Identification of T-Type alpha 1H Ca2+ Channels (Cav3.2) in Major Pelvic Ganglion Neurons

1 Department of Life Science, Sogang University, Shinsu-1Dong, Seoul 121-742, Republic of Korea; and 2 Department of Thoracic and Cardiovascular Surgery; 3 Department of Physiology and Institute of Basic Medical Science, Yonsei University Wonju College of Medicine, Ilsan-Dong 162, Wonju, Kangwon-Do 220-701, Republic of Korea Lee, Jung-Ha, Eun-Gi Kim, Byong-Gon Park, Kyoung-Han Kim, Seung-Kyu Cha, In Deok Kong, Joong-Woo Lee, and Seong-Woo Jeong. Identification of T-Type 1H Ca 2+ Channels (Ca v 3.2) in Major Pelvic Ganglion Neurons. J. Neurophysiol. 87: 2844-2850, 2002. Among autonomic neurons, sympathetic neurons of the major pelvic ganglia (MPG) are unique by expressing low-voltage-activated T-type Ca 2+ channels. To date, the T-type Ca 2+ channels have been poorly characterized, although they are believed to be potentially important for functions of the MPG neurons. In the present study, thus we investigated characteristics and molecular identity of the T-type Ca 2+ channels using patch-clamp and RT-PCR techniques. When the external solution contained 10 mM Ca 2+ as a charge carrier, T-type Ca 2+ currents were first activated at 50 mV and peaked around 20 mV. Besides the low-voltage activation, T-type Ca 2+ currents displayed typical characteristics including transient activation/inactivation and voltage-dependent slow deactivation. Overlap of the activation and inactivation curves generated a prominent window current around resting membrane potentials. Replacement of the external Ca 2+ with 10 mM Ba 2+ did not affect the amplitudes of T-type Ca 2+ currents. Mibefradil, a known T-type Ca 2+ channel antagonist, depressed T-type Ca 2+ currents in a concentration-dependent manner (IC 50 = 3 µM). Application of Ni 2+ also produced a concentration-dependent blockade of T-type Ca 2+ currents with an IC 50 of 10 µM. The high sensitivity to Ni 2+ implicates 1H in generating the T-type Ca 2+ currents in MPG neurons. RT-PCR experiments showed that MPG neurons predominantly express mRNAs encoding splicing variants of 1H (called pelvic Ta and Tb, short and long forms of 1H, respectively). Finally, we tested whether the low-threshold spikes could be generated in sympathetic MPG neurons expressing T-type Ca 2+ channels. When hyperpolarizing currents were injected under a current-clamp mode, sympathetic neurons produced postanodal rebound spikes, while parasympathetic neurons were silent. The number of the rebound spikes was reduced by 10 µM Ni 2+ that blocked 5