Ontogeny of cholinergic neurons in the mouse forebrain

The ameliorative effects of choline on ethanol-induced cell death in the neural tube of susceptible BXD strains of mice

Introduction

Fetal alcohol spectrum disorders (FASD) are the leading preventable cause of intellectual disability, providing the impetus for evaluating various potential treatments to ameliorate ethanol's teratogenic effects, particularly in the nervous system. One treatment is the dietary supplement choline which has been shown to mitigate at least some of ethanol's teratogenic effects. The present study was designed to investigate the effects of genetics on choline's efficacy in ameliorating cell death in the developing neural tube. Previously, we examined BXD recombinant inbred mice, and their parental C57BL/6 J (B6) and DBA/2 J strains, and identified strains that were sensitive to ethanol's teratogenic actions. Thus, we used these strains to identify response to choline treatment.

Materials and methods

Timed pregnant mice from 4 strains (B6, BXD51, BXD73, BXD2) were given either ethanol or isocaloric maltose-dextrin (5.8 g/kg in two administrations separated by 2 h) with choline at one of 3 doses: 0, 100 or 250 mg/kg. Subjects were exposed via intragastric gavage on embryonic day 9 and embryos were collected 7 h after the initial ethanol administrations. Cell death was analyzed using TUNEL staining in the developing forebrain and brainstem.

Results

Choline ameliorated the ethanol-induced cell death across all 4 strains without causing enhanced cell death in control mice. Choline was effective in both the developing telencephalon and in the brainstem. Both doses diminished cell death, with some differences across strains and brain regions, although the 100 mg/kg dose was most consistent in mitigating ethanol-related cell death. Comparisons across strains showed that there was an effect of strain, particularly in the forebrain at the higher dose.

Discussion

These results show that choline is effective in ameliorating ethanol-induced cell death at this early stage of nervous system development. However, there were some strain differences in its efficacy, especially at the high dose, providing further evidence of the importance of genetics in influencing the ability of choline to protect against prenatal alcohol exposure.

Fate mapping reveals mixed embryonic origin and unique developmental codes of mouse forebrain septal neurons

The septum is a key structure at the core of the forebrain that integrates inputs and relays information to other brain areas to support cognition and behaviours such as feeding and locomotion. Underlying these functions is a rich diversity of neuronal types and an intricate complexity of wiring across and within the septal region. We currently have very little understanding of how septal neuronal diversity emerges during development. Using transgenic mice expressing Cre in different subsets of telencephalic precursors we explored the origins of the three main neuronal types of the septal complex: GABAergic, cholinergic and glutamatergic neurons. We find that septal neurons originate from distinct neuroepithelial domains of the developing septum and are born at different embryonic time points. An exception to this is the GABAergic medial septal Parvalbumin-expressing population which is generated outside the septum from surrounding germinal zones. We identify the transcription factor BSX as being expressed in the developing glutamatergic neuron population. Embryonic elimination of BSX in the septum results in a reduction of septal glutamatergic cell numbers and a consequent deficit in locomotion. Further refinement of septal neuron diversity is needed to understand the multiple roles of septal neurons and their contribution to distinct behaviours.

Characterization of social behavior in young and middle-aged ChAT-IRES-Cre mouse

The cholinergic system is an important modulator of brain processes. It contributes to the regulation of several cognitive functions and emotional states, hence altering behaviors. Previous works showed that cholinergic (nicotinic) receptors of the prefrontal cortex are needed for adapted social behaviors. However, these data were obtained in mutant mice that also present alterations of several neurotransmitter systems, in addition to the cholinergic system. ChAT-IRES-Cre mice, that express the Cre recombinase specifically in cholinergic neurons, are useful tools to investigate the role of the cholinergic circuits in behavior. However, their own behavioral phenotype has not yet been fully characterized, in particular social behavior. In addition, the consequences of aging on the cholinergic system of ChAT-IRES-Cre mice has never been studied, despite the fact that aging is known to compromise the cholinergic system efficiency. The aim of the current study was thus to characterize the social phenotype of ChAT-IRES-Cre mice both at young (2–3 months) and middle (10–11 months) ages. Our results reveal an alteration of the cholinergic system, evidenced by a decrease of ChAT, CHT and VAChT gene expression in the striatum of the mice, that was accompanied by mild social disturbances and a tendency towards anxiety. Aging decreased social dominance, without being amplified by the cholinergic alterations. Altogether, this study shows that ChAT-IRES-Cre mice are useful models for studying the cholinergic system‘s role in social behavior using appropriate modulating technics (optogenetic or DREADD).

Embryonic and postnatal development of mouse olfactory tubercle

The olfactory tubercle (OT) is located in the ventral-medial region of the brain where it receives primary input from olfactory bulb (OB) projection neurons and processes olfactory behaviors related to motivation, hedonics of smell and sexual encounters. The OT is part of the dopamine reward system that shares characteristics with the striatum. Together with the nucleus accumbens, the OT has been referred to as the "ventral striatum". However, despite its functional importance little is known about the embryonic development of the OT and the phenotypic properties of the OT cells. Here, using thymidine analogs, we establish that mouse OT neurogenesis occurs predominantly between E11-E15 in a lateral-to-medial gradient. Then, using a piggyBac multicolor technique we characterized the migratory route of OT neuroblasts from their embryonic point of origin. Following neurogenesis in the ventral lateral ganglionic eminence (vLGE), neuroblasts destined for the OT followed a dorsal-ventral pathway we named "ventral migratory course" (VMC). Upon reaching the nascent OT, neurons established a prototypical laminar distribution that was determined, in part, by the progenitor cell of origin. A phenotypic analysis of OT neuroblasts using a single-color piggyBac technique, showed that OT shared the molecular specification of striatal neurons. In addition to primary afferent input from the OB, the OT also receives a robust dopaminergic input from ventral tegmentum (Ikemoto, 2007). We used tyrosine hydroxylase (TH) expression as a proxy for dopaminergic innervation and showed that TH onset occurs at E13 and progressively increased until postnatal stages following an 'inside-out' pattern. Postnatally, we established the myelination in the OT occurring between P7 and P14, as shown with CNPase staining, and we characterized the cellular phenotypes populating the OT by immunohistochemistry. Collectively, this work provides the first detailed analysis of the developmental and maturation processes occurring in mouse OT, and demonstrates the striatal nature of the OT as part of the ventral striatum (vST).

