Hyperalgesia In Diabetic Rats Research Articles

Antinociceptive effect of intrathecal administration of taurine in rat models of neuropathic pain

Taurine is the most abundant amino acid in many tissues. Although taurine has been shown to be antinociceptive, in this report, our focus is to elucidate the mechanism and action site on neuropathic pain. This study used behavioural assessments to determine whether taurine attenuates neuropathic pain in the spinal cord. Chronic constriction injury (CCI) to the sciatic nerve and streptozotocin-induced diabetic neuropathy were introduced to male Sprague-Dawley rats. We then assessed the antinociceptive effect of spinal injections of taurine (100, 200, 400, or 800μg) using electronic von Frey, paw pressure, and plantar tests. To explore the effect of taurine on motor function, a rotarod test was performed, and in order to determine which neurotransmitter pathway is involved in taurine's action, we examined how several antagonists of spinal pain processing receptors altered the effect of taurine 400μg in a paw pressure test. Taurine alleviated mechanical allodynia, mechanical hyperalgesia, and thermal hyperalgesia in CCI rats and suppressed mechanical allodynia and hyperalgesia in diabetic rats. Significant effects were observed at 200μg in both models. On the other hand, taurine dose-dependently affected motor performance, and a significant effect was seen at 400μg. The antinociceptive effects were reversed completely by pretreatment with an intrathecal injection of strychnine, a glycine receptor antagonist. The present study demonstrated that intrathecal administration of taurine attenuates different models of neuropathic pain, and these effects seem to be mediated by the activation of glycinergic neurotransmission. These findings suggest that taurine may be a candidate remedy for neuropathic pain.

Effects of histamine H3 receptors on chemical hyperalgesia in diabetic rats

There is considerable interest in histamine H3 receptors as emerging pharmaceutical targets recently. Diabetic rats display increased pain responses following the injection of formalin into the paw suggesting the presence of hyperalgesia. In this study, the efficacy of systemic administration of selective H3 receptor agonist, immepip (1, 5 and 30 mg/kg), and antagonist, thioperamide (5, 15 and 30 mg/kg), was investigated on hyperalgesia during the formalin test in streptozocin (STZ)-induced diabetic rats. Nociceptive testing was performed in male adult Wistar rats 4 weeks after the onset of hyperglycemia. At the end of the experiment, all rats were weighed and then underwent plasma glucose measurement. Diabetes caused significant hyperalgesia during both phases of the formalin test. 5 and 30 mg/kg doses of immepip reversed chemical hyperalgesia in diabetic rats. The 1 mg dose of immepip did not alter pain behaviors in control and diabetic groups compared to the respective control ones. Immepip at any doses used in this study did not affect the body weight and plasma glucose levels of treated animals. Thioperamide alone at any doses used had no effect on formalin-induced nociceptive behaviors in the control and diabetic rats. The results indicated the efficacy of immepip systemic administration in an experimental model of diabetic hyperalgesia. It may also suggest it as a promising tool for treatment of painful diabetic neuropathy.

Anti-allodynic and anti-hyperalgesic effects of ceftriaxone in streptozocin-induced diabetic rats

Glutamate is the principal excitatory neurotransmitter in the central nervous system. Recent evidence suggests that beta lactam antibiotics offer neuroprotection by increasing glutamate transporter expression. Moreover, these antibiotics have been shown to prevent the development of tolerance and dependence to opioids, and reduce visceral and nerve injury-induced neuropathic nociceptive responses. The aim of this study is to observe the effect of a beta lactam antibiotic, ceftriaxone, on mechanical allodynia and mechanical hyperalgesia in diabetic rats. Diabetes was produced with the injection of a single dose of streptozocin (50 mg/kg, i.p.) and this procedure resulted in neuropathic pain behaviors in the hindpaws. Mechanical allodynia was detected with an electronic aesthesiometer, and mechanical hyperalgesia was studied using the method of Randall-Selitto. With its higher doses, ceftriaxone (100, 200 mg/kg, i.p.) reduced both mechanical allodynia and hyperalgesia. Dihydrokainic acid (10 mg/kg, i.p.), a selective GLT-1 transporter inhibitor, reversed the anti-allodynic and anti-hyperalgesic effects of ceftriaxone, at doses that produced no effect on its own. Our results indicate that ceftriaxone exerts an antinociceptive effect in streptozocin-induced diabetic rats and GLT-1 activation by beta lactam antibiotics may be a promising option in the treatment of diabetic neuropathy.

