Introduction Neuropathic pain occurs in approximately one th
Neuropathic pain occurs in approximately one-third of patients with diabetes and is refractory to currently available analgesic drugs (Abbott et al., 2011). This painful diabetic neuropathy (PDN) is associated with elevated levels of methylglyoxal (MG; a reactive glucose metabolite) and decreased expression and activity of glyoxalase 1 (GLO1), the major detoxification enzyme for MG (Bierhaus et al., 2012; Huang et al., 2016; Jack et al., 2012; Skapare et al., 2013; Sveen et al., 2013). Previous studies suggest MG produces PDN by contributing to the formation of advanced glycation end-products (MG-AGEs) (Skapare et al., 2013; Sveen et al., 2013), and/or by sensitizing pronociceptive ion cholinesterase inhibitors in peripheral afferents (Andersson et al., 2013; Bierhaus et al., 2012; Griggs et al., 2017; Huang et al., 2016; Koivisto et al., 2012). While these studies focused on the pathophysiology of peripheral sensory nerves in the type 1 form of diabetes, type 2 diabetes accounts for 90% of patients, is more frequently associated with PDN (Abbott et al., 2011), and the mechanisms of diabetic neuropathy may differ in these two forms of diabetes (Callaghan et al., 2012a; Callaghan et al., 2012b; Feldman et al., 2017). Furthermore, very little is known regarding the spinal mechanisms that contribute to PDN. Thus, the central mechanisms that generate and maintain PDN in type 2 diabetes is a critical gap in knowledge.
The contribution of a MG-related spinal signaling cascade to PDN in type 2 diabetes remains unknown. Recent studies implicate both transient receptor potential ankyrin subtype 1 (TRPA1) (Andersson et al., 2013; Griggs et al., 2017; Huang et al., 2016) and adenylyl cyclase isoform 1 (AC1) (Griggs et al., 2017) in the pain evoked by intraplantar administration of MG. The contribution of a spinal adenylyl cyclase, cyclic adenosine monophosphate (cAMP), protein kinase A (PKA) pathway was identified in the Zucker Diabetic Fatty rat model of type 2 diabetes (Feng et al., 2017), but the specific adenylyl cyclase isoform (e.g. AC1) was not identified and antagonism of spinal PKA was not tested. Furthermore, spinal AC1-cAMP signaling could be mediated not just by PKA, but also by isoform 1 or 2 of exchange protein directly activated by cAMP (Epac1/2). Both PKA and Epac1/2 are implicated in peripheral nociceptive sensitization in various pain conditions (Aley and Levine, 1999; Eijkelkamp et al., 2013; Gu et al., 2016; Huang and Gu, 2017; Hucho et al., 2005; Matsuda et al., 2017; Wang et al., 2013). Whether spinal TRPA1, AC1, PKA, or Epac1/2 contribute to PDN in type 2 diabetes remains an important question.
To test the hypothesis that spinal MG signals through TRPA1, AC1, PKA, and Epac1/2 to cause PDN in type 2 diabetes, we used two experimental models of neuropathic pain. First, we targeted the spinal cord dorsal horn with intrathecal administration of MG in conventional C57BL/6J mice and tested both reflexive and affective pain-like behaviors. Second, we utilized db/db mice, a model of type 2 diabetes that develops heat hyperalgesia (Bierhaus et al., 2012; Xu et al., 2014) concordant with hyperglycemia and elevated MG (Bierhaus et al., 2012). We determined the behavioral effects of agents that scavenge MG or promote overexpression of GLO1, used calcium imaging to assess MG-evoked central sensitization in ex vivo spinal cord slices, and evaluated the effect of intrathecal administration of inhibitors of MG, TRPA1, AC1, PKA, and Epac1/2 on pain-like hypersensitivity in the intrathecal MG and db/db models.
Materials and methods
Conclusions Despite decades of research, diabetic neuropathy continues to be complicated by neuropathic pain in up to 50% of patients (Abbott et al., 2011). The current study aimed to establish novel targets for the development of new therapies for PDN. First, we establish the therapeutic utility of MG scavengers and GLO1 enhancers in a mouse model of type 2 diabetes, extending a study in streptozotocin-evoked type 1 diabetes (Bierhaus et al., 2012). This is important because prior preclinical studies of PDN tended not to distinguish between type 1 and type 2 patient classes (Callaghan et al., 2012a; Feldman et al., 2017), despite the fact that rigorous glycemic control reduces neuropathy in type 1 but not type 2 diabetic patients (Callaghan et al., 2012b). Second, our results are the first to identify multiple targets within the spinal cord (e.g. the inhibition of MG, TRPA1, AC1, Epac) as potential therapies for PDN associated with type 2 diabetes. This is in line with a recent report highlighting the need to identify the central mechanisms that drive neuropathic pain in diabetes (Feldman et al., 2017).