Glycosylated neuropeptides: A new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.