DEVELOPMENTAL-CHANGES IN THE EXPRESSION OF LOW-VOLTAGE-ACTIVATED CA2-NEURONS( CHANNELS IN RAT VISUAL CORTICAL)

1. The functional properties of low-voltage-activated (LVA) Ca2+ chann els were studied in pyramidal neurones from different rat visual corti cal layers in order to investigate changes in their properties during early postnatal development. Ca2+ currents were recorded in brain slic es using the whole-cell patch-clamp technique in rats from three age g roups: 2, 3 and 12 days old (postnatal day (P) 2, P3 and P12). 2. It w as demonstrated that LVA Ca2+ currents are present in neurones from su perficial (I-II) and deep (V-VI) visual cortex layers of P2 and P3 rat s. 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 bl ocked by nifedipine (100 mu M) and were insensitive to Ni2+ (25 mu M). 3. The density of LVA Ca2+ currents decreased rapidly during the earl y stages of postnatal. development, while the density of high-voltage- activated (HVA) Ca2+ currents progressively increased up to the twelft h 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 LV A Ca2+ currents could be well approximated by the m(2)h Hodgkin-Huxley equation With an inactivation time constant of 24 +/- 6 ms. The stead y-state inactivation curve fitted by a Boltzmann function had the foll owing parameters: membrane potential at half-inactivation, -86.9 mV; s teepness coefficient, 3.4 mV. 5. It is concluded that, in visual corti cal neurones, LVA Ca2+ channels are expressed only in the neurones of deep and superficial layers over a short period during the earliest po stnatal stages. These channels are nifedipine sensitive and similar in functional properties to those in the laterodorsal (LD) thalamic nucl eus. However, the cortical neurones do not express another ('slow') ty pe of LVA Ca2+ channel, which is permanently present in LD thalamic ne urones after the second postnatal week, indicating that the developmen tal time course of cortical and thalamic cells is different.