The formation of axonal sprouts in organ culture and their relationship to sprouting in vivo

Resting membrane potentials and excitability at different regions of rat dorsal root ganglion neurons in culture

To study the role of electrical membrane processes in neuronal regeneration and growth, resting membrane potentials and action potentials of sensory (dorsal root ganglion) neurons growing in culture were measured at the soma, neurite and growth cone using the whole-cell patch-clamp technique. Our results show that resting membrane potentials measured at the soma (-56.8 +/- 8.8 mV), neurite varicosity (-55.8 +/- 5.2 mV) and growth cone (-57.2 +/- 4.1 mV) of growing neurons were not statistically different. The membrane resistance measured around the resting membrane potential at the neurite varicosity (160 +/- 70 M omega) was smaller than those at the soma (687 +/- 540 M omega) and growth cone (922 +/- 825 M omega). The resting membrane potential measured at the soma using a perforated patch (-60.3 +/- 4.4 mV) was not different from that measured in the normal whole cell. In both configurations, isotonic KCl (140 mM) depolarized the membrane potential to above 0 mV. The K+ channel blockers quinine, Cs+, 4-aminopyridine and tetraethylammonium depolarized the membrane potential by 10-40 mV, while Na(+)-free extracellular solution hyperpolarized it by about 10 mV. Extracellularly applied ouabain, intracellular Na(+)-free or low Cl(-)-containing solutions did not affect the resting membrane potential. Similar results were obtained for growth cones. Action potentials could be evoked by current pulses in 81% of somata and in all growth cones, but not in neurite varicosities. Current-induced repetitive firing was found in 19% of somata and in 65% of growth cones.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of sprouting in organ culture after axotomy of an identified molluscan neuron

We examined a variety of factors that might modulate the initiation of neurite outgrowth in an attempt to identify means by which its initiation might be accelerated. We examined this initiation from an identified molluscan neuron, Helisoma trivolvis buccal neuron B5 after axotomy, and determined whether the site of injury, temperature, ion channel blockers, pH, the second messenger cAMP, and protein synthesis affect the initiation of neurite outgrowth. Neurite outgrowth was assayed from axotomized neurons by filling the neurons intracellularly with Lucifer Yellow and examining the percentage of axons that extended (sprouted) new process after 9 or 24 h in organ culture. About one-third (31%) of axotomized neurons sprouted from the site of injury after 9 h (n = 22), and 88% (n = 20) sprouted after 24 h in saline at 22 degrees-24 degrees C when the injury was located 800 microns from the soma. Elevating the temperature to 32 degrees C or moving the lesion site to 400 or 1500 microns from the soma did not significantly alter the incidence of sprouting. Blocking sodium channels with tetrodotoxin [TTX (2 x 10(-5) M)] did not significantly reduce the incidence of sprouting, whereas the sodium channel agonist, veratridine (10(-5) M) did. The calcium channel blocker lanthanum (10(-6)-10(-4) M), stimulated neurite outgrowth; however, the organic calcium channel blocker verapamil (10(-3)-10(-5) M), and the calcium ionophore A23187 (10(-5) M), had no effect on sprouting. Exposure of neurons to the potassium channel blocker tetraethylammonium [TEA (20 mM)], elevation of intracellular pH with NH4Cl (5 mM), or treatment with the adenylate cyclase activator forskolin (10(-5) M) reduced the incidence of sprouting, whereas dideoxy-forskolin (10(-5) M) had no effect. Inhibition of protein synthesis with anisomycin (2 x 10(-4) to 2 x 10(-6) M) did not significantly suppress sprouting 24 h after axotomy. Both D and L isomers of glutamate (300 microM) stimulated sprouting. The present results suggest that the initiation of sprouting is regulated locally at or near the site of injury, and that blocking specific ion channels may either inhibit or enhance the initiation of neurite outgrowth.

Ultrastructure of an identified molluscan neuron in organ culture and cell culture following axotomy

