INHIBITORY EFFECT OF SOMATOSTATIN ON THE MECHANOSENSITIVITY OF ARTICULAR AFFERENTS IN NORMAL AND INFLAMED KNEE JOINTS OF THE RAT

The effect of somatostatin on the sensory activity of primary afferent s was studied in normal and acutely inflamed rat knee joints. Fine aff erent nerve fibers with conduction velocities of 0.9-18.0 m/s were rec orded as single units. All nerve fibers tested responded to local mech anical stimulation, movements of the joint and i.a. injections of KCl (10(-4) mel, 0.1 mi) close to the joint. Somatostatin (10(-4) mel, 0.2 mi) caused no direct response of the units. In normal joints, somatos tatin did not change the discharges evoked by non-noxious movements bu t decreased the responses to noxious movements significantly to about 63% of the responses before the application. In acutely inflamed joint s, somatostatin reduced the discharges of non-noxious and of noxious m ovements to about 55% and 52%, respectively. Injections of somatostati n with lower concentrations (10(-6) mel, 10(-8) mel) i.a. close to inf lamed joints revealed shorter and less pronounced reductions of the re sponses to noxious movements. In a proportion of afferents, substance P (10(-4) mel) and bradykinin (10(-4) mel) were able to increase these responses again. These data indicate that the mechanosensitivity of a rticular afferents in normal joints may also be regulated by several n europeptides based on a balance of pro-inflammatory peptides such as s ubstance P, and anti-inflammatory peptides such as somatostatin. In an inflamed joint, pro-inflammatory peptides seem to predominate resulti ng in a sensitization of the peripheral nerve fibers. In this case, an application of somatostatin or its analogues could be used clinically to compensate this effect. (C) 1997 International Association for the Study of Pain. Published by Elsevier Science B.V.