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Developmental changes in the expression of low-voltage-activated Ca²⁺ channels in rat visual cortical neurones

A. N. Tarasenko, D. S. Isaev, A. V. Eremin and P. G. Kostyuk

1. The functional properties of low-voltage-activated (LVA) Ca²⁺ channels were studied in pyramidal neurones from different rat visual cortical layers in order to investigate changes in their properties during early postnatal development. Ca²⁺ currents were recorded in brain slices using the whole-cell patch-clamp technique in rats from three age groups: 2, 3 and 12 days old (postnatal day (P) 2, P3 and P12).

2. It was demonstrated that LVA Ca²⁺ currents are present in neurones from superficial (I-II) and deep (V-VI) visual cortex layers of P2 and P3 rats. No LVA Ca²⁺ currents were observed in neurones from the middle (III-IV) layers of these rats. The LVA Ca²⁺ currents observed in P2 and P3 neurones from both superficial and deep layers could be completely blocked by nifedipine (100 µM) and were insensitive to Ni²⁺ (25 µM).

3. The density of LVA Ca²⁺ currents decreased rapidly during the early stages of postnatal development, while the density of high-voltage-activated (HVA) Ca²⁺ currents progressively increased up to the twelfth postnatal day. No LVA Ca²⁺ currents were found in P12 neurones from any of the layers. Only HVA Ca²⁺ currents with high sensitivity to F^- applied through the patch pipette were observed.

4. The kinetics of LVA Ca²⁺ currents could be well approximated by the m²h Hodgkin-Huxley equation with an inactivation time constant of 24 ± 6 ms. The steady-state inactivation curve fitted by a Boltzmann function had the following parameters: membrane potential at half-inactivation, -86·9 mV; steepness coefficient,3·4 mV.

5. It is concluded that, in visual cortical neurones, LVA Ca²⁺ channels are expressed only in the neurones of deep and superficial layers over a short period during the earliest postnatal stages. These channels are nifedipine sensitive and similar in functional properties to those in the laterodorsal (LD) thalamic nucleus. However, the cortical neurones do not express another ('slow') type of LVA Ca²⁺ channel, which is permanently present in LD thalamic neurones after the second postnatal week, indicating that the developmental time course of cortical and thalamic cellsdifferent.

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