DIFFERENTIAL-EFFECTS OF DEPOLARIZATION ON THE GROWTH OF CRAYFISH TONIC AND PHASIC MOTOR AXONS IN CULTURE

Previous studies have demonstrated neuron-specific differences in the inhibitory effects of depolarization upon neurite outgrowth, We examin ed whether there is a relationship between the normal impulse activity level of an axon and the effect of depolarization upon its growth, In active phasic motor axons and active tonic motor axons grow from crayf ish abdominal nerve cord explants in culture. Depolarization of these axons with high K+ solutions produced greater inhibition of advancing growth cones from the phasic axons than from the tonic axons, During t he period 20-40 min after the beginning of depolarization, tonic axon growth cones continued to advance, whereas phasic axon growth cones re tracted, During chronic depolarization, all of the phasic axons retrac ted during the first day and approximately half of the phasic axons ha d degenerated after 4 days of depolarization, The majority of tonic ax ons continue to grow after 3 days of depolarization, and all of the to nic axon growth survived the 4 days of depolarization, The different r esponses of the growing phasic and tonic axons to depolarization appea r to be Ca2+ dependent, The inhibitory effects of depolarization upon phasic axon growth were reduced by the Ca2+ channel blockers La3+ and Mg2+. Application of a Ca2+ ionophore, A23187, produces greater inhibi tion of phasic axon growth than tonic axon growth. This study demonstr ates that depolarization produces greater inhibition of growth from in active motor axons than from active motor axons, This is likely due to differences in Ca2+ regulation and/or sensitivity to intracellular Ca 2+. (C) 1997 John Wiley & Sons,Inc.