Glial and Neuronal Connexin Expression Patterns in the Rat Spinal Cord during Development and Following Injury I-HUI LEE, * EVA LINDQVIST, OLE KIEHN, JOHAN WIDENFALK, AND LARS OLSON Department of Neuroscience, Karolinska Institute, 17177 Stockholm, Sweden ABSTRACT Spinal cord injury induces a complex cascade of degenerative and remodeling events evolving over time. The possible roles of changed intercellular communication via gap junctions after spinal cord injury (SCI) have remained relatively unexplored. We investigated the temporospatial expression patterns of gap junctional genes and proteins, connexin 43 (Cx43), Cx36, and Cx32, by in situ hybridization and immunohistochemistry in the rat neonatal, adult normal, and adult injured spinal cord. Cx36 was strongly expressed in immature neurons, and levels declined markedly during development, whereas Cx43 and Cx32 persisted throughout adulthood. After a complete transection of the adult spinal cord, the levels of Cx43 mRNA and protein were up-regulated within hours, especially in gray matter rostral to the lesion, reaching over three times normal levels at 4 weeks postinjury. Cx43 immunoreactivity was seen primarily in astrocytes and rarely in microglia. In contrast, Cx36 and Cx32 mRNA and proteins were relatively sparse and unchanged after spinal cord injury along the entire axis of the spinal cord. Cx43 is the most abundant gap junctional protein in the adult CNS and has been shown to form channels between astrocytes as well as between astrocytes and oligodendrocytes. Long-term up-regulation of Cx43 in reactive astro- cytes may be one critical component in the rearrangement of the local astroglial network following SCI. J. Comp. Neurol. 489:1–10, 2005. © 2005 Wiley-Liss, Inc. Indexing terms: gap junction; astrocyte; neuron; oligodendrocyte; microglia Gap junctions are intercellular channels that directly connect cytoplasm of neighboring cells. They allow electri- cal as well as biochemical (ions and small molecules 1 kDa) communication between adjacent cells (Simon and Goodenough, 1998). Gap junctions can mediate synchro- nous behavior of coupled cells and have a critical role in development, morphogenesis, and pattern formation (Ku- mar and Gilula, 1996). In CNS, astrocytes are connected by a rich presence of gap junctions. The ubiquity of astro- cytic gap junctions allows astrocytes to form a functional syncytium that permeates CNS parenchyma and links various regions in terms of flow of current, ions, and small metabolites (Mugnaini, 1986). The system acts as a K + sink following intense neuronal activity (Holthoff and Witte, 2000) and allows propagation of Ca 2+ waves in response to activation (Cornell-Bell et al., 1990; Charles et al., 1992; Venance et al., 1997). Neuronal gap junctions, or electrical synapses, allow functional coupling of a variety of neurons generating synchronous activity. They have functional properties strikingly different from those of chemical synapses (Bennett, 1977, 1997). Electrical syn- apses occur among neonatal spinal motor neurons and decline steeply during early development (Cheng et al., 1999). The progressive loss of gap junctions among devel- oping motor neurons may reduce their correlated activity, which in turn may trigger synaptic competition at neuro- muscular synapses (Personius and Balice-Gordon, 2001). Recently, a novel ultrafast form of axoaxonal electrical coupling was demonstrated in rat hippocampal pyramidal cells, allowing the recruitment of neurons for fast oscilla- tions by transmitting action potentials between axons. Grant sponsor: Yen Tjing Ling Medical Foundation in Taiwan; Grant sponsor: Swedish Research Council; Grant sponsor: Arbetsmarknadens Fo ¨rsa ¨ kringsaktiebolag; Grant sponsor: National Institute on Drug Abuse. *Correspondence to: I-Hui Lee, Retzius va ¨ g 8, B2: 4, 17177 Stockholm, Sweden. E-mail: i-hui.lee@neuro.ki.se Received 20 October 2004; Revised 17 January 2005; Accepted 16 Feb- ruary 2005 DOI 10.1002/cne.20567 Published online in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 489:1–10 (2005) © 2005 WILEY-LISS, INC.