"...researchers have found that insulin receptors in the brains of mice are involved in appetite, weight gain and fertility.

Insulin is a key hormone in metabolism, and its levels rise in tandem with body fat. Because obese people have chronically high insulin levels, they often become resistant to the hormone and eventually develop diabetes. While tissue such as muscle and fat are known to become insulin resistant, conventional wisdom has held that the brain is not sensitive to insulin.

The current study, however, confirms earlier evidence that insulin receptors in the brain help control food intake and body weight."

(quoted from http://www.lowcarb.ca/articles/article108.html )

Insulin is a hormone that brings blood sugar (glucose) to body tissues to be used as a source of energy. Our body's demand for fuel varies from moment to moment, but the brain needs our blood sugar level to remain stable. So getting the cells the energy they need without changing that level is a critical function — and that's the role that insulin plays. Insulin signals the cells to absorb glucose from the bloodstream. The body monitors what we've digested, blood sugar levels, and cell demands, and releases insulin in just the right amounts."

(from http://www.womentowomen.com/LIBinsulinresistance.asp )

I've heard that insulin triggers growth hormone by bringing the body sugar levels down. In most bodybuilding centers, they advice to eat rice often in order to keep sugar levels low to let the body produce growth hormone. Moreover, i know that some professional bodybuilders misuse insulin to fasten their muscle growth by affecting growth hormon release.