Changes in regional blood-brain transfer of L-leucine elicited by arginine-vasopressin

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Enhanced leptin transport across the blood-brain barrier by alpha 1-adrenergic agents

Leptin is a 16 kDa protein secreted by fat cells which regulates body weight and thermogenesis at sites within the brain. Blood-borne leptin reaches those brain sites because of a saturable transport system located at the blood-brain barrier (BBB). Impaired transport occurs in obese rodents and likely underlies the resistance to the actions of peripheral leptin seen in obesity. Here, we show that leptin transport into the brain is enhanced 2-3-fold by epinephrine and other agents which are more specific for the alpha1 adrenergic receptor. Epinephrine had no effect on the transport across the BBB of insulin or tumor necrosis factor, on BBB integrity, or on the size of the vascular space of the brain. Dopamine, acetylcholine, histamine, serotonin, thyroid hormones, and phentolamine were without effect. Of several amino acids tested, only the catecholamine precursor tyrosine had an effect on leptin transport. Epinephrine was effective after intravenous or intraperitoneal injection, but neither epinephrine nor any of the other monoamines given by intracerebroventricular injection had an effect on leptin transport. These results show that epinephrine likely acts at a site on the luminal surface of the BBB. In conclusion, epinephrine works at an alpha1-like adrenergic, luminal side to enhance the transport of leptin across the BBB.

Expression of large amino acid transporter LAT1 in rat brain endothelium

The expression of the large amino acid transporter, LAT1, was investigated in brain of adult Long-Evans rats. The LAT1 transcript was readily detected in brain microvessels and choroid plexus by reverse transcription polymerase chain reaction analysis using three different gene specific primer pairs. A polyclonal affinity purified antibody against the N-terminus of LAT1 was generated in chickens and used in immunoblot and immunocytochemical analyses of brain tissue sections of adult rats. On immunoblots, the antibody detected a peptide-inhibitable 45 kDa band in a rat brain microvessel membrane preparation. It also identified the same protein band in membrane preparations of different brain structures, as well as in heart and testis, whereas the protein was absent or only faintly detectable in muscle, kidney, and liver. In brain sections, the antibody intensely labeled the luminal and abluminal membranes of brain microvessel endothelial cells in all brain areas examined including cerebral cortex, cerebellum, hippocampus, and in gray and white matter regions. These results suggest that LAT1 is involved in transcellular transport and may play an important role in large, neutral amino acid transfer across the blood-brain barrier.

Developmental exposure to vasopressin increases aggression in adult prairie voles

Although the biological roots of aggression have been the source of intense debate, the precise physiological mechanisms responsible for aggression remain poorly understood. In most species, aggression is more common in males than females; thus, gonadal hormones have been a focal point for research in this field. Although gonadal hormones have been shown to influence the expression of aggression, in many cases aggression can continue after castration, indicating that testicular steroids are not completely essential for the expression of aggression. Recently, the mammalian neuropeptide arginine vasopressin (AVP) has been implicated in aggression. AVP plays a particularly important role in social behavior in monogamous mammals, such as prairie voles (Microtus ochrogaster). In turn, the effects of social experiences may be mediated by neuropeptides, including AVP. For example, sexually naïve prairie voles are rarely aggressive. However, 24 h after the onset of mating, males of this species become significantly aggressive toward strangers. Likewise, in adult male prairie voles, central (intracerebroventricular) injections of AVP can significantly increase intermale aggression, suggesting a role for AVP in the expression of postcopulatory aggression in adult male prairie voles. In this paper, we demonstrate that early postnatal exposure to AVP can have long-lasting effects on the tendency to show aggression, producing levels of aggression in sexually naïve, adult male prairie voles that are comparable to those levels observed after mating. Females showed less aggression and were less responsive to exogenous AVP, but the capacity of an AVP V(1a) receptor antagonist to block female aggression also implicates AVP in the development of female aggression.