A cell autonomous torsinA requirement for cholinergic neuron survival and motor control

Cholinergic dysfunction is strongly implicated in dystonia pathophysiology. Previously (Pappas et al., 2015;4:e08352), we reported that Dlx5/6-Cre mediated forebrain deletion of the DYT1 dystonia protein torsinA (Dlx-CKO) causes abnormal twisting and selective degeneration of dorsal striatal cholinergic interneurons (ChI) (Pappas et al., 2015). A central question raised by that work is whether the ChI loss is cell autonomous or requires torsinA loss from neurons synaptically connected to ChIs. Here, we addressed this question by using ChAT-Cre mice to conditionally delete torsinA from cholinergic neurons ('ChAT-CKO'). ChAT-CKO mice phenocopy the Dlx-CKO phenotype of selective dorsal striatal ChI loss and identify an essential requirement for torsinA in brainstem and spinal cholinergic neurons. ChAT-CKO mice are tremulous, weak, and exhibit trunk twisting and postural abnormalities. These findings are the first to demonstrate a cell autonomous requirement for torsinA in specific populations of cholinergic neurons, strengthening the connection between torsinA, cholinergic dysfunction and dystonia pathophysiology.

Modulation of Neocortical Development by Early Neuronal Activity: Physiology and Pathophysiology

Animal and human studies revealed that patterned neuronal activity is an inherent feature of developing nervous systems. This review summarizes our current knowledge about the mechanisms generating early electrical activity patterns and their impact on structural and functional development of the cerebral cortex. All neocortical areas display distinct spontaneous and sensory-driven neuronal activity patterns already at early phases of development. At embryonic stages, intermittent spontaneous activity is synchronized within small neuronal networks, becoming more complex with further development. This transition is accompanied by a gradual shift from electrical to chemical synaptic transmission, with a particular role of non-synaptic tonic currents before the onset of phasic synaptic activity. In this review article we first describe functional impacts of classical neurotransmitters (GABA, glutamate) and modulatory systems (e.g., acetylcholine, ACh) on early neuronal activities in the neocortex with special emphasis on electrical synapses, nonsynaptic and synaptic currents. Early neuronal activity influences probably all developmental processes and is crucial for the proper formation of neuronal circuits. In the second part of our review, we illustrate how specific activity patterns might interfere with distinct neurodevelopmental processes like proliferation, migration, axonal and dendritic sprouting, synapse formation and neurotransmitter specification. Finally, we present evidence that transient alterations in neuronal activity during restricted perinatal periods can lead to persistent changes in functional connectivity and therefore might underlie the manifestation of neurological and neuropsychiatric diseases.

Stem cell derived basal forebrain cholinergic neurons from Alzheimer's disease patients are more susceptible to cell death

An early substantial loss of basal forebrain cholinergic neurons (BFCNs) is a constant feature of Alzheimer’s disease (AD) and is associated with deficits in spatial learning and memory. Induced pluripotent stem cells (iPSCs) derived from patients with AD as well as from normal controls could be efficiently differentiated into neurons with characteristics of BFCNs. We used BFCNs derived from iPSCs to model sporadic AD with a focus on patients with ApoE3/E4 genotypes (AD-E3/E4). BFCNs derived from AD-E3/E4 patients showed typical AD biochemical features evidenced by increased Aβ42/Aβ40 ratios. AD-E3/E4 neurons also exhibited altered responses to treatment with γ-secretase inhibitors compared to control BFCNs or neurons derived from patients with familial AD. BFCNs from patients with AD-E3/E4 also exhibited increased vulnerability to glutamate-mediated cell death which correlated with increased intracellular free calcium upon glutamate exposure. The ability to generate BFCNs with an AD phenotype is a significant step both for understanding disease mechanisms and for facilitating screening for agents that promote synaptic integrity and neuronal survival.

Frizzled3 controls axonal development in distinct populations of cranial and spinal motor neurons

Disruption of the Frizzled3 (Fz3) gene leads to defects in axonal growth in the VII^th and XII^th cranial motor nerves, the phrenic nerve, and the dorsal motor nerve in fore- and hindlimbs. In Fz3^−/− limbs, dorsal axons stall at a precise location in the nerve plexus, and, in contrast to the phenotypes of several other axon path-finding mutants, Fz3^−/− dorsal axons do not reroute to other trajectories. Affected motor neurons undergo cell death 2 days prior to the normal wave of developmental cell death that coincides with innervation of muscle targets, providing in vivo evidence for the idea that developing neurons with long-range axons are programmed to die unless their axons arrive at intermediate targets on schedule. These experiments implicate planar cell polarity (PCP) signaling in motor axon growth and they highlight the question of how PCP proteins, which form cell–cell complexes in epithelia, function in the dynamic context of axonal growth.

DOI:http://dx.doi.org/10.7554/eLife.01482.001

For the nervous system to become wired up correctly, neurons within the developing embryo must project over long distances to form connections with remote targets. They do this by lengthening their axons—the ‘cables’ along which electrical signals flow—and some axons in adult humans can grow to be more than 1 metre long.

This type of long-range pathfinding activity is particularly common for neurons that control movement, as many of these neurons must establish connections with muscles that are some distance away from the brain. For example, motor neurons in the brainstem form connections with muscles in the face to control facial expressions, while motor neurons in parts of the spinal cord project to muscles in the limbs. Multiple signaling pathways tell the developing axons which direction to grow en route to their final targets.

Now, Hua et al. have shown that an evolutionarily conserved protein called Frizzled3 is also involved in this process. In mouse embryos that lacked Frizzled3, the motor nerves that control breathing and limb movements were thinner than those in normal mice. In the mutant animals, many motor axons failed to form connections with their targets. Instead, these axons came to an abrupt halt midway along their intended paths and the neurons from which they originated died soon afterwards. These experiments support the idea that developing neurons are programmed to die unless their axons progress on the appropriate schedule.

As well as increasing our knowledge of the networks of connections that form within the developing mammalian nervous system, the work of Hua et al. provides new insights into some of the molecular mechanisms by which these connections are established.

DOI:http://dx.doi.org/10.7554/eLife.01482.002

Toxic effects of methylmercury, arsanilic acid and danofloxacin on the differentiation of mouse embryonic stem cells into neural cells

This study was performed to assess the neurotoxic effects of methylmercury, arsanilic acid and danofloxacin by quantification of neural-specific proteins in vitro. Quantitation of the protein markers during 14 days of differentiation indicated that the mouse ESCs were completely differentiated into neural cells by Day 8. The cells were treated with non-cytotoxic concentrations of three chemicals during differentiation. Low levels of exposure to methylmercury decreased the expression of GABAA-R and Nestin during the differentiating stage, and Nestin during the differentiated stage. In contrast, GFAP, Tuj1, and MAP2 expression was affected only by relatively high doses during both stages. Arsanilic acid affected the levels of GABAA-R and GFAP during the differentiated stage while the changes of Nestin and Tuj1 were greater during the differentiating stage. For the neural markers (except Nestin) expressed during both stages, danofloxacin affected protein levels at lower concentrations in the differentiated stage than the differentiating stage. Acetylcholinesterase activity was inhibited by relatively low concentrations of methylmercury and arsanilic acid during the differentiating stage while this activity was inhibited only by more than 40 μM of danofloxacin in the differentiated stage. Our results provide useful information about the different toxicities of chemicals and the impact on neural development.

