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

Abstract

SUMMARY1. 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 (T.) varying between 8 and 2 ms at command potentials of -60 to -10 mV at 23 'C. The rate of inactivation was also voltage dependent, and the time constant Th 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. 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 2-5.9. These properties of the T-current are such that its activation is necessary and sufficient to cause the generation of low-threshold calcium spikes in thalamic relay neurones, although other ionic conductances may modulate such events.

Developmental changes in electrophysiological properties of LGNd neurons during reorganization of retinogeniculate connections

“…1, P30, sag;McCormick and Pape, 1990). Moreover, a low-threshold spike resulting from activation of the calcium current IT (Jahnsen and Llinas, 1984a,b;Coulter et al, 1989;Crunelli et al, 1989;Hernandez-Cruz and Pape, 1989) was present at the end of the hyperpolarizing pulse ( Fig. 1, P30, Ca+ + spikes).…”

“…We have further estimated the effects of age-dependent increase in calcium spike amplitude by counting the number of action potentials evoked by the calcium spikes in each cell and the results are pooled together in the histogram of Figure 6A. To standardize the findings, we compared results obtained at resting membrane potential levels after the application of current pulses that hyperpolarized the cell to approximately -90 mV, where removal of inactivation of Z, is relatively complete (Coulter et al, 1989). During the first postnatal week approximately half of the cells studied did not generate calcium spikes or generated such weak calcium currents that action potentials were not elicited.…”

Calcium currents in pyramidal CA1 neurons in vitro after kindling epileptogenesis in the hippocampus of the rat

“…In the model of absence epilepsy (GAERS), 44 the enhanced LVA calcium currents in the thalamus can drive rhythmic activities and underlie rebound activity. 8,16,19 Antiepileptic drugs like ethosuximide, trimethadione 9 and valproic acid 23 have been reported to affect LVA calcium currents.…”

Molecular Physiology of Low-Voltage-Activated T-type Calcium Channels

“…The temperature sensitivity of T-type currents has been measured in thalamic neurons (85), DRG neurons (305), N1E-115 neuroblastoma cells (298), and GH 3 cells (343). Heating from 22 to 37°C caused over twofold increases in current amplitudes and accelerated channel activation and inactivation.…”