Abstract
Regulation of the “neuronal” nicotinic acetylcholine receptors (nAChRs) is implicated in both tobacco addiction and smoking-dependent tumor promotion. Some of these effects are caused by the tobacco-derived N-nitrosamines, which are carcinogenic compounds that avidly bind to nAChRs. However, the functional effects of these drugs on specific nAChR subtypes are largely unknown. By using patch-clamp methods, we tested 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) on human α4β2 nAChRs. These latter are widely distributed in the mammalian brain and are also frequently expressed outside the nervous system. NNK behaved as a partial agonist, with an apparent EC50 of 16.7 μM. At 100 μM, it activated 16% of the maximal current activated by nicotine. When NNK was co-applied with nicotine, it potentiated the currents elicited by nicotine concentrations ≤ 100 nM. At higher concentrations of nicotine, NNK always inhibited the α4β2 nAChR. In contrast, NNN was a pure inhibitor of this nAChR subtype, with IC50 of approximately 1 nM in the presence of 10 μM nicotine. The effects of both NNK and NNN were mainly competitive and largely independent of Vm. The different actions of NNN and NNK must be taken into account when interpreting their biological effects in vitro and in vivo.
Highlights
The neuronal nicotinic acetylcholine receptor (nAChR) are ligand-gated ion channels permeable to cations, which regulate cell excitability and synaptic transmission
We studied by patch-clamp methods the effect of NNK and NNN on human α4β2 nAChRs stably expressed in human embryonic kidney (HEK) cells
This nAChR subtype is widely distributed in the brain, where it regulates both excitatory and inhibitory transmission (Becchetti et al, 2015) and is implicated in the cognitive and addictive effects of smoking (Changeux, 2010; Faure et al, 2014). It is commonly expressed in non-nervous tissue, including lung cells (Fu et al, 2009), and tumor cell lines (Egleton et al, 2008). We found that these nitrosamines exert distinct actions, as NNK is a partial agonist, whereas NNN is a pure inhibitor of α4β2 nAChRs
Summary
The nAChRs are ligand-gated ion channels permeable to cations, which regulate cell excitability and synaptic transmission. They are formed by different α and β subunits, which assemble to form homo- or heteropentamers (Dani and Bertrand, 2007). The homopentameric receptors, such as the widespread (α7), have low sensitivity to ACh and nicotine (with EC50 higher than 100 μM), high permeability to Ca2+ (PCa) and rapid desensitization. The heteromeric nAChRs generally display higher sensitivity to the agonists, lower PCa and slower desensitization (Dani and Bertrand, 2007). In the peripheral nervous system, Abbreviations: CNS, central nervous system; DHβE, dihydro-β-erythroidine; nAChR, neuronal nicotinic acetylcholine receptor; EC50high and EC50low, EC50 values for two-term Hill equations; HEK, human embryonic kidney; nH, Hill coefficient for one-term Hill equation; nH1, nH2, Hill coefficients for two-term Hill equation; NNK, 4-(methylnitrosamine)-1-(3pyridyl)-1-butanone; NNN, N′-nitrosonornicotine; Vm, membrane potential; Vrev, reversal potential
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
