Cannabinoid-receptor 1 null mice are susceptible to neurofilament damage and caspase 3 activation

Administered cannabinoids have been shown to ameliorate signs of CNS inflammatory disease in a number of animal models, including allergic encephalomyelitis. More recently, neuroprotective actions have been attributed to activation of the cannabinoid 1 receptor in a number of in vitro and in vivo models. One of these, chronic relapsing experimental allergic encephalomyelitis, is considered a robust analog of multiple sclerosis. In this study, spinal cord tissue from cannabinoid receptor 1 knockout mice was analyzed for neurofilament H and myelin basic protein content, as markers of neurons/axons and myelin respectively, during the course of chronic relapsing experimental allergic encephalomyelitis. Dephosphorylation of a neurofilament H epitope, immunoreactive to the SMI32 antibody, was assessed as a marker of axonal damage and levels of the endpoint cell death mediator caspase 3 were evaluated. It was found that both neurofilament and myelin basic protein levels decrease over the course of disease, indicating concomitant neuronal/axonal loss and demyelination. Loss of each marker was more severe in cannabinoid receptor 1 knockout animals. Increased SMI32 reactivity was observed as disease progressed. SMI32 reactivity was significantly increased in knockout animals over wildtype counterparts, an indication of greater axonal dephosphorylation and injury. Active caspase 3 levels were increased in all animals during disease, with knockout animals displaying highest levels, even in knockout animals prior to disease induction. These results indicate that lack of the cannabinoid receptor 1 is associated with increased caspase activation and greater loss and/or compromise of myelin and axonal/neuronal proteins. The increase of caspase 3 in knockout mice prior to disease induction indicates a latent physiological effect of the missing receptor. The data presented further strengthen the hypothesis of neuroprotection elicited via cannabinoid receptor 1 signaling.

Activation of group II metabotropic glutamate receptors underlies microglial reactivity and neurotoxicity following stimulation with chromogranin A, a peptide up-regulated in Alzheimer's disease

Regulation of microglial reactivity and neurotoxicity is critical for neuroprotection in neurodegenerative diseases. Here we report that microglia possess functional group II metabotropic glutamate receptors, expressing mRNA and receptor protein for mGlu2 and mGlu3, negatively coupled to adenylate cyclase. Two different agonists of these receptors were able to induce a neurotoxic microglial phenotype which was attenuated by a specific antagonist. Chromogranin A, a secretory peptide expressed in amyloid plaques in Alzheimer's disease, activates microglia to a reactive neurotoxic phenotype. Chromogranin A-induced microglial activation and subsequent neurotoxicity may also involve an underlying stimulation of group II metabotropic glutamate receptors since their inhibition reduced chromogranin A-induced microglial reactivity and neurotoxicity. These results show that selective inhibition of microglial group II metabotropic glutamate receptors has a positive impact on neuronal survival, and may prove a therapeutic target in Alzheimer's disease.

Myelin phagocytosis and remyelination of macrophage-enriched central nervous system aggregate cultures

An increased level of myelin basic protein (MBP) degradation peptide 80-89, representative of myelin breakdown, is detected in myelinating foetal rat brain aggregate cultures supplemented with peritoneal macrophages at a time coinciding with the onset of myelination. During the period of myelination, the proportion of activated macrophages/microglia in the aggregates decreases, accompanied by a reduction in the content of MBP degradation products. During the recovery period following a demyelinating episode, the rate of MBP synthesis in antibody-treated standard aggregates was greater than in their medium controls. However, the rate of MBP accumulation was not as efficient in macrophage-enriched aggregates and was associated with persistently raised MBP peptide levels. Thus, as occurs in multiple sclerosis lesions, attempts at remyelination appear to be counterbalanced by macrophage-mediated demyelination, with the continued presence of degraded myelin rendering a local environment that is not fully conducive to remyelination.

Myelin/axonal pathology in interleukin-12 induced serial relapses of experimental allergic encephalomyelitis in the Lewis rat

Lewis rats, on recovery from monophasic clinical experimental allergic encephalomyelitis (EAE), can be induced to develop repeated paralytic relapses with a graded reduction in clinical severity following intraperitoneal administration of IL-12. By the time of the third relapse, the number and size of inflammatory cuffs in the spinal cord were reduced with the makeup of the cellular infiltrate shifting to a significantly increased number of B cells. Serum levels of myelin basic protein (MBP)-specific IgG1 and IgG2b were found to rise over time while MBP and MBP peptide-positive macrophages and microglia became evident in perivascular cuffs and in spinal cord parenchyma, indicative of myelin phagocytosis. Axonal death was observed in semithin and EM sections of spinal cord in third relapse animals in association with iNOS and tPA immunostaining throughout gray and white matter. These neurotoxic or excitotoxic agents may contribute to axonal damage directly or indirectly by activated microglia and macrophages, leading to limited damage of the axonal-myelin unit.

