glucagon-like-peptide-1 and Brain-Ischemia

Glucagon-like peptide-1 (GLP-1)-based therapies, GLP-1 receptor agonists (GLP-1RAs) and dipeptidyl peptidase-4 inhibitors (DPP-4Is) are widely used for the treatment of type 2 diabetes. Increasing evidence suggests that they may provide neuroprotection. The aim of this MiniReview was to systematically evaluate the proposed mechanism of action for GLP-1-based therapies in ischaemic brain damage in animals. We performed a literature search using MEDLINE, EMBASE and The Cochrane Library. GLP-1-based therapies administered before, during or after experimental stroke in diabetic and non-diabetic animals were evaluated. We reviewed 27 studies comprised of 20 involving GLP-1RAs and seven involving DPP-4Is. Both GLP-1RAs and DPP-4Is affected the acute inflammatory response secondary to ischaemia by reducing inflammation, endothelial leakage and excitotoxicity. Both treatments also reduced oxidative stress and apoptosis. GLP-1RAs significantly reduced infarct volume when administered acutely, but not later after stroke. The reported effects of DPP-4Is on infarct volume were inconsistent. GLP-1-RAs reliably improved functional outcome, but the effects on cerebral blood flow were inconclusive. These neuroprotective effects were often attributed to activation of the GLP-1 receptor, but non-GLP-1R-mediated effects have also been suggested. Both GLP-1RAs and DPP-4Is significantly affected inflammation, oxidative stress and apoptosis in animal stroke models; however, data from clinical trials only report therapeutic efficacy for GLP-1RAs. Thus, GLP-1RA administration is the most promising treatment to pursue for patients at risk of stroke or immediately after stroke. Future studies should address acute and prophylactic treatments in stroke patients with and without diabetes.

Topics: Animals; Brain Ischemia; Glucagon-Like Peptide 1; Humans; Neuroprotective Agents; Stroke

Hyperglycemia affects approximately one-third of acute ischemic stroke patients and is associated with poor clinical outcomes. In experimental and clinical stroke studies, hyperglycemia has been shown to be detrimental to the penumbral tissue for several reasons. First, hyperglycemia exacerbates both calcium imbalance and the accumulation of reactive oxygen species (ROS) in neurons, leading to increased apoptosis. Second, hyperglycemia fuels anaerobic energy production, causing lactic acidosis, which further stresses neurons in the penumbral regions. Third, hyperglycemia decreases blood perfusion after ischemic stroke by lowering the availability of nitric oxide (NO), which is a crucial mediator of vasodilation. Lastly, hyperglycemia intensifies the inflammatory response after stroke, causing edema, and hemorrhage through disruption of the blood brain barrier and degradation of white matter, which leads to a worsening of functional outcomes. Many neuroprotective treatments addressing hyperglycemia in stroke have been implemented in the past decade. Early clinical use of insulin provided mixed results due to insufficiently controlled glucose levels and heterogeneity of patient population. Recently, however, the latest Stroke Hyperglycemia Insulin Network Effort trial has addressed the shortcomings of insulin therapy. While glucagon-like protein-1 administration, hyperbaric oxygen preconditioning, and ethanol therapy appear promising, these treatments remain in their infancy and more research is needed to better understand the mechanisms underlying hyperglycemia-induced injuries. Elucidation of these mechanistic pathways could lead to the development of rational treatments that reduce hyperglycemia-associated injuries and improve functional outcomes for ischemic stroke patients.

Topics: Brain Ischemia; Ethanol; Glucagon-Like Peptide 1; Humans; Hyperbaric Oxygenation; Hyperglycemia; Hypothermia, Induced; Insulin; Insulin-Like Growth Factor I; Neuroprotective Agents; Stroke

