Histamine excites groups III and IV afferents from the cat knee joint depending on their resting activity

The effect of histamine on the sensory activity of primary afferents was st udied in normal a nd acutely inflamed cat knee joints. A subpopulation of g roups III and IV articular afferents could be activated by close-arterial b olus injections of histamine: units with a high resting activity (about 100 /min) were particular sensitive to histamine and were excited even by 3.3 f g histamine. The lower the resting discharges of groups III and IV units bo th from normal and acutely inflamed joints, the higher the dose of histamin e (up to 3.3 or 33 mug) necessary to excite the nerve fibres. Thirty-seven of 39 units without any resting activity were completely insensitive to his tamine. In contrast to its clear excitatory effect, histamine caused only m inor changes in the responses to joint movements. Movement-evoked activity remained unchanged in 22 of 28 units, 1 unit was sensitized and 5 units sho wed reduced activity after histamine (3.3 mug). The present results support the notion that histamine may participate in the mediation of pain from in jured or inflamed tissue. It is remarkable that histamine has a profound ex citatory action on a proportion of both groups III and IV articular afferen ts without changing their sensitivity to joint movements. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.