Minocycline completely reverses mechanical hyperalgesia in diabetic rats through microglia-induced changes in the expression of the potassium chloride co-transporter 2 (KCC2) at the spinal cord

neuronal hyperactivity at the spinal cord during mechanical hyperalgesia induced by diabetes may result from a decrease in the local expression of the potassium chloride co-transporter 2 (KCC2), which shifts the action of the neurotransmitter γ-amminobutiric acid (GABA) from inhibitory to excitatory. In this study, we evaluated the effects of spinal microglia inhibition or brain-derived neurotrophic factor (BDNF) blockade on KCC2 expression, spinal neuronal activity and mechanically induced pain responses of streptozotocin (STZ)-diabetic rats. four weeks after induction of diabetes, the STZ-diabetic rats received daily intrathecal injections, for 3 days, of minocycline (microglia inhibitor), TrkB/Fc (BDNF sequester) or saline. Behavioural responses to mechanical nociceptive stimulation of STZ-diabetic rats were evaluated by the Randall-Selitto test. The lumbar spinal cord was immunoreacted against the Fos protein (marker of neuronal activation) or KCC2, which was also quantified by western blotting. BDNF levels at the spinal cord were quantified by an enzyme-linked immunosorbent assay (ELISA). minocycline treatment reversed the mechanical hyperalgesia, increased Fos expression and decreased the KCC2 expression detected in STZ-diabetic rats to control levels. Treatment with TrkB/Fc was less effective, inducing moderate effects in mechanical hyperalgesia and Fos expression and only a partial correction of KCC2 expression. BDNF levels were not increased in STZ-diabetic rats. this study demonstrates that the microglial activation at the spinal cord contributes to mechanical hyperalgesia and spinal neuronal hyperactivity induced by diabetes, apparently by regulating the KCC2 expression. These effects do not seem to be mediated by BDNF, which is an important difference from other chronic pain conditions. New targets directed to prevent spinal microglia activation should be considered for the treatment of mechanical hyperalgesia induced by diabetes.

Effects of the endocannabinoid transport inhibitors AM404 and UCM707 on diabetic neuropathy in rats

1. Diabetic rats display increased pain responses following injection of formalin into the paw, suggesting the presence of hyperalgesia. In the present study, we investigated the efficacy of the systemic administration of the endocannabinoid transport inhibitors UCM707 and AM404 (1, 10 and 50 mg/kg, i.p.) on hyperalgesia during the formalin test in streptozocin (STZ)-induced diabetic rats. 2. Nociceptive testing was performed in male adult Wistar rats 4 weeks after the onset of hyperglycaemia. At the end of the experiment, all rats were weighed and then underwent plasma glucose measurements. 3. Diabetes caused significant hyperalgesia during both phases of the formalin test. At 10 and 50 mg/kg, both UCM707 and AM404 reversed chemical hyperalgesia in diabetic rats. UCM707 (10 and 50 mg/kg) caused less intensive nociceptive behaviour during both phases of the test, whereas AM404 (10 and 50 mg/kg) only affected pain scores during Phase 1 of the formalin test. At 1 mg, neither drug had any effect on pain behaviour in control and diabetic groups compared with their respective controls. Neither UCM707 nor AM404 had any effect on bodyweight or plasma glucose levels of treated compared with non-treated rats at any of the doses tested. 4. The results of the present study indicate that systemic administration of UCM707 and AM404 is effective in ameliorating chemical hyperalgesia in STZ-diabetic rats. Thus, endocannabinoid transport inhibitors may have potential in the treatment of painful diabetic neuropathy.

