The accumulation of (3H) enkephalinamide (2-D-alanine-5-methioninamide) in rat brain tissues

Kinetic analysis of leucine-enkephalin cellular uptake at the luminal side of the blood-brain barrier of an in situ perfused guinea-pig brain

The uptake of enkephalin-(5-L-leucine) (Leu-enkephalin) at the luminal side of the blood-brain barrier was measured by means of an in situ vascular brain perfusion technique in the anaesthetized guinea pig. This method allows measurements of cerebrovascular peptide uptake over periods of up to 20 min, and excludes the solute under study from the general circulation and systemic metabolic influences. A capillary unidirectional transfer constant, Kin, for [tyrosyl-3,5-3H]Leu-enkephalin was estimated graphically from the multiple-time brain uptake data in the presence of different concentrations of unlabelled peptide, and dose-dependent self-inhibition was demonstrated. Analysis of unidirectional influx of blood-borne Leu-enkephalin into the brain revealed Michaelis-Menten saturation kinetics in the parietal cortex, caudate nucleus, and hippocampus, with Vmax between 0.14 and 0.16 nmol min-1 g-1 and Km ranging from 34 to 41 microM, for the saturable component, whereas the estimated diffusion constant, Kd, was not significantly different from zero. Entry of [3H]Leu-enkephalin was not inhibited in the presence of either a 5 mM concentration of unlabelled L-tyrosine, tyrosylglycine, and tyrosylglycylglycine, or aminopeptidase inhibitor, bestatin (0.5 mM), suggesting that the saturable mechanism of the tracer at the luminal side of the blood-brain barrier does not involve uptake of the peptide's N-terminal amino acid and/or its tyrosine-containing fragments.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neurotransmitters on the system that transports Tyr-MIF-1 and the enkephalins across the blood-brain barrier: a dominant role for serotonin

Neurotransmitters and neuropeptides interact in several ways. We studied a new type of interaction: the effect of neurotransmitters on the saturable system that transports Tyr-MIF-1 and the enkephalins out of the central nervous system (CNS). The neurotransmitters were introduced into the lateral ventricle of the brain with radioiodinated peptide, using an established method previously shown to accurately quantify the amount of peptide being transported from the CNS to the blood. Serotonin inhibited transport, histamine stimulated transport, and dopamine, acetylcholine, epinephrine, GABA, kainic acid, cAMP and cGMP were without effect. Cyproheptadine, a serotonin antagonist, stimulated transport. Of several psychotropic agents tested, only tranylcypromine had a statistically significant effect and stimulated transport. Of the serotonin receptor specific agents tested, those with 5HT1 activity most consistently affected transport. We conclude that serotonin, and perhaps histamine, are important modulators of the system that transports Tyr-MIF-1 and the enkephalins out of the CNS.

Unidirectional uptake of enkephalins at the blood-tissue interface of the blood-cerebrospinal fluid barrier: a saturable mechanism

The cellular uptake at the blood-tissue interface of the blood-cerebrospinal fluid (CSF) barrier to tyrosyl-3,5-[3H]enkephalin-[5-L-leucine] (abbreviated to Leu-enkephalin) and of its synthetic analogue D-alanine2-tyrosyl-3,5-[3H]enkephalin-[5-D-leucine] (abbreviated to D-Ala2-D-Leu5-enkephalin) was studied in the isolated perfused choroid plexuses from the lateral ventricles of the sheep, using the rapid (less than 30 s), single circulation, paired-tracer dilution technique, in which D-[14C]-mannitol serves as an extracellular marker. Cellular uptake of peptides was estimated by directly comparing venous dilution profiles of [3H] and [14C] radioactivities in the absence and presence of unlabelled peptide, the N-terminal amino acid (L-tyrosine), the typical L-transport system substrate, 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) and the inhibitor of aminopeptidase activity, bacitracin. The cellular uptake of both enkephalins was strongly (65-76%) but not completely inhibited by the addition of 5 mM unlabelled peptide to the bolus; the self-inhibition was significantly higher for D-Ala2-D-Leu5-enkephalin than for Leu-enkephalin. The addition to the bolus of L-tyrosine (5 mM), BCH (10 mM) or bacitracin (2 mM) reduced the 3H-radioactivity uptake by the choroid plexus of both enkephalins by 20-40%, the degree of inhibition being greater for [3H]-Leu-enkephalin than for its analogue. It is concluded that during single passage of enkephalins through the choroid plexus circulation, unidirectional uptake at the blood-tissue interface of the blood-CSF barrier consists of two components; a saturable component, which represents uptake of the intact peptide by the choroid epithelium, and a non-saturable component, which reflects enzymatic degradation of peptide in the blood and/or at the barrier, with a liberation of the N-terminal tyrosyl residue. Higher penetration of the blood-CSF barrier by D-Ala2-D-Leu5-enkephalin can be attributed to its greater resistance to hydrolysis.

