Rapid axonal transport in vitro. Effects of derivatives of cyclic AMP and other agents acting on the cyclic AMP system

cAMP regulates axon outgrowth and guidance during optic nerve regeneration in goldfish

Increased cAMP improves neuronal survival and axon regeneration in mammals. Here, we assess cAMP levels and identify activated pathways in a spontaneously regenerating central nervous system. Following optic nerve crush in goldfish, almost all retinal ganglion cells (RGC) survive and regenerate retinotectal topography. Goldfish received injections of a cAMP analogue (CPT-cAMP), a protein kinase A (PKA) inhibitor (KT5720), both compounds combined, or PBS (control). RGC survival in experimental groups was unaffected at any stage. The rate of axon regeneration was accelerated by the activator and decelerated both by the inhibitor and by combined injections, suggesting a PKA-dependent pathway. In addition, errors in regenerate retinotectal topography were observed when agents were applied in vivo and RGC response to the guidance cue ephrin-A5 in vitro was altered by the inhibitor. Our results highlight that therapeutic manipulation of cAMP levels to enhance axonal regeneration in mammals must ensure that topography, and consequently function, is not disrupted.

Increased cyclic AMP in in vitro regenerating frog sciatic nerves inhibits Schwann cell proliferation but has no effect on axonal outgrowth

In the present study the role of cAMP for axonal outgrowth and Schwann cell proliferation was studied using the cultured frog sciatic nerve. An intrinsic rise in nerve and ganglionic cAMP could be measured as a response to nerve injury, both in vitro and in vivo. Treatment with 0.1-1.0 microM forskolin, an activator of the cAMP-generating enzyme adenylyl cyclase, increased the cAMP content up to 13-fold, but was yet without effect on axonal outgrowth during an 8-day culturing period. HA-1004, an inhibitor of cAMP-dependent protein kinase, also lacked effect on the regeneration. In contrast, the proliferation of Schwann cells, measured as [3H]thymidine incorporation, was inhibited to about 70% of control by forskolin, whereas HA-1004 stimulated proliferation to approximately 130% of control. The results suggest that cAMP is involved in the injury-induced proliferation of Schwann cells of an adult peripheral nerve but that it lacks a central role in the regeneration of sensory axons of such nerves.

Effect of neurotransmitters on axoplasmic transport: acetylcholine effect on superior cervical ganglion cells

The effect of acetylcholine (ACh) on particle movements along axons of cultured superior cervical ganglion cells was analyzed with a computer-assisted video-enhanced differential interference contrast microscope system. ACh suppressed the axoplasmic transport reversibly in both anterograde and retrograde directions. A muscarinic agonist, arecoline, mimicked the ACh effect, but nicotine did not. An experiment with the Ca(2+)-indicator dye, fura-2, revealed that ACh suppressed the transport without any change of intracellular Ca2+ concentration. ACh also suppressed the axoplasmic transport in Ca(2+)-free medium. Islet-activating protein (IAP), pertussis toxin, blocked the ACh effect. These results indicate that ACh activates muscarinic receptors and suppresses fast axoplasmic transport through the activation of IAP-sensitive GTP-binding protein, irrespective of Ca2+ ions.

Effect of acetoxycycloheximide and dibutyryladenosine cyclic 3':5'-monophosphate on axonal regeneration in the goldfish optic nerve

Acetoxycycloheximide (AXM) or dibutyryl cyclic AMP (dbcAMP) was injected unilaterally into the vitreous humor of the eye beginning 5-6 days after bilateral optic nerve crush. Injections were repeated every 12-24 h for a total of 3-5 days; goldfish were sacrificed 10 days after lesioning the nerves. At a low dosage of AXM (0.1 microgram daily for 5 days), the mean outgrowth distance in treated neurons was 60% less than in contralateral control neurons. At a high dosage (0.3 microgram daily for 4 days), outgrowth was immediately blocked in both treated and contralateral control axons. Dibutyryl cyclic AMP, in a dose of 5 microM every 12 h for 3 days, produced a 38% reduction in outgrowth distance, associated with a 30% reduction in protein synthesis by the retinal ganglion cells and a 73% reduction in the amount of protein carried by the fast component of axonal transport.

