Postnatal development of a persistent Na+ current in pyramidal neurons from rat sensorimotor cortex.

1. Whole-cell recordings were performed on acutely isolated pyramidal neurons from rat sensorimotor cortex 2 to 21 days postnatal to study the expression of a tetrodotoxin (TTX) sensitive, voltage dependent, persistent Na+ current (INaP) during different stages of postnatal development. 2. INaP was activated positive to about -60 mV and attained its peak amplitude between -40 and -35 mV. Activation of INaP did not require preceding activation of the transient Na+ current. 3. Peak INaP amplitudes showed a three-fold increase over the first three postnatal weeks, starting from 60.7 +/- 7.5 (SE) pA (n = 6) at postnatal day (P) 2-P5 and reaching 189.1 +/- 20.4 pA (n = 13) at P17-P21. 4. Measurements of peak INaP density, which took concomitant cell growth into account, revealed that a considerable current density already existed in very young neurons (P2-P5: 4.3 +/- 1.0 microA/cm2, n = 6) when compared with INaP density in early adult neurons (P17 - P21: 8.9 +/- 0.8 microA/cm2, n = 5). 5. Our data provide the first direct evidence for the presence of a significant INaP density during early postnatal development of neocortical neurons indicating that this current should play a role in the control of intrinsic excitability at this age.