Des-Tyr1-gamma-endorphin (DT gamma E) and des-enkephalin-gamma-endorphin (DE gamma E): plasma profile and brain uptake after systemic administration in the rat

Behavioural and physiological effects of electrical stimulation in the nucleus accumbens: a review

Electrical stimulation (ES) in the brain is becoming a new treatment option in patients with treatment-resistant obsessive-compulsive disorder (OCD). A possible brain target might be the nucleus accumbens (NACC). This review aims to summarise the behavioural and physiological effects of ES in the NACC in humans and in animals and to discuss these findings with regard to neuroanatomical, electrophysiological and behavioural insights. The results clearly demonstrate that ES in the NACC has an effect on reward, activity, fight-or-flight, exploratory behaviour and food intake, with evidence for only moderate physiological effects. Seizures were rarely observed. Finally, the results of ES studies in patients with treatment-resistant OCD and in animal models for OCD are promising.

Investigation of the metabolism of substance P at the blood-brain barrier using capillary electrophoresis with laser-induced fluorescence detection

Substance P (SP) metabolism was investigated upon exposure to a monolayer of bovine brain microvessel endothelial cells (BBMECs), a cell culture model of the blood-brain barrier. SP was incubated with the BBMECs and its metabolism was followed as a function of time over a 5-h period. The resulting samples were derivatized with naphthalene-2,3-dicarboxaldehyde (NDA)/cyanide, separated, and detected using cyclodextrin-modified electrokinetic chromatography with laser-induced fluorescence detection (CDMEKC-LIF). Upon exposure to the BBMEC monolayer, SP rapidly degraded to produce the N-terminal (1-9), (1-4) and (1-7) and C-terminal (2-11) and (3-11) fragments. These results were compared with those in an earlier report from our laboratory, where SP metabolism was investigated in vivo by microdialysis sampling in rat striatum.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

Drug transport across the blood-brain barrier. III. Mechanisms and methods to improve drug delivery to the central nervous system

This is the third part of a review on the transport of drugs across the blood-brain barrier. In the first two parts, the anatomical and physiological aspects and the various techniques that can be used to study blood-brain transport have been discussed and reviewed. This third part focuses specifically on the mechanisms that are involved in drug transport across the blood-brain barrier. In addition, the opportunities to improve drug transport into the brain will be reviewed. Emphasis is on the transport of peptides.

Delivering peptides to the central nervous system: dilemmas and strategies

Peptides have been shown to cross the blood-brain barrier (BBB) as intact molecules so that they can influence the central nervous system. Peptides cross by saturable and nonsaturable mechanisms in the direction of both brain to blood and blood to brain. Passage of peptides, especially by saturable transport, has been shown to be influenced by pharmacological agents and physiological events. These findings support the view that peptides or their analogues could be useful as therapeutic agents for disorders of the central nervous system. They also suggest strategies in approaching therapeutic goals, including manipulating transport rates, targeting diseases due to altered BBB-peptide interactions, and designing analogues capable of taking advantage of such mechanisms of passage as paracellular transmembrane diffusion and brain-to-blood transport.

Bioanalysis of the neuropeptide des-enkephalin-gamma-endorphin by high-performance liquid chromatography with on-line sample pretreatment using gel permeation and solid-phase isolation

A bioanalytical method is described that allows the determination of a number of beta-endorphin-related peptides. The method is based on the application of fluorescence detection after high-performance liquid chromatography followed by post-column derivatization with o-phthaldialdehyde. Concentrations exceeding 10-25 ng/ml could be determined by using conventional fluorescence detection, whereas lower concentrations demand the use of laser-induced fluorescence detection. The sample pretreatment includes the use of on-line gel permeation, on-line solid-phase isolation and heart cutting of a peak from reversed-phase gradient elution. The sample pretreatment procedure does not discriminate between the dodecapeptide des-enkaphalin-gamma-endorphin (DE gamma E) and its metabolites in order to obtain similar recoveries for all components. The final chromatographic phase system is based on ion-pair formation, which permits the separation of DE gamma E from its metabolites and degradation products. The optimized procedure allows the determination of these peptides in plasma at concentration levels down to about 1 ng/ml, demanding a sample volume of 1 ml.

Bioanalysis of the peptide des-enkephalin-gamma-endorphin. On-line sample pretreatment using membrane dialysis and solid-phase isolation

Des-enkephalin-gamma-endorphin is a neuroleptic endogenous peptide that is active in the central nervous system in extremely low concentrations. The pharmacokinetics of this peptide could not be studied in detail as a bioanalytical method for determining endogenous levels of this peptide in biological matrices was not available. Liquid chromatography with fluorescence reaction detection in principle offers sufficient sensitivity for this application, provided that a selective sample pretreatment can be performed. The development of a pretreatment method for plasma samples is described. After protein precipitation with trichloroacetic acid, high-molecular-weight compounds are removed using on-line continuous-flow dialysis. After dialysis, polar low-molecular-weight compounds, including those containing amino functions, are removed by solid-phase isolation, while simultaneously the analyte is concentrated. By means of valve switching the pretreatment system is coupled on-line to the liquid chromatographic system. With the developed system it is possible to determine desenkephalin-gamma-endorphin in plasma in the range 10-100 ng/ml.

The opioid system in neurologic and psychiatric disorders and in their experimental models

Evidence from experimental and clinical studies suggests the involvement of the endogenous opioid system in several neurologic and psychiatric disorders (Alzheimer's, Huntington's and Parkinson's diseases, drug-induced movement disorders, Gilles de la Tourette syndrome, stroke, ischemia, brain and spinal cord injury, epilepsy, schizophrenia and affective disorders). However, its involvement is rather a secondary one, perhaps being a severe consequence of a primary, nonopioid disturbance. Thus, treatment of an opioidergic manifestation of a disorder of nonopioidergic origin is necessarily symptomatic and targets only the restoration of the opioid system; such treatment may be beneficial in ameliorating the clinical symptoms of the disorder.