Temporal analysis of growth factor mRNA expression in myelinating rat brain aggregate cultures: increments in CNTF, FGF-2, IGF-I, and PDGF-AA mRNA are induced by antibody-mediated demyelination

Myelinogenesis in rat brain aggregate cultures is associated with a pattern of growth factor mRNA expression comparable to that of the developing brain. The rate of increase in platelet-derived growth factor-AA (PDGF-AA) expression was greatest just before the detection of myelin basic protein (MBP) mRNA in the cultures and remained high thereafter, consistent with in vivo observations. Levels of fibroblast growth factor-2 (FGF-2) and of ciliary neurotrophic factor (CNTF) mRNA increased continuously over the period of MBP accumulation. High rates of transforming growth factor beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and neurotrophin-3 (NT-3) expression at early time points during the culture gradually decreased over time, indicative of a key regulatory role during oligodendrocyte development. The addition of demyelinative anti-myelin oligodendrocyte glycoprotein (anti-MOG) antibody resulted in a significant increase in MBP peptide fragments with a C-terminus at phenylalanine 89 indicating proteolytic breakdown of MBP after myelin phagocytosis. Immediately after antibody treatment the expression of CNTF mRNA was significantly increased, compared with controls, while that of FGF-2 and IGF-I, and of PDGF-AA peaked during the early and later stages of recovery respectively. Thus, specific growth factors combine to regulate myelination and remyelination in the aggregates; these data have implications for demyelinating disease in which protective growth factor secretion may be central to regeneration.

Myelin basic protein isoforms in myelinating and remyelinating rat brain aggregate cultures

Recent evidence suggests that myelin basic protein (MBP) exon-2-containing isoforms play a significant role in the onset of myelination because they are more abundant during early development. The pattern of expression of MBP exon-2-containing isoforms was studied in rat brain aggregate cultures during myelination to draw comparisons with the developing brain and at remyelination after demyelinative treatment. The pattern of MBP isoform expression in the aggregate cultures was found to be similar to that of the brain and was recapitulated after demyelination with antimyelin antibodies. Macrophage enrichment, resulting in increased accumulation of total MBP in the cultures, did not alter the isoform distribution. Both control and enriched cultures expressed a 16-kDa protein (26+/-9.8% of total MBP for control samples) that reacted with MBP antisera at the onset of myelination (day in vitro 14) but was barely detectable by day in vitro 21. The expression of this protein, also present in postnatal day 6 rat brain but no longer by day 11, has been predicted by reverse transcription polymerase chain reaction in embryonic mouse brain. The results of the present study reinforce the value of the aggregate culture system as a versatile yet accurate model of myelination and remyelination.

The structural effect of systemic NGF treatment on permanently axotomised dorsal root ganglion cells in adult rats

The effect of systemic NGF treatment on loss and shrinkage of dorsal root ganglion cells was studied in adult male rats after permanent axotomy. Nineteen 16 to 18-wk-old rats had their right 5th lumbar spinal nerve ligated and cut approximately 7 mm peripheral to the ganglion. Two days before the operation, treatment with subcutaneous injections of human recombinant NGF (1.0-0.5 mg/kg/day) was started in 9 test rats; 10 controls were given saline injections. After 1 mo the levels of substance P (SP) and calcitonin gene related peptide (CGRP) were significantly increased in intact sciatic nerve. The number and mean volume of perikarya were estimated using assumption-free stereological techniques including vertical sections, the Cavalieri principle, optical disectors, the planar rotator and systematic sampling techniques. Systemic NGF administration had no influence on survival of primary sensory neurons after axotomy. The number of perikarya was 14300 (S.D. = 1800) in axotomised ganglia in control rats versus 14700 (S.D. = 2100) in axotomised ganglia of NGF treated rats. The reduction of perikarya volume after axotomy was significantly less after NGF treatment (11600 microm3 in the control group versus 8000 microm3 in the NGF treated group). However, the apparent protection of NGF-treatment on perikaryal volume is explained by a hitherto unrecognised size effect on nonaxotomised dorsal root ganglion cells. The untreated rats had a mean volume of 24700 microm3 (S.D. = 2700 microm3) whereas rats treated with NGF had a volume of 20400 microm3 (S.D. = 1700 microm3) on the nonaxotomised side. In conclusion, systemic NGF treatment in adult rats has no effect on dorsal root ganglion cell loss in permanent axotomy whereas perikaryal size of intact nonaxotomised cells is reduced.

