Developmental expression of P-glycoprotein (multidrug resistance gene product) in the rat brain

Elucidation of the GSK3α Structure Informs the Design of Novel, Paralog-Selective Inhibitors

Glycogen synthase kinase 3 (GSK3) remains a therapeutic target of interest for diverse clinical indications. However, one hurdle in the development of small molecule GSK3 inhibitors has been safety concerns related to pan-inhibition of both GSK3 paralogs, leading to activation of the Wnt/β-catenin pathway and potential for aberrant cell proliferation. Development of GSK3α or GSK3β paralog-selective inhibitors that could offer an improved safety profile has been reported but further advancement has been hampered by the lack of structural information for GSK3α. Here we report for the first time the crystal structure for GSK3α, both in apo form and bound to a paralog-selective inhibitor. Taking advantage of this new structural information, we describe the design and in vitro testing of novel compounds with up to ∼37-fold selectivity for GSK3α over GSK3β with favorable drug-like properties. Furthermore, using chemoproteomics, we confirm that acute inhibition of GSK3α can lower tau phosphorylation at disease-relevant sites in vivo, with a high degree of selectivity over GSK3β and other kinases. Altogether, our studies advance prior efforts to develop GSK3 inhibitors by describing GSK3α structure and novel GSK3α inhibitors with improved selectivity, potency, and activity in disease-relevant systems.

Maternal and Fetal Expression of ATP-Binding Cassette and Solute Carrier Transporters Involved in the Brain Disposition of Drugs

Evidence from preclinical and clinical studies demonstrate that pregnancy is a physiological state capable of modifying drug disposition. Factors including increased hepatic metabolism and renal excretion are responsible for impacting disposition, and the role of membrane transporters expressed in biological barriers, including the placental- and blood-brain barriers, has received considerable attention. In this regard, the brain disposition of drugs in the mother and fetus has been the subject of studies attempting to characterize the mechanisms by which pregnancy could alter the expression of ATP-binding cassette (ABC) and solute carrier (SLC) transporters. This chapter will summarize findings of the influence of pregnancy on the maternal and fetal expression of ABC and SLC transporters in the brain and the consequences of such changes on the disposition of therapeutic drugs.

Simple and efficient protocol to isolate and culture brain microvascular endothelial cells from newborn mice

The neurovascular unit (NVU) is a multicellular structure comprising of neurons, glial cells, and non-neural cells, and it is supported by a specialized extracellular matrix, the basal lamina. Astrocytes, brain microvascular endothelial cells (BMECs), pericytes, and smooth muscle cells constitute the blood-brain barrier (BBB). BMECs have a mesodermal origin and invade the nervous system early in neural tube development, forming the BBB anatomical core. BMECs are connected by adherent junction complexes composed of integral membrane and cytoplasmic proteins. In vivo and in vitro studies have shown that, given the proximity and relationship with neural cells, BMECs acquire a unique gene expression profile, proteome, and specific mechanical and physical properties compared to endothelial cells from the general vasculature. BMECs are fundamental in maintaining brain homeostasis by regulating transcellular and paracellular transport of fluids, molecules, and cells. Therefore, it is essential to gain in-depth knowledge of the dynamic cellular structure of the cells in the NVU and their interactions with health and disease. Here we describe a significantly improved and simplified protocol using C57BL/6 newborn mice at postnatal day 1 (PND1) to isolate, purify, and culture BMECs monolayers in two different substrates (glass coverslips and transwell culture inserts). In vitro characterization and validation of the BMEC primary culture monolayers seeded on glass or insert included light microscopy, immunolabeling, and gene expression profile. Transendothelial electrical resistance (TEER) measurement and diffusion test were used as functional assays for adherent junction complexes and integrity and permeability of BMECs monolayers. The protocol presented here for the isolation and culture of BMECs is more straightforward than previously published protocols and yields a high number of purified cells. Finally, we tested BMECs function using the oxygen-glucose deprivation (OGD) model of hypoxia. This protocol may be suitable as a bioscaffold for secondary cell seeding allowing the study and better understanding of the NVU.

Mind the Gaps: Ontogeny of Human Brain P-gp and Its Impact on Drug Toxicity

Available data on human brain P-glycoprotein ontogeny during infancy and childhood are limited. This review discusses the current body of data relating to maturation of human brain P-glycoprotein including transporter expression levels in post-mortem human brain samples, in vivo transporter activity using probe substrates, surrogate marker endpoints, and extrapolations from animal models. Overall, the data tend to confirm that human brain P-glycoprotein activity keeps developing after birth, although with a developmental time frame that remains unclear. This knowledge gap is a concern given the critical role of brain P-glycoprotein in drug safety and efficacy, and the vulnerable nature of the pediatric population. Future research could include the measurement of brain P-glycoprotein activity across age groups using positron emission tomography or central pharmacodynamic responses. For now, caution is advised when extrapolating adult data to children aged younger than 2 years for drugs with P-glycoprotein-dependent central nervous system activity.

Physiologically Based Modeling Approach to Predict Dopamine D2 Receptor Occupancy of Antipsychotics in Brain: Translation From Rat to Human

Receptor occupancy (RO) is a translational biomarker for assessing drug efficacy and safety. We aimed to apply a physiologically based pharmacokinetic (PBPK) modeling approach to predict the brain dopamine D2 RO time profiles of antipsychotics. Clozapine and risperidone were modeled together with their active metabolites, norclozapine and paliperidone, First, in PK‐Sim a rat PBPK model was developed and optimized using literature plasma PK data. Then, blood‐brain barrier parameters including the expression and efflux transport kinetics of P‐glycoprotein were optimized using literature microdialysis data on brain extracellular fluid (brainECF), which were further adapted when translating the rat PBPK model into the human PBPK model. Based on the simulated drug and metabolite concentrations in brainECF, drug‐D2 receptor binding kinetics (association and dissociation rates) were incorporated in MoBi to predict RO. From an extensive literature search, 32 plasma PK data sets (16 from rat and 16 from human studies) and 23 striatum RO data sets (13 from rat and 10 from human studies) were prepared and compared with the model predictions. The rat PBPK‐RO model adequately predicted the plasma concentrations of the parent drugs and metabolites and the RO levels. The human PBPK‐RO model also captured the plasma PK and RO levels despite the large interindividual and interstudy variability, although it tended to underestimate the plasma concentrations and RO measured at late time points after risperidone dosing. The developed human PBPK‐RO model was successfully applied to predict the plasma PK and RO changes observed after risperidone dose reduction in a clinical trial in schizophrenic patients.

