Differential vulnerability of neuropeptides in nerves of the vasa nervorum to streptozotocin-induced diabetes

The neuropeptide substance P/neurokinin-1 receptor system and diabetes: From mechanism to therapy

Diabetes is a significant public health issue known as the world's fastest-growing disease condition. It is characterized by persistent hyperglycemia and subsequent chronic complications leading to organ dysfunction and, ultimately, the failure of target organs. Substance P (SP) is an undecapeptide that belongs to the family of tachykinin (TK) peptides. The SP-mediated activation of the neurokinin 1 receptor (NK1R) regulates many pathophysiological processes in the body. There is also a relation between the SP/NK1R system and diabetic processes. Importantly, deregulated expression of SP has been reported in diabetes and diabetes-associated chronic complications. SP can induce both diabetogenic and antidiabetogenic effects and thus affect the pathology of diabetes destructively or protectively. Here, we review the current knowledge of the functional relevance of the SP/NK1R system in diabetes pathogenesis and its exploitation for diabetes therapy. A comprehensive understanding of the role of the SP/NK1R system in diabetes is expected to shed further light on developing new therapeutic possibilities for diabetes and its associated chronic conditions.

Increased ATP and ADO Overflow From Sympathetic Nerve Endings and Mesentery Endothelial Cells Plus Reduced Nitric Oxide Are Involved in Diabetic Neurovascular Dysfunction

Since the mechanism of human diabetic peripheral neuropathy and vascular disease in type 1 diabetes mellitus remains unknown, we assessed whether sympathetic transmitter overflow is altered by this disease and associated to vascular dysfunction. Diabetes was induced by streptozotocin (STZ)-treatment and compared to vehicle-treated rats. Aliquots of the ex vivo perfused rat arterial mesenteric preparation, denuded of the endothelial layer, were collected to quantify analytically sympathetic nerve co-transmitters overflow secreted by the isolated mesenteries of both groups of rats. Noradrenaline (NA), neuropeptide tyrosine (NPY), and ATP/metabolites were detected before, during, and after electrical field stimulation (EFS, 20 Hz) of the nerve terminals surrounding the mesenteric artery. NA overflow was comparable in both groups; however, basal or EFS-secreted ir-NPY was 26% reduced (p < 0.05) in diabetics. Basal and EFS-evoked ATP and adenosine (ADO) overflow to the arterial mesentery perfusate increased twofold and was longer lasting in diabetics; purine tissue content was 37.8% increased (p < 0.05) in the mesenteries from STZ-treated group of rats. Perfusion of the arterial mesentery vascular territory with 100 μM ATP, 100 nM 2-MeSADP, or 1 μM UTP elicited vasodilator responses of the same magnitude in controls or diabetics, but the increase in luminally accessible NO was 60-70% lower in diabetics (p < 0.05). Moreover, the concentration-response curve elicited by two NO donors was displaced downwards (p < 0.01) in diabetic rats. Parallel studies using primary cultures of endothelial cells from the arterial mesentery vasculature revealed that mechanical stimulation induced a rise in extracellular nucleotides, which in the cells from diabetic rats was larger and longer-lasting when comparing the extracellular release of ATP and ADO values to those of vehicle-treated controls. A 5 min challenge with purinergic agonists elicited a cell media NO rise, which was reduced in the endothelial cells from diabetic rats. Present findings provide neurochemical support for the diabetes-induced neuropathy and show that mesenteric endothelial cells alterations in response to mechanical stimulation are compatible with the endothelial dysfunction related to vascular disease progress.

Sciatic nerve of diabetic rat treated with epoetin delta: effects on C-fibers and blood vessels including pericytes

In diabetes mellitus (DM) reduced motor and sensory properties of peripheral nerves are linked with the dysfunction of neural vasculature. We investigated C-fibers and microvessels of sciatic nerve of normal, DM, and DM + epoetin delta-treated rats. C-fibers immunoreactive for calcitonin gene-related peptide (CGRP), tyrosine hydroxylase (TH), epoetin receptor (EpoR), and common beta receptor subunit of the interleukin 3 receptor (IL-3Rbeta) were present in all rats, whereas in DM and epoetin-treated rats C-fibers also showed neuronal (nNOS) and inducible (iNOS) nitric oxide synthases. The cross-sectional area of CGRP-positive C-fibers was decreased in DM, but it recovered after epoetin treatment. In all conditions, vascular endothelium showed scarce immunolabeling for endothelial nitric oxide synthase (eNOS); the profound immunoreactivity for eNOS, EpoR, and IL-3Rbeta was in pericytes. Some perivascular autonomic nerves were damaged and IL-3Rbeta positive. Findings are discussed in terms of declined sensory conduction velocity in DM, its improvement after epoetin treatment, and the possible vascular contribution to these phenomena.

