Evaluation of Ca²⁺ 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 Ca²⁺ permeable α7 nAChRs. However, the Ca²⁺ permeability of TM nAChR channels has not been determined. To directly evaluate the Ca²⁺ 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 Ca²⁺. A shift in the reversal potentials was determined from the current-voltage relationships and the permeability ratio, P_Ca/P _Na, was estimated within the Goldman-Hodgkin-Katz constant field approximation. TM nAChRs were found to be highly Ca²⁺ permeable with the permeability ratio, P_Ca/P_Na(nAChR) being ∼5.9 and the fractional Ca²⁺ current, P_f(nAChR) being ∼10.1% at -60 mV. As a positive control for the applied methods and analysis, the permeability ratio, P_Ca/P_Na(NMDAR) being ∼8.3 and the fractional Ca²⁺ current, P_f(NMDAR) being ∼13.6 at -60 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 ²⁺ permeable, but ∼1.4 fold less permeable to Ca²⁺ than native hippocampal pyramidal NMDA receptors.