Developmental changes of enzymes associated with energy metabolism and the synthesis of some neurotransmitters in discrete areas of human neocortex

Development of central neurotransmitter systems

Inter-neuronal communication is mediated primarily by chemical neurotransmitters, which are released from the nerve terminal, diffuse across the synaptic cleft and interact with specific receptors on adjacent neurons. The development of the biochemical machinery for neurotransmission is closely linked to the functional maturation of the brain's neuronal circuitry. Components essential for neurotransmission (e.g., synthetic enzymes, endogenous neurotransmitters, re-uptake processes and receptors) serve as specific biochemical markers for neuronal systems. The appearance of and developmental increases in these markers during fetal and postnatal life occur with the cessation of neuronal replication and initiation of neuropil elaboration. Discrete groups of neurotransmitter-specific neurons develop according to different timetables, resulting in a shifting pattern of their relative influence in the maturing brain. Human and animal studies demonstrate an early innervation of the neocortex by catecholaminergic axons while neurons using gamma-aminobutyric acid (GABA) mature somewhat later; and the ontogeny of the acetylcholine neurons lags behind both of these. Within each neuronal group the individual biochemical components for neurotransmission also follow differing time courses of maturation. Animal studies, in which cortical neurons were ablated by administering a toxin to the fetus, illustrate the interplay between intrinsic programmes and environmental influences in the assembly of neuronal circuits. The brain's preparation for independent life is characterized by a continual reorganization of neurotransmitter pathways.

Early pharmacological treatment of autism: a rationale for developmental treatment

Autism is a dynamic neurodevelopmental syndrome in which disabilities emerge during the first three postnatal years and continue to evolve with ongoing development. We briefly review research in autism describing subtle changes in molecules important in brain development and neurotransmission, in morphology of specific neurons, brain connections, and in brain size. We then provide a general schema of how these processes may interact with particular emphasis on neurotransmission. In this context, we present a rationale for utilizing pharmacologic treatments aimed at modifying key neurodevelopmental processes in young children with autism. Early treatment with selective serotonin reuptake inhibitors (SSRIs) is presented as a model for pharmacologic interventions because there is evidence in autistic children for reduced brain serotonin synthesis during periods of peak synaptogenesis; serotonin is known to enhance synapse refinement; and exploratory studies with these agents in autistic children exist. Additional hypothetical developmental interventions and relevant published clinical data are described. Finally, we discuss the importance of exploring early pharmacologic interventions within multiple experimental settings in order to develop effective treatments as quickly as possible while minimizing risks.

Increased regional cerebral blood flow but normal distribution of GABAA receptor in the visual cortex of subjects with early-onset blindness

Before the completion of visual development, visual deprivation impairs synaptic elimination in the visual cortex. The purpose of this study was to determine whether the distribution of central benzodiazepine receptor (BZR) is also altered in the visual cortex in subjects with early-onset blindness. Positron emission tomography was carried out with [(15)O]water and [(11)C]flumazenil on six blind subjects and seven sighted controls at rest. We found that the CBF was significantly higher in the visual cortex for the early-onset blind subjects than for the sighted control subjects. However, there was no significant difference in the BZR distribution in the visual cortex for the subject with early-onset blindness than for the sighted control subjects. These results demonstrated that early visual deprivation does not affect the distribution of GABA(A) receptors in the visual cortex with the sensitivity of our measurements. Synaptic elimination may be independent of visual experience in the GABAergic system of the human visual cortex during visual development.

Mechanisms of postnatal neurobiological development: implications for human development

This review focuses on the postnatal neuroanatomical changes that arise during the first years of human life. Development is characterized by 2 major organizational periods. The first period begins at conception and includes the major histogenetic events such as neurulation, proliferation, migration, and differentiation. It has been proposed that these events may be controlled by genetic and epigenetic events, which give rise to neural structures that are amenable to external influence. The second period is a time of reorganization in the human cortex. These events occur during gestation and continue postnatally, possibly through the 2nd decade of life. This stage is characterized by dendritic and axonal growth, synapse production, neuronal and synaptic pruning, and changes in neurotransmitter sensitivity. Although the initiation of these events is influenced by endogenous signals, further neural maturation is primarily influenced by exogenous signals. To illustrate both the progressive and regressive events during the postnatal period, we use examples from the development of the human cortex.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Neurotransmission and the ontogeny of human brain

The early appearance of neurotransmitters in brain tissue refers to their regulative functions on the neuronal circuits. Many neurotransmitters have direct effects on neuronal outgrowth and differentiation during brain development, which precede their role in synaptic information coding. Both the neurotrophic and neurotoxic properties of excitatory amino acids (EAAs) have focused special interest on glutamatergic neurotransmission during brain development. Therefore, this work intends to review and discuss developmental alterations of the EAA neurotransmitter system in the human brain, their relation to human brain maturation and implications for pathological processes during early human brain development.

