The role of brain-derived neurotrophic factor in cortical motor learning

The molecular mechanisms that mediate learning have not yet been fully identified. The neurotrophic factor brain- derived neurotrophic factor (BDNF) exhibits properties that make it a candidate molecule for investigations of learning-related plasticity. BDNF and its high affinity receptor, TrkB, are widely expressed in the normal adult brain. The expression and secretion of BDNF are regulated by neuronal activity, and it has been shown to induce and facilitate synaptic transmission and synaptic plasticity. BDNF and TrkB are upregulated in association with many learning paradigms, and BDNF has been shown to be involved in and, in some cases, necessary for the formation of certain types of memory. The experiments described in this dissertation tested the hypothesis that BDNF exerts a role in normal cortical learning and map plasticity in the adult animal. The experimental paradigm used cortical motor learning, in which normal adult rats were trained in a skilled forelimb reaching task. To date, no study has examined the involvement of BDNF in the context of cortical motor learning, and cortical reorganization concomitant with this learning. The first set of experiments examined the expression of BDNF and its high- affinity receptor, TrkB, as a function of motor skill learning, to assess whether learning modulates the transcriptional regulation of these molecules. Using quantitative real-time PCR, BDNF was found to be gradually upregulated over the course of learning, in bothskilled and unskilled motor learners. TrkB expression was not modulated. The next set of experiments utilized two methods of attenuating BDNF signaling in the forepaw motor cortex: lentivirus encoding small interfering RNAs (siRNA) directed against BDNF mRNA, and lentivirus encoding a TrkB receptor body which binds BDNF, rendering it unavailable for normal signaling. Both constructs were first tested in vitro. siRNA directed against BDNF reduced BDNF protein levels in vitro by 81% compared to uninfected cells and 85% compared to control virus-infected cells, and the TrkB receptor body was found to significantly attenuate BDNF bioactivity in a cell survival/proliferation assay. In vivo, the siRNA construct reduced motor cortical BDNF protein levels by 22% compared to uninjected cortices and 34% compared to control virus-injected cortices, and the TrkB receptor body sequestered BDNF protein to an unknown extent. A putative BDNF blocking antibody was also tested in vitro, but it was not found to block BDNF bioactivity. When injected into the forepaw motor cortices of rats trained in the skilled reach task, neither the siRNA nor the TrkB receptor body had an effect on skill learning or on motor map organization. The combined results lead to the conclusion that, although BDNF expression is modulated by motor learning, reducing the availability of BDNF protein in the motor cortex by 22% using siRNA, or binding extracellular BDNF to an unknown extent using a TrkB receptor body, does not affect motor learning or motor map organization. Techniques that more extensively reduce BDNF signaling in the motor cortex would likely be required to reach definitive conclusions regarding a potential role of BDNF in modulating motor cortical learning and map plasticit