URB597, an inhibitor of fatty acid amide hydrolase, reduces hyperalgesia in diabetic rats

Diabetic rats display increased pain responses after injection of formalin into the paw or thermal stimulation of the tail, suggesting the presence of hyperalgesia. In this study, we investigated the efficacy of URB597 (0.1, 0.3, and 0.5 mg/kg, i.p.), an inhibitor of endocannabinoids metabolism, on 2 models of experimental hyperalgesia in streptozotocin (STZ)-induced diabetic rats. Animals were divided into control, URB597-treated control (0.1, 0.3, and 0.5 mg/kg), diabetic, and URB597-treated diabetic (0.1, 0.3, and 0.5 mg/kg) groups. Formalin and tail-flick tests were performed 4 and 8 weeks after the onset of hyperglycemia, respectively. Diabetes caused significant hyperalgesia during these tests. URB597 (0.3 and 0.5 mg/kg) reversed chemical and thermal hyperalgesia in diabetic rats. Administration of URB597 at a dose of 0.1 mg/kg did not alter pain-related behaviors in control and diabetic groups compared with those of the respective control groups. URB597 treatment did not affect body weight or plasma glucose level of treated animals compared with nontreated animals. This study shows that increasing endocannabinoid neurotransmission with URB597 displays efficacy in chemical and thermal models of diabetic hyperalgesia. It also suggests that URB597 is a promising tool for treatment of painful diabetic neuropathy.

B vitamins alleviate indices of neuropathic pain in diabetic rats

There are sporadic reports that assorted combinations of B vitamins can alleviate pain in diabetic patients, but there is neither agreement on the relative efficacy of individual B vitamins nor understanding of the mechanisms involved. We therefore investigated the efficacy of a cocktail of the vitamins B1, B6 and B12 in alleviating behavioral indices of sensory dysfunction such as allodynia and hyperalgesia in diabetic rats and also the relative contribution of individual components of the cocktail. Repeated daily treatment with the cocktail of B vitamins for 7–9 days ameliorated tactile allodynia and formalin-evoked hyperalgesia in a dose-dependent manner and also improved sensory nerve conduction velocity in diabetic rats. Investigation of the contribution of individual B vitamins suggested that all three participated with variable efficacy in the alleviation of allodynia after protracted, but not single dose treatment. Only vitamin B6 improved sensory nerve conduction velocity slowing in diabetic rats when given alone. To address potential mechanisms of action, we measured markers of oxidative stress (lipid and protein oxidation) and inflammation (cyclooxygenase-2 (COX-2) and TNFα protein) in the nerve but treatment with the vitamin B cocktail did not significantly affect any of these parameters. The positive effects of B vitamins on functional and behavioral disorders of diabetic rats suggest a potential for use in treating painful diabetic neuropathy.

Allodynia and hyperalgesia in diabetic rats are mediated by GABA and depletion of spinal potassium-chloride co-transporters

Diabetic rats show behavioral indices of painful neuropathy that may model the human condition. Hyperalgesia during the formalin test in diabetic rats is accompanied by the apparently paradoxical decrease in spinal release of excitatory neurotransmitters and increase in the inhibitory neurotransmitter GABA. Decreased expression of the potassium-chloride co-transporter, KCC2, in the spinal cord promotes excitatory properties of GABA. We therefore measured spinal KCC2 expression and explored the role of the GABA A receptor in rats with painful diabetic neuropathy. KCC2 protein levels were significantly reduced in the spinal cord of diabetic rats, while levels of NKCC1 and the GABA A receptor were unchanged. Spinal delivery of the GABA A receptor antagonist bicuculline reduced formalin-evoked flinching in diabetic rats and also dose-dependently alleviated tactile allodynia. GABA A receptor-mediated rate-dependent depression of the spinal H reflex was absent in the spinal cord of diabetic rats. Control rats treated with the KCC2 blocker DIOA, mimicked diabetes by showing increased formalin-evoked flinching and diminished rate- dependent depression. The ability of bicuculline to alleviate allodynia and formalin-evoked hyperalgesia in diabetic rats is consistent with a reversal of the properties of GABA predicted by reduced spinal KCC2 and suggests that reduced KCC2 expression and increased GABA release contribute to spinally mediated hyperalgesia in diabetes.

