Fig. 1.

Two types of thalamic relay neurons with different compositions of whole-cell Ca²⁺ currents. Nimodipine and ω-conotoxin MVIIC were not added inA–D but were added in E.A, The cell was held at −120 mV and stepped every 5 sec to the test pulse (−100, −70, −40, and −10 mV) for 200 msec. A test pulse of −100 mV elicited no Ca²⁺ currents, which started to be discernible with a test pulse of −70 mV. At −40 mV, the Ca²⁺ currents were predominated by a rapidly inactivating or transient component (presumably T-current), whereas a more sustained component (presumably L- or N-type or other high-voltage-activated Ca²⁺ currents) became very manifest at −10 mV. B, The same cell and pulse protocol as that in A, except that the holding potential was changed to −80 mV. Note a significant decrease of the transient component but not the sustained component of the Ca²⁺ currents. C, The same pulse protocol as that in A (holding potential of −120 mV) was repeated in another cell. There was no current elicited by a test pulse of −100 mV, but significant transient current was observed with a test pulse of −70 mV. Most interestingly, no sustained current was observed with test pulses of −40 or even −10 mV. D, The same cell and pulse protocol as that in C, except that the holding potential was changed to −80 mV. No current was elicited with test pulses of −100 to −10 mV. E, The same cell and pulse protocol as that in C, except that 0.5 μm ω-conotoxin MVIIC and 1 μmnimodipine were added to the external solution. Note that the transient Ca²⁺ currents are slightly decreased in amplitude, but kinetics of the currents and the current–voltage relationship remain unchanged. F, The cells in which Ca²⁺ current is larger at a test pulse of −10 mV than at a test pulse of −40 mV are classified as type I neurons (typically represented by the cell in A). The cells in which Ca²⁺ current is smaller at a test pulse of −10 mV than at a test pulse of −40 mV are classified as type II neurons (typically represented by the cell in C), which always contain little discernible high-voltage-activated Ca²⁺ currents.