Neural regulation of submandibular gland blood flow in the streptozotocin-diabetic rat: evidence for impaired endothelium-dependent vasodilatation

Functional changes in vascular tone and reactivity arise early in diabetes, and endothelial dysfunction plays a central role in the development of these microvascular abnormalities. Blood flow in the rat submandibular gland is mainly under neural regulation, which is mediated in part via endothelium-dependent mechanisms. Given the role of the endothelium in regulating blood flow and the deleterious effects of diabetes on endothelial cell function, we hypothesised that diabetes would significantly impair neural regulation of submandibular gland vascular perfusion. Three weeks after the induction of streptozotocin diabetes continuous 2 Hz sympathetic stimulation resulted in a similar degree of vasoconstriction (as measured by a decrease in perfusion) in both diabetic (-31+/-17%) and control rats (-22+/-7%). However, the magnitude and the duration of the after-dilatation were significantly less in diabetic animals. The same number of impulses delivered at 20 Hz in bursts (1s in every 10s) also resulted in vasoconstriction with each burst, but unlike the effects of burst stimulation in control rats the initial vasoconstriction was not converted to a net vasodilatation between bursts. Parasympathetic stimulation (2, 5 and 10 Hz) caused a marked vasodilatation in both control and diabetic rats, but the initial responses were delayed in diabetic animals, the maintained phases were smaller in magnitude (P<0.02) and it took longer to return to resting levels. In conclusion, submandibular gland vascular responses are altered in streptozotocin-induced diabetic rats. Vasoconstrictor responses evoked by sympathetic impulses were unaffected, but vasodilatory responses, particularly those associated with endothelium-dependent mechanisms, were significantly reduced.

Role of calcitonin gene-related peptide in ischaemic preconditioning in diabetic rat hearts

1. It has been suggested that calcitonin gene-related peptide (CGRP) is involved in the protection provided by ischaemic preconditioning in rat hearts and that ischaemic preconditioning is absent in diabetic rat hearts. 2. In the present study, we tested the relationship between sensory nerve function and ischaemic preconditioning in diabetic rats. 3. In 4- and 8-week diabetic rats and age-matched non- diabetic controls, 30 min global ischaemia and 40 min reperfusion caused a significant decrease in cardiac function and a marked increase in creatine kinase (CK) release. Ischaemic preconditioning, by three cycles of 5 min ischaemia and 5 min reperfusion, improved the recovery of cardiac function and decreased CK release during reperfusion in 4-week diabetic rat hearts. However, the cardioprotection afforded by ischaemic preconditioning was lost in 8-week diabetic rat hearts. Pretreatment with CGRP for 5 min also significantly improved the recovery of cardiac function and decreased CK release in rats subjected to 4 or 8 weeks of diabetes. 4. The content of CGRP in the coronary effluent during ischaemic preconditioning was significantly increased in 4-week diabetic rat hearts (P < 0.05). However, only a slight increase in the release of CGRP was shown in 8-week diabetic rat hearts (P > 0.05). 5. In summary, the present results suggest that the protection afforded by ischaemic preconditioning is attenuated in diabetic rats and that the change may be related to the reduction in CGRP release in diabetic rat hearts.

Diabetic neuropathy

Better clinical characteristics and a standardized approach to the definition of neuropathy has enabled us to define more precisely the natural history of diabetic neuropathy. Detailed studies on the pathology and pathogenesis have allowed dissection of important pathogenetic pathways. Effective treatment is currently limited, although a number of new and potentially important therapeutic interventions, including modification of the vascular supply and antioxidant status and growth factors, may prove to be of benefit in preventing damage and also promoting repair of peripheral nerves in human diabetic neuropathy.

Blood flow in the sciatic nerve is regulated by vasoconstrictive and vasodilative nerve fibers originating from the ventral and dorsal roots of the spinal nerves

