Short-term and long-term alterations in neuronal excitability during injury-induced axonal regeneration in ganglia and cell culture

Helisoma neurons B5, regenerating axonal arbors following crush-axotomyin vivo, displayed a transient (<24 h) reduction in excitability followed by a sustained period of hyperexcitability (>13 d). Neurons isolated into outgrowth-permissive cell culture conditions expressed a similar pattern of hypo- and hyperexcitability; however, excitability of neurons B5 in culture was elevated for only 5 d and then declined to a lower, stable level. The expression of these alterations in excitability was neurite outgrowth-independent and required the presence of ganglia-derived conditioning factors in the culture medium. Excitability of neurons in medium lacking conditioning factors fell by day 3 to minimal levels. Conditioned medium was effective in rescuing excitability of neurons deprived of conditioning factors during their first 3 days of cell culture, but not following longer periods of deprivation. Exposure to the protein synthesis inhibitor, anisomycin, blocked the ability of conditioning factors to rescue B5 neuronal excitability. Together, results from cell culture suggest that mechanisms underlying neuronal excitability following nerve injury are independent of process outgrowth state, but require exposure to conditioning factors derived from injured neural tissue within several days of axonal insult. Although changes in B5 neurite outgrowth and neuronal excitability were expressed simultaneously following axonal injury, their independence suggests the existence of an underlying regenerative state that regulates both cellular modifications.