Modulation of the notch signaling by Mash1 and Dlx1/2regulates sequential specification and differentiation of progenitor cell types in the subcortical telencephalon

Notch signaling has a central role in cell fate specification and differentiation. We provide evidence that the Mash1 (bHLH) andDlx1 and Dlx2 (homeobox) transcription factors have complementary roles in regulating Notch signaling, which in turn mediates the temporal control of subcortical telencephalic neurogenesis in mice. We defined progressively more mature subcortical progenitors (P1, P2 and P3) through their combinatorial expression of MASH1 and DLX2, as well as the expression of proliferative and postmitotic cell markers at E10.5-E11.5. In the absence ofMash1, Notch signaling is greatly reduced and `early' VZ progenitors(P1 and P2) precociously acquire SVZ progenitor (P3) properties. Comparing the molecular phenotypes of the delta-like 1 and Mash1 mutants, suggests that Mash1 regulates early neurogenesis through Notch-and Delta-dependent and -independent mechanisms. While Mash1 is required for early neurogenesis (E10.5), Dlx1 and Dlx2 are required to downregulate Notch signaling during specification and differentiation steps of `late' progenitors (P3). We suggest that alternate cell fate choices in the developing telencephalon are controlled by coordinated functions of bHLH and homeobox transcription factors through their differential affects on Notch signaling.

Development of two transmitter release components during the critical period for imprinting in the chick IMHV

Transmitter release at an excitatory synapse has two components, fast synchronous and slow asynchronous transmitter release. Using the whole cell recording technique, we investigated the developmental properties of neurotransmitter release, which is composed of the two components in the intermediate and medial part of the hyperstriatum ventral (IMHV) of chicks during the critical period for imprinting. Analysis of the paired-pulse responses revealed that the depression of the excitatory postsynaptic currents (EPSCs), driven mainly by fast synchronous release, was frequently observed in P0-1 chicks but not in those at P5-8. The spontaneous excitatory postsynaptic currents (sEPSCs) after the paired-pulse stimulation, which were thought to be driven by asynchronous transmitter releases, were observed more frequently in P0-1 chicks than P5-8 chicks. Furthermore, examination of Ca2+ dependency in the evoked EPSCs showed that the amplitudes in P5-8 chicks were more sensitive to reduction of the extracellular Ca2+ concentration than younger chicks. Considering that the Ca2+ dependency of EPSCs is defined by both Ca2+ sensitivity and the proportion of each type of release machineries at the release site, these results indicate that the ratio of fast synchronous to slow asynchronous transmitter release machinery changed during the critical period. These changes may play critical roles in the capacity of the avian brain to consolidate novel experience in the immediate period after hatching.

[Structural basics for development of blood-cerebrospinal fluid barrier in man]

Studies of development of hematoliquorian barrier in man represent significant difficulties, as it is not possible to employ the experimental-physiological approaches. In these conditions, the morphological analysis based on application of modern immunocytochemistry methods acquires the key role in fundamental physiological studies of onthogenesis of barrier central neurology systems. The current article presents an analytical review of publications and results of own authors research of structural organization of the hematoliquorian barrier in man during the prenatal ontogenesis.

Slit promotes branching and elongation of neurites of interneurons but not projection neurons from the developing telencephalon

Proper neuronal migration and establishment of circuitry are key processes for laying down the functional network of cortical neurons. A variety of environmental guidance cues, attractive or repulsive, have been shown to guide cell migration and axon arborization. One of these, Slit, appears to possess contrarian properties; it can either inhibit axon outgrowth or promote branching and elongation. The object of the present study was to assess the effect of Slit on MGE and neocortical neurons in culture and in the developing ventricle. When cocultured with a Slit source, E13.5 MGE explants displayed inhibited neurite outgrowth while GABA neuron dispersion away from Slit was increased. Similar inhibition of neurite outgrowth was seen in dissociated cells from E13.5 MGE, these cells were identified to be interneurons based upon their GABA staining. In contrast, E13.5 interneurons, after culture for another 5 days, were responsive to Slit by neurite branching and elongation. Projection neurons, identified by lack of GABA staining, did not respond to Slit, either by branching or elongation. Furthermore, GABA interneurons but not pyramidal neurons, appeared to avoid neocortical areas close to an implanted source of Slit in the ventricular wall. These results lead us to suggest that interneurons but not projection neurons are responsive to the chemorepellant effect of Slit. However, more mature interneurons appear to respond to Slit by neurite arborization. These results demonstrate a selective response to Slit by GABAergic neurons during neocortical development.

