Glutamine 57 at the Complementary Binding Site Face Is a Key Determinant of Morantel Selectivity for ��7 Nicotinic Receptors

Nicotinic receptors (AChRs) play key roles in synaptic transmission. We explored activation of neuronal α7 and mammalian muscle AChRs by morantel and oxantel. Our results revealed a novel action of morantel as a high efficacy and more potent agonist than ACh of α7 receptors. The EC₅₀ for activation by morantel of both α7 and α7-5HT_3A receptors is 7-fold lower than that determined for ACh. The minimum morantel concentration required to activate α7-5HT_3A channels is 6-fold lower than that of ACh, and activation episodes are more prolonged than in the presence of ACh. By contrast, oxantel is a weak agonist of α7 and α7-5HT_3A, and both drugs are very low efficacy agonists of muscle AChRs. The replacement of Gln⁵⁷ in α7 by glycine, which is found in the equivalent position of the muscle AChR, decreases the efficacy for activation and turns morantel into a partial agonist. The reverse mutation in the muscle AChR (ϵG57Q) increases 7-fold the efficacy of morantel. The mutations do not affect activation by ACh or oxantel, indicating that this position is selective for morantel. In silico studies show that the tetrahydropyrimidinyl group, common to both drugs, is close to Trp¹⁴⁹ of the principal face of the binding site, whereas the other cyclic group is proximal to Gln⁵⁷ of the complementary face in morantel but not in oxantel. Thus, position 57 at the complementary face is a key determinant of the high selectivity of morantel for α7. These results provide new information for further progress in drug design.Nicotinic acetylcholine receptors (AChRs),³ members of the Cys-loop receptor superfamily, are of fundamental importance in synaptic transmission throughout the nervous system in both vertebrates and invertebrates. They are implicated in a wide range of important pathologies and are targets of clinically relevant drugs. AChRs are pentameric proteins composed of highly homologous subunits (1, 2). Subunits are classified as either α, which contain a disulfide bridge formed by two adjacent cysteine residues important for acetylcholine (ACh) binding, or non-α subunits, which lack this motif (3).AChRs assemble from five identical α subunits, forming homomeric receptors, such as neuronal α7 receptors, or from different α and non-α subunits, forming heteromeric receptors, such as the muscle AChR. Human adult muscle AChRs are composed of two α1, one β, one ϵ, and one δ subunits. The five homologous subunits are arranged as barrel staves around a central ion-conducting pore (4). Approximately half of each subunit is extracellular with the remainder comprising transmembrane domains M1–M4 and a large cytoplasmic domain spanning M3 and M4 (4). The neurotransmitter binding sites are formed within the extracellular domain at interfaces between subunits (4, 5). One of the sides, called the principal face, is formed by three discontinuous loops of the α subunit, whereas the complementary face is formed by three discontinuous β-strands of the adjacent subunit. Key residues of the principal face are grouped in regions called loop A (Trp⁸⁶ and Tyr⁹³), loop B (Trp¹⁴⁹ and Gly¹⁵³), and loop C (Tyr¹⁹⁰, Cys¹⁹², Cys¹⁹³, and Tyr¹⁹⁸). The complementary face is formed by residues from α7 or δ or ϵ subunits in the adult muscle AChR. At this face of the muscle AChR, residues are clustered in loop D (Trp⁵⁵), E (Leu¹⁰⁹, Tyr¹¹¹, Tyr¹¹⁷, and Leu¹¹⁹), and F (Asp¹⁷⁴ and Glu¹⁷⁶) (2, 5, 6). Residues of the principal face are highly conserved between α7 and α1 subunits, whereas less conservation is found in residues located at the complementary face (5, 7).The anthelmintic agents levamisole, pyrantel, oxantel, and morantel are full agonists of nematode muscle AChRs, and exert their therapeutic actions by producing muscle paralysis (8). By contrast, levamisole and pyrantel have been shown to be low efficacy agonists of mammalian muscle AChRs (9). A few lines of experimental evidence suggest that these compounds also interact with some types of neuronal AChRs, but instead of acting as agonists, they act as modulators. Morantel and levamisole have been shown to allosterically potentiate responses of α3β2 and α3β4 receptors (10, 11). Thus, the actions of anthelmintic agents seem to be strongly dependent on the AChR subtype. Therefore, these compounds are useful tools for the identification of determinants of drug selectivity, which, in turn, is required for rational design of novel and more specific drugs.We have here determined that, similarly to pyrantel and levamisole (9), morantel and oxantel are low efficacy agonists of mammalian muscle AChRs. However, whereas oxantel is also a weak agonist of α7, morantel is more potent than ACh. By site-directed mutagenesis we determined that position 57, located at the complementary face of the binding site, is involved in the differential selectivity of morantel for α7 and mammalian muscle AChR.Neuronal α7 receptors may be involved in a range of neurological and psychiatric disorders that lead to cognitive impairment, including Alzheimer disease, attention deficit hyperactivity disorder, and schizophrenia (12). Given that its deficit is associated with cognitive impairment in these diseases, enhancement of its activity has recently emerged as a physiological and effective therapeutic strategy. Therefore, the characterization of the novel action of morantel as a potent agonist of α7 together with the identification of the structural basis of this high selectivity become of importance as they provide new information for further progress in drug design.     

