IONIC MECHANISMS OF ELECTRICAL EXCITABILITY IN RAT SENSORY NEURONS DURING POSTNATAL ONTOGENY

Inward currents in the somatic membrane of rat dorsal root ganglion neurons have been studied in three groups of animals (8-10, 45-50 and 90-100 days of postnatal development) by intracellular perfusion and voltage clamp techniques. Altogether, 228 neurons have been examined (76 in each age group). Four components have been identified in the inward current: fast tetrodotoxin-sensitive (I(Na)f) and slow tetrodotoxin-insensitive (I(Na)s) sodium, low threshold (I(Ca)l) and high threshold (I(Ca)h) calcium currents. The existence of certain types of inward currents in the somatic membrane allowed the heterogeneous population of all the investigated neurons to be divided into six homogeneous subpopulations. A significant decrease in the percentage of neurons was found, demonstrating four types of inward currents (I(Na)f, I(Na)s, I(Ca)l, I(Ca)h) during postnatal ontogenesis. The percentage of cells in the subpopulation showing only I(Na)f and I(Ca)h increased from 26% in the first age group to 43% in the second and 62% in the third age groups. Correlations between densities of various types of inward currents were studied. A linear relationship was found between the densities of I(Ca)h and I(Na)s in subpopulations of neurons with I(Na)s. An inverse dependence was observed between the densities of I(Na)f and I(Ca)h in cells showing only two current components. In all cells studied a "washout" of I(Ca)h was observed during intracellular dialysis with saline solutions. Recovery of calcium conductance produced by intracellular application of an cAMP-ATP-Mg2+ complex was different in neurons with various inward current combinations. Intracellular introduction of cAMP-ATP-Mg2+ failed to restore I(Ca)h in most cells of the second and third age groups. However, such restoration could be achieved in most cells of the first age group.