Coupling of human nicotinic acetylcholine receptors α7 to calcium channels in GH3 cells

In the present study expression of homomeric nAChR α7 was achieved in GH3 rat pituitary cells. α7 Subunits were heterologously expressed as components of [125I]-labeled α-bungarotoxin binding nAChRs (Bmax: 1.2 pmol/mg protein). Function of the expressed α7 ion channels was assessed by patch-clamp recording and calcium imaging. While acetylcholine-induced currents desensitized within much less than 1 s, calcium-sensitive fluorescence transients peaked after 5-10 s and returned to background levels within 30 s only. The fluorescence signal was blocked by isradipine and removal of extracellular sodium indicated that in these cells opening of rapidly desensitizing α7 nAChR triggers calcium influx via voltage-gated, DHP-sensitive calcium channels. In this cellular system, agonists revealed the following rank order of potency: epibatidine > anatoxin A > AAR17779 > ABT-594 > DMPP > nicotine > GTS-21 > cytisine > ABT-418 > acetylcholine > choline > ABT-089. All of the signals were inhibited by the α7 antagonists α-bungarotoxin (pIC50: 7.4) and methyllycaconitine (pIC50: 7.8). Further, marketed antidepressants showed antagonistic activity with the following rank order of potency: fluoxetine > imipramine > paroxetine > sertraline. These data illustrate that coupling to voltage-gated calcium channels allows a rapid and reliable functional examination of nAChR α7.