Conserved and divergent patterns of Reelin expression in the zebrafish central nervous system

The protein Reelin is suggested to function in cell-cell interactions and in mediating neuronal migrations in layered central nervous system structures. With the aim of shedding light on the development of the teleost telencephalon, which forms through the process of eversion and results in the formation of a nonlaminar pallium, we isolated a zebrafish ortholog of the reelin gene and studied its expression in developing and adult brain. The pattern of expression is highly dynamic during the first 24-72 hours of development. By 5 days postfertilization, high amounts of reelin mRNA are found in the dorsal telencephalon, thalamic and hypothalamic regions, pretectal nuclei, optic tectum, cerebellum, hindbrain, reticular formation, and spinal cord, primarily confined to postmitotic neurons. This pattern persists in 1- to 3-month-old zebrafish. This study, together with reports on reelin expression in other vertebrates, shows that reelin mRNA distribution is conserved in many regions of the vertebrate brain. A major exception is that reelin is expressed in the majority of the cells of the dorsal regions of the everted telencephalon in zebrafish embryos, whereas it is restricted to specific neuronal populations in the developing telencephalon of amniotes. To better understand the origin of these differences, we analyzed reelin expression in the telencephalon of an amphibian. Telencephalic reelin expression in Xenopus laevis shows more similarities with the sauropsidian than with the teleostean pattern. Thus, the differences in the telencephalic expression of reelin between teleosts and tetrapods are likely to be due to different roles for Reelin during eversion, a process that is specific for the teleost telencephalon.

A1 adenosine receptor activation induces ventriculomegaly and white matter loss

A1 adenosine receptors (A1ARs) are widely expressed in the brain during development. To examine whether A1AR activation can alter postnatal brain formation, neonatal rats from postnatal days 3 to 14 were treated with the A1AR agonist N6-cyclopentyladenosine (CPA) in the presence or absence of the peripheral A1AR antagonist 8-(p-sulfophenyl)-theophylline (8SPT). CPA or CPA + 8SPT treatment resulted in reductions in white matter volume, ventriculomegaly, and neuronal loss. Quantitative electron microscopy revealed reductions in total axon volume following A1AR agonist treatment. We also observed reduced expression of myelin basic protein in treated animals. Showing that functional A1ARs were present over the ranges of ages studies, high levels of specific [3H]CCPA binding were observed at PD 4, 7 and 14, and receptor-G protein coupling was present at each age. These observations show that activation of A1ARs with doses of CPA that mimic the effects of high adenosine levels results in damage to the developing brain.

Determination of cell fate within the telencephalon

The telencephalon (basal ganglia, septum, cerebral cortex and olfactory bulb) contains two general classes of neurons: those that project axons to distant targets and those that make only local connections. While projection neurons can be either excitatory (such as those in the olfactory bulb and cortex) or inhibitory (such as those in the striatum), local circuit neurons (interneurons) are usually inhibitory. Within these two general classes of neurons there are a myriad of cell subtypes based upon axonal and dendritic morphology, chemical markers, neurotransmitters, connectivity and physiology. A crucial issue regarding the development of the telencephalon is the molecular determination of neuronal subtypes. Since important aspects of neuronal fate determination occur within the proliferative zone, the consideration of determinants of a mature neuron's fate requires consideration of that cell's origin.

