IONIC DEPENDENCE OF THE AXOTOMY-INDUCED LONG-LASTING FIRING IN AN IDENTIFIED CRAYFISH MOTONEURON

I have reported previously that axotomy of an identifiable anal motone uron of crayfish Procambarus clarkii induces a long-lasting firing and that a prolonged depolarizing pulse to its cut end can induce a simil ar response. In this study, I confirmed that this stimulus is comparab le to axotomy; the frequency of stimulus-induced firing increases line arly with the stimulus intensity and its firing pattern is the same as that following axotomy. Then, when the cut end was bathed for more th an 1 hr in test solutions, it was examined whether the stimulus to the cut end induces or blocks the response. Na+-free saline (Tris(+) repl aced Na+) or TTX (3 x 10(-7) M) reversibly blocked the response within 30 min. By contrast, Mn2+ saline (40 mM Mn2+ replaced Ca2+) or Ca2+-f ree salines (Mg2+ or 1 mM EDTA replaced Ca2+) cannot block the respons e, but instead increased the firing frequency. These results obtained with stimulus were confirmed also by those with axotomy. I concluded t hat axotomy-induced firing, which occurs locally at its cut region, is primarily responsible for voltage-dependent Na+ conductances, but not for Ca2+ ones.