Green tea [Camellia sinensis (L.) O. Kuntze] extract reverses the despair behaviour in reserpinised and diabetic mice

Green tea (C. sinensis) extract (GTE) dose dependently produced reversal of despair in normal, reserpinised and diabetic mice, thereby demonstrating an antidepressant effect. Although the exact mechanism is yet to be explored, the possible inhibition of catechol-o-methyl transferase and monoamine oxidase enzymes may be responsible for antidepressant activity of GTE.

Current status of clinical and experimental researches on cognitive impairment in diabetes

This article reviews the clinical and experimental researches on cognitive impairment related to diabetes in the recent decade. Most clinical studies indicate that the cognitive impairment in patients with type 1 diabetes mellitus is related to recurrent hypoglycemia closely. There is little research about whether or not hyperglycemia is related to cognitive impairment in patients with type 1 diabetes mellitus. Most studies indicate that the cognitive impairment in type 2 diabetes involves multiple factors through multiple mechanisms, including blood glucose, blood lipid, blood pressure, level of insulin, medication, chronic complication, etc. But, there has been no large-scale, multi-center, randomized controlled clinical trial in China recently. And what is more, some problems exist in this field of research, such as the lack of golden criterion of cognitive function measurement, different population of studied objects, and incomprehensive handling of confounding factors. Experimental studies found that hippocampal long-term potentiation (LTP) was impaired, which were manifested by impairment of spatial memory and decreased expression of LTP, but it's relation to hyperglycemia, the duration of diabetes, learning and memory has always been differently reported by different researches. Thus, there are a lot of unknown things to be explored and studied in order to clarify its mechanism. TCM has abundant clinical experience in treating cerebral disease with medicine that enforces the kidney and promotes wit. However, there has been no research on treating diabetic cognitive impairment, which requires work to be done actively and TCM to be put into full play, in order to improve the treatment of diabetes and enhance living quality of patients.

Effects of insulin treatment on heart rhythm, body temperature and physical activity in streptozotocin-induced diabetic rat

1. Streptozotocin (STZ)-induced diabetic cardiomyopathy is frequently associated with depressed diastolic/systolic function and altered heart rhythm. 2. The effects of insulin treatment on heart rhythm, body temperature and physical activity in STZ-induced diabetic rats were investigated using biotelemetry techniques. 3. Transmitter devices were surgically implanted in the peritoneal cavity of young adult male Wistar rats. Electrodes from the transmitter were arranged in Einthoven bipolar - Lead II configuration. Electrocardiogram, physical activity and body temperature data were recorded with a telemetry system for 10 days before STZ treatment, for 20 days following administration of STZ (60 mg/kg) and thereafter, for 30 days while rats received daily insulin. 4. Heart rate, physical activity and body temperature declined rapidly 3-5 days after administration of STZ. Pre-STZ heart rate was 362 +/- 7 b.p.m., falling to 266 +/- 12 b.p.m. 5-15 days after STZ with significant recovery to 303 +/- 14 b.p.m. 10-20 days after commencement of insulin. Pre-STZ body temperature was 37.5 +/- 0.1C, falling to 37.2 +/- 0.2C 5-15 days after STZ with significant recovery to 37.5 +/- 0.1C 10-20 days after commencement of insulin. Physical activity and heart rate variability were also reduced after STZ but there was no significant recovery during insulin replacement. 5. Defective autonomic regulation and/or mechanisms of control that are intrinsic to the heart may underlie disturbances in heart rhythm in the STZ-induced diabetic rat.

Modulatory effect of sildenafil in diabetes and electroconvulsive shock-induced cognitive dysfunction in rats

The nitric oxide/guanylyl cyclase, cyclic guanosine monophosphate/phosphodiesterase 5 (NO/cGMP/PDE5) pathways play a key role in physiological and pathological situations, such as synaptic plasticity, learning and memory formation, diabetic gastropathy and neuropathy, long-term potentiation (LTP), epilepsy, cerebral ischemia, and neurodegenerative diseases. Several studies have demonstrated the alteration of NO-cGMP pathway in cognitive impairment. The present study was aimed to study the effect of sildenafil, a PDE5 inhibitor on diabetes and electroconvulsive shock (ECS)-induced cognitive dysfunction in rat using one-trial step-through type of passive avoidance and elevated plus-maze task. Diabetic and ECS-treated rats showed poor learning performance in step-through passive avoidance and plus-maze task. Acute administration of sildenafil significantly reversed the diabetes and ECS-induced retention deficits in both the test paradigms. Sildenafil also significantly improved the cognitive performance in young rats in both the paradigms. Furthermore, L-NAME, a non-selective NOS inhibitor and methylene blue, a guanylate cyclase inhibitor blocked the effect of sildenafil. The results thus suggest that cognitive impairment might be due to the modulatory effect of nNOS or PDE5 enzyme on cGMP levels. Moreover, sildenafil-induced reversal of cognitive impairment suggests the protective role of PDE5 inhibitors in neurodegenerative disorders.

