Axonal regeneration in the central nervous system of Aplysia californica determined by anterograde transport of biocytin

Rhythmic biting, a component of consummatory feeding behavior in the sea ha re Aplysia californica, is eliminated following bilateral cerebral-buccal c onnective (CBC) crushes and recovers within 14 days postlesion. To assess a xonal regeneration after CBC lesions, we used biocytin backfills of CBCs fo llowed by fluorescence labeling with streptavidin-lissamine rhodamine. Ante rograde transport of biocytin showed up to 1 mm of outgrowth by regeneratin g axons at 3 days postlesion. At 7 days postlesion, the regenerated axons a pproached or had entered the ipsilateral buccal neuropil and exhibited nume rous varicosities; the average rate of axonal growth was 326 mu m/day for t he longest, most rapidly growing axons labeled in the CBC. The number of va ricosities on labeled axons, suggestive of intercellular interactions, was increased dramatically at all times postlesion. At 14 and 20 days postlesio n, regenerated axons branched extensively in the ipsilateral buccal neuropi l, entered the contralateral buccal neuropil, and entered peripheral nerves on both sides of the midline. At these later times postlesion, some labele d axons encircled unlabeled buccal cell bodies and exhibited branches conta ining numerous varicosities, indicative of axosomatic contacts. Some regene rating axons were observed in the sheath of the CBC, but the vast majority of labeled axone remained confined to the connective core, as in control pr eparations. The bilateral projections within the buccal ganglia of labeled cerebral-to-buccal axons and the large number of varicosities present on th ese processes are indicative of regenerating axons and synapses that likely contribute to the functional recovery of rhythmic biting. J. Comp. Neurol. 406:476-486, 1999. (C) 1999 Wiley-Liss, Inc.