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Background: Pain is an unpleasant feeling which humans experience. It is a warning sign of the damaged tissue. Due to the awful sense of pain, scientists always attempt to relieve it. Retinoic acid (RT), an active metabolite of natural vitamin A has important roles in modulation of the inflammatory responses. The aim of the present study was to analyze the pain threshold of rats which had microinjections of RT, applying acute and chronic models. Methods and Materials: In this study, the tail flick and formalin tests were used to determine pain threshold. In each test, the acute and chronic pain thresholds of 252 Wistar male rats (275 ± 25 gr) were assayed. The druge were injected in the acute model one-dose30 minutes before behavioral testing and in chronic model two-dose for one or two-weeks. The rats of both models divided randomly into six groups (n=7). In four treatment groups retinoic acid (RT) intra cerebro ventricular (i.C.V) were injected as dosagc of 0.5, 3 and 6 (µg/kg) micrograms per kilogram. In control group, was microinjected by ACSF. In vehicle group injected RT solvent (DMSO+ Distil water). Results: The resuits Showed acute injection of RT did not change pain thresholds in the tail-flick methd, but the chronic administration of RT (0.5, 1, 3, 6 µg/kg) reduced tail-flick latencies of the rats (p<0.05) in compare to DMSO group. The threshold of pain in the first phase of formalin test was reduced after injection of 3µg/kg of RT for two weeks. Conclusion: It was concluded that chronic i.c.v. injections of RT can induce significant hyperalgesia in rat.

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Background: Considering to the frequency of occurrence of hypoglycemia in diabetes, alternative substrates for glucose play an important role in maintaining brain metabolism. The aim of the present study was to investigate the long-term effects of endurance training on brain lactate uptake during hypoglycemia in type 1 diabetic rats.

Materials and Methods: In this experimental study, seventy-two male Wistar rats were equally divided into four groups: Control, Trained, Diabetic control and Diabetic trained. Diabetes was induced by intraperitoneally injection of streptozotocin. After 8 weeks of endurance training, Hypoglycemia was induced by an intraperitoneally injection of insulin. One-half hours after hypoglycemia, lactate (4 mmol/kg) was injected intraperitoneally into animals. Animals were killed at intervals of 10, 20, and 30 min after injection, plasma and brain concentration of lactate and glucose were measured.

Results: A significant difference was found for plasma lactate concentration at intervals 20 (p<0.05) and 30 (p<0.05) between healthy groups and also between diabetic groups (p<0.05). Thirty minutes after injection, significant increase in brain lactate concentration was found between control and trained (p<0.05) and control diabetic and diabetic trained (p<0.05). Brain/plasma ratio of lactate had a similar pattern with brain lactate concentration. Increase in brain lactate concentration had no effect on decreased level of brain glucose concentration.

Conclusion: Endurance training can increase the brain lactate uptake of diabetic rats during hypoglycemia.