Radioactively iodinated cyclo(His-Pro) crosses the blood-brain barrier and reverses ethanol-induced narcosis

Abstract

Cyclo(His-Pro) (cHP) is a peptide widely distributed in the central nervous system (CNS) and peripheral tissues that can affect brain function after either peripheral or CNS administration. This suggests that cHP may be a neuromodulator capable of crossing the blood-brain barrier (BBB). We, therefore, studied the ability of radioactively labeled cHP (I-cHP) to cross the BBB. We found that I-cHP can cross the BBB in either the direction of blood to brain or brain to blood by nonsaturable mechanisms. The rate of entry of I-cHP into the CNS was low in comparison with other peptides, especially considering its relatively low molecular weight and high lipid solubility. However, this slow entry was offset by a long half-life in blood and extreme enzymatic resistance, allowing cHP to accumulate in the CNS. This accumulation was sufficient to allow intravenous cHP to reverse ethanol-induced narcosis, an effect mediated through the CNS. The rate of entry of I-cHP was resistant to conditions that alter the passage of some other substances across the BBB or that have been shown to affect cHP metabolism such as aging, diabetes, and pretreatment with aluminum. Entry of cHP into the brain was not retarded by binding to serum proteins. Significant amounts of I-cHP entered the serum, brain, and other tissues after intraperitoneal administration, the route used in many studies of cHP. Taken together, these results show that cHP is a highly stable peptide that, after intravenous injection, slowly enters the brain by a nonsaturable mechanism in amounts large enough to affect such aspects of the CNS as ethanol-induced narcosis.