Permeability of a capsaicin derivative, [14C]DA-5018 to blood-brain barrier corrected with HPLC method

In the present work, the transport mechanism of a capsaicin derivative, DA-5018, through blood-brain barrier (BBB) has been investigated to evaluate the feasibility of potential drug development. The result of pharmacokinetic parameters obtained from the intravenous injection of plasma volume marker, [3H]RSA and [14C]DA-5018, indicated that both AUC, area under the plasma concentration curve and VD, volume of distribution in brain of [3H]RSA agreed with those reported (1620 +/- 10 percentage injected dose minute per milliliter (%IDmin/ml) and 12.0 +/- 0.1 microliters/g, respectively). Elimination half-life and AUC of [14C]DA-5018 is corrected by the HPLC analysis, 19.6 +/- 1.2 min and 7.69 +/- 0.85% IDmin/ml, respectively. The metabolic rate of [14C] DA-5018 was very rapid. The blood-brain barrier permeability surface area (PS) product of [14C]DA-5018 was calculated to be 0.24 +/- 0.05 microliter/min/g. The result of internal carotid artery perfusion and capillary depletion suggested that [14C]DA-5018 pass through BBB with the time increasingly. Investigation of transport mechanism of [14C]DA-5018 using agonist and antagonist suggested that vanilloid (capsaicin) receptor did not exist in the BBB, and nutrient carrier system in the BBB has no effect on the transport of DA-5018. In conclusion, despite the fact that penetration of DA-5018 through BBB is significant, the intact drug found in the brain tissue is small because of a rapid metabolism. Therefore, for the central analgesic effect of DA-5018, the method to increase the metabolic stability in plasma and the brain permeability should be considered.

Simultaneous measurement of [18F]FDOPA metabolism and cerebral blood flow in newborn piglets

Available information on the dopamine (DA) metabolism of the immature brain is rare. In order to establish a useful animal model we have performed PET experiments in anesthetized neonatal pigs using 6-[18F]-fluoro-L-DOPA (FDOPA) as tracer. In this study, we have simultaneously determined the cerebral blood flow and the rate constant of FDOPA conversion by the aromatic amino acid decarboxylase, the ultimate enzyme in the synthesis of dopamine. The estimated values of FDOPA decarboxylation in the basal ganglia were similar to values calculated in adult animals and humans. However, in contrast to those studies a significant decarboxylation was also found in the frontal cortex and the cerebellum. HPLC analysis of brain samples also revealed extensive and rapid metabolism of FDOPA in the five investigated brain regions. At 8 min after tracer injection about 80% of FDOPA was already converted to FDA and its metabolites. Surprisingly, a rather high fraction (16-21%) of [18F]-fluoro-3-methoxytyramine was found which may indicate a low storage capacity of vesicular DA at this perinatal stage. It is suggested that the findings are related to the ontogenetic development of the dopaminergic system. The knowledge of the regulation of the DA metabolism in the immature brain may have implications for the understanding of neurodevelopmental effects of perinatal oxygen deprivation.

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.

Arginine vasopressin reduces the blood-brain transfer of L-tyrosine and L-valine: further evidence of the effect of the peptide on the L-system transporter at the blood-brain barrier

Arginine vasopressin (AVP) coinjected into the carotid artery in physiological concentrations (0.1 nmol/l), with either L-[3H]tyrosine or L-[3H]valine, induced changes in the kinetic parameters of the blood-to-brain transfer of both large neutral amino acids (LNAA) without alterations in brain haemodynamics. The half-saturation constant (Km), the maximum velocity of transport (V(max)) and Kd, the nonsaturable transport constant, were estimated in 9 brain regions of male Wistar rats anaesthetized with ether. Apart from Kd, significant changes in Km and V(max) were observed in all brain regions investigated. On average Km decreased from 0.17 to 0.048 mmol/l for tyrosine, and from 0.61 to 0.059 mmol/l for valine, whereas V(max) declined from 22 to 9.9 nmol/min/g for tyrosine, and from 29 to 3.2 nmol/min/g for valine, respectively. The results provide further evidence that vasopressin-receptor interactions at the blood-brain barrier (BBB) induce changes in the properties of the common transporter, the L-system, which eventually result in a suppression of the blood-to-brain transfer of LNAA. Data analysis of the 5 LNAA tested so far reveals a significant negative correlation (R = 0.98, P < 0.05) between the respective substrate affinity for the transporter and the corresponding magnitude of transport reduction induced by circulating AVP. Calculations of the unidirectional influx (J) of the LNAA indicate that AVP (1) reduces J by approximately one-third for every LNAA, but (2) does not change the relative contribution for each single LNAA to the total influx across the BBB.

