Neurotrophin-3 promotes the survival of a limited subpopulation of cutaneous sensory neurons

In the chick embryo, exogenous neurotrophin-3 (NT3) is sufficient to promot e the differentiation of proprioceptive afferents even in the absence of li mb muscle targets. To determine if NT3 can promote the differentiation of t his phenotype in afferents with cutaneous targets, we analyzed the effects of chronic NT3 on cutaneous and muscle sensory neurons that express trkC, a receptor for NT3. In normal embryos, retrograde labeling and immunohistoch emistry showed that about 75% of large-diameter muscle afferents express tr kC, whereas only about 7% of large-diameter cutaneous afferents express thi s protein. After chronic treatment with NT3 during the cell death period, b oth populations of trkC(+) neurons were increased approximately twofold. Be cause this treatment is known to block cell death in sensory neurons, these results indicate that NT3 can promote the survival of both proprioceptive afferents and cutaneous afferents. To examine the phenotype of the cutaneou s afferents rescued by NT3, we analyzed their projections and connections u sing transganglionic labeling and electrophysiological recording. The resul ts indicate that exogenous NT3 neither altered the pattern of spinal projec tions nor caused cutaneous afferents to form monosynaptic connections with motor neurons. These results demonstrate that selective cell death does not contribute to the modality-specific pattern of spinal innervation and sugg est that proprioceptive afferents may innervate muscle selectively. (C) 200 0 Academic Press.