Depolarizing influences increase low-affinity NGF receptor gene expression in cultured Purkinje neurons

Multiple cellular and molecular interactions are required for the differentiation and development of central neurons. For example, neural activity may modulate trophic function. In the developing cerebellum, establishment of functional excitatory synaptic connections coincides with the expression of NGF and its receptors. We have previously shown that excitatory signals and NGF act in concert to regulate the survival and morphological differentiation of cerebellar Purkinje cells in culture. To begin investigating the molecular mechanisms by which trophic interactions and neural activity modulate cerebellar development, we have now studied the role of excitatory signals on the expression of both NGF and the p75 glycoprotein (the low-affinity component of the NGF receptor) by cerebellar cells in culture. We used p75 as a model of potential responsiveness, since it is well characterized and conveniently monitored. Expression of the NGF and p75 mRNA's was studied in either mixed, neuron-enriched, or pure glial cultures. Expression of the NGF gene was localized to proliferating glial cells, while expression of p75 was restricted to developing Purkinje cells. To evaluate whether presynaptic activation may potentially modulate trophic factor receptor expression, the expression of the p75 gene was studied in cultures exposed to excitatory signals. Depolarization of cultures with high potassium, veratridine, or exposure to the excitatory neurotransmitter aspartate, resulted in a two- to threefold increase in the expression of both the p75 protein and messenger RNA. These increases did not require the presence of glia, indicating a direct effect of the excitatory signals on the neuronal population. Moreover, message and receptor increased per neuron. Our study suggests that local glia provide trophic support for Purkinje cell development, and that impulse activity modulates Purkinje cell responsiveness by regulating expression of trophic receptor subunits.