Permeability of the blood-brain barrier to the neurotensin8-13 analog NT1

Neurotensin (NT) has been suggested to be a neuropeptide with therapeutic potential. We used multiple-time regression analysis to measure the unidirectional influx constant (Ki) of a tritiated analog of NT8-13, NT1, with improved metabolic stability. The Ki of [3H]NT1 across the blood-brain barrier (BBB) was 5.12(10(-4)) ml/g-min and was decreased 66% by unlabeled NT1 system. The amount of NT1 crossing the BBB, 0.087% of the injected dose per gram of brain, is consistent with its exerting central effects after peripheral administration. The stable [3H]NT1 crossed the BBB in intact form as assessed by HPLC and completely crossed the endothelial cells that comprise the BBB as assessed by the capillary depletion method. The presence of a transport system could be important for the development of NT analogs.

Permeability of the blood-brain barrier to melanocortins

Adrenocorticotropin (ACTH)-related compounds, termed melanocortins, produce a large number of effects on the central nervous system (CNS) after their peripheral administration. Some of the CNS effects of ACTH are mediated through the release of glucocorticoids from the adrenal gland, but there are fragments and analogues of ACTH that do not act on the adrenals. This raises the possibility that some blood-borne melanocortins may be acting directly on the brain, which would necessitate their crossing the blood-brain barrier (BBB). We review here the literature showing that melanocortins can affect the BBB in several ways, including an alteration of the permeability of the BBB to other substances.

Changes in amino acid levels in rat plasma, cisternal cerebrospinal fluid, and brain tissue induced by intravenously infused arginine-vasopressin

Circulating arginine-vasopressin (AVP) is known to reduce the blood-to-brain transfer of large neutral amino acids (AA). As a first step to examine whether the reduced uptake by brain endothelial cells is reflected in changes in large neutral amino acid levels of the extracellular fluid environment of cells within the nervous tissue, we measured the concentrations of amino acids in plasma, cerebrospinal fluid (CSF), and hippocampal tissue of rats before and after infusion of AVP (34 and 68 ng/min/kg, respectively) over the time period of 60 min. AA levels changed in all compartments investigated during both saline and AVP infusions. Whereas in the saline-infused controls changes in CSF AA levels paralleled those in plasma, this correlation was abolished by raising AVP concentrations. The effect of AVP was found to be i) dependent on the AA, ii) different with respect to direction and iii) magnitude of changes in AA levels, and iv) in some cases dose dependent. In summary, AVP infusion increased plasma levels of 10 AA, but decreased all 15 AA measured by some 30% in CSF. In contrast to CSF, levels of AA were slightly enhanced in the hippocampal tissue. The results are not solely explicable by a reduced blood-to-brain transfer of AA. We conclude that further mechanisms by which AVP affects the availability of AA to the brain may exist. The physiological significance of the findings might be related to brain osmoregulation, especially in situations of stress.

Developmental changes of enzymes involved in peptide degradation in isolated rat brain microvessels

The specific activities of aminopeptidase A (APA), aminopeptidase M (APM), and dipeptidyl-aminopeptidase IV (DP IV) were determined in isolated brain microvessels and in brain homogenate of rats with different ages (between 1 and 8 weeks old). In addition, the blood-brain barrier (BBB)-specific enzymes gamma-glutamyltranspeptidase (gamma-GT) and alkaline phosphatase (ALP) were measured. As similarly described by others, gamma-GT activity increased during this time period by fourfold, whereas ALP increased between weeks 1 and 2 and declined thereafter. DP IV activity increased fivefold during the first 8 weeks after birth and APM activity increased by twofold. A decrease of APA activity was found between weeks 1 and 2 after birth followed by an increase thereafter. The development of aminopeptidase activities responsible for the processing of specific neuropeptides acting on brain microvessels may be important in the development of regulation processes for cerebral blood flow and BBB permeability in the maturing animal.