Distinct origins of neocortical projection neurons and interneurons in vivo

Recent studies in rodents have suggested that some cortical GABAergic interneurons arise within the neuroepithelium of the subcortical telencephalon then migrate dorsally into the cerebral cortex. These studies have relied heavily on short-term organotypic culture methods and on the analysis of mutant mice that die during the neonatal period. The purpose of this study is to ascertain directly whether cells labeled in the subcortical telencephalon in vivo differentiate into mature cortical interneurons and whether any cortical interneurons arise from the dorsal, cortical neuroepithelium. Mitotic cells within the neonatal cortex or subcortical telencephalon were labeled by focal injections of [(3)H]thymidine into the brains of neonatal ferrets. The fates of labeled cells were assessed in mature animals 6 weeks later. Our results suggest that many cortical interneurons, but not cortical projection neurons, derive from the subcortical telencephalon. Conversely, cortical projection neurons, but few if any interneurons, are generated within the proliferative zones of the neocortex.

Enhanced fos expression in the zebra finch (Taeniopygia guttata) brain following first courtship

Young zebra finch males that court a female for the first time develop a stable preference for the females of that species. On the neuronal level, consolidation of the imprinted information takes place. Here we demonstrate that first courtship or being chased around in the cage leads to enhanced fos expression in forebrain areas implicated in learning and imprinting in zebra finch males compared with birds reared in isolation or in the aviary. Two of the forebrain areas highly active during first courtship (as demonstrated by the 14C-2-deoxyglucose technique), the imprinting locus latral neo/hyperstriatum ventrale (LNH) and the secondary visual area hyperstriatum accessorium/dorsale (HAD), demonstrate enhanced fos expression. Two other imprinting-related areas, the medial neo/hyperstriatum ventrale (MNH) and archistriatum/neostriatum caudale (ANC), do show c-fos induction; however, the areas are not congruous with those demarcated by the 2-DG autoradiographic studies. Additional telencephalic areas include the olfactory lobe, the information storage site lobus parolfactorius (LPO), the memory site hippocampus, the auditory caudomedial neostriatum implicated in the strength of song learning, and the caudolateral neostriatum, which is comparable to the mammalian prefrontal cortex. In addition, c-fos is induced by first courtship and chasing in neurosecretory cell groups of the preoptic area and hypothalamus associated with the repertoire of sexual behavior and stress or enhanced arousal. Enhanced fos expression is also observed in brainstem sources of specific (noradrenergic, catecholaminergic) and nonspecific (reticular formation) activating pathways with inputs to higher brain areas implicated in the imprinting process. Birds reared in isolation or alternatively in the aviary with social and sexual contact to conspecifics showed attenuated or no fos expression in most of the above-mentioned areas. First courtship and chasing both lead to enhanced uptake of 2-DG in the four imprinting areas, as well as subsequent changes in spine density-an anatomical manifestation of the imprinting process. fos expression in the imprinting and other telencephalic, preoptic, hypothalamic, and mesencephalic brain regions indicates processing of stimuli originating from exposure (like chasing) and the analysis of stimuli in a behaviorally relevant, sexually explicit context (like first courtship). c-fos induction in these brain areas indicates its involvement in the triggering of neural changes that accompany the learning process of imprinting, leading eventually to alterations in dendritic spine density in the zebra finch.

Development of tyrosine hydroxylase-immunoreactive systems in the brain of the larval lamprey Lampetra fluviatilis

The development of the catecholaminergic system of the brain of the lamprey (Lampetra fluviatilis) was studied with immunocytochemistry in a series of larvae of different sizes by using two different antibodies directed against tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. In group 1 larvae (length: 29-54 mm, ages: 8 months to 1.5 years), the only TH-immunoreactive somata observed were located in the caudal wall of the recessus praeopticus (RP) and in the nucleus tuberculi posterioris (NTP). In group 2 larvae (length: 55-80 mm, ages: 1.5-2.5 years), the somata of immunolabeled cells of the NTP give rise to fibers, most of which are ascending and terminate in the corpus striatum. Additional immunoreactive cells are observed in the nucleus praeopticus (NP), which has differentiated, and in the spinal cord. In group 3 larvae (length: 81-110 mm, ages: 2.5-4 years), the spatial distribution of TH-immunoreactive elements (somata, fibers, and terminals) bears many resemblances to that seen in the adult. Immunolabeled cells may be observed in the olfactory bulb, in the nucleus commissurae postopticae (NCP), and in the nucleus dorsalis hypothalami (NDH). Nevertheless, some groups of TH-immunoreactive cells found in the adult are not observed in group 3 larvae; these may appear during the metamorphic phase. By comparative analysis, we show that, in spite of several differences, the spatiotemporal sequence of appearance of TH-immunoreactive cell bodies and fibers in the lamprey presents many similarities to that described in gnathostomes.