Plasma uptake and in vivo metabolism of [Leu]enkephalin following its intraperitoneal administration to rats

To understand better how [Leu]enkephalin (LE) acts to modulate learning and memory in rats, the plasma uptake, disappearance, and metabolism of LE were investigated following its intraperitoneal administration. Concentrations of [3H]-LE and its radioactive metabolites were determined by thin layer chromatography in plasma samples withdrawn from rats at various times after injection of peptide. As measured in rats receiving an IP injection of a dose of LE (3 micrograms/kg) that impairs active avoidance conditioning, the LE was very rapidly metabolized, with greater than 95% of plasma [3H] in the form of metabolites by 1 min after injection. Despite this rapid metabolism, low but measurable quantities of intact LE were detectable in plasma at all sampling times. Consistent with a greater potency of D-Ala2-[D-Leu5]enkephalin (DADLE) than of LE in modulating avoidance conditioning, DADLE was less rapidly metabolized than was LE following its IP administration. The metabolism of DADLE and LE in vivo was more rapid than it was in plasma in vitro, suggesting a role for membrane bound enzymes in the metabolism of IP-administered enkephalins. The data demonstrate that, despite a rapid hydrolysis of LE in vivo, sufficient LE is present in plasma following IP administration of a behaviorally active dose to support a role of circulating intact LE in the modulation of avoidance conditioning.

Des-enkephalin-gamma-endorphin (DE gamma E): pharmacokinetics in dogs after intravenous and subcutaneous administration

After intravenous dosing in dogs [3H-Lys9]-DE gamma E (Org 5878) was very rapidly eliminated from the circulation. Disappearance of the neuropeptide from blood followed a biphasic decay with half-lives of 0.6 +/- 0.1 min (+/- S.D.; alpha-phase) and 2.4 +/- 1.0 min (beta-phase). The central volume of distribution ranged between 0.05 and 0.23 l.kg-1. The mean blood clearance rate amounted to 0.15 l.min-1.kg-1, which is indicative of extensive hepatic and extrahepatic metabolism of DE gamma E. In contrast to intravenous dosing, subcutaneous injection of [3H]-DE gamma E in dogs resulted in low but relatively long-lasting peptide levels in blood. Peak values were found at 5-10 min, whereafter they declined to the limit of detection at 1.5-2 h. The bioavailability of DE gamma E for this route of administration was shown to be 20-23%.

Des-enkephalin-gamma-endorphin (DE gamma E): biotransformation in rat, dog and human plasma

Biotransformation of [3H-Lys9] DE gamma E was investigated after in vitro incubation of the tritiated peptide with rat, dog and human plasma. In addition, its metabolite profile in blood was studied following intravenous administration to rats and dogs. Half-lives for the in vitro disappearance of DE gamma E in plasma were 13.0 +/- 0.8 min (dog), 15.7 +/- 1.2 min (rat) and 19.2 +/- 0.9 min (human), indicating very rapid degradation of the peptide by proteolytic enzymes. Biotransformation products were identified on the basis of co-chromatography on HPLC with synthetic reference peptides. The six principal fragments appeared to be beta-endorphin (beta E) sequences 7-17, 8-17, 9-17, 6-15, 7-15 and 8-15. The abundance of beta E6-15, beta E7-15 and beta E8-15 in rat and human plasma suggests preferential, subsequent carboxypeptidase and aminopeptidase mechanisms, whereas in dog plasma DE gamma E is predominantly degraded by aminopeptidase activities (major peptide metabolites: beta E7-17 and beta E8-17). In the in vivo studies with rats and dogs the same radioactive peptide fragments were detected in blood as found in the in vitro experiments with plasma. In both species their blood levels were already maximal within a minute after intravenous administration of the parent peptide, thereafter they declined rapidly. 3H-Lysine was the main radioactive metabolite in vivo, exceeding 70% of total radioactivity in rat and dog blood 10 min after 3H-DE gamma E dosing.

Des-enkephalin-gamma-endorphin: bioavailability in rats following the subcutaneous and intramuscular route of administration

A pharmacokinetic study with [3H]des-enkephalin-gamma-endorphin (3H-DE gamma E) was performed in rats after the intravenous, subcutaneous and intramuscular route of administration. Disappearance of non-metabolized 3H-DE gamma E from blood upon intravenous dosing followed a biphasic decay with half-lives of 0.7 +/- 0.3 (+/- S.D.) min for the initial distribution phase and 6.3 +/- 2.7 min for the terminal elimination phase. The central and peripheral volumes of distribution were strikingly high (0.38 and 0.55 1 X kg-1, respectively). Extensive metabolism occurred already within the first minutes after injection. The blood clearance rate was found to be 0.29 +/- 0.12 1 X min-1 X kg-1, which value points to remarkable extrahepatic elimination of the neuropeptide. As compared to the intravenous route of administration, subcutaneous or intramuscular injection of 3H-DE gamma E resulted in low but longer-lasting peptide levels in blood. These levels reached already peak values at 2 min after both routes of administration and then declined to below the limit of detection at 2-3 h. The absolute bioavailability of DE gamma E after subcutaneous injection amounted to 30.9 +/- 16.3% (range 16.0-46.9%), whereas the bioavailability after intramuscular injection was observed to be 3.5 times lower (8.5 +/- 3.0%; range 4.6-12.0%). These data suggest that subcutaneous dosing of DE gamma E might be more effective in displaying CNS activity than the intramuscular route.