ABSTRACT
Acetylcholine (ACh) activates two types of chloride conductances in Aplysia neurons that can be distinguished by their kinetics and pharmacology. One is a rapidly desensitizing current that is blocked by alpha-conotoxin-ImI and the other is a sustained current that is insensitive to the toxin. These currents are differentially expressed in Aplysia neurons. We report here that neurons that respond to ACh with a sustained chloride conductance also generate 8-lipoxygenase metabolites. The sustained chloride conductance and the activation of 8-lipoxygenase have similar pharmacological profiles. Both are stimulated by suberyldicholine and nicotine, and both are inhibited by alpha-bungarotoxin. Like the sustained chloride conductance, the activation of 8-lipoxygenase is not blocked by alpha-conotoxin-ImI. In spite of the similarities between the metabolic and electrophysiological responses, the generation of 8-lipoxygenase metabolites does not appear to depend on the ion current since an influx of chloride ions is neither necessary nor sufficient for the formation of the lipid metabolites. In addition, the application of pertussis toxin blocked the ACh-activated release of arachidonic acid and the subsequent production of 8-lipoxygenase metabolites, yet the ACh-induced activation of the chloride conductance is not dependent on a G protein. Our results are consistent with the idea that the nicotinic ACh receptor that activates the sustained chloride conductance can, independent of the chloride ion influx, initiate lipid messenger synthesis.
Subject(s)
Arachidonate Lipoxygenases/metabolism , Chloride Channels/metabolism , Chlorides/metabolism , Nerve Tissue Proteins/metabolism , Neurons/drug effects , Pertussis Toxin , Receptors, Nicotinic/drug effects , Virulence Factors, Bordetella/pharmacology , Acetylcholine/pharmacology , Animals , Aplysia , Arachidonic Acids/biosynthesis , Arecoline/pharmacology , Atropine/pharmacology , Bungarotoxins/pharmacology , Choline/analogs & derivatives , Choline/pharmacology , Cholinergic Antagonists/pharmacology , Conotoxins/pharmacology , Depression, Chemical , Enzyme Activation/drug effects , Ganglia, Invertebrate/cytology , Hexamethonium/pharmacology , Hydroxyeicosatetraenoic Acids/biosynthesis , Ion Channel Gating/drug effects , Ion Transport/drug effects , Neurons/physiology , Nicotine/pharmacology , Nicotinic Agonists/pharmacology , Receptor Cross-Talk/physiology , Receptors, Nicotinic/physiology , Tetraethylammonium/pharmacology , Tubocurarine/pharmacologyABSTRACT
Arachidonic acid is converted to (8R)-hydroperoxyeicosa-5,9,11, 14-tetraenoic acid (8-HPETE) during incubations with homogenates of the central nervous system of the marine mollusc, Aplysia californica. 8-HPETE can be reduced to the corresponding hydroxy acid or be enzymatically converted to a newly identified metabolite, 8-ketoeicosa-5,9,11,14-tetraenoic acid (8-KETE). These metabolites were identified by high performance liquid chromatography, UV absorbance, and gas chromatography/mass spectrometry. Stereochemical analysis of the products demonstrate that the neuronal enzyme is an (8R)-lipoxygenase. Previously we have shown that the neurotransmitters, histamine and Phe-Met-Arg-Phe-amide, activate 12-lipoxygenase metabolism in isolated identified Aplysia neurons. We now show that acetylcholine activates the (8R)-lipoxygenase pathway within intact nerve cells. Thus, both (12S)- and (8R)-lipoxygenase co-exist in intact Aplysia nervous tissue but are differentially activated by several neurotransmitters. The precise physiological role of the 8-lipoxygenase products is currently under investigation, but by analogy to the well-described 12-lipoxygenase pathway, we suggest that (8R)-HPETE and 8-KETE may serve as second messengers in Aplysia cholinoceptive neurons.
