The effect of intra-cerebroventricular injection of insulin on nociception of formalin test in non-diabetic and short-term diabetic rat models.

Pain is a complex process in the central nervous system (CNS). Several factors can alter the pain threshold and insulin is one of them which is produced by the beta cells of pancreas and capable of crossing blood-brain barrier. The aim of this study was to evaluate the effects of intra-cerebroventricular (ICV) injection of insulin on the pain response to formalin in short-term induced diabetic and non-diabetic rats. Sixty-four Sprague-Dawley male rats (280 ± 30 g) were divided into non-diabetic and diabetic groups. Diabetes was induced with streptozotocin (STZ, 60 mg/kg, i.p) for elimination of peripheral insulin. After proving diabetes, insulin (5 mU/animal, 5 μL) was injected to the left lateral cerebral ventricle while equal volume of normal saline was injected in control groups. After 10 min, formalin test was performed. Present study showed that ICV injection of insulin possessed anti-nociceptive effect in non-diabetic rats in formalin test while in diabetic rats, it did not have this effect and even decreased pain threshold partially. In conclusion we showed that ICV injection of insulin in non-diabetic rats, in contrast with diabetic rats, has an anti-nociceptive effect in formalin test. In short-term diabetic rats, ICV injection of insulin was not able to reduce pain response and partially decreased pain threshold.