Spatial representation of the glomerular map in the Drosophila protocerebrum

In the fruit fly, Drosophila, olfactory sensory neurons expressing a given receptor project to spatially invariant loci in the antennal lobe to create a topographic map of receptor activation. We have asked how the map in the antennal lobe is represented in higher sensory centers in the brain. Random labeling of individual projection neurons using the FLP-out technique reveals that projection neurons that innervate the same glomerulus exhibit strikingly similar axonal topography, whereas neurons from different glomeruli display very different patterns of projection in the protocerebrum. These results demonstrate that a topographic map of olfactory information is retained in higher brain centers, but the character of the map differs from that of the antennal lobe, affording an opportunity for integration of olfactory sensory input.

The homeobox genes Lhx7 and Gbx1 are expressed in the basal forebrain cholinergic system

The specific combination of homeobox genes is proposed to be decisive in the terminal differentiation of neuronal systems. In order to identify combined expression of homeobox genes in the ventral forebrain, a reverse transcriptase-polymerase chain reaction strategy using degenerated primers was employed. We identified, amongst others, Lhx7 and Gbx1, displaying a marked overlapping expression in septal and pallidal areas. Gbx1 and Lhx7 were both expressed in those adult brain nuclei that collectively form the basal forebrain cholinergic system, a prime target of neurodegeneration in Alzheimer's disease. Indeed, we detected Lhx7 within cholinergic neurons, whereas the related Lhx6 gene was found in adjacent neurons. From these data we suggest that combined expression of Lhx7 and Gbx1 plays a role in the development of the cholinergic system of the basal forebrain. It is speculated that both genes remain participating in molecular processes in the adult cholinergic neurons, and can be employed to study regulation and survival of these neurons under normal and pathological conditions.

Dehydroepiandosterone and its sulphate enhance memory retention in day-old chicks

We report the presence of dehydroepiandosterone (DHEA) and DHEA sulphate (DHEA-S) in the day-old-chick brain, and their possible role in memory formation. DHEA and DHEA-S were present in the brain at higher concentrations than in the plasma. Radioimmunoassay examination of the intermediate medial hyperstriatum ventrale 5 or 30 min after training or the lobus parolfactorius 60 or 120 min after training on the passive avoidance task did not show learning-related differences in absolute levels of DHEA or DHEA-S. However, bilateral intracerebral injections of DHEA or DHEA-S before or after training on the weak passive avoidance task enhanced recall 24 h after training. Memory retention was enhanced by administration of DHEA and DHEA-S 15 min before training or 30 and 60 but not 180 min after training. Neurosteroids are present in high concentrations in regions of the chick brain known to be associated with learning and memory for an aversive one-trial task. Our study demonstrates that memory retention for this task is enhanced by administration of the neurosteroids DHEA-S and DHEA. These findings provide additional evidence that these neurosteroids have memory-enhancing properties and, thus, if common to other tasks and species, that DHEA-S and DHEA may constitute potential therapeutic tools for the treatment of cognitive deficits.

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

Neuroepithelial origin of the insular and endopiriform parts of the claustrum

The lateral and ventral pallia have been proposed as the source of neurons for the insular and endopiriform claustra, respectively. However, this correlation is controversial. Here, we analysed this relationship by labelling radial glia in coronal slices of the telencephalon of paraformaldehyde-fixed rabbit embryos (E18-E28) and newborn rabbits with an anti-vimentin antibody or with the fluorescent dye DiI. The radial glia that crossed the claustrum was anchored to the neuroepithelium of the lateral ventricular angle (LVA) at all ages studied. The LVA was deep at E18, but it subsequently become shallower, because of the apposition of the portion of its walls proximal to the vertex of the LVA. At E18, the radial glia that crossed most of the insular claustrum extended from the lateral wall of the LVA (presumptive lateral pallium), and the radial glia that crossed either the most ventral part of the insular claustrum or the endopiriform claustrum proceeded from the medial wall of the LVA (presumptive ventral pallium). These results suggest that although the endopiriform claustrum originates from the ventral pallium, the insular claustrum originates from both the lateral and the ventral pallial portions.

