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A method is described for studying the morphological features of extensive axonal projections within the central nervous system of the gerbil, Meriones anguiculatus. Potentially long descending axonal projections between the auditory thalamus and lower brainstem were used as a model. The inferior colliculus (IC) in the tectum was injected in vivo with a fluorescent retrograde tracer, Fluoro-Gold, to label cells in the medial geniculate body (MGB) that had descending projections to the IC, and cells in the superior olivary complex (SOC) that had ascending projections to the IC. Another fluorescent retrograde tracer, fast blue, was injected into the cochlea to label olivocochlear (OC) cells in the SOC. Inferomedially curved parasagittal slices containing ipsilateral auditory cell groups from the thalamus to the brainstem were cut and descending axons of the pre-labeled MGB cells were traced anterogradely with Biocytin. After visualizing histologically the injected Biocytin, discretely labeled IC-projecting axons of the MGB cells were traced including their collaterals that extended further into the SOC. In the SOC, these axons terminated on pre-labeled cells including OC cells. The combination of anterograde and retrograde tracing in the slice preparations described here demonstrated extensive descending axonal projections from the thalamus to their targets in the lower brainstem that had known ascending/descending projections within the auditory system. [ABSTRACT FROM AUTHOR]

Whereas basic features of post-axotomy muscle reinnervation have been extensively studied in rats, little is known about axonal regrowth and pathfinding in cats. To address the question, adult cats were subjected to facial-facial anastomosis (FFA). First group served to establish optimal parameters for labeling of the zygomatic and buccal facial branches with 1,1' dioctadecyl - 3,3,3,'3' - tetramethylindo-carbocyanine perchlorate (DiI) and Fast Blue (FB) placed onto respective transected nerves. The second group of animals underwent identical bilateral labeling 3 months after transection and suture of the right facial nerve. This group served to establish the number of motoneurons, which had branched after surgery and projected into both facial branches. On control side. DiI application onto zygomotico-orbital branch labeled 3883 ± 598 (mean ± S.D.) perikarya were confined to the dorsal and intermediate facial subnuclei, meanwhile an application of FB onto the buccal branch labeled 1617 ± 552 perikarya in the lateral and ventrolateral subnuclei. There were no doubled-labeled cells. Three months after FFA all retrogradely labeled motoneurons were scattered throughout the entire facial nucleus. To establish the proportion of perikarya, that er-grew multiple axonal branches into both nerves, double-labeled (FB + DiI) mononeurons were counted from digital images. The zygomatico-orbital nerve contained 3311 ± 430 DiI-labeled whereas the buccal nerve 1500 ± 442 FB-labeled motoneurons. The occurrence of 311 ± 103 double-labeled perikaya (DiI+FB) suggested that approximately 6% of all retrogradely labeled motoneurons branched axons into both nerves. I conclude that malfunctioning axonal pathfinding rather than deviant reinnervation contributed to poor recovery of function after FFa in the cat. [ABSTRACT FROM AUTHOR]