We examined the ultrastructure of neuron 5 from the buccal ganglion of the mollusc Helisoma trivolvis after axotomy and organ culture, and after isolation of the same neuron in culture. Buccal ganglia containing axotomized neurons 5 were cultured either in host snails or in Leibovitz medium conditioned with ganglia. In addition, some neurons 5 were isolated from buccal ganglia by micro-dissection and plated into culture. Neuron 5 and its processes were identified in both whole mounts and plastic sections of buccal ganglia after intracellular injection with Lucifer Yellow or horseradish peroxidase. Five days after axotomy of neuron 5, thick sections of buccal ganglia stained with toluidine blue revealed that densely staining basophilic bodies (Nissl bodies) within the cytoplasm had dispersed, i.e., they had undergone chromatolysis. Coincident with chromatolysis was an overall increase in diffuse basophilic staining within the cytoplasm of neuron 5 when maintained in organ culture. The dispersion of Nissl bodies viewed by light microscopy correlated with a more freely arranged rough endoplasmic reticulum and associated polysomes within neuron 5 as seen by electron microscopy. Isolated neurons 5 did not possess densely staining Nissl bodies when examined after 2 days in vitro, thus indicating that chromatolysis occurred earlier in isolated neurons. Furthermore, no increase in diffuse cytoplasmic basophilia was observed within isolated neurons 5 cultured in vitro. However, isolated neurons 5 exhibited a marked increase in the number of lipid-like bodies (0.5-1.5 micron in diameter) that were particularly evident in scanning electron micrographs. Scanning and transmission electron micrographs revealed that the isolated neurons were free of associated glia, but non-neuronal cells (hemocytes) would attach themselves to the somata and neurites. Glia surrounding neuron 5 within buccal ganglia exhibited a marked hypertrophy following axotomy and organ culture. Hypertrophy of glia was absent, however, if ganglia were axotomized and left within the animal or axotomized ganglia were implanted into host animals and examined 5 days later by electron microscopy. These observations indicate that, following axotomy, a molluscan neuron may exhibit different morphological features depending on its microenvironment. In addition, the hypertrophy of glia surrounding neurons in Helisoma was not associated with axotomy per se, but with organ culture.

Neurofilament elongation into regenerating facial nerve axons

Immunocytochemistry was used to show that neurofilaments advance into regenerating facial nerve axons at 2.5 mm/day, which is less than the rate of axonal elongation (4.3 mm/day), measured from the transport of radiolabeled protein into the axons. Thus, the distal region of the newly-regenerated axons is deficient in neurofilaments, and this was confirmed by electron microscopy. These neurofilament-free regenerating axons could also be detected by immunocytochemistry using antibody to protein B50 (GAP43), a component of growth-cones. Immunoblots of nerve segments, incubated with monoclonal antibodies against the three neurofilament proteins, showed that all three proteins were present in the neurofilaments elongating into the regenerating axons, and confirmed the more distal extensions of B50 immunoreactivity. These results show that neurofilament immunocytochemistry underestimates the extent of axonal regeneration, and it is suggested that this technique should be employed with caution in regeneration studies. When the facial nerve received a conditioning lesion 7 days prior to a test lesion, axonal regeneration rate increased to 6.0 mm/day, and there was a proportional increase in neurofilament elongation rate to 4.4 mm/day. This occurred in spite of the reduction in cell body neurofilament protein synthesis induced by the lesions. It is concluded that the rate of neurofilament extension into regenerating axons is not governed by cell body synthesis but by local interactions with other cytoskeletal materials which support the increased regeneration rate of conditioned axons.

Afterdischarge and interactions among fibers in damaged peripheral nerve in the rat

Sensory fibers trapped in nerve-end neuromas become abnormally excitable, and produce an ectopic discharge which is believed to contribute to paresthesias and pain associated with chronic nerve injury in man. Here we report that stimulation of injured nerves can alter this discharge, directly by antidromic invasion of active neuroma fibers, and indirectly through interactions with neighboring fibers. Antidromic stimulation of spontaneously active fibers in experimental neuromas in the rat sciatic nerve, using single electrical stimulus pulses, produced time-locking of rhythmic spontaneous firing and of spontaneous impulse bursts. Some initially silent fibers generated a burst of rhythmic afterdischarge when stimulated in this way. Stimulation delivered in brief trains (tetani) produced more prolonged alterations in spontaneous neuroma discharge, including excitation, suppression and combinations of the two. In some cases initially silent fibers were activated for extended periods. These responses to tetanic stimulation occurred even when the active fibers were not themselves stimulated, and reflect a novel form of fiber-fiber interaction in neuromas that we term 'crossed afterdischarge'. This interaction probably results from the accumulation of potassium ions within the extracellular compartment adjacent to active neuroma fibers during activation of their neighbors. It differs fundamentally from the high safety factor ephaptic cross-talk seen in acutely cut nerves and in neuromas of 30 or more days standing.

