COLLATERAL SPROUTING IN THE ELECTROSENSORY LATERAL-LINE LOBE OF WEAKLY ELECTRIC TELEOSTS (GYMNOTIFORMES) FOLLOWING RICIN ABLATION

Sprouted collateral axons were observed in the electrosensory lateral line lobe (ELL) of gymnotiform teleosts (Apteronotus leptorhynchus) fo llowing the ablation of the supraorbital branch of the anterior latera l line nerve. Ablation was accomplished by using microinjections of th e toxic lectin ricin. Sprouted axons were followed for up to 26 weeks postablation. Ricin exposure severely reduced axonal numbers and the p eripheral electroreceptors in the region innervated by these fibers. T o visualize sprouted fibers, intact lateral line afferent nerve branch es were anterogradely labelled with the neuronal tract tracers horsera dish peroxidase or cobalt chloride, or the monoclonal antibody Q26A3. Within the four somatotopically-organized ELL segments, sprouted colla terals were first observed two weeks after ricin injection in the medi al and centromedial segments, and four weeks postinjection in the cent rolateral and lateral segments. Sprouting involved intrasegmental, hor izontally directed axons from adjacent nerve branch terminal fields, a nd mixed intra- and extrasegmental, dorsally directed axons from the E LL deep fiber layer. The sprouting response was robust but variable in its timing, peaking between 6 and 12 weeks. Subsequently, the intrase gmental, horizontally directed fibers were retained but the mixed dors ally directed fibers, including all extrasegmental axons, were retract ed. Therefore, this sprouting response appears to consist of a collate ral overproduction followed by a selective axonal retraction. In our v iew, the most likely explanation for this axonal retraction is that th e descending inputs from the isthmus and the cerebellum, as well as co mmissural fibers from the contralateral ELL, maintain established soma totopic relationships by eliminating somatotopically mismatched sprout ed collaterals.