Dynamic regulation of neurotransmitter specification: relevance to nervous system homeostasis

During nervous system development the neurotransmitter identity changes and coexpression of several neurotransmitters is a rather generalized feature of developing neurons. In the mature nervous system, different physiological and pathological circumstances recreate this phenomenon. The rules of neurotransmitter respecification are multiple. Among them, the goal of assuring balanced excitability appears as an important driving force for the modifications in neurotransmitter phenotype expression. The functional consequences of these dynamic revisions in neurotransmitter identity span a varied range, from fine-tuning the developing neural circuit to modifications in addictive and locomotor behaviors. Current challenges include determining the mechanisms underlying neurotransmitter phenotype respecification and how they intersect with genetic programs of neuronal specialization. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

The controlled generation of functional basal forebrain cholinergic neurons from human embryonic stem cells

An early substantial loss of basal forebrain cholinergic neurons (BFCN) is a constant feature of Alzheimer's disease and is associated with deficits in spatial learning and memory. The ability to selectively control the differentiation of human embryonic stem cells (hESCs) into BFCN would be a significant step toward a cell replacement therapy. We demonstrate here a method for the derivation of a predominantly pure population of BFCN from hESC cells using diffusible ligands present in the forebrain at developmentally relevant time periods. Overexpression of two relevant human transcription factors in hESC-derived neural progenitors also generates BFCN. These neurons express only those markers characteristic of BFCN, generate action potentials, and form functional cholinergic synapses in murine hippocampal slice cultures. siRNA-mediated knockdown of the transcription factors blocks BFCN generation by the diffusible ligands, clearly demonstrating the factors both necessary and sufficient for the controlled derivation of this neuronal population. The ability to selectively control the differentiation of hESCs into BFCN is a significant step both for understanding mechanisms regulating BFCN lineage commitment and for the development of both cell transplant-mediated therapeutic interventions for Alzheimer's disease and high-throughput screening for agents that promote BFCN survival.

Ethanol inhibits neuritogenesis induced by astrocyte muscarinic receptors

In utero alcohol exposure can lead to fetal alcohol spectrum disorders, characterized by cognitive and behavioral deficits. In vivo and in vitro studies have shown that ethanol alters neuronal development. We have recently shown that stimulation of M(3) muscarinic receptors in astrocytes increases the synthesis and release of fibronectin, laminin, and plasminogen activator inhibitor-1, causing neurite outgrowth in hippocampal neurons. As M(3) muscarinic receptor signaling in astroglial cells is strongly inhibited by ethanol, we hypothesized that ethanol may also inhibit neuritogenesis in hippocampal neurons induced by carbachol-stimulated astrocytes. In the present study, we report that the effect of carbachol-stimulated astrocytes on hippocampal neuron neurite outgrowth was inhibited in a concentration-dependent manner (25-100 mM) by ethanol. This effect was because of the inhibition of the release of fibronectin, laminin, and plasminogen activator inhibitor-1. Similar effects on neuritogenesis and on the release of astrocyte extracellular proteins were observed after the incubation of astrocytes with carbachol in the presence of 1-butanol, another short-chain alcohol, which like ethanol is a competitive substrate for phospholipase D, but not by tert-butanol, its analog that is not a substrate for this enzyme. This study identifies a potential novel mechanism involved in the developmental effects of ethanol mediated by the interaction of ethanol with cell signaling in astrocytes, leading to an impairment in neuron-astrocyte communication.

Developmental aspects of the cholinergic system

Beyond its importance in sustaining or modulating different aspects of the activity of the central nervous system (CNS), the cholinergic system plays important roles during development. In the current review, we focus on the developmental aspects associated with major components of the cholinergic system: Acetylcholine, choline acetyltransferase, vesicular acetylcholine transporter, high-affinity choline transporter, acetylcholinesterase, nicotinic and muscarinic receptors. We describe when and where each one of these components is first identified in the CNS and the changes in their levels that occur during the course of prenatal and postnatal development. We also describe how these components are relevant to many events that occur during the development of the CNS, including progenitor cells proliferation and differentiation, neurogenesis, gliogenesis, neuronal maturation and plasticity, axonal pathfinding, regulation of gene expression and cell survival. It will be noticed that evidence regarding the developmental aspects of the cholinergic system comes mostly from studies that used agonists, such as nicotine, and antagonists, such as hemicholinium-3. Studies using immunohistochemistry and genetically altered mice also provided valuable information.

Acetylcholine-induced neuronal differentiation: muscarinic receptor activation regulates EGR-1 and REST expression in neuroblastoma cells

Neurotransmitters are considered part of the signaling system active in nervous system development and we have previously reported that acetylcholine (ACh) is capable of enhancing neuronal differentiation in cultures of sensory neurons and N18TG2 neuroblastoma cells. To study the mechanism of ACh action, in this study, we demonstrate the ability of choline acetyltransferase-transfected N18TG2 clones (e.g. 2/4 clone) to release ACh. Analysis of muscarinic receptors showed the presence of M1-M4 subtypes and the activation of both IP(3) and cAMP signal transduction pathways. Muscarinic receptor activation increases early growth response factor-1 (EGR-1) levels and treatments with agonists, antagonists, and signal transduction enzyme inhibitors suggest a role for M3 subtype in EGR-1 induction. The role of EGR-1 in the enhancement of differentiation was investigated transfecting in N18TG2 cells a construct for EGR-1. EGR-1 clones show increased neurite extension and a decrease in Repressor Element-1 silencing transcription factor (REST) expression: both these features have also been observed for the 2/4 clone. Transfection of this latter with EGR zinc-finger domain, a dominant negative inhibitor of EGR-1 action, increases REST expression, and decreases fiber outgrowth. The data reported suggest that progression of the clone 2/4 in the developmental program is dependent on ACh release and the ensuing activation of muscarinic receptors, which in turn modulate the level of EGR-1 and REST transcription factors.

