Minireview. Brain peptides: the dangers of constricted nomenclatures

Peptides and the blood-brain barrier

The demonstration that peptides and regulatory proteins can cross the blood-brain barrier (BBB) is one of the major contributions of Dr. Abba J. Kastin. He was the first to propose that peptides could cross the BBB, the first to show that an endogenous peptide did so, and the first to describe a saturable transport system at the BBB for peptides. His work shows that in crossing the BBB, peptides and regulatory proteins act as informational molecules, informing the brain of peripheral events. Brain-to-blood passage helps to control levels of peptides with the brain and can deliver information in the brain-to-blood direction. He showed that the transporters for peptides and proteins are not static, but respond to developmental and physiological changes and are affected by disease states. As such, the BBB is adaptive to the needs of the CNS, but when that adaption goes awry, the BBB can be a cause of disease. The mechanisms by which peptides and proteins cross the BBB offer opportunities for drug delivery of these substances or their analogs to the brain in the treatment of diseases of the central nervous system.

Intranasal administration of PACAP: uptake by brain and regional brain targeting with cyclodextrins

Pituitary adenylate cyclase activating polypeptide (PACAP) is a potent neurotrophic and neuroprotectant that is transported across the blood-brain barrier in amounts sufficient to affect brain function. However, its short half-life in blood makes it difficult to administer peripherally. Here, we determined whether the radioactively labeled 38 amino acid form of PACAP can enter the brain after intranasal (i.n.) administration. Occipital cortex and striatum were the regions with the highest uptake, peaking at levels of about 2-4% of the injected dose per gram of brain region. Inclusion of unlabeled PACAP greatly increased retention of I-PACAP by brain probably because of inhibition of the brain-to-blood efflux transporter for PACAP located at the blood-brain barrier. Sufficient amounts of PACAP could be delivered to the brain to affect function as shown by improvement of memory in aged SAMP8 mice, a model of Alzheimer's disease. We found that each of three cyclodextrins when included in the i.n. injection produced a unique distribution pattern of I-PACAP among brain regions. As examples, β-cyclodextrin greatly increased uptake by the occipital cortex and hypothalamus, α-cyclodextrin increased uptake by the olfactory bulb and decreased uptake by the occipital cortex and striatum, and (2-hydropropyl)-β-cyclodextrin increased uptake by the thalamus and decreased uptake by the striatum. These results show that therapeutic amounts of PACAP can be delivered to the brain by intranasal administration and that cyclodextrins may be useful in the therapeutic targeting of peptides to specific brain regions.

The Cdk5/p35 kinases modulate leptin-induced STAT3 signaling

Cyclin-dependent kinase (Cdk) 5 is ubiquitously expressed in the brain and plays an essential role in central nervous system development and synaptic plasticity. The p35 kinase is a neuronal specific activator of Cdk5. Here, we show for the first time that Cdk5 activation modulates leptin signaling. P35 and its metabolite p25 were colocalized with the leptin receptor ObR in selective neurons in the hypothalamus. Overexpression of p35 alone was sufficient to induce the transcriptional activation of signal transducer and activator of transcription 3 (STAT3) in a cellular model. In retinoic acid-differentiated SH-SY5Y neuronal cells where ObRb was induced, leptin increased the expression of Cdk5, p35, and p25 kinases. The time course of induction coincided with that of phosphorylated (p)-STAT3. When Cdk5 activity was inhibited, either by roscovitine or overexpression of dominant negative Cdk5, there was a reduction of pSTAT3 activation. The results show that the activation of Cdk5 by p35 sustained leptin-induced pSTAT3 at 3-6 h. Thus, p35 is a novel modulator of leptin-induced STAT3 signaling.

Relation of beta-casomorphin to apnea in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the most common cause of death in infants and its pathogenesis is complex and multifactorial. The aim of this review is to summarize recent novel findings regarding the possible association of beta-casomorphin (beta-CM) to apnea in SIDS, which has not been widely appreciated by pediatricians and scientists. beta-CM is an exogenous bioactive peptide derived from casein, a major protein in milk and milk products, which has opioid activity. Mechanistically, circulation of this peptide into the infant's immature central nervous system might inhibit the respiratory center in the brainstem leading to apnea and death. This paper will review the possible relationship between beta-CM and SIDS in the context of passage of beta-CM through the gastrointestinal tract and the blood-brain barrier (BBB), permeability of the BBB to peptides in infants, and characterization of the casomorphin system in the brain.

