Genetically Triggered Sexual Differentiation of Brain and Behavior

The dominant theory of sexual differentiation of the brain holds that sex differences in brain anatomy and function arise because of the action of gonadal steroids during embryonic and neonatal life. In mammals, testicular steroids trigger masculine patterns of neural development, and feminine patterns of neural development occur in the absence of such testicular secretions. In contrast, gonadal differentiation in mammals is not initiated by hormonal mechanisms, but is regulated by the action of gene products such as SRY, a testis-determining gene on the Y chromosome. This paper argues that such genetic, nonhormonal signals may also trigger specific examples of sexual differentiation of the brain. This thesis is supported by two arguments. The first is that “direct genetic” (i.e., nonhormonal) control of sexual differentiation may be as likely to evolve as hormonal control. The second line of argument is that neural and nonneural dimorphisms have already been described that are not well explained by classical theories of steroid-dependent sexual differentiation and for which other factors need to be invoked.     

<Emphasis Type="Italic">In vivo</Emphasis> Quinolinic Acid Increases Synaptosomal Glutamate Release in Rats: Reversal by Guanosine

Glutamate, the main excitatory neurotransmitter in the mammalian central nervous system (CNS), plays important role in brain physiological and pathological events. Quinolinic acid (QA) is a glutamatergic agent that induces seizures and is involved in the etiology of epilepsy. Guanine-based purines (GBPs) (guanosine and GMP) have been shown to exert neuroprotective effects against glutamatergic excitotoxic events. In this study, the influence of QA and GBPs on synaptosomal glutamate release and uptake in rats was investigated. We had previously demonstrated that QA “in vitro” stimulates synaptosomal L-[³H]glutamate release. In this work, we show that i.c.v. QA administration induced seizures in rats and was able to stimulate synaptosomal L-[³H]glutamate release. This in vivo neurochemical effect was prevented by i.p. guanosine only when this nucleoside prevented QA-induced seizures. I.c.v. QA did not affect synaptosomal L-[³H]glutamate uptake. These data provided new evidence on the role of QA and GBPs on glutamatergic system in rat brain.     

Neurochemistry,Neuropathology, and Heredity in SAMP8: A Mouse Model of Senescence

The SAMP8 strain spontaneously develops learning and memory deficits with characteristics of aging, and is a good model for studying the mechanism of cognitive dysfunction with age. Oxidative stress occurs systemically in SAMP8 from early on in life and increases with aging. Neuropathological changes such as the deposition of Aβ, hyperphosphorylation of tau, impaired development of dendritic spines, and sponge formation, and neurochemical changes were found in the SAMP8 brain. These changes may be partially mediated by oxidative stress. Oxidative damage is a major factor in neurodegenerative disorders and aging. A decline in the respiratory control ratio suggesting mitochondrial dysfunction was found in the brain of SAMP8. The rise in oxidative stress following mitochondrial dysfunction may trigger neuropathological and neurochemical changes, disrupting the development of neural networks in the brain in SAMP8. Special issue article in Honour of Dr. Akitane Mori.     

Loss of EphA4 impairs short‐term spatial recognition memory performance and locomotor habituation

EphA4 receptor (EphA4) tyrosine kinase is an important regulator of central nervous system development and synaptic plasticity in the mature brain, but its relevance to the control of normal behavior remains largely unexplored. This study is the first attempt to obtain a behavioral profile of constitutive homozygous and heterozygous EphA4 knockout mice. A deficit in locomotor habituation in the open field, impairment in spatial recognition in the Y‐maze and reduced probability of spatial spontaneous alternation in the T‐maze were identified in homozygous EphA4^?/? mice, while heterozygo us EphA4^+/? mice appeared normal on these tests in comparison with wild‐type (WT) controls. The multiple phenotypes observed in EphA4^?/? mice might stem from an underlying deficit in habituation learning, reflecting an elementary form of nonassociative learning that is in contrast to Pavlovian associative learning, which appeared unaffected by EphA4 disruption. A deficit in motor coordination on the accelerating rotarod was also demonstrated only in EphA4^?/? mice – a finding in keeping with the presence of abnormal gait in EphA4^?/? mice – although they were able to improve performance over training. There was no evidence for substantial changes in major neurochemical markers in various brain regions rich in EphA4 as shown by post‐mortem analysis. This excludes the possibility of major neurochemical compensation in the brain of EphA4^?/? mice. In summary, we have demonstrated for the first time the behavioral significance of EphA4 disruption, supporting further investigation of EphA4 as a possible target for behavioral interventions where habituation deficits are prominent.     