Modulation of neuritogenesis by astrocyte muscarinic receptors

Astrocytes have been shown to release factors that have promoting or inhibiting effects on neuronal development. However, mechanisms controlling the release of such factors from astrocytes are not well established. Astrocytes express muscarinic receptors whose activation stimulates a robust intracellular signaling, although the role of these receptors in glial cells is not well understood. Acetylcholine and acetylcholine receptors are present in the brain before synaptogenesis occurs and are believed to be involved in neuronal maturation. The present study was undertaken to investigate whether stimulation of muscarinic receptors in astrocytes would modulate neurite outgrowth in hippocampal neurons. Rat hippocampal neurons, co-cultured with rat cortical astrocytes previously exposed to the cholinergic agonist carbachol, displayed longer neurites. The effect of carbachol in astrocytes was due to the activation of M3 muscarinic receptors. Exposure of astrocytes to carbachol increased the expression of the extracellular matrix proteins fibronectin and laminin-1 in these cells. This effect was mediated in part by an increase in laminin-1 and fibronectin mRNA levels and in part by the up-regulation of the production and release of plasminogen activator inhibitor-1, an inhibitor of the proteolytic degradation of the extracellular matrix. The inhibition of fibronectin activity strongly reduced the effect of carbachol on the elongation of all the neurites, whereas inhibition of laminin-1 activity reduced the elongation of minor neurites only. Plasminogen activator inhibitor-1 also induced neurite elongation through a direct effect on neurons. Taken together, these results demonstrate that cholinergic muscarinic stimulation of astrocytes induces the release of permissive factors that accelerate neuronal development.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

The role of the interhemispheric pathway in hearing

The corpus callosum consists of heavily myelinated fibres connecting the two hemispheres. Its caudal portion and splenium contain fibres that originate from the primary and second auditory cortices, and from other auditory responsive areas. The anterior commissure in humans is much smaller than the corpus callosum, and it also contains interhemispheric fibres from auditory responsive cortical areas. The corpus callosum is exclusively present in placental mammals, while in acallosal mammals, most of the corpus callosum-related functions are carried out by the anterior commissure. The exact contribution of these two structures and of interhemispheric transfer in hearing in humans is still a matter of debate. In more recent years, human behavioural studies which employ psychoacoustic tasks designed to tap into interhemispheric transfer, combined with sophisticated neuroimaging paradigms, have helped to interpret information from animal experiments and post-mortem studies. This review will summarize and discuss the available information of the contributions of the human interhemispheric pathway in hearing in humans from behavioural, neuroimaging and histopathological studies in humans.

Involvement of the Olig2 transcription factor in cholinergic neuron development of the basal forebrain

Cholinergic neurons, which express choline acetyltransferase (ChAT), are a major neuron subset generated in the basal forebrain. Areas presumed to be sites of origin of cholinergic neurons are roughly demarcated by expression of Olig2, a basic helix-loop-helix transcription factor, which includes the medial ganglionic eminence, septal area, and anterior entopeduncular/preoptic area. In the present study, we examined the involvement of Olig2 in cholinergic differentiation. When the Olig2-expressing cells at E12.5 were permanently modified to express the lacZ or EGFP gene by tamoxifen-induced Cre-mediated recombination, the cells marked by reporter gene expression were widely distributed in the basal forebrain by E18.5, some of which expressed neuronal markers. We showed that a small number of cells were double-positive for ChAT and X-gal or EGFP in almost all cases. In addition, the number of ChAT+ cells was reduced to 60% in the Olig2 knockout mouse basal forebrain. No evidence of elevated apoptosis or reduced proliferation was observed in the knockout mouse forebrain. The present study provides the first direct evidence for involvement of the Olig2 gene in cholinergic differentiation in the basal forebrain.

Muscarinic acetylcholine receptors mediate oligodendrocyte progenitor survival through Src-like tyrosine kinases and PI3K/Akt pathways

The function of muscarinic acetylcholine receptors expressed in oligodendrocytes and in myelin has remained largely undetermined. Here we present evidence that incubation of oligodendrocyte progenitors, deprived of growth factor, with the acetylcholine analog carbachol significantly reduced cell death by apoptosis and blocked caspase-3 cleavage. This protective effect was reversed by atropine, a muscarinic acetylcholine receptor antagonist, as well as by specific inhibitors of intracellular signaling molecules, including phosphatidylinositol 3-kinase (Wortmannin and LY294002), Akt (Akt inhibitor III) and Src-like tyrosine kinases (PP2), but not by the mitogen-activated protein kinase kinase inhibitor, PD98059. Activation of Akt by carbachol was antagonized by atropine and inhibited by LY294002 and PP2. The Src-like tyrosine kinase inhibitor, PP2, also reduced carbachol stimulation of extracellular signal-regulated kinases 1/2 and cAMP-response element binding protein in a dose-dependent manner. Furthermore, carbachol increased tyrosine-phosphorylation of Fyn, a member of the Src-like tyrosine kinases. These results indicate that muscarinic acetylcholine receptors play an important role in oligodendrocyte progenitor survival through transduction pathways involving activation of Src-like tyrosine kinases and phosphatidylinositol 3-kinase/Akt.

Neurogenesis of the cholinergic medial septum in female and male C57BL/6J mice

Sex differences exist in the structure and function of the cholinergic septo-hippocampal system throughout the lifespan of mammals. How and when these sex differences originate is unclear. Because estrogen modulates sexual differentiation of several brain regions during development and influences neurogenesis in adult mammals, we hypothesized that sexual dimorphism of the cholinergic septo-hippocampal system would extend to its neurogenesis. A birthdating agent 5'-bromo-2'-deoxyuridine (BrdU) was injected into pregnant dams on one of eight gestational days, ranging from embryonic day (E)10 to E17. The offspring were euthanized at 2 months of age, and brains were processed for BrdU and choline acetyltransferase (ChAT) immunoreactivity to label cholinergic neurons that became postmitotic on a given embryonic day and survived to adulthood. Unbiased stereology was used to compare the number of double-labeled neurons in the medial septum (MS) of female and male offspring. Cholinergic neurons in the MS were generated primarily between E11 and E14, similar to other published reports. We found sex differences in the pattern of peak neurogenesis but not in the length of neurogenesis, or in total number of neurons generated in the MS. Additionally, in adult female and male mice, we estimated the total number of cholinergic neurons using unbiased stereology and found no sex differences in the number of cholinergic neurons or in the volume of the MS in adulthood. These results suggest that sex differences noted in the function of the postnatal cholinergic septo-hippocampal system may originate from its neurogenesis.

Olfactory structures in staged human embryos

The olfactory region was investigated in 303 serially sectioned human embryos, 23 of which were controlled by precise graphic reconstructions. The following findings in the embryonic period are new for the human. (1) The nasal plates arise at the neurosomatic junction, as do also the otic placodes. (2) Crest comes from the nasal plates later (stage 13) than the maximum production in the neural folds (stage 10). (3) The crest arises and migrates during a much longer time (at least until the end of the embryonic period) than the neural crest of the head, where origin and migration end at stage 12. (4) Olfactory nerve fibres enter the brain at stage 17, the vomeronasal fibres and those of the nervus terminalis at stages 17 and 18. (5) Fibre connections between the olfactory tubercle and the olfactory bulb, as well as those to the amygdaloid nuclei, forebrain septum, and hippocampus, develop during and after stage 17. (6) Mitral cells appear late in the embryonic period. (7) Localized, although incomplete, lamination of the olfactory bulb is detectable at the embryonic/fetal transition. (8) Tangential migratory streams of neurons, from stage 22 to the early fetal period, proceed from the subventricular zone of the olfactory bulb towards the future claustrum; they remain within the insular region but are separated from the cortical plate. (9) In future cebocephaly morphological indications may be visible as early as stage 13. The various findings are integrated by means of staging, and current information for the fetal period is tabulated from the literature.

