Differences between mammalian ventral and dorsal spinal roots in response to blockade of potassium channels during maturation

Abstract

Differences in potassium channel organization between motor and sensory fibers have been described in amphibians but have not previously been examined in mammals. In the present investigation, we studied whole nerve and single axon responses following pharmacological blockade of potassium conductance in rat ventral and dorsal spinal roots during maturation. Our results indicate a differential sensitivity in maturing mammalian motor and sensory fibers which is most apparent in younger roots. Specifically, application of 4-aminopyridine (4-AP) results in a broadening of the compound action potential in ventral roots which is associated with a delayed repolarization of the individual action potential of single fibers. In contrast, blockade of potassium channels in young dorsal roots results in a late negativity in the compound response which is correlated with multispike bursting activity recorded from single sensory fibers. The effects of 4-AP on ventral root fibers diminish earlier in the course of maturation than do the effects of 4-AP in dorsal root fibers. These results demonstrate developmental differences in the functional organization of potassium channels in mammalian motor and sensory axons which may have implications for differences in coding properties between these two classes of axons.