Development of cholinergic markers in mouse forebrain. II. Muscarinic receptor binding in cortex

Role of Brain Modulators in Neurodevelopment: Focus on Autism Spectrum Disorder and Associated Comorbidities

Autism spectrum disorder (ASD) and associated neurodevelopmental disorders share similar pathogenesis and clinical features. Pathophysiological changes in these diseases are rooted in early neuronal stem cells in the uterus. Several genetic and environmental factors potentially perturb neurogenesis and synaptogenesis processes causing incomplete or altered maturation of the brain that precedes the symptomology later in life. In this review, the impact of several endogenous neuromodulators and pharmacological agents on the foetus during pregnancy, manifested on numerous aspects of neurodevelopment is discussed. Within this context, some possible insults that may alter these modulators and therefore alter their role in neurodevelopment are high-lighted. Sometimes, a particular insult could influence several neuromodulator systems as is supported by recent research in the field of ASD and associated disorders. Dopaminergic hy-pothesis prevailed on the table for discussion of the pathogenesis of schizophrenia (SCH), atten-tion-deficit hyperactivity disorder (ADHD) and ASD for a long time. However, recent cumulative evidence suggests otherwise. Indeed, the neuromodulators that are dysregulated in ASD and comorbid disorders are as diverse as the causes and symptoms of this disease. Additionally, these neuromodulators have roles in brain development, further complicating their involvement in comorbidity. This review will survey the current understanding of the neuromodulating systems to serve the pharmacological field during pregnancy and to minimize drug-related insults in pa-tients with ASD and associated comorbidity disorders, e.g., SCH or ADHD.

Dual recombinase fate mapping reveals a transient cholinergic phenotype in multiple populations of developing glutamatergic neurons

Cholinergic transmission shapes the maturation of glutamatergic circuits, yet the developmental sources of acetylcholine have not been systematically explored. Here, we have used Cre-recombinase-mediated genetic labeling to identify and map both mature and developing CNS neurons that express choline acetyltransferase (ChAT). Correction of a significant problem with a widely used ChatCre transgenic line ensures that this map does not contain expression artifacts. ChatCre marks all known cholinergic systems in the adult brain, but also identifies several brain areas not usually regarded as cholinergic, including specific thalamic and hypothalamic neurons, the subiculum, the lateral parabrachial nucleus, the cuneate/gracilis nuclei, and the pontocerebellar system. This ChatCre fate map suggests transient developmental expression of a cholinergic phenotype in areas important for cognition, motor control, and respiration. We therefore examined expression of ChAT and the vesicular acetylcholine transporter in the embryonic and early postnatal brain to determine the developmental timing of this transient cholinergic phenotype, and found that it mirrored the establishment of relevant glutamatergic projection pathways. We then used an intersectional genetic strategy combining ChatCre with Vglut2Flp to show that these neurons adopt a glutamatergic fate in the adult brain. The transient cholinergic phenotype of these glutamatergic neurons suggests a homosynaptic source of acetylcholine for the maturation of developing glutamatergic synapses. These findings thus define critical windows during which specific glutamatergic circuits may be vulnerable to disruption by nicotine in utero, and suggest new mechanisms for pediatric disorders associated with maternal smoking, such as sudden infant death syndrome.

Postnatal development of the cholinergic innervation in the dorsal hippocampus of rat: Quantitative light and electron microscopic immunocytochemical study

Choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution and ultrastructural features of the acetylcholine (ACh) innervation in the dorsal hippocampus of postnatal rat. The length of ChAT-immunostained axons was measured and the number of ChAT-immunostained varicosities counted, in each layer of CA1, CA3, and dentate gyrus, at postnatal ages P8, P16, and P32. At P8, an elaborate network of varicose ChAT-immunostained axons was already visible. At P16, the laminar distribution of this network resembled that in the adult, but adult densities were reached only by P32. Between P8 and P32, the mean densities for the three regions increased from 8.4 to 14 meters of axons and 2.3 to 5.7 million varicosities per cubic millimeter of tissue. At the three postnatal ages, the ultrastructural features of ChAT-immunostained axon varicosities from the strata pyramidale and radiatum of CA1 were similar between layers and comparable to those in adult, except for an increasing frequency of mitochondria (up to 41% at P32). The proportion of these profiles displaying a synaptic junction was equally low at all ages, indicating an average synaptic incidence of 7% for whole varicosities, as previously found in adult. The observed junctions were small, usually symmetrical, and made mostly with dendritic branches. These results demonstrate the precocious and rapid maturation of the hippocampal cholinergic innervation and reveal its largely asynaptic nature as soon as it is formed. They emphasize the remarkable growth capacities of individual ACh neurons and substantiate a role for diffuse transmission by ACh during hippocampal development.