Pathogenesis of Spinally Mediated Hyperalgesia in Diabetes

Hyperalgesia to noxious stimuli is accompanied by increased spinal cyclooxygenase (COX)-2 protein in diabetic rats. The present studies were initiated to establish causality between increased spinal COX-2 activity and hyperalgesia during diabetes and to assess the potential involvement of polyol pathway activity in the pathogenesis of spinally mediated hyperalgesia. Rats with 1, 2, or 4 weeks of streptozotocin-induced diabetes exhibited significantly increased levels of spinal COX-2 protein and activity, along with exaggerated paw flinching in response to 0.5% paw formalin injection. Increased flinching of diabetic rats was attenuated by intrathecal pretreatment with a selective COX-2 inhibitor immediately before formalin injection, confirming the involvement of COX-2 activity in diabetic hyperalgesia. Chronic treatment with insulin or ICI222155, an aldose reductase inhibitor (ARI) previously shown to prevent spinal polyol accumulation and formalin-evoked hyperalgesia in diabetic rats, prevented elevated spinal COX-2 protein and activity in diabetic rats. In contrast, the ARI IDD676 had no effect on spinal polyol accumulation, elevated spinal COX-2, or hyperalgesia to paw formalin injection. In the spinal cord, aldose reductase immunoreactivity was present solely in oligodendrocytes, which also contained COX-2 immunoreactivity. Polyol pathway flux in spinal oligodendrocytes provides a pathogenic mechanism linking hyperglycemia to hyperalgesia in diabetic rats.

Impaired formalin-evoked changes of spinal amino acid levels in diabetic rats

To investigate mechanisms by which diabetes alters sensory processing, we measured levels of amino acid neurotransmitters in spinal dialysates from awake, unrestrained control and diabetic rats under resting conditions and following hind paw formalin injection. Under resting conditions, glutamate concentrations in spinal dialysates were significantly ( P < 0.05) decreased in diabetic rats compared to those of control rats whereas aspartate, taurine, glycine and citrulline remained unchanged and GABA was significantly ( P < 0.05) increased. Noxious stimulation of the hind paw by subcutaneous injection of 0.5% formalin into the dorsum caused a defined flinching behavior in the afflicted paw, and the amount of flinching was significantly ( P < 0.05) greater in diabetic rats than in controls. Paw formalin injection significantly ( P < 0.05) increased dialysate levels of glutamate, aspartate, taurine, glycine and citrulline by 3- to 4-fold above basal in both control and diabetic rats. The concentration of glutamate in dialysate samples collected immediately after paw formalin injection remained significantly ( P < 0.05) lower in diabetic rats compared to those in controls. Formalin injection did not alter dialysate GABA concentrations in control rats, whereas in diabetic rats there was an increase of 151 ± 15% above basal levels. These findings indicate that the selective depression of basal and stimulus-evoked glutamate levels in the spinal cord of diabetic rats occurs in parallel with elevated spinal GABA levels. Because increased pain-associated behavior is accompanied by an attenuated spinal glutamate spike following paw formalin injection, hyperalgesia in diabetic rats does not appear to be secondary to enhanced glutamatergic input to the spinal cord.

The effect of WIN 55,212-2, a cannabinoid agonist, on tactile allodynia in diabetic rats

The antinociceptive action of cannabinoids in acute and inflammatory pain states have been well-documented. There is also accumulating evidence suggesting that cannabinoids are effective analgesics in chronic pain conditions. WIN 55,212-2, a mixed CB1 and CB2 cannabinoid receptor agonist, has been shown to be effective against hyperalgesia and allodynia in painful peripheral mononeuropathy. Recently, in addition to their spinal and supraspinal antinociceptive action, cannabinoids have also reported to exert local analgesic effects. The aim of this study is to observe the effect of a high affinity cannabinoid, WIN 55,212-2, on tactile allodynia and thermal hyperalgesia in diabetic rats. Diabetes was produced with the injection of a single dose of streptozocin (50 mg/kg, i.p.) and this procedure resulted in neuropathic pain behaviors in the hindlimbs. Mechanical allodynia was detected by application of von Frey filaments to the plantar surface of the foot, and thermal hyperalgesia was studied using the Hargreaves’ method; however, thermal hyperalgesia did not develop in diabetic rats. With its higher doses, both systemic (3 and 10 mg/kg, i.p.) and peripheral (30 μg, i.p.l.) injections of WIN 55,212-2 reduced mechanical allodynia. These results suggest that WIN 55,212-2 has an antiallodynic effect in streptozocin-induced diabetic rats and may be a promising approach in the treatment of diabetic neuropathy.

Elevated spinal cyclooxygenase and prostaglandin release during hyperalgesia in diabetic rats.