Phe(13),Tyr(19)-melanin-concentration hormone and the blood-brain barrier: role of protein binding

Melanin-concentrating hormone (MCH), found both peripherally and centrally, is involved in food ingestion. Although its expression in brain is increased by fasting, it is not known whether it crosses the blood-brain barrier (BBB). Use of the sensitive method of multiple-time regression analysis has shown that almost all of the peptides and polypeptides tested cross the BBB at a rate faster than the vascular marker albumin. With this same method, however, we found that the 19-amino acid 125I-Phe13,Tyr19-MCH did not cross faster than 99mTc-albumin. Several mechanisms were excluded as possible explanations for the slow rate of influx. These included degradation, association with capillary endothelial cells, and transport from brain to blood. When Phe13,Tyr19-MCH was perfused in blood-free buffer, however, it entered the brain significantly faster than albumin. This suggested protein binding as an explanation for the slow rate of influx when the MCH was administered in blood. Protein binding was confirmed by capillary zone electrophoresis, which showed that almost all of the Phe13,Tyr19-MCH added to blood migrated with a large-molecular-weight substance. Sodium dodecyl sulfate-capillary gel electrophoresis of Phe13,Tyr19-MCH in buffer additionally showed that the MCH aggregated as a trimer, a factor not preventing its influx by blood-free perfusion. Thus, the results show that blood-borne Phe13,Tyr19-MCH does not significantly cross the BBB, probably because of its binding to serum proteins.

Oxytocin and addiction: a review

Neuropeptides affect adaptive central nervous system processes related to opiate ethanol and cocaine addiction. Oxytocin (OXT), a neurohypophyseal neuropeptide synthesized in the brain and released at the posterior pituitary, also is released in the central nervous system (CNS). OXT acts within the CNS and has been shown to inhibit the development of tolerance to morphine, and to attenuate various symptoms of morphine withdrawal in mice. In rats, intravenous self-administration of heroin was potently decreased by OXT treatment. In relation to cocaine abuse, OXT dose-dependently decreased cocaine-induced hyperlocomotion and stereotyped grooming behavior. Following chronic cocaine treatment, the behavioral tolerance to the sniffing-inducing effect of cocaine was markedly inhibited by OXT. Behavioral sensitization to cocaine, on the other hand, was facilitated by OXT. OXT receptors in the CNS--mainly those located in limbic and basal forebrain structures--are responsible for mediating various effects of OXT in the opiate- and cocaine-addicted organism. Dopaminergic neurotransmission--primarily in basal forebrain structures--is another important biochemical mediator of the central nervous system effects of OXT. Tolerance to ethanol (e.g. hypothermia-inducing effect of ethanol) also was inhibited by OXT.

Ontogenesis of cell surface mu-opioid ([3H]DAGO) binding sites in rat hypothalamus and ex vivo determination of blood-brain barrier penetration by opioid peptide FK 33-824

Previous studies have demonstrated that the LH response to naloxone changes during development but the reason(s) for this are unknown. In the present work we have investigated the possibility that variations in cell surface opioid receptor levels (determined in tissue slice/punches) or changes in the ability of opioids to enter the CNS might be responsible. Opioid binding data indicate that both [3H]naloxone and [3H]DAGO-labelled binding sites remain at low levels until 10 days of age after which there is a progressive rise to adult levels at 15 days ([3H]DAGO) and 21 days ([3H]naloxone). Although several peaks and nadirs were observed in this detailed profile of receptor ontogeny, no exact correlation with the time course of LH response to opioid drugs was found. In an adaption of the slice binding assay we are able to quantify drug penetration into the brain (ex vivo binding). Ex vivo binding studies of blood-brain barrier (BBB) ontogeny indicate that there are changes with age in the ability of opioid peptides, injected subcutaneously, to inhibit binding at the mu-receptor. FK 33-824 induced a reduction in [3H]DAGO binding in the mediobasal hypothalamus until 15 days of age. FK in older rats had no effect on [3H]DAGO binding suggesting that formation of the BBB is complete at this age. In contrast, FK injection reduced binding in the median eminence-arcuate nucleus area (outside BBB) until 30 days of age. Surprisingly, this area also became refractory to FK injection after this age.(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.

