Peptidergic modulation of insect voltage-gated Ca2+ currents:: Role of resting Ca2+ current and protein kinases A and C

The modulation of voltage-gated Ca2+ 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 Ca2+ 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) Ca2+ 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 Go 6976 abolished the NHD effect. These results indicate that also PKC, too, may play a role in the peptidergic modulation of the M-LVA Ca2+ current. The reduction of Ca2+ currents in the high-voltage-range is caused by the NHD-induced upregulation of a voltage-independent Ca2+ resting current, I-Ca,I-R, which most probably leads to enhanced Ca2+-dependent inactivation of voltage-gated Ca2+ currents. To assess the major consequences of the Ca2+ current changes, current-clamp investigations were performed. Experiments with iberiotoxin, a specific blocker of BK-type Ca2+-dependent K+ currents, and the M-LVA current-blocking omega -toxins suggested that NHD causes-via increasing Ca2+-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 Ca2+ current resulting from NHD-induced upregulation of the M-LVA current component.