Making Connections: Role of Neurotrophic Factors in the Development of Connections between the Brain and Inner Ear. A. Chem and Dwayne Simmons, Department of Otolaryngology and the Central Institute for the Deaf, Washington University School of Medicine Millions of people worldwide suffer from sensorineural hearing loss. Yet little is known about the many factors that play a role in the development of hearing and which factors might contribute to hearing recovery. In the nervous system, neurotrophic factors affect the survival of neurons and sensory cells and modulate their expression of neurotransmitters. Absence of neurotrophic factors such as nerve growth factor (NGF) and glial cell-line derived neurotrophic factor (GDNF) from the brain or sensory organs leads to various sensorineural deficits including hearing loss. However, the neurotrophic factors involved in cholinergic mechanisms of hearing are not known. Previous studies in our laboratory suggest that TrkB and TrkC neurotrophin receptors are seen in many regions of the brainstem auditory regions in adult rats and mice. During early developmental periods, however, none of the Trk neurotrophin receptors investigated were detected in the auditory brainstem regions. Cranial motor nuclei displayed various levels of p75NTR, TrkA, TrkB and TrkC expression during these same developmental periods. We hypothesize that GDNF may be involved in connecting brainstem cholinergic neurons to the inner ear. Employing transgenic mice models, we attempted to determine the presence of the GDNF receptor in cholinergic regions of the brainstem during development with two separate approaches. One strategy employed Ret-GFP transgenic mice and immunocytochemistry. These are mice that have the gene for GFP inserted in the place of the gene for Ret tyrosine kinase. The Ret tyrosine kinase is the common receptor subunit for the GDNF family of receptors. Mice that are homozygous for this transgene are postnatal lethal; therefore, we used Ret-GFP heterozygous mice. With immunocytochemistry, antibody labeling specifies the location of cells expressing GFP, where Ret tyrosine kinase would be found, in P0 transgenic mice. We also labeled vesicular acetylcholine transporter (VAChT), a cholinergic marker whose location is already known. The other strategy used was to define the developmental expression of cholinergic markers and GDNF receptors using RT-PCR. By designing primers for GDNF, GDNF family receptor a1 (GFRa1), and cholinergic markers including ChT1, ChAT, AChE, VAChT, and p75NTR, we could run RT-PCR on brain tissue samples to detect the presence of these molecules. In both immunocytochemistry and RT-PCR, we look for colocalization of GDNF family receptors and cholinergic markers to show that GDNF plays a role in cholinergic synaptogenesis. Our research will hopefully demonstrate the role of GDNF in both the forming of cholinergic synapses critical for proper auditory functions and the possible restoration of efferent terminals, and thus, recovery from sensorineural hearing deficits. Scholar Index Next   Natural Sciences Learning Center Washington University - Biology All contents copyright © 2004