Type 2 diabetes and hyperglycemia with the resulting increase of glucose concentrations in the brain impair the outcome of ischemic stroke, and may increase the risk of developing Alzheimer's disease (AD). Reports indicate that glucagon-like peptide-1 (GLP-1) may be neuroprotective in models of AD and stroke: Although the mechanism is unclear, glucose homeostasis appears to be important. We conducted a randomized, double-blinded, placebo-controlled crossover study in nine healthy males. Positron emission tomography was used to determine the effect of GLP-1 on cerebral glucose transport and metabolism during a hyperglycemic clamp with (18)fluoro-deoxy-glucose as tracer. Glucagon-like peptide-1 lowered brain glucose (P=0.023) in all regions. The cerebral metabolic rate for glucose was increased everywhere (P=0.039) but not to the same extent in all regions (P=0.022). The unidirectional glucose transfer across the blood-brain barrier remained unchanged (P=0.099) in all regions, while the unidirectional clearance and the phosphorylation rate increased (P=0.013 and 0.017), leading to increased net clearance of the glucose tracer (P=0.006). We show that GLP-1 plays a role in a regulatory mechanism involved in the actions of GLUT1 and glucose metabolism: GLP-1 ensures less fluctuation of brain glucose levels in response to alterations in plasma glucose, which may prove to be neuroprotective during hyperglycemia.

Topics: Adult; Alzheimer Disease; Biological Transport; Blood-Brain Barrier; Brain Chemistry; Brain Ischemia; Cross-Over Studies; Diabetes Mellitus, Type 2; Double-Blind Method; Fluorodeoxyglucose F18; Glucagon-Like Peptide 1; Glucose; Glucose Clamp Technique; Glucose Transporter Type 1; Hexokinase; Humans; Hyperglycemia; Male; Positron-Emission Tomography; Radiography; Radiopharmaceuticals; Stroke

Glucagon-like peptide-1 (GLP-1) treatment has been shown to reduce stroke incidence in diabetes and also to be neuroprotective in experimental stroke models. The prognostic value of endogenous levels of GLP-1 in the recovery phase after stroke remains to be elucidated. The aim of the study was to investigate the potential association between GLP-1 levels and functional outcome after stroke and to determine whether GLP-1 is altered in the acute phase of stroke compared to 3 months post stroke and to healthy controls.. Fasting GLP-1 was measured on hospital day 2-4 in patients without previously known diabetes (n = 59) that received recombinant tissue plasminogen activator (rtPA) for ischemic stroke. Fasting GLP-1 was measured again after 3 months and neurologic outcome was measured as modified Rankin Scale (mRS). mRS ≥ 2 was considered as unfavorable outcome. A control group of healthy individuals (n = 27) was recruited and their fasting GLP-1 was measured.. Fasting GLP-1 was higher in the patients that suffered a stroke compared to healthy controls (25.1 vs. 18.0 pmol/L; p = 0.004). The GLP-1 levels did not change significantly at the 3-month follow up OGTT (25.8 vs. 25.6; p = 0.80). There was no significant association between GLP-1 levels and unfavorable mRS (OR 1.03, 95% CI 0.95-1.12, p = 0.50).. Endogenous GLP-1 levels in patients that recently suffered an ischemic stroke are higher than in healthy controls and remained unchanged at the 3 months follow-up, possibly indicating an elevation of the levels of GLP-1 already pre-stroke. However, no association between endogenous GLP-1 and functional outcome of stroke 3 months post stroke was found.

Topics: Adult; Aged; Aged, 80 and over; Biomarkers; Brain Ischemia; Case-Control Studies; Disability Evaluation; Female; Fibrinolytic Agents; Glucagon-Like Peptide 1; Humans; Male; Middle Aged; Registries; Stroke; Thrombolytic Therapy; Time Factors; Tissue Plasminogen Activator; Up-Regulation

Type 2 diabetes mellitus (T2DM) is a major risk factor for ischemic stroke accompanied by vascular dysfunction and poor cerebrovascular outcome. Lixisenatide is a glucagon like peptide-1 (GLP-1) analog that is recently used for T2DM treatment with established neuroprotective properties. This study investigated and compared the neuroprotective effect of lixisenatide against glimepiride on diabetic rats subjected to global cerebral ischemia/reperfusion (I/R) injury. T2DM-induced adult male Wistar rats were administered lixisenatide or glimepiride prior to induction of global cerebral I/R-induced injury. Results showed a disturbance in oxidative stress parameters (catalase, reduced glutathione, and malondialdehyde) along with increasing in caspase-3 and tumor necrosis factor-alpha protein expressions in ischemic diabetic brain tissues. An upregulation of protein level of inducible nitric oxide (iNOS) synthase and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit, NOX2 gene expression associated with significant suppression of endothelial nitric oxide synthase (eNOS) protein expression are recorded in carotid arteries of diabetic I/R-injured rats. Apart from ameliorating glucose intolerance and insulin resistance, lixisenatide was found to be superior to glimepiride as protective treatment in terms of enhancing behavioral/neurological functions and suppressing cerebral oxidative stress, inflammation, and apoptosis in cerebral I/R-injured diabetic rats. Unlike glimepiride, lixisenatide relieved carotid endothelial dysfunction by increasing eNOS expression. It also dampened vascular nitrosative/oxidative stress via suppression of iNOS and NADPH oxidase expressions. This study supposed that lixisenatide represents a more suitable anti-diabetic therapy for patients who are at risk of ischemic stroke, and even so, the mechanisms of lixisenatide-mediated vascular protection warrant further experimental and clinical investigations.

