ALPHA-BUNGAROTOXIN-SENSITIVE HIPPOCAMPAL NICOTINIC RECEPTOR-CHANNEL HAS A HIGH-CALCIUM PERMEABILITY

The hippocampal nicotinic acetylcholine receptor (nAChR) is a newly identified ligand-gated ion channel that is blocked by the snake toxin alpha-bungarotoxin (alpha-BGT) and that probably contains the alpha 7 nAChR subunit in its structure. Here its ion selectivity was characterized and compared with that of the N-methyl-D-aspartate (NMDA) receptor channel. The reversal potentials (V-R) of acetylcholine- and NMDA-activated whole-cell currents were determined under various ionic conditions. Using ion activities and a Goldman-Hodgkin-Katz equation for V-R shifts in the presence of Ca2+, permeability ratios were calculated. For the alpha-BGT-sensitive nAChR, P-Na/P-Cs was close to 1 and Cl- did not contribute to the currents. Changing the [Ca2+](0), from 1 to 10 mM, the V(R)s of the nAChR and NMDA currents were shifted by +5.6 +/- 0.4 and +8.3 +/- 0.4 mV, respectively, and the nAChR current decay was accelerated. These shifts yielded P-Ca/P(Cs)s of 6.1 +/- 0.5 for the nAChR channel and 10.3 +/- 0.7 for the NMDA channel. Thus, the neuronal alpha-BGT-sensitive nAChR is a cation channel considerably selective to Ca2+ and may mediate a fast rise in intracellular Ca2+ that would increase in magnitude with membrane hyperpolarization.