Differential responses to the application of exogenous NT-3 are observed for subpopulations of motor and sensory neurons depending on the presence of skeletal muscle

We examined the effects of a single injection of exogenous NT-3, administered at embryonic day (E) 13.5, on the survival of two populations of motor neurons and two populations of sensory neurons. Both wild-type and double knockout, Myf5-/-:MyoD-/-, mutant embryos were examined to determine the effects of the aforementioned neurotrophin on motor and sensory neuron survival in the presence and absence, respectively, of skeletal muscle. We found that, although NT-3 rescues select populations of motor neurons in the absence of muscles, there is a lack of increase in neuron survival when skeletal muscle is present. Additionally, NT-3 was found to rescue a select population of proprioceptive sensory neurons in the absence of target tissue, while, at times, exacerbating neuron cell death when target tissues are present. Lastly, we found that neurons in the spinal cord and brainstem show both a regional and functional specificity in their response to the administration of NT-3 in utero. Our results indicate the possibility that different pathways are involved in the survival of neurons during naturally occurring programmed cell death and during excessively occurring programmed cell death.