Ca2+ permeability in rat hippocampal of nicotinic acetylcholine receptors CA1 interneurones

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain where they are involved in a variety of physiological processes, including cognition and development. The nAChRs are ligand-gated cationic channels, and different subtypes are known to be differentially permeable to Ca2+; the alpha 7-containing nAChRs are generally considered to be the most permeable. Ca2+ can activate and regulate a variety of signal transduction cascades, and the influx of Ca2+ through these receptors may have implications for synaptic plasticity. To determine the Ca2+ permeability of the nAChRs in rat hippocampal interneurones in the slice, which contain diverse subtypes of alpha 7- and non-alpha 7-containing nAChRs, we combined patch-clamp electrophysiology recordings with conventional fura-2 fluorescence imaging techniques. We estimated the relative Ca2+ permeability of the channels by determining the ratio of the increase in [Ca2+](i) level (Delta [Ca2+](i)) in the soma to the integrated transmembrane current (charge, Q) induced by the activation of the nAChRs, and compared this ratio to the highly Ca2+ permeable NMDA subtype of glutamate receptor channel. In all cells tested, the Delta[Ca2+](i)/Q ratio was significantly larger (i.e. more than twice as big) for responses activated by NMDA than for alpha 7-containing nAChRs in interneurones; the activation of the non-alpha 7 nAChRs did not produce any significant increase in [Ca2+](i). Interestingly, the Ca2+ permeability of native alpha 7 nAChRs in PC12 cells was significantly larger than in hippocampal interneurones, and not significantly different from NMDA receptors. Therefore, the a7-containing nAChRs in rat hippocampal interneurones are significantly less permeable to Ca2+ than not only NMDA receptors but also alpha 7 nAChRs in PC12 cells.