Environmental Factors that Influence the Differentiation and the Development of Voltage-Dependent Sodium Channel in Cultured Dorsal Root Ganglion Cells of Newborn Rats. In: Gilad GM, Gorio A, Kreutzberg GW, editors. Processes of Recovery from Neural Trauma. Berlin: Springer Berlin Heidelberg; 1986. pp. 231-45. [DOI: 10.1007/978-3-642-70699-8_21]

Structural and developmental differences between three types of Na channels in dorsal root ganglion cells of newborn rats

The changes in Na current during development were studied in the dorsal root ganglion (DRG) cells using the whole-cell patch-clamp technique. Cells obtained from rats 1-3 and 5-8 days after birth were cultured and their Na currents were compared. On top of the two types of Na currents reported in these cells (fast-FA current and slow-S current) a new fast current was found (FN). The main characteristics of the three currents are: (i) The voltages of activation are -37, -36 and -23 mV for the FN, FA and S currents, respectively. (ii) The activation and inactivation kinetics of FN and FA currents are about five times faster than those of the S current. (iii) The voltages at which inactivation reaches 50% are -139, -75 and -23 mV for the FN, FA and S currents, respectively. The kinetics and voltage-dependent parameters of the three currents and their density do not change during the first eight days after birth. However, their relative frequency in the cells changes. In the 1-3 day-old rats the percent of cells with S, FA, and mixed S + FN currents is 22, 18, and 60% of the cells, respectively. In the 5-8 day-old, the percent of cells with S, FA, and FN + S is 10, 66 and 22%. The relative increase in the frequency of cells with FA current during development can contribute to the ease of action potential generation compared with cells with FN currents, which are almost completely inactivated under physiological conditions. The predominance of FA cells also results in a significant decrease in the relative frequency of cells with the high-threshold, slow current. Antibodies directed against a part of the S4 region of internal repeat I of the sodium channel (C1+, amino acids 210-223, eel channel numbering) were found to shift the voltage dependence of FA current inactivation (but not of FN or S currents) to more negative potentials. The effect was found only when the antibodies were applied externally. The results suggest that FN, FA and S types of Na currents are generated by channels, which are different in the topography of the C1+ region in the membrane.

Characterization of sodium currents in mammalian sensory neurons cultured in serum-free defined medium with and without nerve growth factor

The influence of nerve growth factor (NGF) on Na currents of rat dorsal root ganglia (DRG) was studied in neurons obtained from newborns and cultured for 2-30 hr in serum-free defined medium (SFM). Cell survival for the period studied was 78-87% both with and without NGF. Na currents were detected in all cells cultured for 6-9 hr. They were also detected after 2 hr in culture in 21.5% of the cells cultured without NGF (-NGF cells), and in 91.5% of the cells cultured with NGF (+NGF cells). Current density of the -NGF cells was 2.3 and 2 pA/microns 2 after growth for 2 and 6-9 hr, respectively, compared to 3.0 and 3.9 pA/microns 2 for the +NGF cells. The +NGF cells were separated into fast (F), Intermediate (I) and slow (S) cells, based on the Na current they expressed, while -NGF cells were all of the I type. F, I and S currents differed in their voltage-dependent inactivation (Vh50 = -79, -28 and -20 mV), kinetics of inactivation (tau h = 0.55, 1.3 and 7.75 msec), and TTX sensitivity (Ki = 60, 550 and 1100 nM). All currents were depressed by [Ca]0 with a KdCa of 22, 17 and 8 mM for F, I and S currents, respectively. Current density of F and S currents was 5.5 and 5 pA/micron 2 for the I current. The concentration-dependent curve of I current vs. TTX indicated that I current has two sites: one with F-like and another with S-like Ki for TTX. Hybridization of F and S currents yielded I-like currents. Thus, the major effect of NGF on Na currents in SFM is the acceleration of Na current acquisition and diversity, reflected in an increase of either the S or F type in a cell.

Abnormal impulse discharge in primary afferent axons injured in the peripheral versus the central nervous system

Chronic injury to sensory axons in the rat peripheral nerve induces pathophysiologic changes in the axolemma at the cut nerve end, which are reflected in spontaneous ectopic impulse discharge and hyperexcitability to a range of depolarizing stimuli. We asked whether sensory axons injured in the central nervous system (CNS) also respond in this way. Primary afferent axons were severed in the sciatic nerve and, alternatively, in the midcervical or upper lumbar dorsal column (DC). Measurements of abnormal discharge from myelinated afferents showed high levels of spontaneous activity generated at the nerve injury site, especially during the period 3-16 days postoperatively, but comparatively little activity generated at the DC lesion site at any postoperative time. There was a corresponding difference in ectopic hyperexcitability to mechanical and adrenergic stimulation, and to depolarization with topical K+. DC lesion sites were not made more excitable by concurrent transection of the sciatic nerve, or by placing an autologous graft of excised sciatic nerve tissue into the DC defect at the time of initial surgery. Transection sites on dorsal roots L4 and L5 yielded abnormal discharge similar to that of sciatic nerve neuromas, indicating that the relative silence of DC transection sites was related to the CNS environment and not to position with respect to the sensory cell body.