Increased nerve growth factor mRNA in lateral calf skin biopsies from diabetic patients

AIMS

This study set out to establish a novel procedure for the measurement of human nerve growth factor (NGF) messenger ribonucleic acid (mRNA) and to use this method to measure NGF expression in skin biopsies from control subjects and from patients with early neuropathies. NGF mRNA levels were related to functional measures of the competence of NGF-responsive nerves.

METHODS

mRNA levels were measured by competitive reverse transcription with polymerase chain reaction amplification (cRT-PCR). Functional correlates of this observation were assessed by indices of thermal sensitivity--mediated by C-fibres, whose phenotype is regulated by NGF.

RESULTS

NGF mRNA was increased in skin biopsies from 19 diabetic patients (5.12+/-3.88 (SD)) compared with samples from eight controls (1.57+/-0.95; P=0.001). Diabetic patients showed significantly (P < 0.001) diminished detection of cool and warm stimuli compared to age matched control group (n=24), but there were no differences in detection of heat as pain, or correlation with NGF mRNA levels.

CONCLUSIONS

These findings suggest abnormally increased expression of NGF in diabetic neuropathy, which may represent a compensatory mechanism for impaired phenotype in NGF-responsive neurones.

Stimulation of nerve growth-factor and substance P expression in the iris-trigeminal axis of diabetic rats--involvement of oxidative stress and effects of aldose reductase inhibition

In rats with streptozotocin-induced diabetes, we measured increased (by 61%; P < 0.05) mRNA for nerve growth factor (NGF) in the iris together with increased (by 82%; P < 0.05) mRNA for preprotachykinin (the substance P precursor) in the trigeminal ganglion, suggesting that increased NGF was driving increased substance P gene expression. In other diabetic rats, these changes were prevented by treatment with either an antioxidant (butylated hydroxytoluene; 1% by diet) or an aldose reductase inhibitor (ARI) (sorbinil; 25 mg/kg/day p.o.) and the sorbinil treatment was associated with significant inhibition of polyol pathway intermediates in both lens and sciatic nerve. This suggests that polyol pathway activity in the lens may translate to oxidative stress-driving stimulation of NGF gene expression in the iris. The change is selective for NFG, because expression of the analogous neurotrophin, neurotrophin-3 (NT-3), was unaltered in the same irises. These changes suggest that oxidative stress and/or inflammation can drive up NGF expression in diabetes--a mechanism that might participate in iritis.

Neurogenic cutaneous vasodilatation and plasma extravasation in diabetic rats: effect of insulin and nerve growth factor

1. Neurogenic vasoactive responses in rat skin were investigated following 8 weeks of streptozotocin-induced diabetes to determine the effect of diabetes and of treatment with insulin and nerve growth factor (NGF) treatment. 2. Diabetic rats were divided into three groups: untreated; insulin (4 IU day(-1) by s.c. implant weeks 4-8) treated; Nerve Growth Factor, NGF, (0.2 mg kg(-1) three times weekly, weeks 4-8) treated. A fourth group served as a non-diabetic control. 3. Electrical stimulation of the saphenous nerve (10 V, 2 Hz, 1 ms for 30 s) increased blood flow in the ipsilateral paw skin, as measured by laser Doppler flowmetry. The peak increase was similar between groups, but the time taken for flow to return to a steady baseline was significantly (P < 0.01) reduced in untreated diabetic rats, when compared with non-diabetic controls, but not significantly reduced in the insulin- or NGF-treated diabetic groups. 4. A second stimulation of the saphenous nerve (10 V, 2 Hz, 1 ms for 5 min) produced plasma extravasation, measured by the extravascular accumulation of 125I-albumin, in the skin. Plasma extravasation was significantly attenuated (P < 0.001) in the untreated diabetic group, but not the insulin-treated group, compared to non-diabetic controls. Plasma extravasation was present, though reduced, in the NGF-treated group. 5. Plasma extravasation induced by intradermal injections of substance P with and without CGRP was similar in all groups indicating no decrease in vascular responsiveness to exogenously applied neuropeptides. The results suggest that release of neuropeptides is diminished in diabetes and that treatment with either insulin or NGF can restore neurogenic microvascular vasoactive responses towards normal.

