Calcium currents in rat thalamocortical relay neurones: kinetic properties of the transient, low‐threshold current.

Abstract

1. Calcium currents were recorded with whole‐cell voltage‐clamp procedures in relay neurones of the rat thalamus which had been acutely isolated by an enzymatic dissociation procedure. 2. Low‐threshold and high‐threshold Ca2+ currents were elicited by depolarizing voltage steps from holding potentials more negative than ‐60 mV. A transient current, analogous to the T‐current in sensory neurones, was activated at low threshold near ‐65 mV and was completely inactivating at command steps up to ‐35 mV. Voltage steps to more depolarized levels activated a high‐threshold current that inactivated slowly and incompletely during a 200 ms step depolarization. 3. The high‐threshold current contained both non‐inactivating and slowly inactivating components which were insensitive and sensitive to holding potential, respectively. 4. A ‘T‐type’ current was prominent in relay neurones, in both absolute terms (350 pA peak current average) and in relation to high‐threshold currents. The average ratio of maximum transient to maximum sustained current was greater than 2. 5. T‐current could be modelled in a manner analogous to that employed for the fast Na+ current underlying action potential generation, using the m3h format. The rate of activation of T‐current was voltage dependent, with a time constant (tau m) varying between 8 and 2 ms at command potentials of ‐60 to ‐10 mV at 23 degrees C. The rate of inactivation was also voltage dependent, and the time constant tau h varied between 50 and 20 ms over the same voltage range. With command potentials more positive than ‐35 mV, the inactivation of Ca2+ current could no longer be fitted by a single exponential. 6. Steady‐state inactivation of T‐current could be well fitted by a Boltzman equation with slope factor of 6.3 and half‐inactivated voltage of ‐83.5 mV. 7. Recovery from inactivation of T‐current was not exponential. The major component of recovery (70‐80% of total) was not very voltage sensitive at potentials more negative than ‐90 mV, with tau r of 251 ms at ‐92 mV and 23 degrees C, compared to 225 ms at ‐112 mV. A smaller, voltage‐sensitive component accounted for the remainder of recovery. 8. All kinetic properties, including rates of activation, inactivation, and recovery from inactivation, as well as the amplitude of T‐current, were temperature sensitive with Q10 (temperature coefficient) values of greater than 2.5.(ABSTRACT TRUNCATED AT 400 WORDS)