A T‐type Ca2+ current underlies low‐threshold Ca2+ potentials in cells of the cat and rat lateral geniculate nucleus.

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Abstract
1. The characteristics of a transient inward Ca2+ current (IT) underlying low‐threshold Ca2+ potentials were studied in projection cells of the cat and rat dorsal lateral geniculate nucleus (LGN) in vitro using the single‐electrode voltage‐clamp technique. 2. In cat LGN slices perfused at 25 degrees C with a solution which included 1 mM‐Ca2+ and 3 mM‐Mg2+, IT could be evoked by depolarizing voltage steps to ‐55 mV from a holding potential (Vh) of ‐95 mV and was abolished by reducing [Ca2+]o from 1 to 0.1 mM. IT was also blocked by 8 mM‐Mg2+ and 500 microM‐Ni2+, but 500 microM‐Cd2+ was a significantly less effective antagonist. 3. The inactivation of IT, which occurred at Vh positive to ‐65 mV, was removed as Vh approached ‐100 mV. The process of inactivation removal was also time dependent, with 800‐1000 ms needed for total removal. Activation curves for IT showed a threshold of ‐70 mV and illustrated that IT was extremely voltage sensitive over the voltage range from ‐65 to ‐55 mV. 4. The decay phase of IT followed a single‐exponential time course with a time constant of decay which was voltage sensitive and ranged from 20 to 100 ms. The mean peak conductance increase associated with IT was 8.4 nS (+/‐0.9, S.E.M.). 5. In more ‘physiological’ conditions (35 degrees C and 1.5 mM‐Ca2+, 1 mM‐Mg2+) the voltage dependence of activation and inactivation were unaffected. However, the development and decay of IT proceeded more rapidly and only 500‐600 ms were needed for total removal of inactivation. Under these conditions, the use of voltage ramps showed that depolarization rates of greater than 30 mV/s were necessary for IT activation. 6. The use of multiple voltage‐step protocols illustrated that the process of inactivation removal was rapidly reversed by brief returns to a Vh of ‐50 mV. Furthermore, any delay in IT activation, once the LGN cell membrane potential was in the IT activation range, resulted in a current of reduced amplitude. 7. Although IT in rat LGN cells was briefer and had a shorter latency to peak, it was otherwise similar to that seen in cat LGN cells. 8. The characteristics of IT are very similar to those of the T‐type Ca2+ currents of other excitable membranes. The properties of IT are discussed with respect to its role in generating the low‐threshold Ca2+ potentials which are central to the oscillatory behaviour of thalamic projection cells.