Topics: Animals; Brain Ischemia; Carotid Arteries; Diabetes Mellitus, Experimental; Glucagon-Like Peptide 1; Hypoglycemic Agents; Insulin Resistance; Male; NADPH Oxidase 2; NADPH Oxidases; Neuroprotective Agents; Nitric Oxide Synthase Type II; Nitric Oxide Synthase Type III; Oxidative Stress; Peptides; Rats, Wistar; Reperfusion Injury

Preclinical Research & Development The purpose of the present study is to evaluate the neuroprotective effect of recombinant human glucagon-like peptide-1 (rhGLP-1) as well as to explore corresponding mechanisms in diabetic rats with cerebral ischemia/reperfusion injury induced by middle cerebral artery occlusion (MCAO). Diabetes mellitus was induced by intraperitoneal injection of streptozotocin. The rats were pretreated with rhGLP-1 (20 μg/kg intraperitoneally, thrice a day) for 14 days. Thereafter, the rats were subjected to MCAO 90 min/reperfusion 24 hr. At 2 and 24 hr of reperfusion, the rats were assessed for neurological deficits and subsequently executed for the evaluation of cerebral infarct volume, oxidative stress parameters, and the expression of excitatory amino acid transporter 2 (EAAT2) and apoptotic markers. Results indicate that rhGLP-1 significantly ameliorated neurological deficits and reduced cerebral infarct volume in diabetic MCAO rats. In addition, oxidative stress parameters in ischemic penumbra were significantly alleviated in rhGLP-1-pretreated diabetic MCAO rats. rhGLP-1 significantly upregulated the ratio of Bcl-2/Bax and EAAT2 expression and downregulated cleaved caspase-3 expression in ischemic penumbra of diabetic MCAO rats. Our results suggest that rhGLP-1 could significantly ameliorate neurological deficits and reduce cerebral infarct volume in diabetic MCAO rats, which may be due to the inhibition of oxidative stress and apoptosis and the promotion of EAAT2 expression.

Topics: Animals; Apoptosis; Brain Ischemia; Diabetes Complications; Diabetes Mellitus; Excitatory Amino Acid Transporter 2; Glucagon-Like Peptide 1; Humans; Incretins; Infarction, Middle Cerebral Artery; Neuroprotective Agents; Oxidative Stress; Rats; Recombinant Proteins; Reperfusion Injury

Preclinical Research The aim of the present study was to evaluate the neuroprotective benefits of rhGLP-1 in diabetic rats subjected to acute cerebral ischemia/reperfusion injury induced by middle cerebral artery occlusion/reperfusion (MCAO/R). Streptozotocin (STZ)-induced diabetic rats were pretreated with rhGLP-1 (10, 20, or 40 μg/kg ip, tid) for 14 days. During this time, body weight and fasting blood glucose levels were assessed. Rats were then subjected to MCAO 90 min/R 24 h. At 2 and 24 h of reperfusion, rats were evaluated for neurological deficits and blood samples were collected to analyze markers of brain injury. Rats were then sacrificed to assess the infarction volume. rhGLP-1 pretreatment lowered blood glucose levels, improved neurological scores, attenuated infarct volumes, and reduced the blood levels of S100 calcium-binding protein B (S100B), neuron-specific enolase (NSE), and myelin basic protein (MBP). rhGLP-1 has neuroprotective benefits in diabetic rats with cerebral ischemia/reperfusion injury and could potentially be used as a prophylatic neuroprotectant in diabetic patients at high risk of ischemic stroke. Drug Dev Res 77 : 124-133, 2016.   © 2016 Wiley Periodicals, Inc.

