D-aspartate release was monitored in parallel; the release evoked by 10 nM compound A corresponded in magnitude to the response to 8 mM KCl, whereas 12 mM KCl produced a substantially larger response (Fig.…”

“…This approach demonstrated unequivocally that ␣7 and ␤2* nAChRs are functionally coupled to distinct downstream targets: internal Ca 2ϩ stores and VOCCs, respectively. We have observed the same dichotomy in PC12 cells, in which compound A evoked increases in intracellular Ca 2ϩ that were independent of VOCC inhibitors but blocked by ryanodine, whereas 5-I-A-85380 elicited Ca 2ϩ increases that were attenuated by verapamil but insensitive to ryanodine (Dickinson et al, 2007). Taken together, these various reports are consistent with the hypothesis that in diverse cell types and in different subcellular domains, highly Ca 2ϩ -permeable ␣7 nAChRs are preferentially localized to directly elicit CICR from intracellular stores.…”

“…We have used a novel ␣7 nAChR-selective agonist, compound A (Cilia et al, 2005;Dickinson et al, 2007; Fig. 1A, left inset), to explore the contribution of ␣7 nAChRs to […”

“…However, Ca 2ϩ entry can further augment intracellular Ca 2ϩ concentration by Ca 2ϩ -induced Ca 2ϩ release (CICR) from internal Ca 2ϩ stores, via Ca 2ϩ release channels: notably, ryanodine receptors, and also inositol 1,4,5-trisphosphate receptors (Verkhratsky, 2005;Bardo et al, 2006). Indeed, increases in Ca 2ϩ in response to ␣7 nAChR activation are amplified by CICR in diverse preparations (Dajas-Bailador and Wonnacott, 2004), including hippocampal mossy fiber terminals (Sharma et al, 2008) and neuronal cell lines (Dickinson et al, 2007).…”

RG3487, a Novel Nicotinic α7 Receptor Partial Agonist, Improves Cognition and Sensorimotor Gating in Rodents

“…Not only have ␣7nAChRs been identified postsynaptically, but their high Ca 2ϩ permeability suggests that metabotropic Ca 2ϩ -mediated second messenger signaling could underlie its cognitive enhancing properties (Berg and Conroy, 2002). In support of this concept, activation of ␣7nAChRs can trigger Ca 2ϩ -induced Ca 2ϩ release from internal stores (Dickinson et al, 2007), which has been linked to transcriptional regulation and synaptic plasticity (Berg and Conroy, 2002). To this end, Bitner et al (2007) report activation of the extracellular signal-regulated kinase 1/2 pathway and subsequent downstream phosphorylation of cAMP response element binding protein, a critical mediator of synaptic plasticity after administration of the selective ␣7nAChR agonist A-582941.…”