O'Brien, Claire; (1990) Phenotypic differences among primary sensory neurons in vivo and in vitro. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An influence of the peripheral target tissue on primary afferent neuron phenotype in the adult dorsal root ganglion has been studied using in vivo and in vitro techniques. Firstly the chemical phenotype of dorsal root ganglion neurons innervating skin, muscle or joint of the rat hindlimb, was studied by retrogradely labelling afferent neurons from the different target tissues with fluorescent dye. The target-identified afferents were then counterstained, in sections of dorsal root ganglia, for four putative transmitters or transmitter-related markers of subpopulations of primary afferent neurons: the enzyme thiamine monophosphatase, and the neuropeptides substance P, calcitonin gene-related peptide and somatostatin. Retrogradely-identified afferents were also immunostained with RT97, which labels large light neurons. The distribution of the four markers among primary sensory neurons was found to be related to the peripheral target they innervated, as was the proportion of large light neurons labelled from each tissue. The distribution of low-affinity receptors for nerve growth factor was studied among retrogradely-labelled skin and muscle afferents. A higher proportion of muscle afferents than skin afferents were immunoreactive for the HGF receptor. The possibility that the target-phenotype relationship was modifiable in response to a change in the peripheral environment, was studied by producing a chronic sterile inflammation in an area of skin from which skin afferents were retrogradely labelled. The results suggest that an increased proportion of RT97-positive skin afferents contained substance P immunoreactivity ipsilateral to the inflammation. An alternative system was used to investigate dynamic target influences on chemical expression by retrogradely-labelled sensory neurons: maintenance of dissociated dorsal root ganglia (incorporating the target-identified neurons) in vitro, where diffusible factors representing target-derived influences could be added to, or removed from, the culture medium. The first experiment indicated that nerve growth factor may regulate thiamine monophosphatase enzyme activity in vitro in the total neuron population (as suggested previously in vivo), and in muscle afferents. In the second experiment, medium conditioned by myotubes was found to increase neuronal survival, with a selective effect on RT97-positive neurons, and any effects on peptide expression by skin afferents were secondary to this. A relationship between phenotype and peripheral target has therefore been demonstrated in adult rat primary afferent neurons, along with the possibility that this relationship is plastic.

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