Target tissue production and axonal transport of neurotrophin-3 are reduced in streptozotocin-diabetic rats

Neurotrophin-3 (NT-3) acts as a target-derived neurotrophic factor for large calibre sensory neurones and plays a role in the maintenance of the adult phenotype of proprioceptive and mechanoreceptive fibres. Large fibre sensory neuropathy is common in diabetes mellitus and the aim of this study was to determine whether endogenous NT-3-dependent neurotrophic support was sub-optimal in the streptozotocin-diabetic rat. NT-3 gene expression was analysed by Northern blotting and ELISA in hindlimb skeletal muscle and found to be decreased by up to 70% (p < 0.05) in rats with 4-6 weeks of diabetes compared to aged-matched controls. Treatment of other diabetic rats with insulin prevented development of deficits of both NT-3 protein and of its mRNA. The deficits in target tissue production of NT-3 were coincident with significant decreases in its anterograde and retrograde axonal transport in sciatic nerve at 6 weeks of diabetes. The mRNA expression in lumbar dorsal root ganglia of the specific receptor for NT-3, trkC, was also down-regulated at 12 weeks of diabetes by 50% (p < 0.05). The observed decreases in NT-3 target tissue production and related axonal transport suggest that large calibre sensory neurones expressing trkC may be receiving sub-optimal neurotrophic support in experimental diabetes.

Macrophages in CNS remyelination: friend or foe?

Hematogenous macrophages and resident brain microglia are agents of demyelination in multiple sclerosis (MS) and paradoxically may also participate in remyelination. In vitro studies have shown that macrophage enrichment of aggregate brain cultures promotes myelination per se and enhances the capacity to remyelinate following a demyelinating episode. It has been hypothesized that remyelination in MS is implemented by surviving dedifferentiated oligodendrocytes or by newly recruited progenitors that migrate, proliferate and synthesize myelin in response to signalling molecules in the local environment. We postulate that macrophage-derived cytokines or growth factors may directly or indirectly promote oligodendroglial proliferation and differentiation, contributing to myelin repair in inflammatory demyelinating disease.

Role of neurotrophins in diabetic neuropathy and treatment with nerve growth factors

In rodent models of diabetes, there are expression deficits in nerve growth factor (NGF) and in mRNA for its high-affinity receptor, trkA, leading to decreased retrograde axonal transport of NGF and decreased support of NGF-dependent sensory neurons, with reduced expression of their neuropeptides, substance P and calcitonin gene-related peptide (CGRP). Treatment of diabetic rats with intensive insulin normalized these deficits, and treatment with exogenous NGF caused dose-related increases, giving levels of NGF and neuropeptides that were greater than those of controls. Neurotrophin-3 (NT-3) mRNA was also deficient in leg muscle from diabetic rats, and administration of recombinant NT-3 to diabetic rats increased the conduction velocity of sensory nerves without affecting motor conduction velocity. In regenerating nerves after experimental crush injury, expression of NGF in the nerve trunk is increased in diabetes to a greater extent than in controls, but this is offset by a greater reduction in the neuronal expression of trkA in dorsal root ganglia of diabetic rats. Nonetheless, targeted administration of exogenous NGF via impregnated conduits stimulated regeneration in both control and diabetic rats. These findings implicate deficient neurotrophic support in diabetic neuropathy and suggest that its correction should be a paramount therapeutic target.

Enteric neuropeptides in streptozotocin-diabetic rats; effects of insulin and aldose reductase inhibition

The aim of the present study was to determine whether diabetes-induced changes in the distribution of enteric neuropeptides, could be prevented in 12-week streptozotocin-diabetic rats, by rigorous control of glycaemia, using daily adminstration of insulin, or an aldose reductase inhibitor (ponalrestat). The pattern of distribution of nerve fibres and cell bodies, containing immunoreactive vasoactive intestinal polypeptide (VIP), galanin (GAL), calcitonin gene-related peptide (CGRP) and substance P was examined in the myenteric plexus of ileum from control, untreated diabetic, insulin-treated diabetic and aldose reductase inhibitor-treated diabetic rats. The increase in VIP- and GAL-like immunoreactivity, seen in the myenteric plexus of untreated diabetic rat ileum, was not present in the myenteric plexus of ileum from insulin- and aldose reductase inhibitor-treated diabetic rats. With CGRP-like immunoreactive fibres, there was a clear decrease in the ileum of untreated diabetic rats. This was prevented by insulin treatment, but aldose reductase inhibitor treatment had no effect. No alterations in substance P-like immunoreactivity were seen in the myenteric plexus of ileum from any of the groups investigated. Generally, the similarity of effect of ponalrestat and insulin on VIP and galanin expression in this study supports a primary effect of insulin via glycaemic control. The dissimilarity of the effect of the two treatments on CGRP expression may imply a neurotrophic effect of insulin, although there are certainly consequences of hyperglycaemia other than exaggerated flux through the polyol pathway.