Effects of vasopressin on blood-brain transfer of methionine in dogs

We used a simplified probe detection system for positron-emitting radionuclides in order to measure blood-brain barrier transport of amino acids in anesthetized dogs. Plasma and brain time-activity curves were recorded after intravenous bolus injection of L-[11C]methionine before and after administration of 1 microgram of vasopressin. Three-compartment models with three or four transfer coefficients were used to derive the kinetics of L-[11C]methionine uptake in brain. The blood-brain clearance of the tracer (K1) was 0.075 ml ml-1 min-1 before and 0.041 ml ml-1 min-1 after injection of vasopressin. The partition volume and the initial distribution (plasma) volume of methionine were unchanged and within the expected limits. The net accumulation rate of methionine (K), estimated by both the four-parameter (kinetic) and three-parameter (graphic) approaches, decreased after vasopressin injection in all six studies.

Transfer of [3H]leucine across the blood-brain barrier at high blood-side oxytocin concentrations in normal and morphine-dependent rats

The effects of circulating oxytocin on permeability of the blood-brain barrier (BBB) to L-[3H]leucine were studied in anaesthetized rats using the intracarotid, single pass, bolus injection technique. After bolus intracarotid oxytocin injection (10(-9) M), there were no differences in [3H]leucine uptake, compared with controls, in any of eight brain regions with a 'tight' BBB (olfactory bulb, frontal cortex, visual cortex, corpus striatum, hippocampus, thalamus, hypothalamus and colliculi) or in BBB-free, 'leaky' structures (pineal gland, choroid plexus, neuro-intermediate pituitary, anterior pituitary). [3H]leucine uptake by the 'leaky' structures was 2.4x and 2.6x uptake by 'tight' regions in the oxytocin and control groups respectively. In morphine-dependent rats, naloxone increased oxytocin secretion 28-fold within 5 min, but did not affect [3H]leucine uptake for any BBB-protected brain region or BBB-free 'leaky' structure. Accumulation of [3H]leucine was 8.3x and 7.0x greater in the 'leaky' structures than in the 'tight' regions in the naloxone and control groups respectively; [14C]inulin accumulation by each 'tight' region (measured simultaneously with [3H]leucine to determine the vascular space) was not affected by naloxone. It is concluded that even very high blood plasma concentrations of oxytocin do not affect BBB permeability for leucine. It is unlikely that altered BBB permeability, at least for amino acids, contributes to CNS changes during naloxone-provoked morphine withdrawal.

Decrease of extracellular taurine in the rat dorsal hippocampus after central nervous administration of vasopressin

The extracellular amino acid concentrations in the left and right dorsal hippocampus of male rats were studied before and during application of vasopressin into the right hippocampus. The method of intracerebral microdialysis was used for both arginine vasopressin administration and monitoring of the composition of the extracellular fluid. The concentrations of 16 amino acids were measured by HPLC in the perfusate samples. The level of taurine declined 20% in the right hippocampus during perfusion with vasopressin, whereas o-phosphoethanolamine decreased in both sides, the left 20% and the right 24%. These alterations may be related to cerebral osmoregulation. Also, the levels of tyrosine and phenylalanine increased 15% and 35%, respectively, during administration of vasopressin. No changes of other amino acids were observed.

Peptide receptors of the blood-brain barrier and substrate transport into the brain

The BBB is a target for some peptide signals, as demonstrated by our group for arginine-vasopressin (AVP) and atriopeptin (ANP). Peptide molecules contacting the luminal surface of endothelial cells interact with specific high-affinity binding sites. The minimal simple diffusion of peptide molecules across the layer of endothelial cells which are connected by tight junctions is most probably without any significance under physiological conditions, although that question should be checked for brain regions like the olfactory bulb in which some leakiness of the BBB can be demonstrated. The AVP- and ANP-receptors at least partly localized at the luminal surface of the endothelial cells are heterogeneously distributed in the vessels of the brain. The number of AVP receptors is up-regulated by ligand deficiency, which induces furthermore a decrease in the receptor affinity. At physiological concentrations AVP and ANP do not affect the tightness of the BBB, but regulate the transcellular transfer of essential substances from blood to brain. AVP decreases the Km and Vmax of the transporter of large neutral amino acids, and ANP alters the water permeability of the endothelial cell layer. The phenomenon that the cells of the tight epithelium representing the BBB need information from blood-borne peptide signals for the regulation of intercompartmental transport processes seems to be only a special case of a general principle concerning tight epithelial cell layers which separate compartments containing fluids of different composition; amino acid transport across the intestine is regulated by specific peptides contacting that barrier, the casomorphins.

