Binding of Tyr-MIF-1 to isolated brain capillaries

Opiate modulation of IL-1alpha, IL-2, and TNF-alpha transport across the blood-brain barrier

Interleukin-1alpha (IL-1alpha), interleukin-2 (IL-2), and tumor necrosis factor alpha (TNF-alpha) are proinflammatory cytokines with potent neuromodulatory effects and are implicated in the etiology and pathogenesis of various psychological and neurological disorders. The findings that chronic morphine treatment alters both blood-brain barrier (BBB) function and cytokine production raises the possibility that morphine can also modulate cytokine transport across the BBB. Here, we found that acute morphine treatment (12 mg/kg i.p.) did not alter blood-to-brain transport of IL-1alpha, IL-2 or TNF-alpha. Whereas chronic morphine treatment (48 h after implantation of 75 mg morphine pellets) and withdrawal from morphine (10-15 min after an i.p. injection of 1mg/kg of naltroxone 48 h after implantation of 75 mg morphine pellets) did not alter blood-to-brain transport of IL-1alpha or TNF-alpha, both the chronic morphine treatment and withdrawal from morphine groups had increased blood-to-brain transport of IL-2. Typically, the permeability of the BBB to IL-2 is dominated by brain-to-blood efflux, with only limited blood-to-brain transport. Here, we found that chronic morphine and withdrawal from morphine did not alter brain-to-blood efflux, but induced a novel saturable blood-to-brain transport system. Whereas IL-1alpha, IL-2, and TNF-alpha are all proinflammatory cytokines, morphine exposure has individualized effects on their blood-to-brain transport.

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH(2)) was the first hypothalamic peptide shown to act "up" on the brain, not just "down" on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose-response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH(2)) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH(2) and endomorphin-2 is Tyr-Pro-Phe-Phe-NH(2). Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood-brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.

Permeability of the mouse blood-brain barrier to murine interleukin-2: predominance of a saturable efflux system

Interleukin (IL)-2, a T helper (TH)1 cell-derived glycoprotein with potent neuromodulatory effects, is implicated in the etiology and pathogenesis of various psychiatric and neurological disorders. Paralleling these findings, chronic IL-2 intravenous immunotherapy may induce similar psychopathological outcomes. The findings that acute or repeated injections of IL-2 induce motor and cognitive abnormalities in rodents are consistent with these clinical findings, and raise the possibility that IL-2 crosses the blood-brain barrier (BBB) to alter brain function. However, little is known about the ability of IL-2 to enter the brain or whether its effects vary with the chronicity of IL-2 treatment. Here, we found that radioactively labeled mouse IL-2 (I-IL-2) given intravenously entered the brain at a low rate (Ki=0.142+/-0.044microl/g-min) by a non-saturable process. Repeated injections of either IL-2 or vehicle altered the kinetics of entry without producing a net effect on IL-2 entry. When I-IL-2 was given by brain perfusion, the entry rate greatly increased over 10-fold to 2.2+/-0.805microl/g-min. This suggests a circulating factor is retarding the entry of IL-2 into the brain. A paradoxic increase in the rate of I-IL-2 entry into brain occurred when an excess of unlabeled IL-2 was included in the brain perfusate, suggesting a saturable CNS-to-blood efflux system. Intracerebroventricular injection of I-IL-2 with and without unlabeled IL-2 confirmed the presence of a saturable efflux system. We conclude that IL-2 entry into the brain is low because of the absence of a blood-to-brain transporter and further retarded by circulating factors and a CNS-to-blood efflux system. This is the first description of a saturable CNS-to-blood efflux system for a cytokine. We postulate that this efflux system may protect the brain from circulating IL-2.