Neurotrophins and peripheral neuropathy

The most common form of peripheral neuropathy is that associated with diabetes mellitus. In rodent models of diabetes there are expression deficits in nerve growth factor (NGF) and in its high-affinity receptor, trkA, leading to decreased retrograde axonal transport of NGF and decreased support of NGF-dependent sensory neurons, with reduced expression of their neuropeptides, substance P and calcitonin gene-related peptide (CGRP). Treatment of diabetic rats with intensive insulin normalized these deficits and treatment with exogenous NGF caused dose-related increases, giving levels of NGF and neuropeptides which were greater than those of controls. Neurotrophin-3 (NT-3) mRNA was also deficient in leg muscle from diabetic rats and administration of recombinant NT-3 to diabetic rats increased the conduction velocity of sensory nerves without affecting motor conduction velocity. These findings implicate deficient neurotrophic support in diabetic neuropathy and suggest that its correction should be a paramount therapeutic target.

Nerve growth factor treatment of adult rats selectively enhances innervation of urinogenital tract rather than vascular smooth muscle

Following treatment of adult rats with nerve growth factor (0.5 mg/rat, three times a week for 3 weeks), the innervation of cardiovascular and urinogenital tract smooth muscle was investigated using immunoassay and immunohistochemical techniques. Substance P and calcitonin gene-related peptide levels were increased in the vas deferens, but not in the atria or femoral artery. Neuropeptide Y and vasoactive intestinal polypeptide levels were unchanged. In penile tissues, there was a marked increase in the density of substance P-, calcitonin gene-related peptide-, neuropeptide Y-, tyrosine hydroxylase- and vasoactive intestinal polypeptide-containing nerves innervating the urethra and in SP-containing nerves in the tunica with little changes in the innervation of the deep dorsal vein and artery and corpus cavernosum. In the bladder, there was increased innervation of the detrusor by neuropeptide Y- and vasoactive intestinal polypeptide-containing nerves, but a decrease in innervation by substance P-containing nerves in the trigone. There were no changes in the density of innervation of the femoral artery after nerve growth factor treatment. Thus, in the mature rat, sensory and sympathetic nerve innervating urinogenital tract smooth muscle appear to be more responsive to exogenous nerve growth factor than those innervating cardiovascular smooth muscle. This may reflect an ongoing requirement of plasticity of innervation in the urinogenital tract of the sexually mature animal.

Human recombinant nerve growth factor replaces deficient neurotrophic support in the diabetic rat

Manipulation of neurotrophic support is a developing strategy for new therapy aimed at neurodegenerative diseases. This study demonstrates reduced content and retrograde transport of endogenous nerve growth factor (NGF) in sciatic nerve of diabetic rats. There were also reductions in the diabetic rats in NGF protein and mRNA in skin and muscle of the hindlimb. These deficits correlated with reductions in substance P and calcitonin gene-related peptide--both products of NGF-influenced genes in primary afferents. These manifestations of deficient neurotrophic support were corrected by intensive insulin treatment and surmounted by administration of exogenous human recombinant NGF in a dose-related manner. Impaired neurotrophic support may, therefore, participate in the pathogenesis of diabetic and other peripheral neuropathies.