High-cholesterol diets impair short-term retention of memory in alloxan-induced diabetic mice, but not acquisition of memory nor retention of memory in prediabetic mice

Whether high-cholesterol diets (HCD) induce a high incidence of memory deficits in diabetes requires to be established; if so, whether they induce impairments of memory acquired in the pre-diabetic stage as well as in the diabetic stage also needs to be elucidated, and part of the related mechanisms involved in this dysfunction should be determined. The mice were grouped into: normal mice fed normal diets (NN), diabetic mice fed normal diets (DN), normal mice fed HCD (NH), and diabetic mice fed HCD (DH). Animals were subjected to Morris water maze testing: 1) Learning in the pre-diabetic stage and memory retrieval in the diabetic stage; 2) Learning and memory retrieval in the diabetic stage. Following water maze testing, biochemical parameters were estimated in the animals. The results showed that significant impairments of memory retrieval, acquired in the diabetic stage, were observed only in DH group, neither in DN nor NH group in a short term compared with NN group. Biochemical parameters including fasting blood glucose, lipid peroxidation productions and acetylcholinesterase activities in frontal cortex and hippocampus increased more rapidly in DH group than those in the rest. These results indicate that HCD impair the diabetic retention of memory, but neither the diabetic acquisition of memory nor the pre-diabetic retention of memory in diabetic mice in a short term. Controlled HCD may be a strategy to prevent the loss of memory in diabetic individuals after they have acquired new information.

Antinociceptive Effect of Shakuyakukanzoto, a Kampo Medicine, in Diabetic Mice

In this study, the antinociceptive effect of shakuyakukanzoto was investigated using streptozotocin-induced diabetic mice to certify its analgesic effect on diabetic patients. Shakuyakukanzoto (0.5 and 1.0 g/kg, p.o.) significantly increased the nociceptive threshold in diabetic mice. The antinociceptive activity of shakuyakukanzoto in diabetic mice was not antagonized by beta-funaltrexamine, naltrindole, or nor-binaltorphimine. The increased antinociceptive activity of (1.0 g/kg, p.o.) in diabetic mice was abolished by yohimbine (15 microg, i.t.), but not by NAN-190 (1 microg, i.t.), methysergide (15 microg, i.t.), or MDL-72222 (15 microg, i.t.). In shakuyakukanzoto diabetic mice treated with 6-hydroxydopamine (20 microg, i.t.) chemically lesioned noradrenergic pathways, shakuyakukanzoto (1.0 g/kg, p.o.) failed to exhibit an antinociceptive effect. Furthermore, the antinociceptive activity induced by norepinephrine (0.06 - 2 microg, i.t.) was markedly more potent in diabetic mice than in non-diabetic mice at the same dose. These results suggest that the antinociceptive effect of shakuyakukanzoto in diabetic mice is not mediated by the opioid systems and that this effect appears via selective activation of the spinal descending inhibitory alpha2-adrenergic systems without activating the serotonergic systems. The spinal alpha2-adrenoceptor-mediated analgesic mechanism was enhanced in diabetic mice, suggesting that shakuyakukanzoto exhibits its effect by activating the descending noradrenergic neurons.

Biphasic effects of stress upon GLUT8 glucose transporter expression and trafficking in the diabetic rat hippocampus

Disease states such as diabetes mellitus are known to impair hippocampal glucoregulatory activities, which may contribute to cognitive deficits observed in diabetic subjects. Stress or exposure to stress levels of glucocorticoids (GCs) are also intimately involved in hippocampal glucoregulatory activities and the actions of GCs are often most evident in hyperglycemic states. Glucose transporter (GLUT) expression, activity and translocation represent components of the glucoregulatory activities of the hippocampus that may be disrupted by diabetes and stress. Accordingly, the current study examined the effects of stress, streptozotocin (STZ)-induced diabetes and the combined actions of stress and hyperglycemia upon GLUT8 mRNA expression, protein levels and intracellular trafficking in the rat hippocampus. Short-term stress in euglycemic rats had no effect upon GLUT8 mRNA, while restraint stress normalized diabetes mediated increases in GLUT8 mRNA expression in STZ treated rats. Radioimmunocytochemical analysis revealed that total GLUT8 protein levels were not altered by diabetes, short-term stress or the combined actions of hyperglycemia and stress. However, subcellular compartmentalization of GLUT8 was modulated by stress in that hippocampal GLUT8 protein levels were increased in high-density microsomal (HDM) fractions isolated from rats subjected to stress. In contrast, STZ-diabetes decreased GLUT8 protein levels in the HDM fraction, an effect that was potentiated by stress. Collectively, these results demonstrate that the actions of GCs may be dramatically different in euglycemic and hyperglycemic/insulinopenic states, suggesting that stress may increase hippocampal neuronal responsiveness under normal physiological conditions while increasing hippocampal neuronal vulnerability in pathophysiological settings such as in type 1 diabetes.

