Peptidergic modulation of insect voltage-gated Ca(2+) currents: role of resting Ca(2+) current and protein kinases A and C.
The modulation of voltage-gated Ca(2+) currents in isolated dorsal unpaired median (DUM) neurons of cockroach was investigated using whole cell patch clamp. The neuropeptide neurohormone D (NHD), a member of the adipokinetic hormone family, affected Ca(2+) currents at pico- to nanomolar concentrations. It strongly enhanced currents activating at lower depolarizations, whereas those activating at strong depolarizations were slightly attenuated. The first effect results from upregulation of a previously characterized omega-conotoxin MVIIC- and omega-agatoxin IVA-sensitive "mid/low voltage-activated" (M-LVA) Ca(2+) current. The cAMP-analogue 8-bromo-cAMP, forskolin, and the catalytic subunit of protein kinase A (PKA) mimicked the stimulating action of NHD. In addition, preincubation of neurons with the PKA inhibitor KT 5720 abolished the action of NHD. Thus NHD seems to upregulate the M-LVA current via channel phosphorylation by PKA. Activation of protein kinase C by oleoylacetylglycerol (OAG) mimicked the effect of NHD, and subsequent NHD application only enhanced the current to a moderate extent. On the other hand, inhibition of protein kinase C (PKC) by Gö 6976 abolished the NHD effect. These results indicate that also PKC, too, may play a role in the peptidergic modulation of the M-LVA Ca(2+) current. The reduction of Ca(2+) currents in the high-voltage-range is caused by the NHD-induced upregulation of a voltage-independent Ca(2+) resting current, I(Ca,R), which most probably leads to enhanced Ca(2+)-dependent inactivation of voltage-gated Ca(2+) currents. To assess the major consequences of the Ca(2+) current changes, current-clamp investigations were performed. Experiments with iberiotoxin, a specific blocker of BK-type Ca(2+)-dependent K(+) currents, and the M-LVA current-blocking omega-toxins suggested that NHD causes-via increasing Ca(2+)-dependent K(+) currents-a larger hyperpolarization of action potentials. The lowering in the action potential threshold produced by NHD, however, seems to be a direct consequence of the hyperpolarizing shift of the activation curve of total Ca(2+) current resulting from NHD-induced upregulation of the M-LVA current component.
Wicher
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Sächsische Akademie der Wissenschaften zu Leipzig, D-07743 Jena, Germany. b6widi@pan.zoo.uni-jena.de
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D003058:Cockroaches / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D004553:Electric Conductivity; D004594:Electrophysiology; D017952:Ganglia, Invertebrate / Q000502:physiology; D006706:Homeostasis; D019476:Insect Proteins / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology*; D009479:Neuropeptides / Q000494:pharmacology; D009994:Osmolar Concentration; D011493:Protein Kinase C / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology
D015220:Calcium Channels; D019476:Insect Proteins; D009479:Neuropeptides; C043192:glutamyl-valyl-asparaginyl-phenylalanyl-seryl-prolyl-asparaginyl-tryptophanamide; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D002118:Calcium