Molecular Determinants of Agonist Selectivity in Glutamate-Gated Chloride Channels Which Likely Explain the Agonist Selectivity of the Vertebrate Glycine and GABAA-ρ Receptors

Orthologous Cys-loop glutamate-gated chloride channels (GluClR’s) have been cloned and described electrophysiologically and pharmacologically in arthropods and nematodes (both members of the invertebrate ecdysozoan superphylum). Recently, GluClR’s from Aplysia californica (a mollusc from the lophotrochozoan superphylum) have been cloned and similarly studied. In spite of sharing a common function, the ecdysozoan and lophotrochozoan receptors have been shown by phylogenetic analyses to have evolved independently. The recent crystallization of the GluClR from C. elegans revealed the binding pocket of the nematode receptor. An alignment of the protein sequences of the nematode and molluscan GluClRs showed that the Aplysia receptor does not contain all of the residues defining the binding mode of the ecdysozoan receptor. That the two receptors have slightly different binding modes is not surprising since earlier electrophysiological and pharmacological experiments had suggested that they were differentially responsive to certain agonists. Knowledge of the structure of the C. elegans GluClR has permitted us to generate a homology model of the binding pocket of the Aplysia receptor. We have analyzed the differences between the two binding modes and evaluated the relative significance of their non-common residues. We have compared the GluClRs electrophysiologically and pharmacologically and we have used site-directed mutagenesis on both receptor types to test predictions made from the model. Finally, we propose an explanation derived from the model for why the nematode receptors are gated only by glutamate, whereas the molluscan receptors can also be activated by β-alanine, GABA and taurine. Like the Aplysia receptor, the vertebrate glycine and GABA_A-ρ receptors also respond to these other agonists. An alignment of the sequences of the molluscan and vertebrate receptors shows that the reasons we have given for the ability of the other agonists to activate the Aplysia receptor also explain the agonist profile seen in the glycine and GABA_A-ρ receptors.     

Hyperoxia Exposure Impairs Nephrogenesis in the Neonatal Rat: Role of HIF-1α

Preterm neonates are exposed at birth to high oxygen concentrations relative to the intrauterine environment. We have previously shown in a rat model that a hyperoxic insult results in a reduced nephron number in adulthood. Therefore, the aim of this study was to determine the effects of transient neonatal hyperoxia exposure on nephrogenesis. Sprague-Dawley rat pups were raised in 80% O₂ or room air from P3 to P10. Pups (n = 12/group, 6 males and 6 females) were sacrificed at P5 (during active nephrogenesis) and at P10 (after the completion of nephrogenesis). Hyperoxia exposure resulted in a significant reduction in both nephrogenic zone width and glomerular diameter at P5, and a significantly increased apoptotic cell count; however, nephron number at P10 was not affected. HIF-1α expression in the developing kidney was significantly reduced following hyperoxia exposure. Systemic administration of the HIF-1α stabilizer dimethyloxalylglycine (DMOG) resulted in enhanced expression of HIF-1α and improved nephrogenesis: kidneys from hyperoxia-exposed pups treated with DMOG exhibited a nephrogenic zone width and glomerular diameter similar to room-air controls. These findings demonstrate that neonatal hyperoxia exposure results in impaired nephrogenesis, which may be at least in part HIF-1α-mediated. Although nephron number was not significantly reduced at the completion of nephrogenesis, early indicators of maldevelopment suggest the potential for accelerated nephron loss in adulthood. Overall, this study supports the premise that prematurely born neonates exposed to high oxygen levels after birth are vulnerable to impaired renal development.     

