Our laboratory studies the function of neurotrophic factors (NTF) in the brain and their potential for use as therapeutic agents in treatment of human neurodegenerative disease. Animal models of neurodegeneration are being used to study the potential therapeutic value of neurotrophic factors in treatment of neurodegenerative disease. Careful characterization of eachanimal model including analysis of the mechanisms and time course of neuron death is an important step in evaluating novel therapeutic interventions. Our work is focused on characterization of the neonatal facial nerve axotomy model of motor neuron degeneration. This model is often used in preclinical studies to support clinical trials for testing of therapeutic agents in treatment of adult motor neuron disease. Transection of the rat facial nerve on the day of birth results in apoptosis of neurons in the facial nucleus (FN) within seven days. Our preliminary investigations of the mechanism of cell death in this model reveal numerous profiles in the FN exhibiting morphological characteristics of apoptosis within 5-7 days of axotomy. To confirm biochemically that apoptosis occurs in neonatal FN neurons after axotomy, DNA was extracted from the axotomized and normal FN and analyzed by gel electrophoresis. Results revealed the ~ladder~ effect characteristic of apoptosis in samples from both lesioned and unlesioned nuclei 6 days after axotomy or sham surgery, respectively. This result suggests that ongoing apoptosis is present in the normal neonatal facial nucleus independent of axotomy. Therefore, we hypothesized that developmental cell death in the FN persists during the first postnatal week, much later than previously reported. To test this hypothesis, we performed a developmental study in rats (ages E18-P15) using unbiased sterology to quantify the total number of motor neurons at each age. Results show a 40% loss in motor neurons in the FN between E20 and P15. These results show that developmental cell death continues through the first 2 postnatal weeks and suggests that the neonatal facial nucleus is not fully developed at this time. Therefore, the neonatal axotomy model may not be appropriate for predicting efficacy of novel therapies for adult motor neuron disease. To understand why adult motor neurons are less vulnerable to injury than neonatal motor neurons, we used in situ hybridization to measure the levels of acidic fibroblast growthfactor (aFGF) mRNA in the FN at selected developmental ages ( E20-adult). Preliminary results show that aFGF mRNA is strongly expressed in motor neurons in the adult FN but very little is found before P6. At P15, aFGF mRNA levels approach levels seen in the adult. These data suggest that aFGF may play a role in motor neuron survival after injury. These data will be presented at the 24th Society for Neuroscience meeting, November, 1998.