Neural stem cells redefined: a FACS perspective

Using the generally accepted ontogenetic definition, neural stem cells (NSCs) are characterized as undifferentiated cells originating from the neuroectoderm that have the capacity both to perpetually self-renew without differentiating and to generate multiple types of lineage-restricted progenitors (LRP). LRPs can themselves undergo limited self-renewal, then ultimately differentiate into highly specialized cells that compose the nervous system. However, this physiologically delimited definition of NSCs has been increasingly blurred in the current state of the field, as the great majority of studies have retrospectively inferred the existence of NSCs based on their deferred functional capability rather than prospectively identifying the actual cells that created the outcome. Further complicating the matter is the use of a wide variety of neuroepithelial or neurosphere preparations as a source of putative NSCs, without due consideration that these preparations are themselves composed of heterogeneous populations of both NSCs and LRPs. This article focuses on recent attempts using FACS strategies to prospectively isolate NSCs from different types of LRPs as they appear in vivo and reveals the contrasting differences among these populations at molecular, phenotypic, and functional levels. Thus, the strategies presented here provide a framework for more precise studies of NSC and LRP cell biology in the future.

Transcription of the M1 muscarinic receptor gene in neurons and neuronal progenitors of the embryonic rat forebrain

Development of the nervous system is accompanied by expansion and differentiation of the neuronal progenitors within the embryonic neuroepithelium. Although the role of growth factors in this process is well documented, there is increasing evidence for a role of neurotransmitters. Acetylcholine is known to exert many actions on developing neural cells, but its potential role in neurogenesis is unclear. Here, we show that the M1 muscarinic acetylcholine receptor is expressed in the neuroepithelium of the rat forebrain, where it is found on both nestin+ progenitor cells and TuJ1+ newly differentiated neurons. Furthermore, transcription is governed, at least in part, by regulatory cis elements that are also responsible for driving transcription in neuroblastoma cells. This represents the first demonstration of M1 receptors on neuronal progenitor cells and supports the notion that M1 muscarinic receptors may play a role in development of the nervous system prior to the onset of synaptogenesis and their subsequent role in neurotransmission.

The ganglionic eminence--a putative intermediate target of amygdaloid connections

The superior part of the ganglionic eminence has been shown to act as an intermediate target for outgrowing axons of projections between the thalamus and the cerebral cortex. This study aims at investigating whether amygdaloid projections transiently contact the inferior portion of the human ganglionic eminence which directly borders upon the amygdala. Between 16 and 20 weeks of gestation a high number of small fiber bundles which were immunolabelled with anti-MAP1b and anti-SNAP-25 could be traced from the amygdala towards the mantle zone of the ganglionic eminence. These fiber bundles left a fiber system which coursed from the amygdala towards the entorhinal cortex. Within the mantle zone of the ganglionic eminence immunoreactive puncta indicative of fiber termination were observed. After 22 weeks of gestation the number of fibers entering the ganglionic eminence gradually decreased. These results provide the first evidence that the marginal zone of the inferior ganglionic eminence is likely to constitute an intermediate target for growing axons which belong the amygdaloid projection to the entorhinal cortex.

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

Muscarinic acetylcholine receptors induce neurite outgrowth and activate the synapsin I gene promoter in neuroblastoma clones

The possible role of acetylcholine as a modulator of neuronal differentiation has been tested using a neuroblastoma cell line (N18TG2), which does not synthesize any neurotransmitter. Acetylcholine synthesis has been activated in this line by transfection with a construct containing a choline acetyltransferase (ChAT) cDNA; ChAT-positive clones share a higher ability to grow fibers and an activation of synapsin I expression compared to the parental cells. Atropine, a muscarinic antagonist, abolishes the higher ability to grow fibers of ChAT-positive transfected clones, and the cholinergic agonist carbachol induces higher neurite outgrowth in the parental line. In transient transfections of ChAT-positive clones, the expression of a reporter gene under the control of synapsin I promoter is considerably reduced by atropine, while it is not modified by carbachol; in contrast, in the parental cells, which do not synthesize acetylcholine, the reporter gene expression is induced by carbachol and this effect is abolished by atropine. The data presented provide evidence for the existence of a direct modulation of fiber outgrowth and synapsin I expression by muscarinic receptor activation, which may be related to early growth response gene-1 (EGR-1) levels.

Calcium-dependent regulation of cholinergic cell phenotype in the hypothalamus in vitro

Glutamate is a major fast excitatory neurotransmitter in the CNS including the hypothalamus. Our previous experiments in hypothalamic neuronal cultures showed that a long-term decrease in glutamate excitation upregulates ACh excitatory transmission. Data suggested that in the absence of glutamate activity in the hypothalamus in vitro, ACh becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. Here, using neuronal cultures, fura-2 Ca(2+) digital imaging, and immunocytochemistry, we studied the mechanisms of regulation of cholinergic properties in hypothalamic neurons. No ACh-dependent activity and a low number (0.5%) of cholinergic neurons were detected in control hypothalamic cultures. A chronic (2 wk) inactivation of N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors, L-type voltage-gated Ca(2+) channels, calmodulin, Ca(2+)/calmodulin-dependent protein kinases II/IV (CaMK II/IV), or protein kinase C (PKC) increased the number of cholinergic neurons (to 15-24%) and induced ACh activity (in 40-60% of cells). Additionally, ACh activity and an increased number of cholinergic neurons were detected in hypothalamic cultures 2 wk after a short-term (30 min) pretreatment with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid tetrakis(acetoxy-methyl) ester (BAPTA AM; 2.5 microM), a membrane permeable Ca(2+)-chelating agent that blocks cytoplasmic Ca(2+) fluctuations. An increase in the number of cholinergic neurons following a chronic NMDA receptor blockade was likely due to the induction of cholinergic phenotypic properties in postmitotic noncholinergic neurons, as determined using 5-bromo-2'-deoxyuridine (BrdU) labeling. In contrast, a chronic inactivation of non-NMDA glutamate receptors or cGMP-dependent protein kinase had little effect on the expression of ACh properties. The data suggest that Ca(2+), at normal intracellular concentrations, tonically suppresses the development of cholinergic properties in hypothalamic neurons. However, a decrease in Ca(2+) influx into cells (through NMDA receptors or L-type Ca(2+) channels), inactivation of intracellular Ca(2+) fluctuations, or downregulation of Ca(2+)-dependent signal transduction pathways (CaMK II/IV and PKC) remove the tonic inhibition and trigger the development of cholinergic phenotype in some hypothalamic neurons. An increase in excitatory ACh transmission may represent a novel form of neuronal plasticity that regulates the activity and excitability of neurons during a decrease in glutamate excitation. This type of plasticity has apparent region-specific character and is not expressed in the cortex in vitro; neither increase in ACh activity nor change in the number of cholinergic neurons were detected in cortical cultures under all experimental conditions.