Diisopropylfluorophosphate administration in the pre-weanling period induces long-term changes in anxiety behavior and passive avoidance in adult mice

The developing brain may be particularly vulnerable to exposure to acetylcholinesterase (AChE) inhibitors because of the role of AChE on neuronal development and the effects of cholinergic pathways in mediating behavioral and hormonal responses to stress. C57BL/65 mice of both sexes were injected with 1 mg/kg s.c. diisopropylfluorophosphate (DFP) or saline in three separate experiments, on postnatal days (PNDs) 4-10, 14-20, or 30-36. Anxiety and conditioned avoidance were assessed on the elevated-plus maze (EPM) and step-down passive avoidance (PA) paradigms, respectively, at age 4-5 months. In addition, locomotion and reactivity to pain on the hot plate were assessed. Mice treated on PNDs 4-10 or PNDs 14-20 spent relatively more time and made more entries to the open arms on the first, but not second, exposure to the EPM. Females, but not males, treated with DFP showed deficits in PA retention after 24 h when treated on PNDs 4-10 and on PNDs 14-20. Mice treated on PNDs 30-36 were not impaired in either behavior. Administration of DFP in the preweanling period did not affect locomotor activity or pain reactivity. The results suggest that preweanling exposure to DFP results in anxiolysis in novel conflict situations but exacerbated context-enhanced anxiety.

Ultrastructural features of the acetylcholine innervation in the developing parietal cortex of rat

To follow on a recent quantitative study of the developing cholinergic (ACh) innervation in rat neocortex, axon varicosities identified by electron microscopic immunocytochemistry with a highly sensitive antibody against choline acetyltransferase (ChAT) were examined in the primary somatosensory area (Par1) of rats at postnatal ages (days) P8, P16, and P32. As visualized and measured in single thin sections, and compared with those of unlabeled varicosities selected at random in the same photomicrographs, the ChAT-immunostained profiles displayed intrinsic and relational features very similar to those previously described in the same cortical area of adult rat (Umbriaco et al. [1994] J. Comp. Neurol. 348:351-373). At the three postnatal ages, the immunoreactive profiles were comparable in shape, size, and vesicular content in all cortical layers, but showed an increasing frequency of mitochondria with age, reaching 44% at P32. Synaptic junctions were observed on 6.3 to 8.7% of these sectional profiles, indicating an average synaptic incidence of 17% for whole varicosities, again comparable to that in the adult (14%). As in adult, the junctions made by the rare synaptic ChAT-immunostained varicosities were always single, usually symmetrical, and more frequently found on dendritic branches than spines. Thus, cortical ACh varicosities displayed intrinsic and relational features similar to adult ones as soon as this innervation was installed, suggesting that a diffuse mode of transmission and ambient level of ACh could play a major role in the diverse effects of this neuromodulator during cortical development.

Molecular ontogeny of major neurotransmitter receptor systems in the mammalian central nervous system: norepinephrine, dopamine, serotonin, acetylcholine, and glycine

Neurotransmitter receptors are critical elements in intercellular signaling within the central nervous system and are divided into two major types based on their molecular structure and biophysical properties. The first are ionotropic receptors--ligand-gated ion channels that directly affect the membrane potential via passage of permeant ions (such as sodium and calcium) and mediate fast synaptic transmission. The second type are slower metabotropic receptors that are also ligand gated but depend on an interaction with guanine nucleotide-binding proteins and mediate signal transduction by activating second-messenger systems within the cell. In the past two decades, a wealth of information has emerged regarding the molecular biology and pharmacology of classic neurotransmitter receptors (including adrenergic, dopaminergic, serotonergic, cholinergic, glycine, gamma-aminobutyric acid [GABA(A)], and glutamate receptors). Further, the distribution of subunits comprising these receptors has been extensively studied. This review focuses on the molecular ontogeny of several of the major neurotransmitter receptor systems in the mammalian central nervous system, highlighting the role that some of these may play during brain development and in certain pathologic states.

