Analgesic, learning and memory and anxiolytic effects of insulin in mice.

Insulin is a polypeptide hormone that is present in mammals and its main function is the maintenance of adequate blood sugar level. Insulin receptors are widely but unevenly distributed in the brain. Insulin has been reported to be involved in the regulation of neurotransmitters release. It has also been linked to the pathogenesis of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Although there is abundant literature on the study of biochemical and molecular properties of insulin, there has been no literature on its central behavioural effects on anxiety and pain relief among other behavioural effects. This study therefore investigates whether insulin has any anxiolytic and other CNS effects. This experiment was carried out in mice using animal behavioural models including a hot plate analgesic test, holeboard and elevated plus maze for anxiolytic test. A Y-maze was used for the locomotor activity and spontaneous alternation investigations. Mice were administered intraperitoneally with insulin at different doses of 0.5, 1.0 and 2.0IU/kg. The results obtained showed that insulin has no analgesic activity, however, it caused significant central inhibitory effects by decreasing both locomotor activity in both holeboard and Y-maze models and also decreased the exploratory behaviour in holeboard at doses administered dose-dependently indicating its sedative effects. In elevated plus maze, insulin had no effects on percentage of open arm entries at all doses but had a significant effect on percentage of open arm duration at the dose of 1.0IU/kg only. Insulin administration at lower doses (0.5 and 1.0IU/kg, i.p.) had no effect on spatial working memory, however, it had significant spatial working memory impairment at the dose of 2.0IU/kg, i.p. in mice. The study showed that insulin has several neuropharmacological effects at doses used.