The Journal of Neurophysiology Vol. 77 No. 1 January 1997, pp. 186-199
Copyright ©1997 The American Physiological Society

Ca²⁺ Currents in Central Insect Neurons: Electrophysiological and Pharmacological Properties

Dieter Wicher and Heinz Penzlin

Sächsische Akademie der Wissenschaften zu Leipzig, Forschungsgruppe Neurohormonale Wirkungsmechanismen,D-07743 Jena, Germany

Wicher, Dieter, and Heinz Penzlin. Ca²⁺ currents in central insect neurons: electrophysiological and pharmacological properties. J. Neurophysiol. 77: 186-199, 1997. 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 µM), but they also had a remarkable sensitivity to Ni²⁺ (IC₅₀ = 19 µM). M-LVA currents were insensitive to vertebrate LVA channel blockers like flunarizine and amiloride. The currents were, however, potently blocked by -conotoxin MVIIC (1 µM) and -agatoxin IVA (50 nM). The blocking effects of -toxins developed fast (time constant  = 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 µM) but less affected by Ni²⁺ (IC₅₀ = 40 µM). These currents were reduced by phenylalkylamines like verapamil (10 µM) and benzothiazepines like diltiazem (10 µM), but they were insensitive to dihydropyridines like nifedipine (10 µM) and BAY K 8644 (10 µM). Furthermore, HVA currents were sensitive to -conotoxin GVIA (1 µM). The toxin-induced reduction of currents appeared slowly ( ~ 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 currents 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.

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