GABA and Central Neuropathic Pain following Spinal Cord Injury
Young S. Gwak and Claire E. Hulsebosch
Spinal cord injury induces maladaptive synaptic transmission in the somatosensory system that results in chronic central neuropathic pain. Recent literature suggests that glial-neuronal interactions are important modulators in synaptic transmission following spinal cord injury. Neuronal hyperexcitability is one of the predominant phenomenon caused by maladaptive synaptic transmission via altered glial-neuronal interactions after spinal cord injury. In the somatosensory system, spinal inhibitory neurons counter balance the enhanced synaptic transmission from peripheral input. For a decade, the literature suggests that hypofunction of GABAergic inhibitory tone is an important factor in the enhanced synaptic transmission that often results in neuronal hyperexcitability in dorsal horn neurons following spinal cord injury. Neurons and glial cells synergistically control intracellular chloride ion gradients via modulation of chloride transporters, extracellular glutamate and GABA concentrations via uptake mechanisms. Thus, the intracellular ?GABA-glutamate-glutamine cycle? is maintained for normal physiological homeostasis. However, hyperexcitable neurons and glial activation after spinal cord injury disrupts the balance of chloride ions, glutamate and GABA distribution in the spinal dorsal horn and results in chronic neuropathic pain. In this review, we address spinal cord injury induced mechanisms in hypofunction of GABAergic tone that results in chronic central neuropathic pain.
Keywords: Central neuropathic pain, GABA, Glia, Hyperexcitability, Spinal cord injury
Spinal cord injury (SCI) causes central neuropathic pain (CNP) in as high as 70% of people with SCI (Beric et al., 1988; Rintala et al., 1988). In terms of clinical pain behavior, allodynia (pain behavior evoked by non-noxious stimuli) and hyperalgesia (exaggerated pain behavior evoked by noxious stimuli) characterize CNP syndromes (Merskey and Bogduk, 1994). Patients with CNP suffer from altered quality of life that negatively influences the individual and often results in depression and suicide (Siddall and Loeser, 2001; Werhagen et al., 2004). The majority of patients with SCI suffer from CNP; however, current treatment strategies are inadequate and refractive because the cellular mechanisms that provide the substrate for CNP are poorly understood (Beric et al., 1988; Davidoff et al., 1987; Hulsebosch, 2005; Siddall and Loeser, 2001).
Descending and local interneuron inhibitory pathways, such as GABAergic pathways, critically contribute to the modulation of the balance between excitatory and inhibitory tone in synaptic transmission. The GABAergic interneurons produce synaptic inhibition, and thereby prevent or inhibit CNP, via both GABAA and GABAB receptors in the spinal dorsal horn (Hwang and Yaksh, 1997; Munro et al., 2008). Pharmacological blockade of GABAA or GABAB receptor in rats (Gwak et al., 2006; Malan et al., 2002) and transgenic mice of GABAA (Knabl et al., 2008, 2009) or GABAB (Gangadharan et al., 2009) receptor demonstrate increased sensitivity to external stimuli that results in various pain conditions induced by spinal cord injury, peripheral nerve injury and inflammation. Mounting evidence indicates that SCI induces a hypofunction of GABAergic tone in the spinal dorsal horn and results in CNP (Gwak et al., 2006; Liu et al., 2004; Zhang et al., 1994). Thus, it is possible that the hypofunction of spinal GABAergic inhibitory tone in the spinal dorsal horn is a key factor in CNP after SCI (Drew et al., 2004; Liu et al., 2004).
Spinal systems are composed of both neuronal and non-neuronal cells, including astrocytes and microglia. Spinal glia cells outnumber neurons and play important roles in maintaining ionic balance, as well as glutamate and GABA concentrations in the central nervous system (Anderson and Swanson, 2000; Chesler and Kaila, 1992; Largo et al., 1996; Schlue and Deitmer, 1988). However, recent literature consistently reports that activation of astrocytes and microglia contributes significantly to CNP following SCI (Gwak et al., 2008, Gwak and Hulsebosch, 2009; Hains and Waxman, 2006). Neurons and astrocytes both contribute to GABA uptake (also released from both neurons and glia) to control extracellular concentrations of GABA (Chatton et al., 2003; Schousboe et al., 2004). Although, it is known that SCI alters neuronal and glial activity; little is known about the mechanisms underlying hypofunction of spinal GABAergic inhibitory tone following SCI. In this review, we focus on mechanisms that lead to hypofunction of GABAergic tone that contributes to CNP following SCI.