The superior olivary complex of the hamster has multiple periods of cholinergic neuron development

Cholinergic neurons of the superior olivary complex share a common embryological and phylogenetic origin with brainstem motor neurons and serve as the major descending efferent pathway either to the cochlea as part of the olivocochlear system or to the cochlear nucleus. In this study, we investigated the developmental expression patterns of choline acetyltransferase (ChAT) and its co-localization with calcitonin gene-related peptide within the superior olivary complex and neighboring brainstem motor nuclei. At embryonic day 12, neurons in the ventral nucleus of the trapezoid body were first to express ChAT. The temporal expression pattern of both ChAT mRNA and immunoreactivity in this periolivary region mimicked motor neurons in the facial and trigeminal motor nuclei. Just before birth, shell neurons surrounding the lateral superior olive expressed ChAT. Neither ChAT-positive periolivary neurons nor shell neurons co-expressed calcitonin gene-related peptide during development or in the adult. Immediately following birth, intrinsic neurons within the lateral superior olive expressed ChAT but not calcitonin gene-related peptide. However, a transient increase in the number of ChAT-positive neurons in the lateral superior olive coincided with the onset of the calcitonin gene-related peptide co-expression within these neurons. We conclude that ChAT expression appears first in periolivary regions containing medial olivocochlear neurons, precedes the expression of calcitonin gene-related peptide in the superior olivary complex, and is co-expressed with calcitonin gene-related peptide within the lateral superior olive containing lateral olivocochlear neurons. These data suggest that the lateral olivocochlear system co-expresses ChAT and calcitonin gene-related peptide, whereas the medial olivocochlear system does not.

Functional nicotinic acetylcholine receptor expression in stem and progenitor cells of the early embryonic mouse cerebral cortex

The adult cerebral cortex contains nicotinic acetylcholine (ACh) receptors vital to cortical function. However, little is known about the assembly of embryonic nicotinic receptor subunits into functional receptors or whether they play an active role in cortical development. We now report evidence of functional nicotinic acetylcholine receptor channels in fetal mouse cerebral cortex as early as embryonic day 10 (E10), when the cortex consists of dividing stem and progenitor cells. Patch-clamp electrophysiological measurements indicate that nicotine and ACh evoke sizable inward currents characteristic of nicotinic receptors, that are strongly rectifying with a reversal potential near 0 mV. Three different nicotinic agonists, ACh, nicotine, and dimethylphenylpiperazinium, evoked cytosolic Ca(2+) signals. Agonist-evoked Ca(2+) signals and electrophysiological responses were found in greater than 70% of all E10-E11 cells tested and were blocked by nicotinic receptor antagonists. The Ca(2+) response to nicotinic agonists was markedly prolonged in cells from early embryonic stages relative to later stages of development. alpha3, alpha4, and alpha7 receptor subunit proteins were detected immunocytochemically in cortical cells from E10 to birth. The incidence of each subunit declined with embryonic age, suggesting a role in early development. We discuss the possible function of nicotinic receptors in early cortical development and their role as a target for nicotine in the developmental pathologies associated with the fetal tobacco syndrome.

Postnatal development of cholinergic system in mouse basal forebrain: acetylcholinesterase histochemistry and choline-acetyltransferase immunoreactivity

The distribution of acetylcholinesterase histochemistry and choline-O-acetyltransferase immunohistochemistry in the basal forebrain was studied in newborn mice (P0) and until 60 days of postnatal life (P60). A weak acetylcholinesterase activity was found at P0 and P2 in the anterior and intermediate parts of the basal forebrain, and higher in the posterior region. The intensity of labeling, neuronal size and dendritic growth seems to increase progressively in all regions of basal forebrain from P4 to P10. The AChE+ cell count shows that in the anterior portion of the magnocellular basal nucleus the number of cells does not vary significantly from birth to the second month of postnatal life. However, in the intermediate and posterior portions of the nucleus the mean number of labeled cells increases significantly from birth to the end of the second week of postnatal life (P13). The choline-acetyltransferase immunoreactivity appears only detectable at the end of the first week (P6) as a slight immunoreaction, which increases progressively in intensity at P8, and at P10 seems to attain the same intensity of labeling found at P60. These results seem to indicate that the acetylcholinesterase could have a non-classic cholinergic role in the first stages of postnatal development, acting as a growth and cellular differentiation factor.

Sex steroids modulate luteinizing hormone-releasing hormone secretion in a cholinergic cell line from the basal forebrain

The function of a particular neuronal population is in part determined by its neurotransmitter phenotype. We have found that a neuronal-derived septal cell line (SN56), known for its cholinergic properties, also synthesizes and releases luteinizing hormone-releasing hormone. In addition, these cells express the messenger RNAs encoding estrogen and progesterone receptors. The activation of these receptors by their respective ligands cooperatively modulates the depolarization-induced release of luteinizing hormone-releasing hormone in these cells. We have also found that a number of septal neurons in postnatal (1-week-old) mice are immunoreactive to both choline acetyltransferase and luteinizing hormone-releasing hormone. These results indicate that both neurotransmitters, acetylcholine and luteinizing hormone-releasing hormone, may co-exist in septal neurons of the CNS and that they could be modulated by gonadal hormones, and suggest that luteinizing hormone-releasing hormone could be involved in some of the actions of sex steroids on cholinergic neurotransmission.

Acetylcholine becomes the major excitatory neurotransmitter in the hypothalamus in vitro in the absence of glutamate excitation

Glutamate and GABA are two major fast neurotransmitters (excitatory and inhibitory, respectively) in the CNS, including the hypothalamus. They play a key role in the control of excitation/inhibition balance and determine the activity and excitability of neurons in many neuronal circuits. Using neuronal cultures, whole-cell recording, Ca(2+) imaging, and Northern blots, we studied the compensatory regulation of neuronal activity during a prolonged decrease in glutamate excitation. We report here that after a chronic (6-17 d) blockade of ionotropic glutamate receptors, neurons in hypothalamic cultures revealed excitatory electrical and Ca(2+) synaptic activity, which was not elicited in the control cultures that were not subjected to glutamate blockade. This activity was suppressed with acetylcholine (ACh) receptor antagonists and was potentiated by eserine, an inhibitor of acetylcholinesterase, suggesting its cholinergic nature. The upregulation of ACh receptors and the contribution of ACh to the control of the excitation/inhibition balance in cultures after a prolonged decrease in glutamate activity were also demonstrated. Enhanced ACh transmission was also found in chronically blocked cerebellar but not cortical cultures, suggesting the region-specific character of glutamate-ACh interactions in the brain. We believe that in the absence of glutamate excitation in the hypothalamus in vitro, ACh, a neurotransmitter normally exhibiting only weak activity in the hypothalamus, becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. The increase in excitatory ACh transmission during a decrease in glutamate excitation may represent a novel form of neuronal plasticity that regulates activity and excitability of neurons during the glutamate/GABA imbalance.

