Saturable brain-to-blood transport of endomorphins.

Opiate-modulating tetrapeptides such as tyrosine-melanocyte-stimulating hormone-release inhibiting factor-1 (Tyr-MIF-1; Tyr-Pro-Leu-Gly-NH2) and Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2) are saturably transported from brain to blood. We examined whether two recently described endogenous opiate tetrapeptides with similar structures, the mu-specific endomorphins, also are transported across the blood-brain barrier (BBB). We found that the efflux rates of endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) were each self-inhibited by an excess of the respective endomorphin, thereby defining saturable transport. Cross-inhibition of the transport of each endomorphin by the other indicated shared transport. By contrast, no inhibition of the efflux of either endomorphin resulted from coadministration of Tyr-MIF-1, indicating that peptide transport system-1 (PTS-1) was not involved. Tyr-W-MIF-1, which is partially transported by PTS-1, significantly (P<0.01) decreased the transport of endomorphin-1 and tended (P=0.051) to decrease the transport of endomorphin-2, consistent with its role as both an opiate and antiopiate. Although involved in modulation of pain, coinjection of calcitonin gene-related peptide or constriction of the sciatic nerve did not appear to inhibit endomorphin efflux. Thus, the results demonstrate the existence of a new efflux system across the BBB which saturably transports endomorphins from brain to blood.

Transport of brain-derived neurotrophic factor across the blood-brain barrier.

Brain-derived neurotrophic factor (BDNF) is a potential therapeutic agent for degenerative disorders of the central nervous system. In this report, we investigated the ability of BDNF to cross the blood-brain barrier (BBB). BDNF was stable in blood up to 60 min after i.v. injection, with evidence for aggregation, and had an early, rapid influx into brain. By 10 min, most of the BDNF sequestered by the cerebral cortex was associated with the parenchyma rather than with the endothelial cells, demonstrating complete passage across the BBB. A small dose of unlabeled BDNF enhanced the entry of 125I-BDNF from blood to brain after an i.v. bolus injection, whereas larger doses had no effect. In contrast, a large dose of unlabeled BDNF inhibited the influx of 125I-BDNF during in situ brain perfusion. After intracerebroventricular injection, the efflux of BDNF from brain to blood occurred at a rate similar to that for reabsorption of cerebrospinal fluid, and no evidence for self-inhibition was found. Therefore, we conclude that intact BDNF in the peripheral circulation crosses the BBB by a high-capacity, saturable transport system.

Effects of various reproductive hormones on the penetration of LHRH across the blood-brain barrier.

A previous study has shown bidirectional saturable transport of LHRH across the blood-brain barrier. Here, the effects of the steroids progesterone and beta-estradiol and the pituitary glycoproteins luteinizing hormone (LH) and follicle stimulating hormone (FSH) on the bidirectional transport rate were determined. No statistically significant difference in brain to blood transport of 125I-LHRH was found in mice given ICV progesterone (1 and 100 pmol/mouse), beta-estradiol (1 and 100 pmol/mouse), FSH (10 and 1000 pmol/mouse) or LH (100 and 1000 pmol/mouse). Blood to brain transport of 125I-LHRH, tested in rats with a carotid artery perfusion method, was not affected by inclusion of progesterone (100 nmol/ml), beta-estradiol (100 nmol/ml), LH (2 and 10 nmol/ml), or FSH (10 nmol/ml) in the perfusate. We conclude, therefore, that unlike its release from the hypothalamus, the exchange of LHRH between the CNS and blood is unlikely to be influenced by reproductive hormones.

Bidirectional saturable transport of LHRH across the blood-brain barrier.