Modulation of bilirubin neurotoxicity by the Abcb1 transporter in the Ugt1-/- lethal mouse model of neonatal hyperbilirubinemia

Moderate neonatal jaundice is the most common clinical condition during newborn life. However, a combination of factors may result in acute hyperbilirubinemia, placing infants at risk of developing bilirubin encephalopathy and death by kernicterus. While most risk factors are known, the mechanisms acting to reduce susceptibility to bilirubin neurotoxicity remain unclear. The presence of modifier genes modulating the risk of developing bilirubin-induced brain damage is increasingly being recognised. The Abcb1 and Abcc1 members of the ABC family of transporters have been suggested to have an active role in exporting unconjugated bilirubin from the central nervous system into plasma. However, their role in reducing the risk of developing neurological damage and death during neonatal development is still unknown.To this end, we mated Abcb1a/b-/- and Abcc1-/- strains with Ugt1-/- mice, which develop severe neonatal hyperbilirubinemia. While about 60% of Ugt1-/- mice survived after temporary phototherapy, all Abcb1a/b-/-/Ugt1-/- mice died before postnatal day 21, showing higher cerebellar levels of unconjugated bilirubin. Interestingly, Abcc1 role appeared to be less important.In the cerebellum of Ugt1-/- mice, hyperbilirubinemia induced the expression of Car and Pxr nuclear receptors, known regulators of genes involved in the genotoxic response.We demonstrated a critical role of Abcb1 in protecting the cerebellum from bilirubin toxicity during neonatal development, the most clinically relevant phase for human babies, providing further understanding of the mechanisms regulating bilirubin neurotoxicity in vivo. Pharmacological treatments aimed to increase Abcb1 and Abcc1 expression, could represent a therapeutic option to reduce the risk of bilirubin neurotoxicity.

Enduring, Sexually Dimorphic Impact of In Utero Exposure to Elevated Levels of Glucocorticoids on Midbrain Dopaminergic Populations

Glucocorticoid hormones (GCs) released from the fetal/maternal glands during late gestation are required for normal development of mammalian organs and tissues. Accordingly, synthetic glucocorticoids have proven to be invaluable in perinatal medicine where they are widely used to accelerate fetal lung maturation when there is risk of pre-term birth and to promote infant survival. However, clinical and pre-clinical studies have demonstrated that inappropriate exposure of the developing brain to elevated levels of GCs, either as a result of clinical over-use or after stress-induced activation of the fetal/maternal adrenal cortex, is linked with significant effects on brain structure, neurological function and behaviour in later life. In order to understand the underlying neural processes, particular interest has focused on the midbrain dopaminergic systems, which are critical regulators of normal adaptive behaviours, cognitive and sensorimotor functions. Specifically, using a rodent model of GC exposure in late gestation (approximating human brain development at late second/early third trimester), we demonstrated enduring effects on the shape and volume of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) (origins of the mesocorticolimbic and nigrostriatal dopaminergic pathways) on the topographical organisation and size of the dopaminergic neuronal populations and astrocytes within these nuclei and on target innervation density and neurochemical markers of dopaminergic transmission (receptors, transporters, basal and amphetamine-stimulated dopamine release at striatal and prefrontal cortical sites) that impact on the adult brain. The effects of antenatal GC treatment (AGT) were both profound and sexually-dimorphic, not only in terms of quantitative change but also qualitatively, with several parameters affected in the opposite direction in males and females. Although such substantial neurobiological changes might presage marked behavioural effects, in utero GC exposure had only a modest or no effect, depending on sex, on a range of conditioned and unconditioned behaviours known to depend on midbrain dopaminergic transmission. Collectively, these findings suggest that apparent behavioural normality in certain tests, but not others, arises from AGT-induced adaptations or compensatory mechanisms within the midbrain dopaminergic systems, which preserve some, but not all functions. Furthermore, the capacities for molecular adaptations to early environmental challenge are different, even opponent, in males and females, which may account for their differential resilience or failure to perform adequately in behavioural tests. Behavioural "normality" is thus achieved by the midbrain dopaminergic network operating outside its normal limits (in a state of allostasis), rendering it at greater risk to malfunction when challenged in later life. Sex-specific neurobiological programming of midbrain dopaminergic systems may, therefore, have psychopathological relevance for the sex bias commonly found in brain disorders associated with these systems, and which have a neurodevelopmental component, including schizophrenia, ADHD (attention/deficit hyperactivity disorders), autism, depression and substance abuse.

The biological significance of brain barrier mechanisms: help or hindrance in drug delivery to the central nervous system?

Barrier mechanisms in the brain are important for its normal functioning and development. Stability of the brain's internal environment, particularly with respect to its ionic composition, is a prerequisite for the fundamental basis of its function, namely transmission of nerve impulses. In addition, the appropriate and controlled supply of a wide range of nutrients such as glucose, amino acids, monocarboxylates, and vitamins is also essential for normal development and function. These are all cellular functions across the interfaces that separate the brain from the rest of the internal environment of the body. An essential morphological component of all but one of the barriers is the presence of specialized intercellular tight junctions between the cells comprising the interface: endothelial cells in the blood-brain barrier itself, cells of the arachnoid membrane, choroid plexus epithelial cells, and tanycytes (specialized glial cells) in the circumventricular organs. In the ependyma lining the cerebral ventricles in the adult brain, the cells are joined by gap junctions, which are not restrictive for intercellular movement of molecules. But in the developing brain, the forerunners of these cells form the neuroepithelium, which restricts exchange of all but the smallest molecules between cerebrospinal fluid and brain interstitial fluid because of the presence of strap junctions between the cells. The intercellular junctions in all these interfaces are the physical basis for their barrier properties. In the blood-brain barrier proper, this is combined with a paucity of vesicular transport that is a characteristic of other vascular beds. Without such a diffusional restrain, the cellular transport mechanisms in the barrier interfaces would be ineffective. Superimposed on these physical structures are physiological mechanisms as the cells of the interfaces contain various metabolic transporters and efflux pumps, often ATP-binding cassette (ABC) transporters, that provide an important component of the barrier functions by either preventing entry of or expelling numerous molecules including toxins, drugs, and other xenobiotics. In this review, we summarize these influx and efflux mechanisms in normal developing and adult brain, as well as indicating their likely involvement in a wide range of neuropathologies. There have been extensive attempts to overcome the barrier mechanisms that prevent the entry of many drugs of therapeutic potential into the brain. We outline those that have been tried and discuss why they may so far have been largely unsuccessful. Currently, a promising approach appears to be focal, reversible disruption of the blood-brain barrier using focused ultrasound, but more work is required to evaluate the method before it can be tried in patients. Overall, our view is that much more fundamental knowledge of barrier mechanisms and development of new experimental methods will be required before drug targeting to the brain is likely to be a successful endeavor. In addition, such studies, if applied to brain pathologies such as stroke, trauma, or multiple sclerosis, will aid in defining the contribution of brain barrier pathology to these conditions, either causative or secondary.

Imperatorin-pharmacological meaning and analytical clues: profound investigation

Imperatorin, a furanocoumarin derivative, has many documented pharmacological properties which make it a candidate for possible drug development. In this review, the activity on the central nervous system, the anticancer and antiviral properties and the influence on the cardiovascular system are described. The aim of this review is also to present an overview of the techniques used for the analysis, isolation, and separation of imperatorin from plant material from the practical perspective.

Counteractive effects of antenatal glucocorticoid treatment on D1 receptor modulation of spatial working memory

RATIONALE

Antenatal exposure to the glucocorticoid dexamethasone dramatically increases the number of mesencephalic dopaminergic neurons in rat offspring. However, the consequences of this expansion in midbrain dopamine (DA) neurons for behavioural processes in adulthood are poorly understood, including working memory that depends on DA transmission in the prefrontal cortex (PFC).

OBJECTIVES

We therefore investigated the influence of antenatal glucocorticoid treatment (AGT) on the modulation of spatial working memory by a D₁ receptor agonist and on D1 receptor binding and DA content in the PFC and striatum.

