Uptake and degradation of blood-borne insulin by the olfactory bulb.

Insulin found within the brain is derived from the blood and can affect various central nervous system (CNS) functions. The olfactory bulb contains one of the highest concentrations of insulin and insulin receptors within the CNS. To determine the mechanism underlying this high concentration of insulin, we used radioactively iodinated insulin to compare the blood to tissue transport rates and tissue degradation rates for the olfactory bulb, whole brain and spinal cord. We found that the olfactory bulb had both the highest transport rate across the blood-brain barrier (BBB) and the highest rate of degradation. Because a higher degradation rate would decrease, not increase, tissue concentrations of insulin, BBB transport may be the primary mechanism by which high concentrations of insulin are maintained within the olfactory bulb. This illustrates an adaptive aspect of the BBB in its regulation of the exchange of information molecules between the blood and the CNS.