Sprouting in the avian brainstem auditory pathway: dependence on dendritic integrity.

The brainstem auditory pathway of the chicken were used to examine the relationship between the maintenance of dendrites following denervation and the successful reinnervation (sprouting) by surviving afferents. In the system the third-order cells in n. laminaris receive spatially segregated binaural innervation from n. magnocellularis. Afferents from the ipsilateral n. magnocellularis innervate the dendrites on the dorsal aspect of n. laminaris cells, while afferents from contralateral magnocellular neurons innervate ventral dendrites via the crossed dorsal cochlear tract. Denervation of the ventral dendrites of n. laminaris cells by transection at the midline results in rapid and severe atrophy of the denervated dendrite. Unilateral cochlea removal induces transneuronal degeneration of 30-45% of the ipsilateral magnocellular cells, thereby partially denervating one dendrite of the n. laminaris cells on each side of the brain. In animals with long-standing transections of the crossed dorsal cochlear tract there is no evidence of sprouting the fibers from the ipsilateral n. magnocellularis when the projections of the surviving magnocellular neurons are traced with degeneration methods after a secondary cochlea removal. However, when dendrites of n. laminaris are partially denervated dendrites do not disappear. Furthermore, secondary lesions of the crossed dorsal cochlear tract or secondary cochlea removal reveal that these denervated dendrites are reinnervated by the afferents from the opposite n. magnocellularis which are normally restricted to the opposite dendrite of the n. laminaris cells.