Ionic dependence of the axotomy-induced long-lasting firing in an identified crayfish motoneuron

I have reported previously that axotomy of an identifiable anal motoneuron of crayfish Procambarus clarkii induces a long-lasting firing and that a prolonged depolarizing pulse to its cut end can induce a similar response. In this study, I confirmed that this stimulus is comparable to axotomy; the frequency of stimulus-induced firing increases linearly with the stimulus intensity and its firing pattern is the same as that following axotomy. Then, when the cut end was bathed for more than 1 hr in test solutions, it was examined whether the stimulus to the cut end induces or blocks the response. Na(+)-free saline (Tris(+) replaced Na(+)) or TTX (3 x 10(-7) M) reversibly blocked the response within 30 min. By contrast, Mn(2+) saline (40 mM Mn(2+) replaced Ca(2+)) or Ca(2+)-free salines (Mg(2+) or 1 mM EDTA replaced Ca(2+)) cannot block the response, but instead increased the firing frequency. These results obtained with stimulus were confirmed also by those with axotomy. I concluded that axotomy-induced firing, which occurs locally at its cut region, is primarily responsible for voltage-dependent Na(+) conductances, but not for Ca(2+) ones.