Mauthner cells (M-cells) occur as a pair of large, uniquely identifiable neurons at ear level in the hindbrain of premetamorphic amphibians. Each receives synapses from the ipsilateral vestibular nerve (nVIII); these morphologically distinctive terminals, or club endings, are confined to the proximoventral surface and branches of the M-cell lateral dendrite. We have superinnervated this portion of the M-cell to examine the extent to which forming afferent contacts regulate the growth and branching of the lateral dendrite. Superinnervation was brought about in the developing axolotl (Ambystoma mexicanum) by unilaterally implanting an extra vestibular primordium rostral to the in situ one. The contralateral side served as control. When the larvae reached 21 mm in length, the ectopic nerve was labeled with HRP. Subsequent microscopic examination revealed that the grafts developed into anatomically normal ears. The HRP-labeled ectopic axons entered the medulla at the level of nV and confined to the nVIII tract, coursed caudad toward the ipsilateral M-cell. Electron microscopic analysis demonstrated labeled club endings on the appropriate region of the M-cell lateral dendrite. The number of club endings on experimental M-cells was significantly greater than that on the contralateral controls, and the extra terminals appeared to be distributed randomly among unlabeled ones. Comparison of reconstructed experimental and control M-cells revealed that superinnervation produced a localized enhancement of dendritic branching in the region receiving the extra nVIII synapses. In the donor embryos (those from which the vestibular primordium was removed), M-cells were unilaterally deprived of nVIII afferents. Comparison of reconstructed experimental and control M-cells in 21 mm donor larvae demonstrated that deprivation produced a localized decrease of dendritic surface in the region that normally receives nVIII synapses. Together, these data show that ingrowing axons stimulate dendritic growth and thus regulate the development of a normal dendritic branching pattern on target neurons.

In the medulla of the axolotl (Ambystoma mexicanum), Mauthner cells (M-cells) occur as a pair of large identifiable neurons at the level of entry of the vestibular nerve (nVIII). Each receives synapses from the ipsilateral nVIII; the terminals can be identified as club endings and are restricted to a specific set of M-cell dendritic branches. We have examined these branches for morphologic changes following long-term deafferentation in the presence and absence of nerve regeneration. Deafferentation was brought about in a group of young larvae by unilaterally severing nVIII. The nerve was allowed to regenerate in half of the larvae. In those remaining, the nVIII ganglion was damaged to preclude or limit nerve regeneration. The contralateral side served as control. After 3 months survival, the larvae were killed and the medullae prepared for microscopy. To estimate the extent of nerve regeneration, axons in the experimental nVIII tract were counted and compared with the number in the control. The mean number of axons in the nVIII tract ipsilateral to intact ganglia indicated that 69% of the fibers had regenerated. In contrast, only 31% regenerated in larvae with damaged ganglia. Electron microscopic analysis of selected sections revealed that the mean number of nVIII terminals per section through M-cells ipsilateral to destroyed ganglia was significantly less than the mean number in analogous sections through either control cells or cells ipsilateral to intact ganglia. Control and experimental M-cells were reconstructed from serial sections. Deprived M-cells had significantly reduced dendritic branching patterns in the region that normally receives nVIII input. On the other hand, the extent of branching on cells receiving regenerated afferents from intact ganglia was like that of their contralateral controls. The distribution of dendritic branches on many reinnervated M-cells, however, was broader than that on control cells. Electron microscopic examination of the displaced dendritic branches (those extending into adjacent tracts) revealed that they received vestibular synapses. Thus, in some animals, regenerated vestibular fibers were not restricted to the nVIII tract. Deafferentation of the M-cells results in a reduction of dendritic branches in the region deprived of vestibular contacts.(ABSTRACT TRUNCATED AT 400 WORDS)