Cellular Mechanisms Mediating the Actions of Nerve Growth Factor in Sensory Neurons

Kellie Adrienne Park CELLULAR MECHANISMS MEDIATING THE ACTIONS OF NERVE GROWTH FACTOR IN SENSORY NEURONS Nerve growth factor (NGF) is a neurotrophin upregulated with injury and inflammation. Peripheral administration of NGF causes hyperalgesia and allodynia in animals. Blocking NGF signaling reverses these effects. At the cellular level, chronic exposure of sensory neurons to NGF enhances expression the neurotransmitter, calcitonin gene-related peptide (CGRP). Acute exposure to NGF increases capsaicin-evoked CGRP release from sensory neurons in culture. Thus, NGF increases peptide release from neurons by: (1) increasing expression of peptides, and/or (2) altering their sensitivity. The increase in peptide outflow by either mechanism could contribute to development of hyperalgesia and allodynia. The signaling cascades mediating the actions of NGF in sensory neurons are unclear. Therefore, experiments were designed to determine which pathways regulate changes in iCGRP content and evoked release from primary sensory neurons in culture. The Ras/MEK/ERK cascade was identified as a possible regulator of iCGRP expression in response to NGF. To test this pathway, it was manipulated in neurons by (1) expression of dominant negative or constitutively active isoforms of Ras, (2) farnesyltransferase inhibition, (3) manipulation of the RasGAP, synGAP, and (4) blocking MEK activity. When the pathway was blocked, the NGF-induced increase in iCGRP expression was attenuated. When the Ras pathway was activated, iCGRP expression increased. These data