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.

Analgesia and the blood-brain barrier transport system for Tyr-MIF-1/enkephalins: evidence for a dissociation

The blood-brain barrier is capable of transporting peptides with anti-opiate (Tyr-MIF-1) and opiate (enkephalins) activity out of the central nervous system. The relationship of this transport system to the various actions of opiates remains unexplored. This study examined the relationship between the rate of transport and opiate-induced analgesia. Both restraint, a stress that provokes an opiate-mediated analgesia, and the administration of morphine (12 mg/kg, i.p.) each induced an inhibition in the rate of transport. Such inhibition exhibited specificity, since the saturable, brain to blood transport of iodide remained unaltered. However, it was possible to dissociate analgesia and inhibition of transport. The onset and peak of analgesia, as measured by tail-flick latency induced by morphine, preceded the onset and peak of the inhibition of transport. Naltrexone, which blocks opiate-mediated analgesia, also induced inhibition of transport without any significant effect on tail-flick latency. (-) Naloxone but not (+) naloxone also weakly inhibited transport. Deprivation of food and water, associated with analgesia possibly mediated by the opiate, beta-endorphin, which is not transported out of the brain by this system, did not alter transport. These results suggest that while inhibition of transport and analgesia may occur together, these events probably represent two separate aspects of the action of opiates, that may even be mediated by separate receptor sites or peptides in the opiate family.

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.

Delta-sleep-inducing peptide (DSIP): an update

The isolation and characterization of delta-sleep-inducing peptide (DSIP) achieved from 1963 to 1977 were reviewed in 1984. The first reports describing sleep as well as extra-sleep effects of DSIP also were included in that work. Only two years later, much additional literature concerning DSIP has accumulated. Besides further sleep-inducing and/or -supporting effects of DSIP in animals, considerable work has been carried out to evaluate the potential use of the peptide for therapeutic purposes such as treatment of insomnia, pain, and withdrawal. Immunohistochemical as well as radioimmunochemical studies provided further insights into the natural occurrence of the nonpeptide and the distribution of DSIP-like material in the body, suggesting possible relations of the peptide to certain diseases. Various physiological functions of DSIP and a possible mechanism of action involving the modulation of adrenergic transmission remain to be established.

Aging and the blood-brain barrier: changes in the carrier-mediated transport of peptides in rats

Age-related changes in the brain's saturable, specific, carrier-mediated transport system for the small, N-tyrosinated peptides Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and methionine-enkephalin (Met-Enk) were studied in Fischer 344 rats aged 4 and 26 months. These studies showed statistically significant differences between the two age groups for both the Tmax (transport maximum) [3.22 +/- 0.013 nmol/min/g (young rats, mean +/- S.E.M.) vs 2.41 +/- 0.009 nmol/min/g (age rats)] and T50 (the amount required to achieve 50% of that maximum) [84.9 +/- 1.0 nmol/g (young) vs 65.1 +/- 0.60 nmol/g (aged)]. The T50:Tmax ratio was nearly equal to the two groups: 26.4 (young) vs 26.9 (aged), consistent with the uncompetitive type of inhibition indicative of alterations in the substrate-carrier complex. In addition, blood concentrations of Tyr-MIF-1-like immunoactivity were nearly doubled in aged rats (3.24 +/- 0.373 vs 1.67 +/- 0.0904 pM/ml), while blood concentrations of Met-Enk-like immunoactivity and brain concentrations of immunoactive Tyr-MIF-1 and Met-Enk showed no statistically significant difference between age groups. Thus, a carrier-mediated system responsible for the transport of peptides across the blood-brain barrier undergoes changes with aging.