METHODS

Pregnant rats received AGT on gestational days 16-19 by adding dexamethasone to their drinking water. Male offspring reared to adulthood were trained on a delayed alternation spatial working memory task and administered the partial D₁ agonist SKF38393 (0.3-3 mg/kg) by systemic injection. In separate groups of control and AGT animals, D₁ receptor binding and DA content were measured post-mortem in the PFC and striatum.

RESULTS

SKF38393 impaired spatial working memory performance in control rats but had no effect in AGT rats. D₁ binding was significantly reduced in the anterior cingulate cortex, prelimbic cortex, dorsal striatum and ventral pallidum of AGT rats compared with control animals. However, AGT had no significant effect on brain monoamine levels.

CONCLUSIONS

These findings demonstrate that D₁ receptors in corticostriatal circuitry down-regulate in response to AGT. This compensatory effect in D₁ receptors may result from increased DA-ergic tone in AGT rats and underlie the resilience of these animals to the disruptive effects of D₁ receptor activation on spatial working memory.

Treating disorders of the neonatal central nervous system: pharmacokinetic and pharmacodynamic considerations with a focus on antiepileptics

A major consideration in the treatment of neonatal disorders is that the selected drug, dose and dosage frequency is safe, effective and appropriate for the intended patient population. Thus, a thorough knowledge of the pharmacokinetics and pharmacodynamics of the chosen drug within the patient population is essential. In paediatric and neonatal populations two additional challenges can often complicate drug treatment - the inherently greater physiological variability, and a lack of robust clinical evidence of therapeutic range. There has traditionally been an overreliance in paediatric medicine on extrapolating doses from adult values by adjusting for bodyweight or body surface area, but many other sources of variability exist which complicate the choice of dose in neonates. The lack of reliable drug dosage data in neonates has been highlighted by regulatory authorities, as only ~50% of the most commonly used paediatric medicines have been examined in a paediatric population. Moreover, there is a paucity of information on the pharmacokinetic parameters which affect drug concentrations in different body tissues, and pharmacodynamic responses to drugs in the neonate. Thus, in the present review, we draw attention to the main pharmacokinetic factors that influence the unbound brain concentration of neuroactive drugs. Moreover, the pharmacodynamic differences between neonates and adults that affect the activity of centrally-acting therapeutic agents are briefly examined, with a particular emphasis on antiepileptic drugs.

P-glycoprotein in the developing human brain: a review of the effects of ontogeny on the safety of opioids in neonates

The human blood brain barrier is responsible for maintaining brain homeostasis and protecting against potentially toxic substances. The ATP-binding cassette drug efflux protein, P-glycoprotein (P-gp) is a key player in actively extruding a wide range of xenobiotics such as opioids from the brain. Because the blood brain barrier is structurally and functionally immature in neonates, opioids may have a greater penetration to the central nervous system. This may influence the efficacy and safety of opioids in the newborn. Understanding the extent of P-gp's expression in the brain in the embryo, fetus, and newborn will facilitate rational opioid use during pregnancy and the neonatal period. This review aims to summarize the current evidence that associates the ontogeny of P-gp and the susceptibility to opioid-induced adverse respiratory effects in neonates. To date, evidence suggests that the expression of P-gp in the human brain is low at birth, contributing to increased susceptibility.

Sex-specific disruption of murine midbrain astrocytic and dopaminergic developmental trajectories following antenatal GC treatment

The mammalian midbrain dopaminergic systems arising in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) are critical for coping behaviours and are implicated in neuropsychiatric disorders where early life challenges comprise significant risk factors. Here, we aimed to advance our hypothesis that glucocorticoids (GCs), recognised key players in neurobiological programming, target development within these systems, with a novel focus on the astrocytic population. Mice received antenatal GC treatment (AGT) by including the synthetic GC, dexamethasone, in the mothers' drinking water on gestational days 16-19; controls received normal drinking water. Analyses of regional shapes and volumes of the adult SNc and VTA demonstrated that AGT induced long-term, dose-dependent, structural changes that were accompanied by profound effects on astrocytes (doubling/tripling of numbers and/or density). Additionally, AGT induced long-term changes in the population size and distribution of SNc/VTA dopaminergic neurons, confirming and extending our previous observations made in rats. Furthermore, glial/neuronal structural remodelling was sexually dimorphic and depended on the AGT dose and sub-region of the SNc/VTA. Investigations within the neonatal brain revealed that these long-term organisational effects of AGT depend, at least in part, on targeting perinatal processes that determine astrocyte density and programmed cell death in dopaminergic neurons. Collectively, our characterisation of enduring, AGT-induced, sex-specific cytoarchitectural disturbances suggests novel mechanistic links for the strong association between early environmental challenge (inappropriate exposure to excess GCs) and vulnerability to developing aberrant behaviours in later life, with translational implications for dopamine-associated disorders (such as schizophrenia, ADHD, autism, depression), which typically show a sex bias.

A rat model of nerve agent exposure applicable to the pediatric population: The anticonvulsant efficacies of atropine and GluK1 antagonists

Inhibition of acetylcholinesterase (AChE) after nerve agent exposure induces status epilepticus (SE), which causes brain damage or death. The development of countermeasures appropriate for the pediatric population requires testing of anticonvulsant treatments in immature animals. In the present study, exposure of 21-day-old (P21) rats to different doses of soman, followed by probit analysis, produced an LD50 of 62μg/kg. The onset of behaviorally-observed SE was accompanied by a dramatic decrease in brain AChE activity; rats who did not develop SE had significantly less reduction of AChE activity in the basolateral amygdala than rats who developed SE. Atropine sulfate (ATS) at 2mg/kg, administered 20 min after soman exposure (1.2×LD50), terminated seizures. ATS at 0.5mg/kg, given along with an oxime within 1 min after exposure, allowed testing of anticonvulsants at delayed time-points. The AMPA/GluK1 receptor antagonist LY293558, or the specific GluK1 antagonist UBP302, administered 1h post-exposure, terminated SE. There were no degenerating neurons in soman-exposed P21 rats, but both the amygdala and the hippocampus were smaller than in control rats at 30 and 90days post-exposure; this pathology was not present in rats treated with LY293558. Behavioral deficits present at 30 days post-exposure, were also prevented by LY293558 treatment. Thus, in immature animals, a single injection of atropine is sufficient to halt nerve agent-induced seizures, if administered timely. Testing anticonvulsants at delayed time-points requires early administration of ATS at a low dose, sufficient to counteract only peripheral toxicity. LY293558 administered 1h post-exposure, prevents brain pathology and behavioral deficits.