The Gating Kinetics of the Slow Vacuolar Channel. A Novel Mechanism for SV Channel Functioning?

Although there is consensus that the slow vacuolar or SV channel is a Ca²⁺ release channel, the underlying mechanism of operation is still controversial. The main reason is that the voltage sensitivity of SV gating seems to exclude activation at hyperpolarized (physiological) membrane potentials. Inspired by a study of Gambale et al. (1993) and supported by simulation studies presented here, we interpreted SV activation and deactivation kinetics in terms of a cyclic state diagram originally applied to animal cation-selective channels. A cyclic state diagram allows two pathways of activation operating in opposite directions. One pathway represents the frequently observed slow activation at moderate depolarization (<130 mV). With the open state (O) next to the closed state initially occupied (C ₁), direct transitions from C ₁ to O can account for the fast activation observed at higher depolarized potentials (>130 mV). We hypothesize that similar state transitions directly to O may also occur during hyperpolarization. The implication of this proposed mechanism is that SV accomplishes its physiological role during hyperpolarization-evoked deactivation. Despite their rare occurrence and possibly short duration, these opening events may last long enough to substantially raise the local cytosolic free Ca²⁺ level at the channel mouth by as much as 600 nM/ms. Because under in vivo conditions the Ca²⁺ flux is inwardly directed, the mechanism presented here revives the notion that the SV channel can be subject to calcium-induced calcium release. Present address for H. M.: BioMade Technology Foundation, Nijenborgh 4, 9747 AG Groningen, The Netherlands     

Heart Failure in Chronic Myocarditis: A Role for microRNAs?

Myocarditis is an inflammatory disease of the heart, which can persist over a long time. During this time, known as the chronic phase of myocarditis, ongoing inflammation damages the cardiomyocytes. The loss of cardiac cells culminates in the development of dilated cardiomyopathy, often followed by non-ischemic heart failure due to diminished cardiac function. During the course of the disease, expression levels of non-coding small RNAs, called microRNAs (miRNAs), change. Although mainly studied in the acute setting, some of these changes in expression level appear to persist in the chronic phase. In addition to being a much-needed diagnostic tool, these miRNA could provide new treatment options. miRNA-based intervention strategies already showed promising results in the treatment of ischemic cardiovascular diseases in preclinical animal models. By implementing more knowledge on the role of miRNAs in the progression towards heart failure, this can potentially be used in the development of miRNA-based therapeutic interventions in the treatment of myocarditis and thereby preventing the progression towards heart failure. The first part of this review will focus on the natural course of myocarditis and the progression towards heart failure. Secondly, we will discuss the current knowledge on alterations of miRNA expression patterns, and suggest some possible future interventions.     

A Role for the PKC Signaling System in the Pathophysiology and Treatment of Mood Disorders: Involvement of a Functional Imbalance?

Mood disorders, such as bipolar and major depressive disorders, are frequent, severe, and often disabling neuropsychiatric diseases affecting millions of individuals worldwide. Available mood stabilizers and antidepressants remain unsatisfactory because of their delayed and partial therapeutic efficacy. Therefore, the development of targeted therapies, working more rapidly and being fully effective, is urgently needed. In this context, the protein kinase C (PKC) signaling system, which regulates multiple neuronal processes implicated in mood regulation, can constitute a novel therapeutic target. This paper reviews the currently available knowledge regarding the role of the PKC signaling pathway in the pathophysiology of mood disorders and the therapeutic potential of PKC modulators. Current antidepressants and mood stabilizers have been shown to modulate the PKC pathway, and the inhibition of this intracellular signaling cascade results in antimanic-like properties in animal models. Disrupted PKC activity has been found both in postmortem brains and platelet from patients with mood disorders. Finally, the PKC inhibitor tamoxifen has recently demonstrated potent antimanic properties in several clinical trials. Overall, emerging data from preclinical and clinical research suggest an imbalance of the PKC signaling system in mood disorders. Thus, PKC may be a critical molecular target for the development of innovative therapeutics.     

Alterations in the Cell Cycle in the Cerebellum of Hyperbilirubinemic Gunn Rat: A Possible Link with Apoptosis?

