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

Regulation of the Blood-Brain Barrier by Circadian Rhythms and Sleep

The blood-brain barrier (BBB) is an evolutionarily conserved, structural, and functional separation between circulating blood and the central nervous system (CNS). By controlling permeability into and out of the nervous system, the BBB has a critical role in the precise regulation of neural processes. Here, we review recent studies demonstrating that permeability at the BBB is dynamically controlled by circadian rhythms and sleep. An endogenous circadian rhythm in the BBB controls transporter function, regulating permeability across the BBB. In addition, sleep promotes the clearance of metabolites along the BBB, as well as endocytosis across the BBB. Finally, we highlight the implications of this regulation for diseases, including epilepsy.

Neonatal oxytocin and vasopressin manipulation alter social behavior during the juvenile period in Mongolian gerbils

Oxytocin and vasopressin are important modulators of a wide variety of social behaviors, and increasing evidence is showing that these neuropeptides are important organizational effectors of later-life behavior as well. We treated day-old gerbil pups with oxytocin, vasopressin, an oxytocin receptor antagonist, a vasopressin V1a receptor antagonist, or saline control, and then measured received parental responsiveness during the early postnatal period and juvenile social behavior during weaning. Neonatal vasopressin treatment enhanced sociality in males, but not females, at both developmental time points. When pups were individually placed outside the nest, parents were more responsive to male pups treated with vasopressin compared with littermates, and vasopressin treated male pups exhibited increased play with littermates as juveniles. These results show that vasopressin during very early life can enhance social interactions throughout early development.

Social cognition

Social cognition is a major problem underlying deficiencies in interpersonal relationships in several psychiatric populations. And yet there is currently no gold standard for pharmacological treatment of psychiatric illness that directly targets these social cognitive areas. This chapter serves to illustrate some of the most innovative attempts at pharmacological modulation of social cognition in psychiatric illnesses including schizophrenia, borderline personality disorder, autism spectrum disorders, antisocial personality disorder and psychopathy, social anxiety disorder, and posttraumatic stress disorder. Pharmacological modulation includes studies administering oxytocin, ecstasy (MDMA), modafinil, methylphenidate, and D-cycloserine. Furthermore, some background on social cognition research in healthy individuals, which could be helpful in developing future treatments, is provided as well as the potential for each drug as a long-term treatment option.

Neuroendocrine and behavioural responses to exposure to an infant in male prairie voles

Paternal behaviour and pair-bond formation are defining characteristics of social monogamy. However, in comparison to pair-bonding, the endocrine factors associated with the male care of young are not well studied. In the present study, plasma concentrations of oxytocin, vasopressin and corticosterone (CORT) were measured in reproductively naïve male prairie voles as a function of exposure to an infant or control manipulations (i.e. handling or exposure to a wooden dowel). Plasma oxytocin concentrations were transiently elevated within 10 min of pup exposure. Although plasma CORT concentration typically increases after handling, after 10 min of pup exposure, the concentration of plasma CORT was not increased, suggesting an attenuation of CORT release by pup exposure. Group differences in the concentrations of plasma hormones were no longer detected at 20 or 60 min after treatment. These patterns of rapid change in the concentrations of plasma oxytocin and CORT were observed in both juvenile and adult males but not detected after control procedures. Plasma vasopressin, assessed only in adult males, did not vary as a function of pup exposure or other manipulations. In the paraventricular nucleus of the hypothalamus, pup exposure also increased activation (as assessed by the measurement of c-Fos) of neurones that stained for either oxytocin or vasopressin, whereas it decreased c-Fos expression in neurones stained for corticotrophin-releasing hormone. In addition, brief pup exposure (20 min) facilitated subsequent partner preference formation when alloparental males and pup attackers were considered as a group. In the context of other studies, these data support the hypothesis that neuroendocrine changes associated with male alloparental behaviour are related to those implicated in pair-bonding.

Cocaine sensitization does not alter SP effects on locomotion or excitatory synaptic transmission in the NAc of rats

Substance P (SP) and cocaine employ similar mechanisms to modify excitatory synaptic transmission in the nucleus accumbens (NAc), a region implicated in substance abuse. Here we explored, using NAc slices, whether SP effects on these synaptic responses were altered in rats that have been sensitized to cocaine and whether SP could mimic cocaine in triggering increased locomotion in sensitized rats. Intraperitoneal (IP) injection of naïve rats with cocaine (15 mg/kg) caused increased locomotion by 408.5 ± 85.9% (n = 5) which further increased by 733.1 ± 157.8% (n = 5) following a week of cocaine sensitization. A similar challenge with 10 mg/kg of SP after cocaine sensitization did not produce significant changes in locomotion (170.6 ± 61.0%; n = 4). In contrast to cocaine, IP injection of rats with SP or SP(5-11) (10-100 mg/kg) with or without phosphoramidon did not elicit changes in locomotion. In electrophysiological studies, both cocaine and SP depressed evoked NMDA and non-NMDA receptor-mediated excitatory synaptic currents (EPSCs) in slices obtained from naïve rats. In slices derived from cocaine-sensitized rats, cocaine but not SP produced a more profound decrease in non-NMDA compared to NMDA responses. Similar to that in naïve rats, cocaine's effect on the EPSCs in these sensitized rats occluded those of SP. Thus, although SP and cocaine may employ similar mechanisms to depress EPSCs in the NAc, IP injection of SP does not mimic cocaine-induced hyperlocomotion indicating that not all of cocaine's effects are mimicked by SP. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Experimental peripheral administration of oxytocin elevates a suite of cooperative behaviours in a wild social mammal

