We will evaluate the functional significance of gender differences in the phenotype of fibers of the adult rabbit masseter muscle. Torques applied to the mandible by different neuromuscular compartments in adult males and females will be measured as an assay of masseter function. Torques produced by intramuscular microstimulation or during natural rhythmic activations of the masseter will be studied. We will study both orthograde and retrograde signaling mechanisms in the development of these gender differences. We will use high spatial resolution EMG methods to study the activity patterns of masseteric motoneurons in young and fully mature animals to determine whether changed in this orthograde signal accompany or precede changes in muscle fiber phenotype. We will attenuate or eliminate this orthograde signal by blocking neuromuscular synaptic transmission with botulinum toxin in young animals. Based on the available evidence, neurotrophin 4 (NT-4) is currently the best candidate for a retrograde signaling molecule used by motoneurons. We will determine whether NT-4 regulation accompanies or precedes the development of gender differences in masseter muscle fiber phenotype at both the protein and mRNA levels. We will use exogenous NT-4 and blockers of both NT-4 and other trk B ligands to manipulate available quantities of this putative retrograde signal and then study its effect on the development of gender differences in masseter muscle fiber phenotype. The results of these experiments will generate important new knowledge that will contribute significantly to our understanding of the structure and function of the muscles of mastication. Study of this animal model of human mastication will provide important new insights into the cellular mechanisms used by these unique muscles to regulate both their structure and function in different genders. The new knowledge generated will impact significantly several areas of clinical dentistry.