Endogenous neurotrophin-3 supports the survival of a subpopulation of sensory neurons in neonatal rat

Neurotrophin-3 promotes the differentiation and supports the survival of neuroblasts derived from the neural crest in early development. Neurotrophin-3 also plays an important role in the differentiation and survival of a subpopulation of large sensory neurons after their axons arrive at their targets. Proprioception and mechanoception are lost after gene deletion of neurotrophin-3 or its high-affinity receptor, TrkC. However, the function of neurotrophin-3 during late development and in mature animals is not clear. We have used an antiserum, specific for neurotrophin-3, to neutralize endogenous neurotrophin-3 in postnatal rats to determine its role in late sensory neuron development. Administration of the antiserum for a period of two weeks, but not one week, resulted in a 20% reduction in the number of primary sensory neurons in the dorsal root ganglia and a 19% reduction in the number of myelinated axons in the saphenous nerve. The size distribution histogram also indicated that a subpopulation of large neurons was lost by the neurotrophin-3 antiserum treatment. This neuronal loss was accompanied by reduced cell soma sizes and weights of the ganglia. Immunoreactivities for calbindin and calretinin were reduced in the trigeminal and dorsal root ganglia and nerve fibres surrounding whisker hair follicles. The number of Merkel cells in touch domes labelled with quinacrine and the number of parvalbumin-immunoreactive neurons in the dorsal root ganglia were significantly reduced by the antibody treatment. In contrast, the number of muscle spindles in the gastrocnemius muscle is not reduced by the neurotrophin-3 antiserum. Together, these results indicate that a subpopulation of primary sensory neurons in the neonatal rat requires neurotrophin-3 for their survival and expression of calcium binding proteins. In addition, Merkel cells in touch domes also require neurotrophin-3 for their survival. Thus, endogenous neurotrophin-3 in neonatal rats is critical for the survival and function of a subpopulation of primary sensory neurons and Merkel cells.