The evolution and expression of different forms of cooperative behaviour (e.g. feeding, guarding, sentinel duties, etc.) are usually studied independently, with few studies considering them as a single syndrome. However, studies investigating individuals' investment across a suite of different behaviours reveal that they are correlated, suggesting a single mechanism determining the evolution and expression of cooperative behaviours. A hormonal mechanism could achieve this, and one possibility is oxytocin (OT), which affects several prosocial or alloparental behaviours independently. We show, using a double-blind experiment, that peripheral administration of OT to social, free-living meerkats Suricata suricatta elevates a suite of cooperative behaviours. Treated individuals increase their contributions to communal, cooperative activities (digging, guarding, pup-feeding and associating with pups) and decrease initiation of aggressive interactions, compared with a saline control. This suggests that different forms of cooperative behaviour form a single syndrome with a common causal basis. If our peripherally administered OT acts in the same way as the naturally released hormone, then a general tendency to prosociality may be modulated by this hormonal system. Therefore, it may be difficult for an individual to decouple expression of cooperative behaviours that provide the practitioner with benefits from those that provide the recipient with benefits. It may also explain why social species typically exhibit a suite of cooperative behaviours, without having to invoke independent evolution of each.

Variation in oxytocin is related to variation in affiliative behavior in monogamous, pairbonded tamarins

Oxytocin plays an important role in monogamous pairbonded female voles, but not in polygamous voles. Here we examined a socially monogamous cooperatively breeding primate where both sexes share in parental care and territory defense for within species variation in behavior and female and male oxytocin levels in 14 pairs of cotton-top tamarins (Saguinus oedipus). In order to obtain a stable chronic assessment of hormones and behavior, we observed behavior and collected urinary hormonal samples across the tamarins' 3-week ovulatory cycle. We found similar levels of urinary oxytocin in both sexes. However, basal urinary oxytocin levels varied 10-fold across pairs and pair-mates displayed similar oxytocin levels. Affiliative behavior (contact, grooming, sex) also varied greatly across the sample and explained more than half the variance in pair oxytocin levels. The variables accounting for variation in oxytocin levels differed by sex. Mutual contact and grooming explained most of the variance in female oxytocin levels, whereas sexual behavior explained most of the variance in male oxytocin levels. The initiation of contact by males and solicitation of sex by females were related to increased levels of oxytocin in both. This study demonstrates within-species variation in oxytocin that is directly related to levels of affiliative and sexual behavior. However, different behavioral mechanisms influence oxytocin levels in males and females and a strong pair relationship (as indexed by high levels of oxytocin) may require the activation of appropriate mechanisms for both sexes.

The role of the blood-CNS barrier in CNS disorders and their treatment

The physical barrier between blood and the CNS (the blood-brain barrier, the blood-spinal cord barrier and the blood-CSF barrier) protects the CNS from both toxic and pathogenic agents in the blood. It is now clear that disruption of the blood-CNS barrier plays a key role in a number of CNS disorders, particularly those associated with neurodegeneration. Such disruption is inevitably accompanied by inflammatory change, as immune cells and immune mediators gain access to the brain or spinal cord. The blood-CNS barrier also presents a major obstacle for potential CNS medicines. Robust methods to assess CNS permeation are therefore essential for CNS drug discovery, particularly when brain pharmacokinetics are taken into account and especially when such measures are linked to neurochemical, physiological, behavioural or neuroimaging readouts of drug action. Drug candidates can be successfully designed to cross the blood-CNS barrier, but for those that can't there is the possibility of entry with a delivery system that facilitates the movement of drug candidate across the blood-CNS barrier.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Effect of aminophylline on aspirin penetration into the central nervous system in rats

This study investigated with the effect of aminophylline on the penetration of aspirin through the blood-brain barrier (BBB) into the central nervous system (CNS) in rats. Acetylsalycylic was injected into the right axillary artery, to avoid the drug affecting the peripheral organs before it reached the CNS. The test animals received subcutaneously (s.c.) aminophylline 30 min before aspirin injection, while the control animals received an equimolar dose of physiological solution s.c. At time intervals of 30, 60, 90, 120, and 240 s after aspirin injection, the animals were decapitated and blood samples from the left jugular vein, as well as samples from the brainstem, cerebellum and left and right cerebral hemispheres, were taken to determine aspirin concentrations in all of them by a standard method. It was found that aspirin concentrations in the CNS were even 30 times lower than in the blood, with the concentrations being higher in the brainstem and cerebellum than in the left and right hemispheres. The presence of aminophylline did not alter aspirin concentrations either in the blood or the brain, and therefore did not affect significantly the aspirin penetration through the BBB into the CNS.

Oxytocin facilitates female sexual maturation through a glia-to-neuron signaling pathway

It has been earlier proposed that oxytocin could play a facilitatory role in the preovulatory LH surge in both rats and humans. We here provide evidence that oxytocin also facilitates sexual maturation in female rats. The administration of an oxytocin antagonist for 6 d to immature female rats decreased GnRH pulse frequency ex vivo and delayed the age at vaginal opening and first estrus. The in vitro reduction in GnRH pulse frequency required chronic blockade of oxytocin receptors, because it was not acutely observed after a single injection of the antagonist. Hypothalamic explants exposed to the antagonist in vitro showed a reduced GnRH pulse frequency and failed to respond to oxytocin with GnRH release. Prostaglandin E(2) (PGE(2)) mimicked the stimulatory effect of oxytocin on GnRH pulse frequency, and inhibition of PG synthesis blocked the effect of oxytocin, suggesting that oxytocin accelerates pulsatile GnRH release via PGE(2). The source of PGE(2) appears to be astrocytes, because oxytocin stimulates PGE(2) release from cultured hypothalamic astrocytes. Moreover, astrocytes express oxytocin receptors, whereas GnRH neurons do not. These results suggest that oxytocin facilitates female sexual development and that this effect is mediated by a mechanism involving glial production of PGE(2).