Increased density of M1 receptors in the hippocampus of juvenile rats chronically deprived of NGF

Binding studies were used to assess the changes in affinity and/or number of M1 muscarinic receptors in hippocampi from juvenile rats chronically deprived of NGF. NGF deprivation was obtained by implanting into right ventricle at postnatal day 2 (P2) hybrydoma cells secreting high levels of monoclonal antibodies against NGF (alphaD11). Parenteral myeloma cells (P3U) were used as controls. Competition experiments were used to characterise the [3H]-PNZ binding sites in membrane preparations of hippocampi from rats sacrificed at P15. [3H]-PNZ bound M1 receptors both in P3U and alphaD11 group as shown by displacing potency order of antagonists: TLZ=4-DAMP>PNZ>p-F-HHSiD>MTC. The deprivation of NGF for two weeks significantly increased the number of M1 receptors without changing the Ki values of antagonists with exception of methoctramine which showed an increase in affinity in alphaD11 group. Similar changes in binding parameters were already observed after the first week of anti-NGF treatment. In contrast, a treatment for a week with implant at postnatal day 15 failed to produce any changes in M1 binding parameters. These results provide further physiological evidence for developmentally regulated modulatory role of NGF in the cholinergic function in the hippocampus.

Developmental profiles of various cholinergic markers in the rat main olfactory bulb using quantitative autoradiography

The existence of possible relationships among the developmental profile of various cholinergic markers in the main olfactory bulb (OB) was assessed by using in vitro quantitative autoradiography. Muscarinic receptors were visualized with [3H]pirenzepine (muscarinic M1-like sites) and [3H]AF-DX 384 (muscarinic M2-like sites); nicotinic receptors by using [3H]cytisine (nicotinic 42-like subtype) and [125I] alpha-bungarotoxin (nicotinic 7-like subtype); cholinergic nerve terminals by using [3H]vesamicol (vesicular acetylcholine transport sites) and [3H]hemicholinium-3 (high-affinity choline uptake sites). These various cholinergic markers exhibited their lowest levels at birth and reached adult values by the end of the 4-5 postnatal weeks. However, the density of presynaptic cholinergic markers and nicotinic receptors at postnatal day 2 represented a large proportion of the levels observed in adulthood, and displays a transient overexpression around postnatal day 20. In contrast, the postnatal development of cholinergic muscarinic M1-like and M2-like receptors is apparently regulated independently of the presynaptic cholinergic markers and nicotinic receptors. Two neurochemically and anatomically separate olfactory glomeruli subsets were observed in the posterior OB of the developing rat. These atypical glomeruli expressed large amounts of [3H]vesamicol-and [3H]hemicholinium binding sites without significant amounts of muscarinic M1, M2, or nicotinic alpha 4 beta 2 receptor binding sites. A significant density of [125I] alpha-bungarotoxin binding sites could be detected only at early postnatal ages. A few olfactory glomeruli specifically restricted to the dorsal posterior OB expressed a high density of [3H]cytisine binding sites but lacked significant binding of the two presynaptic cholinergic markers used here, suggesting their noncholinergic but cholinoceptive nature.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Development of muscarinic receptor subtypes in the forebrain of the mouse

Cholinergic mechanisms are involved in the regulation of developmental events in the nervous system. Muscarinic cholinergic receptors are thought to be the predominant mediator of cholinergic neurotransmission in the forebrain; however, their developmental role is less well understood. The present study takes advantage of subtype-specific antibodies to muscarinic receptor proteins to investigate the cellular localization of the subtypes in developing mouse forebrain. Receptor protein expression was assessed between postnatal day (PND) 5 and adulthood by immunocytochemical methods with antibodies to m1, m2, and m4 receptors, the most abundant subtypes in rodent brain. We have found dramatic developmental changes in the distribution of all three receptors. In the adult mouse, m1 and m2 receptor immunoreactivity displayed complementary staining patterns in most forebrain areas with m4 sharing similarities in pattern with both m1 and m2. Furthermore, each receptor was expressed transiently in gray matter areas or fiber bundles at various developmental stages. The m4 receptor was also expressed in developing blood vessels. Such transient immunoreactivity was usually associated with times and areas of dynamic morphogenesis, thus suggesting distinct roles for the receptor subtypes in ontogenetic events.