Cerebrovascular permeability and cognition in the aging rat

Regional cerebrovascular permeability-capillary surface area products (rPS) and brain vascular space (BVS) were measured in aging, conscious, unrestrained Sprague-Dawley rats. Three groups of animals were examined: young-mature (6 months), middle-aged (12-14 months), and old (24-26 months) rats. Complex maze learning had been previously characterized in these same animals. Maze learning declined with age. Brain vascular space did not differ significantly with age in any brain region. However, small, but significant age-dependent decreases in rPS (25-33%) were observed. These decreases occurred mainly in the old animals in the basal ganglia and parietal cortex, and in the middle-aged and old rats in the olfactory bulbs. Significant and unexpected positive average correlations between brain permeability-capillary surface area products (PS) and learning errors occurred primarily in young rats and were attributable mainly to changes in 5 of 14 brain regions; hypothalamus, hippocampus, parietal cortex, septal area and superior colliculus. The higher correlations between maze learning errors and PS in young animals may indicate dynamic regulation of this cerebrovascular parameter which is lessened with aging. Average correlations between PS and cerebral blood flow also were determined and found to be generally small and not significant for most brain regions and age groups.

Atrial natriuretic peptide augments the blood-brain transfer of water but not leucine and glucose

Recent evidence predicts an effect of atrial natriuretic peptide (ANP) on the blood-brain transfer of water. To test this prediction, we measured the blood-brain transfer of water, L-leucine, and D-glucose in 9 brain regions of male rats after intravenous injection of 10 pmol ANP. The peptide elicited an increase of the permeability-surface area (PaS) product of labeled water by 28-108% while the PaS products of leucine and glucose remained unchanged. Cerebral blood flow increased 15-48% while cardiac output and plasma volume in brain did not alter, indicating no change of capillary surface area (CSA). Regionally, the CSA varied from 63 cm2/g (striatum) to 97 cm2/g (colliculi) and the fraction of capillaries contributing to the total vascular volume varied from 29% (olfactory bulb/lobe) to 62% (striatum). The blood-brain barrier (BBB) permeability to water (5.7 micron/s) was an order of magnitude higher than to glucose (0.4 micron/s) or to leucine (0.3 micron/s).

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.

The vasopressin receptor of the blood-brain barrier in the rat hippocampus is linked to calcium signalling

The signal transduction system of the vasopressin receptor in cerebral microvessels is not known but appears not to be adenylate cyclase/cyclic AMP. We determined the effect of arginine vasopressin (AVP) on the intracellular free calcium concentration [Ca2+]i in endothelial cells of isolated hippocampal microvessels of rats, using the fura-2 fluorescence technique. AVP administration caused a rapid and transient rise of cytosolic free calcium which was absent after extracellular calcium was removed, and could be blocked with the vasopressin V1 receptor antagonist, d(CH2)5 Tyr(Me)AVP. The vasopressin V2 receptor agonist, 1-deamino-8,D-AVP, on the contrary, failed to affect the intracellular free calcium level, and was unable to inhibit the AVP-induced rise of [Ca2+]i in the preparation. Our results, therefore, demonstrate the presence of a calcium-signalling, i.e. V1 vasopressin receptor at the blood-brain barrier in the hippocampus of the rat.

Leucine modulates peptide transport system-1 across the blood-brain barrier at the stereospecific site within the central nervous system

Previous results have shown that leucine injected into a cerebral ventricle (i.c.v.) can act as an allosteric regulator of peptide transport system-1 (PTS-1), the system that transports Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1) and the enkephalins out of the central nervous system (CNS). D-Leucine appeared more potent than L-leucine. In the current study, dose-response curves were constructed for each compound after both intravenous (i.v.) and i.c.v. injection. Based on ED50 values after i.c.v. injection, D-leucine was about 200 times more potent than L-leucine in its inhibition of PTS-1, thereby confirming stereospecificity of the allosteric site. D- and L-Leucine were also more potent when given i.c.v. than when given i.v., suggesting that the site is located on the CNS side of the blood-brain barrier (BBB). The finding that D-leucine was less potent than L-leucine when given i.v. is also consistent with a CNS site of action because the L-isomer of leucine has been shown to be preferentially transported into the brain. These findings agree with the previous suggestion that some of the neurotoxic effects of leucine may be mediated through PTS-1 and could help explain how D-amino acids can exert opiate-related effects on the CNS.