Topics: Animals; Body Weight; Brain Ischemia; Diabetes Mellitus, Experimental; Disease Models, Animal; Glucagon-Like Peptide 1; Hypoglycemic Agents; Male; Myelin Basic Protein; Neuroprotective Agents; Phosphopyruvate Hydratase; Rats; Rats, Sprague-Dawley; Recombinant Proteins; Reperfusion Injury; S100 Calcium Binding Protein beta Subunit; Streptozocin; Treatment Outcome

In recent years, GLP-1 and its analogs have been developed for the treatment of type 2 diabetes. It has been reported that stimulating the GLP-1 receptor can protect neurons against metabolic and oxidative insults, and therefore can be used in the treatment of stroke and Parkinson׳s disease. The present study aimed to examine the neuroprotective effects of rhGLP-1 (7-36) and its possible mechanisms against acute ischemia/reperfusion injuries induced by middle cerebral artery occlusion (MCAO) in diabetic rats. The type 2 diabetic rat model was established by a combination of a high-fat diet and low-dose streptozotocin (STZ). RhGLP-1 (7-36) (20, 40, 80μg/kg) was given intraperitoneally before reperfusion. The neuroprotective effects of rhGLP-1 (7-36) were evaluated by changes in neurological deficit scores and 2,3,5-Triphenyltetrazolium chloride (TTC) staining. Changes in blood glucose were used to assess hypoglycemic effects. The content of malondialdehyde (MDA) and the activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-PX), inducible nitric oxide syntheses (iNOS) and endothelial nitric oxide syntheses (eNOS) after MCAO/R administration (2h and 46h) were examined to investigate the possible mechanisms of RhGLP-1 (7-36). Haematoxylin and eosin (H&E) staining was used for histopathological observation. Compared with the control group, rhGLP-1 (7-36)-treated groups decreased nerve function deficiency scores; significantly reduced infarction volume percentage, MDA, iNOS and blood glucose; and significantly increased SOD, GSH-PX and eNOS. In addition, rhGLP-1 (7-36) groups enhanced the density of surviving neurons and increased vascular proliferation. The current study suggests a neuroprotective effect of rhGLP-1 (7-36) in diabetic MCAO/R rats since anti-oxidative and anti-nitrosative stress effects can contribute to beneficial effects against ischemia/reperfusion injury.

Topics: Animals; Blood Glucose; Brain; Brain Ischemia; Diabetes Mellitus, Experimental; Dose-Response Relationship, Drug; Glucagon-Like Peptide 1; Glutathione Peroxidase; Hypoglycemic Agents; Infarction, Middle Cerebral Artery; Male; Malondialdehyde; Neuroprotective Agents; Nitric Oxide Synthase Type II; Nitric Oxide Synthase Type III; Peptide Fragments; Random Allocation; Rats, Sprague-Dawley; Reperfusion Injury; Severity of Illness Index; Superoxide Dismutase

Pro-Glucagon-like peptide-1 (Pro-GLP-1), a long-lasting GLP-1 receptor (GLP-1R) agonist, was developed using a polymeric pro-drug strategy and its neuroprotective effects on ischemic stroke were investigated in C57BL/6 mice. Pro-GLP-1 was injected into the intraperitoneal cavity of C57BL/6 mice once a day for 7days before middle cerebral artery occlusion (MCAO) surgery. The neurological deficit score and TTC staining were determined 24h after ischemia. The results demonstrated that Pro-GLP-1 was slowly degraded in the plasma and brain of the mice, and GLP-1 could be detected even 12h after administration. Pro-GLP-1 was significantly neuroprotective in C57BL/6 mice subjected to MCAO. In cultured cortical neurons, treatment with Pro-GLP-1 attenuated apoptosis induced by oxygen-glucose deprivation (OGD). The neuroprotective effects of Pro-GLP-1 were blocked by a selective GLP-1 receptor antagonist and knockdown of GLP-1 receptor with shRNA. However, Pro-GLP-1 had no effect on blood glucose and insulin levels which indicated that neuroprotection was mediated by the activation of GLP-1 receptor in the brain. Pro-GLP-1 repaired the balance of pro- and anti-apoptotic proteins and decreased the expression of caspase-3. The anti-apoptotic effect was mediated by the cAMP/PKA and PI3K/Akt pathway. Our research provides evidence that pre-treatment of MCAO mice with Pro-GLP-1 exerts a neuroprotective effect mediated by a blockade of apoptosis and that Pro-GLP-1 might be a potential neuroprotective agent candidate against ischemic stroke.