Saturable transport of peptides across the blood-brain barrier

Peptides can be transported across the blood-brain barrier by saturable transport systems. One system, characterized with radioactively labeled Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide), is specific for some of the small peptides with an N-terminal tyrosine, including Tyr-MIF-1, the enkephalins, beta-casomorphin, and dynorphin (1-8). Another separate system transports vasopressin-like peptides. The choroid plexus has at least one system distinguishable from those above that is capable of uptake and possibly transport of opiate-like peptides. The possibility of saturable transport of other peptides has been investigated to a varying degree. Specificity, stereo-specificity, saturability, allosteric regulation, modulation by physiologic and pharmacologic manipulations, and noncompetitive inhibition have been demonstrated to occur in peptide transport systems and suggest a role for them in physiology and disease.

Tyr-MIF-1 and Met-enkephalin share a saturable blood-brain barrier transport system

Previous studies have shown that methionine enkephalin and Tyr-MIF-1 are transported from the brain to the blood by a saturable, stereospecific, carrier-mediated process. It was not established by these studies whether Tyr-MIF-1 and methionine enkephalin were transported by the same system or by separate, but overlapping systems. This issue was investigated in anesthetized mice receiving injections containing both 131I-methionine enkephalin and 125I-Tyr-MIF-1 into the lateral ventricle of the brain. Mice were decapitated and the brain to blood transport rate was derived from the residual counts in the brain. It was found that in individual mice, the transport rate for Tyr-MIF-1 correlated highly with the transport rate for methionine enkephalin but not with the transport of iodide. This shows that the transport of Tyr-MIF-1 is closely coupled to the transport of methionine enkephalin but dissociable from the brain to blood transport of iodide. Furthermore, the inability of varying doses of Tyr-MIF-1 or of methionine enkephalin to preferentially self-inhibit is radiolabeled form in comparison with the other peptide shows that, functionally, only a single system exists. Aluminum, a noncompetitive inhibitor of Tyr-MIF-1 transport, was also without preferential inhibition. Thus, under the conditions of these studies, only a single system could be functionally demonstrated for the transport of both Tyr-MIF-1 and methionine enkephalin.

Binding of Tyr-MIF-1 to isolated brain capillaries

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) and the enkephalins have previously been shown with in vivo studies to be transported out of the brain by a saturable, carrier-mediated system. The possibility that this transport system occurs at the level of the endothelial cells was tested by measuring the ability of isolated brain capillaries to bind radioiodinated Tyr-MIF-1. The binding of labeled Tyr-MIF-1 was displaced by unlabeled Tyr-MIF-1 (50% inhibition at 1.5 X 10(-6) M) and by nonradioactive (I127) iodinated Tyr-MIF-1, but not by tyrosine, MIF-1 (Pro-Leu-Gly-amide), Tyr-Pro, or D-Tyr-MIF-1. This pattern of inhibition is similar to that found for the in vivo transport of Tyr-MIF-1 out of the brain. Tyr-MIF-1 did not appear to be bound by albumin or degraded by cells aged 3-7 days at 4 degrees C. These results suggest that the transport system previously described in vivo for Tyr-MIF-1 and the enkephalins may be associated with the brain endothelial cells.

Carrier-mediated transport of enkephalins and N-Tyr-MIF-1 across blood-brain barrier

The saturable, carrier-mediated system capable of the brain-to-blood transport of small peptides with an N-terminal tyrosine was characterized. The rate of disappearance of intraventricularly injected iodinated peptide in the presence or absence of the inhibitor being tested was determined from formulas based on the residual radioactivity in the brains of mice after decapitation. The injection of 100 nmol/mouse of unlabeled N-Tyr-MIF-1 (TMIF) increased the half-time disappearance of 125I-TMIF (ITMIF) in the central nervous system (CNS) from 14.1 to 88.7 min (P less than 0.00005). Technetium, a substance transported out of the brain by the same system that transports iodine, was used as a control; the half-time disappearance of technetium pertechnetate was unaffected by unlabeled TMIF. With two related but distinct techniques, the maximum transport rate out of the CNS (Vmax) for TMIF was 0.266 nmol X g of brain per min (method 1) and 0.297 nmol X g-1 X min-1 (method 2), while the amount of unlabeled material needed to achieve 50% of Vmax (Km) was 15.2 nmol/g (method 1) and 15.1 nmol/g (method 2). The lack of effect of the tyrosinated fragments of TMIF as inhibitors indicates that TMIF is being transported in intact form. The Vmax for methionine enkephalin determined with labeled and unlabeled methionine enkephalin was 0.630 nmol X g-1 X min-1 and the Km was 24.95 nmol/g. Studies with the metabolic modulators furosemide, acetozolamide, reserpine, ouabain, and theophylline suggest that the system is sodium dependent and probably independent of ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptides and the blood-brain barrier: lipophilicity as a predictor of permeability

