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 mu M 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 mu M Ni2+ indicated that this current was involved in the initial part of the predepolarization. 4. The maintained LVA Ca2+ current, resistant to 100 mu M 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 mu M 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.