1. Single isolated neurones were prepared from the spinal trigeminal nucleus and the dorsal horn of cervical spinal cord of the rat. Spinothalamic and trigeminothalamic neurones were identified using rhodamine‐labelled fluorescent latex microspheres. 2. Calcium currents in these cells were examined by the whole‐cell patch‐clamp technique. Three types of calcium currents, transient (T) and slow inactivating (N and L) types, were identified by their sensitivities to inorganic blockers and rates of inactivation at two different holding potentials (temperature = 21‐25 degrees C). 3. From a holding potential of ‐100 mV, the ICa,T began to activate at ‐60 mV. The current reached its maximum amplitude around ‐30 mV and was inactivated completely when the cell was held more positive than ‐60 mV. The time constant of the inactivation was between 10 and 50 ms. 4. The slow inactivating component of ICa was dissociated into two components by eliciting ICa from two holding potentials of ‐100 and of ‐40 mV. The current (ICa,L) activated from ‐40 mV was characterized by positive activation potentials and a very slow inactivation (time constant, 700‐4000 ms). The current (ICa,N) elicited from a holding potential of ‐100 mV started to activate at ‐30 mV and inactivated slowly with time constants ranging between 400 and 1000 ms. 5. Compared with the ICa,T, the inactivation curve for ICa,N was shifted about 30 mV in the depolarizing direction. ICa,N inactivated over a broader range of potentials, and its inactivation and activation curves overlapped. 6. Cadmium blocked ICa,T at a concentration 24 times higher than that which was needed to block slow inactivating currents. The apparent dissociation constant of nickel for ICa,T is twofold lower than that for the slow inactivating currents. 7. Nimodipine (2 microM) decreased the slow inactivating currents, but had no effect on ICa,T. (‐)‐Bay K 8644 (200 nM) increased both ICa,N and ICa,L and shifted the current activation in the hyperpolarizing direction. This result is different from that obtained in sensory and sympathetic neurones in which ICa,N is insensitive to Bay K 8644.
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