The organizational effects of oxytocin on the central expression of estrogen receptor alpha and oxytocin in adulthood

Background

Previous studies have demonstrated that neonatal manipulation of oxytocin (OT) has effects on the expression of estrogen receptor α (ERα) and the central production of oxytocin observed in juveniles (at weaning, 21 days of age). The goal of this study was to determine whether the effects of neonatal manipulation of OT last into adulthood, and if the effects differ from those observed during the early postnatal period. On the first day of life, prairie voles (Microtus ochrogaster) received one of three doses of OT (High, 3 μg; Med, 0.3 μg; Low, 0.03 μg), an OT antagonist, or isotonic saline. Another group was handled, but not injected. Then as adults, brains were collected, sectioned, and stained for ERα or OT using immunocytochemistry.

Results

In females, treatment with OT increased the expression of ERα immunoreactivity in the ventral lateral septum (0.03 μg) and the ventromedial nucleus of the hypothalamus and central amygdala (0.3 μg). In males, OT antagonist increased ERα expression in the bed nucleus of the stria terminalis. There was no apparent effect of OT on the number of cells producing OT in the paraventricular nucleus of the hypothalamus.

Conclusion

The current results suggest that neonatal manipulation of OT has long-term organizational effects on the expression of ERα in both males and females. The lack of effect on OT neurons in the paraventricular nucleus suggests that some developmental effects of OT previously observed in weanlings do not persist into adulthood. Developmental effects of OT on ERα patterns were sexually dimorphic, dose-dependent, and site-specific.

Oxytocin selectively increases ERalpha mRNA in the neonatal hypothalamus and hippocampus of female prairie voles

During neonatal development exogenous oxytocin increases ERalpha immunoreactivity in the hypothalamus of female prairie voles. The purpose of this study was to determine if the increase in ERalpha is associated with an increase in ERalpha mRNA expression and to determine if the effect is specific to ER subtype or if oxytocin also influences ERbeta mRNA expression. On the day of birth female prairie vole pups were treated with oxytocin, an oxytocin antagonist, or saline. Brains were collected and RT-PCR was used to determine the effect of treatment on ER mRNA production in the hypothalamus, hippocampus, and cortex. Within 2h of treatment oxytocin significantly increased ERalpha mRNA expression in the hypothalamus and hippocampus, but not the cortex, while inhibiting the effects of endogenous oxytocin reduced the expression of ERalpha mRNA in the hippocampus. Neonatal treatment did not affect the expression of ERbetamRNA. The results demonstrate that the effects of oxytocin treatment are region and ER subtype specific and that during the neonatal period oxytocin can affect the expression of ERalpha by altering message production. The regional specific changes in ERalpha mRNA expression in females are consistent with studies examining the behavioral and physiological effects of neonatal manipulation of oxytocin in females.

Effect of caffeine on quinidine transport to the central nervous system in rats

We studied the effect of caffeine on the transport of quinidine through the blood-brain barrier (BBB) to the central nervous system (CNS) in rats. The anesthetized animals received quinidine in the form of a retrograde intra-arterial bolus injection (15 s) into the right axillary artery 30 min after receiving a subcutaneous injection of caffeine (test group) or physiological solution (control group). Rats were decapitated at 30, 60, 90, 120, and 240 s after quinidine administration. Blood samples were taken from the left jugular vein. Upon washing, the brain, was divided into the brainstem, cerebellum, and cerebral hemispheres to determine the quinidine content in each section, using a standard spectrofluorimetric method. Quninidine attained maximal concentrations in the CNS with a latency compared with that in blood; the CNS values were higher. Quinidine kinetics showed two compartments in the CNS, one consisting of the brainstem and cerebellum, in which quinidine concentrations were higher, and the other the cerebral hemispheres. Caffeine caused a significant deceleration of quinidine transition through the BBB to the CNS.

Neonatal oxytocin manipulations have long-lasting, sexually dimorphic effects on vasopressin receptors

Developmental exposure to oxytocin (OT) or oxytocin antagonists (OTAs) has been shown to cause long-lasting and often sexually dimorphic effects on social behaviors in prairie voles (Microtus ochrogaster). Because regulation of social behavior in monogamous mammals involves central receptors for OT, arginine vasopressin (AVP), and dopamine, we examined the hypothesis that the long-lasting, developmental effects of exposure to neonatal OT or OTA might reflect changes in the expression of receptors for these peptides. On postnatal day 1, prairie voles were injected intraperitoneally with either OT (1 mg/kg), an OTA (0.1 mg/kg), saline vehicle, or were handled only. At approximately 60 days of age, vasopressin V1a receptors, OT receptors (OTR) and dopamine D2 receptor binding were quantified using receptor autoradiography in brain tissue taken from males and females. Significant treatment effects on V1a binding were found in the bed nucleus of the stria terminalis (BNST), cingulate cortex (CgCtx), mediodorsal thalamus (MdThal), medial preoptic area of the hypothalamus (MPOA), and lateral septum (LS). The CgCtx, MPOA, ventral pallidum, and LS also showed significant sex by treatment interactions on V1a binding. No significant treatment or sex differences were observed for D2 receptor binding. No significant treatment difference was observed for OTR receptor binding, and only a marginal sex difference. Changes in the neuropeptide receptor expression, especially the V1a receptor, may help to explain sexually dimorphic changes in behavior that follow comparable neonatal manipulations.