Adult neurogenesis and neuronal regeneration in the brain of teleost fish

Whereas adult neurogenesis appears to be a universal phenomenon in the vertebrate brain, enormous differences exist in neurogenic potential between “lower” and “higher” vertebrates. Studies in the gymnotiform fish Apteronotus leptorhynchus and in zebrafish have indicated that the relative number of new cells, as well as the number of neurogenic sites, are at least one, if not two, orders of magnitude larger in teleosts than in mammals. In teleosts, these neurogenic sites include brain regions homologous to the mammalian hippocampus and olfactory bulb, both of which have consistently exhibited neurogenesis in all species examined thus far. The source of the new cells in the teleostean brain are intrinsic stem cells that give rise to both glial cells and neurons. In several brain regions, the young cells migrate, guided by radial glial fibers, to specific target areas where they integrate into existing neural networks. Approximately half of the new cells survive for the rest of the fish’s life, whereas the other half are eliminated through apoptotic cell death. A potential mechanism regulating development of the new cells is provided by somatic genomic alterations. The generation of new cells, together with elimination of damaged cells through apoptosis, also enables teleost fish rapid and efficient neuronal regeneration after brain injuries. Proteome analysis has identified a number of proteins potentially involved in the individual regenerative processes. Comparative analysis has suggested that differences between teleosts and mammals in the growth of muscles and sensory organs are key to explain the differences in adult neurogenesis that evolved during phylogenetic development of the two taxa.     

Microtubules and brain development: The effects of thyroid hormones

The data accumulated during the past twenty years suggest that thyroid hormones have a direct effect on the differentiation of both the neurons and the glial cell during the critical period of brain development. A fast survey of the available data (which is presented in the introduction of this article) on the mechanism of action of thyroid hormones and on their different effects during brain development suggests that the most dramatic effect of hypothyroidism is a hypoplastic neuropile. Both in vivo, during the critical period of nerve cell differentiation and in vitro, when added to primary cultures of embryonic nerve cells thyroid hormones stimulate neurite outgrowth. Since neurite outgrowth requires massive microtubule assembly the assumption was made that thyroid hormones stimulate nerve cell differentiation by changing the concentration and/or activity of the different proteins (tubulin and “microtubule associated proteins”, MAPs) which co-polymerize to form microtubules.

Preliminary information was obtained by following the kinetics of microtubule assembly in crude brain supernatants. The data showed that: (1) the rate of in vitro microtubule assembly increases with age during brain development; (2) hypothyroidism, when produced in the rat at late pregnancy, slows this evolution; (3) early replacement therapy with thyroid hormones restores normal rates of assembly; (4) the addition of purified MAPs to normal young or 15-day-old hypothyroid brain preparations restores normal rates of polymerization. These and other data suggested that thyroid hormones regulate microtubule assembly by changing the concentration and/or activity of one or more of the MAPs.

Further analysis revealed that striking qualitative changes in MAPs composition occur during brain development. For instance, the TAU fraction, a group of 4–5 proteins with a molecular weight of 60–68 K which is present in adult brain, is absent at early stages of postnatal development: two other entities are present, TAU slow and TAU fast, with different molecular weights, lower activity and different peptide mapping. This latter observation suggests that different TAU genes are expressed during brain development; a conclusion which has been confirmed by cell-free translation of the mRNas coding for these proteins. Analysis of the TAU fraction prepared from hypothyroid rat brains also revealed that a group of TAU proteins. “TAU₃”, is almost missing, whereas thyroid hormone administration markedly increases its concentration. Two-dimensional gel electrophoresis showed that the TAU fraction is composed with more than 15 entities, with at least five of them being under thyroid hormone control.

The precise physiological significance of the heterogeneity of MAPs and of the changes in MAPs composition seen during development and in hypothyroid rat brain remains to be determined. The assumption is made that these changes might be of utmost importance to regulate the number and length of the microtubules, and therefore the number and length of the neurites which are formed during the differentiation process of the different neurons. Thyroid hormones would be in these respects one of the epigenic factors required to synchronize sequentially the expression of the genes coding for these proteins in the different nerve cells.     

