The distribution of free calcium in transected spinal axons and its modulation by applied electrical fields

Abstract

Intracellular free-calcium concentration ([Ca2+]i) was measured in lamprey spinal axons using the fluorescent calcium indicator fura 2. We used both a photomultiplier tube and a video-image processing system to measure the temporal and spatial distributions of [Ca2+]i in the proximal segments of transected axons. Within 3 min following transection, a spatially graded increase in the [Ca2+]i was apparent in the last few millimeters of the axons. Superimposed on the initial gradient was a moving front of calcium that progressed up the axon, reaching 1.6 mm from the cut end in 3 hr. The [Ca2+]i behind the moving front exceeded 10 microM. This movement of Ca2+ was greatly reduced by an externally applied electrical field with the cathode distal to the lesion and was increased by an applied field of the opposite polarity. When axons were transected in Ca2(+)-free medium, no increases in [Ca2+]i occurred. One d after transection, [Ca2+]i was at or below the precut levels, except in the distal 250 microns, where it remained slightly elevated. Therefore, axons can survive the high levels of [Ca2+]i that occur after transection and can reestablish normal [Ca2+]i levels within 24 hr. Measurements of both the diffusion coefficient and the fluorescence polarization of fura 2 indicate that the dye is not significantly bound to axoplasmic components.

Microtubule-based filopodium-like protrusions form after axotomy

“…The emergence of blebs after axotomy suggests local cortical weakening (Albrect-Buehler, 1987). The apparent dependence of the formation of the FLPs on C&+ influx, which is known to occur as a result of axotomy (Strautman et al, 1990) and to be particularly destructive of actin filaments (Lankford and Letourneau, 1989) is also consistent with cortical weakening, as is the increase in the number of transporting PLPs when transection was done in the presence of dihydrocytochalasin B.…”

Apoptotic cell death induced by optic nerve lesion in the neonatal rat

“…A recent hypothesis formulated by Cheng and Mattson proposes that the mechanisms by which trophic factors, such as NGF and bFGF, exert their protective effect on injured neurons could be through a stabilization in calcium homeostasis (Cheng and Mattson,199 1). The notion that disparate types of neuronal injuries are associated with calcium deregulation (Mattson, 1990;Strautman et al, 1990;Choi, 1992;Heizmann and Braun, 1992;Landfield et al, 1992) and the observation that their effects can be partially prevented by trophic factors (Fisher et al, 1987, 199 1;Cheng and Mattson, 1991) suggest that these different pathologies could share similar mechanisms of degeneration. Our findings of apoptosis in axotomized retinal ganglion cells raise the question as to whether a similar type of active, apoptotic death could occur in other neurodegenerative conditions.…”

Localized and Transient Elevations of Intracellular Ca2+Induce the Dedifferentiation of Axonal Segments into Growth Cones

“…Axotomy causes a large increase of the free intracellular Ca 2ϩ concentration in the transected axon, mainly because of Ca 2ϩ influx through the cut end (Borgens et al, 1980;Happel et al, 1981;Mata et al, 1986;Strautman et al, 1990;Rehder et al, 1991Rehder et al, , 1992Spira, 1993, 1995). This influx forms a steep [Ca 2ϩ ] i gradient along the severed axon, in which Ca 2ϩ concentrations Ͼ1 mM are recorded near the cut end.…”