Ionic Dependence of the Axotomy-Induced Long-Lasting Firing in an Identified Crayfish Motoneuron

Abstract

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.

Spontaneous Firing in the Isolated Anucleate Axonal Segment of an Identified Crayfish Motoneuron

“…The present axonal segment preparation contained other units than AML (Muramoto, 1977(Muramoto, , 1993, but they did not exhibit SF in this study. In crayfish and lobsters, axons can survive, conduct action potentials and release transmitters for months after being separated from their somata (Hoy et al, 1967;Wine, 1973;Bittner and Johnson, 1974;Atwood et al, 1989;Blundon et al, 1990;Parnas et al, 1991), yet none of them have shown prolonged spontaneous firing (Titmus and Faber, 1990;Bittner, 1991).…”

“…Although the AML axon can show a prolonged firing following axotomy (Muramoto, 1993), axotomy-induced firing (AIF) and the present SF activity differ greatly. The distinction is as follows.…”

“…Then, the P-P' delay in the AIF following the A6-cut was compared with that in the SF after the A6-cut on the same preparations (n=4), in which the positions of the proximal and distal electrodes remained unchanged during the experiment, because the AIF following the A6-cut is thought to occur at the proximal end of an isolated AML axon (Muramoto, 1993). P-P' delays for AIFs and SFs were calculated from the initial (Initial), the last (Last) or the total (Total) parts of an AIF or a SF.…”

“…This paper on an isolated anucleate axon of an identifiable anal contractor motoneuron of the crayfish, Procambarus clarkii, presents the first evidence that this anucleate axon can exhibit prolonged spontaneous firing without stimulus. A previous study revealed that transection (axotomy) of any level of this axon can elicit prolonged firing (≤67 min) at its axotomized end (Muramoto, 1993), but spontaneous firing after cessation of such axotomy-induced firing has not been clarified. I reported previously that this contractor motoneuron has its soma in the terminal abdominal ganglion and that the axon extends to the anal musculature out from the ganglion (Muramoto, 1977).…”

“…1 shows, the nerve preparation was immobilized by fixing both distal ends of the AINs with silicon grease and the discharge activity was recorded extracellularly with a suction electrode from either of the cut ends of the PIN distal branches and a reference electrode was located in the bath fluid, as described previously (Muramoto, 1993). By transecting the nerve with scissors at various positions between A6 and the electrode, various lengths of an isolated anucleate segment of the AML axon (axonal segment preparation) were obtained.…”