Eighteen peptides were examined for penetration across the blood-brain barrier (BBB) in rats. Iodinated peptides were injected via the carotid artery and 5 sec later the rats were decapitated. The results were expressed as a brain to blood ratio. The results showed that small amounts of most peptides enter the brain. The octanol coefficient, a measurement of lipophilicity, was a good predictor of penetration for most peptides. The low molecular weight, N-tyrosinated peptides (leucine-enkephalin, methionine enkephalin, N-Tyr-MIF-1, and N-Tyr-FMRF), that have been shown to be transported out of the brain by a saturable, carrier-mediated system, had much lower penetration rates than those predicted by their octanol coefficients. Molecular weight, percent of unbound peptide, total charge, net charge, and absolute charge were not good predictors of peptide penetration. Pretreatment of the rats with IP aluminum, which has been suggested to increase lipophilic permeability of the BBB, enhanced passage of 15 of the 18 peptides but not radioiodinated serum albumin or radioactive red blood cells. Thus, of the various physicochemical characteristics of peptides tested here, lipophilicity was most important in determining penetration of peptides across the BBB. However, the existence of one class of peptides that deviates from this trend (those of low molecular weight with an N-tyrosine) and the variability among the remaining peptides suggests that other, unidentified factors may be important in the prediction of peptide penetration across the BBB.

Permeability of the blood-brain barrier to neuropeptides: the case for penetration

Evidence that peptides can cross the blood-brain barrier (BBB) is reviewed. Penetration is suggested by the observations that blood levels correlate with cerebrospinal fluid levels for many peptides and that peripheral administration of peptides results in effects on the CNS. Passage is confirmed by experiments involving administration of a peptide (immunoactive or radioactive) in one compartment and identification of its appearance in the other, supported by such methods as selective labeling, cross-reactivity with highly specific antibodies, and chromatography. The degree of passage varies among peptides and their analogs. The major route of passage is probably by a non-competitive, non-saturable mechanism, wih the physicochemical characteristics of the peptide (e.g. lipophilicity, charge, molecular weight, and protein binding) determining the degree of passage. A competitive transport mechanism also exists for some peptides. Penetration of the BBB via large pores or by pinocytosis does not appear to be of major importance for peptides. Permeability of the BBB to peptides, but not to the larger iodinated albumin, is affected by intraperitoneal administration of aluminum, apparently by an increase in the permeability of the membrane to lipophilic materials.

A brain-to-blood carrier-mediated transport system for small, N-tyrosinated peptides

Recently, we found that lipophilicity is a good predictor of the degree to which most peptides cross the blood-brain-barrier. Small (MW less than 1000) peptides with an N-terminal tyrosine, however, penetrated to a much smaller degree than was predicted by their measurements of lipophilicity. We show here that two such peptides, N-Tyr-MIF-1 and Met-enkephalin, can significantly inhibit transport of 125I-N-Tyr-MIF-1 out of the rat brain in vivo in a saturable, dose-dependent way. The half-time disappearance of injected 125I-N-Tyr-MIF-1 from the rat brain was 12.4 min but when injected with 200 nmol/animal of unlabeled N-Tyr-MIF-1 was 23.6 min (p less than 0.01). The Km was calculated to be 0.123 nmol. At higher doses, leucine, but not tyrosine, alanine, glutamine, MIF-1, or the dipeptide Gly-Gly, also significantly inhibited transport out of the brain.

Accumulation of peptides by choroid plexus in vitro: Tyr-D-Ala-Gly as a model

Accumulation of Tyr-D-Ala-Gly (TAG) in rat choroid plexus was studied in vitro. Choroid plexus of the lateral ventricles and the fourth ventricle accumulated TAG against a concentration gradient. This accumulation was inhibited by some metabolic inhibitors and some peptides, but not by amino acids. The change and stereo-configuration of peptides had great influence on the accumulation. Metenkephalin was one of the strongest inhibitors. Absence of sodium ions in the medium did not affect the accumulation, but increase or decrease of potassium ions reduced it. Injection of reserpine for chemical denervation of sympathetic nerves or bilateral removal of the superior cervical ganglion had no effect. These results indicate that choroid plexus has different transport systems for amino acids and peptides, which are not affected by denervation of the sympathetic nerves that innervate choroid plexus.