Administered peptides inhibit the degradation of endogenous peptides. The dilemma of distinguishing direct from indirect effects

Virtually all peptides are biologically active following central administration as a consequence of both direct and indirect cellular actions. Direct effects are mainly interactions with specific membrane receptors but may include unions with other components of the receptor/effector complex. Significant indirect biological effects of exogenous peptides, including apparent secretagogue effects on endogenous peptides largely overlooked in practice, result from extensive competition with endogenous peptides for degradative enzymes (peptidases). A consequence of this competition is enhancement of tonic or intermittent activity of endogenous peptides. The pharmacological profile of any peptide reflects or includes, therefore, the spectrum of endogenous peptides that is protected from peptidase action. It is likely that certain pharmacologically active peptides, including a large number of di-, tri- and oligo-peptides, elicit responses mainly or exclusively by competing for peptidases. Therefore, reliable estimates of the relative contributions of direct and indirect actions of exogenous peptides may be difficult, if not impossible, to obtain.

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

The plasma disappearance, metabolism and uptake in the brain of [3H-Phe4]-DT gamma E and [3H-Lys9]-DE gamma E were investigated following systemic administration of these neuroleptic-like peptides to rats. 3H-DT gamma E, 3H-DE gamma E and their radioactive metabolites in plasma and brain extracts were determined by reversed-phase HPLC. Plasma disappearance of DT gamma E upon intravenous (IV) dosing followed a biphasic pattern with half-lives of 0.7 min (distribution phase) and 5.5 min (elimination phase). For DE gamma E the plasma disappearance curve was best characterized by a one-compartment model since a second elimination phase was hardly detectable by our methods. The corresponding half-life was 0.6 min, probably representative for the initial distribution phase of DE gamma E. Both neuropeptides distributed rapidly over the larger part of the extracellular fluid. Following the IV route of administration, brain uptake of DT gamma E and DE gamma E appeared to be low. Brain levels of DT gamma E decreased from 0.0075% to 0.0031% of the administered dose/g tissue at 2-15.5 min after injection, whereas those of DE gamma E decreased very rapidly from 0.0174% of the dose/g brain tissue to below the detection limit at 2-4.5 min after injection. As compared to the IV route of administration, subcutaneous (SC) injection of DE gamma E resulted into lower but remarkably longer-lasting peptide concentrations in plasma as well as in brain, possibly because of a sustained release from the SC site of injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental, behavioral, and opiate receptor changes after prenatal or postnatal beta-endorphin, CRF, or Tyr-MIF-1

Developmental and long-term behavioral effects of perinatal injection of beta-endorphin (BE), CRF and Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1) in male rats were investigated along with the possibility that opiate receptors may be altered by the injection of BE during this critical time. Daily injections of peptide were given to pregnant females (100 micrograms/rat) in the week before birth or to the offspring (50 micrograms/rat) of untreated mothers during the first week of life. Prenatal BE and CRF reduced body weight on day 1, in contrast to Tyr-MIF-1 which produced a significant increase over controls by day 7 as well as a slight but significant acceleration of eye opening. Among the postnatal treatments, CRF-treated animals showed the most dramatic changes. These included decreased body weight, accelerated eye opening, and, in adulthood, increased open field rearing behavior and a tendency for a monotonic body temperature response to low doses of morphine, in contrast to the biphasic response shown by controls. BE, when given to pregnant mothers, increased the number (Bmax) of [3H]naloxone-labeled (mu) receptors in whole brains of offspring assayed on day 14, but it did not significantly alter [3H]D-Ala-D-Leu-enkephalin-labeled (delta) receptors. In contrast, a significant decrease in both mu and delta receptors was observed on day 14 in rats given BE postnatally. These differences in receptors were no longer apparent in adulthood, and no significant differences in tail-flick response were detectable at this time. Nevertheless, some of the effects of these three peptides endured well beyond their presence, and for BE included changes in the number of opiate receptors.

