Barium action potentials in regenerating axons of the lamprey spinal cord

Intracellular recordings were obtained from growing tips of regenerating giant axons in the lamprey spinal cord, the recording sites verified by Lucifer yellow injection. In the presence of extracellular Ba++ (3-6 mM), tetraethylammonium (10-15 mM), and 4-aminopyridine (4-6 mM), action potentials showed prolonged plateaus. The fast initial phase of the action potential, but not the plateau (Ba++-spike), was blocked by tetrodotoxin (10(-6) gm/ml). The Ba++ spike was associated with increased membrane conductance and could be terminated with hyperpolarizing current pulses. Normal axons did not generate similar Ba++ spikes. However, TTX-resistant, voltage-dependant conductance changes could be elicited in normal axons if much higher concentrations of Ba++ (18-30 mM) were used. Their rate of rise was slower than in regenerating axons (0.6 V/sec vs 3.2 V/sec; n = 5), and the response did not outlast the current pulse. The Ba++ responses in normal and regenerating axons were blocked by ions known to block voltage-gated Ca++ conductances (Co++, Ni++, or Cd++). Therefore, these spikes probably represent Ba++ entry through voltage-dependent Ca++ channels, suggesting the presence of a higher-than-average voltage-dependent Ca++ conductance in the growing axon. However, Ca++-dependent spikes could not be obtained under any conditions in either normal or regenerating axons. Simultaneous intracellular recordings from growth cones and axons indicated that the Ba++ spike was initiated, in most cases, at the growth cone. The Ba++ spikes were recorded in regenerating axons for as long as 50 days following cord transection and were not correlatable with the "dying-back" phenomenon in cut axons, which usually is over before day 6. The concept of a higher-than-average voltage-dependent Ca++ conductance in growing tips of regenerating axons is in agreement with the hypothesis that Ca++ is important in regeneration and that regeneration may be related to the process of chemical synaptic transmission.