Peripheral ghrelin transmits orexigenic signals through the noradrenergic pathway from the hindbrain to the hypothalamus

Ghrelin, a gastrointestinal peptide, stimulates feeding when administered peripherally. Blockade of the vagal afferent pathway abolishes ghrelin-induced feeding, indicating that the vagal afferent pathway may be a route conveying orexigenic ghrelin signals to the brain. Here, we demonstrate that peripheral ghrelin signaling, which travels to the nucleus tractus solitarius (NTS) at least in part via the vagus nerve, increases noradrenaline (NA) in the arcuate nucleus of the hypothalamus, thereby stimulating feeding at least partially through alpha-1 and beta-2 noradrenergic receptors. In addition, bilateral midbrain transections rostral to the NTS, or toxin-induced loss of neurons in the hindbrain that express dopamine beta hydroxylase (an NA synthetic enzyme), abolished ghrelin-induced feeding. These findings provide new evidence that the noradrenergic system is necessary in the central control of feeding behavior by peripherally administered ghrelin.

Reciprocal interactions of insulin and insulin-like growth factor I in receptor-mediated transport across the blood-brain barrier

Although the blood-brain barrier limits free passage of peptides and proteins from the peripheral circulation to the central nervous system, specific transport systems for insulin and IGF-I have been identified. To further determine whether insulin and IGF-I share the same transport system, and if not, whether the two transport systems interact with each other, we performed multiple-time regression analysis in mice after iv injection and in situ brain perfusion of these peptides. Insulin and IGF-I caused reciprocal inhibition of each other's transport, although the effect of insulin was detected only by the in situ brain perfusion system. The interaction took place mainly at the step of cell surface binding as seen in cultured rat brain endothelium 4 brain microvessel endothelial cells. Further studies in 3T3 cells stably overexpressing the insulin receptor showed that the sharing of the transport systems was only partial. We conclude that insulin and IGF-I are mainly transported by their own transport systems, but a small amount can enter the brain by their "noncognate" transporters. The redundancy of their transport systems illustrates the regulatory function of the blood-brain barrier and reflects the importance of blood-borne insulin and IGF-I in the central nervous system.

The many lives of leptin

Leptin is a 16,000-Da protein which is secreted by fat but acts within the brain to regulate adiposity. Our Peptides Classic addressed the mystery of how such a large molecule could negotiate the blood-brain barrier (BBB), a structure which normally excludes proteins from the brain. We found that leptin was transported across the BBB by a saturable transport system. This finding was important to understanding how satiety-related peptides and proteins worked, but it was also important to the concept that the BBB is a regulatory interface important in brain-body communication. Obesity in humans and many animals is associated with a leptin resistant state rather than a leptin deficiency. Subsequent work has shown that a defect in the BBB transport of leptin is key in producing and reinforcing this state of resistance. Leptin is pluripotent and the concept of it being primarily an adipostat is being discarded for more encompassing views. Consideration of the BBB data would favor the view that ancestral levels of leptin were much lower than those currently considered normal and are consistent with leptin acting as a metabolic switch, informing the brain when fat reserves are adequate to direct energy expenditures towards activities other than seeking calories.

Neonatal manipulations of oxytocin alter expression of oxytocin and vasopressin immunoreactive cells in the paraventricular nucleus of the hypothalamus in a gender-specific manner

Early postnatal manipulations of oxytocin have long-term behavioral and physiological consequences; the present study examined the hypothesis that oxytocin or its absence influences the subsequent expression of either oxytocin or arginine vasopressin in the CNS. On postnatal day 1 female and male prairie voles (Microtus ochrogaster) received a single i.p. injection of oxytocin (3 microg), oxytocin antagonist (0.3 microg), or 50 microl of isotonic saline or were only handled. On postnatal days 1, 8 and 21, brains were fixed, sectioned and stained for oxytocin or vasopressin immunoreactivity and analyzed as a function of age, treatment and sex. Both oxytocin and vasopressin immunoreactivity were observed on day 1 in the supraoptic and paraventricular nuclei (PVN) of the hypothalamus. Numbers of oxytocin and vasopressin neurons increased with age in both nuclei. Females treated on postnatal day 1 with oxytocin or oxytocin antagonist displayed a significant increase in oxytocin immunoreactivity on day 21 in the PVN. In contrast, males treated with antagonist tended to have decreased vasopressin immunoreactivity in the same region. These results revealed that the effects of neonatal manipulation of oxytocin are age-dependent, site-specific and sexually dimorphic. The long-lasting effects of neonatal exposure to exogenous oxytocin and oxytocin antagonist indicate a role for oxytocin in the development of the CNS during the neonatal period, affecting the development of the oxytocinergic system in females and the vasopressinergic system in males. The developmental effects observed suggest one possible mechanism by which neonatal exposure to oxytocin or neonatal inhibition of endogenous oxytocin produces long-lasting behavioral and physiological alterations and could play a role in the development of male- and female-typical behavior.

Characterization of blood-brain barrier permeability to PYY3-36 in the mouse

Peptide YY3-36 (PYY) has emerged as an important signal in the gut-brain axis, with peripherally administered PYY affecting feeding and brain function. For these effects to be direct, PYY would have to cross the blood-brain barrier (BBB). Here, we determined the permeability of the BBB to PYY radioactively labeled with 131I (I-PYY). Multiple-time regression analysis showed the unidirectional influx rate (Ki) from blood-to-brain for I-PYY to be 0.49 +/- 0.19 microl/g-min, a rate similar to that previously measured for leptin. Influx was not inhibited by 1 microg/mouse of unlabeled PYY, suggesting PYY crosses the BBB by transmembrane diffusion. About 0.176% of the i.v.-injected dose of I-PYY was taken up by brain, an amount similar to that for other peptides important in gut-brain communication. Capillary depletion showed that 69% of I-PYY crossed the BBB to enter the parenchymal space of the brain, and high-performance liquid chromatography demonstrated that the radioactivity in this space represented intact I-PYY. After intracerebroventricular injection, I-PYY crossed from brain to blood by the mechanism of bulk flow. We conclude that PYY crosses in both the blood-to-brain and brain-to-blood directions by nonsaturable mechanisms. Passage across the BBB provides a mechanism by which blood-borne PYY can affect appetite and brain function.