Decrease in expression of α2a-adrenoreceptors in the brain of neonatal rats modulates exploratory behavior on the stage of its development

Reduction in α2A-adrenoreceptors (α2A-AR) expression in the rat brain during neonatal ontogenesis has been observed to influence neurochemical, hormonal, and behavioral characteristics on the stage of exploratory behavior development. Injection of antisense nucleotide complimentary to α2A-AR mRNA on the 2^nd–4^th day after birth has been demonstrated to decrease the quantity of α2-AR in the brain at this stage significantly. Subsequent increase in receptor’s expression (inhibited after the treatment) and its excess in the brainstem and hippocampus over the control meanings at 3 weeks of age were detected. Reduction in the basal corticosterone level in the blood and significant increase in exploratory behavior in the open-field test (number of risings on the hind legs) has been demonstrated in three-week-old pups. No differences in motor activity of these rats assessed by the number of grid crossings were indicated compared to control animals. In total, the reported results have been shown to indicate the presence of a deferred influence of short-term decrease in the brain α2A-AR expression in the first day of life on the subsequent development of α2A-AR system and exploratory behavior.     

Amyloid beta: Functional protein or biological junk?

During a decade there was a dogma that Alzheimer’s amyloid beta (Aβ) is produced only upon the disease, and that this protein is neurotoxic for neurons and brain tissue. Current scientific evidence demonstrates that Aβ is an essential molecule in synaptic plasticity that underlines learning and memory. Therefore, it was hypothesized that the change of Aβ biology in Alzheimer’s disease (as well as in a number of other human pathologies, including cardiovascular disease, Niemann-Pick type C disease and Down syndrome) represents a physiological mechanism serving to compensate the impaired brain structure or function. This review summarizes experimental evidence on Aβ as a functional player in synaptic plasticity and neurochemical pathways.     

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH₂) was the first hypothalamic peptide shown to act “up” on the brain, not just “down” on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose–response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH₂) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH₂) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH₂ and endomorphin-2 is Tyr-Pro-Phe-Phe-NH₂. Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood–brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.     

The C57BL/6J Mouse Exhibits Sporadic Congenital Portosystemic Shunts

C57BL/6 mice are the most widely used strain of laboratory mice. Using in vivo proton Magnetic Resonance Spectroscopy (¹H MRS), we have repeatedly observed an abnormal neurochemical profile in the brains of both wild-type and genetically modified mice derived from the C57BL/6J strain, consisting of a several fold increase in cerebral glutamine and two fold decrease in myo-inositol. This strikingly abnormal neurochemical “phenotype” resembles that observed in chronic liver disease or portosystemic shunting and appeared to be independent of transgene, origin or chow and was not associated with liver failure. As many as 25% of animals displayed the abnormal neurochemical profile, questioning the reliability of this model for neurobiology. We conducted an independent study to determine if this neurochemical profile was associated with portosystemic shunting. Our results showed that 100% of the mice with high brain glutamine displayed portosystemic shunting by concomitant portal angiography while all mice with normal brain glutamine did not. Since portosystemic shunting is known to cause alterations in gene expression in many organs including the brain, we conclude that portosystemic shunting may be the most significant problem associated with C57BL/6J inbreeding both for its effect on the central nervous system and for its systemic repercussions.     

Categories and gatherings: Group selection and the mythology of cultural anthropology

If the term “group selection” is to have any meaning beyond mere semantics, it must refer to situations where individuals live in groups. Although the terminology of cultural anthropology suggests that humans live in bounded and enduring gatherings that might serve as group “vehicles” of selection, we argue that none of the terms asserted to be such an entity (i.e., clans, lineages, villages, bands, tribes, populations, societies, and cultures) fulfill this requirement. This is because these terms refer to: (1) reified abstractions, (2) groups only in the sense of categories of people instead of groups in the sense of people gathered together, or (3) gatherings that are much too fluid and fuzzy in their membership to be “vehicles.” Following Murdock (1972), we refer to this obsession with groups as “anthropology's mythology,” and we suggest that it is the result of our evolved capacity for categorical perception. Although classifying phenomena into categories is useful in many situations, it has hindered our understanding of human social organization and human evolution.     

The Janus face of caffeine

Caffeine is certainly the psychostimulant substance most consumed worldwide. Over the past years, chronic consumption of caffeine has been associated with prevention of cognitive decline associated to aging and mnemonic deficits of brain disorders. While its preventive effects have been reported extensively, the cognitive enhancer properties of caffeine are relatively under debate. Surprisingly, there are scarce detailed ontogenetic studies focusing on neurochemical parameters related to the effects of caffeine during prenatal and earlier postnatal periods. Furthermore, despite the large number of epidemiological studies, it remains unclear how safe is caffeine consumption during pregnancy and brain development. Thus, the purpose of this article is to review what is currently known about the actions of caffeine intake on neurobehavioral and adenosinergic system during brain development. We also reviewed other neurochemical systems affected by caffeine, but not only during brain development. Besides, some recent epidemiological studies were also outlined with the control of “pregnancy signal” as confounding variable. The idea is to tease out how studies on the impact of caffeine consumption during brain development deserve more attention and further investigation.     