Severe hyperbilirubinemia causes neurological damage both in humans and rodents. The hyperbilirubinemic Gunn rat shows a marked cerebellar hypoplasia. More recently bilirubin ability to arrest the cell cycle progression in vascular smooth muscle, tumour cells, and, more importantly, cultured neurons has been demonstrated. However, the involvement of cell cycle perturbation in the development of cerebellar hypoplasia was never investigated before. We explored the effect of sustained spontaneous hyperbilirubinemia on cell cycle progression and apoptosis in whole cerebella dissected from 9 day old Gunn rat by Real Time PCR, Western blot and FACS analysis. The cerebellum of the hyperbilirubinemic Gunn rats exhibits an increased cell cycle arrest in the late G0/G1 phase (p < 0.001), characterized by a decrease in the protein expression of cyclin D1 (15%, p < 0.05), cyclin A/A1 (20 and 30%, p < 0.05 and 0.01, respectively) and cyclin dependent kinases2 (25%, p < 0.001). This was associated with a marked increase in the 18 kDa fragment of cyclin E (67%, p < 0.001) which amplifies the apoptotic pathway. In line with this was the increase of the cleaved form of Poly (ADP-ribose) polymerase (54%, p < 0.01) and active Caspase3 (two fold, p < 0.01). These data indicate that the characteristic cerebellar alteration in this developing brain structure of the hyperbilirubinemic Gunn rat may be partly due to cell cycle perturbation and apoptosis related to the high bilirubin concentration in cerebellar tissue mainly affecting granular cells. These two phenomena might be intimately connected.     

Muscarinic Receptor During Postnatal Development of Rat Cerebellum: An Index of Cholinergic Synapse Formation?

Abstract: Quantitative and qualitative modifications of the specific binding sites for [³H]quinuclidinyl benzylate (QNB), a muscarinic antagonist, were studied during rat cerebellar postnatal development. Specific binding sites for QNB (QNB-sbs), regardless of whether they correspond to muscarinic acetylcholine receptors, are present with the highest density in the archicerebellar cortex, but the total amount per region is about the same in the archi-, paleo-, and neocerebellar cortex regions. Large amounts of QNB-sbs are also present in a cerebellar fraction including central white matter and deep cerebellar nuclei. QNB-sbs are low but present at birth and then accumulate during ontogenic development according to a curve which duplicates, with a delay of a few days, the curve of DNA accumulation. Dissection studies indicated that this curve does not depend on the preferential localization of QNB-sbs in a specific cerebellar region nor on the particular development of this region. The similarity of the QNB-sbs and the DNA developmental curves might indicate that the QNB-sbs are present on granule cells; however, a comparative analysis of the data in the literature suggests that a great many QNB-sbs are located on the Purkinje cell dendrites in the molecular layer, where all or some of them might correspond to the ex-trajunctional muscarinic acetylcholine receptor detected there by electrophysiology. It would appear that only a small percentage of cerebellar QNB-sbs corresponds to the cholinergic synapses present in cerebellar cortex; hence, the question of muscarinic receptors in the cerebellum should be re-examined.     

The Essential Role of Cytosolic Cl− in Ca2+ Regulation of an Amiloride-Sensitive Channel in Fetal Rat Pneumocyte