Efflux transporters in blood-brain interfaces of the developing brain

The cerebral microvessel endothelium forming the blood-brain barrier (BBB) and the epithelium of the choroid plexuses forming the blood-CSF barrier (BCSFB) operate as gatekeepers for the central nervous system. Exposure of the vulnerable developing brain to chemical insults can have dramatic consequences for brain maturation and lead to life-long neurological diseases. The ability of blood-brain interfaces to efficiently protect the immature brain is therefore an important pathophysiological issue. This is also key to our understanding of drug entry into the brain of neonatal and pediatric patients. Non-specific paracellular diffusion through barriers is restricted early during development, but other neuroprotective properties of these interfaces differ between the developing and adult brains. This review focuses on the developmental expression and function of various classes of efflux transporters. These include the multispecific transporters of the ATP-binding cassette transporter families ABCB, ABCC, ABCG, the organic anion and cation transporters of the solute carrier families SLC21/SLCO and SLC22, and the peptide transporters of the SLC15 family. These transporters play a key role in preventing brain entry of blood-borne molecules such as drugs, environmental toxicants, and endogenous metabolites, or else in increasing the clearance of potentially harmful organic ions from the brain. The limited data available for laboratory animals and human highlight transporter-specific developmental patterns of expression and function, which differ between blood-brain interfaces. The BCSFB achieves an adult phenotype earlier than BBB. Efflux transporters at the BBB appear to be regulated by various factors subsequently secreted by neural progenitors and astrocytes during development. Their expression is also modulated by oxidative stress, inflammation, and exposure to xenobiotic inducers. A better understanding of these regulatory pathways during development, in particular the signaling pathways triggered by oxidative stress and xenobiotics, may open new opportunities to therapeutic manipulation in view to improve or restore neuroprotective functions of the blood-brain interfaces in the context of perinatal injuries.

Absence of irreversible effects of aliskiren in standard juvenile rat toxicity studies

BACKGROUND

Aliskiren is the first orally bioavailable direct renin inhibitor approved for the treatment of hypertension in adults. Juvenile toxicity studies in rats were initiated to support treatment in the pediatric population.

METHODS

In Study 1, aliskiren oral administration was initiated on postpartum day (PPD) 14, after nephrogenesis was completed, and continued through PPD 70 at doses of 0, 30, 100, and 300 mg/kg/day. In-life, clinical pathology, anatomic pathology, developmental, behavioral, reproductive, and toxicokinetics evaluations were performed. In Study 2, oral administration was initiated on PPD 8, before completion of nephrogenesis, and continued through PPD 35/36. In-life, clinical pathology, anatomic pathology, developmental, and toxicokinetics evaluations were performed.

RESULTS

With dosing initiated on PPD 8, mortality at 100 and 300 mg/kg/day and slightly increased kidney weight at 100 mg/kg/day occurred. Decreased absolute lymphocyte count at 300 mg/kg/day at the end of dosing occurred with dosing initiated on PPD 14. There were clinical signs and transient effects on body weight gains in both studies. There were no changes in other parameters. Systemic exposure was much higher on PPD 8 and 14 compared with adult rats on PPD 64.

CONCLUSIONS

All effects produced by aliskiren, including kidney effects, were reversible. Increased exposure in very young animals is considered to be the result of immature drug transporter systems.

Aliskiren toxicity in juvenile rats is determined by ontogenic regulation of intestinal P-glycoprotein expression

Juvenile rat toxicity studies with the direct renin inhibitor aliskiren were initiated to support treatment in the pediatric population. In Study 1, aliskiren was administered orally to juvenile rats at doses of 0, 30, 100 or 300 mg/kg/day with repeated dosing from postpartum day (PPD) 8 to PPD 35/36. In-life, clinical pathology, anatomic pathology, and toxicokinetics evaluations were performed. In Study 2, single oral doses of aliskiren (0, 100 or 300 mg/kg) were given to 14-, 21-, 24-, 28-, 31- or 36-day-old rats; in-life data and toxicokinetics were evaluated. Study 3 was a single dose (3 mg/kg i.v.) pharmacokinetic study in juvenile rats on PPD 8, 14, 21 and 28. In Study 4, naïve rats were used to investigate ontogenic changes of the multidrug-resistant protein 1 (MDR1) and the organic anion transporting polypeptide (OATP) mRNA in several organs. Oral administration of aliskiren at 100 and 300 mg/kg caused unexpected mortality and severe morbidity in 8-day-old rats. Aliskiren plasma and tissue concentrations were increased in rats aged 21days and younger. Expression of MDR1 and OATP mRNA in the intestine, liver and brain was significantly lower in very young rats. In conclusion, severe toxicity and increased exposure in very young rats after oral administration of aliskiren are considered to be the result of immature drug transporter systems. Immaturity of MDR1 in enterocytes appears to be the most important mechanism responsible for the high exposure.

In vitro blood-brain barrier models using brain capillary endothelial cells isolated from neonatal and adult rats retain age-related barrier properties

The blood-brain barrier (BBB) restricts the entry of circulating drugs and xenobiotics into the brain, and thus its permeability to substances is a critical factor that determines their central effects. The infant brain is vulnerable to neurotoxic substances partly due to the immature BBB. The employment of in vitro BBB models to evaluate permeability of compounds provides higher throughput than that of in vivo animal experiments. However, existing in vitro BBB models have not been able to simulate the intrinsic neonatal BBB. To establish a neonatal BBB model that mimics age-related BBB properties, the neonatal and adult in vitro BBB models were constructed with brain endothelial cells isolated from 2- and 8-week-old rats, respectively. To evaluate BBB functions, transendothelial electrical resistance, permeability of sodium fluorescein and Evans blue-albumin, and transport of rhodamine123 were measured. Radiolabelled drugs were used for BBB permeability studies in the neonatal and adult BBB models (in vitro) and in age-matched rats (in vivo). The neonatal BBB model showed lower barrier and p-glycoprotein (P-gp) functions than the adult BBB model; these were well associated with lower expressions of the barrier-related proteins and P-gp, and a different distribution pattern of immunostained barrier-related proteins. Verapamil (a P-gp inhibitor) significantly increased the influx of rhodamine 123, supporting functional P-gp expression in the neonatal BBB model. Valproic acid, but not nicotine, showed higher BBB permeability in the neonatal BBB model, which was well in accordance with the in vivo BBB property. We established a neonatal BBB model in vitro. This could allow us to assess the age-dependent BBB permeability of drugs.

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

The pharmacokinetics of commonly used antiepileptic drugs in immature CD1 mice

Rodents eliminate antiepileptic drugs (AEDs) faster than humans, creating challenges for designing clinically relevant protocols. Half-lives of AEDs in immature mice are unknown. The pharmacokinetics of commonly used AEDs were examined in CD1 mice using a single-dose protocol at postnatal day 19. After intraperitoneal therapeutic dosing, blood serum concentrations spanning 1-48 h post-administration and corresponding brain tissue concentrations at 4 h were analyzed. Half-lives of valproate, phenobarbital, diazepam (and metabolites), phenytoin, and levetiracetam were 2.6, 15.8, 22.3, 16.3, and 3.2 h, respectively, compared to 0.8, 7.5, 7.7, 16.0, and 1.5 h reported for adult mice. Brain-to-blood ratios were comparable with adult ratios. AEDs tested had longer half-lives and maintained therapeutic plasma concentrations longer than reported in mature mice, making clinically relevant protocols feasible.

