Rapid Axonal Transport of Glycerophospholipids in Regenerating Hypoglossal Nerve of the Rabbit

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Abstract
The intraaxonal transport of phospholipids in regenerating hypoglossal nerve of the rabbit was investigated by administration of labeled lipid precursors into the medulla oblongata. At various time intervals after crushing the left hypoglossal nerve at the level of the digastric muscle, a mixture of 60 .mu.Ci of [2-3H]glycerol and 15 .mu.Ci of [1-14C]palmitate, dissolved in 15% bovine serum albumin, was injected into the calamus scriptorius of the 4th ventricle. The amount and the pattern of labeling of glycerophospholipids synthesized in the motor neurons were determined. Three days after nerve crush there was an accumulation of labeled glycerophospholipids immediately proximal to the injury site. Seven days after crushing, the regenerating nerve incorporated rapidly transported labeled lipids in greater amounts than the contralateral normal nerve; the incorporation was elevated along the entire length of the nerve containing both regenerating axons and the postcrush sprouting terminals. The difference between 2 sides increased up to 14 days, but disappeared as regeneration proceeded (21-45 days). The pool of radioactive lipids remaining in the cell bodies of hypoglossal nuclei, in the segments of nerve, both proximal and distal to the crush site and in all the segments of uncrushed nerve was similar 6-12 h after labeling. Among the phospholipids, the highest 3H and 14C radioactivity was observed in phosphatidylcholine and phosphatidylethanolamine. An increase in the amount of glycerophospholipids, conveyed by rapid axonal transport, probably takes place in the first 2 wk during nerve regeneration. The increased transport of lipids presumably reflects an augmented demand for membrane precursors during the sprouting process.