Roles of AMPA/KA-type glutamate receptor activation in differentiation and survival during neuronal development

Spontaneous neuronal activity in the immature brain is an integral component of differentiation. Developing neurons express specific arrays of neurotransmitter receptors and ion channels to regulate membrane depolarization, and intracellular signaling pathways transform activity into specific differentiation programs. To produce and detect the appropriate activity patterns, channels in immature neurons display slower kinetics and higher conductances that favor depolarization, leading to synchronization of large neuronal populations. The heightened excitability of the developing brain is a liability, as it lowers the threshold of activation for epileptic discharges during trauma. Nonetheless, immature neurons are less sensitive to excitotoxicity than mature cells, suggesting that protective mechanisms may be in place in this excitable environment.; Despite extensive work analyzing receptor function during development in slices, reductionist in vitro systems had not been fully exploited, as the differentiation stages of the neurons in culture were not known. Cerebellar granule cells are an ideal model for developing neurons because their well-defined differentiation programs can be recapitulated in vitro. I characterized granule cell maturation in culture to establish an in vitro model of developing neurons and I correlated developmental changes in expression of glutamate receptor subtypes, AMPA and KA receptors, to specific developmental stages. I found that immature neurons do not die following doses of AMPA/KA receptor agonists that are lethal for more mature cells. Therefore, I compared immature and mature neurons to define how developing cells are different in their response to excitotoxicity and found that immature granule neurons are resistant to excitotoxic AMPA/KA receptor activation because they express low levels of AMPA receptors on the membrane. However, even in the absence of cell death, axonal and dendritic differentiation is impaired in immature neurons, suggesting that excitotoxicity may cause long-term deficits in these cells.; Finally, revisiting a coculture model that has been studied in our laboratory for the last two decades, I combined granule cells at different developmental stages with their pontine afferents, the mossy fibers, to reanalyze afferent-target interactions in a more defined setting. These models can be used in the future to explore activity-dependent and excitotoxic modulation of circuit formation in the cerebellum.