Diabetic rats display exaggerated hyperalgesic behavior in response to noxious stimuli that may model aspects of painful diabetic neuropathy. This study examined the contribution of spinal prostaglandin production to this exaggerated hyperalgesic behavior. Rats were implanted with spinal dialysis probes and received noxious stimulation to the hind paw by subcutaneous injection of 0.5% formalin solution. Prostaglandin E(2) (PGE(2)) was measured in dialysates of lumbar spinal cerebrospinal fluid concurrent with behavioral responses to formalin injection. In separate experiments, formalin-evoked behavioral responses were measured after intrathecal delivery of either a cyclooxygenase inhibitor or an EP(1) receptor antagonist, and cyclooxygenase protein was measured in spinal cord homogenates. Diabetic rats exhibited exaggerated behavioral responses to paw formalin injection and a concurrent prolongation of formalin-evoked PGE(2) release. Formalin-evoked behavioral responses were dose-dependently reduced in diabetic rats by spinal delivery of a cyclooxygenase inhibitor or an EP(1) receptor antagonist. Protein levels of cyclooxygenase-2 were elevated in the spinal cord of diabetic rats, whereas cyclooxygenase-1 protein was reduced. Hyperalgesic behavior in diabetic rats is associated with both increased cyclooxygenase-2 protein and cyclooxygenase-mediated PGE(2) release. Spinal delivery of selective inhibitors of cyclooxygenase-2 or antagonists of prostaglandin receptors may have therapeutic potential for treating painful diabetic neuropathy.

Protection of sensory function and antihyperalgesic properties of a prosaposin-derived peptide in diabetic rats.

Short-term diabetes causes sensory disorders in rats ranging from thermal hypoalgesia to exaggerated behavioral responses to other sensory stimuli. As impaired neurotrophic support may promote sensory nerve disorders during diabetes, the authors investigated whether TX14(A), a neurotrophic peptide derived from prosaposin, was able to ameliorate nerve disorders in diabetic rats. TX14(A) was delivered by intraperitoneal or intrathecal injection to control or streptozotocin-diabetic rats in either single or multiple (three times weekly) dose regimens. Efficacy was measured against diabetes-induced disorders of sensory nerve conduction velocity, paw withdrawal latency to radiant heat, tactile response thresholds to von Frey filaments, and flinching after paw formalin injection. Prolonged TX14(A) treatment of diabetic rats prevented the progressive decline in large sensory fiber conduction velocity in the sciatic nerve, development of paw thermal hypoalgesia, and increased flinching after paw formalin injection. The effect on formalin hyperalgesia persisted for 48 h but not 72 h after injection. No effects were noted in control rats. A single injection of TX14(A) 30 min before testing did not alter thermal response latencies in control or diabetic rats but prevented formalin hyperalgesia in diabetic rats. Tactile allodynia and the prolonged paw thermal hyperalgesia to radiant heat after intrathecal delivery of substance P were also dose-dependently ameliorated in diabetic rats by a single injection of TX14(A), whereas no effects were observed on the responses to these tests in control rats. TX14(A) exhibits both neuroprotective and acute antihyperalgesic properties in diabetic rats without altering normal nociceptive function.

Hyperalgesia In Diabetic Rats Is Alleviated By A Prosaposin‐Derived Peptide

The saposin C‐derived peptide TX14(A) prevents onset of functional and structural disorders in the peripheral nerve of diabetic rats. We have now investigated the ability of TX14(A) to alleviate behavioral indices of abnormal pain perception in adult female rats 4‐6 weeks after onset of STZ‐induced diabetes. Untreated diabetic rats exhibited tactile allodynia (response threshold = 3 ± 1 g) compared to age‐matched controls (10 ± 1g). A single ip injection of TX14(A) transiently alleviated tactile allodynia, with an effect that was maximal 6 hours (11 ± 1g) after injection and diminished within 48 hours. Maximal efficacy was seen with a 1 mg/kg dose while no effects were noted in control rats. Control rats exhibited a transient thermal hyperalgesia (77 ± 5% of baseline paw withdrawal latency) 15 minutes after intrathecal delivery of substance P (30 nmol) that resolved within 30 minutes. Untreated diabetic rats exhibited substance P evoked thermal hyperalgesia of similar magnitude (82 ± 5% at 15 minutes) but of greater duration (83 ± 4% at 1 hour). Intrathecal delivery of TX14(A) 30 minutes before intrathecal substance P was without effect on the transient thermal hyperalgesia in control rats (74 ± 9% at 15 minutes). In diabetic rats, the prolonged thermal hyperalgesia was abolished by prior intrathecal delivery of TX14(A), although the transient thermal hyperalgesia (72 ± 8% at 15 minutes) remained. These studies show that TX14(A) can rapidly allevate diabetes‐induced allodynia and hyperalgesia for up to 48 hours.

