SOMATOSTATIN-LIKE IMMUNOREACTIVITY IN PRIMARY AFFERENTS OF THE MEDIALARTICULAR NERVE AND COLOCALIZATION WITH SUBSTANCE-P IN THE CAT

The proportion of somatostatin-containing dorsal root ganglion cells i nnervating the knee joint of the cat via the medial articular nerve wa s determined by using retrograde labeling with fast blue and immunohis tochemistry. Immunoreactivity was found in 8.6% of labeled cell bodies . In colchicine-treated ganglia, the proportion increased to 16.8%. On ly small and intermediate-sized perikarya showed somatostatin-like imm unoreactivity, indicating that this neuropeptide is synthesized predom inantly in primary afferent units with unmyelinated sensory axons but may also be present in primary afferents with thinly myelinated sensor y fibers. Colchicine treatment had no influence on the cell size distr ibution. Colocalization of somatostatin with substance P was determine d by comparing the proportions of immunopositive dorsal root ganglion cells after incubation with antibodies against substance P or somatost atin or with a mixture of both. Substance P-like immunoreactivity was found in 18.1% (untreated ganglia) and 19.6% (colchicine treated gangl ia) of the labeled neurons. After incubation with a mixed antibody sol ution, 18.2% of joint afferents in untreated and 19.9% of the cells in colchicine-treated ganglia were immunopositive. Comparing this result with the results obtained using somatostatin and substance P antibodi es alone, one can calculate that both neuropeptides are colocalized in about 17% of the cat's knee joint afferents. About 3% of the neurons contain only substance P, whereas almost none of the neurons contain o nly somatostatin. Based on this fact, one can assume that both neurope ptides are coreleased in peripheral tissue as well as in the central n ervous system. (C) 1995 Wiley-Liss, Inc.