Menadione-dependent alpha glycerophosphate and succinate dehydrogenases in the developing canine retina

Reducing equivalents for the electron transport chain are generated within the mitochondria by the Krebs cycle and in cytoplasm by processes like lipid metabolism. Two mitochondrial enzymes, succinate dehydrogenase (SDH), a prominent enzyme in the Krebs cycle, and alpha-glycerophosphate dehydrogenase (alpha-GPDH), half of the glycerophosphate shuttle system for bringing reducing equivalents from cytoplasm to mitochondria, were examined enzyme histochemically to assess the contribution of each to metabolism of the developing canine retina. SDH activity, a common marker for oxidative metabolism, was insignificant at birth. By 4 days of age, activity was observed only in developing photoreceptor inner segments. By 21 days of age SDH activity was present throughout the retina, especially in photoreceptor inner segments and plexiform layers, and approached the level observed in the adult dog. Menadione-linked alpha-GPDH (M-alpha-GPDH) activity, however, was prominent in developing vasculature and outermost portion of the neuroblastic layer of the 1 day-old retina. Most notable was localization in vascular precursors, angioblasts, found distant from formed vessels in the peripheral nerve fiber layer. Retinal dependence on an oxidative metabolism in neuronal elements, as represented by SDH activity, occurs only when the vasculature is well established.

Mechanisms of alcohol abuse and alcoholism in adolescents: a case for developing animal models

This paper reviews the ontogeny of neurotransmitter systems and neuropharmacological challenge within transmitter systems and discusses the actions of alcohol on such systems during the juvenile through adolescent periods. To place the animal research within the context of human development, an attempt is made to first examine some fundamental principles of developmental research as they relate to the adolescent period. Evidence presented from animal studies indicates that unique neurochemical and behavioral changes are occurring during postnatal development, including adolescence, that could mediate the response to alcohol. The limited number of studies on the neurochemical and behavioral response to alcohol during adolescence has employed rats and has been restricted by the relatively brief adolescent period in that species. While one alternative is to use primates, it is suggested that innovative behavioral paradigms be developed for adolescent animals in other species to study behaviors such as alcohol self-administration or alcohol stimulus discrimination. It is also suggested that existing behavioral models that are more easily adapted to younger age ranges (e.g., conditioned place preference, conditioned taste aversion, thermal response to ethanol) be extended to make ontogenetic comparisons through adolescence and adulthood. This may further our understanding of alcohol's immediate consequences during each maturational stage and, more important, the contribution of early alcohol exposure to excessive drinking and abnormal cognitive and social functioning during subsequent stages of development.

Quantitative histochemical changes in enzymes involved in energy metabolism in the rat brain during postnatal development. II. Glucose-6-phosphate dehydrogenase and beta-hydroxybutyrate dehydrogenase

The postnatal maturation of glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activity was assessed by histochemistry in rats at eight postnatal stages, P0, P5, P10, P14, P17, P21, P35 and the adult stage. Enzyme activities were revealed on cryostat brain sections with nitroblue tetrazolium. Both enzyme activities were low and homogeneous at birth, and increased to reach a peak in all areas studied, at P17 for beta-hydroxybutyrate dehydrogenase and at P21 for glucose-6-phosphate dehydrogenase. Then, glucose-6-phosphate dehydrogenase activity decreased regularly by 20-49% from P21 to adult stage, except in cerebellar white matter where activity did not change after P21. beta-hydroxybutyrate dehydrogenase activity decreased regularly from P17 to adult stage in globus pallidus, hippocampus, thalamus, brainstem, genu of corpus callosum and cerebellar white matter. It sensorimotor cortex, medial geniculate body, caudate nucleus, hypothalamus and inferior colliculus, beta-hydroxybutyrate dehydrogenase activity stayed stable between P17 and P35 and decreased thereafter to adult levels. Finally, in parietal, auditory and cerebellar cortices, beta-hydroxybutyrate dehydrogenase activity either stayed stable or slightly increased after P17. The present study shows that there is a quite good correlation between postnatal changes in cerebral glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activities and the importance of pentose phosphate pathway and ketone body utilization in the developing brain. Our results also reflect the regional heterogeneity of beta-hydroxybutyrate utilization in the adult rat brain, translating into a remaining high activity of beta-hydroxybutyrate dehydrogenase in cerebral cortex.

