Effect of galantamine on the human alpha7 neuronal nicotinic acetylcholine receptor, the Torpedo nicotinic acetylcholine receptor and spontaneous cholinergic synaptic activity

Abstract

1. Various types of anticholinesterasic agents have been used to improve the daily activities of Alzheimer's disease patients. It was recently demonstrated that Galantamine, described as a molecule with anticholinesterasic properties, is also an allosteric enhancer of human alpha4beta2 neuronal nicotinic receptor activity. We explored its effect on the human alpha7 neuronal nicotinic acetylcholine receptor (nAChR) expressed in Xenopus oocytes. 2. Galantamine, at a concentration of 0.1 microM, increased the amplitude of acetylcholine (ACh)-induced ion currents in the human alpha7 nAChR expressed in Xenopus oocytes, but caused inhibition at higher concentrations. The maximum effect of galantamine, an increase of 22% in the amplitude of ACh-induced currents, was observed at a concentration of 250 microM Ach. 3. The same enhancing effect was obtained in oocytes transplanted with Torpedo nicotinic acetylcholine receptor (AChR) isolated from the electric organ, but in this case the optimal concentration of galantamine was 1 microM. In this case, the maximum effect of galantamine, an increase of 35% in the amplitude of ACh-induced currents, occurred at a concentration of 50 microM ACh. 4. Galantamine affects not only the activity of post-synaptic receptors but also the activity of nerve terminals. At a concentration of 1 microM, quantal spontaneous events, recorded in a cholinergic synapse, increased their amplitude, an effect which was independent of the anticholinesterasic activity associated with this compound. The anticholinesterasic effect was recorded in preparations treated with a galantamine concentration of 10 microM. 5. In conclusion, our results show that galantamine enhances human alpha7 neuronal nicotinic ACh receptor activity. It also enhances muscular AChRs and the size of spontaneous cholinergic synaptic events. However, only a very narrow range of galantamine concentrations can be used for enhancing effects.

Figure 1

Effect of galantamine on ACh-induced currents supported by α7 AChRs expressed in Xenopus oocytes. (a) Fast inactivating inward currents were stimulated by ACh; holding potential was −70 mV. (b, c) Galantamine at very low concentration (100 nM) induced a very small but significant increase in the amplitude of ACh-activated currents. Higher concentrations did not cause a significant increase in currents. (d) Dose–response relationship for ACh obtained in the absence and presence of 100 nM galantamine. The averaged amplitudes (expressed as mean±s.e.) of the currents recorded from 10 oocytes from different donors were plotted versus the respective concentration of ACh applied. ^*P<0.05.

Figure 2

Effect of galantamine on ACh-induced currents supported by Torpedo AChRs from the electric organ. Membranes of the electric organ were transplanted to Xenopus oocytes. (a) Fast inactivating inward currents were stimulated by ACh; holding potential was −70 mV. (b, c) Galantamine at low concentration (1 μM) induced a significant increase in the amplitude of ACh-activated currents. Higher concentrations did not cause a significant increase in currents. (d) Dose–response relationship for ACh obtained in the absence and presence of 1 μM galantamine. The averaged amplitudes (expressed as mean±s.e.) of the currents recorded from eight oocytes from different donors were plotted versus the respective concentration of ACh applied. ^*P<0.05.

Figure 3

Effect of galantamine on spontaneous cholinergic synaptic activity of Torpedo electric organ. Low-resistance pipettes placed extracellularly were used for the focal recording of miniature endplate currents. Oscilloscope traces of spontaneous MEPPs were superimposed. (a) Untreated fragments. (b) 1 μM galantamine. (c) 10 μM Galantamine. The inhibition of acetylcholinesterase activity is translated into synaptic activity recording by the prolongation of the decay phase of spontaneous events, which was only observed when high concentrations of galantamine were used. (d) Effect of galantamine treatment on MEPP frequency.

Figure 4

Analysis of MEPPs in 1 and 10 μM galantamine-treated fragments of Torpedo electric organ. Data are presented as cumulative plots and bar histograms (inset). (a) Electrical charge mobilised by spontaneous MEPP: effect of galantamine on electrical charge mobilised by spontaneous ACh release. The area delimited below a MEPP contour corresponds to the total electrical charge passed through the nicotinic ACh receptors as a consequence of spontaneous quanta. (b) Effect of galantamine on the amplitude of MEPP (peak). (c) Rise time of MEPP. (d) Rate of rise time of MEPP. (e) half-width of MEPP, width at 50% of amplitude. Galantamine (10 μM) prolonged the decay phase of MEPPs. The number of MEPPs analysed for control conditions was 2741, 2501 for 1 μM galantamine and 1133 for 10 μM galantamine. The data shown came from three experiments.