Blood-brain barrier transport under different physiological and pathophysiological circumstances including ischemia

The blood-brain barrier (BBB) transport of amino acids, glucose and choline was studied under different experimental conditions. The influence of the neuropeptide arginine-vasopressin (AVP) on the transport of leucine and phenylalanine was investigated after peripheral and central nervous application of the peptide and in rats with different endogenous levels of the hormone. AVP elicited changes of the kinetics of the neutral amino acid transport across the BBB accompanied by a decrease of the permeability/surface area (PaS)-product. Also the influence of different nootropic drugs on the BBB transport was investigated under various circumstances of impaired brain function, i.e. after treatment with scopolamine or ethanol and after unilateral carotid artery occlusion. Changes of the kinetics of leucine transport and of the PaS-product of leucine, choline and glucose were found. The results are discussed as part of complex actions of the peptides and nootropics including alterations of the cerebral hemodynamics and brain metabolism.

Blood-brain transfer of L-phenylalanine declines after peripheral but not central nervous administration of vasopressin

To determine whether a previously reported effect of vasopressin on blood-brain transfer of leucine extends to other large neutral amino acids, we measured the regional blood-brain transfer of L-phenylalanine with the integral technique. Intravenous co-injection of L-phenylalanine and arginine vasopressin (30 nmol to 10 pmol) resulted in a decrease of the permeability-surface area (PaS) product of phenylalanine of between 11 and 39%. In addition, the peptide elicited a decrease of the cerebral blood flow of between 11 and 56% combined with a drastic decrease of the cardiac output (32-64%) and an elevation of the blood pressure to approximately 150% of control values. However, we found no changes of the cardiac output, the blood pressure, or the PaS product of phenylalanine after microdialysis (30 min, 5 microliters min-1) of arginine vasopressin (15 mumol L-1) into the dorsal hippocampus, but cerebral blood flow was decreased. The results support the hypothesis that arginine vasopressin receptors at the blood-brain barrier are involved in the regulation of large neutral amino acid transfer from blood to brain and indicate that these receptors are located at the luminal membrane of the endothelial cells.

beta-Casomorphins alter the intestinal accumulation of L-leucine

Everted sacs of the rat jejunum change the accumulation of [3H]leucine when beta-casomorphins (BCMs) or synthetic analogs, in a concentration range of 10(-8) mol/l, are coincubated with the amino acid. BCM5 (BCM fragment 1-5, Tyr-Pro-Phe-Pro-Gly) and [D-Ala2]-BCM5-NH2 (Tyr-D-Ala-Phe-Pro-Gly) increase, whereas [D-Pro4]-BCM5 (Tyr-Pro-Phe-D-Pro-Gly) decreases the leucine accumulation and [Arg8]-vasopressin has no effect. No effect of BCM5 could be observed on the accumulation of the space marker [14C]inulin. Specific binding sites for casomorphins were detected microautoradiographically, exclusively at the epithelial cell layer using [3H][D-Pro4]-BCM5 in competition studies as a model. HPLC analysis revealed that under the experimental conditions about 50% of the studied [D-Pro4]-BCM5 was enzymatically degraded and no intact peptide is accumulated within the samples of everted sacs. From the results we postulate a brush-border receptor contact of the BCMs which induces an alteration of the amino acid uptake. A contraluminal binding of the chemical signals is not likely, because there is no evidence for a transepithelial transport of intact BCMs. The observed effects of the BCMs demonstrate as yet unknown peptide-receptor interactions, probably at the brush-border membrane, with subsequent effects on the nutrient supply. Furthermore, the results support the general hypothesis of distinct peptide-receptor interactions in those types of epithelia in which the cells are connected by tight junctions.