Modulation of immunoactive levels of DSIP and blood-brain permeability by lighting and diurnal rhythm

The brain and plasma levels of immunoactive delta sleep-inducing peptide (DSIP) as well as the permeability of the blood-brain barrier (BBB) to radioiodinated N-Tyr-DSIP (125I-DSIP) were measured at 0400, 0800, 1200, 1600, 2000, and 2400 hr in rats in a normal 12-hr-light/12-hr-dark cycle and at 0800 in rats in constant light or constant dark. Both brain and blood levels of immunoactivity showed statistically significant diurnal changes, whereas the measurement of BBB permeability varied in a regular fashion over time without the changes reaching statistical significance. Immunoactive levels of DSIP in both the plasma and the brain were higher and permeability of the BBB to 125I-DSIP increased in both the constant light and especially the constant dark groups in comparison with the cycled 0800 group. Diurnal variations continued to occur in the blood levels of immunoactive DSIP in the constant dark animals. Studies with radioiodinated serum albumin (RISA) showed that these findings did not result from a change in brain hemodynamics. Immunoactive levels of DSIP in the plasma correlated with brain immunoactive levels and with BBB permeability to 125I-DSIP. The increase in penetration of 125I-DSIP into the brain that occurred with changes in the lighting cycle appeared to be magnified by pre-treatment with aluminum. The results show interrelationships among various aspects of the neuroendocrine axis for DSIP and their modulation by physiological factors.

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.

The aluminum-induced increase in blood-brain barrier permeability to delta-sleep-inducing peptide occurs throughout the brain and is independent of phosphorus and acetylcholinesterase levels

The effect of aluminum on levels of inorganic phosphorus and acetylcholinesterase in blood and brain and on permeability of the blood-brain barrier (BBB) in different regions of the brain to the neuropeptide delta-sleep-inducing peptide (DSIP) was studied in adult rats. Aluminum (100 mg/kg) significantly increased the permeability of the BBB to intracarotid 125I-N-Tyr-DSIP so that levels of radioactivity in whole brain were 45% higher than in control animals. The pattern of regional distribution of radioactivity in the brain was, however, unaffected, demonstrating that the affect of aluminum occurs throughout the BBB. Aluminum also significantly decreased inorganic phosphorus levels in the serum by 19%, but this effect did not correlate with BBB permeability to DSIP. Aluminum did not decrease brain levels of phosphorus despite the drop in blood levels of phosphorus nor affect brain or blood levels of acetylcholinesterase. Experiments with radioactive 32P reinforced the finding that blood but not brain levels of phosphorus are reliably affected by aluminum. The lack of correlation between changes in BBB permeability and decreased levels of inorganic phosphorus in the blood suggests that the effect of aluminum may not be mediated by its effects on phosphorus metabolism. Also, the change in BBB permeability after administration of aluminum does not appear to depend on changes in brain cholinergic activity but does occur throughout the brain.

Evidence that [125I]N-Tyr-delta sleep-inducing peptide crosses the blood-brain barrier by a non-competitive mechanism

Delta sleep-inducing peptide (DSIP), a nonapeptide, has previously been shown to cross the blood-brain barrier (BBB) in rats and the blood-CSF barrier in dogs. New experiments were conducted to determine if this crossing was competitive. Neither DSIP nor several analogs, including non-radioactive [127I]N-Tyr-DSIP, injected by the jugular vein or carotid artery, inhibited passage of radioactive [125I]N-Tyr-DSIP across the rat BBB. Column chromatography of brain samples confirmed that the radioactivity in the brain represented intact [125I]N-Tyr-DSIP and that the non-radioactive competing materials did not interfere with the degradation or binding of [125I]N-Tyr-DSIP. In addition, N-Tyr-DSIP was unable to inhibit the appearance of radioactive [125I]N-Tyr-DSIP in the CSF of dogs. In conclusion, the evidence from these experiments suggests that [125I]N-Tyr-DSIP crosses the rat BBB and dog blood-CSF barrier by a non-competitive mechanism.

