Neuropeptidergic and autonomic control of the vasculature of the rat knee joint revealed by laser Doppler perfusion imaging.

An imaging technique (laser Doppler perfusion imaging, LDI), based on measurement of backscattered Doppler-broadened laser radiation, was used to provide two-dimensional images of perfusion in the exposed rat knee joint capsule. In control animals, frequency-dependent constriction of knee joint blood vessels occurred during electrical stimulation of sympathetic nerve fibres whilst dose-dependent vasodilatation of these vessels was elicited by direct application of the sensory neuropeptide substance P (SP) to the capsule. Intra-articular injection of the neurotoxic agent capsaicin did not affect vasoconstrictor responses when tested 5-7 days later, but substantially reduced dilator responses to SP. These findings indicate that capsaicin is selectively neurotoxic for sensory unmyelinated fibres but not sympathetic postganglionic fibres and is also capable of modifying receptor-mediated effects of SP. LDI is a useful method for mapping tissue perfusion, particularly in structures such as joints where the spatial distribution of blood flow is heterogeneous.