[Diabetes attenuates the antidepressant-like effect mediated by the activation of 5-HT1A receptors in the mouse tail suspension test]

Several lines of evidence have indicated that the prevalence of psychiatric disorders in diabetic subjects is higher than that in the general population, however, little information is available on the effects of antidepressants in diabetes. In the present review, we summarized the effect of diabetes on the central serotonergic systems and the efficacy of serotonergic antidepressants. Streptozotocin-induced diabetic mice showed prolonged duration of immobility compared to non-diabetic mice in the tail suspension test. This behavioral change was unrelated to the transient increases in blood glucose concentrations or decreased body weights by diabetes. Fluoxetine, a selective serotonin (5-HT) reuptake inhibitor, reduced the duration of immobility in both non-diabetic and diabetic mice. However, a selective 5-HT1A receptor antagonist WAY-100635 reversed the antidepressant-like effect of fluoxetine only in non-diabetic mice. In addition, a 5-HT1A receptor agonist 8-OH-DPAT reduced the duration of immobility in non-diabetic mice, but not in diabetic mice. These results suggest a possibility that the antidepressant-like effect mediated by the activation of 5-HT1A receptors may be attenuated by diabetes.

Effect of recurrent hypoglycemia on spatial cognition and cognitive metabolism in normal and diabetic rats

The effects of recurrent hypoglycemia (RH) on cognition in human subjects remain controversial, perhaps in part due to difficulty in completely controlling previous hypoglycemic history. We used a model of RH in nondiabetic and diabetic rats to examine the effects of short-term (3 h daily for 3 days) RH on subsequent hippocampally dependent spatial memory, tested either at euglycemia or under acute hypoglycemia. Hippocampal metabolism was simultaneously measured using microdialysis. Antecedent RH improved task performance (79 +/- 2% alternation in nondiabetic RH animals vs. 63 +/- 3% in controls; P < 0.001) at euglycemia, accompanied by reversal of the task-associated dip (20 +/- 1% below baseline) in hippocampal extracellular fluid (ECF) glucose seen in control animals. RH rats also had a larger rise in hippocampal ECF glucose, after intraperitoneal glucose injection, than did controls. However, RH animals tested at acute hypoglycemia ( approximately 2.8 mmol/l) performed significantly worse than control animals. Results were similar in diabetic and nondiabetic rats. Our data suggest that RH causes improvement in subsequent cognitive performance at euglycemia, accompanied by alterations in cognitive metabolism. When glucose availability is limited, complex cognitive functioning seems to be adversely effected in RH animals, perhaps to better maintain and preserve basic brain functions.

Effects of i.c.v. Administration of Leptin on Copulatory and Ingestive Behavior in STZ-induced Diabetic Male Rats

It is well known that circulating leptin concentrations correlate with adiposity in both humans and rodents and decrease after fasting, energy restriction, or weight loss. The goal of the present study was to confirm whether the decreases of copulatory behavior and the increases of ingestive behavior in STZ-induced diabetic male rats could be reversed by i.c.v. administration of leptin. Adult male Wistar-Imamichi rats aged 9 weeks were used for the studies. Males received a single injection of STZ (60 mg/kg, i.p.) and vehicle. During the experiment, individual body weight, and food and water intake were measured. The copulatory and ingestive behaviors in STZ-induced diabetic males were observed at 2 and 4 weeks after STZ. At 6 weeks after STZ, leptin (10 microg/10 microl) or aCSF (artificial cerebrospinal fluid) was injected through a lateral ventricle cannula and the above two behaviors were observed again. The i.c.v. leptin injection to STZ-induced diabetic males resulted in a significant increase of ejaculation frequencies (3.6 +/- 0.26 vs. 2.9 +/- 0.30 times) and a significant decrease in amount of food ingested (36.2 +/- 1.93 vs. 23.2 +/- 3.76 g), compared with the aCSF-injected control (p<0.01). These findings suggest that the copulatory and ingestive behaviors in i.c.v. leptin-injected STZ diabetic males were restored to levels equivalent to those in control males.

