Galanin influences the mechanosensitivity of sensory endings in the rat knee joint

The aim of the present study was to examine the effect of galanin on group III and IV afferent nerve fibres (n = 53) innervating normal and acutely inflamed knee joints in rats. They responded to local mechanical stimulation, movements of the joint and i.a. injections of KCl close to the joint. Single i.a. bolus injections of galanin (0.1 mM, 0.2 mL) caused no direct responses of the units. In normal and acutely inflamed joints, about half of the units did not change the responses to knee joint rotation. A significant reduction of the responses to noxious movements was found in approximately 40% of the units reaching a mean value of 57% in normal joints and 70% in inflamed joints compared with control movements. In approximately 10% the responses increased to 143% in normal joints and 120% in inflamed joints. Injection of a galanin receptor antagonist (M35) doubled the responses to noxious movements in 36% of the units in normal joints and reduced it in 18% to 86% of the control movements, indicating a tonic release and influence on the mechanosensitivity of a proportion of primary afferents by galanin. In conclusion, these data further support the hypothesis that the mechanosensitivity of fine afferent nerve fibres is regulated by a mixture of different substances being released into the innervated tissue. Besides the action of several pro-inflammatory peptides there seems to exist a tonic inhibitory system.