Saturable mechanism for delta sleep-inducing peptide (DSIP) at the blood-brain barrier of the vascularly perfused guinea pig brain

Cellular uptake of [125I] labelled DSIP at the luminal interface of the blood-brain barrier (BBB) was studied in the ipsilateral perfused in situ guinea pig forebrain. Regional unidirectional transfer constants (Kin) calculated from the multiple-time brain uptake analysis were 0.93, 1.33 and 1.66 microliter.min-1 g-1 for the parietal cortex, caudate nucleus and hippocampus, respectively. In the presence of 7 microM unlabelled DSIP the brain uptake of [125I]-DSIP (0.3 nM) was inhibited, the values of Kin being reduced to 0.23-0.38 microliter.min-1 g-1, values that were comparable with the Kin for mannitol. The rapidly equilibrating space of brain, measured from the intercept of the line describing brain uptake versus time on the brain uptake ordinate, Vi, was greater for [125I]-DSIP than for mannitol; in the presence of unlabelled DSIP this was reduced to that of mannitol, and it was suggested that the larger volume for [125I]-DSIP represented binding at specific sites on the brain capillary membrane. L-tryptophan, the N-terminal residue of DSIP, in concentrations of 7 microM and 1 mM, inhibited Kin without affecting Vi. A moderate inhibition of Kin was obtained by vasopressin ([Arg8]-VP), but only at a concentration as high as 0.2 mM. The results suggest the presence of a high affinity saturable mechanism for transport of DSIP across the blood-brain barrier, with subsequent uptake at brain sites that are highly sensitive to L-tryptophan, and may be modulated by [Arg8]-VP.

Potassium as a signal for both proliferation and differentiation of rabbit retinal (Müller) glia growing in cell culture

Retinal glial (Müller) cells were grown from explants of early postnatal rabbit retinae. The resulting monolayers of flat cells were exposed to control media (containing 5.85 mM K+), and to media with enhanced K+ concentrations (10 and 20 mM) or arginine-vasopressin (AVP, 20 micrograms/ml) or epithelial growth factor (EGF, 10 ng/ml). Autoradiographically, protein synthesis was quantified as L-[3H]-lysine incorporation, and DNA synthesis as [3H]-thymidine incorporation. Furthermore, the activity of Na+,K(+)-ATPase was measured radiochemically. Short exposure to either moderately enhanced K+ concentrations (10 mM) or to AVP, stimulated L-[3H]-lysine incorporation into the cells. Long-lasting exposure to either high K+ concentrations (20 mM) or to EGF stimulated [3H]-uptake. The Na+,K(+)-ATPase activity of cell cultures increased with increasing K+ concentration of the media. It is suggested that release of K+ by active neuronal compartments stimulates local protein synthesis of glial cells, resulting in the formation of glial sheaths with active K+ uptake capacity. Strong K+ release may even induce glial proliferation.

Immunocytochemically stained vasopressin binding sites on blood vessels in the rat brain

Following two weeks of cerebroventricular administration of arginine-vasopressin (AVP) by Accurel implants, two types of binding sites for this peptide were immunocytochemically visualized in blood vessels in the brain of Brattleboro (di/di) rats: (1) endothelial cells of capillaries were stained with the highest density in hippocampus, striatum, and locus coeruleus (LC), whereas only few such stained cells were present in the septum and cerebral cortex. (2) Bound AVP was also present on endothelial cells and pericyte-like cells in larger blood vessels in striatum and the LC. Both types of vasopressin binding site staining on blood vessels were dose-dependent and could be further enhanced by additional in vitro preincubation of the fixed sections with AVP. This staining was not present in rats implanted with either oxytocin or alpha-melanocyte-stimulating hormone.

Interactions between the blood-brain barrier and endogenous peptides: emerging clinical implications

The effects of peptides on brain function suggest therapeutic and pathologic roles for these substances. Many peptides cross the blood-brain barrier (BBB) by transmembrane diffusion as a function of their lipid solubilities. Other peptides, such as the enkephalins, Tyr-MIF-1, vasopressin-related peptides, and peptide T-like peptides, are transported by carrier-mediated systems. Passage is influenced by aging, stress, lighting, drugs, amino acids, and neurotoxins. Disruption of the BBB results in complex changes in the blood and CSF levels of peptides. Peptides influence the passage of glucose, amino acids, and inorganic acids and may affect the integrity of the BBB. Peptide-BBB interactions have been suggested to play direct roles in dialysis dementia and maple syrup urine disease; they may be expected to be involved in other disorders of the CNS.