Quantitative targeted absolute proteomics-based ADME research as a new path to drug discovery and development: methodology, advantages, strategy, and prospects

An understanding of the functional roles of proteins, for example, in drug absorption, distribution, metabolism, elimination, toxicity, and efficacy (ADMET/efficacy), is important for drug discovery and development. Equally, detailed information about protein expression is required. Recently, a new protein quantification method, called quantitative targeted absolute proteomics (QTAP), has been developed on the basis of separation and identification of protein digests by liquid chromatography-linked tandem mass spectrometry with multiple reaction monitoring. Target peptides for quantification are selected only from sequence information, so time-consuming procedures such as antibody preparation and protein purification are unnecessary. In this review, we introduce the technical features of QTAP and summarize its advantages with reference to recently reported results. These include the evaluation of species differences of blood-brain barrier protein levels among human, monkey, and mouse. The high selectivity of QTAP and its ability to quantify multiple proteins simultaneously make it possible to determine the absolute expression levels of many proteins in tissues and cells in both physiological and disease states. Knowledge of absolute expression amounts, together with data on intrinsic protein activity, allows us to reconstruct in vivo protein function, and this should be an efficient strategy to predict ADMET/efficacy of drug candidates in humans in various disease states.

Modulation of Mrp1 (ABCc1) and Pgp (ABCb1) by bilirubin at the blood-CSF and blood-brain barriers in the Gunn rat

Accumulation of unconjugated bilirubin (UCB) in the brain causes bilirubin encephalopathy. Pgp (ABCb1) and Mrp1 (ABCc1), highly expressed in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) respectively, may modulate the accumulation of UCB in brain. We examined the effect of prolonged exposure to elevated concentrations of UCB on expression of the two transporters in homozygous, jaundiced (jj) Gunn rats compared to heterozygous, not jaundiced (Jj) littermates at different developmental stages (2, 9, 17 and 60 days after birth). BBB Pgp protein expression was low in both jj and Jj pups at 9 days (about 16-27% of adult values), despite the up-regulation in jj animals (2 and 1.3 fold higher than age matched Jj animals at P9 and P17-P60, respectively); Mrp1 protein expression was barely detectable. Conversely, at the BCSFB Mrp1 protein expression was rather high (60-70% of the adult values) in both jj and Jj at P2, but was markedly (50%) down-regulated in jj pups starting at P9, particularly in the 4(th) ventricle choroid plexuses: Pgp was almost undetectable. The Mrp1 protein down regulation was accompanied by a modest up-regulation of mRNA, suggesting a translational rather than a transcriptional inhibition. In vitro exposure of choroid plexus epithelial cells obtained from normal rats to UCB, also resulted in a down-regulation of Mrp1 protein. These data suggest that down-regulation of Mrp1 protein at the BSCFB, resulting from a direct effect of UCB on epithelial cells, may impact the Mrp1-mediated neuroprotective functions of the blood-cerebrospinal fluid barrier and actually potentiate UCB neurotoxicity.

Safety and efficacy of ABT-089 in pediatric attention-deficit/hyperactivity disorder: results from two randomized placebo-controlled clinical trials

OBJECTIVE

To assess the safety and efficacy of ABT-089, a novel α(4)β(2) neuronal nicotinic receptor partial agonist, vs. placebo in children with attention-deficit/hyperactivity disorder (ADHD).

METHOD

Two multicenter, randomized, double-blind, placebo-controlled, parallel-group studies of children 6 through 12 years of age were conducted. Study 1 (n = 274) assessed six treatment groups over 8 weeks: 4 once-daily (QD) ABT-089 doses (0.085-0.700 mg/kg), QD atomoxetine, and placebo. Study 2 (n = 119) assessed three treatment groups over 6 weeks: 2 QD ABT-089 doses (0.7 mg/kg, 1.4 mg/kg) and placebo. The primary efficacy variable was the investigator-administered Attention-Deficit/Hyperactivity Disorder Rating Scale-IV: Home Version (ADHD-RS-IV [HV]) Total Score. Safety was assessed by adverse event (AE) monitoring, laboratory tests, vital signs, physical examinations, and electrocardiogram measures.

RESULTS

There was no statistically significant difference between ABT-089 and placebo in mean change from baseline to final evaluation of ADHD-RS-IV (HV) Total Score or other outcome measures at any dose in either study. In Study 1, atomoxetine showed statistically significant improvement for the primary and most secondary endpoints. ABT-089 was generally safe and well tolerated, with no statistically significant difference between any ABT-089 dose and placebo in the overall incidence of any specific AE, and no clinically significant changes in other safety measures.

CONCLUSIONS

ABT-089 did not show efficacy on the primary efficacy variable, the ADHD-RS-IV (HV) Total Score, or other measures of ADHD symptomatology in children with ADHD, and had a safety profile similar to placebo. These results contrast with published reports of efficacy of nicotinic modulators in adults with ADHD.

Age and sex differences in the effects of the immunosuppressants cyclosporine, sirolimus and everolimus on rat brain metabolism

Application of the widely used immunosuppressant (ISS) cyclosporine (CsA) is severely limited by a number of serious side-effects such as kidney and neurotoxicity. As we have shown before, CsA exhibits metabolic toxicity in brain-models. The macrolide ISSs sirolimus (SRL) and everolimus (RAD) are capable of modulating these CsA-induced effects. It was our aim to study the age-dependent metabolic changes in the rat brain after ISS-treatment and the possible role of the blood-brain-barrier in modulation of CsA metabolic toxicity. Young and adult rats were treated orally with one ISS alone or in combination with CsA for six days. Metabolic changes were assessed by nuclear magnetic resonance (NMR) spectroscopy of brain extracts as toxicodynamic endpoints. Brain P-glycoprotein (P-gp) and ISS concentrations were determined as pharmacokinetic endpoints. Young rats were more susceptible to CsA-induced inhibition of the Krebs cycle (glutamate: 78% of controls, glutamine: 82%, GABA: 71% in young vs. 85%, 89%, 92% in adult rats). Increased glycolysis after CsA-treatment was sufficient to maintain the energy state at control levels in adult brains, but not in the young rat brains (phosphocreatine: 35%). Tissue concentrations of CsA and SRL within the brain of young rats were three-fold higher, while concentrations of P-gp were three-fold higher in adult rat brains. Our results suggest that age-dependent differences in the blood-brain barrier led to increased ISS brain concentrations and hence inhibition of brain energy metabolism.

Age-dependent decline of blood-brain barrier P-glycoprotein expression in the canine brain

The efflux transporter P-glycoprotein serves as a major molecular gatekeeper at the blood-brain barrier. It has been suggested that a reduction of P-glycoprotein activity with aging might enhance exposure of brain tissue to exogenous and endogenous compounds thereby contributing to the development of neurodegenerative diseases. Brain tissue from owner-kept dogs renders an excellent tool to study the impact of aging on the background of variable environmental and genetic influencing factors. Therefore, we determined expression rates of P-glycoprotein in canine post-mortem tissue from 23 non-laboratory dogs. P-glycoprotein expression in the parahippocampal cortex exhibited a negative correlation with age. Analysis of the area labeled for P-glycoprotein in dogs aged >100 months revealed a 72% drop in P-glycoprotein expression as compared to young adults aged 23-36 months. Respective data from the dentate hilus and dentate gyrus indicated an earlier drop with a reduction by 77 and 80% in dogs aged 37-99 months in comparison with younger individuals. In contrast to the decline observed with aging in dogs without plaques, P-glycoprotein expression rates rather tended to increase with further aging in dogs with plaque formation. In conclusion, the thorough analysis of P-glycoprotein expression rates in non-laboratory dogs revealed a significant decline with aging. The data strongly support the concept that age-dependent changes might predispose to neurodegenerative diseases. In the early pathogenesis of Alzheimer's disease which is modelled by diffuse plaques in the canine brain, an up-regulation of P-glycoprotein might act as a compensatory mechanism to enhance Abeta efflux from the brain. Future studies are necessary to further evaluate the correlation between Abeta deposits and P-glycoprotein expression in different phases of the disease.

