ARTHRITIS & RHEUMATISM Vol. 62, No. 12, December 2010, pp 3677–3685 DOI 10.1002/art.27713 © 2010, American College of Rheumatology Phenotypic Alterations of Neurons That Innervate Osteoarthritic Joints in Rats Joana Ferreira-Gomes, Sara Ada ˜es, Jana Sarkander, and Jose ´ M. Castro-Lopes Objective. Pain is a prominent feature of osteoar- thritis (OA). To further understand the primary mech- anisms of nociception in OA, we studied the expression of the phenotype markers calcitonin gene-related pep- tide (CGRP), isolectin B4 (IB4), and neurofilament 200 (NF200) in sensory neurons innervating the OA knee joint in rats. Methods. OA was induced in rats by intraarticu- lar injection of 2 mg of mono-iodoacetate (MIA) into the knee. Neurons innervating the joint were identified by retrograde labeling with fluorogold in dorsal root gan- glia (DRG) and colocalized with neurochemical markers by immunofluorescence. The total number of DRG cells was determined by stereologic methods in Nissl-stained sections. Results. A 37% decrease in the number of fluorogold-backlabeled cells was observed in rats with OA when compared with control rats, even though no decrease in the total number of cells was observed. However, an increase in the number of medium/large cell bodies and a decrease in the number of the smallest cells were observed, suggesting the occurrence of perikarya hypertrophy. The percentage of CGRP- positive cells increased significantly, predominantly in medium/large cells, suggesting the occurrence of a phe- notypic switch. Colocalization of CGRP and NF200 revealed no significant changes in the percentage of double-labeled cells, but an increase in the number of medium/large double-labeled cells was observed. No differences in the expression of either IB4 or NF200 were observed in fluorogold-backlabeled cells. Conclusion. These results indicate that MIA- induced OA causes an up-regulation of CGRP in differ- ent subpopulations of primary afferent neurons in DRG due to a phenotypic switch and/or cell hypertrophy which may be functionally relevant in terms of the onset of pain in this pathologic condition. Osteoarthritis (OA) is a highly prevalent chronic degenerative joint disorder characterized by destruction of articular cartilage and remodeling of subchondral bone, leading to structural and functional degradation of the affected joint (1). Altered joint biomechanics con- tribute to the loss of joint function and disability (2). The major symptom of OA is pain that worsens with weight- bearing and movement (3). Although it has been sug- gested that afferent innervation of the subchondral bone, periosteum, synovium, ligaments, and joint cap- sule could be sources of nociceptive stimuli (4), the etiology of pain in OA has not been fully determined. Currently available analgesics have limited efficacy for OA-related pain; therefore, it is essential to increase the knowledge of the basic mechanisms of such pain in order to develop new, more efficient therapeutic strategies. The generation of joint pain results from activa- tion of primary afferent nerve fibers (5). Joint afferent neurons are sensory neurons, the cell bodies of which are located in the dorsal root ganglia (DRG). These cells can be classified into various subpopulations based on their morphology, physiologic characteristics, and neu- rochemical characteristics (6,7). The large-diameter sen- sory neurons with myelinated axons, which in physio- logic conditions do not convey noxious input, and the medium-diameter neurons with thinly myelinated (A) axons can be distinguished by their content of phosphor- ylated heavy-chain neurofilament 200 (NF200) (8). The Supported by grant POCI/SAU-NEU/60853/2004, which is funded by the Portuguese Science and Technology Foundation, the Operational Programme Science and Innovation 2010 of the Portu- guese Ministry of Science, Technology and Higher Education, and FEDER (Portugal). Joana Ferreira-Gomes, MSc, Sara Ada ˜es, BSc, Jana Sar- kander, BSc, Jose ´ M. Castro-Lopes, MD, PhD: University of Porto, Porto, Portugal. Address correspondence and reprint requests to Joana Ferreira-Gomes, MSc, University of Porto, Instituto de Histologia e Embriologia, Faculdade de Medicina do Porto, Alameda Professor Hernani Monteiro, 4200 Porto, Portugal. E-mail: jogomes@med. up.pt. Submitted for publication June 3, 2010; accepted in revised form August 10, 2010. 3677