Induction and maintenance of the neuronal cholinergic phenotype in the central nervous system by BMP-9

Bone morphogenetic proteins (BMPs) have multiple functions in the developing nervous system. A member of this family, BMP-9, was found to be highly expressed in the embryonic mouse septum and spinal cord, indicating a possible role in regulating the cholinergic phenotype. In cultured neurons, BMP-9 directly induced the expression of the cholinergic gene locus encoding choline acetyltransferase and the vesicular acetylcholine transporter and up-regulated acetylcholine synthesis. The effect was reversed upon withdrawal of BMP-9. Intracerebroventricular injection of BMP-9 increased acetylcholine levels in vivo. Although certain other BMPs also up-regulated the cholinergic phenotype in vitro, they were less effective than BMP-9. These data indicate that BMP-9 is a differentiating factor for cholinergic central nervous system neurons.

Acetylcholine stimulates cortical precursor cell proliferation in vitro via muscarinic receptor activation and MAP kinase phosphorylation

Increasing evidence has shown that some neurotransmitters act as growth-regulatory signals during brain development. Here we report a role for the classical neurotransmitter acetylcholine (ACh) to stimulate proliferation of neural stem cells and stem cell-derived progenitor cells during neural cell lineage progression in vitro. Neuroepithelial cells in the ventricular zone of the embryonic rat cortex were found to express the m2 subtype of the muscarinic receptor. Neural precursor cells dissociated from the embryonic rat cortical neuroepithelium were expanded in culture with basic fibroblast growth factor (bFGF). reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of m2, m3 and m4 muscarinic receptor subtype transcripts, while immunocytochemistry demonstrated m2 protein. ACh and carbachol induced an increase in cytosolic Ca2+ and membrane currents in proliferating (BrdU+) cells, both of which were abolished by atropine. Exposure of bFGF-deprived precursor cells to muscarinic agonists not only increased both cell number and DNA synthesis, but also enhanced differentiation of neurons. These effects were blocked by atropine, indicating the involvement of muscarinic ACh receptors. The growth-stimulating effects were also antagonized by a panel of inhibitors of second messengers, including 1,2-bis-(O-aminophenoxy)-ethane-N,N,N', N'-tetraacetic acid (BAPTA-AM) to chelate cytosolic Ca2+, EGTA to complex extracellular Ca2+, pertussis toxin, which uncouples certain G-proteins, the protein kinase C inhibitor H7 and the mitogen-activated protein kinase (MAPK) inhibitor PD98059. Muscarinic agonists activated MAPK, which was significantly inhibited by atropine and the same panel of inhibitors. Thus, muscarinic receptors expressed by neural precursors transduce a growth-regulatory signal during neurogenesis via pathways involving pertussis toxin-sensitive G-proteins, Ca2+ signalling, protein kinase C activation, MAPK phosphorylation and DNA synthesis.

Postnatal development of the basal forebrain cholinergic projections to the medial prefrontal cortex in mice

The postnatal development of basal forebrain cholinergic projections to the medial prefrontal cortex in mice was analyzed by means of the double labeling track-tracing study. The tracer was injected into the medial prefrontal cortex of mice, on the day of birth (P0) to 60 days after birth. The total number of basal forebrain neurons increased from P4 to P8, and began to decrease until P13 (52.9% vs. the maximal average (P8)). After P13, the mean average remains stable up to P60. On the other hand, differential pattern of frontocortical projections of the anterior, intermediate, and posterior regions can be observed.

Striatal transplants prevent AF64A-induced retention deficits

The relevance of the cholinergic system in mnemonic processes has been repeatedly demonstrated. In addition to the cholinergic systems that project to the telencephalon, there are subcortical nuclei with intrinsic cholinergic cells which appear to be involved in memory consolidation; among these is the striatum. Intrastriatal administration of anticholinergic drugs, as well as excitotoxic and electrolytic lesions have been shown to disrupt the acquisition and retention of instrumentally conditioned behaviors. In the present study male Wistar rats were used to confirm the reported detrimental effects of striatal lesions produced by the cholinotoxin AF64A on long-term retention (LTR) of inhibitory avoidance and spontaneous locomotor activity, to determine its effects on short-term retention (STR) and to investigate whether intrastriatal homotopic transplants can reverse the AF64A-induced behavioral deficits. AF64A-striatal lesions did not interfere with STR but disrupted LTR of the inhibitory avoidance task, and striatal transplants prevented this deficit. Spontaneous locomotor activity increased after the lesion but promptly returned to baseline levels. These results support previous findings showing striatal involvement in long-term but not short-term retention and indicate that homotopic transplants induce behavioral recovery of a learning task in striatal lesioned rats.

Effects of elevated levels of nerve growth factor on the septohippocampal system in transgenic mice

Elevating target-derived levels of nerve growth factor (NGF) in peripheral organs of postnatal mammals is known to enhance the survival of postganglionic sympathetic neurons and to promote the terminal arborization of sympathetic axons within such NGF-rich target tissues. Although increasing levels of NGF in the central nervous system can ameliorate cholinergic function of damaged and aged neurons of the medial septum, it remains undetermined whether the postnatal development of this neuronal population and their projections that innervate the hippocampus are likewise affected by elevated levels of target-derived NGF. To address this question, the cholinergic septohippocampal pathway was examined in adult transgenic mice which display elevated levels of NGF protein production in the dorsal hippocampus during postnatal development. Adult transgenic mice possessed a cholinergic population of septal neurons approximately 15% larger than that seen in age-matched control animals. Despite increased numbers of cholinergic septal neurons, as well as elevated levels of hippocampal NGF, the density of cholinergic septal axons in the outer molecular layer of the hippocampal dentate gyrus of adult transgenic animals was comparable with that found in wild-type controls. These results reveal that elevating levels of target-derived NGF during postnatal development can increase the population size of the cholinergic septal neurons but does not alter their pattern of afferent innervation in the hippocampus of adult mice.