Altered neurotrophin mRNA levels in peripheral nerve and skeletal muscle of experimentally diabetic rats

The levels of neurotrophin mRNA in sensory ganglia, sciatic nerve, and skeletal muscle were measured in the streptozotocin-diabetic rat using northern blotting. Periods of diabetes of 4, 6, and 12 weeks significantly elevated brain-derived neurotrophic factor (BDNF) mRNA levels in soleus muscle compared with age-matched controls, the increase being highest at 6 weeks. At all time periods studied, the levels of nerve growth factor (NGF) mRNA in soleus muscle were decreased by 21-47%. Following 12 weeks of diabetes, BDNF mRNA levels were increased approximately two- to threefold in L4 and L5 dorsal root ganglia (DRG), and in sciatic nerve, NGF mRNA levels were raised 1.65-fold. Intensive insulin treatment of diabetic rats for the final 4 weeks of the 12-week period of diabetes reversed the up-regulation of BDNF mRNA in DRG and muscle and NGF mRNA in sciatic nerve. All diabetes-induced changes in neurotrophin mRNA were not paralleled by similar alterations in the levels of beta-actin mRNA in muscle and nerve, or of GAP-43 mRNA in DRG and nerve. It is proposed that the up-regulation of neurotrophin mRNA is an endogenous protective and/or repair mechanism induced by insult and, as such, appears as an early marker of peripheral nerve and muscle damage in experimental diabetes.

Changes in nerve growth factor and preprotachykinin messenger RNA levels in the iris and trigeminal ganglion in diabetic rats; effects of treatment with insulin or nerve growth factor

This study was designed to explore effects of experimental diabetes mellitus on expression of substance P in the trigeminal ganglion and of nerve growth factor (NGF) in the iris. Rats with streptozotocin-diabetes showed an increase in NGF mRNA in the iris (P < 0.01) which was corrected by insulin treatment. There was also an increase in the levels of mRNA for the substance P precursor, preprotachykinin (PPT), in the trigeminal ganglion of these animals, despite there being no change in GAP-43 mRNA. Since expression of both of these peptides is sensitive to NGF in vitro, we examined the effect of treatment of diabetic rats with NGF at three different doses (0.2, 0.5 and 1.0 mg/kg body weight). There was a dose-dependent increase in both gamma-PPT and GAP-43 mRNA in the trigeminal ganglia of NGF-treated diabetic rats. The findings indicate that increased NGF may be responsible for raised substance P levels in the iris.

Deficits in sciatic nerve neuropeptide content coincide with a reduction in target tissue nerve growth factor messenger RNA in streptozotocin-diabetic rats: effects of insulin treatment

The role of sub-optimal neurotrophic support in the aetiology of the sensory neuron dysfunction associated with diabetic neuropathy was investigated. The status of sciatic nerve neuropeptide content was related to target tissue nerve growth factor messenger RNA levels in streptozotocin-diabetic rats. The levels of substance P and calcitonin gene-related peptide in diabetic sciatic nerve were significantly lowered by approximately 50% and 28%, respectively, compared with aged matched controls and insulin-treated diabetic rats (P < 0.01) for both peptides and both comparisons). Measurements of nerve growth factor messenger RNA levels in sensory neuron target tissues, namely foot-skin and soleus muscle, revealed deficits of approximately 50% in diabetic rats, with insulin treatment reversing the decrease in foot-skin but not in soleus muscle. The results show a possible correlation between deficient neuropeptide gene expression in sensory neurons and reduced nerve growth factor messenger RNA levels in target tissue.

Diabetic neuropathy, nerve growth factor and other neurotrophic factors

Diabetic neuropathy typically presents as an insidious symmetrical distal degenerative disease of peripheral nerves. A failure of neurotrophic factors to regulate neuronal phenotype might be expected to result in such a clinical picture. Experimentally, diabetic rats show reduced expression of target-derived nerve growth factor as well as reduced expression of neuronal genes that are responsive to nerve growth factor. The latter is corrected by administration of exogenous nerve growth factor. Thus, deficient neurotrophic support might contribute to the pathogenesis of diabetic neuropathy, and any successful treatment might include exogenous neurotrophins or other strategies to correct their deficiency of action.

Aldose reductase inhibitors and their potential for the treatment of diabetic complications

Aldose reductase converts glucose to sorbitol, which is further processed to fructose. The enzyme is present in most tissues and its possible physiological role is to produce an electrically neutral, non-diffusible osmolyte in cells exposed to hypertonicity, as typified by the renal medullary cells of the loop of Henlé. The enzyme has a low affinity for glucose, and under normal conditions it processes little substrate. However, in diabetes mellitus, the marked rise in intracellular glucose that occurs in some cells causes marked production of sorbitol. The increased flux and accumulation of sorbitol is damaging to cells and may result in some of the long-term complications of diabetes. In this review, David Tomlinson, Elizabeth Stevens and Lara Diemel discuss the role of aldose reductase and the potential of its inhibitors as therapeutic agents targeted at chronic diabetic complications.

