Tail spasms in rat spinal cord injury: Changes in interneuronal connectivity Sandra Kapitza a, 1 , Björn Zörner a, ⁎ , 1 , Oliver Weinmann a , Marc Bolliger b , Linard Filli a , Volker Dietz b , Martin E. Schwab a a Brain Research Institute, University and ETH Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland b Spinal Cord Injury Center, Balgrist University Hospital, Forchstrasse 340, 8008 Zurich, Switzerland abstract article info Article history: Received 22 January 2012 Revised 16 April 2012 Accepted 23 April 2012 Available online 1 May 2012 Keywords: Spasticity Spinal cord injury Spinal networks Interneuron Immunohistochemistry Uncontrolled muscle spasms often develop after spinal cord injury. Structural and functional maladaptive changes in spinal neuronal circuits below the lesion site were postulated as an underlying mechanism but re- main to be demonstrated in detail. To further explore the background of such secondary phenomena, rats re- ceived a complete sacral spinal cord transection at S 2 spinal level. Animals progressively developed signs of tail spasms starting 1 week after injury. Immunohistochemistry was performed on S 3/4 spinal cord sections from intact rats and animals were sacrificed 1, 4 and 12 weeks after injury. We found a progressive decrease of cholinergic input onto motoneuron somata starting 1 week post-lesion succeeded by shrinkage of the cholinergic interneuron cell bodies located around the central canal. The num- ber of inhibitory GABAergic boutons in close contact with Ia afferent fibers was greatly reduced at 1 week after injury, potentially leading to a loss of inhibitory control of the Ia stretch reflex pathways. In addition, a gradual loss and shrinkage of GAD65 positive GABAergic cell bodies was detected in the medial portion of the spinal cord gray matter. These results show that major structural changes occur in the connectivity of the sacral spinal cord interneuronal circuits below the level of transection. They may contribute in an im- portant way to the development of spastic symptoms after spinal cord injury, while reduced cholinergic input on motoneurons is assumed to result in the rapid exhaustion of the central drive required for the perfor- mance of locomotor movements in animals and humans. © 2012 Elsevier Inc. All rights reserved. Introduction Muscle cramps and uncontrolled, rhythmic, clonic muscle spasms are frequent complications that develop in the first few months after a spinal cord injury (SCI) (Maynard et al., 1990). These cramps can se- verely affect the quality of life of paraplegic patients (Adams and Hicks, 2005). Although the underlying mechanisms are poorly under- stood at present, it is widely accepted that the pathogenesis of spastic symptoms is multifactorial (Dietz and Sinkjaer, 2007; Nielsen et al., 2007). Hence, the appearance of muscle spasms cannot be exclusively explained by the loss of inhibitory supraspinal projections since spas- tic symptoms appear gradually with a delay of several weeks or months after injury suggesting the presence of additional, rather slowly and secondarily developing processes of adaptation. Results from electrophysiological studies conducted over the last 40 years in humans with brain or spinal cord injury indicate that spontaneous secondary maladaptive changes, such as inefficient inhibition of local reflex pathways, occur in spinal circuits distal to the lesion (Delwaide and Penders, 1969; Dietz, 2010; Faist et al., 1994; Kitzman, 2006; Little and Halar, 1985; Mailis and Ashby, 1990). Acti- vation of motoneurons, e.g., via excitatory Ia afferents from muscle spindles, is regulated by presynaptic inhibition from GABAergic inter- neurons, autogenic inhibition from Golgi tendon organs, disynaptic reciprocal inhibition from muscle spindles of antagonistic muscles and recurrent inhibition via Renshaw cells (Nielsen et al., 2007). Fol- lowing injury, a progressive decline of these control mechanisms could result in reduced spinal inhibition (disinhibition) and, there- fore, unrestrained reflex transmission. In addition, altered intrinsic properties of motoneurons, e.g., increased excitability due to changed receptor expression profiles, have been linked to the development of spastic symptoms (Khristy et al., 2009; Li et al., 2004; Murray et al., 2010a, 2010b; Ryge et al., 2010; Wienecke et al., 2010). Changes in gene expression can lead to the spontaneous reappearance of prolonged motoneuronal depolarizations (plateau potentials) as observed after SCI in animal models (Bennett et al., 2001; Gorassini et al., 2004; Li et al., 2004). Information about the possible pathophysiological roles of axonal or dendritic sprouting or pruning and circuit reorganization below the injury site and their contribution to the development of spastic symptoms and signs is sparse. After damage to the central nervous system (CNS), neuronal networks can spontaneously reorganize via the formation of new connections and fiber sprouting, generally referred to as structural plasticity. Recent animal studies have Experimental Neurology 236 (2012) 179–189 ⁎ Corresponding author at: Department of Neurology, University Hospital Zurich, Frauenklinikstr. 26, 8091 Zurich, Switzerland. Fax: + 41 44 255 43 80. E-mail address: bjoern.zoerner@usz.ch (B. Zörner). 1 Both authors contributed equally to the study. 0014-4886/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.expneurol.2012.04.023 Contents lists available at SciVerse ScienceDirect Experimental Neurology journal homepage: www.elsevier.com/locate/yexnr