Altered expression of NCAM in hippocampus and cortex may underlie memory and learning deficits in rats with streptozotocin-induced diabetes mellitus

Neurological and structural changes are paralleled by cognitive deficits in diabetes mellitus. The present study was designed to evaluate the expression of neural cell adhesion molecules (NCAM) in the hippocampus, cortex and cerebellum and to examine cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a passive avoidance test and a spatial version of the Morris water maze test. NCAM expression was detected in the hippocampus, cortex and cerebellum by an immunoblotting method. The diabetic rats developed significant impairment in learning and memory behaviours as indicated by deficits in passive avoidance and water maze tests as compared to control rats. Expression of NCAM 180 and 120 kDa were found to be higher in hippocampus and cortex of diabetic rat brains compared to those of control, whereas expression of NCAM 140 kDa decreased in these brain regions. Our findings suggest that streptozotocin-induced diabetes impairs cognitive functions and causes an imbalance in expression of NCAM in those brain regions involved in learning and memory. Altered expression of NCAM in hippocampus may be an important cause of learning and memory deficits that occur in diabetes mellitus.

Effects of selective serotonin reuptake inhibitors on immobility time in the tail suspension test in streptozotocin-induced diabetic mice

We examined the effects of fluoxetine and fluvoxamine, selective serotonin reuptake inhibitors (SSRIs), and desipramine, a selective noradrenaline (NA) reuptake inhibitor, given alone or in combination with diazepam on immobility time in the tail suspension test in diabetic mice. Immobility time was significantly longer in diabetic than in nondiabetic mice. Diazepam (0.1 and 0.3 mg/kg s.c.) dose-dependently decreased immobility time in diabetic mice to the level observed in saline-treated nondiabetic mice. However, diazepam had no significant effect on immobility time in nondiabetic mice. Fluoxetine (3-56 mg/kg i.p.) and desipramine (1-30 mg/kg i.p.) produced marked, dose-dependent suppression of immobility time in both nondiabetic and diabetic mice. However, anti-immobility effects of fluoxetine and desipramine in diabetic mice were less than those in nondiabetic mice. Fluvoxamine (3-30 mg/kg i.p.) produced a dose-dependent suppression of immobility time in nondiabetic mice but not in diabetic mice. The anti-immobility effects of fluoxetine, fluvoxamine and desipramine in nondiabetic mice were antagonized by pretreatment with diazepam (0.3 mg/kg s.c.). Furthermore, fluoxetine, fluvoxamine and desipramine had no effect on the immobility time in diazepam (0.3 mg/kg s.c.)-treated diabetic mice. These results indicate that the anti-immobility effects of SSRIs and desipramine are less in diabetic mice than in nondiabetic mice in the tail suspension test. Furthermore, in diabetic mice, SSRIs and selective NA reuptake inhibitors did not affect immobility time even though the prolonged duration of immobility was suppressed by pretreatment with diazepam.

Effect of amyloid precursor protein 17mer peptide on microtubule structure and tau protein hyperphosphorylation in hippocampal neurons of experimental diabetic mice

The objective of this study was to investigate whether microtubular structure changes and tau protein hyperphosphorylation exist in hippocampal neurons of experimental diabetic mice, and to study the effect of amyloid precursor protein 17mer peptide. The results showed that the microtubules of hippocampal neurons of diabetic mice manifested prominent signs of fragmentation and dissolution, tau protein is hyperphosphorylated at Ser 199/Thr 202 sites, enzymes related to the phosphorylation and dephosphorylation of tau protein were diminished. The administration of amyloid precursor protein 17mer peptide could ameliorate the foregoing changes in diabetic mice. These results indicated that protein synthesis in the brain tissue of diabetic mice decreased. Amyloid precursor protein 17mer peptide acted as a neuroprotective agent that globally alleviates the disturbances due to impaired energy metabolism in diabetic mice.