Reduced sciatic nerve substance P and calcitonin gene-related peptide in rats with short-term diabetes or central hypoxaemia co-exist with normal messenger RNA levels in the lumbar dorsal root ganglia

Endoneurial hypoxia of ischaemic origin is believed to cause the reduction in sciatic nerve substance P levels in experimentally diabetic rats. The first part of this study was designed to determine whether the changes seen extended to another neuropeptide, calcitonin gene-related peptide, and to reveal any correlation between substance P and calcitonin gene-related peptide levels in the sciatic nerve of both diabetic and centrally hypoxaemic rats. Comparison of streptozotocin diabetic rats (four-week duration) with their control group showed clear reductions in both substance P-like immunoreactivity (control = 225 +/- 20 pg/mg protein, diabetic = 139 +/- 19; P < 0.01) and calcitonin gene-related peptide-like immunoreactivity (control = 9.08 +/- 0.65 ng/mg protein, diabetic = 4.43 +/- 0.44; P < 0.001). A similar pattern was seen with the comparison of five-week centrally hypoxaemic rats (housed in 10% O2) with their controls for both substance P-like immunoreactivity (control = 222 +/- 10 pg/mg protein, hypoxic = 148 +/- 13; P < 0.001) and calcitonin gene-related peptide-like immunoreactivity (control = 6.58 +/- 0.42 ng/mg protein, hypoxic = 3.01 +/- 0.45; P < 0.001). Calcitonin gene-related peptide levels correlated closely with substance P levels in both the diabetes and central hypoxaemia studies (r2 = 0.69 and 0.62, respectively). The second part of this study measured the messenger RNA levels of the substance P precursor, preprotachykinin-A, and calcitonin gene-related peptide messenger RNA in the L4 and L5 dorsal root ganglia of control, diabetic and centrally hypoxaemic rats. There was no change in preprotachykinin-A or calcitonin gene-related peptide messenger RNA levels between any of the groups, suggesting that the sciatic nerve decreases in substance P and calcitonin gene-related peptide described above are post-transcriptional in origin.

Expression of neuropeptides in experimental diabetes; effects of treatment with nerve growth factor or brain-derived neurotrophic factor

Rats with streptozotocin-induced diabetes of 4 to 6 weeks duration showed a depletion of both substance P (P < 0.01) and calcitonin gene-related peptide (P < 0.01) in the sciatic nerve. Since expression of both peptides is sensitive to nerve growth factor (NGF) in vitro we examined the effect of treatment of diabetic rats with NGF, which significantly increased the levels of both peptides in treated diabetic animals (P < 0.01 for both). Treatment of non-diabetic rats with a similar NGF regime raised the mean peptide levels to a value similar to that seen in treated diabetic rats but the change was not statistically significant. In vehicle-treated diabetic rats the depletions of sciatic nerve neuropeptides were accompanied by a significant (P < 0.05) reduction in the level of CGRP mRNA in the 4th and 5th lumbar dorsal root ganglia, this was accompanied by an analogous reduction in the mRNA for gamma-preprotachykinin A (gamma-PPT), which did not attain statistical significance. Treatment of diabetic rats with NGF also prevented the deficits in the levels of CGRP and gamma-PPT mRNA in the lumbar dorsal root ganglia (P < 0.05). Treatment of other diabetic rats with the related neurotrophin, brain-derived neurotrophic factor (BDNF), had no effect on the levels of substance P and calcitonin gene-related peptide in the sciatic nerve.

The extraction and assay of substance P in the rat sciatic nerve

Published data for substance P (SP) content of sciatic nerves in control rats vary greatly. This study sought possible reasons for this variability by examining the influence of homogenisation procedures, freezing and selection of left/right nerve or proximal/distal segments. Substance P-like immunoreactivity (SP-LI) content (pg/mg nerve protein +/- SD) was significantly greater in sciatic nerves which had been homogenised using motor-powered equipment (615.4 +/- 146.3) as opposed to a hand-held pestle (445.4 +/- 111.8). Our second investigation revealed that freshly homogenised nerve tissue yielded greater SP-LI (508.8 +/- 88.7) than either tissue snap frozen in liquid nitrogen (307.6 +/- 77.9), snap-frozen and stored at -70 degrees C for 7 days (331.8 +/- 53.5), or tissue allowed to remain in the cadaver for 1 h and subsequently dissected and homogenised immediately (412.6 +/- 105.8). These data also show that storage at -70 degrees C imposes no further losses on those caused by freezing and extraction of frozen tissue. Two further studies indicated no variation between left and right sciatic nerves nor any proximal-distal gradients. Hence, this study illuminated the need for samples to be homogenised using motor-powered equipment immediately upon dissection, followed by SP extraction, for complete avoidance of losses of SP-LI.