Developmental exposure to oxytocin facilitates partner preferences in male prairie voles (Microtus ochrogaster)

The authors investigated the effects of postnatal manipulations of oxytocin (OT) on the subsequent tendency to form a partner preference in male prairie voles (Microtus ochrogaster). Neonatally, males received either an injection of OT, an oxytocin antagonist (OTA), 0.9% saline vehicle, or handling without injection. As adults, males were tested for partner preference following 1 hr of cohabitation with a nonestrous female. In a 3-hr preference test, males neonatally exposed to exogenous OT exhibited a significant partner preference, not seen in males receiving OTA or saline. Both OT and OTA voles had significantly higher levels of social contact than saline controls. A single neonatal injection of OT increased both total and selective social behaviors in male prairie voles.

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.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. III: pharmacokinetic and pharmacodynamic considerations

We review pharmacokinetic and pharmacodynamic factors that should be considered in the design and interpretation of developmental neurotoxicity studies. Toxicologic effects on the developing nervous system depend on the delivered dose, exposure duration, and developmental stage at which exposure occurred. Several pharmacokinetic processes (absorption, distribution, metabolism, and excretion) govern chemical disposition within the dam and the nervous system of the offspring. In addition, unique physical features such as the presence or absence of a placental barrier and the gradual development of the blood--brain barrier influence chemical disposition and thus modulate developmental neurotoxicity. Neonatal exposure may depend on maternal pharmacokinetic processes and transfer of the xenobiotic through the milk, although direct exposure may occur through other routes (e.g., inhalation). Measurement of the xenobiotic in milk and evaluation of biomarkers of exposure or effect following exposure can confirm or characterize neonatal exposure. Physiologically based pharmacokinetic and pharmacodynamic models that incorporate these and other determinants can estimate tissue dose and biologic response following in utero or neonatal exposure. These models can characterize dose--response relationships and improve extrapolation of results from animal studies to humans. In addition, pharmacologic data allow an experimenter to determine whether exposure to the test chemical is adequate, whether exposure occurs during critical periods of nervous system development, whether route and duration of exposure are appropriate, and whether developmental neurotoxicity can be differentiated from direct actions of the xenobiotic.

David and Goliath - the slingshot that started the neuropeptide revolution

This review in honor of David de Wied summarizes the work done in my laboratory that first indicated that adrenocorticotropic hormone (ACTH) has a direct effect on the neuromuscular system. Cold stress or ACTH and its related peptides alpha-melanocyte-stimulating hormone (alpha-MSH ) and beta-lipotropin improve the electromechanical characteristics of adrenalectomized and hypophysectomized rats. ACTH-(1-39) accelerates the return of motor and sensory function and improves the morphological characteristics of the motor endplate after peripheral nerve crush. The non-corticotropic fragments ACTH-(4-10), alpha-MSH, the ACTH-(4-9) analogue Organon 2766 (Org 2766) or the ACTH-(4-10) analogue Biomeasure 22015 (BIM 22015) improve electrophysiological and morphological parameters of the regenerating neuromuscular system. ACTH-(4-10) immunoreactivity, present in ventral horn motor neurons in low levels, is decreased ipsilaterally following ipsilateral nerve crush but increases both ipsilaterally and contralaterally if injured animals are treated with ACTH-(4-10) indicating a neuroprotective action. Similarly, Org 2766 appears to have a protective action in the brain following nigrostriatal lesions. In developmental studies, perinatal exposure to ACTH peptides improves the structure of the neuromuscular junction, accelerates the maturation of electromechanical properties and enhances nerve-muscle integration and nerve regeneration. Perinatal exposure to these peptides decreases adult male sexual behavior, a change correlated with increased serotinergic input within the medial preoptic area. Similar changes occur in female rats and appear to be long-lasting. In tissue culture studies, both Org 2766 and BIM 22015 promote neurite outgrowth in the absence of nerve growth factor, indicating a neurotrophic role for these peptides.

C-fos protein expression in the nucleus of the solitary tract correlates with cholecystokinin dose injected and food intake in rats

C-fos protein expression was investigated in the nucleus of the solitary tract (NTS) in response to increasing cholecystokinin (CCK) doses and food intake in rats by counting the number of c-fos protein positive cells in the NTS. C-fos protein expression in the NTS dose-dependently increased in response to CCK, the lowest effective dose being 0.1 microg/kg. The ED(50) for c-fos protein expression in the NTS in response to CCK was calculated to be 0.5 to 1.8 microg/kg, depending on the anatomical level of the NTS investigated. Food intake increased c-fos protein expression in the NTS, the maximum number of c-fos protein positive cells being reached at 90 min after the start of food intake. Regression analysis identified a positive correlation between c-fos protein expression and the amount of food intake. Our data indicate that subpopulations of the NTS that are activated by CCK or food intake are involved into the short-term regulation of food intake and the neural control of feeding by the caudal brainstem.

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.