Structure and function of the blood-brain barrier

Neural signalling within the central nervous system (CNS) requires a highly controlled microenvironment. Cells at three key interfaces form barriers between the blood and the CNS: the blood-brain barrier (BBB), blood-CSF barrier and the arachnoid barrier. The BBB at the level of brain microvessel endothelium is the major site of blood-CNS exchange. The structure and function of the BBB is summarised, the physical barrier formed by the endothelial tight junctions, and the transport barrier resulting from membrane transporters and vesicular mechanisms. The roles of associated cells are outlined, especially the endfeet of astrocytic glial cells, and pericytes and microglia. The embryonic development of the BBB, and changes in pathology are described. The BBB is subject to short and long-term regulation, which may be disturbed in pathology. Any programme for drug discovery or delivery, to target or avoid the CNS, needs to consider the special features of the BBB.

Differential expression of the multidrug resistance-related proteins ABCb1 and ABCc1 between blood-brain interfaces

Cerebral homeostasis results from the presence of the protective blood-brain and blood-cerebrospinal fluid barriers located respectively at the brain capillary endothelium and the choroid plexus epithelium. ABCb1 (Pgp) and ABCc1 (Mrp1) transporters are two major proteins of neuroprotection whose localization and functional significance at both barriers remain partly unsettled. We conducted a comparative analysis of their relative protein content between the two blood-brain interfaces. Microvessels and choroid plexuses located in the fourth and lateral ventricles were isolated from developing and adult rat brains, and whole homogenates were submitted to quantitative Western blot analysis by using standard curves generated from one of the samples. In adult, choroid plexus-associated Pgp content was less than 0.5% of the level in microvessels, whereas Mrp1 content in microvessels was 4% of that in the fourth ventricle choroid plexus. Pgp but not Mrp1 was enriched in microvessels over parenchyma. In choroid plexuses, Mrp1 displayed a basolateral epithelial localization, and reached its high adult protein level, early during postnatal development. In postnatal as in adult microvessels, Pgp localization appeared luminal. However, by contrast to Mrp1, the level of this transporter increased 4.6-fold between 9-day-old and adult animals. Western blot analysis of human samples confirmed the mirror image of Pgp and Mrp1 expression between the two barriers. We conclude that there are major differences in the mechanisms by which blood-brain interfaces fulfill their neuroprotective functions. The data also highlight the significance of the neuroprotective function of the choroid plexus during brain maturation, when the microvasculature is still developing.

Pulmonary targeting of sustained release formulation of budesonide in neonatal rats

Systemic corticosteroids are widely used for the treatment/prevention of chronic lung disease (CLD) in premature infants. The use of the inhalation route for delivering corticosteroids has been widely recognized, however so far pre-term babies continue to be treated with oral glucocorticoids, such as dexamethasone. We hypothesize that the pulmonary administration of sustained release formulations of inhaled corticosteroids to pre-term infants will result in a higher benefit/risk ratio as compared to traditional inhalation therapy. To achieve a slow release formulation budesonide particles were coated with a very thin film of polylactic acid using a pulse laser ablation technique. Coated material was characterized with respect to the dissolution behavior and particle size. Ex vivo receptor binding studies were performed to monitor the cumulative lung, liver and brain receptor occupancies after adminstration in neonatal (10-11 days old) rats after intratracheal instillation of either uncoated budesonide or poly (l-lactic acid) (PLA) coated budesonide. The mean dissolution timed for the uncoated and the polymer coated formulations were 1.2 +/- 0.5 and 4.7 +/- 0.1 h, respectively (p < 0.05). No significant differences in the respirable fraction were found between coated and uncoated formulation (p>0.05). The average receptor occupancies in the lung, liver and brain after administration of uncoated budesonide were 58.4 +/- 12.9, 56.4 +/- 6.8 and 38.3 +/- 6.7%, respectively. However, after administration of PLA coated budesonide, the average AUC estimates in the lung, liver and brain were 75.8 +/- 3.7%, 46.6 +/- 14.5 and 29 +/- 7, respectively. The results from our study suggest sustained receptor occupancy in the lungs of neonatal rats after administration of PLA coated budesonide results in lower systemic exposure (as indicated by low liver receptor occupancy). The data strongly underscore the urgent need to develop sustained release pulmonary-targeted delivery systems of corticosteroids for the treatment of CLD in pre-term infants. The administration of inhaled corticosteroids using targeted drug-delivery systems will potentially result in higher local effects and a reduction in systemic exposure.

ABC transporter (P-gp/ABCB1, MRP1/ABCC1, BCRP/ABCG2) expression in the developing human CNS

P-glycoprotein (P-gp/ABCB1), multidrug resistance associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) are plasma membrane efflux pumps that limit the intracellular uptake and retention of numerous lipophilic, amphipathic xeno- and endobiotics. Little is known about the neonatal and developmental expression of P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 in the human central nervous system (CNS), therefore post-mortem CNS tissue from infants born at 22 (0/7)-42 (0/7) weeks of gestation and adults was immunostained to determine their ontogeny and cellular localization. P-gp/ABCB1 immunostaining was observed in microvessel endothelial cells as early as 22 (0/7) weeks, increasing in prevalence and intensity with maturation, and later in gestation in large pyramidal neurons. MRP1/ABCC1 immunostaining was prominent early in the choroid plexus and ventricular ependyma, and noted later in large pyramidal neurons. BCRP/ABCG2 expression was limited to microvessel endothelial cells. P-gp/ABCB1, MRP1/ABCC1 and BCRP/ABCG2 in adult brain matched term newborn CNS but with more intense immunostaining. We conclude that P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 are expressed in a developmental, cell specific, fashion in the human CNS. The complementary pattern of P-gp/ABCB1 and BCRP/ABCG2 at the blood-brain with MRP1/ABCC1 at the blood-CSF barriers may limit CNS uptake and retention of drugs and toxins in neonates.

Cyclosporine, a P-glycoprotein modulator, increases [18F]MPPF uptake in rat brain and peripheral tissues: microPET and ex vivo studies

PURPOSE

Pretreatment with cyclosporine, a P-glycoprotein (P-gp) modulator increases brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethylpiperazine ([(18)F]MPPF) for binding to hydroxytryptamine(1A) (5-HT(1A)) receptors. Those increases were quantified in rat brain with in vivo microPET and ex vivo tissue studies.

MATERIALS AND METHODS

Each Sprague-Dawley rat (n = 4) received a baseline [(18)F]MPPF microPET scan followed by second scan 2-3 weeks later that included cyclosporine pretreatment (50 mg/kg, i.p.). Maximum a posteriori reconstructed images and volumetric ROIs were used to generate dynamic radioactivity concentration measurements for hippocampus, striatum, and cerebellum, with simplified reference tissue method (SRTM) analysis. Western blots were used to semiquantify P-gp regional distribution in brain.

