Voltage-dependent calcium currents in trigeminal chick neurons

Classification of voltage-dependent Ca2+ channels in trigeminal ganglion neurons from neonatal rats

We examined the subtypes and characteristics of the Ca(2+) channel in small (diameter < 30 microm) trigeminal ganglion (TG) neurons from neonatal rats by means of whole cell patch clamp techniques. There were two current components, low-voltage activated (LVA) and high-voltage activated (HVA) I(Ba), with different activation ranges and waveforms. LVA I(Ba) elicited from a depolarizing step pulse at a holding potential (HP) of -80 mV was inhibited by 0.25 mM amiloride (62%), which did not produce any significant inhibition of the peak amplitude of HVA I(Ba). The application of 0.5 mM amiloride inhibited 10% of the HVA I(Ba). The LVA I(Ba) was also reduced by changing the HP from -80 to -60 mV (61%), and under these conditions the peak amplitude of HVA I(Ba) did not change significantly. In addition, HVA I(Ba) and LVA I(Ba) showed marked differences in their inactivation properties. Experiments with several Ca(2+) channel blockers revealed that on average, 26% of the HVA I(Ba) was nifedipine (10 microM) sensitive, 55% was sensitive to omega-conotoxinGVIA (1 microM), 4% was blocked by omega-agatoxinIVA (1 microM), and the remainder of the current that was resistant to the co-application of all three Ca(2+) channel blockers was 15% of the total current. These results suggest that the application of amiloride and the alteration of the holding potential level can discriminate between HVA and LVA Ba(2+) currents in TG neurons, and that TG neurons expressed T-, L-, N-, P-/Q- and R-type Ca(2+) channels.

Voltage-dependent sodium and calcium currents in acutely isolated adult rat trigeminal root ganglion neurons

Voltage-dependent sodium (INa) and calcium (ICa) currents in small (<30 microM) neurons from adult rat trigeminal root ganglia were characterized with a standard whole cell patch-clamp technique. Two types of INa showing different sensitivity to tetrodotoxin (TTX) were recorded, which showed marked differences in their activating and inactivating time courses. The activation and the steady-state inactivation kinetics of TTX-resistant INa were more depolarized by about +20 and +30 mV, respectively, than those of TTX-sensitive INa. Voltage-dependent ICa was recorded under the condition that suppressed sodium and potassium currents with 10 mM Ca2+ as a charge carrier. Depolarizing step pulses from a holding potential of -80 mV evoked two distinct inward ICa, low-voltage activated (LVA) and high-voltage activated (HVA) ICa. LVA ICa was first observed at -60 to -50 mV and reached a peak at about -30 mV. Amiloride (0.5 mM) suppressed approximately 60% of the LVA ICa, whereas approximately 10% of HVA ICa was inhibited by the same concentration of the amiloride. LVA ICa was far less affected by the presence of external Cd2+ or the replacement of Ca2+ by 10 Ba2+ than HVA ICa. The omega-conotoxin GVIA (omega-CgTx), an N-type ICa blocker, suppressed approximately 65% of the whole cell HVA ICa at the concentration of 1 microM. The omega-CgTx-resistant HVA ICa was sensitive to nifedipine (10 microM), a dihydropyridine (DHP) calcium channel antagonist, which produced an additional blockade by approximately 25% of the drug-free control ( approximately 70% of the omega-CgTx-resistant ICa). The combination of 10 microM nifedipine and 1 microM omega-CgTx left approximately 13% of the drug-free control ICa unblocked. The DHP agonist S(-)-BayK8644 (5 microM) shifted the activation of the HVA ICa to more negative potentials and increased its maximal amplitude. Additionally, S(-)-BayK8644 caused the appearance of a slowed component of the tail current. These results clearly demonstrate that the presence of two types of sodium channels, TTX sensitive and resistant, and three types of calcium channels, T, L, and N type, in the small-sized adult rat trigeminal ganglion neurons.