The enkephalins. Peptides with morphine-like activity

Role of the basal ganglia in innate and learned behavioural sequences

Integrating individual actions into coherent, organised behavioural units, a process called chunking, is a fundamental, evolutionarily conserved process that renders actions automatic. In vertebrates, evidence points to the basal ganglia - a complex network believed to be involved in action selection - as a key component of action sequence encoding, although the underlying mechanisms are only just beginning to be understood. Central pattern generators control many innate automatic behavioural sequences that form some of the most basic behaviours in an animal's repertoire, and in vertebrates, brainstem and spinal pattern generators are under the control of higher order structures such as the basal ganglia. Evidence suggests that the basal ganglia play a crucial role in the concatenation of simpler behaviours into more complex chunks, in the context of innate behavioural sequences such as chain grooming in rats, as well as sequences in which innate capabilities and learning interact such as birdsong, and sequences that are learned from scratch, such as lever press sequences in operant behaviour. It has been proposed that the role of the striatum, the largest input structure of the basal ganglia, might lie in selecting and allowing the relevant central pattern generators to gain access to the motor system in the correct order, while inhibiting other behaviours. As behaviours become more complex and flexible, the pattern generators seem to become more dependent on descending signals. Indeed, during learning, the striatum itself may adopt the functional characteristics of a higher order pattern generator, facilitated at the microcircuit level by striatal neuropeptides.

Metabolism and absorption enhancement of methionine enkephalin in human nasal epithelium

The objective of this study was to investigate absorption enhancing approaches for systemic delivery of methionine enkephalin via the nose. Absorption promotion of methionine enkephalin in the presence of protease inhibitors (bestatin, puromycin) and absorption enhancers (glycocholate, dimethyl-beta-cyclodextrin) were investigated in human nasal epithelium. Co-administration of the peptide with protease inhibitors and absorption enhancers resulted in a remarkable increase in Met-Enk permeation (4- to 94-fold). The increase was proportional to transepithelial resistance reduction and permeation of paracellular marker dye. Perturbation of the epithelial tight junctions seen in vitro may not occur in vivo due to mucus protection and mucociliary clearance.

Transmucosal delivery of leucine enkephalin: stabilization in rabbit enzyme extracts and enhancement of permeation through mucosae

Leucine enkephalin (Tyr-Gly-Gly-Phe-Leu; Leu-Enk) is a naturally occurring peptide that has been shown to have pain modulating properties. To evaluate the feasibility of using various absorptive mucosae as a route of systemic delivery, the stability of Leu-Enk and the effect of enzyme inhibitors (e.g., amastatin, EDTA, and thimerosal) on stabilization and permeation of Leu-Enk through rabbit mucosae in the presence of dihydrofusidates were investigated. Enzymes in the nasal, rectal, and vaginal mucosae were extracted and Leu-Enk (50 micrograms/mL) was added to each of the enzyme extracts and incubated to determine the kinetics and mechanism of degradation. The rate of degradation in the extracts in the absence of inhibitors followed the order: rectal > vaginal > nasal. Whereas EDTA had the best stabilizing effect on Leu-Enk, thimerosal was the best stabilizer for the degradation intermediates. A combination of amastatin (50 microM), EDTA (5 mM), and thimerosal (50 microM) had the greatest stabilizing effect on Leu-Enk and its degradation intermediates. For permeation studies, each mucosa was mounted onto a Valia-Chien permeation cell with Leu-Enk (200 micrograms/mL) in isotonic phosphate buffer (as donor solution). The enhancers used for the study were sodium tauro-dihydrofusidate (STDHF), sodium glycodihydrofusidate (SGDHF), and phosphato-dihydrofusidate (PHDHF). The greatest effect was achieved by PHDHF for all the mucosae. STDHF had a significant effect only on the rectal permeation, whereas SGDHF had significant effects on rectal and vaginal mucosae. Mechanisms by which the dihydrofusidates enhance permeating may involve micelle formation.(ABSTRACT TRUNCATED AT 250 WORDS)

The binding of opioid peptides to the Mcg light chain dimer: flexible keys and adjustable locks

Enkephalins and beta-casomorphins (opioid peptides) were found to bind in a variety of conformations to a human light chain (Bence-Jones) dimer from a patient (Mcg) with amyloidosis. The peptides were diffused into crystals of the protein and their positions, relative occupancies and modes of binding were determined at 2.7 A resolution by difference Fourier analyses. Collectively, the opioid peptides occupied practically all of the available space in the concave, internal parts of the binding region, as well as flat or convex external surfaces around the rim of the binding cavity. Suitable ligands ranged in size from four to seven residues. As many as five residues could be accommodated inside the binding region, and there was space for at least four residues on the external surfaces. External binding was influenced by solvent effects and local packing interactions among adjacent protein molecules in the crystal lattice. In the enkephalin series the presence of amino-terminal tyrosine was necessary, but not sufficient for binding. [Met]-enkephalin, a pentapeptide, showed two different modes of binding in overlapping subsites. In one subsite, preferred over the second in a ratio of 1.3:1.0, the side chain of amino-terminal tyrosine penetrated through the floor of the main cavity to lodge in the deep binding pocket about 20 A from the entrance. The remainder of the peptide spanned the length of the main cavity in an extended conformation. In the second subsite the amino end was restricted to the main cavity and the peptide backbone turned abruptly upward at residue 3 to interact with external surfaces. An (Arg-6, Phe-7) heptapeptide extension of [Met]-enkephalin entered the deep pocket and assumed an extended conformation in the main cavity like the pentapeptide. Its last two residues flattened against the external surfaces. [Leu]-enkephalin and its analogues displayed a combination of internal and external binding like [Met]-enkephalin in its secondary subsite. Enkephalin analogues with D-amino acids in position 2 generally adopted conformations which were more convoluted than those in the L-isomers. Moreover, external interactions tended to be more prominent in the D-derivatives. The beta-casomorphin-7 heptapeptide penetrated into the deep pocket and traversed the main cavity in as extended a conformation as the presence of two proline residues would permit. On removal of the ligand there was an unexpected hysteresis effect involving permanent structural alterations in the walls of the binding region. beta-casomorphins-4 and -5 were bound in the main cavity with the carboxyl ends protruding from the entrance.(ABSTRACT TRUNCATED AT 400 WORDS)