2. Central Neuropathic Pain and GABA
2.1 Central Neuropathic pain following SCI
Traumatic spinal cord injuries (SCI) directly and indirectly produce dramatic changes of neuroanatomical and neurochemical shifts that result in maladaptive synaptic circuits in the spinal dorsal horn. Upregulated glutamate receptors and ion channels, increased release of proinflammatory cytokines and reactive oxygen species (ROS), activation of glial cells and subsequent activation of intracellular downstream/upstream cascades (Crown et al., 2006; Gwak et al., 2008; 2009a; Hains et al., 2003, Leem et al., 2010; Tan et al., 2008; Zinck et al., 2007) are predominant events that lead to enhanced pain transmission following SCI.
Maladaptive synaptic circuits in the spinal dorsal horn induced by SCI, individually or synergistically, contribute to the neuronal hyperexcitability in response to mechanical, chemical and thermal stimuli. Electrophysiologically, neuronal hyperexcitability or central sensitization is characterized by the enhanced spontaneous or evoked neuronal response properties to external stimuli applied to peripheral receptive fields with lowered thresholds for the activation, increased peripheral receptive field size and increased afterdischarge activity (Drew et al., 2004; Gwak et al., 2008; Hains et al., 2003). Thus, neuronal hyperexcitability is a substrate of central neuropathic pain following SCI. SCI is categorized by below-, at-, and above-level pain based on dermatomal regions that involves neuropathic pain symptoms, such as mechanical allodynia and thermal hyperalgesia, following SCI (Siddall et al., 1997, 1999, 2002). Clinically, below-level pain is defined as pain that occurs several dermatomes caudal to the lesion. At-level pain is defined as pain that occurs at dermatomes adjacent to the lesion. Above-level pain is defined as pain that occurs several dermatomes rostral to the lesion (Siddall et al., 1997, 1999). The development of central neuropathic pain syndromes take several weeks to months after SCI, but once developed, persists for life. Consequently, the abnormal pain syndromes are important to understand for effective treatment strategies (Carlton et al., 2009; Hulsebosch et al., 2009).
Mechanically and chemically induced mammalian SCI models were developed to study the development and the maintenance of CNP after SCI. The models include; 1) spinal ischemic injury via intravascular photochemical reaction (Hao et al., 1991); 2) clip compression of the thoracic cord (Bruce et al., 2002); 3) excitotoxic injury via quisqualic acid injection into the dorsal horn (Yezierski et al., 1998); 4) spinal hemisection injury (Christensen et al., 1996; Gwak et al., 2006); 5) spinal contusion injury (Hulsebosch et al., 2000; Siddall et al., 1997). It is a worthy to note that these SCI models showed CNP that correlates with hypofunction of GABAergic inhibitory tone in the spinal dorsal horn (Drew et al., 2004; Eaton et al., 2007; Gwak et al., 2006; Xu et al., 1993). Taken together, hypofunction of GABAergic tone may provides a representative mechanism in CNP syndromes after SCI, although Polar and Todd reported the GABAergic loss is not sufficient to induce neuropathic pain following peripheral nerve injury (Polgar et al., 2003, 2005).
2.2. GABA Synthesis and Release
In the somatosensory system, GABAergic descending pathways originate from locus ceruleus (LC), nucleus raphe-magnus (NRM) and periaquctal gray (PAG) and terminate in the spinal cord (Willis and Westlund, 1997). Gamma-aminobutyric acid (GABA) is a widely distributed inhibitory neurotransmitter in the spinal cord and plays a ?counter balance? role against enhanced synaptic transmission in the spinal cord as a result of glutamate-mediated excitation of neurons following SCI. GABA is produced by the decarboxylation of L-glutamate by glutamic acid decarboxylase (GAD, a rate-limiting enzyme) that is distributed in GABA interneurons and glial cells (Bu et al., 1992; Erlander et al., 1991; Kaufman et al., 1991; Mackie et al., 2003). GAD65 (65 kDa) is a membrane associated protein that produces vesicular GABA, released by exocytosis, and contributes to the rapid and focal communication to individual postsynaptic site on neurons. GAD67 (67kDa) is a cytosolic protein that produces cytosolic GABA release and contributes to both paracrine signaling and intracellular metabolites. However, neurons are not the only cells that synthesize GABA in the central nervous system. After ischemic injury, forebrain regions show increased GFAP immunoreactivity (activated astrocytes) co-labeled with GABA and GAD that indicate that glial cells also synthesize GABA, since GAD is the enzyme necessary in GABA synthesis (Bellier et al., 2000; Kozlov et al., 2006; Lin et al., 1993; Liu et al., 2007; New and Rabkin, 1998).
