Abstract
Developmental changes in the sensitivity of neurons to tetrodotoxin (TTX) were studied in relation to the cell size in rat dorsal root ganglia (DRG). Na + currents were recorded from neurons of various stages of development. Two types of Na + channels were identified on the basis of their sensitivity to TTX. One type was insensitive to a very high concentration (0.1 mM) of TTX, while the other type was blocked by a low concentration (1 nM) of TTX. These two types of Na + channels were observed throughout the developmental stages examined from day 17 of gestation and adulthood. Thus, both types of Na + channels are already established at the early stage of neuronal development and appear to be retained throughout the life-span of the DRG neuron. The concentration-response relationships for the block of TTX-sensitive Na + current by TTX did not appreciably change during development. Although two types of Na + channels had strikingly different kinetic properties,of each channel type were basically similar throughout development. The TTX-sensitive Na + channels were mainly concentrated in cells with large cell diameters throughout developmental stages examined. These large cells appear to correspond to the ‘large-light’ cells. On the contrary, the TTX-insensitive Na + channels were found in smaller diameter cells which may correspond to the ‘small-dark’ cells. Thus, it is concluded that there are heterogeneous categories of neurons which have Na + channels with different physiological and pharmacological properties. Since Na + channels play a pivotal role in the action potential generation, these heterogeneity of DRG neurons appear to be instrumental in integrating the sensory signals.
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