Evaluation of Ca permeability of nicotinic acetylcholine receptors in hypothalamic histaminergic neurons

Hypothalamic histaminergic tuberomammillary (TM) neurons express nicotinic acetylcholine receptors (nAChRs) with kinetic and pharmacological properties resembling those of highly Ca permeable a7 nAChRs. However, the Ca permeability of TM nAChR channels has not been determined. To directly evaluate the Ca permeability of TM nAChRs, patch-clamp recordings were conducted using non-cultured acutely dissociated TM neurons and external solutions containing low (2 mM) and high (20 mM) concentrations of Ca. A shift in the reversal potentials was determined from the current–voltage relationships and the permeability ratio, PCa/PNa, was estimated within the Goldman-HodgkinKatz constant field approximation. TM nAChRs were found to be highly Ca permeable with the permeability ratio, PCa/PNa(nAChR) being 5.9 and the fractional Ca current, Pf(nAChR) being 10.1% at 260 mV. As a positive control for the applied methods and analysis, the permeability ratio, PCa/PNa(NMDAR) being 8.3 and the fractional Ca current, Pf(NMDAR) being 13.6% at 260 mV for NMDA receptors were determined using non-cultured acutely dissociated hippocampal pyramidal neurons and found similar to previously reported values. Therefore, these results demonstrate that native TM nAChRs are highly Ca permeable, but 1.4 fold less permeable to Ca than native hippocampal pyramidal NMDA receptors.