Quantitative histochemical changes in enzymes involved in energy metabolism in the rat brain during postnatal development. I. Cytochrome oxidase and lactate dehydrogenase

The postnatal maturation of cytochrome oxidase and lactate dehydrogenase activity was assessed by histochemistry in rats at 8 postnatal stages, P0, P5, P10, P14, P17, P21, P35 and the adult stage. Enzyme activities were revealed on cryostat brain sections with diaminobenzidine for cytochrome oxidase and nitroblue tetrazolium for lactate dehydrogenase. Lactate dehydrogenase activity remained unchanged between P0 and P10, significantly increased in 8 areas of the 14 studied between P10 and P14 and in 6 structures from P14 to P17. These were mainly parietal, auditory and cerebellar cortices, hippocampus, thalamus, hypothalamus and medial geniculate body. There was no further change until P35 and lactate dehydrogenase activity increased then significantly to reach higher adult levels in hippocampus and medial geniculate body. Cytochrome oxidase activity was low from P0 to P10 and increased in 8 regions between P10 and P14. These were all cortices, caudate nucleus, hippocampus, inferior colliculus and genu. Enzyme activity further increased between P14 and P17 in auditory cortex, medial geniculate body and brainstem, did not vary from P17 to P21 but increased by 92 to 371% in all areas between P21 and P35. Cytochrome oxidase activity rose further from P35 to adult stage in hippocampus and medial geniculate body. From birth to adulthood, cytochrome oxidase activity increased 5 to 19 fold and lactate dehydrogenase activity 1.8 to 3.0. The present study shows that there is a quite good correlation between postnatal changes in regional cerebral glucose utilization and activity of enzymes involved in glycolytic and oxidative glucose metabolism in the rat.

Effects of postnatal hypoxia-ischemia on cholinergic neurons in the developing rat forebrain: choline acetyltransferase immunocytochemistry

We studied the effect of early postnatal hypoxia-ischemia on cholinergic neurons in the developing rat forebrain using immunohistochemistry for choline acetyltransferase (ChAT). In 7-day-old rat pups, hypoxia-ischemia was induced in one cerebral hemisphere by combining unilateral carotid ligation with exposure to 8% oxygen for 2.5 h. This procedure caused brain injury in the hemisphere ipsilateral to ligation, most prominent in the corpus striatum, hippocampus and overlying cortex. In animals sacrificed 2-3 weeks after the insult, at approximately 3 weeks of age, the density of cholinergic cell bodies was slightly higher in the lesioned rostral caudate-putamen than the opposite side (+12%, P less than 0.05). In the more caudal portion of caudate-putamen, this effect was greater. In contrast, the size of the cholinergic perikarya in the injured striatum was significantly reduced. Cholinergic neurons in the septum (Ch1, Ch2), globus pallidus and nucleus basalis (Ch4) were relatively unaffected. Considered together with previously reported neurochemical data, these observations suggest that the immature cholinergic neurons are less vulnerable to death from hypoxia-ischemia than other components of the striatum. However, differentiation of surviving cholinergic perikarya and possibly their axonodendritic processes may be disrupted by the early insult.

Nuclear and mitochondrial DNA synthesis and energy metabolism in primary rat glial cell cultures

DNA synthesis in nuclei and mitochondria purified from serum-supplemented rat glial cell cultures at different days after plating was studied. Furthermore in mitochondria, some enzymatic activities related to energy transduction (citrate synthase, malate dehydrogenase, total NADH-cytochrome c reductase, cytochrome oxidase and glutamate dehydrogenase) were measured. For DNA labeling [methyl-3H]thymidine was added to the culture medium at different days after plating. During the culture times studied the specific activity of total, nuclear, and mitochondrial DNA decreased from 8 days in vitro (DIV) to 21 DIV and increased at 30 DIV. The specific activity of nuclear DNA was always higher than that of mitochondrial DNA. The specific activity of the above mentioned mitochondrial enzymes increased from 8 DIV up to 21 DIV and decreased at 30 DIV, suggesting a relationship between the energy metabolism and the differentiation of glial cells in culture.

