Functional parameters of voltage-activated Ca2+ currents from olfactory interneurons in the antennal lobe of Periplaneta americana.

Toward our goal to better understand the physiological parameters that mediate olfactory information processing on the cellular level, voltage-activated calcium currents (I(Ca)) in olfactory interneurons of the antennal lobe from adult cockroaches were analyzed under two conditions: 1) in acutely dissociated cells (in vitro) and 2) in an intact brain preparation (in situ). The study included an analysis of modulatory effects of potential inorganic and organic Ca(2+) channel blockers. I(Ca) was isolated and identified using pharmacological, voltage, and ion substitution protocols. I(Ca) consisted of two components: transient and sustained. The decay of the transient component was largely Ca(2+) dependent. In vitro, I(Ca) had an activation threshold of -50 mV with a maximal peak current at -7 mV and a half-maximal voltage (V(0.5act)) for tail-current activation of -18 mV. In situ these parameters were significantly shifted to more depolarized membrane potentials: I(Ca) activated at -40 mV with a maximal peak current at 8 mV and a V(0.5act) for tail-current activation of -11 mV. The sensitivity of I(Ca) to the divalent cations Cd(2+), Co(2+), and Ni(2+) was dose dependent. The most effective blocker was Cd(2+) with an IC(50) of 10(-5) M followed by Ni(2+) (IC(50)=3.13 x 10(-3) M) and Co(2+) (IC(50)=1.06 x 10(-3) M). The organic channel blockers verapamil, diltiazem, and nifedipine also blocked I(Ca) in a dose-dependent way and had differential effects on the current waveform. Verapamil blocked I(Ca) with an IC(50) of 1.5 x 10(-4) M and diltiazem had an IC(50) of 2.87 x 10(-4) M. Nifedipine blocked I(Ca) by 33% at a concentration of 10(-4) M.