Date of Award
Doctor of Philosophy (PhD)
This thesis examines the roles of nerve growth factor (NGF) in developing and mature dorsal root ganglion neurons, especially in the context of reparative nerve growth mechanisms in the adult peripheral nervous system (PNS). Peripheral nerve injury evokes two types of growths in the periphery, regeneration and/or collateral sprouting. However, these growth processes differ in a number of respects. Axonal regeneration is triggered by the injury and occurs independently of NGF whereas collateral sprouting is evoked and sustained by an increase in a target-derived signal, NGF. Indeed, cutaneous denervation is shown to result in a significant and prolonged increase in the level of NGF mRNA compared to the level in innervated skin. In addition, NGF mRNA is demonstrated to be expressed in not only the distal nerve pathways but also in non-nerve associated epidermally- and dermally-located cells. These findings indicate that an increase in NGF synthesis is associated with the increased availability of NGF in denervated skin, and that cutaneous nerves play a role in regulating NGF synthesis. Other findings in this thesis serve to strengthen the distinction between the injury-induced nerve growth responses. mRNA expression of the two NGF receptors, p75ᴺᴳᶠᴿ and trkA, are shown to increase in undamaged DRG neurons whose axons are sprouting into denervated skin, a proposed response that is most-likely related to the increased availability of target-derived NGF based on the findings that (i) NGF mRNA is increased in denervated skin, and (ii) polyclonal anti-NGF antiserum blocks the increase in the mRNA level of at least p75ᴺᴳᶠᴿ. In contrast to these findings, there was little or no change in receptor mRNA levels in regenerating neurons, consistent with the observations that NGF does not play a role in this process. Finally, here it is demonstrated that the NGF-driven collateral sprouting is severely impaired in adult rats that had received daily injections of anti-NGF serum during the first 2 weeks of postnatal life. However, after nerve crush the same nociceptive axons regenerated normally. Since NGF expression levels was found to be normal in innervated and denervated skin, NGF insufficiency was not responsible for the impairment in sprouting. Moreover, the possibility of a permanently defective axoplasmic transport was eliminated since sensory thresholds and axon calibers were also normal. Lastly, the finding that systemically-injected NGF promoted nociceptive sprouting, implies that the neurons had not become NGF-insensitive, nor had they reached a sprouting "ceiling". Indeed, the difference between the effects of exogenous and endogenous NGF is understandable if the latter accesses only the nerve terminals, and if the terminals' ability to take up NGF is defective. Thus, the exogenous NGF may work entirely through regions of the neuron that are outside this functional "compartment". To this end, although NGF uptake must have been adequate to account for the healthy state of the neurons, the terminals apparently failed to take up enough of the increased NGF produced in the collateral sprouting paradigm either to evoke sprouting, or to cause the usual upregulation of p75ᴺᴳᶠᴿ in the neurons of the dorsal root ganglion (DRG); there was, however, enough increased uptake to bring about the usual upregulation of trkA. This last finding suggests that neurotrophin deficiencies induced by adverse conditions during development might permanently compromise the organism's ability to mount neurotrophin-based reparative nerve growth in response to neuronal loss or peripheral nerve injury.
Kril, Yvonne Maria Anne, "Roles of nerve growth factor in developing and mature dorsal root ganglion neurons" (1997). Open Access Dissertations and Theses. Paper 3394.