CA2- ELECTROPHYSIOLOGICAL AND PHARMACOLOGICAL PROPERTIES( CURRENTS INCENTRAL INSECT NEURONS )

Ca2+ currents in dorsal unpaired median (DUM) neurons isolated from th e 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) curr ents 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 wer e effectively blocked by Cd2+ median inhibiting concentration (IC50 = 9 mu M), but they also had a remarkable sensitivity to Ni2+ (IC50 = 19 mu M). M-LVA currents were insensitive to vertebrate LVA channel bloc kers Like flunarizine and amiloride. The currents were, however, poten tly blocked by omega-conotoxin MVIIC (1 mu M) and omega-agatoxin IVA ( 50 nM). The blocking effects of omega-toxins developed fast (time cons tant tau = 15 s) and were fully reversible after wash. HVA currents ac tivated positive to -30 mV and showed maximal peak currents at +10 mV. They were resistant to depolarized holding potentials up to -50 mV an d decayed in a less pronounced manner than M-LVA currents. HVA current s were potently blocked by Cd2+ (IC50 = 5 mu M) but less affected by N i2+ (IC50 = 40 mu M). These currents were reduced by phenylalkylamines like verapamil (10 mu M) and benzothiazepines like diltiazem (10 mu M ), but they were insensitive to dihydropyridines like nifedipine (10 m u M) and BAY K 8644 (10 mu M). Furthermore, HVA currents were sensitiv e to omega-conotoxin GVIA (1 mu M). The toxin-induced reduction of cur rents appeared slowly (tau similar to 120 s) and the recovery after wa sh was incomplete in most cases. The dihydropyridine insensitivity of the phenylalhylamine-sensitive HVA currents is a property the cockroac h DUM cells share with other invertebrate neurons. Compared with Ca2currents in vertebrates, the DUM neuron currents differ considerably f rom the presently known types. Although there are some similarities co ncerning kinetics, the pharmacological profile of the cockroach Ca2+ c urrents especially is very different from profiles already described f or vertebrate currents.