RESULTS

MicroPET studies showed that hippocampus uptake of [(18)F]MPPF was increased after cyclosporine; ex vivo studies showed similar increases in hippocampus and frontal cortex at 30 min, and for heart and kidney at 2.5 and 5 min, without concomitant increases in [(18)F]MPPF plasma concentration. P-gp content in cerebellum was twofold higher than in hippocampus or frontal cortex.

CONCLUSIONS

These studies confirm and extend prior ex vivo results (J. Passchier, et al., Eur J Pharmacol, 2000) that showed [(18)F]MPPF as a substrate for P-gp. Our microPET results showed that P-gp modulation of [(18)F]MPPF binding to 5-HT(1A) receptors can be imaged in rat hippocampus. The heterogeneous brain distribution of P-gp appeared to invalidate the use of cerebellum as a nonspecific reference region for SRTM modeling. Regional quantitation of P-gp may be necessary for accurate PET assessment of 5-HT(1A) receptor density when based on tracer uptake sensitive to P-gp modulation.

P-glycoprotein restricts the penetration of oseltamivir across the blood-brain barrier

Oseltamivir is an ethyl ester prodrug of [3R,4R,5S]-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate phosphate (Ro 64-0802), the anti-influenza drug. Abnormal behavior has been suspected to be associated with oseltamivir medication in Japan. The purpose of the present study is to examine the involvement of transporters in the brain distribution of oseltamivir and its active form Ro 64-0802 across the blood-brain barrier (BBB). The brain-to-plasma concentration ratio (K(p,brain)) of oseltamivir after i.v. infusion of oseltamivir in FVB mice was increased by pretreatment with N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), a dual inhibitor for P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp), whereas that of Ro 64-0802 was only slightly increased. Furthermore, the distribution volume of Ro 64-0802 following i.v. administration of Ro 64-0802 in the brain was similar to the capillary volume, suggesting its minimal distribution. The K(p,brain) value of oseltamivir in multidrug-resistant (Mdr) 1a/1b P-gp knockout mice was 5.5-fold higher than that in wild-type mice and comparable with that obtained by pretreatment with GF120918, whereas it was unchanged in Bcrp knockout mice. The K(p,brain) value of oseltamivir was significantly higher in newborn rats, which is in good agreement with the ontogenetic expression profile of P-gp. Intracellular accumulation of oseltamivir was lower in human and mouse P-gp-expressing cells, which was reversed by P-gp inhibitor valspodar (PSC833). These results suggest that P-gp limits the brain uptake of oseltamivir at the BBB and that Ro 64-0802 itself barely crosses the BBB. However, it may be possible that Ro 64-0802 is formed in the brain from the oseltamivir, considering the presence of carboxylesterase in the brain endothelial cells.

Microvascular P-glycoprotein expression at the blood-brain barrier following focal astrocyte loss and at the fenestrated vasculature of the area postrema

The multidrug transporter, P-glycoprotein, expressed at the blood-brain barrier is thought to be important for limiting access of toxic agents to the brain, but its relationship to astrocyte expression is unclear. We have studied P-glycoprotein expression in the inferior colliculus after a temporary loss of blood-brain barrier integrity following chemically induced astrocyte loss and at the fenestrated vascular endothelium of the area postrema. Male Fisher F344 rats given 3-chloropropanediol showed astrocyte loss from 12 to 24 h until the lesion was repopulated 8-28 days later. In non-dosed tissue, P-glycoprotein expression was seen the entire length of platelet endothelial cell adhesion molecule immunoreactive vessels. Within 6 h of dosing, a significant (p<0.05) reduction in the total length of P-glycoprotein immunoreactive vasculature was evident. By 48 h, P-glycoprotein immunoreactivity was heavily fragmented. The total length of P-glycoprotein immunoreactive vessels became minimal at 4 days (p<0.001) but was still present in many vessels. From 6 to 28 days, P-glycoprotein immunoreactivity returned across the inferior colliculus, in parallel with astrocytic repopulation of the lesion, and by 28 days resembled that seen in control tissue. The area postrema showed GFAP immunoreactive astrocytes but which made limited contact with the vasculature, while the platelet endothelial cell adhesion molecule immunoreactive vasculature showed no expression of P-glycoprotein. These findings provide evidence supporting a link between GFAP-astrocyte and P-glycoprotein expression in the mature brain vasculature in vivo.

The size and distribution of midbrain dopaminergic populations are permanently altered by perinatal glucocorticoid exposure in a sex- region- and time-specific manner

Central dopaminergic (DA) systems appear to be particularly vulnerable to disruption by exposure to stressors in early life, but the underlying mechanisms are poorly understood. As endogenous glucocorticoids (GCs) are implicated in other aspects of neurobiological programming, this study aimed to characterize the effects of perinatal GC exposure on the cytoarchitecture of DA populations in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA). Dexamethasone was administered non-invasively to rat pups via the mothers' drinking water during embryonic days 16-19 or postnatal days 1-7, with a total oral intake circa 0.075 or 0.15 mg/kg/day, respectively; controls received normal drinking water. Analysis of tyrosine hydroxylase-immunoreactive cell counts and regional volumes in adult offspring identified notable sex differences in the shape and volume of the SNc and VTA, as well as the topographical organization and size of the DA populations. Perinatal GC treatments increased the DA population size and altered the shape of the SNc and VTA as well as the organization of the DA neurons by expanding and/or shifting them in a caudal direction. This response was sexually dimorphic and included a feminization or demasculinization of the three-dimensional cytoarchitecture in males, and subtle differences that were dependent on the window of exposure. These findings demonstrate that inappropriate perinatal exposure to GCs have enduring effects on the organization of midbrain DA systems that are critically important for normal brain function throughout life.

Melanocortinergic activation by melanotan II inhibits feeding and increases uncoupling protein 1 messenger ribonucleic acid in the developing rat

The hypothalamic neurocircuitry that regulates energy homeostasis in adult rats is not fully developed until the third postnatal week. In particular, fibers from the hypothalamic arcuate nucleus, including both neuropeptide Y (NPY) and alpha-MSH fibers, do not begin to innervate downstream hypothalamic targets until the second postnatal week. However, alpha-MSH fibers from the brainstem and melanocortin receptors are present in the hypothalamus at birth. The present study investigated the melanocortin system in the early postnatal period by examining effects of the melanocortin receptor agonist melanotan II (MTII) on body weight, energy expenditure, and hypothalamic NPY expression. Rat pups were injected ip with MTII (3 mg/kg body weight) or saline on postnatal day (P) 5 to P6, P10-P11, or P15-P16 at 1700 and 0900 h and then killed at 1300 h. Stomach weight and brown adipose tissue uncoupling protein 1 mRNA were determined. In addition, we assessed central c-Fos activation 90 min after MTII administration and hypothalamic NPY mRNA after twice daily MTII administration from P5-P10 or P10-P15. MTII induced hypothalamic c-Fos activation as well as attenuating body weight gain in rat pups. Stomach weight was significantly decreased and uncoupling protein 1 mRNA was increased at all ages, indicating decreased food intake and increased energy expenditure, respectively. However, MTII had no effect on NPY mRNA levels in any hypothalamic region. These findings demonstrate that MTII can inhibit food intake and stimulate energy expenditure before the full development of hypothalamic feeding neurocircuitry. These effects do not appear to be mediated by changes in NPY expression.

