Deafferentation-induced terminal field expansion of myelinated saphenous afferents in the adult rat dorsal horn and the nucleus gracilis following pronase injection of the sciatic nerve.

We have previously demonstrated sprouting of small diameter saphenous afferents, labelled with wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and (HRP), into the sciatic territory of the adult rat superficial dorsal horn following destruction of sciatic afferents by injection of the sciatic nerve with pronase (a combination of proteolytic enzymes). In the present experiments, we examined the response of myelinated saphenous axons, which terminate in lamina I and the deep dorsal horn (laminae III-V) under the same conditions, with the tracer B subunit of cholera toxin conjugated to HRP (B-HRP) which specifically labels myelinated primary afferents when injected into a peripheral somatic nerve. We also examined changes in the nucleus gracilis, another site of sciatic degeneration and a target of saphenous afferents. Four months after injection of the pronase, the area of label determined by measurement of the width of the saphenous territory in lamina III was expanded by 24% on the pronase side. Since there was also expansion throughout the deep dorsal horn, the area measured by tracing the labelled region in transverse sections was actually twice that of the control side, and the intensity of labelling within the traced area increased by 18%. There was no change in grey matter area due to the lesion. The traced area of labelling in the nucleus gracilis increased by 40%, and increased in intensity by 17%. The substantia gelatinosa is not normally supplied by B-HRP-labelled afferents, and there was no expansion of these sprouted saphenous afferents into the gelatinosa. These results indicate that myelinated afferents can sprout as vigorously in lamina I and the deep dorsal horn as the small diameter afferents do in the substantia gelatinosa; that there is no invasion of the substantia gelatinosa by the myelinated afferents at least as long as the small diameter afferents also have the opportunity to sprout; and that primary afferents have the potential to sprout at more than one site of termination, i.e., both the dorsal horn and the dorsal column nuclei.