Retrograde tracing and neuropeptide immunohistochemistry of sensory neurones projecting to the cartilaginous distal femoral epiphysis of young rats

Although cartilage is considered to be devoid of innervation, axons occur i n the perichondrium and during development in cartilage canals, thereby hav ing a relatively close spatial relationship to chondroblasts and chondrocyt es. The present study locates the source of the sensory innervation of the femoral cartilaginous epiphyses of young rats and investigates whether the neuropeptide calcitonin gene-related peptide (CGRP) can influence chondrocy tes. Retrograde tracing from the distal femoral epiphysis of young rats wit h Fast Blue (FB) showed labelled neuronal profiles in the L2-L5 dorsal root ganglia. Sample countings indicated that 50% of the FB-labelled neuronal p rofiles were located at the L3 level and 25% at the L4 level. The labelled neurones had diameters of 15-40 mu m, with a peak at 25-30 mu m Immunohisto chemistry showed that about 50% of the FB-labelled profiles contained CGRP. Together with the finding that CGRP influences bone cells to generate the second messenger cAMP, this result suggested the hypothesis that chondrocyt es might be similarly influenced by CGRP. However, stimulation of cartilage slices with CGRP in vitro followed by an assay of the cAMP content did not provide support for this hypothesis. We conclude that primary sensory neur ones containing CGRP project to the perichondrium and to cartilage canals o f growing cartilage, and that exogenous CGRP does not elevate the cAMP cont ent of cartilage slices in vitro.