Effect of immunization against melatonin on prolactin concentrations and the timing of reproductive transitions in ewes

The objective of this experiment was to develop a procedure for immunizing ewes against melatonin that would alter the effects of changing photoperiod on seasonal reproduction and prolactin secretion. Ewes were immunized against human serum albumin (HSA) as controls (n = 9) or a melatonin-human serum albumin conjugate (0.25 mg; n = 10) on December 14th (Day 0) and boosted 9 times. They were maintained on natural photoperiod and then transferred indoors and exposed to long days for 35 d, followed by short days for 146 d, long days for 93 d, and short days for a further 123 d. Antibody titers to melatonin (at a serum dilution of 1:1,250) were significantly higher in immunized ewes (27.3 +/- 6.6%) than controls (0.7 +/- 0.1%; P < 0.001). At the end of the experiment, antibody titers in immunized ewes (at dilution of 1:50) were higher in blood (43.7 +/- 8.2%) than in cerebrospinal fluid (10.8 +/- 3.9%; P < 0.05), and highly correlated (r2 = 0.746). Onset of the breeding season was advanced slightly after the second transfer from long to short days in immunized ewes (April 12 +/- 3 d) compared with controls (April 25 +/- 3 d; P < 0.05). Mean serum prolactin concentrations were lower (P < 0.05) in melatonin-immunized ewes compared with controls on natural photoperiod, after transfer from long to short days, during long days, and after the second transfer from long to short days. In conclusion, despite melatonin-immunization increasing antibody titers in blood and cerebrospinal fluid, and decreasing prolactin concentrations over much of the experiment, minimal effects on the timing of reproductive transitions in the ewes were evident. This discrepancy between the response of the prolactin and reproductive axes to melatonin immunization supports the hypothesis of a dual site of action of melatonin, with melatonin acting in the pituitary gland to mediate the effects of photoperiod on prolactin secretion and in the mediobasal hypothalamus to affect reproductive responses.

[Transport of drugs across the blood-brain barrier]

The blood-brain barrier prevents an indifferent medicine existing in the blood to enter also in the brain. This barrier has got an anatomical base: it is first consisting in a cerebrovascular layer of endothelial capillary vessels of the peripheral tissue. It is moreover covered by outgrowths of the flial cells, which are called astrocytes. There are, for that reason, important limits to a size of molecules which can reach the cerebral tissue through a paracellular way (through what is called in English "tight-junctions"). Most medicines must use the transcellular way. Lipophily is necessary to follow that way. Year after year, it appeared, thanks to a comparative study of the substances, that there exists--grosso modo--a positive correlation between the lipophilic level and the permeation-level of a substance in the cerebral tissue. There are, however, several exceptions: it is so that hydrophilic substances, possessing an important nourishing function (such as glucosis, amino-acids) seem to penetrate much more easily than we could expect when we consider their physicochemical characteristics. This is the result of the fact that there exist specifical transport-mechanisms for these substances at the level of the endothelial cell-membranes, allowing the penetration of such substances. There exist, on the contrary, lipophilic components that penetrate the cerebral tissue much less strongly than we should expect. This happens because there also exist pumping-mechanisms at the level of the hemato-encephalic barrier. The concerning substance, which was recently discovered is the "glycoprotein P", which is also responsible for the "multi-drug-resistance" and for the resistance of tumors to cytostatics. This phenomenon relies on a very efficient pumping of substances which have penetrated cells in which this protein expressed itself in the membranous structure. In order to obtain a better understanding of the function of the hemato-encephalic barrier, comprising the transport of medicines, it is most important to have reliable experimental models. It is to that aim that, during former years, the technique of cultivating endothelial cerebrovascular cells was developed. These cells are isolated from brains of calves or rats and, subsequently, cultivated on a laboratory medium; about a week later, they have grown a single and confluent layer. This layer represents a kint of "hemato-encephalic barrier" in vitro, which allows us to study the transfer of substances through the layer and thus also the details concerning the transport mechanisms, as well as the factors influencing the permeability of the cells-layer (for instance the inflammatory stimuli). Concerning the "in vivo" research, the technique of intracerebral microdialysis in lab-animals proved to be very promising. In order to effect this microdialysis, a semipermeable microcannula is introduced in the brain tissue, across which an iso-osmotic liquid is being injected continuously. The substances staying in the interstitial liquid of the cerebral tissue will diffuse under the influence of a concentration gradient, into the dialysing liquid and they will also be ready to be analysed. Thanks to this technique, it is possible to follow, in the same animal, the evolution of the concentration in the brain of a substance which has, for instance been injected in a peripheral region. In this way, we obtain, indirectly and in vivo, informations about the functioning-process of the "hemato-encephalic barrier". We can, moreover, effect measures on a specific spot, for instance in tumoral brain tissue: this allows us to study the influence of specific transport-mechanisms. These rather recent techniques, as well in vitro as in vivo, will allow us, in consequence, to increase, during the next years, our understanding of the way the hemato-encephalic barrier functions as to the transfer of medicines towards the central nervous system. This understanding may lead us to new strategies allowing

Brain substrates of infant-mother attachment: contributions of opioids, oxytocin, and norepinephrine

The aim of this paper is to review recent work concerning the psychobiological substrates of social bonding, focusing on the literature attributed to opioids, oxytocin and norepinephrine in rats. Existing evidence and thinking about the biological foundations of attachment in young mammalian species and the neurobiology of several other affiliative behaviors including maternal behavior, sexual behavior and social memory is reviewed. We postulate the existence of social motivation circuitry which is common to all mammals and consistent across development. Oxytocin, vasopressin, endogenous opioids and catecholamines appear to participate in a wide variety of affiliative behaviors and are likely to be important components in this circuitry. It is proposed that these same neurochemical and neuroanatomical patterns will emerge as key substrates in the neurobiology of infant attachments to their caregivers.

