NGF MODULATION OF SENSORY NEURON ACTION POTENTIALS

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These studies concern regulation of the excitability properties of dorsal root ganglion (DRG) neurons by growth factors (GFs). The DRG neuron AP is characterized by a "plateau" on the repolarizing phase which is due to an inward Ca++ current. We have demonstrated that nerve growth factor (NGF) shortens the duration of the AP (APD) within 2 days for exposure when the DRG neurons no longer require NGF for survival. The APD reverses to longer duration within 1 day after exposure to anti-NGF. One aim of this study is to identify the ionic conductance changes underlying the effect of NGF on the APD. Analysis will use 3-week old dissociated cultures of embryonic, neonatal and adult DRG neurons. First, by intracellular and whole-cell recording (current clamp) measurements of the APD, size of the overshoot and amplitude and duration of the after hyperpolarizing phase (AHP) will be compared in control and NGF-treated neurons. Second, systematic analysis of the ionic conductances mediating these parameters will be carried out using whole-cell voltage clamp recording. Possible NGF-dependent alterations to be quantitated will include those n voltage-dependent Ca++ (N,L,T) and K+ (delayed rectifier, A current, K(ca)) conductances. Macroscopic currents affected by NGF will identified with specific channel blockers. The earliest time at which APD shortening can be detected will be determined. Possible mediation of this effect by synthetic mechanisms will be tested using RNA and protein synthesis inhibitors. Other GFs with neurotrophic activities (basic fibroblast growth factor, epidermal growth factor, insulin-like growth factor, insulin, and brain-derived neurotrophic factor) will be tested for their capacities to alter DRG excitability properties. Also, compounds which are not growth factors per se, but which have neurotrophic activities on DRG neurons (8-substituted cAMP analogs) will be tested. If such molecules are active, the ionic basis of their effects will be compared with those for NGF. Synergistic effects between NGF and other GFs on DRG excitability properties will be investigated. These studies could establish that NGF and other neurotrophic GFs can modulate physiological properties of neurons independently from effects on survival. A long-term objective is to understand the molecular regulation of channels by neurotrophic factors in the mature nervous system. This may be useful in the treatment of certain neurodegenerative diseases.