Comparative analysis of ionic currents in the somatic membrane of embryonic and newborn rat sensory neurons

Inward currents in the somatic membrane of dissociated rat dorsal root ganglion neurons have been studied in two groups of animals (17 days of embryonic development and the first day after birth) by suction pipette (whole-cell configuration) and voltage-clamp techniques. Altogether 157 neurons were examined. Four components in the inward currents have been identified: fast tetrodotoxin-sensitive (INaf) and slow tetrodotoxin insensitive (INas) sodium, low-(ICal) and high-threshold (ICah) calcium currents. The percentage of neurons demonstrating four types of inward currents INaf, INas, ICal, ICah increased from 21% in embryo to 61% in newborn. The percentage of neurons with INaf, INas and ICah increased from 4% in embryonic to 14% in the first day after birth. The percentage of cells with INaf, ICal and ICah (without tetrodotoxin-insensitive INas) decreased from 56 to 11% in embryo and newborn rats, respectively. A statistically significant linear correlation was found between the densities of INaf and ICah currents for both ages. A correlation also occurred between the densities of INas and ICah. A reciprocal relation between the densities of both types of calcium currents and the size of cell soma was found in the neurons with all four types of inward currents from newborn animals. A comparison of these data with previous study of inward currents during postnatal development indicates that the most dramatic changes in their distributions and mean densities takes place some time after the birth of the animals.