Local protein synthesizing activity in axonal fields regenerating in vitro

The goldfish retinal explant system of Landreth and Agranoff was used to study endogenous protein synthesizing activity of retinal ganglion cell axons regenerating in culture. Light and electron microscopic examination of axonal fields showed that axons were free of nonneural cell investment. Decentralized axons were incubated with a mixture of tritiated amino acids, and direct quantitative microanalysis of protein and tritium radioactivity was carried out on individual axonal fields. Our findings showed that radioactive amino acids were incorporated into axonal protein in a manner that inhibited significantly by cycloheximide, but not by chloramphenicol. Decentralized axons appeared to maintain their viability for at least 3-4 h. Axonal fields maintaining their central connections to the explant incorporated 3H-amino acids at an apparent rate that was similar to decentralized axonal fields. Labeled material transported into axonal fields from ganglion cell bodies appeared in significant amounts after a delay of 2-3 h. Fluorographic patterns of axonal proteins after labeling with either 3H-amino acids or [35S]methionine and separated by microelectrophoresis indicated that primarily tubulin and, to a lesser extent, actin were labeled. Our findings indicate that goldfish retinal ganglion cell axons regenerating in vitro exhibit measureable endogenous protein-synthesizing activity.

Axonal transport of adenylate cyclase activity in normal and axotomized frog sciatic nerve

Adenylate cyclase activity accumulated proximal to a constriction placed around the frog sciatic nerve. The rate of accumulation was linear between 8 and 24 h following placement of the constriction; accumulation rate declined substantially after 24 h. Accumulation of activity distal to the constriction in normal nerve was not significantly different from control for the first 72 h, but increased at 5 days. These data are interpreted as indicating that adenylate cyclase is transported from the cell body to the nerve terminals in normal frog nerve, but not in the reverse direction. Following axon transection, anterograde transport of adenylate cyclase activity declined, but a transient retrograde transport of adenylate cyclase activity appeared. In addition, adenylate cyclase activity accumulated in the proximal transected nerve stump during the period when Schwann cell proliferation and the initiation of nerve regeneration both appear. The pattern of response of adenylate cyclase activity to nerve injury suggests that the adenylate cyclase: cAMP system could play some role in peripheral nerve regeneration.

Batrachotoxin blocks saltatory organelle movement in electrically excitable neuroblastoma cells

Batrachotoxin has been reported to inhibit fast axonal transport. We have examined the effect of batrachotoxin on saltatory organelle movements in N115 neuroblastoma cells (a model of fast axonal transport) using time-lapse video intensification microscopy. Batrachotoxin (0.1-1.0 microM) inhibits saltatory organelle movement. Contrary to previously published hypotheses, this inhibition of intra-axonal movement depends upon the action of batrachotoxin on action potential Na+ channels. Evidence for this conclusion is: (1) the range of concentrations of batrachotoxin which inhibit saltatory organelle movement is consistent with the dose-response curve for the activation of action potential Na+ channels by batrachotoxin in N18 neuroblastoma cells; (2) tetrodotoxin, which blocks action potential Na+ channels, prevents the inhibition of organelle movements by batrachotoxin; (3) batrachotoxin has no effect on saltatory movement in cells, including some neuroblastoma cell lines, which lack action potential Na+ channels; and (4) in Na+-free or low Na+ media, batrachotoxin does not block organelle movement. We suggest that changes in internal ion concentrations which follow the influx of Na+ are responsible for the inhibition of fast axonal transport by batrachotoxin.

Axoplasmic transport in vagal afferent fibres in normal and alloxan-diabetic rabbits

Fast and slow components of anterograde axoplasmic transport have been studied in the sensory fibres of the vagus nerve of alloxan-diabetic rabbits and age-matched controls by incorporation of tritiated leucine into nodose ganglion cells. The diabetic rabbits were maintained for 2 months with blood glucose levels in the range 20--40 mmol/l. They showed growth retardation and one third developed cataracts. No alteration of either fast or slow axoplasmic transport was detected in the diabetic animals. These results are discussed in the light of the present understanding of the role of axoplasmic transport, of the findings in other axoplasmic transport studies, and of other data available on the pathogenesis of human and experimental diabetic neuropathy.

Adenylate cyclase in the developing rat cerebral cortex and olfactory bulb

Activities of adenylate cyclase, measured either in the absence or presence of sodium fluoride and Triton X-100, are determined in cerebral cortex and olfactory bulb homogenate of rats of 1 to 35 days of postnatal age. Differences in properties of the enzyme in the 2 structures are demonstrated.