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

Abstract

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.

Calcium Current Diversity in Physiologically Different Local Interneuron Types of the Antennal Lobe

“…In all investigated cell types, the current waveforms and physiological properties of I Ca were typical for voltage-activated Ca 2ϩ currents and in the range of other insect preparations [A. mellifera, antennal motorneurons (Kloppenburg et al, 1999a) and Kenyon cells (Schäfer et al, 1994); D. melanogaster, embryonic neurons (Byerly and Leung, 1988;Saito and Wu, 1991); Gryllus bimaculatus, giant interneurons (Kloppenburg and Hörner, 1998); P. americana, embryonic cockroach neurons (Benquet et al, 1999) and DUM neurons (Wicher and Penzlin, 1994); Manduca sexta, motor neurons (Hayashi and Levine, 1992); S. americana, thoracic neurons (Laurent et al, 1993); Schistocerca gregaria, DUM neurons (Heidel and Pflüger, 2006) and thoracic neurons (Pearson et al, 1993)]. However, a quantitative comparison of the functional properties from I Ca between the different cell types revealed significant differences in some physiologically important parameters (for summary, see supplemental Table 1, available at www.jneurosci.org as supplemental material).…”

“…5B). This is a relatively low activation threshold compared with I Ca in uPNs and in many other insect neurons (Byerly and Leung, 1988;Saito and Wu, 1991;Hayashi and Levine, 1992;Laurent et al, 1993;Pearson et al, 1993;Schä-fer et al, 1994;Wicher andPenzlin, 1994, 1997;Kloppenburg and Hörner, 1998;Benquet et al, 1999;Kloppenburg et al, 1999a;Heidel and Pflüger, 2006). In type II LNs, despite the similar activation threshold in both LN types, the half-maximal voltage for activation is significantly more hyperpolarized than in type I LNs (Fig.…”

Neurite‐specific Ca2+ dynamics underlying sound processing in an auditory interneurone

“…Accordingly, conclusions drawn about possible ionic currents underlying the observed changes in cell physiology in response to acoustic stimulation are based on characterized Ca 2þ effects in similar studies (Wicher and Penzlin, 1997;Single and Borst, 1998;Nakamura et al, 1999;Augustine et al, 2003).…”

“…1(D)]. We could not distinguish between contributions from high and low voltage-gated channels (Wicher and Penzlin, 1997). The much slower Ca 2þ dynamics at soma and axon varied as a function of distance from these primary entry areas and therefore indicates passive diffusion of Ca 2þ [ Fig.…”

Development of depolarization‐induced calcium transients in insect glial cells is dependent on the presence of afferent axons

“…In vertebrates, verapamil-sensitive L-type calcium channels are also sensitive to dihydropyridines such as nifedipine (Hille, 1992;Hockerman et al, 1997). In invertebrates, verapamil-sensitive calcium channels lack this sensitivity to dihydropyridines, while dihydropyridine-sensitive channels are insensitive to verapamil (Pelzer et al, 1989;Gielow et al, 1995;Takeuchi et al, 1996;Wicher and Penzlin, 1997;Morales et al, 1999;Beck et al, 2001). With only limited information about the molecular structure of invertebrate (mainly Drosophila) calcium channels, we are not yet able to classify the calcium channels by their pharmacologic properties.…”