Molecular forms and solubility of acetylcholinesterase during the embryonic development of rat and human brain

Acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase (EC 3.1.1.8) form homologous sets of multiple molecular forms. The central nervous system of mammals contains mostly tetramers (G4) and monomers (G1). Their proportions have been shown to vary during maturation in rat brain. In order to examine whether a similar evolution occurs in the human, we performed parallel studies of the activity, solubility and molecular forms of acetylcholinesterase in rat and human brains at various stages. We find both similarities and differences: in rat brain, the enzyme increases mostly postnatally but in human brain acetylcholinesterase reaches a maximum at birth. There is an increase in the proportion of G4 and a decrease in the solubility of this from in the absence of detergent in human as well as in rat brain. These changes occur around birth in rat, but during early pregnancy, before 11 weeks in human brain. In both species, the solubility of the enzyme in detergent-free buffers decreases progressively from more than 50% before birth to about 10-20% in the adult. In addition we analyzed butyrylcholinesterase as well as the levels of the neuron-specific enolase and of the glial S-100 protein. In human, gamma gamma-enolase rises to its adult level after birth, but before the S-100 protein.

Muscarinic cholinergic receptors in human infant forebrain: [3H]quinuclidinyl benzilate binding in homogenates and quantitative autoradiography in sections

The ontogeny of muscarinic receptors in human brain was studied by comparing [3H]quinuclidinyl benzilate [( 3H]QNB) binding in postmortem tissue from infants 1 week to 3 months of age with binding in adult specimens. Saturation analysis with [3H]QNB and displacement studies with muscarinic antagonists and agonists in tissue homogenates demonstrated that binding sites in the infants' forebrain regions were present in adult or higher than adult concentrations (Bmax). Binding affinity (Kd) and pharmacological characteristics were nearly identical at the two ages. Quantitative receptor autoradiography demonstrated more [3H]QNB binding in the gray matter of infants than adults and revealed a marked difference between the two ages in the laminar distribution of binding sites in neocortex. In contrast to the adult pattern with higher binding in superficial layers 1-3 than in layers 4-6, the distribution in the immature cortex was inverted. These results suggest that muscarinic receptors in infants resemble closely those in mature brain. However, the topography of receptors in the immature neocortex is distinct and they are redistributed in a gradient from inside outward during postnatal development.

Unique aspects of human newborn cerebral metabolism evaluated with phosphorus nuclear magnetic resonance spectroscopy

In vivo phosphorus nuclear magnetic resonance spectroscopy (31P NMR) was used to evaluate the pattern of phosphate compounds in seven newborn babies (mean gestational age, 32 weeks; birth weight, 1,430 gm; age, 37 days) with a history of perinatal asphyxia. Spectra were collected in a 1.9 Tesla superconductive magnet with surface coil techniques. The spectra had characteristic peaks for phosphorylated monoesters (PME), inorganic phosphate (Pi), phosphodiesters (PD), phosphocreatine (PCr), and ATP. In contrast to cortical spectra from mature animals, these newborn infant 31P NMR spectra were dominated by a large PME peak and had small PCr, Pi, and PD peaks. Intracellular pH, as measured from the chemical shift of the Pi peak relative to the PCr peak, was 7.1 +/- 0.1 (SD). We studied one infant postmortem, and a large PME peak was present in his spectrum. The presence of PME 3 hours after death strongly suggests that it is not a sugar phosphate. In NMR spectroscopy, compounds are identified by their chemical shift relative to a known standard (PCr); the chemical shift of the PME peak was 6.5 ppm, suggesting that it is a mixture of phosphoryl ethanolamine and phosphoryl choline. The PCr/Pi ratio (1.3 +/- 0.7) and the PCr/ATP ratio (0.7 +/- 0.4) were lower in these babies than in mature animals (greater than 2 and greater than 1.4, respectively); the PME/PD ratio (1.2 +/- 0.6), however, was much greater in the infants (mature animals, less than 0.2). These findings suggest that there are unique aspects of human newborn cerebral metabolites and bioenergetic reserve.

