Release of neurite outgrowth promoting factors by Helisoma central ganglia depends on neural activity

Regeneration as an application of gastropod neural plasticity

Gastropod research is providing many insights into mechanisms of neural regeneration. These observations were made possible by the pioneering work of individuals who described the nervous systems of gastropods, mapped prominent neurons and determined their roles and connections, and developed the techniques for culturing them. This information has allowed questions about injury responses, target selection, and pathway cues to be explored at the level of individually identified neurons. Because of gastropod studies, more is known about axon sealing, growth cone formation and behavior, signals that travel from the site of axotomy to the soma, and the second messengers that are activated there. The responses in neurons and non-neuronal cells during neural development and injury are coordinated by chemical messenger systems that are highly conserved, including neurotransmitters, cytokines, and neurotrophins. The nervous system is modified in learning paradigms by some of the same messenger systems activated by injury, because learning and injury both challenge neurons to change. The conservation of basic mechanisms that coordinate neuronal plasticity allows us to approach basic questions in relatively simple nervous systems with reasonable confidence that the findings will be relevant for other nervous systems, including possible applications to the mammalian nervous system.

Nifedipine-induced morphological differentiation of rat pheochromocytoma cells

Extension of neuronal processes is fundamental for the establishment of the intricate network of the nervous system. The rat PC12 pheochromocytoma cell line is a well-known model system for the study of neuronal differentiation. In the present study the effect of a high-voltage-activated calcium channel blocker, nifedipine, on the cell differentiation was investigated. In cells cultured in a modified medium containing nifedipine (at 2.5 microM or 5.0 microM) a decrease of mitotic activity took place on the third day of culturing. An increase of total length and number of processes occurred on the fifth day. A higher concentration of nifedipine (10.0 microM) considerably suppressed vital functions of the cells. Retraction of processes together with blocking of proliferation activity was observed. In parallel with these changes, a reduction of [Ca2+]i from 100 +/- 6 nM to 50 +/- 2 nM was detected on the second day of cultivation in 2.5 microM and 5.0 microM nifedipine containing medium. At 10.0 microM concentration of nifedipine in the medium a decrease in [Ca2+]i was observed on the first day only (to 76 +/- 8 nM); then the level of free calcium concentration rose dramatically. The data obtained allow us to suggest that the decrease of [Ca2+]i during first two days of PC12 cells culturing in a nifedipine-containing medium could be a factor which stimulated their morphological differentiation.

The regulation of acetylated microtubules during outgrowth from cultured neurons of the snail, Helisoma

Axonal stumps of cultured Helisoma trivolvis neurons express abundant acetylated microtubules, as a subset of total microtubules. Label completely disappears from the axonal remnants within approximately 1 day, and reappears in newly extended neurites over the course of the next 3-4 days, first in the proximal neurite as short, isolated segments. Acetylated microtubules occur in the neuritic shaft, but never in growth cones or membranous veils. Thus, acetylated microtubules are very labile to the signals generated by axotomy, and their proximodistal re-expression occurs at well separated sites within the neurite as it matures.

Neurite outgrowth is enhanced by conditioning factor(s) released from central ganglia of Aplysia californica

Single neurons of Aplysia californica were cultured on control and conditioned medium-treated cover slips (CM, derived from central ganglia of Aplysia) and fed with L-15 medium plus 20% hemolymph. After 5 h 36.9 +/- 3.3% of all neurons plated on CM-treated cover slips sprouted, whereas only 22.9 +/- 2.7% sprouted in the control group. After 24 h the average neurite length in the presence of putative conditioning factor(s) was 621 +/- 42 microns versus 411 +/- 20 microns in control, but the number of primary processes was the same (4.3 +/- 0.5 versus 4.2 +/- 0.9).

Growth cone collapse and neurite retraction from cultured Helisoma neurons in response to antibody Fab fragments against an extracellular matrix protein

Helisoma neurons require a factor(s) present in conditioned medium (CM), for successful neurite outgrowth in vitro. A approximately 300 kDa Helisoma extracellular matrix (ECM) protein has been identified in CM and is necessary for neurite initiation. Here we show that purified approximately 300 kDa ECM protein supports outgrowth. Furthermore, anti- approximately 300 kDa Fab fragments cause a rapid, dose-dependent decrease in outgrowth when added to neurons already growing in CM, culminating in growth cone collapse and neurite retraction at 200 micrograms/ml. Collapsing growth cones rapidly lost lamellipodia and filopodia transformed into long filamentous strands. Contortion of microtubules in retracting neurites into serpentine shapes, apparently by compressive forces, suggests that large-scale microtubule depolymerization is not a prerequisite for growth cone retraction. These results imply that substrate-bound approximately 300 kDa CM protein is necessary and sufficient for CM-stimulated growth cone initiation and neurite elongation from Helisoma neurons.

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.