Topics: Animals; Brain Ischemia; Cells, Cultured; Dose-Response Relationship, Drug; Glucagon-Like Peptide 1; Infusions, Intraventricular; Male; Mice; Mice, Inbred C57BL; Neurons; Neuroprotective Agents; Prodrugs; RNA, Small Interfering

Sitagliptin, a new antidiabetic drug that inhibits dipeptidyl peptidase (DPP)-4 enzyme activity, has been reported to possess neuroprotective property. We tested the protective effects of sitagliptin against chronic cerebral hypoperfusion (CHP) in mice after bilateral carotid artery stenosis (BCAS).. Thirty C57BL/6 mice were divided into three groups: sham control (n = 10), CHP (n = 10) and CHP-sitagliptin (orally 600 mg/kg/day) (n = 10). Working memory was assessed with novel-object recognition test. MRI was performed at day 0 and day 90 after BCAS procedure prior to sacrifice.. Immunohistochemical (IHC) staining showed significantly enhanced white matter lesions, microglia activation and astrocytosis of white matter in CHP group than in sham control, but the changes were significantly suppressed after sitagliptin treatment (all P < 0.01). The mRNA expressions of inflammatory [tumour necrosis factor-alpha (TNF-α), monocyte chemoattractant protein (MCP-1) and matrix metalloproteinase (MMP)-2] and apoptotic (Bax) biomarkers showed an identical pattern, whereas the anti-inflammatory (interleukin, IL-10) and antiapoptotic (Bcl-2) biomarkers showed an opposite pattern compared with that of IHC among all groups (all P < 0.01). The protein expressions of oxidative stress (NOX-I, NOX-II, nitrotyrosin, oxidized protein), inflammatory [nuclear factor-kappa B (NF-κB), TNF-α and MMP-2], apoptotic [mitochondrial Bax, cleaved poly(ADP-ribose) polymerase (PARP)] and DNA-damage (γ-H2AX) markers showed an identical pattern, while expression pattern of antiapoptotic marker (Bcl-2) was opposite to that of IHC (all P < 0.01). Glycogen-like peptide-1 receptor protein expression progressively increased from sham control to CHP-sitagliptin (P < 0.01). The short-term working-memory loss and MRI/diffusion tensor imaging (DTI) showed a pattern identical to that of IHC in all groups (all P < 0.01).. Sitagliptin protected against cognitive impairment and brain damage in a murine CHP model.

Topics: Animals; Apoptosis; Brain; Brain Ischemia; Cognition Disorders; Drug Evaluation, Preclinical; Glucagon-Like Peptide 1; Hypoglycemic Agents; Inflammation; Interleukin-10; Male; Mice; Mice, Inbred C57BL; Oxidative Stress; Random Allocation; Sitagliptin Phosphate; Tumor Necrosis Factor-alpha