Succinate dehydrogenase, monoamine oxidase and glutamine synthetase in developing human foetal brain regions

In human foetal brain ontogeny the cerebral activity of succinate oxidoreductase (EC 1.3.99.1), i.e. succinate dehydrogenase (SDH), is higher than the cerebellar activity. With rise in foetal body weight the activity in all the brain regions gradually declines. SDH in all the brain regions shows two high-activity periods, one at 20-35 g and another at 110-220 g body weight. The enzyme exhibits a craniocaudal pattern of development. At all times of gestation, L-glutamate:ammonia ligase (EC 6.3.1.2), i.e. glutamine synthetase, activity in the spinal cord and medulla is higher than in the other three regions. At 190 g body weight glutamine synthetase shows an activity peak in all brain regions. Monoamine:oxygen oxidoreductase (EC 1.4.3.4). i.e. monoamine oxidase (MAO), is present much before the onset of electrical activity. It develops caudocranially and exhibits a biphasic pattern of development in all the regions. It increases considerably in the medulla and the spinal core towards late gestational periods.

Modifications in energy metabolism during the development of chick glial cells and neurons in culture

Developmental changes in lactate dehydrogenase (LDH), enolase, hexokinase (HK), malate dehydrogenase (MDH), and glutamate dehydrogenase (GDH) activities were measured in cultures of pure neurons and glial cells prepared from brains of chick embryos (8 day-old for neurons, 14 day-old for glial cells) as a function of cellular development with time in culture. The modifications observed in culture were compared to those measured in brain extracts during the development of the nervous tissue in the chick embryo and during the post-hatching period. A significant increase of MDH, GDH, LDH, and enolase activities are observed in neurons between 3 and 6 days of culture, whereas simultaneously a decrease of HK values occurs. In the embryonic brain between 11 and 14 days of incubation, which would correspond for the neuronal cultures to day 3 through 6, modifications of MDH, GDH, HK, and enolase levels are similar to those observed in neurons in culture. Only the increase of LDH activity is less pronounced in vivo than in cultivated cells. The evolution of the tested enzymatic activities in the brain of the chick during the period between 7 days before and 10 days after hatching is quite similar to that observed in cultivated glial cells (prepared from 14 day-old embryos) between 6 and 18 days of culture. All tested activities increased in comparable proportions. The modifications of the enzymatic profile indicate that some maturation phenomena affecting energy metabolism of neuronal and glial elements in culture, are quite similar to those occuring in the total nervous tissue. A relationship between the development of the energy metabolism of the brain and differentiation processes affecting neuroblasts and the glial-forming cells is discussed.

Glucose-6-phosphate dehydrogenase and NADP-linked malate dehydrogenase during posthatching development of brain of altricial birds

Glucose-6-phosphate dehydrogenase and NADP-linked malate dehydrogenase were studied in different areas of the brain of three altricial birds during posthatching development. The birds were pigeon and swift, having a posthatching nestling period of 30 days; and sparrow, having a posthatching nestling period of 14 days. The activity of the two enzymes was high during development. G-6-PD activity may be high because of the need for pentoses in the early part of development and the need for reducing equivalents (NADPH2) for synthesis of lipids and other compounds in the later stages of development. Malic enzyme activity also seems to be high because of the need for reducing equivalents. The activity of malic enzyme was found to be higher than that of G-6-PD.

Development of glial cells in primary cultures: energy metabolism and lactate dehydrogenase isoenzymes

Primary cultures of glial cells prepared from brains of newborn rats were grown for periods of 1-5 weeks. After a proliferative phase of between 2 and 3 weeks, the cultures were maintained in stationary phase, during which a significant increase of oxygen consumption and of the activities of lactate dehydrogenase, succinate dehydrogenase, and mitochondrial glycerolphosphate dehydrogenase could be observed. Furthermore, qualitative changes in the lactate dehydrogenase isoenzyme pattern were found with time, characterized by a shift toward an enhanced synthesis of H subunits. A similar development was found in comparing the LDH isoenzyme pattern in the brain of 15-day-old rat embryo with those of newborn and adult rat brains. It is suggested that some aspects of maturation of glial cells in culture are comparable to those occurring in whole brain in vivo, namely a shift towards an enhanced aerobic metabolism.