Quantitative examination of calcitonin gene-related peptide immunoreactive nerve fibres in the cat knee joint capsule

The knee joint of the cat has been used extensively to study the morphology and function of primary afferents in a deep somatic tissue. A proportion of these neurones synthesizes various neuropeptides, with calcitonin gene-related peptide being the most prominent. In the present study we examined the distribution and density of nerve fibres immunoreactive for calcitonin gene related peptide within the medial articular capsule. The fibres were predominantly located in the superficial layer of the capsule. They formed a dense innervation pattern, mainly accompanying blood vessels. Electron microscopy showed that most fibres were in close proximity to small arteries. The highest innervation density was found in parts of the capsule that were located over the epicondyle of the femur with 21+/-12 fibres per mm(2) (mean+/-SD). In the tissue over the joint cleft this density was lower, with 11+/-6 fibres per mm(2). In conclusion, the high innervation density of the knee joint capsule by nerve fibres containing calcitonin gene-related peptide supports the hypothesis of an important regulatory function of this peptide in normal tissue.