New Horizons in Diabetic Neuropathy: Mechanisms, Bioenergetics, and Pain

Systemic dexmedetomidine attenuates mechanical allodynia through extracellular sign db type 2 diabetic mice

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus. However, the treatment for PDN is limited in clinical practice. In the present study, we investigated the effect of systemic administration dexmedetomidine (DEX), a selective alpha 2 adrenergic receptor (α2AR) agonist, on mechanical allodynia and its underlying mechanism in db/db mice, an animal model of type 2 diabetes mellitus. Our data demonstrated that db/db mice develop mechanical allodynia at the early stage of diabetes. During the period of mechanical allodynia, we detected increased release of norepinephrine (NE) and decreased levels of α2A-Adrenoceptors in db/db mice. Immunohistochemistry showed that the α2A-Adrenoceptor is predominantly expressed in neurons in the spinal cord. Acute injection of dexmedetomidine significantly decreased mechanical allodynia, which was blocked by its selective antagonist BRL44408. Furthermore, the upregulation of pERK1 and pERK2 in db/db mice were attenuated by preadministration of dexmedetomidine. We provide the first evidence that the functional alternation of spinal noradrenergic system might underlie exaggerated nociception in PDN. Systemic dexmedetomidine inhibits the mechanical allodynia which is related to ERK signaling pathway in type 2 diabetes, implying that the α2-Adrenoceptor might be a potential therapeutic strategy for PDN.

Longitudinal study of neuropathy, microangiopathy, and autophagy in sural nerve: Implications for diabetic neuropathy

OBJECTIVES

The progression and pathophysiology of neuropathy in impaired glucose tolerance (IGT) and type 2 diabetes (T2DM) is poorly understood, especially in relation to autophagy. This study was designed to assess whether the presence of autophagy-related structures was associated with sural nerve fiber pathology, and to investigate if endoneurial capillary pathology could predict the development of T2DM and neuropathy.

PATIENTS AND METHODS

Sural nerve physiology and ultrastructural morphology were studied at baseline and 11 years later in subjects with normal glucose tolerance (NGT), IGT, and T2DM.

RESULTS

Subjects with T2DM had significantly lower sural nerve amplitude compared to subjects with NGT and IGT at baseline. Myelinated and unmyelinated fiber, endoneurial capillary morphology, and the presence and distribution of autophagy structures were comparable between groups at baseline, except for a smaller myelinated axon diameter in subjects with T2DM and IGT compared to NGT. The baseline values of the subjects with NGT and IGT who converted to T2DM 11 years later demonstrated healthy smaller endoneurial capillary and higher g-ratio versus subjects who remained NGT. At follow-up, T2DM showed a reduction in nerve conduction, amplitude, myelinated fiber density, unmyelinated axon diameter, and autophagy structures in myelinated axons. Endothelial cell area and total diffusion barrier was increased versus baseline.

CONCLUSIONS

We conclude that small healthy endoneurial capillary may presage the development of T2DM and neuropathy. Autophagy occurs in human sural nerves and can be affected by T2DM. Further studies are warranted to understand the role of autophagy in diabetic neuropathy.

Axon degeneration: make the Schwann cell great again

Axonal degeneration is a pivotal feature of many neurodegenerative conditions and substantially accounts for neurological morbidity. A widely used experimental model to study the mechanisms of axonal degeneration is Wallerian degeneration (WD), which occurs after acute axonal injury. In the peripheral nervous system (PNS), WD is characterized by swift dismantling and clearance of injured axons with their myelin sheaths. This is a prerequisite for successful axonal regeneration. In the central nervous system (CNS), WD is much slower, which significantly contributes to failed axonal regeneration. Although it is well-documented that Schwann cells (SCs) have a critical role in the regenerative potential of the PNS, to date we have only scarce knowledge as to how SCs ‘sense’ axonal injury and immediately respond to it. In this regard, it remains unknown as to whether SCs play the role of a passive bystander or an active director during the execution of the highly orchestrated disintegration program of axons. Older reports, together with more recent studies, suggest that SCs mount dynamic injury responses minutes after axonal injury, long before axonal breakdown occurs. The swift SC response to axonal injury could play either a pro-degenerative role, or alternatively a supportive role, to the integrity of distressed axons that have not yet committed to degenerate. Indeed, supporting the latter concept, recent findings in a chronic PNS neurodegeneration model indicate that deactivation of a key molecule promoting SC injury responses exacerbates axonal loss. If this holds true in a broader spectrum of conditions, it may provide the grounds for the development of new glia-centric therapeutic approaches to counteract axonal loss.

Clinical application of ghrelin for diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes, and its progression significantly worsens the patient's quality of life. Although several drugs are available for DPN, all of these provide only symptomatic relief. We investigated the therapeutic effects of ghrelin for DPN, based on its various physiological functions. Seven patients with type 2 diabetes with typical clinical signs and symptoms of DPN were hospitalized. Synthetic human ghrelin (1.0 μg/kg) was administered intravenously for 14 days. Motor nerve conduction velocity (MCV) of the posterior tibial nerve improved significantly after the treatment, compared to that at baseline (35.1 ± 1.8 to 38.6 ± 1.8 m/s, p < 0.0001), while the MCV in six untreated patients did not change throughout hospitalization. The subjective symptoms assessed based on the total symptom score also significantly improved (15.6 ± 3.1 to 11.1 ± 2.2, p = 0.047). Although sensory nerve conduction velocity (SCV) of the sural nerve could not be detected in three patients at baseline, it was detected in two of the three patients after 14 days of ghrelin administration. Overall, SCV did not change significantly. Plasma glucose, but not serum C peptide, levels during a liquid meal tolerance test significantly improved after treatment. These results suggest that ghrelin may be a novel therapeutic option for DPN; however, a double-blind, placebo-controlled trial is needed in the future.