Cerebrospinal fluid levels of substance P and calcitonin-gene-related peptide: correlation with sural nerve levels and neuropathic signs in sensory diabetic polyneuropathy

1. Lumbar spinal substance P and calcitonin-gene-related peptide derive from spillage out of the dorsal horns associated with activity of small primary sensory afferents (C- and A delta-fibres). Cerebrospinal fluid and sural nerve levels of substance P and calcitonin-gene-related peptide have been measured in patients with diabetic polyneuropathy to determine whether differences in small primary sensory afferent activity are related to the presence or absence of painful symptoms. 2. Calcitonin-gene-related peptide was undetectable in the cerebrospinal fluid of the majority of diabetic patients (14 out of 22); it was lower overall in diabetic patients as compared with control subjects (P < 0.01), it was lower in those diabetic patients with painless neuropathy (100% undetectable) as compared with those with painful neuropathy (50% undetectable; P < 0.05) and it correlated conversely with warming threshold (r = 0.50; P < 0.01). 3. Substance P showed no overall numerical intergroup differences or correlation with other measured variables, but six diabetic patients as compared with one control subject had undetectable cerebrospinal fluid levels and the proportion of patients with undetectable levels was higher in the group with painless neuropathy than in the group with painful neuropathy (P < 0.05). 4. The levels of each peptide in cerebrospinal fluid correlated with its equivalent in sural nerve (P < 0.01 for calcitonin-gene-related peptide and P < 0.03 for substance P). Calcitonin-gene-related peptide correlated with substance P in the sural nerve (r = 0.84; P < 0.002) and in the cerebrospinal fluid (r = 0.30; P < 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)

Depletion of substance P and calcitonin gene-related peptide in sciatic nerve of rats with experimental diabetes; effects of insulin and aldose reductase inhibition

This study was designed to determine whether deficient substance P in the sciatic nerve of diabetic rats was associated with a similar reduction in calcitonin gene-related peptide and whether the depletion of either or both peptides could be affected by insulin treatment or by aldose reductase inhibition. Substance P and calcitonin gene-related peptide were measured as immunoreactivities in the same nerve extracts. The sciatic nerve content of substance P was significantly reduced in diabetic rats (0.063 +/- 0.011; all data are mean +/- 1 standard deviation in ng peptide/mg nerve protein; n = 9 for all groups) compared to controls (0.093 +/- 0.026). The calcitonin gene related peptide content was similarly reduced (2.14 +/- 0.49) compared to controls (3.78 +/- 1.21). Tight glycaemic control with insulin prevented completely the deficit in both peptides (substance P = 0.096 +/- 0.021, calcitonin gene-related peptide = 4.66 +/- 0.92). Treatment with the aldose reductase inhibitor, imirestat, corrected the substance P deficit (0.08 +/- 0.018) and attenuated the calcitonin gene-related peptide (3.55 +/- 1.03) depletion seen in the untreated diabetic animals. This indicates that the polyol pathway may play a role in the peptide status of the sciatic nerve. Regression analysis of all data gave r2 = 0.53, indicating a comparable effect of diabetes and the treatments on both peptides.

Central hypoxaemia in rats provokes neurological defects similar to those seen in experimental diabetes mellitus: evidence for a partial role of endoneurial hypoxia in diabetic neuropathy

Endoneurial hypoxia has been put forward as a factor contributing to diabetic neuropathy. The aim of this study was to determine whether alterations in motor nerve conduction velocity, Na+/K(+)-ATPase activity and substance P content of nerve and skin tissue, characteristic of the diabetic rat, could develop in non-diabetic animals subjected to a central hypoxaemia for five weeks. Compared to normoxic controls, five weeks of central hypoxaemia caused a fall in motor nerve conduction velocity of 30% (P less than 0.01), a decrease in sciatic nerve substance P content (68%; P less than 0.001) combined with elevated substance P content per unit area foot skin (44%; P less than 0.01). This pattern of change is qualitatively similar to that seen in diabetic rats. The Na+/K(+)-ATPase activity, however, was unaltered by the hypoxic environment. These findings support strongly a partial role for hypoxia in the pathogenesis of diabetic neuropathy.