Potential impact of ABCB1 (p-glycoprotein) polymorphisms on avermectin toxicity in humans

Several members of the ATP binding cassette (ABC) transporter protein superfamily perform xenobiotic efflux functions in mammals, limiting gut absorption, mediating excretion, and controlling entry of a wide range of chemicals to sensitive compartments such as brain, testes and foetus. Perhaps the best characterised of these is p-glycoprotein (gene name ABCB1/MDR1), a barrier epithelia expressed protein with structurally diverse substrates, including the avermectin pesticides. In specific mouse and dog strains, ABCB1 mutations have been identified that result in loss of p-glycoprotein function in the blood brain barrier (BBB) and increased susceptibility to avermectin neurotoxicity. As yet no large rearrangements of the human ABCB1 gene analogous to those in the mouse and dog have been identified. However, numerous human ABCB1 single nucleotide polymorphisms (SNPs) have been identified, the allelic frequencies of which vary with ethnicity. There is no clear consensus on whether or not SNPs, or combinations of SNPs, reduce human p-glycoprotein functionality. However, recent in vivo human data indicate that the two commonest ABCB1 haplotypes both exhibit full BBB functionality. We discuss here the role of p-glycoprotein in limiting brain absorption of avermectin pesticides, as well as the potential impact of the reported functional effects and population frequencies of known ABCB1 polymorphisms on avermectin pesticide risk assessments.

New insights into glycosphingolipid functions--storage, lipid rafts, and translocators

Glycosphingolipids are key components of eukaryotic cellular membranes. Through their propensity to form lipid rafts, they are important in membrane transport and signaling. At the cell surface, they are required for caveolar-mediated endocytosis, a process required for the action of many glycosphingolipid-binding toxins. Glycosphingolipids also exist intracellularly, on both leaflets of organelle membranes. It is expected that dissecting the mechanisms of cell pathology seen in the glycosphingolipid storage diseases, where lysosomal glycosphingolipid degradation is defective, will reveal their functions. Disrupted cation gradients in Mucolipidosis type IV disease are interlinked with glycosphingolipid storage, defective rab 7 function, and the activation of autophagy. Relationships between drug translocators and glycosphingolipid synthesis are also discussed. Mass spectrometry of cell lines defective in drug transporters reveal clear differences in glycosphingolipid mass and fatty acid composition. The potential roles of glycosphingolipids in lipid raft formation, endocytosis, and cationic gradients are discussed.

Prenatal dexamethasone exposure, postnatal development, and adulthood prepulse inhibition and latent inhibition in Wistar rats

Prenatal stress is an important risk factor in schizophrenia, and the aetiological factors mediating this relationship are central to the neurodevelopmental hypothesis of schizophrenia. The glucocorticoid receptor (GR) agonist dexamethasone (DEX) is commonly prescribed for prenatal conditions, and results in GR activation, which is part of the stress response. To investigate animal evidence for whether prenatal DEX leads to development of schizophrenia-like phenotypes, Wistar rats were prenatally exposed to DEX (0.1mg/kg/day) between the gestational days 15 and 21, and tested in two paradigms known to be disrupted in schizophrenia patients: prepulse inhibition (PPI) and latent inhibition (LI). A cross-fostering design was used to allow dissociation of any direct prenatal effects on offspring from effects dependent on DEX exposure of the rearing dam. Pup birth weight was reduced by prenatal DEX treatment. DEX-treated dams demonstrated increased pup-directed behaviour. There were additive effects of prenatal DEX treatment and DEX treatment of rearing dam in terms of reduced body weight in adulthood. In one of two replications, PPI was increased by prenatal DEX in males only and specific to the highest prepulse intensity. There was no evidence that LI was disrupted by prenatal DEX treatment. This study does not provide support for the hypothesis that prenatal DEX exposure leads to schizophrenia-like deficits in PPI or LI, suggesting that GR prenatal programming is not a mechanism of direct relevance to the neurodevelopmental hypothesis of schizophrenia.

CONTRARY TO ADULT, NEONATAL RATS SHOW PRONOUNCED BRAIN UPTAKE OF CORTICOSTEROIDS

Neurotoxic adverse effects after systemic corticosteroid administration are elevated in preterm infants. To test whether this might be related to an immature blood-brain barrier (BBB) that permits corticosteroids to enter the brain and induce neurotoxic effects, this study assessed the differences in brain permeability of triamcinolone acetonide after intratracheal administration to neonatal (10- to 11-day-old) and adult rats. Triamcinolone acetonide (or the phosphate prodrug in the case of neonatal rats) was administered intratracheally to neonatal rats at doses of 2.5, 25, or 50 microg/kg and to adult rats at 100 microg/kg. An ex vivo receptor binding assay was used to monitor the cumulative brain and liver glucocorticoid receptor occupancies over 6 h. Brain and liver receptor occupancies in neonates were similar for the 25 and 50 microg/kg triamcinolone acetonide phosphate (brain/liver receptor occupancy ratio, 1.10 +/- 0.14 and 0.87 +/- 0.13, respectively), whereas some reduction in the brain permeability was seen at the lower dose. After intratracheal administration of 100 microg/kg triamcinolone acetonide to adult rats, receptor occupancies in the brain were significantly lower (brain/liver ratio, 0.21 +/- 0.14; p < 0.001). The study demonstrated that glucocorticoids enter the brain of neonatal rats because of an immature BBB. The results of this study support the hypothesis that neurotoxic adverse effects in preterm infants after systemic corticosteroid administration might be related to an immature BBB.

Brain cyclosporin A levels are determined by ontogenic regulation of mdr1a expression

Cyclosporin A (CyA) toxicity is a common occurrence in pediatric organ transplant patients. We hypothesized that reduced mdr1a expression in newborn and developing mice would affect CyA accumulation within organs and/or toxicity. For functional studies, CyA was administered (5 mg kg(-1) i.p.) to 1-, 12-, and 19-day, and adult male and female mdr1a+/+ and mdr1a-/- mice. Peak blood CyA was lower in 1-, 12-, and 19-day-old (1000 ng ml(-1)) versus adult (1500 ng ml(-1)) mice but was similar in mdr1a+/+ and mdr1a-/- mice. Kidney mdr1a expression (measured by quantitative polymerase chain reaction) increased 2.5-fold in 19-day-old male and female mice and increased another 4-fold in adult females compared with adult males. Liver mdr1a expression increased 6-fold by day 12 compared with neonatal mice. Thereafter, maintenance of hepatic mdr1a expression in females and a reduction to neonatal levels in males was observed. Kidney/blood (8- to 9-fold) and liver/blood (12- to 15-fold) CyA levels were highest on days 12 and 19 and were not dependent on maturational changes in mdr1a mRNA levels. Adults had higher brain expression of mdr1a mRNA (3-fold), a corresponding 5-fold increase in immunodetectable P-glycoprotein, and 80% lower brain accumulation of CyA compared with 1-day-old mice. Conversely, in mdr1a-null mice, brain/blood CyA was similar in newborn and adult mice. A similar pattern was observed for the brain accumulation of the mdr1a substrate 3H-digoxin. We conclude that the risk for central nervous system drug toxicity could be higher in neonates or young children as a consequence of underdeveloped P-glycoprotein.