Phospholipid Distribution and Fatty Acid Composition in Different Brain Regions During Chick Embryo Development

Abstract: The phospholipid profile of different chick embryo brain regions was studied from 11 to 21 days of development, revealing interesting changes in content and distribution. Total phospholipid phosphorus (P), in micrograms of P per microgram of DNA, increases significantly during development of cerebral hemispheres (CHs), optic lobes (OLs), and brainstem (BS). Compared with CH and OL, the BS shows at all stages a significantly higher concentration of phospholipid P, which in contrast decreases in the cerebellum (CB) during development. Moreover, the data show interesting differences between the right and the left portion of the brain. The distribution of phospholipid P and the fatty acid composition of phospholipids were asymmetric between left and right OL and CH, as were the concentrations of DNA and cholesterol, demonstrating lateralized neurochemical development in these structures, i.e., left OL, right OL, left CH, and right CH. The data are discussed also in relation to the potential importance of neurochemical lateralization for determining lateralized embryonic and postnatal behavior of this species.     

The relation of protein synthesis to the concentrations of free and membrane-bound ribosomes in brain at different ages

Free and membrane-bound ribosomes were prepared from the brains of young (3- and 8-day-old) and adult (30 day) rats by the method of Ramsey and Steele (1977). Though the concentration of RNA in young brain is higher than that in adult brain, the fraction of the RNA which is ribosomal is virtually the same (64%) as is the ratio of free ribosomes total ribosomes (61%) at all ages studied. The rate of protein synthesis measured in vivo, expressed in the usual terms of “% per h”, is much higher in young compared to adult brain, but when expressed as the ribosomal specific activity, i.e. “mg protein synthesized per hour per mg ribosomal RNA”, is the same in the three age groups (0.61, 0.58 and 0.60, respectively). Thus, even during early development, when protein is increasing rapidly, ribosomes are no more active than in adult brain, suggesting that synthesis rates in brain are limited by ribosomal content.     

Comparative study of bioelectric brain activity in drug and internet addicts

The analysis of EEG wave intensity index has been used for comparative assessment of specific changes in brain activity in the cases of chronic ethanol or drug (heroin, opium) abuse, exposure to toxic agents (volatile organic compounds, household chemistry), polydrug use, and nonchemical (Internet) dependence. These data may be relevant to the problem of general and specific mechanisms of influence of various addiction risk factors on neurophysiological and neurochemical processes.     

The influence of genomic imprinting on brain development and behavior

Genomic imprinting, a newly discovered and significant form of gene regulation, refers to the differential expression of a gene depending on whether it is inherited from the male or female parent. The genetic conflict theory of genomic imprinting postulates that conflicts between the genetic interests of mothers, fathers, and their offspring, as well as asymmetric genetic relationships with maternal and paternal kin, led to an evolutionary “arms race” within the genome, which resulted in the expression of these conflicts at the phenotypic level. This paper provides background and evidence regarding genomic imprinting and its role in brain development, describes the cognitive and behavioral phenomena that have been interpreted in terms of the genetic conflict model, and points to potential avenues of further research.     

Changes in kynurenic, anthranilic, and quinolinic acid concentrations in rat brain tissue during development

Kynurenic, anthranilic, and quinolinic acid, brain tissue concentrations and indoleamine 2,3-dioxygenase [EC 1 13.11.17] activity were determined in rat brain, during pre- and postnatal development. Quinolinic acid brain tissue concentration was significantly increased at birth as compared with the prenatal level, then it declined rapidly in the postnatal period. By the contrary, kynurenic and anthranilic acids brain tissue concentrations in rat brain were significantly lower at birth as compared with those found prenatally; then kynurenic acid concentration decreased in the first postnatal week and increased thereafter, while anthranilic acid concentration increased in the first postnatal week and decreased thereafter. Indoleamine 2,3-dioxygenase [EC 1 13.11.17] activity were found unchanged in pre and post natal rat brain. The described opposite changes in quinolinic and kynurenic acids concentrations, occurring in pre- and postnatal period, despite the lack of knowledge on the precise role played by these compounds on the different neurotransmitter systems in the brain, could be involved in brain ontogenetic development.     