Immunocytochemical demonstration of developmental distribution of muscarinic acetylcholine receptors in rat parietal cortex

The present investigation reveals many cortical neurons immunopositive for M35, the monoclonal antibody raised against purified muscarinic acetylcholine receptor (mAChR) proteins, in the early postnatal rat brain. The ontogeny of mAChR expression, exemplified on the parietal neocortex, was studied in a series of rat pups from postnatal days (PD) 1, 3, 7, 14 and 21. Immunoprecipitation in the parietal somatosensory cortex was manifest in the population of pyramidal neurons during postnatal development. In particular during the early postnatal ages, until 2 weeks after birth, M35 immunoreactivity (M35-ir) was present in all neuronal compartments, indicating transportation of mAChR protein in axonal and dendritic processes as observed in light and electron microscopic analysis. The immunoprecipitation in the apical dendrites yielded dense labeling in layer 1 where the distal processes of the pyramidal dendrites branched extensively forming a plexus that intermingled with horizontal fibers in this superficial layer. At PD21, immunolabeling in layer 1 and in axons of pyramidal cells was reduced compared to earlier ages suggesting a transient expression of mAChRs in these neuronal structures. The development of M35-ir in the cortex appeared to antedate that of its cholinergic afferentation as indicated by AChE histochemical study.

Lesions of the nucleus basalis magnocellularis in immature rats: short- and long-term biochemical and behavioral changes

Short- and long-term effects of unilateral lesions of the nucleus basalis magnocellularis (NBM) on cortical choline acetyltransferase (ChAT) activity and passive avoidance conditioned responses were examined in immature rats. The lesions were made by stereotaxic injection of quisqualic acid on postnatal days 14 (P14), 17 (P17), and 21 (P21). A marked loss of ChAT activity was found 7 days after surgery in all age groups of lesioned rats. Unoperated P14 rats were unable to perform the passive avoidance conditioned responses. Acquisition began on P17. Lesions made on P17 and P21 strongly impaired the acquisition and retention of the task, evaluated 7 days postoperation. No biochemical but a partial behavioral recovery was observed 3 months after surgery in rats lesioned on P14. On the contrary, despite a persistent decrease in cortical ChAT activity, rats lesioned on P21 were able to acquire and retain the passive avoidance conditioned response. These results indicate that destruction of NBM cholinergic neurons shortly after birth is not compensated for by the developmental plasticity of the residual neurons but results in permanent cholinergic hypofunction. They also demonstrate that cholinergic NBM neurons play an important role in the acquisition and retention of a passive avoidance task; nevertheless, a behavioral recovery may take place 3 months after the lesion, even in the presence of a persistent cholinergic hypofunction.

Distribution of [3H]QNB and [125I]alpha-bungarotoxin binding and acetylcholinesterase activity in visual system and hippocampal structures of eleven mammalian species

This study assessed interspecies differences in regional brain distribution of [3H]QNB binding, [125I]alpha-bungarotoxin binding and acetylcholinesterase activity, by autoradiographic and histochemical methods. Eleven mammalian species were examined, including carnivores (cat, dog), a lagomorph (rabbit), and rodents (squirrel, guinea pig, gerbil, hamster, vole, lemming, rat, mouse). Comparisons were based on primary visual system structures (superior colliculus, lateral geniculate nucleus, primary visual cortex) and the hippocampal formation. The two radioligands differed greatly in the degree of interspecies variation: while the pattern of [3H]QNB binding was quite similar across species, [125I]alpha-bungarotoxin showed striking interspecies diversity. This contrast was most obvious in laminar patterns of the visual cortex and hippocampal formation. Regional distributions of acetylcholinesterase staining were fairly diverse, and were unlike the patterns of either [3H]QNB or [125I]alpha-bungarotoxin. The two ligands showed more consistency in overall levels across species than did acetylcholinesterase. Possible correlates of the differences in interspecies diversity are discussed.