Loss of polysialic residues accelerates CNS neural precursor differentiation in pathological conditions

Using the model of lysolecithin-induced demyelination of the corpus callosum in wild-type, NCAM-deficient, and endoneuraminidase-injected mice, we have analyzed the consequences of the loss of expression of NCAM or PSA residues on the migration and proliferation capacities of neural precursors of the subventricular zone (SVZ). We showed that the absence of PSA or NCAM delayed migration of neural precursors to the olfactory bulb and consequently enhanced their recruitment at the lesion site. Moreover, after demyelination, the lack of NCAM but not PSA promoted proliferation in the SVZ and the lesion while the lack of PSA favored the differentiation of the traced cells into the oligodendroglial fate both in the SVZ and in the lesion. As previously demonstrated in vitro (L. Decker et al., 2000, Mol. Cell. Neurosci. 16, 422-439), these data illustrate the involvement of PSA and NCAM in neural precursor motility and differentiation in the normal and injured central nervous system, suggesting distinct roles for these two molecules under pathophysiological conditions.

Inversion of the anatomical lateralization of chick thalamofugal visual pathway by light experience

It has been reported that light exposure to one eye induces functional lateralization, which can be inverted by exposing the opposite eye to the light. However, the anatomical basis of the functional inversion by the light has not been shown. To address this issue, we labeled cells in the dorsolateral anterior thalamus (DLA) using retrograde fluorescent tracers injected into visual Wulst, counted the labeled cell number, and compared the anatomical asymmetry of DLA between the left eye occluded and the right eye occluded chickens. We found that a rostral part of DLA (DLAda) and a lateral/ventral part of DLA differentially projected to the visual cortex ipsilaterally and contralaterally, respectively. These regions showed anatomical asymmetry that was inverted by the light. An antibody against a nicotinic acetylcholine receptor subunit more intensively and widely stained the side of DLA receiving the light stimulation and the cell labeled by the tracers co-localized with the immunoreactive neuropil. These results indicated that the light experience induced the anatomical lateralization of thalamofugal visual pathway.

The teleostean forebrain: a comparative and developmental view based on early proliferation, Pax6 activity and catecholaminergic organization

An improved comparative interpretation of the teleostean forebrain suggests that the dorsal tier (Vd,Vc) and ventral tier (Vv,Vl) nuclei of the ventral telencephalic area (subpallium) represent the striatum and septum, respectively. Among other arguments, a dopaminergic innervation originating in the diencephalic posterior tubercle reaches Vd and dense efferents of Vv project to the midline hypothalamus in the adult zebrafish subpallium. The adult area dorsalis telencephali represents the teleostean pallium. Regulatory genes typically expressed in the early amniote subpallium (e.g., Dlx-1) are also restricted to the presumptive zebrafish ventral telencephalic area. Further, early Pax6 protein distribution in the zebrafish telencephalon corresponds to the migrating stream noted at the pallial-subpallial boundary in amniotes, but a ventricular, radial glia-based expression in the pallium is absent. The peripherally migrated, adult diencephalic preglomerular complex of the basal plate posterior tubercle (early: M2) provides sensory inputs to the pallium. Early Pax6 protein distribution indicates that at least part of M2 may directly originate from alar plate ventral thalamic Pax6-expressing cells. Dopaminergic cells of the basal plate posterior zebrafish forebrain (P1-P3) are restricted to the ventral thalamic prosomere (P3), including those forming the adult ascending dopaminergic system. Moreover, the latter likely depend developmentally on the dorsally adjacent alar plate Pax6-expressing cells.