Anesthetized rats were subjected to repetitive electrical stimulation of either the ventral or dorsal root of the spinal nerves between the 11th thoracic and 2nd sacral spinal segments. The response of nerve blood flow (NBF) in the sciatic nerve was examined using laser Doppler flowmetry. For all nerve fibers stimulation was for a 10-30-s period at a supramaximal intensity. (1) Stimulation of the T11-L1 ventral roots produced an increase in mean arterial pressure (MAP) and a biphasic NBF response was comprised of an initial increase and a subsequent decrease. The initial increase was a passive vasodilation due to the increase in MAP, while the following decrease in NBF resulted from active vasoconstriction of the vasa nervorum due to the activation of sympathetic nerves innervating the sciatic vasa nervorum. (2) Stimulation of the ventral root of the L6 segment produced an increase in NBF, even though MAP decreased. This increase in NBF was apparently mediated by activation of parasympathetic cholinergic vasodilators, because the response was abolished by i.v. injection of atropine, a muscarinic cholinergic receptor antagonist. (3) Stimulation of the dorsal roots between the L3 and S1 segments produced an increase in NBF, independent of changes in MAP. This increase in NBF appeared to be mediated by activation of a calcitonin gene-related peptide (CGRP) containing afferent fibers innervating the vasa nervorum, because the response was abolished by topical application of hCGRP (8-37), a CGRP receptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Beneficial effect of the Ca2+ antagonist, nimodipine, on existing diabetic neuropathy in the BB/Wor rat

1. Neuropathy is a frequently diagnosed complication of diabetes mellitus. Effective pharmacotherapy is not available. 2. The spontaneously diabetic BB/Wor rats develop secondary complications like neuropathy as do human diabetic patients. 3. BB/Wor rats treated with insulin via a subcutaneous implant show a significant impairment of sensory and motor nerve conduction velocity 6 weeks after the onset of diabetes mellitus. 4. Intraperitoneal treatment of diabetic BB/Wor rats with the Ca2+ antagonist, nimodipine (20 mg kg-1), from week 6 onwards every 48 h for a period of 6 weeks resulted in a significant increase of sensory and motor nerve conduction velocity. 5. Twelve weeks after the onset of diabetes mellitus BB/Wor rats show a 40% impairment of sciatic nerve blood flow as compared to the non-diabetic age-matched controls. Treatment with nimodipine (20 mg kg-1) from week 6 onwards significantly increased the sciatic nerve blood flow as compared to placebo-treated diabetic BB/Wor rats. 6. The adrenergic responsiveness of the vasa nervorum of the sciatic nerve to tyramine and phenylephrine was investigated as a parameter for autonomic neuropathy. 7. The fact that nimodipine treatment restored the reduced response to tyramine independently of the reduced postsynaptic phenylephrine responsiveness indicates that nimodipine improves adrenergic responsiveness mainly at the presynaptic level.

Innervation of normal human sural and optic nerves by noradrenaline- and peptide-containing nervi vasorum and nervorum: effect of diabetes and alcoholism

Histochemical, immunohistochemical and neurochemical techniques were used to examine the innervation of epineurial nerve sheaths and fascicular nerve bundles of human sural and optic nerves from controls and patients with peripheral neuropathy due to diabetes or alcoholism. The normal distribution of autonomic nerves in both nerve trunk sheaths consisted of a dense innervation by noradrenaline (NA)-containing nerves of the vasa nervorum, together with some fibres in the nervi nervorum. Intrafascicular NA-containing nerves were only present in the sural nerve. Vasoactive intestinal polypeptide (VIP)- and neuropeptide Y (NPY)-containing nerves also innervated the vasa nervorum and nervi nervorum of the nerve sheaths, although their density was considerably less. Substance P (SP)-containing nerves were sparse and primarily intrafascicular. Neurochemical assays for NA, VIP, NPY and SP in fascicular and epineurial preparations from the sural and optic nerves confirmed the light microscopical observations. Post mortem delay significantly affected the NA levels in the sural nerve but not in the optic nerve while the NA fascicular/epineurial ratio for the sural nerve was independent of this factor. Age, sex and the presence of alcohol at time of death had no effect on transmitter levels in normal sural nerves. In the optic nerve fascicles NA levels were higher in females than in males. In patients with peripheral neuropathy there was a significant reduction in the SP fascicular/epineurial ratio in both the optic nerve, which was histologically normal, and in the sural nerve, where there was evidence of neuropathy. The NA fascicular/epineurial ratio was also significantly reduced in the sural nerve from patients with peripheral neuropathy with a possible greater effect in diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nimodipine on sciatic nerve blood flow and vasa nervorum responsiveness in the diabetic rat

Evidence is accumulating that impairment of nerve blood flow is a key factor in the pathogenesis of diabetic neuropathy. Nimodipine, a 1,4-dihydropyridine type Ca2+ channel antagonist, has been shown to ameliorate an existing neuropathy in the streptozotocin-induced diabetic rat. In the present study the effect of diabetes mellitus itself and the effect of chronic nimodipine treatment on the sciatic nerve blood flow of streptozotocin-induced diabetic rats were investigated. Nerve blood flow was assessed using laser-Doppler flowmetry. Nerve blood flow gradually decreased during the first 10 weeks of diabetes mellitus and remained relatively stable thereafter. Intervention with nimodipine significantly improved the flow deficit observed in the diabetic rats. Vasa nervorum adrenergic responsiveness was also investigated. Diabetic rats showed a postsynaptic adrenergic hyporesponsiveness. Treatment with nimodipine restored the reduced presynaptic responsiveness independent of the postsynaptic adrenergic hyporesponsiveness. It was concluded that, in addition to direct neuroprotective effects, nimodipine exerts beneficial effects on disturbed nerve blood flow and on reduced presynaptic adrenergic responsiveness of the vasa nervorum in experimental diabetic neuropathy.

