Frozen spinal cord sections (10 μm thick) were blocked and incubated in a cocktail of rabbit anti-TrkA and goat anti-TrkC antibodies (gift of Dr. Reichardt; Huang et al. 1999), followed by a cocktail of CY3 conjugated donkey anti-rabbit and FITC conjugated donkey anti-goat antibodies (Chemicon, Temecula, California, United States) in the presence of 0.3% TritonX-100 and 10% normal donkey serum. For PV immunohistochemistry, sections were reacted with monoclonal mouse anti-PV antibody (Sigma, St. Louis, Missouri, United States), and developed by Vector fluorescein mouse on mouse kit (Vector Laboratories, Burlingame, California, United States). For quantification purposes, TrkA- and TrkC-labeled cells from four different DRGs of each genotype and age studied were counted, and ratios plotted.