ABSTRACT
The distribution of iodinated margatoxin ([125I]margatoxin) binding sites in rat was investigated by autoradiography. Rat striatum expresses a high density of margatoxin binding sites and, therefore, the effects of margatoxin, charybdotoxin and iberiotoxin have been studied on [3H]dopamine release from rat striatal slices in vitro. Margatoxin (0.1-100 nM) and charybdotoxin (10-1000 nM), but not iberiotoxin increased the spontaneous and the electrically evoked [3H]dopamine release. [3H]dopamine release by margatoxin was inhibited by tetrodotoxin and omega-conotoxin GVIA, but not by atropine, naloxone, N(omega)-nitro-L-arginine and neurokinin or neurotensin receptor antagonists. In the buffer solution used for release experiments, [125I]margatoxin labels a maximum of 0.12 pmol of sites/mg protein in rat striatal membranes with a Kd of 5 pM. [125I]margatoxin binding was inhibited by margatoxin (Ki of 4 pM), charybdotoxin (Ki of 162 pM) but not by iberiotoxin. We conclude that inhibition of margatoxin-sensitive voltage-gated K+ channels increases [3H]dopamine release demonstrating their role in repolarization of nigrostriatal projections. In contrast, iberiotoxin-sensitive, high-conductance Ca2+-activated K+ channels are not involved in release of [3H]dopamine.
Subject(s)
Corpus Striatum/drug effects , Dopamine/metabolism , Neurotoxins/pharmacology , Potassium Channels/drug effects , Animals , Binding Sites , Charybdotoxin/pharmacology , Corpus Striatum/metabolism , Iodine Radioisotopes , Ion Channel Gating , Male , Peptides/pharmacology , Potassium Channels/metabolism , Radioligand Assay , Rats , Rats, Sprague-Dawley , Scorpion Venoms , TritiumABSTRACT
Secretoneurin is a newly discovered peptide found in high concentrations in brain. We have studied the release of secretoneurin and noradrenaline from superfused hypothalamic slices from rat brain. Both electrical stimulation and potassium induced depolarisation released secretoneurin and noradrenaline from these slices in a calcium-dependent manner. Electrical stimulation caused a preferential release of noradrenaline when compared to the secretion elicited by high potassium. The time course of secretoneurin release was more protracted than that of noradrenaline. The calcium channel blocker omega-conotoxin inhibited only the electrically induced release of noradrenaline, whereas nifedipine inhibited only that of secretoneurin. These results establish that secretoneurin is secreted from neurons. Inhibition of this release by nifedipine is consistent with the concept that secretion from large dense core vesicles occurs at sites different from that of small vesicles and depends on calcium influx via L-type calcium channels.
