Sensory nerve fibres, neuropeptides and cartilage : Experimental studies in the rat

Sammanfattning: During development, maintenance and repair after injury, reciprocal interactions occur between the peripheral nervous system and the target tissues. In the Papers presented in this thesis, different aspects of such netvetarget influences between peripheral nerve fibres and skeletal tissues dtuing development and repair have been investigated in the rat. Developing rat cartilaginous bone primordia have a richly innervated and vascularised perichondriwn. In addition, larger bones exhibit cartilage canals containing blood vessels and putative sensory nerve fibres. Tills evoked the question if there is a nervous regulation of skeletal development. Denervation of the hind paws of young rats resulted in a deficient length growth but had no influence on the progress of secondary ossification. Since growth is mainly due to events in cartilage, cartilage projecting sensory neurones were identified and examined. Sensory neurones projecting to the rat cartilaginous distal femoral epiphyses were located mainly in the dorsal root ganglia (DRG) L3 and L4 and exhibited small or medium-sized diameters. A large proportion of these neurones contained the neuropeptides CGRP and/or SP. However, application of CGRP to cartilage explants in vitro did not stimulate the chondrocytes in terms of an elevation of the level of cyclic AMP. Another possibility would be that the neuropeptides affect the developmental growth of bone and chondrocytes indirectly via effects on the blood vessels. Experiments .involving tracing as above and eo-culture of labelled DRG neurones and perichondrial cells in combination with immunohistochenllstty or electrophysiology showed that the traced cultured neurones contained CGRP and/or SPin in vivo-like proportions and that most of the cartilage-projecting neurones were proton sensitive, This prompted the suggestion that the nerve fibres in the perichondrium and in cartilage canals might release CGRP and SP in response to local tissue acidosis, thereby promoting tissue homeostasis by monitoring the balance between vascular supply and metabolic load and by influencing angiogenesis and blood flow. Subsequently, possible target influences on the local presence of perichondrial sensory nerve fibres were investigated. Application of inflammation related cytokines (IL-1ß, IL-6 and LIF) affected sensory neurones eo-cultured with perichondrium- or skin-derived fibroblast-like cells in terms of survival and neurite growth. These effects were strongly influenced by the origin of the target cells. Finally, experiments using the adult rat patella showed that osteochondral defects heal spontaneously but incompletely and that healing is not accompanied by an increase of local nerve fibres at the times examined. In conclusion, the present results indicate that cartilagerelated sensory nerve fibres influence skeletal growth, that a high proportion of these neurones contain CGRP and SP, that CGRP does not activate chondrocytes in cartilage slices, that many cartilage related sensory nerve fibres are proton-sensitive· and likely have a vasoregulatory role, that inflammatory mediators have distinct effects on sensory neurones eo-cultivated with perichondrial cells and that healing of an osteochondral defect in the rat patella does not involve a local increase of cartilage-related nerve fibres.