Chapter Four - Restoring the Spinal Pain Gate: GABAA Receptors as Targets for Novel Analgesics
Introduction
Chronic pain is a severe medical condition affecting millions of patients worldwide. It is almost generally accepted that neuronal and synaptic plasticity occurring at different levels of the neuraxis are major contributors to chronic pain (Luo et al., 2014, Sandkühler, 2009, Zeilhofer, Witschi and Johansson, 2009) (for a schematic illustration of the pain pathway, see Fig. 1). Some of these neuroplastic changes occur already in the peripheral terminals of nociceptors, which sense noxious stimuli arriving at the skin or in other peripheral tissues and convey them to the central nervous system. The central terminals of these nociceptors innervate the substantia gelatinosa (lamina II) of the spinal dorsal horn, or the trigeminal nucleus of the brainstem in case of those nociceptors coming from the facial skin or the meninges. From there, signals are propagated through various relay stations in the brainstem, midbrain, and thalamus to several cortical areas which give rise to the conscious sensation of pain. One site that has attracted particular attention in pain-related neuroplasticity is the spinal dorsal horn, which constitutes as the first site of synaptic integration in the pain pathway. Neurons located in the spinal dorsal horn integrate primary afferent sensory signals of painful and non-painful modalities with input from descending fiber tracts, which can either inhibit or facilitate pain. Inhibitory interneurons have been attributed a critical role in this process already in the gate-control-theory of pain (Fig. 2; Melzack & Wall, 1965). Although some of the synaptic connections proposed in the original scheme do apparently not exist, plenty of evidence indicates that compromising the function of inhibitory dorsal horn neurons induces symptoms reminiscent of chronic pain syndromes in humans. Animals develop an exaggerated sensitivity to painful stimuli (hyperalgesia), they respond with withdrawal responses upon exposure to stimuli, which are normally not felt as painful (allodynia), and they also show signs of spontaneous discomfort. Many lines of evidence indicate that typical causes of chronic pain such as inflammation or neuropathies compromise the function of inhibitory interneurons in the spinal dorsal horn through different mechanisms (for a review, see Zeilhofer, Benke, & Yévenes, 2012). According to this concept, a facilitation of inhibitory neurotransmission should be a rational strategy for the treatment of many chronic pain states. Yet, none of the established analgesics act- through a facilitation of inhibitory neurotransmission. In the following text, we will review mechanisms of pain-related spinal disinhibition and evidence supporting the concept that novel subtype-selective benzodiazepine agonists would be suitable for the treatment of chronic pain syndromes. In the context of this review, we use the term “benzodiazepine” for all agonists at the benzodiazepine binding site of γ-aminobutyric acid type A receptors (GABAARs) independent of their chemical structure. It should also be mentioned here that GABAARs exist, which are resistant to modulation by classical benzodiazepines. These receptors contain α4 or α6 subunits instead of α1, α2, α3, or α5, or a γ1 or δ subunit instead of the γ2 subunit. These benzodiazepine-insensitive receptors are quite abundant in several brain regions (e.g., thalamus and cerebellum), but their expression in the spinal cord is sparse.
Section snippets
Synaptic Disinhibition in Pathological Pain
Fast synaptic inhibition in the spinal dorsal horn is mediated by GABA and glycine acting respectively at GABAAR and strychnine-sensitive GlyRs. Plenty of evidence indicates that blockade of spinal GABAARs or GlyRs produces signs of allodynia and spontaneous pain (Beyer et al., 1985, Miraucourt et al., 2007, Roberts et al., 1986). More recent studies provided insights into the mechanism of this sensitization on the level of dorsal horn neuronal circuits. The most consistent observation in these
Spinal GABAAR Subtypes Mediating Antihyperalgesia: Evidence from Genetically Engineered Mice
Analgesic or antihyperalgesic actions of benzodiazepines occur after local spinal injection suggesting that these effects are mediated by GABAARs expressed in the spinal cord. To identify the GABAAR subtypes responsible for these antihyperalgesic effects, “knock-in” mice were investigated, in which the α1, α2, α3, or α5-GABAAR subunits had been rendered diazepam-insensitive through the introduction of a histamine to arginine (H/R) point mutation (Knabl et al., 2008; for information on the
Mechanisms of Spinal Benzodiazepine-Mediated Antihyperalgesia
Immunohistochemistry studies have identified specific spinal distribution patterns of GABAAR subunits (Bohlhalter et al., 1996, Paul et al., 2012; Fig. 4A–D). These receptors are expressed on intrinsic dorsal horn neurons and on the central terminals of primary sensory nociceptors. Spinal antihyperalgesia may therefore originate either from classical postsynaptic inhibition mediated by GABAARs on intrinsic dorsal horn neurons or from GABAARs on nociceptor terminals which mediate presynaptic
Antihyperalgesic Action of Benzodiazepines with Improved Subtype Specificity: Preclinical Studies
A number of benzodiazepines with reduced activity at α1-GABAARs have been developed in the last two decades mainly in the quest for nonsedating anxiolytics (for a comprehensive list, see Rudolph & Knoflach, 2011). Because benzodiazepine-mediated anxiolysis and antihyperalgesia share a similar dependence on GABAAR subtypes, some of these compounds were also tested in pain studies (Table 1). It should be mentioned here that α1-sparing compounds are sometimes referred to as “α2/3 selective” (e.g.,
Clinical Studies on Antihyperalgesia by Benzodiazepines
The preclinical studies discussed above performed in mice resistant to the sedative effects of benzodiazepines demonstrate that classical benzodiazepines do in principle exert profound antihyperalgesic actions but only at doses, which normally induce strong sedation. Less-sedating benzodiazepines exhibited antihyperalgesic efficacy at nonsedating doses also in wild-type mice.
In human patients, classical nonselective benzodiazepines do not exert relevant analgesic (or antihyperalgesic) actions
Which GABAAR subtypes should be targeted for optimal analgesia with minimal side-effects?
Work in the GABAAR H/R point-mutated mice suggests that α2-, α3-, and α5-GABAARs contribute to spinal benzodiazepine antihyperalgesia. It is however not clear whether only one subtype (i.e., α2-GABAARs) should be targeted for optimal antihyperalgesia or whether simultaneous activity at more than one subunit would be advantageous. In the absence of fully selective subtype-specific drugs, investigations on mice carrying more than one point-mutated GABAAR subtype should be informative. When taking
Conclusion
There is compelling evidence from preclinical studies in rodents to support that non-sedative benzodiazepines with improved subtype specificity exert antihyperalgesic effects. Available clinical data are consistent with this view. Current knowledge suggests that robust antihyperalgesic activity with low sedative properties requires a high intrinsic activity at α2-GABAARs (or possibly also at α3-/α5-GABAARs) and very low activity at α1-GABAARs. The optimal profile of such a drug in terms of GABAA
Conflict of Interest Statement
The authors declare that they have no conflict of interest.
Acknowledgment
The research of H. U. Z. has been supported by grants from the Swiss National Science Foundation and by an Advanced Investigator Grant from the European Research Council (DHISP; 250128).
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