Effects of isaxonine on skeletal muscle reinnervation in the rat: an electrophysiologic evaluation

Electrophysiologic detection of the first signs of gastrocnemius muscle reinnervation shows that after a single localized freezing of the rat sciatic nerve, isaxonine does not significantly increase the rate of axonal regeneration. However, this drug does significantly enhance axonal sprouting, as it induces an increase of 50% in the number of muscle fibers with multiple innervation 10-30 days after freezing. After 30 days, the proportion of multiinnervated fibers decreases in both the control and isaxonine-treated rats, and the differences between them gradually disappear.

The morphology of growth cones of regenerating optic nerve axons

The morphology of growth cones of regenerating optic nerve axons was examined by light and electron microscopy in adult frogs (Rana pipiens), using a horseradish peroxidase (HRP) fiber-filling method, during early and later phases of regeneration. Optic nerve regeneration was initiated unilaterally by crushing the optic nerve in mid-orbit. Fiber filling was accomplished by severing the affected nerve closer to the eye 24-48 hrs. prior to sacrifice and applying HRP to the central stump. Regenerating axons and their growth cones were observed in the optic nerves, chiasma, tract, pretectal neuropil, and optic tectum. Growth cones of normal-appearing axons varied in shape and size. Flattened, foliate growth cones similar to those commonly described in vitro were observed in the pretectal neuropil and optic tectum. Other growth cones having vermiform, lanceolate, spatulate, and bulbous forms were observed throughout the optic pathway at all stages examined, although the longer (up to 70 micrograms) wormlike structures appeared only in the optic tract during the early period of outgrowth. Nearly complete serial-section reconstructions were obtained for two growth cones in the contralateral optic tectum at 8 wks. regeneration time. One was thinly flattened (to 30-50 nm in places) and extended broadly (8 micrograms in diameter) in contact with a neuronal perikaryon. The other formed a hood over the blind end of a severed, nonregenerating myelinated axon, which was normal-appearing except at its end within the confines of the growth cone. Morphological variation among the growth cones is discussed in relation to other descriptive in vivo studies and views concerning growth cone motility.

'Zap axotomy': localized fluorescent excitation of single dye-filled neurons induces growth by selective axotomy

The response of populations of neurons to axotomy has traditionally been studied by crushing or sectioning whole nerve trunks. The present communication describes a technique by which single neurons can be reliably and selectively axotomized in the absence of damage to other axons and non-neuronal cells within the nerve. To obtain selective axotomy, identified neurons of the buccal ganglia of the snail, Helisoma, were first filled with fluorescent dye. Next, the preparation was positioned in a restricted beam of blue light using low light video fluorescence microscopy. Finally, the selected region of axon was briefly exposed to light levels normally employed for fluorescence microscopy. Shortly after irradiation of the identified neuron 5, antidromic action potentials no longer propagated past the region of exposure in the dye-filled cell, whereas adjacent axons were physiologically intact. Several days after exposure, profuse neurite outgrowth was observed from the proximal region of axon of neuron 5, but never in neighboring axons which were not filled with dye at the time of irradiation. When the axons of both neurons 5R and 5L were spot irradiated neurite outgrowth resulted in the formation of a novel electrical connection between these cells. These changes in growth and connectivity which were induced by selective axotomy of single axons were indistinguishable from the changes which are produced by crushing entire nerve trunks.

Sprouting inducing activity of human sera: low capacity in sera from patients with cranial polyneuropathy

Cranial idiopathic acute peripheral facial nerve paralysis (Bell palsy) has been used as a model disease in the present study mainly to investigate trophic capacity of serum components. The ability to induce sprouting from chick embryo dorsal root ganglia was used as the bioassay for trophic activity. Sera from normal patients induce neuritic outgrowth from dorsal root ganglia of chick embryo in the absence of any exogenous factor. Sprouting, to a very limited extent or none, has been induced by 80% of the sera derived from the Bell palsy patients. Further analysis of the biochemical nature of the reduced activity has revealed that in sera of Bell palsy patients the substances that are fractionated within the Ig fraction are responsible for the reduced capacity to induce sprouting without loss of supportive molecules.

Acceleration of muscle re-innervation in rats by ganglioside treatment: an electromyographic study

The sciatic nerve in the right thigh was divided in 40 rats and the nerve stumps then sewn together with two microsutures. The treated animals, 20 rats, had daily subcutaneous administration of a bovine cortex ganglioside mixture. The controls, 20 rats, received the solvent alone. At 21 days after nerve division it was found that more treated rats than controls had regenerated nerve fibres capable of producing re-innervation potentials in the gastrocnemius muscle. This result supports the view that cortex gangliosides act to accelerate muscle re-innervation.