An amiloride-sensitive, Ca²⁺-activated nonselective cation (NSC) channel in the apical membrane of fetal rat alveolar epithelium plays an important role in stimulation of Na⁺ transport by a beta adrenergic agonist (beta agonist). We studied whether Ca²⁺ has an essential role in the stimulation of the NSC channel by beta agonists. In cell-attached patches formed on the epithelium, terbutaline, a beta agonist, increased the open probability (P _o ) of the NSC channel to 0.62 ± 0.07 from 0.03 ± 0.01 (mean ±se; n= 8) 30 min after application of terbutaline in a solution containing 1 mm Ca²⁺. The P _o of the terbutaline-stimulated NSC channel was diminished in the absence of extracellular Ca²⁺ to 0.26 ± 0.05 (n= 8). The cytosolic Ca²⁺ concentration ([Ca²⁺] _c ) in the presence and absence of extracellular Ca²⁺ was, respectively, 100 ± 6 and 20 ± 2 nm (n= 7) 30 min after application of terbutaline. The cytosolic Cl⁻ concentration ([Cl⁻] _c ) in the presence and absence of extracellular Ca²⁺ was, respectively, 20 ± 1 and 40 ± 2 mm (n= 7) 30 min after application of terbutaline. The diminution of [Ca²⁺] _c from 100 to 20 nm itself had no significant effects on the P _o if the [Cl⁻] _c was reduced to 20 mm; the P _o was 0.58 ± 0.10 at 100 nm [Ca²⁺] _c and 0.55 ± 0.09 at 20 nm [Ca²⁺] _c (n= 8) with 20 mm [Cl⁻] _c in inside-out patches. On the other hand, the P _o (0.28 ± 0.10) at 20 nm [Ca²⁺] _c with 40 mm [Cl⁻] _c was significantly lower than that (0.58 ± 0.10; P < 0.01; n= 8) at 100 nm [Ca²⁺] _c with 20 mm [Cl⁻] _c , suggesting that reduction of [Cl⁻] _c is an important factor stimulating the NSC channel. These observations indicate that the extracellular Ca²⁺ plays an important role in the stimulatory action of beta agonist on the NSC channel via reduction of [Cl⁻] _c . Received: 11 August 2000/Revised: 4 December 2000     

Role of Hsp90 in Biogenesis of the β-Cell ATP-sensitive Potassium Channel Complex

The pancreatic β-cell ATP-sensitive potassium (K_ATP) channel is a multimeric protein complex composed of four inwardly rectifying potassium channel (Kir6.2) and four sulfonylurea receptor 1 (SUR1) subunits. K_ATP channels play a key role in glucose-stimulated insulin secretion by linking glucose metabolism to membrane excitability. Many SUR1 and Kir6.2 mutations reduce channel function by disrupting channel biogenesis and processing, resulting in insulin secretion disease. To better understand the mechanisms governing K_ATP channel biogenesis, a proteomics approach was used to identify chaperone proteins associated with K_ATP channels. We report that chaperone proteins heat-shock protein (Hsp)90, heat-shock cognate protein (Hsc)70, and Hsp40 are associated with β-cell K_ATP channels. Pharmacologic inhibition of Hsp90 function by geldanamycin reduces, whereas overexpression of Hsp90 increases surface expression of wild-type K_ATP channels. Coimmunoprecipitation data indicate that channel association with the Hsp90 complex is mediated through SUR1. Accordingly, manipulation of Hsp90 protein expression or function has significant effects on the biogenesis efficiency of SUR1, but not Kir6.2, expressed alone. Interestingly, overexpression of Hsp90 selectively improved surface expression of mutant channels harboring a subset of disease-causing SUR1 processing mutations. Our study demonstrates that Hsp90 regulates biogenesis efficiency of heteromeric K_ATP channels via SUR1, thereby affecting functional expression of the channel in β-cell membrane.     

A Role for the αvβ3 Integrin in the Transmigration of Monocytes

The β2 integrins and intercellular adhesion molecule-1 (ICAM-1) are important for monocyte migration through inflammatory endothelium. Here we demonstrate that the integrin αvβ3 is also a key player in this process. In an in vitro transendothelial migration assay, monocytes lacking β3 integrins revealed weak migratory ability, whereas monocytes expressing β3 integrins engaged in stronger migration. This migration could be partially blocked by antibodies against the integrin chains αL, β2, αv, or IAP, a protein functionally associated with αvβ3 integrin. Transfection of β3 integrin chain cDNA into monocytes lacking β3 integrins resulted in expression of the αvβ3 integrin and conferred on these cells an enhanced ability to transmigrate through cell monolayers expressing ICAM-1. These monocytes also engaged in αLβ2-dependent locomotion on recombinant ICAM-1 which was enhanced by αvβ3 integrin occupancy. Antibodies against IAP were able to revert this αvβ3 integrin-dependent cell locomotion to control levels. Finally, adhesion assays revealed that occupancy of αvβ3 integrin could decrease monocyte binding to ICAM-1.In conclusion, we show that αvβ3 integrin modulates αLβ2 integrin-dependent monocyte adhesion to and migration on ICAM-1. This could represent a novel mechanism to promote monocyte motility on vascular ICAM-1 and initiate subsequent transendothelial migration.     

Changes in body temperature in king penguins at sea: the result of fine adjustments in peripheral heat loss?

