Ca2+ currents in central insect neurons: electrophysiological and pharmacological properties.
Ca2+ currents in dorsal unpaired median (DUM) neurons isolated from the fifth abdominal ganglion of the cockroach Periplaneta americana were investigated with the whole cell patch-clamp technique. On the basis of kinetic and pharmacological properties, two different Ca2+ currents were separated in these cells: mid/low-voltage-activated (M-LVA) currents and high-voltage-activated (HVA) currents. M-LVA currents had an activation threshold of -50 mV and reached maximal peak values at -10 mV. They were sensitive to depolarized holding potentials and decayed very rapidly. The decay was largely Ca2+ dependent. M-LVA currents were effectively blocked by Cd2+ median inhibiting concentration (IC50 = 9 microM), but they also had a remarkable sensitivity to Ni2+ (IC50 = 19 microM). M-LVA currents were insensitive to vertebrate LVA channel blockers like flunarizine and amiloride. The currents were, however, potently blocked by omega-conotoxin MVIIC (1 microM) and omega-agatoxin IVA (50 nM). The blocking effects of omega-toxins developed fast (time constant tau = 15 s) and were fully reversible after wash. HVA currents activated positive to -30 mV and showed maximal peak currents at + 10 mV. They were resistant to depolarized holding potentials up to -50 mV and decayed in a less pronounced manner than M-LVA currents. HVA currents were potently blocked by Cd2+ (IC50 = 5 microM) but less affected by Ni2+ (IC50 = 40 microM). These currents were reduced by phenylalkylamines like verapamil (10 microM) and benzothiazepines like diltiazem (10 microM), but they were insensitive to dihydropyridines like nifedipine (10 microM) and BAY K 8644 (10 microM). Furthermore, HVA currents were sensitive to omega-conotoxin GVIA (1 microM). The toxin-induced reduction of currents appeared slowly (tau approximately 120 s) and the recovery after wash was incomplete in most cases. The dihydropyridine insensitivity of the phenylalkylamine-sensitive HVA currents is a property the cockroach DUM cells share with other invertebrate neurons. Compared with Ca2+ currents in vertebrates, the DUM neuron current differ considerably from the presently known types. Although there are some similarities concerning kinetics, the pharmacological profile of the cockroach Ca2+ currents especially is very different from profiles already described for vertebrate currents.
Wicher
D
D
Sächsische Akademie der Wissenschaften zu Leipzig, Forschungsgruppe Neurohormonale Wirkungsmechanismen, Jena, Germany.
Penzlin
H
H
D000818:Animals; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology*; D004594:Electrophysiology; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000187:drug effects; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects* / Q000502:physiology*; D009498:Neurotoxins / Q000633:toxicity; D018408:Patch-Clamp Techniques; D010526:Periplaneta / Q000502:physiology*
D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D009498:Neurotoxins