Effects of colchicine applied to the peripheral nerve on the thermal hyperalgesia evoked with chronic nerve constriction

Loose ligatures placed unilaterally on the sciatic nerve results in a thermal hyperalgesia. We applied 5 or 50 mM colchicine (COL, blocker of the fast axonal transport) to the sciatic nerve and examined the effects of COL on thermal hyperalgesia and the levels of substance P (sP), calcitonin gene-related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) in the spinal cord and the sciatic nerve. These data showed that the application of 50 mM COL to the sciatic nerve in this study functioned as an axonal transport blocker. COL abolished the hyperalgesic state in a concentration-dependent manner when applied proximal to the constriction injury. COL (50 mM), when applied distal to the injury, had no effect on the hyperalgesia. COL did not alter motor function or paw withdrawal response in the non-lesioned animal. Examination of peptide levels in nerve shows that COL resulted in an accumulation of sP, CGRP and VIP in the nerve. In the dorsal horn, COL resulted in a modest reduction in levels of sP and CGRP as compared to the non-lesioned side while VIP levels were elevated. These data suggest that active factors generated by the focal nerve compression and carried by fast axonal transport from the lesioned site to the spinal cord and/or dorsal ganglion are important in the development of thermal hyperalgesia after constriction injury in rats.

Impaired induction of vasoactive intestinal polypeptide after sciatic nerve injury in the streptozotocin-diabetic rat

This study was designed to assess the effects of experimental diabetes and nerve crush injury upon vasoactive intestinal polypeptide (VIP) content and axonal transport in the dorsal root ganglia (DRG) and sciatic nerve. Sciatic nerve crush injury in control and 3-week streptozotocin-diabetic rats was followed 6.5 days later by placement of 2 constricting ligatures above the site of injury. After 12 h, the L4 and L5 DRG and sciatic nerve were removed for VIP radioimmunoassay. Similar samples were also taken from control and diabetic rats whose nerve had been ligated without a preceding crush. VIP was increased over 2-fold in ganglia and 4-fold in nerves of crush-injured controls compared to uninjured controls (both P < 0.01). Crush injury also increased ganglion and nerve VIP in diabetic rats (P < 0.05 and 0.01, respectively) but the increase was less than what occurred in crush-injured controls (both P < 0.05). The accumulation of VIP proximal to a sciatic ligature was similar in control and diabetic rats and was not altered in either group by crush injury, while retrograde transport of VIP was initiated by crush injury in both control and diabetic rats. These data show that short-term diabetes does not alter the amount and peripheral axonal transport of VIP in the sciatic nerve but impairs the ability of peripheral nerve to respond to injury.

Diabetes mellitus prevents capsaicin from inducing hyperaemia in the rat sciatic nerve

Loss of neurogenic inflammation in response to tissue injury may be an important complication of diabetes mellitus. We studied local neurogenic inflammation in the peripheral nerve trunk of Sprague-Dawley rats 4 months following the induction of diabetes by streptozotocin injection. To assess neurogenic inflammation, the epineurial plexus of the sciatic nerve was exposed to topical capsaicin, an agent that releases vasoactive neuropeptides from perivascular afferent terminals. Under normal circumstances, local vasodilation results in endoneurial hyperaemia or a 'flare'. We evaluated the influence of capsaicin in diabetic sciatic nerve by making serial measurements of endoneurial blood flow using microelectrodes sensitive to hydrogen clearance. After 4 months of hyperglycaemia (glucose > 16.0 mmol/l), diabetic animals had slowing of unmyelinated and myelinated sural sensory conduction velocity compared to citrate buffer injected controls. Baseline sciatic endoneurial blood flow was unaltered by diabetes, and was comparable to controls. There was an expected hyperaemic response of endoneurial blood flow to capsaicin in control rat sciatic endoneurium but no consistent 'flare' response in diabetic rats. Our findings indicate that there is loss of capsaicin-related neurogenic inflammation in the vasa nervorum of experimental diabetes. It is possible that a similar deficit following nerve injury could impair the milieu for axonal regeneration in diabetes.