Systemic administration of luteinizing hormone-releasing hormone (LHRH) in rats has been found to influence behavior independently of pituitary or ovarian function. A previous study has shown that LHRH can cross the blood-brain barrier in one direction, but it was not known whether this was due to a saturable transport system. The rate of entry of 125I-labeled LHRH from blood to brain was determined by two different single-pass methods of carotid perfusion. The first, a multiple time point method, measures Ki from the slope of the linear regression when brain-to-blood ratios of radioiodinated LHRH are plotted against time. Saturable transport was determined by the difference between the Ki of rats perfused with 125I-LHRH (12.51 X 10(-3) mg.g-1.min-1) vs. rats perfused with 125I-LHRH and unlabeled LHRH (10 nmol/ml; 2.20 X 10(-3) ml.g-1.min-1). The inhibition by the unlabeled peptide was statistically significant (P less than 0.001). The second method, a single time point technique, measures the cerebrovascular permeability-surface area coefficient (PA). Saturable transport was determined in rats by the competition of unlabeled LHRH with 125I-LHRH. The PA value for 125I-LHRH (20.00 X 10(-3) ml.g-1.min-1) was significantly greater (P less than 0.05) than for 125I-LHRH with the addition of 10 nmol/ml unlabeled LHRH (4.14 X 10(-3) ml.g-1.min-1). Saturable transport of LHRH from brain to blood in mice was also determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Permeability of the murine blood-brain barrier to some octapeptide analogs of somatostatin.

Analogs of somatostatin are being investigated clinically for the treatment of various malignancies, including brain tumors. We studied the ability of three therapeutically promising radioactively labeled somatostatin octapeptide analogs, RC-160, RC-121, and RC-161, to cross the blood-brain barrier (BBB) after peripheral or central injection. After i.v. injection, intact RC-160 was recovered from the blood and the brain. The entry rates were different from each compound but were generally low. By contrast, entry across the intact BBB increased 220 times when RC-160 was given in a serum-free perfusate. This suggests that some serum-related factor, probably the previously described protein binding or an aggregation-promoting factor, is the main determinant in limiting the blood-to-brain passage of somatostatin analogs. Entry into the brain was not inhibited by the addition of unlabeled analog to the perfusate, showing that passage was probably by diffusion across the membranes that comprise the BBB rather than by saturable transport. By contrast, a saturable system was found to transport peptide out of the central nervous system (CNS). The clearance from the CNS of RC-160 and RC-121, but not RC-161, was faster than could be accounted for by reabsorption of cerebrospinal fluid. Transport of radioactively labeled RC-160 out of the CNS was inhibited by unlabeled RC-160 or somatostatin but was not affected by some other peptide known to cross the BBB by their own transport systems. More than 80% of the radioactivity recovered from the blood after intracerebroventricular injection of RC-160 was eluted by HPLC at the position of the labeled analog, showing that the peptide had crossed the BBB in intact form. Our results indicate the presence of a saturable transport system in one direction across the BBB for some superactive analogs of somatostatin.

Studies of the slow bidirectional transport of iron and transferrin across the blood-brain barrier.

Although iron is involved in brain function, very little is known about the regulation of its concentrations in the central nervous system. We quantitatively measured the entry and exit rates of iron, transferrin (its major transport protein), and albumin in mice. The blood to brain transport of iron greater than transferrin greater than albumin and the brain to blood transport of transferrin greater than albumin greater than iron. The results suggest that iron and transferrin have slow, bidirectional, probably saturable, and to some degree independent transport systems, although iron introduced directly into the brain is not readily available for brain to blood transport.

Differential effect of aluminum on the blood-brain barrier transport of peptides, technetium and albumin.

Aluminum is a neurotoxin capable of altering membrane structure and function. We investigated whether aluminum also can affect saturable transport across membranes using the blood-brain barrier as our model. Mice were given i.p. or i.v. aluminum (up to 100 mg/kg) as the chloride salt and the disappearance from the brain of several centrally administered substances was measured. We found that aluminum rapidly and profoundly inhibited the saturable system that transports the small, N-tyrosinated peptides Tyr-MIF-1 and the enkephalins from the brain to the blood by acting as a noncompetitive inhibitor. In contrast, the disappearance from the brain of technetium pertechnetate (a substance also transported out of the brain by a different saturable system), albumin or D-Tyr-MIF-1 (a stereoisomer of Tyr-MIF-1 that was confirmed not to be transported by the carrier system) was not affected by aluminum. Aluminum also did not alter either the saturable or nonsaturable component of the uptake of Tyr-MIF-1 by erythrocytes. These findings suggest that one mechanism by which aluminum may induce neurotoxicity is by selective alteration of the transport systems of the blood-brain barrier.