Nitric oxide involvement in the trigeminal hyperalgesia in diabetic rats

Trigeminal hyperalgesia frequently appears in diabetic neuralgia altering the transmission of orofacial sensory information. This study was designed to explore the effects of trigeminal hyperalgesia in streptozotocin-induced diabetes monitoring the expression of nitric oxide synthase in the trigeminal ganglion cells. The threshold to heat noxious stimuli decreased in diabetic animals. The number of NADPH-diaphorase (NADPH-d)-positive neurons significantly decreased in the diabetic rats compared with controls. Insulin treatment prevented the decreased nociceptive threshold and reduction of the number of NADPH-d-positive neurons. These findings point out that there is a relationship between the trigeminal nociceptive perception and NADPH-d neuronal expression suggesting that NO may play a role in the pathogenesis of trigeminal sensory neuropathy.

Different effects of two aldose reductase inhibitors on nociception and prostaglandin E

This study examined the effect of two structurally dissimilar aldose reductase inhibitors, N-[[5-(trifluoromethyl)-6-methoxy-1-napthalenyl]thioxomethyl]- N-methlyglycine (tolrestat) and 4-amino-2,6-dimethylphenyl-sulphonyl nitromethane (ICI 222155), on formalin-evoked behavioural responses in control and diabetic rats and on capsaicin-evoked release of prostaglandin E from spinal cord slices in vitro. Both compounds, given orally for 4 weeks, prevented hyperalgesia in diabetic rats 5–20 min after hindpaw formalin injection. ICI 222155 also prevented hyperalgesia in diabetic rats 21–60 min after formalin, whereas tolrestat suppressed activity in diabetic rats below controls and also suppressed activity in controls when given orally or intrathecally. Capsaicin-evoked release of prostaglandin E from spinal cord slices of control rats was significantly reduced by tolrestat, but not ICI 222155. These data suggest that hyperalgesia in diabetic rats is related to glucose metabolism by aldose reductase, whereas tolrestat has specific effects on formalin-evoked nociception associated with an ability to reduce spinal prostaglandin release.

RP-67580, a specific tachykinin NK1 receptor antagonist, relieves chronic hyperalgesia in diabetic rats

The effect of a non-peptide NK1 receptor antagonist, RP-67580, and of its inactive enantiomer, RP-68651, on nociceptive thresholds in normal and streptozocin-induced diabetic rats submitted to paw pressure is reported. RP-67580 (1, 3, 9 mg/kg s.c.) dose dependently reduced the diabetes-induced mechanical hyperalgesia observed 4 weeks after induction of diabetes. In normal rats, RP-67580 failed to increase nociceptive thresholds. RP-68651 was inactive in both diabetic and normal rats. These results suggest that (i) substance P is involved in the diabetes-induced chronic hyperalgesia, and (ii) NK1 receptor antagonists merit to be studied more extensively in relation with this pathological condition.

Pain threshold in diabetic rats: effects of good versus poor diabetic control

Pain threshold was assessed via tail flick latency in streptozotocin diabetic rats following a 1-month period of either good or poor diabetic control. Additionally, tail flick latencies were determined under test conditions of euglycemia (60–120 mg/dl) and hyperglycemia (>250 mg/dl) for both groups of diabetic rats. Conditions of hyperglycemia resulted in a significant decrease in tail flick latency in diabetic animals maintained in good as well as poor diabetic control. However, tail flick latencies for animals in the good but not in the poor diabetic control group increased when animals were euglycemic at the time of testing. Animals in the poor diabetic control group continued to have reduced tail flick latencies even after acute normalization of blood glucose levels. These results suggest that chronic states of hyperglycemia that attend prolonged periods of poor diabetic management may lead to persistent alterations in pain threshold. In contrast, the reduced pain threshold of rats maintained in good diabetic control was reversed by a normalization of blood glucose levels. These findings provide evidence for two patterns of hyperalgesia in diabetic rats: 1. (1) a readily reversible form in well-maintained diabetic rats that is altered by reducing circulating blood glucose levels to control values and 2. (2) a more persistent form found in poorly controlled diabetic rats that does not respond to acute normalization of blood glucose.