Arginine vasopressin receptors in pig cerebral microvessels, cerebral cortex and hippocampus

We tested for the presence of arginine vasopressin (AVP) receptors in pig cerebral microvessels, cerebral cortex and hippocampus by specific binding methods with [3H]AVP as the ligand. The specific binding of [3H]AVP to all preparations was saturable and Scatchard analysis indicated a single class of high affinity binding sites (dissociation constant of 1-2 nM). Maximal binding capacity in cerebral microvessels was about 60% that of the cerebral cortex; and there were no apparent differences in the maximal binding capacity between cerebral cortex and hippocampus. These findings suggest the existence of AVP receptor sites in cerebral microvessels and support the hypothesis that AVP has a role in the control of the brain microcirculation.

Saturable retention of vasopressin by hippocampus vessels in vivo, associated with inhibition of blood-brain transfer of large neutral amino acids

Vasopressin receptors have been reported in the endothelium of brain capillaries. The function of these receptors is not known. To test the prediction that vasopressin receptors in brain capillary endothelium affect amino acid transport across the blood-brain barrier and to assess the role of vasopressin transport across the cerebral vascular endothelium, we measured (a) the endothelial permeability to the large neutral amino acid leucine in the absence and presence of arginine vasopressin (AVP) and (b) the permeability of the blood-brain barrier to AVP relative to manitol. In brain regions protected by the blood-brain barrier, after circulation for 20 s, coinjection of leucine and AVP intravenously led to a decrease of leucine transport unrelated to changes of blood flow. The decrease was most pronounced in hippocampus (42%) and least pronounced in olfactory bulb and colliculi (17 and 19%, respectively). In the latter regions, the endothelial permeability to AVP did not significantly exceed that of mannitol. In hippocampus and in regions with no blood-brain barrier (pituitary and pineal glands), AVP retention in excess of mannitol retention was blocked by unlabeled AVP. The findings do not contradict the hypothesis of a role for AVP in the regulation of large neutral amino acid transfer into brain tissue.

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.

Carrier-mediated transport of vasopressin across the blood-brain barrier of the mouse

A brain to blood carrier-mediated transport system for arginine vasopressin (AVP) was investigated in mice after intraventricular injection of iodinated AVP and varying amounts of unlabeled material or candidate inhibitors. Residual activity in the brain detected after decapitation was used as the main determinant of transport activity. The half-time disappearance of iodinated AVP from the brain was 12.4 min, the Vmax was 1.41 nmol/g-min, and the apparent Km was 28.7 nmol/g. A 30-nmol dose of AVP, mesotocin, arginine vasotocin, pressinoic amide, pressinoic acid, tocinoic acid, and lysine vasotocin, but not oxytocin, lysine vasopressin, AVP free acid, tocinoic amide, Tyr-MIF-1, or cyclo Leu-Gly, significantly (P less than 0.05) inhibited the transport of iodinated AVP out of the brain. The 30 nmol dose of AVP had no effect on the transport of iodide or iodotyrosine out of the brain. High-performance liquid chromatography showed that 59.2% of the radioactivity found in the blood 2 min after an i.c.v. injection of labeled AVP eluted at the same position as labeled AVP compared with 68.8% of radioactivity eluting at that position after material was infused i.v. for 2 min. This indicates that intact peptide is transported across the blood-brain barrier and that most of the degradation of AVP occurs during circulation in the blood. Calculations based on the appearance of radioactivity in the periphery showed that 56.2% of the material injected centrally would have been transported into the periphery by 10 min. This appearance of material in the periphery was inhibited by the simultaneous injection of an excess of unlabeled peptide. Water loading significantly decreased the brain to blood transport rate of AVP by 40%. It is concluded that a saturable system exists for brain to blood transport of AVP and some structurally similar peptides.