Agonist and antagonist effects of tobacco-related nitrosamines on human α4ß2 nicotinic acetylcholine receptors.

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 α4ß2 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 α4ß2 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.

Targeting neuronal nicotinic receptors in cancer: new ligands and potential side-effects.

In the nervous system, the neuronal nicotinic acetylcholine receptors (nAChRs) mediate fast excitatory postsynaptic potentials as well as slower paracrine actions of ACh. They are also widely expressed in non-nervous tissue, including the neoplastic, which is intriguing as smoking is an established risk factor for cancer. Moreover, recent evidence attributes to the gene cluster coding for the α3/α5/ß4 nAChR subunits a role in both development of lung cancer and nicotine addiction. Many cellular effects of nicotine and the tobacco-derived carcinogenic N-nitrosamines are probably caused by nAChR activation, which regulates cell proliferation, migration, apoptosis and neoangiogenesis. Nonetheless, the precise nAChR roles in tumors are difficult to determine because cancer cells express a wide variety of nicotinic subunits, whose function is unclear. Patented compounds which selectively target nAChRs subtypes are increasingly available and will hopefully allow better understanding of the physiology of these channels in specific cell types, as well as suggest novel diagnostic and therapeutic approaches. At the present state, however, thorough functional studies of these compounds are still limited and whether they act as agonists, antagonists or partial agonists is often unclear. Such a blurred distinction between activators and inhibitors makes detailed studies in expression systems sorely needed for both physiological understanding and outlining the possible side-effects.

Effect of carbamazepine and oxcarbazepine on wild-type and mutant neuronal nicotinic acetylcholine receptors linked to nocturnal frontal lobe epilepsy.

Carbamazepine (5H-dibenz[b,f]azepine-5-carboxamide) and oxcarbazepine (10,11-dihydro-10-oxo-5H-dibenz[b,f]azepine-5-carboxamide) are widely used for the treatment of partial epilepsy. Recent work indicates that these drugs, in addition to targeting voltage-gated Na(+) channels, can modulate ligand-gated channels. These compounds appear to be particularly effective for treatment of nocturnal frontal lobe epilepsy, which can be caused by mutant neuronal nicotinic receptors. We compared the effects of carbamazepine and oxcarbazepine on heteromeric nicotinic receptors to better understand the underlying mechanism of the effect of these drugs in epileptic patients. Receptors were expressed in cell lines and studied by patch-clamp methods at -60 mV. For alpha2beta4 receptors activated with 100 microM nicotine, IC(50) for carbamazepine was 49 microM. Receptors in which alpha2 was substituted with alpha2-I279 N, linked to autosomal dominant nocturnal frontal lobe epilepsy, had an IC(50) of 21 microM. For oxcarbazepine, the IC(50) was larger than 500 microM for wild-type receptors and approximately 100 microM for mutant receptors. A similar inhibition was observed in the presence of 10 microM nicotine, indicating a non-competitive mechanism. The monohydroxy derivative (MHD) of oxcarbazepine, clinically the most relevant compound, was tested on both alpha2beta4 and alpha4beta2 receptors, to obtain a broader view of its possible physiological effects. At the typical concentration present in blood (100 microM), MHD produced an approximate 40% channel block on alpha4beta2, but no significant effect on alpha2beta4 receptors. Oxcarbazepine and MHD retarded the channel deactivation, suggesting that these compounds produce open channel block. These results may explain the particular efficacy of these drugs in nocturnal frontal lobe epilepsy.