Two distinct low-voltage-activated Ca2+ currents contribute to the pacemaker mechanism in cockroach dorsal unpaired median neurons.
1. The contribution of Ca2+ currents to the endogenous firing properties of cockroach isolated adult dorsal unpaired median neurons was investigated using whole cell patch-clamp technique with 5 mM Ca2+ as the charge carrier. At least three types of Ca2+ currents, a high-voltage-activated Ca2+ current and two low-voltage-activated (LVA) Ca2+ currents, have been found in these neurons. This study focused on the LVA Ca2+ currents, which are suitable candidates in the generation of the slow predepolarization because of their low threshold of activation. 2. The global LVA Ca2+ current could be dissociated by means of nickel sensitivity, deactivation time constant and voltage dependence of time to peak, tail current amplitude and inactivation, as transient and maintained LVA Ca2+ currents. 3. The transient LVA Ca2+ current, sensitive to 100 microM Ni2+, was isolated by using a subtraction procedure. It was activated at -70 mV and half-inactivated at -59.5 mV. The inactivation was purely voltage dependent. Current-clamp experiments performed with 150 microM Ni2+ indicated that this current was involved in the initial part of the predepolarization. 4. The maintained LVA Ca2+ current, resistant to 100 microM Ni2+, was activated in a range of potential 10 mV more positive than the transient LVA Ca2+ current, and its voltage dependence of inactivation displayed a U-shaped-curve. 5. Replacing Ca2+ with Ba2+ in equimolar amount or low internal Ca2+ concentration [5 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) in the pipette] induced a monotonic voltage dependence of inactivation and increased the rate of relaxation of this current. These effects were mimicked by high internal Ca2+ concentration [0.1 mM Ca2+ and no ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the pipette]. This demonstrated an unusual Ca2+-sensitive inactivation process that varied over a narrow range of Ca2+ concentrations. 6. Current-clamp experiments performed under 150 microM Ni2+, with 15 mM external Ca2+ concentration (which potentiated the maintained LVA current within 30 s of superfusion) or with 5 mM BAPTA in the pipette demonstrated the participation of this current in the last two-thirds of the slow predepolarizing phase. 7. Our findings demonstrated, for the first time in neurosecretory cells, the coexistence of two distinct LVA Ca2+ currents, which have specialized function in the generation of the pacemaker activity.
Grolleau
F
F
Laboratoire de Neurophysiologie, Centre Nationale de la Recherche Scientifique, Université d'Angers, France.
Lapied
B
B
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D003058:Cockroaches; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009532:Nickel / Q000494:pharmacology; D018408:Patch-Clamp Techniques
D015220:Calcium Channels; C022838:nickel chloride; D009532:Nickel; D002118:Calcium