GABAergic neurons predominantly synapse axodendritically and axosomatically and only a small number have axoaxonic synapses. Activation of NMDA receptors and other calcium channels, largely located on neuronal dendritic or somatic membranes, trigger large influxes of calcium ions, dependent on the depolarization of the membrane and initiate subsequent Ca2+ dependent GABA release via vesicular exocytosis (Isaacson, 2001; Koch and Magnusson, 2009). Thus, the somatic and dendritic localized GABA release may produce widespread inhibition in nociceptive transmission in synaptic and extrasynaptic terminals.
2.3 GABA Receptors
GABA, released by neurons and glia, provides an inhibitory role via neighboring GABA receptors that are distributed in presynaptic and postsynaptic membranes on both neurons and glial cells (Barakat and Bordey, 2002; Malcangio and Bowery, 1996). There are three types of GABA receptors: 1) GABAA, 2) GABAB and 3) GABAC. The GABAA receptor is an ionotropic ligand-gated Cl− channel and is present throughout the spinal cord gray matter on both neurons and glial cells. Activation of GABAA receptor increases the permeability of chloride ions and hyperpolarizes postsynaptic neurons, which results in increases in the resting membrane conductance of the cell (Jensen et al., 2002; Sieghart and Sperk, 2002). The GABAB receptor is a metabotropic receptor, coupled to a G-protein, and concentrated in the superficial layers of the spinal dorsal horn on both neurons and glial cells (Albrecht et al., 1986; Bowery et al., 1980; Charles et al., 2003a,b). Activation of the GABAB receptor inhibits synaptic transmission at primary afferent terminals in the spinal cord via reduction of calcium entry at the presynatic terminal and hyperpolarization at the postsynaptic terminal through increased conductance of potassium ions (Bowery et al., 1980). The GABAC receptor is a subtype of GABAA and insensitive to benzodiazepine (Johnston, 1996; Shimada et al., 1992). Currently, the role of the ionotropic GABAc receptor in somatosensory systems is unknown but it appears to be important in cognition and neuronal processing (Johnston, 1996; Lukasiewicz et al., 1994).
2.4 Hypofunction of GABA and CNP
The evidence that hypofunction of GABAergic tone contributes to central neuropathic pain following SCI is well documented by several experimental obervations. First, pharmacological treatments that enhance GABAergic function attenuate central neuropathic pain behavior and neuronal hyperexcitability following SCI. For example, intrathecal administration of GABA attenuated mechanical allodynia and hyperexcitability of spinal dorsal horn neurons following SCI (Figure 1). The attenuation of pain behavior and neuronal hyperexcitability are mediated by both GABAA and GABAB receptors (Drew et al., 2004; Gwak et al., 2006). In addition, treatment with bicuculline (a GABAA receptor antagonist) produced neuronal hyperexcitability and pain behavior in normal rats (Drew et al., 2004). Second, immunohistochemical studies demonstrated decreased numbers of GABAergic interneurons and GAD expression in the spinal dorsal horn following SCI that correlated well with central neuropathic pain behavior (Gwak et al., 2008; Meisner et al., 2010; Zhang et al., 1994). Third, transplantation of herpes simplex virus (HSV)-mediated GAD65 and GAD67 producing gene vector or transfer of human foamy virus (HFV) mediated GAD67 gene, attenuated central neuropathic pain following SCI (Liu et al., 2004, 2008; New and Rabkin, 1998). In addition, transplantation of the human neuronal NT cell line (NT2.17), which synthesizes and releases GABA, attenuated SCI-induced mechanical allodynia and thermal hyperalgesia (Eaton et al., 2007). Taken together, these pharmacological and molecular approaches suggest that hypofunction of GABAergic tone is a substrate for the central neuropathic pain following SCI.
Attenuation of mechanical allodynia and neuronal hyperexcitability by administration of GABA. Spinal T13 hemisection injury results in bilateral mechanical allodynia on the contralateral (A, uninjured side) and the ipsilateral (B, injured side) hindlimbs ...