A Pertussis Toxin-Sensitive 8-Lipoxygenase Pathway Is Activated by a Nicotinic Acetylcholine Receptor inAplysia Neurons

Nicotine inhibits potassium currents in Aplysia bag cell neurons

Acetylcholine and the archetypal cholinergic agonist, nicotine, are typically associated with the opening of ionotropic receptors. In the bag cell neurons, which govern the reproductive behavior of the marine snail, Aplysia californica, there are two cholinergic responses: a relatively large acetylcholine-induced current and a relatively small nicotine-induced current. Both currents are readily apparent at resting membrane potential and result from the opening of distinct ionotropic receptors. We now report a separate current response elicited by applying nicotine to cultured bag cell neurons under whole cell voltage-clamp. This current was ostensibly inward, best resolved at depolarized voltages, presented a noncooperative dose-response with a half-maximal concentration near 1.5 mM, and associated with a decrease in membrane conductance. The unique nicotine-evoked response was not altered by intracellular perfusion with the G protein blocker GDPβS or exposure to classical nicotinic antagonists but was occluded by replacing intracellular K(+) with Cs(+) Consistent with an underlying mechanism of direct inhibition of one or more K(+) channels, nicotine was found to rapidly reduce the fast-inactivating A-type K(+) current as well as both components of the delayed-rectifier K(+) current. Finally, nicotine increased bag cell neuron excitability, which manifested as reduction in spike threshold, greater action potential height and width, and markedly more spiking to continuous depolarizing current injection. In contrast to conventional transient activation of nicotinic ionotropic receptors, block of K(+) channels could represent a nonstandard means for nicotine to profoundly alter the electrical properties of neurons over prolonged periods of time.

The immunomodulation of nicotinic acetylcholine receptor subunits in Zhikong scallop Chlamys farreri

Nicotinic acetylcholine receptor (nAChR), the best-studied ionotropic neuron receptor protein, is a key player in neuronal communication, and it has been reported to play an important role in immunomodulation of vertebrates. Although nAChRs have also been identified in most invertebrates, the knowledge about their immunomodulation is still limited. In the present study, two scallop nAChR genes were identified from Chlamys farreri (designed as CfnAChR1 and CfnAChR2), which encoded 384 and 443 amino acids, respectively. The conserved disulfide-linked cystines, ion selectivity residues and the hydrophobic gating residues (L251, V255 and V259) were identified in CfnAChR1 and CfnAChR2. The immunoreactivities of CfnAChR1 and CfnAChR2 were observed in all the tested scallop tissues, including adductor muscle, mantle, gill, hepatopancreas, kidney and gonad. After LPS (0.5 mg mL(-1)) stimulation, the expression of CfnAChR1 mRNA in haemocytes increased significantly by 9.83-fold (P < 0.05) and 12.93-fold (P < 0.05) at 3 h and 24 h, respectively. While the expression level of CfnAChR2 mRNA increased 43.94% at 12 h after LPS stimulation (P < 0.05). After TNF-α (50 ng mL(-1)) stimulation, the expression levels of CfnAChR1 and CfnAChR2 both increased significantly at 1 h, which were 21.33-fold (P < 0.05) and 2.44-fold (P < 0.05) of that in the PBS group, respectively. The results collectively indicated that the cholinergic nervous system in scallops could be activated by immune stimulations through CfnAChR1 and CfnAChR2, which function as the links between the cholinergic nervous system and immune system.

Acetylcholine-evoked afterdischarge in Aplysia bag cell neurons

A brief synaptic input to the bag cell neurons of Aplysia evokes a lengthy afterdischarge and the secretion of peptide hormones that trigger ovulation. The input transmitter is unknown, although prior work has shown that afterdischarges are prevented by strychnine. Because molluscan excitatory cholinergic synapses are blocked by strychnine, we tested the hypothesis that acetylcholine acts on an ionotropic receptor to initiate the afterdischarge. In cultured bag cell neurons, acetylcholine induced a short burst of action potentials followed by either return to near baseline or, like a true afterdischarge, transition to continuous firing. The current underlying the acetylcholine-induced depolarization was dose dependent, associated with increased membrane conductance, and sensitive to the nicotinic antagonists hexamethonium, mecamylamine, and α-conotoxin ImI. Whereas nicotine, choline, carbachol, and glycine did not mimic acetylcholine, tetramethylammonium did produce a similar current. Consistent with an ionotropic receptor, the response was not altered by intracellular dialysis with the G protein blocker guanosine 5′-(β-thio)diphosphate. Recording from the intact bag cell neuron cluster showed acetylcholine to evoke prominent depolarization, which often led to extended bursting, but only in the presence of the acetylcholinesterase inhibitor neostigmine. Extracellular recording confirmed that exogenous acetylcholine caused genuine afterdischarges, which, as per those generated synaptically, rendered the cluster refractory to further stimulation. Finally, treatment with a combination of mecamylamine and α-conotoxin ImI blocked synaptically induced afterdischarges in the intact bag cell neuron cluster. Acetylcholine appears to elicit the afterdischarge through an ionotropic receptor. This represents an expedient means for transient stimulation to elicit prolonged firing in the absence of ongoing synaptic input.

The modulating role of nuclear factor-kappaB in the action of alpha7-nicotinic acetylcholine receptor and cross-talk between 5-lipoxygenase and cyclooxygenase-2 in colon cancer growth induced by 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone

4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), the tobacco-specific nitrosamine, induces lung cancer in all animal species tested and is thought to contribute significantly to the high lung cancer burden associated with smoking. However, there is no report whether NNK could promote colon cancer growth. To address this hypothesis and the possible signaling pathways involved, we used SW1116 colon cancer cell line to study these biological events in vitro. Results showed that NNK, after 5-h treatment, stimulated cell proliferation, enhanced alpha7-nicotinic acetylcholine receptor (alpha7-nAChR) mRNA levels and nuclear factor-kappaB (NF-kappaB) DNA binding activity, as well as 5-lipoxygenase and cyclooxygenase-2 protein expressions. alpha-Bungarotoxin, the specific alpha7-nAChR antagonist, inhibited these biological effects. However, 5-lipoxygenase inhibition had no effect on alpha7-nAChR mRNA expression, but significantly inhibited cell proliferation and activation of NF-kappaB and cyclooxygenase-2, whereas NF-kappaB-specific inhibitor caffeic acid phenethyl ester reduced both cell proliferation and cyclooxygenase expression induced by NNK without affecting alpha7-nAChR mRNA level and 5-lipoxygenase expression. Together, the present study demonstrated that NNK promoted colon cancer growth in vitro. NF-kappaB not only conveys the biological effect of alpha7-nAChR activation but is also involved in the cross-talk between 5-lipoxygenase and cyclooxygenase-2 in response to NNK in colon cancer cell development.

Nicotinic receptor-mediated effects on appetite and food intake

It is well known, although not well understood, that smoking and eating just do not go together. Smoking is associated with decreased food intake and lower body weight. Nicotine, administered either by smoking or by smokeless routes, is considered the major appetite-suppressing component of tobacco. Perhaps the most renowned example of nicotine's influence on appetite and feeding behavior is the significant weight gain associated with smoking cessation. This article presents an overview of the literature at, or near, the interface of nicotinic receptors and appetite regulation. We first consider some of the possible sites of nicotine's action along the complex network of neural and non-neural regulators of feeding. We then present the hypothesis that the lateral hypothalamus is a particularly important locus of the anorectic effects of nicotine. Finally, we discuss the potential role of endogenous cholinergic systems in motivational feeding, focusing on cholinergic pathways in the lateral hypothalamus.