Patch/matrix patterns of gray matter differentiation in the telencephalon of chicken and mouse

The mammalian striatum, a subpallial area, consists of two compartments (patches/striosomes and matrix) that differ in their neuronal birth dates, connectivity, neurochemistry, and molecular make-up. For example, members of the cadherin family of adhesion molecules (cadherin-8 and OL-protocadherin) are differentially expressed by the striosomes and the striatal matrix. A patch/matrix type of organization also has recently been found in the ventral hyperstriatum and the neostriatum of the chicken pallium, where cell clusters of similar birthdates ("isochronic" clusters) are surrounded by a matrix of cells that are born at a different time. Immunostaining with antibodies against cadherins reveals a similar arrangement of cell clusters. In the avian neostriatum, cadherin-7-positive cell clusters ("islands") are surrounded by a matrix of cells that express R-cadherin. The islands coincide, at least in part, with the isochronic cell clusters, as shown by pulse-labeling with bromodeoxyuridine. Likewise, isochronic clusters of the hyperstriatum ventrale relate to patchy heterogeneities in the cadherin-7 immunoreactivity pattern. Cadherins are known to mediate the aggregation and sorting of cells during development in many organs. Their differential expression by isochronic cell populations in the mammal subpallium and avian pallium suggests a common morphogenetic mechanism that regulates the formation of the patch/matrix patterns in these regions.

Single-cell RT-PCR detects shifts in mRNA expression profiles of basal forebrain neurons during aging

The medial septum and nucleus of the diagonal band (MS/nDB) contain cholinergic and GABAergic neuronal populations that have been identified based on immunohistochemical staining and/or electrophysiological properties. We explored the molecular diversity of MS/nDB neurons using single-cell reverse transcription-polymerase chain reaction (scRT-PCR) to assess gene expression profiles during aging in individual neurons acutely isolated from young (2-4 months) and aged (26-27 months) F344 rats. Neuronal gene expression profiles were characterized by detection of mRNAs for choline acetyltransferase (ChAT, cholinergic) and glutamate decarboxylase (GAD67, GABAergic), as well as mRNAs for calcium binding proteins (CaBPs) calbindin-D28k, calretinin and parvalbumin. Four major neuronal populations were identified: ChAT-positive (ChAT+) cells, GAD-positive (GAD+) cells, ChAT+/GAD+ cells and ChAT negative/GAD negative (ChAT-/GAD-) cells. With age, the percentage of cells expressing ChAT mRNA decreased from 53% in young to 40%, and the expression of GAD67 mRNA was reduced from 56 to 35% of the cells tested. The percentage of cells with detectable levels of both ChAT and GAD67 mRNA was reduced from 24% in young to 9% in aged. Concomitantly, the percentage of ChAT-/GAD- cells increased from 15 to 34% with age. Of the CaBPs, calretinin expression was observed most frequently in this study, and its detection decreased from 33 to 22% of the cells with age. Observations concerning the CaBPs were confirmed using in situ hybridization. These results suggest a shift in the mRNA expression profiles of MS/nDB neuronal populations during aging and exemplify the molecular diversity of cholinergic and GABAergic cells.

Spatiotemporal expression patterns of slit and robo genes in the rat brain

Diffusible chemorepellents play a major role in guiding developing axons toward their correct targets by preventing them from entering or steering them away from certain regions. Genetic studies in Drosophila revealed a repulsive guidance system that prevents inappropriate axons from crossing the central nervous system midline; this repulsive system is mediated by the secreted extracellular matrix protein Slit and its receptors Roundabout (Robo). Three distinct slit genes (slit1, slit2, and slit3) and three distinct robo genes (robo1, robo2, rig-1) have been cloned in mammals. However, to date, only Robo1 and Robo2 have been shown to be receptors for Slits. In rodents, Slits have been shown to function as chemorepellents for several classes of axons and migrating neurons. In addition, Slit can also stimulate the formation of axonal branches by some sensory axons. To identify Slit-responsive neurons and to help analyze Slit function, we have studied, by in situ hybridization, the expression pattern of slits and their receptors robo1 and robo2, in the rat central nervous system from embryonic stages to adult age. We found that their expression patterns are very dynamic: in most regions, slit and robo are expressed in a complementary pattern, and their expression is up-regulated postnatally. Our study confirms the potential role of these molecules in axonal pathfinding and neuronal migration. However, the persistence of robo and slit expression suggests that the couple slit/robo may also have an important function in the adult brain.