Delta-sleep-inducing peptide (DSIP): a review

Since the turn of the century, it has been postulated that humoral factors induce sleep. Many compounds were proposed as sleep-factors, but only two of the sleep-peptides have been purified to homogeneity and characterized, so far. One of them, DSIP, was shown to be a nonapeptide of MW 849 and to induce mainly delta-sleep in rabbits, rats, mice, and humans, whereas in cats, the effect on REM sleep was more pronounced. A U-shaped activity curve was determined for the dose as well as for the time of infusion. DSIP-like material was found by RIA and immunohistochemistry in brain and by RIA in peripheral organs of the rat as well as in plasma of several mammals. In addition to sleep, the peptide also has been observed to affect electrophysiological activity, neurotransmitter levels in the brain, circadian and locomotor patterns, hormonal levels, psychological performance, and the activity of neuropharmacological drugs including their withdrawal.

Alpha-melanocyte stimulating hormone increases plasma levels of glucagon and insulin in rabbits

Intravenous injections of 25 and 2.5 micrograms alpha-melanocyte stimulating hormone (alpha-MSH) increased plasma levels of glucagon, insulin and free fatty acids in fasted and fed rabbits. 45 micrograms beta-melanocyte stimulating hormone (beta-MSH) had similar effects, whereas 22 micrograms gamma-2-melanocyte stimulating hormone (gamma-MSH) was inactive. The alpha-MSH-induced increases in the plasma levels of glucagon, insulin and free fatty acids were not inhibited by alpha- or beta-adrenergic blocking drugs. The alpha-MSH-induced increases in the plasma levels of insulin were, however, augmented by phentolamine (an alpha-adrenergic receptor blocking drug). The plasma levels of glucose were increased by 25 micrograms alpha-MSH in fed rabbits, only, and were decreased by alpha-MSH during alpha-receptor blockade. The acute in vivo effects of alpha-MSH and beta-MSH on the plasma levels of glucagon, insulin and free fatty acids were rather similar to those previously reported for corticotropin (ACTH). It is possible that the 4-10 ACTH sequence, present in alpha-MSH, beta-MSH and ACTH, but not in gamma-MSH, is a message sequence for the observed effects. However, ORG 2766, a 4-9 ACTH analogue, was inactive. The mechanism by which alpha-MSH increased the plasma levels of glucagon and insulin in rabbits remains to be determined. It is possible, that the effects were mediated by both a central nervous action and a direct action on the endocrine pancreas.

Misleading concepts in the field of brain peptides

Peptide research, like all of science, requires a careful balance between building upon previously accumulated knowledge and exploring perspectives that are in conflict with prevailing views. Many of the concepts discussed here originally stimulated certain lines of research. The downfall of most of these concepts, and thus their misleading nature, lies in the limitation of possible modes by which peptides are perceived to affect the functioning of the central nervous system (CNS).

Characterization of binding sites for N-Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) in rat brain

Binding sites for N-Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1), a novel peptide structurally related but immunoreactively different from Pro-Leu-Gly-NH2 (MIF-1), were investigated. The presence of Tyr-MIF-1-like material in brain tissue has previously been demonstrated by RIA and its levels were shown to vary with the diurnal cycle and after pinealectomy. We now demonstrate a high affinity binding site in rat brain that is saturable and specific for Tyr-MIF-1. Crude P2 synaptosomal fractions from rat brains were incubated with 125I-Tyr-MIF-1 in the presence or absence of 10 microM unlabeled Tyr-MIF-1 (nonspecific binding). Binding reached equilibrium at 30-40 min at 23 degrees C and at about 4 hr on ice, after which it was relatively stable for at least 18 hr. None of the other peptides (including MIF-1) or amino acid residues tested were found to effectively compete for 125I-Tyr-MIF-1 binding. Binding was linear with protein from 280 micrograms to at least 1.1 mg protein per tube. Scatchard analysis of the striatum-thalamus revealed the presence of binding sites with an apparent Kp of 91 nM and maximum number of sites in the range of 45 fmol/mg tissue. Analysis of several brain areas revealed a differential distribution of the binding sites with relatively high concentrations in cortex, striatum, and amygdala and low concentrations in pons-medulla. Together with the previously published RIA results, the demonstration of a receptor for Tyr-Pro-Leu-Gly-NH2 supports the concept of the presence of this novel peptide and its receptor in the brain.