Signaling pathways that mediate nerve growth factor-induced increase in expression and release of calcitonin...

Nerve growth factor (NGF) can augment transmitter release in sensory neurons by acutely sensitizing sensory neurons and by increasing the expression of calcitonin gene-related peptide (CGRP) over time. The current study examined the intracellular signaling pathways that mediate these two temporally distinct effects of NGF to augment CGRP release from sensory neurons. Growing sensory neurons in 30 or 100 ng/mL of NGF for 7 days increases CGRP content and this increase augments the amount of CGRP that is released by high extracellular potassium. Overexpressing a dominant negative Ras, Ras(17N) or treatment with a farnesyltransferase inhibitor attenuates the NGF-induced increase in CGRP content. Conversely, overexpressing a constitutively active Ras augments the NGF-induced increase in content of CGRP. Inhibiting mitogen activated protein kinase (MEK) activity also blocks the ability of NGF to increase CGRP expression. In contrast to the ability of chronic NGF to increase peptide content, acute exposure of sensory neurons to 100 ng/mL NGF augments capsaicin-evoked release of CGRP without affecting the content of CGRP. This sensitizing action of NGF is not affected by inhibiting Ras, MEK, or PI3 kinases. In contrast, the NGF-induced increase in capsaicin-evoked release of CGRP is blocked by the protein kinase C (PKC) inhibitor, BIM and the Src family kinases inhibitor, PP2. These data demonstrate that different signaling pathways mediate the alterations in expression of CGRP by chronic NGF and the acute actions of the neurotrophin to augment capsaicin-evoked release of CGRP in the absence of a change in the content of the peptide.