Dopaminergic system modulation of nociceptive response in long-term diabetic rats

The present study examines the effects of dopaminergic system modulation on nociceptive response time in male diabetic rats. In this study, diabetes was induced by streptozotocin (STZ, 45 mg/kg) in adult male Sprague-Dawley rats. Insulin replacement therapy was initiated 6 weeks after the induction of diabetes for one-half of the diabetic group (1.5-2.5 IU/12 h/rat) and was continued throughout the duration of the study (up to 14 weeks). After 6 weeks of daily insulin replacement therapy, eight rats from each experimental group (STZ-diabetic, STZ-diabetic+insulin and nondiabetic control) were injected with either bromocriptine (BROM, 3 mg/kg/12 h), haloperidol (HALO, 1.5 mg/kg/12 h) or vehicle. Nociceptive response was measured by the hot plate (HP) latency test before the induction of diabetes (baseline), every 3 weeks for the first 12 weeks and then on days 5, 9 and 14 of treatment with dopaminergic agents. Animals were sacrificed 3 or 4 days after the last HP test and the brain, blood, spinal cord (SC), pituitary and adrenal glands (AD) were dissected for Met-enkephalin (ME) assay. The results show that nociceptive response of untreated diabetic animals increased gradually and significantly over the duration of this study. Administration of BROM and HALO significantly decreased and increased the nociceptive response, respectively, in all groups. However, the response of the diabetic group was more pronounced than that of the other two groups, especially for those treated with BROM. Daily insulin administration normalized nociceptive response to that of the nondiabetic controls. Diabetic animals receiving insulin replacement+BROM also showed normalized nociceptive response while the diabetic animals+HALO did not. Moreover, the administration of HALO and BROM resulted in an increase and decrease ME concentrations, respectively, in most tissues and brain regions examined. The effect of these dopaminergic agents on ME levels was greater in brain regions and tissues of the diabetic rats than in the diabetic groups receiving vehicle or in the nondiabetic control receiving these two agents. These data suggest that diabetes alters the sensitivity of the dopaminergic receptors and that altered response of the dopaminergic system could be indirectly involved in the modulation of nociception in diabetic rats possibly through the enhancement and/or deactivation of the endogenous Met-enkephalinergic system.

Food-anticipatory activity and liver per1-luc activity in diabetic transgenic rats

The mammalian Per1 gene is an important component of the core cellular clock mechanism responsible for circadian rhythms. The rodent liver and other tissues rhythmically express Per1 in vitro but typically damp out within a few cycles. In the liver, the peak of this rhythm occurs in the late subjective night in an ad lib-fed rat, but will show a large phase advance in response to restricted availability of food during the day. The relationship between this shift in the liver clock and food-anticipatory activity (FAA), the circadian behavior entrained by daily feeding, is currently unknown. Insulin is released during feeding in mammals and could serve as an entraining signal to the liver. To test the role of insulin in the shift in liver Per1 expression and the generation of FAA, per-luciferase transgenic rats were made diabetic with a single injection of streptozotocine. Following 1 week of restricted feeding and locomotor activity monitoring, liver was collected for per-luc recording. In two separate experiments, FAA emerged and liver Per1 phase-shifted in response to daytime 8-h food restriction. The results rule out insulin as a necessary component of this system.

Learning and memory in streptozotocin-induced diabetic rats in a novel spatial/object discrimination task

Diabetes mellitus is associated with disturbances of cognitive functioning. The aim of this study was to examine cognitive functioning in diabetic rats using the 'Can test', a novel spatial/object learning and memory task, without the use of aversive stimuli. Rats were trained to select a single rewarded can from seven cans. Mild water deprivation provided the motivation to obtain the reward (0.3 ml of water). After 5 days of baseline training, in which the rewarded can was marked by its surface and position in an open field, the animals were divided into two groups. Diabetes was induced in one group, by an intravenous injection of streptozotocin. Retention of baseline training was tested at 2-weekly intervals for 10 weeks. Next, two adapted versions of the task were used, with 4 days of training in each version. The rewarded can was a soft-drink can with coloured print. In a 'simple visual task' the soft-drink can was placed among six white cans, whereas in a 'complex visual task' it was placed among six soft-drink cans from different brands with distinct prints. In diabetic rats the number of correct responses was lower and number of reference and working memory errors higher than in controls in the various versions of the test. Switches between tasks and increases in task complexity accentuated the performance deficits, which may reflect an inability of diabetic rats to adapt behavioural strategies to the demands of the tasks.

