Plasticity of primary sensory neurons and their central connections: Normal development and the effects of neonatal axotomy

Neonatal peripheral sensory neurons undergo considerable developmental and injury induced plasticity. This thesis investigates this plasticity and the mechanisms that underlie it. The first part describes the results of investigations into the central consequences of neonatal peripheral nerve injury. While nerve section in the adult mammal results in the death of axotomised sensory neurons and a rearrangement of their central connections, the same procedure performed neonatally results in more rapid and extreme changes. In Chapter 1 the issue of axotomy-induced cell death is addressed. Using unbiased stereological counting techniques a full quantitative measure of the extent and time course of dorsal root ganglion neuronal death is presented and shows a critical period of postnatal plasticity exists during which massive cell loss rapidly occurs. In addition, immunocytochemical methods were used to identify the relative loss of identified subpopulations of axotomised sensory neurons. The results indicate that all cell types are equally susceptible to axotomy and that the surviving population retains a normal phenotypic distribution. Chapter 2 describes a novel method of revealing and quantifying the collateral sprouting of primary sensory afferents within the spinal cord following axotomy. Using the tracer DiI, post-mortem labelling of central terminal fields can be achieved without the disadvantages of transport delay and possible nerve damage that occur with in vivo dye application. Chapter 3 provides evidence for a role of BDNF in mediating the axotomy-induced sprouting response. Immunocytochemistry and northern blot analysis show an upregulation of BDNF mRNA and protein within the dorsal horn of the spinal cord following sciatic axotomy. Further evidence comes from BDNF knockout mice that exhibit reduced sprouting. The second part of the thesis investigates the normal developmental plasticity of primary sensory neurons in the postnatal period, namely the refinement of afferent connections within the dorsal horn of the spinal cord. Initially diffuse Aβ-fibre projections gradually withdraw from superficial laminae until the adult pattern is achieved. In chapter 4, chronic NMDA receptor blockade from the day of birth is shown to halt this normal developmental process. Control experiments confirm that other aspects of dorsal horn development are unaffected. It is therefore concluded that postnatal reorganisation of afferent terminals is an NMDA receptor-mediated activity-dependent process and that peripheral afferent input is required to fine-tune central connectivity. This series of experiments provides further understanding of the postnatal development of spinal cord connectivity. The issues of trophic control of sensory neuron survival and growth are discussed with reference to target removal and mechanisms are proposed for collateral sprouting that incorporate the normal developmental and axotomy-induced reactive processes within the spinal cord