Analysis of cholinergic markers, biogenic amines, and amino acids in the CNS of two APP overexpression mouse models

Two transgenic mouse models expressing mutated human amyloid precursor protein and previously found to display cognitive and behavioural alterations, reminiscent of Alzheimer patients' symptomatology, were scrutinised for putative brain region-specific changes in neurochemical parameters. Brains of NSE-hAPP751m-57, APP23 and wild-type mice were microdissected to perform brain region-specific neurochemical analyses. Impairment of cholinergic transmission, the prominent neurochemical deficit in Alzheimer brain, was examined; acetylcholinesterase and choline acetyltransferase activity levels were determined as markers of the cholinergic system. Since Alzheimer neurodegeneration is not restricted to the cholinergic system, brain levels of biogenic amines and metabolites, and amino acidergic neurotransmitters and systemic amino acids were analysed as well. Cholinergic dysfunction, reflected in reduced enzymatic activity in the basal forebrain nuclei, was restricted to the APP23 model, which also exhibited more outspoken and more widespread changes in other neurotransmitter systems. Significant changes in compounds of the noradrenergic and serotonergic system were observed, as well as alterations in levels of the inhibitory neurotransmitter glycine and systemic amino acids. These observations were clearly in occurrence with the more pronounced histopathological and behavioural phenotype of the APP23 model. As transgenic models often do not represent an end-stage of the disease, some discrepancies with results from post-mortem human Alzheimer brain analyses were apparent; in particular, no significant alterations in excitatory amino acid levels were detected. Our findings of brain region-specific alterations in compound levels indicate disturbed neurotransmission pathways, and greatly add to the validity of APP23 mice as a model for Alzheimer's disease. Transgenic mouse models may be employed as a tool to study early-stage neurochemical changes, which are often not accessible in Alzheimer brain.     

Back to the future: The organizational–activational hypothesis adapted to puberty and adolescence

Phoenix, Goy, Gerall, and Young first proposed in 1959 the organizational–activational hypothesis of hormone-driven sex differences in brain and behavior. The original hypothesis posited that exposure to steroid hormones early in development masculinizes and defeminizes neural circuits, programming behavioral responses to hormones in adulthood. This hypothesis has inspired a multitude of experiments demonstrating that the perinatal period is a time of maximal sensitivity to gonadal steroid hormones. However, recent work from our laboratory and others demonstrates that steroid-dependent organization of behavior also occurs during adolescence, prompting a reassessment of the developmental time-frame within which organizational effects are possible. In addition, we present evidence that adolescence is part of a single protracted postnatal sensitive period for steroid-dependent organization of male mating behavior that begins perinatally and ends in late adolescence. These findings are consistent with the original formulation of the organizational/activational hypothesis, but extend our notions of what constitutes “early” development considerably. Finally, we present evidence that female behaviors also undergo steroid-dependent organization during adolescence, and that social experience modulates steroid-dependent adolescent brain and behavioral development. The implications for human adolescent development are also discussed, especially with respect to how animal models can help to elucidate the factors underlying the association between pubertal timing and adult psychopathology in humans.     

Melanin-concentrating hormone producing neurons: Activities and modulations

Regulation of energy homeostasis in animals involves adaptation of energy intake to its loss, through a perfect regulation of feeding behavior and energy storage/expenditure. Factors from the periphery modulate brain activity in order to adjust food intake as needed. Particularly, “first order” neurons from arcuate nucleus are able to detect modifications in homeostatic parameters and to transmit information to “second order” neurons, partly located in the lateral hypothalamic area. These “second order” neurons have widespread projections throughout the brain and their proper activation leads them to a coordinated response associated to an adapted behavior. Among these neurons, melanin-concentrating hormone (MCH) expressing neurons play an integrative role of the various factors arising from periphery, first order neurons and extra-hypothalamic arousal systems neurons and modulate regulation of feeding, drinking and seeking behaviors. As regulation of MCH release is correlated to regulation of MCH neuronal activity, we focused this review on the electrophysiological properties of MCH neurons from the lateral hypothalamic area. We first reviewed the knowledge on the endogenous electrical properties of MCH neurons identified according to various criteria which are described. Then, we dealt with the modulations of the electrical activity of MCH neurons by different factors such as glucose, glutamate and GABA, peptides and hormones regulating feeding and transmitters of extra-hypothalamic arousal systems. Finally, we described the current knowledge on the modulation of MCH neuronal activity by cytokines and chemokines. Because of such regulation, MCH neurons are some of the best candidate to account for infection-induced anorexia, but also obesity.