To investigate thermoregulatory adjustments at sea, body temperatures (the pectoral muscle and the brood patch) and diving behavior were monitored during a foraging trip of several days at sea in six breeding king penguins Aptenodytes patagonicus. During inactive phases at sea (water temperature: 4-7 degrees C), all tissues measured were maintained at normothermic temperatures. The brood patch temperature was maintained at the same values as those measured when brooding on shore (38 degrees C). This high temperature difference causes a significant loss of heat. We hypothesize that high-energy expenditure associated with elevated peripheral temperature when resting at sea is the thermoregulatory cost that a postabsorptive penguin has to face for the restoration of its subcutaneous body fat. During diving, mean pectoral temperature was 37.6 +/- 1.6 degrees C. While being almost normothermic on average, the temperature of the pectoral muscle was still significantly lower than during inactivity in five out of the six birds and underwent temperature drops of up to 5.5 degrees C. Mean brood patch temperature was 29.6 +/- 2.5 degrees C during diving, and temperature decreases of up to 21.6 degrees C were recorded. Interestingly, we observed episodes of brood patch warming during the descent to depth, suggesting that, in some cases, king penguins may perform active thermolysis using the brood patch. It is hypothesized that functional pectoral temperature may be regulated through peripheral adjustments in blood perfusion. These two paradoxical features, i.e., lower temperature of deep tissues during activity and normothermic peripheral tissues while inactive, may highlight the key to the energetics of this diving endotherm while foraging at sea.     

Astrogliosis in CNS Pathologies: Is There A Role for Microglia?

Astrogliosis, a cellular reaction with specific structural and functional characteristics, represents a remarkably homotypic response of astrocytes to all kinds of central nervous system (CNS) pathologies. Astrocytes play diverse functions in the brain, both harmful and beneficial. Mounting evidence indicates that astrogliosis is an underlying component of a diverse range of diseases and associated neuropathologies. The mechanisms that lead to astrogliosis are not fully understood, nevertheless, damaged neurons have long been reported to induce astrogliosis and astrogliosis has been used as an index for underlying neuronal damage. As the predominant source of proinflammatory factors in the CNS, microglia are readily activated under certain pathological conditions. An increasing body of evidence suggests that release of cytokines and other soluble products by activated microglia can significantly influence the subsequent development of astrogliosis and scar formation in CNS. It is well known that damaged neurons activate microglia very quickly, therefore, it is possible that activated microglia contribute factors/mediators through which damaged neuron induce astrogliosis. The hypothesis that activated microglia initiate and maintain astrogliosis suggests that suppression of microglial overactivation might effectively attenuate reactive astrogliosis. Development of targeted anti-microglial activation therapies might slow or halt the progression of astrogliosis and, therefore, help achieve a more beneficial environment in various CNS pathologies.     

A Potential Role for Creatine in Drug Abuse?

Supplemental creatine has been promoted for its positive health effects and is best known for its use by athletes to increase muscle mass. In addition to its role in physical performance, creatine supplementation has protective effects on the brain in models of neuronal damage and also alters mood state and cognitive performance. Creatine is found in high protein foods, such as fish or meat, and is also produced endogenously from the biosynthesis of arginine, glycine, and methionine. Changes in brain creatine levels, as measured using magnetic resonance spectroscopy, are seen in individuals exposed to drugs of abuse and depressed individuals. These changes in brain creatine indicate that energy metabolism differs in these populations relative to healthy individuals. Recent work shows that creatine supplementation has the ability to function in a manner similar to antidepressant drugs and can offset negative consequences of stress. These observations are important in relation to addictive behaviors as addiction is influenced by psychological factors such as psychosocial stress and depression. The significance of altered brain levels of creatine in drug-exposed individuals and the role of creatine supplementation in models of drug abuse have yet to be explored and represent gaps in the current understanding of brain energetics and addiction.     

What Is the Optimal Value of the g-Ratio for Myelinated Fibers in the Rat CNS? A Theoretical Approach

Background

The biological process underlying axonal myelination is complex and often prone to injury and disease. The ratio of the inner axonal diameter to the total outer diameter or g-ratio is widely utilized as a functional and structural index of optimal axonal myelination. Based on the speed of fiber conduction, Rushton was the first to derive a theoretical estimate of the optimal g-ratio of 0.6 [1]. This theoretical limit nicely explains the experimental data for myelinated axons obtained for some peripheral fibers but appears significantly lower than that found for CNS fibers. This is, however, hardly surprising given that in the CNS, axonal myelination must achieve multiple goals including reducing conduction delays, promoting conduction fidelity, lowering energy costs, and saving space.