Modification of the effects of benzodiazepines on the exploratory behaviors of mice on a hole-board by diabetes

The effect of diabetes on the emotional behavior of mice was examined using an automatic hole-board apparatus. Changes in the emotional state of mice were evaluated in terms of changes in exploratory activity; i.e., total locomotor activity, numbers and duration of rearing and head-dipping, and latency to the first head-dipping. The number and duration of head-dipping in diabetic mice were less than those in non-diabetic mice. Diazepam (0.1-0.56 mg/kg, i.p.) dose-dependently increased the number and duration of head-dipping at doses that did not produce sedation in both diabetic and non-diabetic mice. In contrast, methyl-beta-carboline-3-carboxylate (1 and 2 mg/kg, i.p.) decreased the number and duration of head-dipping in non-diabetic mice, but not in diabetic mice. The number and duration of head-dipping in diabetic mice were increased by treatment with flumazenil (0.1 and 0.3 mg/kg, i.v.). These doses of flumazenil did not affect the number or duration of head-dipping in non-diabetic mice. The present data indicate that diabetic mice exhibited anxiety in the hole-board test and that a benzodiazepine receptor antagonist affected the attenuated number and duration of head-dipping in diabetic mice. The heightened anxiety in diabetic mice may be due to the dysfunction of the benzodiazepine receptor and/or of central inhibitory systems.

Intracerebroventricular insulin-like growth factor-1 decreases feeding in diabetic rats

Insulin-like growth factor-1 (IGF-1) is a hormone that is important in the regulation of growth processes and additionally has been demonstrated to modulate metabolic and autonomic responses. Some of its effects are mediated by the central nervous system (CNS), and there are IGF-1 receptors dispersed throughout the CNS. Both IGF-1 and insulin alter peripheral metabolic and autonomic nervous activity by a central mechanism, and the well-defined role of insulin in the regulation of feeding, especially in diabetes, led us to investigate the effect of chronic central administration of IGF-1 on metabolic and feeding parameters in normal and diabetic rats. Normal and diabetic rats with intracerebroventricular cannulas were given IGF-1, insulin (0.5 nmol/animal), or artificial cerebrospinal fluid via cannula twice daily for 4 d. Blood samples were collected on d 2 and 4, and the body weights and food intake were recorded daily. IGF-1 administered intracerebroventricularly did not alter plasma glucose, insulin, body weight, or food intake in normal rats. However, in diabetic animals, IGF-1 decreased food intake but did not alter blood glucose or plasma insulin. In correlated studies, intracerebroventricular insulin decreased food intake in both normal and diabetic animals. From these studies, we conclude that IGF-1 may act centrally to decrease food intake in the hyperphagic diabetic animals but not in normal animals. This suggests that diabetic animals have an increased sensitivity to CNS IGF-1.

Ethopharmacology of the antidepressant effect of clonazepam in diabetic rats

Diabetes-associated depression may occur due to changes in the quality of life imposed by treatment, or may be a consequence of the biochemical changes accompanying the disease. It was our objective to evaluate the behaviors of diabetic rats through an animal model of depression, and determine if a positive GABA modulator agent, clonazepam, is an effective antidepressant. Wistar male rats were submitted to the forced-swimming test after 26 days of the induction of diabetes with streptozotocin (60 mg/kg). Test and retest days analyzed with an ethological approach. Clonazepam (control, 0.25, 0. 5, and 1.0 mg/kg) was administered IP 24, 5, and 1 h before the retest. Diabetic rats presented longer immobility duration during test and retest of forced swimming. Diabetic rats dived significantly less during the test. Clonazepam 0.25 and 0.5 mg/kg decreased immobility of diabetic rats with no consequences on the behaviors of nondiabetic rats. These results demonstrate that diabetic rats present more intense depressive-like behavior, such as immobility and lack of interest in exploring the environment, when exposed to the forced-swimming test. It is possible that decreased GABA function is involved in depression associated with diabetes, because a benzodiazepine partially counteracts these changes without modifying blood glucose and glycogen parameters.

Socio-psychological stress-induced antinociception in diabetic mice

RATIONALE

Although it is well established that different forms of stress produce a pronounced antinociception, the effect of diabetes on psychological stress-induced antinociception is not yet clear.

OBJECTIVES

The effect of diabetes on psychological stress-induced antinociceptive effect was assessed in mice.

METHODS

Animals were rendered diabetic by an injection of streptozotocin (200 mg/kg, i.v.). Mice were exposed to psychological stress in the compartment of a communication box. The antinociceptive response was evaluated by the tail-flick test, using radiant heat as a stimulus, which was performed before stress (pre-stress latency) and 0, 30 and 60 min after stress.

