Late appearance and deprivation‐sensitive growth of permanent dendrites in the avian cochlear nucleus (Nuc. magnocellularis)

Abstract The second‐order auditory neurons in the avian nucleus magnocellularis (NM) do not begin to grow permanent dendrites until about embryonic day 17 (E17), a few days before hatching. Since the auditory periphery and brain‐stem auditory nuclei are functional by E14 at the latest, the late appearance and growth of permanent dendrites in NM provides an unusual opportunity to examine the role of sensory experience in the very early stages of dendritic growth. We studied the development of NM dendrites in normal chickens and in animals with sustained monaural acoustic deprivation. In Golgi‐Hortega preparations at E17 and posthatching days 4 (P4), 10 (P10), and 60 (P60), the proportion of stained NM neurons with a dendritic process (“dendritic neurons”) and the length of these dendrites were measured. In normal animals the mean proportion of dendritic neurons rose from about 14% at E17 to 40% at P4 and thereafter. The mean length of NM dendrites in normal animals, however, grew significantly until P10, rising from about 14 μm at E17 to 48 μm by P4, and about 100 μm at P10 and P60. A 40‐dB monaural conductive hearing loss maintained with plastic earplugs from E18 on produced substantial retardation of dendritic growth after P4; at P10 the difference in length between deprived and nondeprived dendrites was 32% and at P60 was 38%. The proportion of dendritic neurons in the deprived NM and the mean diameter of the deprived dendrites were not affected by deprivation. Deprived dendrites in experimental animals were also significantly shorter than those in normal control animals. The length of nondeprived dendrites in experimental animals did not differ from that of controls, arguing against any compensatory hypertrophy. The functions of NM dendrites in the central processing of auditory information are as yet unknown. The facts that these dendrites (1) undergo their most rapid growth during the period of the animal's first exposure to airborne sound and (2) are markedly stunted by a nondeafferenting moderately severe acoustic deprivation suggest that acoustic experience has a strong facilitative function in the posthatching development of the cochlear nucleus. These results also suggest that afferent synaptic stimulation can affect the subsequent elongation, but not the initial outgrowth, of dendrites.