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Influence of chronic prenatal ethanol on cholinergic neurons of the septohippocampal system

This study characterized the influence of full-term gestational ethanol exposure on choline acetyltransferase (ChAT)-immunoreactive neurons that project to the hippocampus, within the medial septal (MS) nucleus and the vertical limb of the diagonal band of Broca (DBv). On gestation days 1-22, pregnant dams were fed either a vitamin fortified ethanol-containing liquid diet, pair fed a calorically equivalent sucrose-containing diet, or given rat chow ad libitum. In a previous study, we found that chronic prenatal exposure to ethanol, in this manner, resulted in a significant decline in the ontogenetic upregulation of ChAT activity in the septal area during the second postnatal week, but was followed by recovery to control levels by adulthood. On postnatal days 14 and 60 (P14 and P60) the brains were prepared for ChAT immunocytochemistry. Ethanol exposure had little influence on the number of ChAT-positive neurons in the MS nucleus of animals at either age. Ethanol exposure had no effect on neuronal size or ChAT staining intensity of MS or DBv neurons when compared to chow-fed offspring. Although age-related increases in cholinergic neuronal numbers and decreases in neuronal size were observed between juvenile and adult animals, prenatal ethanol exposure did not appear to influence these postnatal changes in the population as a whole. Overall, these findings suggest that the anatomical maturation of septal cholinergic neurons may be relatively insensitive to prenatal ethanol exposure under conditions of a vitamin-rich dietary supplementation, while biochemical development within this region may be more susceptible to early ethanol influences.

Neuronal overmaturation in dysraphism: ontogenic expression of neuropeptides in the fetal brain and developmental anomalies in exencephaly

Starting from knowledge obtained in our previous studies of experimental dysraphism in chick embryos, the entity of neuronal overgrowth observed in exencephaly was further investigated. The ontogenic expression of neuron-specific enolase (NSE), vasoactive intestinal polypeptide (VIP), and somatostatin was analyzed both in chick exencephaly of the natural product and in normal chick fetuses by carrying out immunohistochemical studies. In normal fetuses, immunostained elements positive for NSE first appeared in the spinal gray matter on postincubation day 16 and increased in intensity during the fetal period. By postnatal day 2, the cerebral peduncle, brain stem neurofibers, molecular layer of the cerebellum, corpus striatum, and piriform cortex became immunoreactive. No immunohistochemical reaction to VIP was observed during these stages. Somatostatin-positive elements were not identified during the fetal period, except in limited regions, such as the corpus striatum, which appeared to have weakly positive staining on day 21. The exencephalic fetuses, however, demonstrated extremely advanced neuronal maturation, with intense immunoreactivity already being manifest in various regions, including the corpus striatum, piriform cortex, spinal gray matter, and brain stem nuclei, on day 16 of the fetal period. Somatostatin-positive elements also appeared at this stage in chick exencephaly, but such immunoreactivity was localized, particularly in the overgrown foci. The present study showed that the neuronal maturation process in some neurons of exencephalic brain and spinal cord was definitely further advanced than that in normal controls. A possible clinical application of NSE and somatostatin measurement as markers for dysraphic states in the fetus is suggested.

Synaptic and non-synaptic cholinergic innervation of the various types of neurons in the main olfactory bulb of adult rat: immunocytochemistry of choline acetyltransferase

The cholinergic neuronal structures and their synaptic connections in the main olfactory bulb of adult rats were analysed by using choline acetyltransferase immunocytochemistry. Within the glomeruli, cholinergic nerve fibers were restricted to strands which subdivided the neuropil into small compartments, the interior of which contained sensory axons but was devoid of cholinergic axons. Small numbers of choline acetyltransferase neurons were detected in all layers. Ultrastructural analysis revealed selective triadic synaptic relationships with different neuron classes in the intraglomerular area and in the external plexiform layer. These triads were made up of (i) a cholinergic axon, (ii) one or several periglomerular or granule cell dendrites, and (iii) usually one relay cell dendrite. In these triads, asymmetric cholinergic synapses were selectively focused on dendrites (gemmules and spines) of periglomerular or granule cells. Within the glomerulus, mitral and tufted cell dendrites were closely apposed to some cholinergic axon varicosities, most abundantly near arborizations of the apical dendrites. However, cholinergic synapses were never seen on any relay cell dendrite. In the external plexiform layer, cholinergic synapses were present on all parts of the superficial short-axon cells. In the internal plexiform layer and the granule cell layer, cholinergic axon varicosities exhibited close apposition or asymmetric synapses with granule cell gemmules. The data suggest that cholinergic projections from the basal forebrain to the main olfactory bulb focus synaptic innervation on interneurons. On relay cells, direct acetylcholine effects may occur, but these must be based on non-synaptic acetylcholine release at the surface of their dendrites.

Postnatal maturation of GABA-immunoreactive neurons of rat medial prefrontal cortex

A light microscopic immunocytochemical approach has been used to examine the distribution and maturation of gamma-aminobutyric acid- (GABA) containing cells in rat medial prefrontal cortex (mPFC) at progressive postnatal stages. Between P1 and P5, labeled cells in the cortical plate show less differentiated morphological characteristics when compared to cells in the deeper laminae. By P10, however, most labeled cells in superficial laminae show more differentiated characteristics with some having a distinctive multipolar appearance. Between P1 and P5, there is a significant increase (50%) in the density of GABA-containing cells in the superficial laminae, while concurrently there is an overall decreases in the subjacent deeper laminae. As the cortex continues to expand, there is a corresponding decrease in the density of GABA-immunoreactive cells in the outer two-thirds of the cortical mantle until approximately P15, stabilizing at 20-25 cells/100,000 microns2 for all laminae. Between P1 and P15, there is also a significant increase (133%) in the average size of labeled cells, followed by a gradual decrease of 30% between P15 and P41. During P1-7, there is a marked increase in the density of labeled axosomatic terminals in both the superficial (200%) and deep laminae (116%). In the superficial layers, however, the density of labeled terminals again increases by 86% between P12 and P18. In general, the present findings are consistent with the idea that there is a progressive maturation of the intrinsic GABAergic system in rat mPFC in a classic "inside-out" pattern, and this involves extensive postnatal changes occurring during the first 3 postnatal weeks.

Septal cell lines derived from the trisomy 16 mouse: generation, characterization, and response to NGF

The trisomy 16 mouse is a genetic model of Down syndrome. Clonal cell lines were developed from trisomic as well as euploid embryonic mouse septal cells by introduction of thermolabile large T antigen mutant of SV 40. The cell lines underwent morphological differentiation at the non-permissive temperature and in response to a differentiating agent. Immunocytochemical staining indicated that cells of neuronal lineage were immortalized. The addition of beta-nerve growth factor (100 ng/ml) increased the survival rate of a trisomy cell line in differentiated state, as measured by Trypan blue exclusion. These cell lines may prove useful in studies of neuronal abnormalities in this mouse model of Down syndrome.