Alpha motoneurons are present in normal numbers but with reduced soma size in neurotrophin-3 knockout mice

Neurotrophin-3 (NT-3) is essential for survival of proprioceptive and mechanoreceptor neurons, but its role in motoneuron development in vivo has seemed slight. Recent evidence however has suggested that NT-3 may be involved in motoneuron maturation. Here, we quantitatively assess the number and state of development of motoneurons within the lumbar lateral motor column (LLMC) of newborn NT-3 (-/-), (+/-) and (+/+) mutant mice. We find that the number of alpha motoneurons in the LLMC is the same in all genotypes, but soma size is significantly reduced in (-/-) mutants. This suggests that NT-3 is not required for normal production and survival of alpha motoneurons, but is essential for their full maturation and/or maintenance. We also confirm a previous report that gamma motoneurons are absent in NT-3 (-/-) mice.