2 TYPES OF LOW-VOLTAGE-ACTIVATED CA2+ CHANNELS IN NEURONS OF RAT LATERODORSAL THALAMIC NUCLEUS

1. The pharmacological and kinetic properties of two types of low-volt age-activated (LVA) Ca2+ currents were studied in thalamocortical neur ones of the laterodorsal (LD) thalamic nucleus during early postnatal development. The whole-cell patch-clamp technique was used on brain sl ices from rats of three age groups: 12, 14 and 17 days old (postnatal day (P) 12, P14 and P17). 2. In P12 neurones, the population of LVA Ca 2+ channels was homogeneous. LVA Ca2+ current elicited by depolarizing voltage steps from a holding potential more negative than -70 mV was sensitive to nifedipine (K-d = 2.6 mu M). This current reached a maxim um at about -55 mV and had a fast monoexponential decay with a time co nstant, tau(h,f), of 32.3 +/- 4.0 ms. 3. The population of LVA Ca2+ ch annels in P14 and P17 neurones was found to be heterogeneous. A subpop ulation of nifedipine-insensitive LVA Ca2+ channels was observed. The current-voltage curve of the Ca2+ current had a characteristic hump wi th two peaks at about -65 and -55 mV. As well as the fast component (d esignated I-T,I-f), the decay of the LVA current also included a slow component (designated I-T,I-s), with inactivation time constants (tau( h,s)) of 54.2 +/- 4.5 and 68.6 +/- 3.17 ms for P14 and P17 neurones, r espectively. 4. The kinetics of both components could be well approxim ated by the m(2)h Hodgkin-Huxley equation. No significant difference i n activation kinetics was observed. The activation time constants for the fast (tau(m,f)) and slow (tau(m,s)) components were 6.3 +/- 1.0 an d 7.3 +/- 1.5 ms, respectively. 5. La3+ at a concentration of 1 mu M e ffectively blocked the I-T,I-f component but Ni2+ (25 mu M) completely eliminated the I-T,I-s component. 6. Steady-state inactivation curves of both components could he best fitted by a Boltzmann function with membrane potential values at half-maximal inactivation of -85.5 and -9 8.1 mV for the fast and slow components, respectively. 7. It was concl uded that two different subtypes of LVA Ca2+ channel are present in LD neurones. Only the fast type is well expressed at the earliest postna tal stage (P12). The slow type could be found at the end of the second week (P14). The amplitude of the slow current increased progressively up to P17, obviously coinciding with dendritic expansion as judged by progressive increase of the membrane capacitance of the corresponding neurones. This property appears to differentiate neurones of the asso ciative nuclei from neurones of other thalamic nuclei.