Stroke is a leading cause of death and serious, long-term disability worldwide. We report that rats receiving liraglutide show markedly attenuated infarct volumes and neurological deficit following ischemic insult. We have also investigated the effect of liraglutide on apoptosis and oxidative stress pathways after ischemic injury in diabetic and non-diabetic rats. Male Sprague-Dawley rats weighing 300-350g were used. Diabetes was induced by streptozotocin. Rats were pretreated with either vehicle or liraglutide (50μg/kg, s.c.) for 14days and thereafter subjected to middle cerebral artery occlusion (MCAO). Twenty-four hours after occlusion, rats were assessed for neurological deficit, motor function and subsequently sacrificed for estimation of infarct volume, oxidative stress and apoptotic markers. Vehicle-treated non-diabetic and diabetic rats showed significant (p<0.001) neurological deficit following cerebral ischemia. Liraglutide pretreatment resulted in significantly (p<0.001) less neurological deficit compared to vehicle-treated MCAO rats. Cerebral ischemia produced significant (p<0.0001) infarction in vehicle-treated rats; however, the infarct volume was significantly (p<0.001) less in liraglutide-pretreated rats. Oxidative stress markers were increased following ischemia but were attenuated in liraglutide-treated rats. Anti-apoptotic protein Bcl-2 expression was decreased and pro-apoptotic protein Bax expression was increased in vehicle-treated MCAO rats compared to sham (p<0.0001). On the other hand liraglutide pretreatment showed significantly (p<0.01) increased expression of Bcl-2 and decreased expression of Bax in MCAO rats. In vehicle-treated group, the number of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells significantly (p<0.0001) increased in the ischemic hemisphere compared to sham-operated group. The number of TUNEL-positive cells in vehicle group was 73.5±3.3 and 85.5±5.2/750μm(2) in non-diabetic and diabetic vehicle-treated MCAO rats, respectively. Following liraglutide treatment the number of TUNEL-positive cells was remarkably attenuated to 25.5±2.8 and 41.5±4.1/750μm(2) (p<0.001) in non-diabetic and diabetic rats, respectively. The results demonstrate that glucagon-like peptide 1 (GLP-1) agonist, liraglutide, is a neuroprotective agent and attenuates the neuronal damage following cerebral ischemia in rats by preventing apoptosis and decreasing oxidative stress.

Topics: Animals; Apoptosis; Brain Ischemia; Diabetes Mellitus, Experimental; Disease Models, Animal; Glucagon-Like Peptide 1; Hypoglycemic Agents; Infarction, Middle Cerebral Artery; Liraglutide; Male; Neuroprotective Agents; Oxidative Stress; Rats; Rats, Sprague-Dawley

Previous studies demonstrated that exendin-4 (Ex-4) may possess neurotrophic and neuroprotective functions in ischemia insults, but its mechanism remained unknown. Here, by using real-time PCR and ELISA, we identified the distribution of active GLP-1Rs in the rat primary cortical neurons. After establishment of an in vitro ischemia model by oxygen/glucose deprivation (OGD), neurons were treated with various dosages of Ex-4. The MTT assay showed that the relative survival rate increased with the dosage of Ex-4 ranging from 0.2 to 0.8 μg/ml (P<0.001, vs. OGD group). The apoptosis rate was reduced from (49.47±2.70)% to (14.61±0.81)% after Ex-4 treatment (0.4 μg/ml) 12h after OGD (P<0.001). Moreover, immunofluorescence staining indicated that Ex-4 increased glucose-regulated proteins 78 (GRP78) and reduced C/EBP-homologous protein (CHOP). Western blot analysis demonstrated that, after neurons were treated with Ex-4, GRP78 was up-regulated over time (P<0.01, vs. OGD group), while CHOP levels rose to a peak 8h after OGD and then decreased (P<0.05, vs. OGD group). This effect was changed by both the protein kinase A (PKA) inhibitor H89 (P<0.01, P<0.05, respectively, vs. Ex-4 group) and the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (P<0.01, P<0.01, respectively, vs. Ex-4 group) but not by the mitogen-activated protein kinase (MAPK) inhibitor U0126. Our study also revealed that, compared with the Ex-4 group, inhibition of the PKA signaling pathway significantly decreased the survival rate of neurons, down-regulated the expression of B-cell lymphoma 2 (Bcl-2) and up-regulated the Bax expression 3h after ODG (P<0.05, P<0.01, respectively), while neither PI3K nor MAPK inhibition exerted such effects. Furthermore, Western blotting exhibited that PKA expression was elevated in the presence or absence of OGD insults (P<0.05). This study indicated that Ex-4 protected neurons against OGD by modulating the unfolded protein response (UPR) through the PKA pathway and may serve as a novel therapeutic agent for stroke.

Topics: Animals; Animals, Newborn; Apoptosis; Blotting, Western; Brain Ischemia; Cell Survival; Cerebral Cortex; Coloring Agents; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Endoplasmic Reticulum; Exenatide; Flow Cytometry; Fluorescent Antibody Technique; Glucagon-Like Peptide 1; Glucose; Hypoxia; Neurons; Neuroprotective Agents; PC12 Cells; Peptides; Rats; Rats, Sprague-Dawley; Real-Time Polymerase Chain Reaction; Signal Transduction; Tetrazolium Salts; Thiazoles; Venoms