RESULTS

Exposure to socio-psychological stress for 5, 10 and 15 min produced duration-dependent antinociception in diabetic mice. However, in non-diabetic mice, no appreciable antinociception was found even in the case of socio-psychological stress for 15 min. Pretreatment with diazepam (0.3 mg/kg, i.p.) significantly attenuated socio-psychological stress-induced antinociception in diabetic mice (vehicle: 62.9 +/- 5.5%, n = 10; diazepam: 22 +/- 1%, n = 10). Furthermore, pretreatment with flumazenil (1 mg/kg, i.v.), a benzodiazepine receptor antagonist, also significantly reduced socio-psychological stress-induced antinociception in diabetic mice (vehicle: 77.9 +/- 5.0%, n = 10; flumazenil: 5.8 +/- 1.2%, n = 10). In contrast, pretreatment with methyl beta-carboline-3-carboxylate (beta-CCM, 2 mg/kg, i.v.), a benzodiazepine receptor inverse agonist, significantly enhanced socio-psychological stress-induced antinociception in non-diabetic mice (vehicle: 4.9 +/- 0.6%, n = 10; beta-CCM: 61.5 +/- 5.9%, n = 10), but not in diabetic mice (vehicle: 50.7 +/- 4.5%, n = 10; beta-CCM: 64.4 +/- 7.2%, n = 10).

CONCLUSIONS

These results indicate that emotional stress can readily induce antinociception in diabetic mice. Furthermore, this enhanced emotional stress-induced antinociception might be attributable to an increase in the production and/or release of endogenous ligands for benzodiazepine receptors, such as diazepam binding inhibitor, which act as inverse benzodiazepine receptor agonists.

Neurological changes induced by stress in streptozotocin diabetic rats

Previous studies from our laboratory demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid impairment of glucose utilization is proposed as a causative factor involved in stress-induced changes. Current studies have examined the neurological changes induced by stress in rats with a preexisting strain upon their homeostatic load--namely, in streptozotocin (stz)-diabetic rats. Administration of stz (70 mg/kg, i.v.) produced diabetic symptoms such as weight loss, polyuria, polydipsia, hyperglycemia, and neuroendocrine dysfunction. Morphological analysis of hippocampal neurons revealed that diabetes alone produced dendritic atrophy of CA3 pyramidal neurons, an effect potentiated by 7 days of restraint stress. Analysis of genes critical to neuronal homeostasis revealed that glucose transporter 3 (GLUT3) mRNA and protein levels were specifically increased in the hippocampus of diabetic rats, while stress had no effect upon GLUT3 expression. Insulin-like growth factor (IGF) receptor expression was also increased in the hippocampus of diabetic rats subjected to stress. In spite of the activation of these adaptive mechanisms, diabetic rats subjected to stress also had signs of neuronal damage and oxidative damage. Collectively, these results suggest that the hippocampus of diabetic rats is extremely susceptible to additional stressful events, which in turn can lead to irreversible hippocampal damage.

Antinociceptive effects of morphine were different between experimental and genetic diabetes

This study was designed to investigate the effect of morphine on formalin-induced nociceptive responses in streptozotocin (STZ) induced-diabetic mice, noninsulin-dependent genetically diabetic db/db mice and their respective controls (ddY and (+/+)). In nondiabetic (ddY and (+/+)) mice, morphine (1-10 mg/kg, PO) dose dependently attenuated the biphasic nociceptive responses induced by s.c. injection of formalin to the hindpaw, demonstrating equipotency on both the first and second phases. Para-chlorophenylalanine (800 mg/kg x 2, PO) and pindolol (1 mg/kg, i.p.) reduced the effect of morphine on the first phase, sulpiride (10 mg/kg, i.p.) abolished the effect on both phases, while ketanserin (1 mg/kg, i.p.) had no effect. In STZ (200 mg/kg, i.p.)-diabetic mice, morphine weakly attenuated the nociception in comparison to control ddY mice, whereas it had comparable effects in both the first and second phases of control (+/+) mice and db/db mice. The serotonergic agonist, meta-chlorophenylpiperazine (0.32-3.2 mg/kg, PO), dose dependently attenuated the biphasic nociceptive responses to formalin in both phases of diabetic mice; however, FR64822, a dopaminergic compound (0.1-10 mg/kg, PO), had little effect. We speculate that activation of both dopaminergic (DA)- and serotonergic-mediated mechanisms are potentially responsible for the effect of morphine on the first phase, while the DA-mediated effect is involved in the second phase. The DA-mediated mechanism, but not the serotonin-mediated one, appears to be altered in both STZ-diabetic and db/db mice. These results suggest that the attenuated effects of morphine might be due to a dopaminergic dysfunction in STZ mice, and that there might be other mechanisms compensating for this attenuation of dopaminergic function in db/db mice.