GABA, taurine and learning: release of amino acids from slices of chick brain following filial imprinting

The intermediate and medial hyperstriatum ventrale (IMHV) is a forebrain region in the domestic chick that is a site of information storage for the learning process of imprinting. We enquired whether imprinting is associated with learning-related increases in calcium-dependent, potassium-stimulated release of neurotransmitter amino acids from the IMHV. Chicks were hatched and reared in darkness until 15-30 h after hatching. They then either remained in darkness or were trained for 2 h by exposure to an imprinting stimulus. One hour later, the chicks were given a preference test and a preference score was calculated from the results of this test, as a measure of imprinting. Chicks were killed 2 h after training. Slices from the left and right IMHV of trained and untrained chicks were superfused with Krebs' solution either with or without calcium and the superfusate assayed for arginine, aspartate, citrulline, GABA, glutamate, glycine and taurine using high-performance liquid chromatography. For calcium-containing superfusates from the left IMHV, preference score was significantly correlated with potassium-stimulated release of (i) GABA (r=0.51, 23 d.f., P=0.008) and (ii) taurine (r=0.77, 23 d.f., P<0.0001). There was no significant difference between the mean values of trained and untrained chicks for either compound. However, examination of the variance of the data indicated that release of both GABA and taurine increased as a result of learning. No significant correlation between preference score and release was found for any of the amino acids from the right IMHV, nor for control tissue from the left IMHV superfused with calcium-free solution. These results demonstrate that the learning process of imprinting is associated with increases in releasable pools of GABA and taurine and/or membrane excitability in the left IMHV.

Ontogeny of angiotensin II type 1 receptor mRNAs in fetal and neonatal rat brain

Studies have demonstrated a specific function of the angiotensin II (Ang II) type 1 receptor (AT(1)) in regulation of adult central cardiovascular, fluid, and pituitary hormone release and a predominant role of the renin-angiotensin system in fetal and neonatal cardiovascular homeostasis. The pattern of brain AT(1) mRNA expression during fetal and neonatal development is currently unknown. We used radiolabeled cRNA probes for in situ hybridization histochemistry to determine the ontogenic development of the two AT(1) subtypes (AT(1a) and AT(1b)) mRNA in rat brain, from 11 days of gestation (E11) to 28 days after birth (P28). No AT(1b) mRNA was detected in the developing brain, whereas AT(1a) mRNA was first detected at E19. The age at which AT(1a) mRNA is first detected varied among different brain areas and expression predominates in areas involved in fluid homeostasis, pituitary hormone release, and cardiovascular regulation, where it persists until P28. AT(1a) mRNA expression is present from E19 onward in the median preoptic nucleus, the vascular organ of the lamina terminalis, the paraventricular nucleus, the periaqueductal gray, the nucleus raphe pallidus, the motor facial nucleus, and very weakly in the nucleus of the solitary tract and the ambiguous nucleus, and at E21 in the subfornical organ, the anterior olfactory nucleus and the piriform cortex. AT(1a) mRNA expression is present after birth in many regions, including the preoptic and lateral hypothalamic areas, the area postrema and medullary reticular nuclei. In conclusion, during brain development, expression of AT(1a) mRNA, appears in late gestation at E19, predominantly in forebrain areas involved in fluid homeostasis and cardiovascular regulation. In contrast, AT(1a) mRNA expression is absent or present only in very small amounts until after birth in many medullary nuclei, known to play an important role in cardiovascular modulation. Our results suggest that, in perinatal life, AT(1a) is involved in fluid and perhaps cardiovascular homeostasis and that the role of Ang II in modulating medullary cardiovascular centers matures later in postnatal life.