Methodology/Principal Findings

In this study we explore the notion that a balanced set-point can be achieved at a functional level as the micro-structure of individual axons becomes optimized, particularly for the central system where axons tend to be smaller and their myelin sheath thinner. We used an intuitive yet novel theoretical approach based on the fundamental biophysical properties describing axonal structure and function to show that an optimal g-ratio can be defined for the central nervous system (≈0.77). Furthermore, by reducing the influence of volume constraints on structural design by about 40%, this approach can also predict the g-ratio observed in some peripheral fibers (≈0.6).

Conclusions/Significance

These results support the notion of optimization theory in nervous system design and construction and may also help explain why the central and peripheral systems have evolved different g-ratios as a result of volume constraints.     

Development of SCN Connectivity and the Circadian Control of Arousal: A Diminishing Role for Humoral Factors?

The suprachiasmatic nucleus (SCN) is part of a wake-promoting circuit comprising the dorsomedial hypothalamus (DMH) and locus coeruleus (LC). Although widely considered a “master clock,” the SCN of adult rats is also sensitive to feedback regarding an animal''s behavioral state. Interestingly, in rats at postnatal day (P)2, repeated arousing stimulation does not increase neural activation in the SCN, despite doing so in the LC and DMH. Here we show that, by P8, the SCN is activated by arousing stimulation and that selective destruction of LC terminals with DSP-4 blocks this activational effect. We next show that bidirectional projections among the SCN, DMH, and LC are nearly absent at P2 but present at P8. Despite the relative lack of SCN connectivity with downstream structures at P2, day-night differences in sleep-wake activity are observed, suggesting that the SCN modulates behavior at this age via humoral factors. To test this hypothesis, we lesioned the SCN at P1 and recorded sleep-wake behavior at P2: Day-night differences in sleep and wake were eliminated. We next performed precollicular transections at P2 and P8 that isolate the SCN and DMH from the brainstem and found that day-night differences in sleep-wake behavior were retained at P2 but eliminated at P8. Finally, the SCN or DMH was lesioned at P8: When recorded at P21, rats with either lesion exhibited similarly fragmented wake bouts and no evidence of circadian modulation of wakefulness. These results suggest an age-related decline in the SCN''s humoral influence on sleep-wake behavior that coincides with the emergence of bidirectional connectivity among the SCN, DMH, and LC.     

BCG Vaccination: A Role for Vitamin D?

Background

BCG vaccination is administered in infancy in most countries with the aim of providing protection against tuberculosis. There is increasing interest in the role of vitamin D in immunity to tuberculosis. This study objective was to determine if there was an association between circulating 25(OH)D concentrations and BCG vaccination status and cytokine responses following BCG vaccination in infants.

Methods

Blood samples were collected from UK infants who were vaccinated with BCG at 3 (n = 47) and 12 (n = 37) months post BCG vaccination. These two time-points are denoted as time-point 1 and time-point 2. Two blood samples were also collected from age-matched unvaccinated infants (n = 32 and 28 respectively), as a control group. Plasma vitamin D concentrations (25(OH)D) were measured by radio-immunoassay. The cytokine IFNγ was measured in supernatants from diluted whole blood stimulated with M.tuberculosis (M.tb) PPD for 6 days.

Results

58% of infants had some level of hypovitaminosis (25(OH)D <30ng/ml) at time-point 1, and this increased to 97% 9 months later. BCG vaccinated infants were almost 6 times (CI: 1.8–18.6) more likely to have sufficient vitamin D concentrations than unvaccinated infants at time-point 1, and the association remained strong after controlling for season of blood collection, ethnic group and sex. Among vaccinees, there was also a strong inverse association between IFNγ response to M.tb PPD and vitamin D concentration, with infants with higher vitamin D concentrations having lower IFNγ responses.

Conclusions

Vitamin D may play an immuno-regulatory role following BCG vaccination. The increased vitamin D concentrations in BCG vaccinated infants could have important implications: vitamin D may play a role in immunity induced by BCG vaccination and may contribute to non-specific effects observed following BCG vaccination.