Water maze learning and hippocampal synaptic plasticity in streptozotocin-diabetic rats: effects of insulin treatment

Streptozotocin-diabetic rats express deficits in water maze learning and hippocampal synaptic plasticity. The present study examined whether these deficits could be prevented and/or reversed with insulin treatment. In addition, the water maze learning deficit in diabetic rats was further characterized. Insulin treatment was commenced at the onset of diabetes in a prevention experiment, and 10 weeks after diabetes induction in a reversal experiment. After 10 weeks of treatment, insulin-treated diabetic rats, untreated diabetic rats and non-diabetic controls were tested in a spatial version of the Morris water maze. Next, hippocampal long-term potentiation (LTP) was measured in vitro. To further characterize the effects of diabetes on water maze learning, a separate group of rats was pre-trained in a non-spatial version of the maze, prior to exposure to the spatial version. Both water maze learning and hippocampal LTP were impaired in diabetic rats. Insulin treatment commenced at the onset of diabetes prevented these impairments. In the reversal experiment, insulin treatment failed to reverse established deficits in maze learning and restored LTP only partially. Non-spatial pre-training abolished the performance deficit of diabetic rats in the spatial version of the maze. It is concluded that insulin treatment may prevent but not reverse deficits in water maze learning and LTP in streptozotocin-diabetic rats. The pre-training experiment suggests that the performance deficit of diabetic rats in the spatial version of the water maze is related to difficulties in learning the procedures of the maze rather than to impairments of spatial learning.

A pharmacologic analysis of mechanical hyperalgesia in streptozotocin/diabetic rats

This study used streptozotocin (STZ; 50 mg/kg i.p.) diabetic rats and monitored weekly thermal and mechanical nociceptive thresholds for 8 weeks diabetes. Rats developed mechanical hyperalgesia as soon as 2 weeks after STZ injection. Thermal nociceptive threshold was not altered up to 8 weeks after STZ injection. Four week-diabetic rat mechanical hyperalgesia showed reduced sensitivity to the antinociceptive effect of morphine (5-20 mg/kg i.p.). Furthermore, a reduced sensitivity to the antinociceptive effect of the GABA(B) agonist, (+/-)baclofen, was observed. A dose as high as 16 mg/kg i.p. of (+/-)baclofen was necessary to reverse 4 week-diabetic rat hyperalgesia, whereas in control rats the highest antinociceptive dose devoid of muscle-relaxant effect was 4 mg/kg i.p. The non-peptide antagonist for the substance P, neurokinin, (NK1) receptor, RP 67580 (3-9 mg/kg i.p.) was not effective in reversing the mechanical hyperalgesia associated with 4 week-diabetes. A six day-treatment with an antagonist for the N-methyl-D-aspartate (NMDA) receptor for glutamate, (+)MK-801 (0.1 mg/kg i.p. twice a day), gradually but completely reversed 4 week-diabetes-induced mechanical hyperalgesia. These data suggest that diabetes-induced hyperalgesia may be the consequence of increased activity of primary afferent fibres leading to an increased excitatory tone within the spinal cord. An increased release of glutamate and activation of the NMDA receptor, would maintain the hyperalgesic state. Reduced activity of both opioidergic and GABA(B)ergic inhibitory systems, might exacerbate the increased excitation thus contributing to the ongoing pain. It is suggested that NMDA receptor antagonists may constitute an alternative therapy for diabetic neuropathic pain.

Effects of the experimental diabetes on dopamine D1 receptor-mediated locomotor-enhancing activity in mice

The effects of diabetes on the dopamine-related locomotor-enhancing activities were studied in mice. Although spontaneous locomotor activity in diabetic mice was significantly greater than that in nondiabetic mice, the locomotor-enhancing effects of methamphetamine (4 mg/kg, s.c.), cocaine (20 mg/kg, s.c.) and SKF82958 (1 mg/kg, s.c.), a selective dopamine D1-receptor agonist, in diabetic mice were significantly lower than those in nondiabetic mice. When dopamine level in the whole brain was reduced by pretreatment with 6-hydroxydopamine (6-OHDA), spontaneous locomotor activity was significantly reduced in both nondiabetic and diabetic mice. There was no significant difference in the total spontaneous locomotor activity counts within 3 h between 6-OHDA-treated nondiabetic and 6-OHDA-treated diabetic mice. Furthermore, the locomotor-enhancing effect of SKF82958 in 6-OHDA-treated diabetic mice was also significantly lower than that in 6-OHDA-treated nondiabetic mice. In a binding assay, the Bmax values of [3H]SCH23390 binding to whole-brain membranes of diabetic mice were significantly lower than those in nondiabetic mice. However, there was no significant difference in the Kd values between nondiabetic and diabetic mice. These results suggest that the decreased density of dopamine D1 receptors in diabetic mice may result in hyporesponsiveness to dopamine-related locomotor enhancement.