Neonatal exposure to DDT induces increased susceptibility to pyrethroid (bioallethrin) exposure at adult age.--Changes in cholinergic muscarinic receptor and behavioural variables

We have recently reported that DDT and the pyrethroid bioallethrin cause similar changes in the brain muscarinic cholinergic receptors (MAChR) and behavioural disturbances in the neonatal and adult mouse when given to neonatal mice during the peak of rapid brain growth. In the present study the interaction between neonatal and adult exposure to DDT and bioallethrin, respectively, is explored. Ten-day-old NMRI mice received a single low oral dose of DDT (0.5 mg/kg body wt). At adult age (5 months) the mice received bioallethrin 0.7 mg/kg body wt./day per os for 7 days. Mice used as controls received a 20% fat emulsion vehicle. The spontaneous behavioural tests revealed significant differences, both in mice treated neonatally with DDT and receiving bioallethrin as adults and in mice receiving the vehicle as neonates and bioallethrin as adults, compared with their corresponding controls. However, the behavioural changes developed in mutually opposite directions. Significant changes in MAChR, assayed in the P2 fraction of the cerebral cortex by using the muscarinic antagonist, quinuclidinyl benzilate ([3H]QNB) and agonist carbachol, was only observed in animals receiving DDT as neonates and bioallethrin as adults. The present study indicates an increased susceptibility in the cholinergic muscarinic receptors and a different behaviour reaction in animals already exposed to DDT (at a physiologically relevant dose), when again exposed to a similar neurotoxic agent as adults.

Development of high-affinity choline transport sites in rat forebrain: a quantitative autoradiography study with [3H]hemicholinium-3

The development of cholinergic terminals in rat brain has been quantitatively analyzed by [3H]hemicholinium-3 autoradiography. [3H]Hemicholinium-3 binds to high affinity choline transport sites, a specific marker for cholinergic neurons. In neonatal animals, kinetic and pharmacologic binding characteristics and regional distribution of [3H]hemicholinium-3 sites are consistent with specific cholinergic localization, as in the adult. The distribution of cholinergic terminals is described in the adult rat brain and during development, including heterogeneity of binding within several regions such as the striatum, nucleus accumbens, olfactory tubercle, cortex, and hippocampus. Early development and maturation vary greatly between brain regions. At embryonic day E18 and day 0, specific binding density is high only in the medial habenula. Development occurs primarily during the postnatal period in most brain regions examined. Many brain regions exhibit a lull in development between days 5 and 10, although the rate of development is highly region specific. Specific binding increases 2-12-fold between day 5 and adult animals, with adult density being achieved anywhere from day 15 to after day 21. The ontogeny of [3H]hemicholinium-3 binding sites generally occurs in a rostral to caudal direction. In the striatal body the characteristic lateral to medial gradient of binding site density is apparent by day 5, and development is more rapid in the lateral striatum. Patches of dense [3H]hemicholinium-3 binding coincident with acetylcholinesterase are observed on day 5 in the caudal striatum. The various patterns of cholinergic terminal development suggest that factors regulating cholinergic development are regional and complex.

Exposure to DDT during a defined period in neonatal life induces permanent changes in brain muscarinic receptors and behaviour in adult mice

DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] is a potent neurotoxicant in both vertebrate and invertebrate species. We have previously reported that neonatal exposure to DDT affects the muscarinic cholinergic receptors (MAChR) in the cerebral cortex in the neonatal mouse, leading to permanent disturbances in the cholinergic system and behaviour of the animals as adults. In order to determine if there is a critical period for these effects, mice at the ages of 3-days, 10-days and 19-days were given a single low oral dose of DDT (0.5 mg/kg b.wt.). At adult age (4 months) the mice were tested for spontaneous behaviour: 'locomotion', 'rearing' and 'total activity', and were subsequently sacrificed for measurement of the density of MAChR and subpopulations of MAChR in the cerebral cortex by using the muscarinic antagonist quinuclidinyl benzilate, [3H]QNB, and agonist carbachol, respectively. A significant increase in spontaneous motor behaviour and a significant decrease in MAChR density in the cerebral cortex was only observed in adult mice receiving DDT at the age of 10 days. The induction of these disturbances is limited to peaks in the development of spontaneous behavioural activity and MAChR in the neonatal rodent.

