The source of cerebral insulin.

Insulin and its receptor are found throughout the central nervous system (CNS). Insulin administered into the CNS can exert powerful effects, yet the consensus is that little or no insulin is produced in the CNS. Therefore, CNS insulin is essentially dependent on the ability of peripheral insulin to cross the blood-brain barrier (BBB). Insulin is known to cross the BBB by a saturable transport mechanism. This transporter shows some thematic similarities to other transporters for peptides or regulatory proteins. It is unevenly distributed throughout the CNS with the olfactory bulbs having the fastest transport rate of any brain region. It is partially saturated at euglycemic levels, suggesting that its main signaling function occurs at physiological blood levels, rather than as a brake to hypoglycemic events. One probable function of the BBB transporter is to allow CNS insulin to act as a counter-regulatory hormone to peripheral insulin. The transporter is regulated, with the transport rate of insulin being altered during development and by fasting, obesity, hibernation, diabetes mellitus and Alzheimer's disease. Enhancement of insulin transport by lipopolysaccharide could be the basis for the insulin resistance seen with bacterial infections. Inhibition of insulin transport across the BBB by dexamethasone could be the basis for the enhanced appetite seen with glucocorticoid treatments. Insulin itself also has effects on the BBB, altering enzymatic and transporter functions. Overall, BBB transport of insulin provides a mechanism for peripheral insulin to act within the CNS as a regulatory peptide.