Characterization of hypothalamic low-voltage-activated Ca channels based on their functional expression in Xenopus oocytes

Ca-channel currents expressed in Xenopus oocytes by means of messenger RNA extracted from rat thalamohypothalamic complex were studied using the double microelectrode technique. Currents were recorded in C1(-)-free extracellular solutions with 40 mM Ba2+ as a charge carrier. In response to depolarizations from a very negative holding potential (V-b = -120 mV), inward Ba2+ current activated at around -80 mV, peaked at -30 to -20 mV and reversed at + 50 mV indicating that it may be transferred through the low voltage-activated calcium channels. The time-dependent inactivation of the current during prolonged depolarization to -20 mV was quite slow and followed a single exponential decay with a time-constant of 1550 ms and a maintained component constituting 30% of the maximal amplitude. The current could not be completely inactivated at any holding potential. As expected for low voltage-activated current, steady-state inactivation curve shifted towards negative potentials. It could be described by the Boltzmann equation with half inactivation potential -78 mV, slope factor 15 mV and maintained level 0.3. Expressed Ba2+ current could be blocked by flunarizine with K-d = 0.42 mu M, nifedipine, K-d = 10 mu M, and amiloride at 500 mu M concentration. Among inorganic Ca-channel blockers the most potent was La3+ (K-d = 0.48 mu M) while Cd2+ and Ni2+ were not very discriminative and almost 1000-fold less effective than La3+ (K-d = 0.52 mM and K-d = 0.62 mM, respectively). Our data show that messenger RNA purified from thalamohypothalamic complex induces expression in the oocytes of almost exclusively low voltage-activated calcium channels with voltage-dependent and pharmacological properties very similar to those observed for T-type calcium current in native hypothalamic neurons, though kinetic properties of the expressed and natural currents are somewhat different.