Peptide transport system-1 (PTS-1) for Tyr-MIF-1 and Met-enkephalin differs from the receptors for either

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and Met-enkephalin share a saturable transport system (peptide transport system-1, PTS-1) across the blood-brain barrier but do not readily bind to each other's receptors. This information allows the unique opportunity to differentiate the transport protein(s) from the receptors for either peptide in brain endothelial cells. PTS-1 was studied in vitro by allowing radiolabeled Tyr-MIF-1 (125I-Tyr-MIF-1) to bind to the solubilized proteins of isolated murine brain microvessels in the presence or absence of potential inhibitors. Sephadex chromatography separated bound from free labeled peptide. The binding was saturable as shown by inhibition with increasing concentrations of unlabeled Tyr-MIF-1. 125I-Tyr-MIF-1 binding was not inhibited by an unrelated peptide or iodo-tyrosine. D-Tyr-MIF-1 had no effect, demonstrating the stereospecificity of the system. Met-enkephalin decreased the binding of 125I-Tyr-MIF-1 to 84.4% of total, whereas Leu-enkephalin was without effect. Agonists for the mu, delta, and kappa opiate receptors did not change the binding, indicating that the proteins which bound to 125I-Tyr-MIF-1 were not endogenous opiate receptors. The results indicate that, in vitro, Tyr-MIF-1 binds to brain microvessel proteins with characteristics similar to PTS-1.

Transmission routes of HIV-1 gp120 from brain to lymphoid tissues

The blood-brain barrier (BBB) restricts the entry of antiviral agents into the CNS thereby facilitating the creation of a reservoir of HIV that could potentially reinfect peripheral tissues. We characterized the efflux from brain of radioactively labeled viral coat HIV-1 gp120 (I-gp120) after intracerebroventricular (i.c.v.) injection. The half-time disappearance rate of I-gp120 from brain was 12.6 min, which was faster than could be explained by the reabsorption of cerebrospinal fluid into blood but could not be explained by a saturable transporter. After i.c.v. injection, I-gp120 appeared in the serum and was sequestered by spleen and the cervical nodes, demonstrating a potential for virus within the CNS to reinfect peripheral tissues. However, the amount of I-gp120 appearing in serum was less than that expected based on the efflux rate, whereas uptake by the cervical nodes was much greater after i. c.v. than after i.v. injection of I-gp120. These findings were explained by drainage from the brain directly to the cervical lymph nodes through the brain's primitive lymphatic system. These lymphatics potentially provide a pathway through which CNS reservoirs of HIV-1 could directly reinfect lymphoid tissue without being exposed to circulating antiviral agents.

Periventricular penetration and disappearance of ICV Tyr-MIF-1, DAMGO, tyrosine, and albumin

The penetration of four radioiodinated materials-Tyr-MIF-1, DAMGO, tyrosine, and albumin-into the periventricular tissue after ICV injection was studied in rats by film autoradiography. Rates of disappearance from the CNS for the injected compounds were also determined by computer-assisted image analysis of the autoradiographic images. The four materials showed distinct patterns of dispersion from the ventricular system, with Tyr-MIF-1 moving farthest into the parenchyma of the brain and albumin primarily restricted to the ventricular space. The other two compounds, tyrosine and DAMGO, had intermediate values. Tyr-MIF-1 also displayed the fastest rate of removal from the brain, which may represent the ability of the peptide to gain access to sites of saturable transport. By contrast, the exit from the brain of DAMGO was minimal, whereas the efflux of albumin and tyrosine was intermediate. These results show the utility of these methods in the simultaneous measurement of both the patterns of distribution within the CNS and the rates of removal from the CNS of compounds injected into the brain.

Selective transport of blood-borne interleukin-1 alpha into the posterior division of the septum of the mouse brain

Film autoradiography was used to demonstrate the transport and sites of accumulation of blood-borne radioiodinated interleukin-1 alpha (II-1 alpha) and other cytokines into the brain after intravenous administration. [125 I]Il-1 alpha, [125I]Il-1 beta, [125I]interleukin-1 receptor antagonist (II-1ra), and [125I]tumor necrosis factor-alpha (TNF alpha) labeled the choroid plexus and the capillary network 30 min after injection into the blood, suggesting that these areas may serve as sites of blood-to-brain transport. [125I]Il-1alpha, but not [125I]Il-1beta, [125I]Il-1ra, [125I]TNF alpha, or [125I]interleukin-2 (Il-2), was also found localized to the caudal region of the septal nuclei. Only unlabeled II-1 alpha was able to inhibit this accumulation. These findings provide further evidence for the passage of select cytokines across the blood-brain barrier (BBB) and are the first to identify a target site within the central nervous system (CNS) for a transported cytokine.

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.

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.