Neonatal ACTH administration elicits long-term changes in forebrain monoamine innervation. Subsequent disruptions in hypothalamic-pituitary-adrenal and gonadal function

The findings from this study demonstrated that the manipulation of the HPA system resulting from ACTH administration during neonatal development produces long-term, differential effects, not only on adrenocortical activity, but also on the activity and integrity of the forebrain monoamine systems. Increased concentrations of the monoamines within the forebrain regions studied at days 7 and 15, suggest a hastened maturation of these neural systems in animals neonatally treated with ACTH. The observed neurochemical alterations in these animals at one year are suggestive of an accelerated aging in the monoamine systems. A further consequence of these disturbances during development is an altered functioning of the HPG axis, as demonstrated by a delayed onset of puberty as previously reported, as well as significantly decreased proestrus plasma estradiol. Although deficits in sexual behavior also existed, it seems probable that these behavioral changes are a manifestation of altered neural systems regulating the ability to cope with a novel stimulus or situation, rather than a disruption of the "feminization" of the brain during sexual differentiation. This is in contrast to the male rat which exhibits permanent deficits in male typical sexual behavior following developmental ACTH treatment. The clinical relevance of these findings may be extensive. Perinatal exposure to events or agents that markedly increase ACTH and the corticosteroids may cause significant immediate and long-term changes in central monoamine functioning. These changes may constitute some of the most deleterious effects of stress exposure in infants and children. The alterations may be especially devastating in individuals with predispositions to stress-sensitive disorders such as anxiety, depression, and Tourette's syndrome. Finally, the use of ACTH in the treatment of infantile spasms may need to be reassessed in light of the possible long-term effects of ACTH on central monoamine functioning.

Cytokine-to-brain communication: a review & analysis of alternative mechanisms

It is becoming well accepted that products of the immune system (cytokines) can signal the brain that infection has occurred. This cytokine-to-brain communication can result in marked alterations in brain function and behavior. This review examines alternative mechanisms that have been proposed to explain how such immune products can reach the brain via the blood to cause centrally-mediated "illness" responses. Finally, we describe a new view which argues that cytokines signal brain in quite a different manner, by stimulating afferent terminals of peripheral nerves at local sites of synthesis and release.

Effects of substance P on cardiovascular regulation in the rabbit

The effect of substance P on blood pressure and aortic reflex was investigated in rabbits. Microinjections of substance P and Sar9, Met(O2)11-SP (a selective NK1-receptor agonist) into the floor of the fourth ventricle led to a dose-dependent increase of blood pressure and a sharp enhancement of the baroreflex. These effects were abolished by pretreatment with SR 140333 (a selective NK1-receptor antagonist). Intraventricular injection of the antagonist alone significantly decreased the amplitude of the aortic reflex. After bivagotomy, the amplitude of the parasympathetic component of the baroreflex decreased dramatically and substance P injections were no longer effective. Our results demonstrate that substance P activation of NK1 receptors plays a major role in the modulation of the parasympathetic component of the baroceptor reflex.

Specificity versus redundancy of melanocortins in nerve regeneration

The results of the present study demonstrate that administration of the ACTH-(4-9) analogue Org 2766 acutely enhances behavioral, morphological, and biochemical recovery after nigrostriatal destruction. Animals treated with Org 2766 (10 micrograms/kg every 24 hr) demonstrated an acceleration of denervation supersensitivity and a significantly decreased ipsilateral rotational response, as compared to their saline counterparts. Upon evaluation of the mesolimbic DA system using open field behavior, peptide-treated rats demonstrated a compensatory response in their rearing behavior. Furthermore, tyrosine hydroxylase immunocytochemical analysis indicated an enhanced staining in the Org 2766-treated groups. This evaluation was confirmed and quantified using specific high-affinity dopamine uptake. The brains of animals treated with Org 2766 maintained higher uptake levels, suggesting a greater fiber density than the saline-treated animals. Although recovery via reinnervation is very unlikely in this short period of time, improved recovery may be the result of a protective effect of Org 2766 after administration of 6-OHDA into the substantia nigra. Thus, it appears that Org 2766 provides the rapid effects in this system, by both accelerating some compensatory mechanisms necessary for functional recovery and promoting cell survival by providing neuronal protection. However, it does not appear that this protection is due to NMDA receptor manipulation. Org 2766 neither mimicked the NMDA antagonist MK-801 behaviorally nor biochemically in binding displacement studies. Interestingly, other studies have suggested that only the full ACTH molecule, and fragments larger than ACTH-(1-17), demonstrated binding activity at micromolar concentrations, whereas the shorter, noncorticotropic fragments were either less active or inactive (Table 2). As for ACTH-(4-10) immunoreactivity, it appears that this neurotrophic fragment of ACTH reappears in adults following injury to the nigrostriatal system. In addition, the systemically administered ACTH-(4-9) analogue, Org 2766, seems to be gaining access to the CNS, but is only effective in the injured system. Therefore, based on the immunocytochemical localization of the ACTH-(4-10) fragment in neonatal brains and in the injured adult rat CNS, the interesting possibility may be raised that endogenous ACTH peptides appear during both ontogeny and regeneration. These studies demonstrate once again that biological responses to the family of ACTH/MSH peptides depend on the specific peptide fragment administered, its dosage, and the timing of the administration. Consequently, since early intervention is of vital importance in CNS recovery processes, synergistic administration of ACTH fragments and other neurotrophic agents may offer a viable approach with which to combat degeneration in the CNS.

Anteroventral third ventricle lesions abolish lumbar sympathetic responses to insulin

Insulin has been shown to increase sympathetic nerve activity. Because evidence shows that insulin acts within the central nervous system, we hypothesized that lesions of the anteroventral third ventricle region, an area rich in insulin receptors, would abolish sympathetic responses to hyperinsulinemia. We measured mean arterial pressure and lumbar sympathetic nerve activity in fasted, anesthetized sham-lesioned (n = 8) and lesioned (n = 8) rats before and after intravenous insulin infusion at 0.13 U/h during euglycemic clamp. Additional sham-lesioned (n = 10) and lesioned (n = 5) rats received vehicle infusion. Insulin-infused sham-lesioned rats had substantially greater increases in lumbar sympathetic nerve activity (+83 +/- 18%) than vehicle-infused sham-lesioned rats (+27 +/- 4%). Most importantly, insulin-infused lesioned rats had increases in sympathetic activity (+32 +/- 11%) that were no greater than lesioned rats receiving vehicle (+23 +/- 16%). Blood pressure was not altered by insulin or vehicle. To test the possibility that lesions of the anteroventral third ventricle region nonspecifically suppress sympathetic excitatory responses, we evaluated reflex increases in lumbar sympathetic activity to nitroglycerin in sham-lesioned (n = 5) and lesioned (n = 8) rats. Rats with lesions and sham lesions showed comparable increases in lumbar nerve activity during nitroglycerin-induced hypotension. In summary, increases in sympathetic nerve activity to intravenous insulin infusion are abolished by anteroventral third ventricle lesions. These data indicate that the integrity of this brain region is necessary for activation of lumbar sympathetic nerve activity by systemic administration of insulin.

Physiological consequences of the passage of peptides across the blood-brain barrier

Peptides given peripherally have been shown to affect the central nervous system (CNS). Peptides are also capable of crossing the blood-brain barrier (BBB). It is unclear, however, whether such crossing underlies the ability of peptides to affect the CNS. We review specific examples in which a peptide must cross the BBB to produce its effect. The effect elicited by passage often duplicates the effect elicited at peripheral sites of action. Other examples, however, are reviewed in which peptides have opposite effects after central and peripheral administration. Such paradoxical effects suggest that passage of peptides may be involved in feedback or counter-regulatory loops.

Permeability of the blood-brain barrier to peptides: an approach to the development of therapeutically useful analogs

Peptides have been shown in both in vivo and in vitro systems to cross the blood-brain barrier (BBB) and so affect function on the side contralateral to their origin. Some peptides cross primarily by transmembrane diffusion, a nonsaturable mechanism largely dependent on the lipid solubility of the peptide. Other peptides are transported by saturable systems across the BBB. These transport systems can be in the CNS to blood direction, as in the cases of Tyr-MIF-1 and methionine enkephalin, in the blood to CNS direction, as in the case of peptide T, or bidirectional, as in the case of LHRH. Other factors that also affect the amount of peptide crossing the BBB include binding in blood, volume of distribution, enzymatic resistance, and half-time disappearance from the blood. An in vitro model of the BBB has been characterized and used to confirm that peptides can cross the BBB. Results with the model agree with those obtained in vivo and have been used to study the permeability of the BBB to peptides, the effect of peptides on BBB integrity, the cellular pathway peptides and proteins use to cross the BBB, and the ability of the BBB to degrade peptides. The in vivo and in vitro methods have been used together to develop halogenated enkephalin analogs that are enzymatically resistant, cross the BBB readily to accumulate in areas of the brain rich in opiate receptors, and are powerful analgesics.(ABSTRACT TRUNCATED AT 250 WORDS)

Capsaicin does not attenuate bombesin-induced suppression of operant responding for food reward

Systemic treatment with capsaicin, a neurotoxin which damages unmyelinated peptide-containing sensory neurons, has been shown to attenuate bombesin (BBS)-induced suppression of food intake. To determine whether capsaicin-sensitive fibers mediate the effect of BBS on appetitive motivation, we examined BBS-induced suppression of operant responding in rats pretreated neonatally with capsaicin (50 mg/kg; SC) or control vehicle. At 8-10 weeks of age, rats were trained to bar press for food. After achieving a stable level of performance, the animals were injected with BBS (10 micrograms/kg), normal saline, or prefed with 20 Noyes 45-mg pellets. Animals were then tested in an operant chamber on an FR 5 schedule of reinforcement for one hour. The results indicated that BBS suppressed bar pressing, regardless of whether animals were pretreated with capsaicin or control vehicle. These findings are inconsistent with the hypothesis that BBS induces satiety via capsaicin-sensitive neurons. The results suggest the possibility that more than one mechanism may mediate the effects of BBS: a neural mechanism involved in consummatory responses and a humoral mechanism involved in the operant response.

In vitro penetration of des-tyrosine1-D-phenylalanine3-beta-casomorphin across the blood-brain barrier

The blood-brain barrier transport and metabolism of the synthetic beta-casomorphin (beta CM) derivative des-tyrosine1-D-phenylalanine3-beta-casomorphin (DT-D-Phe3-beta CM) were investigated using an in vitro model consisting of primary cultures of bovine cerebrovascular endothelial cells. DT-D-Phe3-beta CM was transported across the endothelial monolayer without significant metabolism. The endothelial permeability expressing the transport rate ranged between 1.4 and 2.2 cm x 10(-3)/min and was neither affected by luminal concentration changes (1 nM and 1 microM) nor different after luminal and abluminal administration. The metabolic inhibitor 2-desoxy-D-glucose did not affect the permeability of DT-D-Phe3-beta CM. These results suggest that DT-D-Phe3-beta CM is able to cross the blood-brain barrier by paracellular transport without using a carrier system.