Development of the basal forebrain cholinergic system: phenotype expression prior to target innervation

We measured choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities in the rat to determine the time course of development, maturity, and senescence of ChAT activity. Tissue was obtained from Sprague-Dawley rats ranging in age from embryonic day 14 through 23 months. Seven regions were examined, including the magnocellular preoptic/substantia innominata region, frontal cortex, medial septal region, hippocampus, diagnoal band, and medial and lateral striatum. ChAT and AChE activities were first detected as early as E18 in the medial septum, diagonal band and magnocellular preoptic area, all regions of cholinergic cell bodies. Enzyme activity subsequently developed in terminal fields of these cholinergic perikarya (hippocampus and frontal cortex) as well as in the striatum. For all regions, enzyme activity rose during the first four postnatal weeks. This increase in enzyme activity was transient and, in most instances, decreases were observed between postnatal days 30 and 60. Most dramatic were the decreases in enzyme activity in the magnocellular preoptic/substantia innominata and diagonal band regions. Age-related declines also occurred in the frontal cortex, hippocampus, magnocellular preoptic/substantia innominata region, and the striatum. Cholinergic systems undergo dynamic changes especially during development and adulthood.

Neurotoxic effects of two different pyrethroids, bioallethrin and deltamethrin, on immature and adult mice: changes in behavioral and muscarinic receptor variables

We have recently shown that two pyrethroids, bioallethrin and deltamethrin, affect muscarinic cholinergic receptors (MAChR) in the neonatal mouse brain when given to suckling mice during the period of rapid brain growth. Such early exposure to these pyrethroids can also lead to permanent changes in the MAChR and behavior in the mice as adults. In the present study, male NMRI mice were given bioallethrin (0.7 mg), deltamethrin (0.7 mg), or a 20% fat emulsion vehicle (10 ml) per kilogram of body weight per os once daily between the 10th and 16th postnatal day. The mice were subjected to behavioral tests upon reaching the age of 17 days and at 4 months. Within 1-2 weeks after the behavioral tests the mice were killed by decapitation and crude synaptosomal fractions (P2) were prepared from the cerebral cortex, hippocampus, and striatum. The densities of MAChR were assayed by measuring the amounts of quinuclidinyl benzilate ([3H]QNB) specifically bound in the P2 fraction. The proportions of high-affinity (HA) and low-affinity (LA) binding sites of MAChR were assayed in a displacement study using [3H]QNB/carbachol. The behavioral tests at an adult age of 4 months indicated a significant increase in spontaneous motor behavior in both bioallethrin- and deltamethrin-treated mice. There was also a significant decrease and a tendency toward a decrease in the density of MAChR in the cerebral cortex in mice receiving bioallethrin and deltamethrin, respectively. The proportions of HA- and LA-binding sites of MAChR were not changed. This study further supports that disturbances of the cholinergic system during rapid development in the neonatal mouse can lead to permanent changes in cholinergic and behavioral variables in the animals as adults.

Neonatal exposure to 3,3',4,4'-tetrachlorobiphenyl: changes in spontaneous behaviour and cholinergic muscarinic receptors in the adult mouse

We have previously reported that 3,3',4,4'-tetrachlorobiphenyl (TCB) affects muscarinic cholinergic receptors (MAChR) in the neonatal mouse brain when given to suckling mice at the age of 10 days. As shown in the present study, such early exposure to TCB may also lead to a permanent change in the MAChR and disturbed behaviour of the mice as adults. Male NMRI mice were given two single oral doses of TCB, 0.41 or 41 mg/kg body wt, and a 20% fat emulsion vehicle (10 ml)/kg body wt. The behavioural test at adult age of 4 months indicated a significant change in spontaneous motor behaviour in the TCB-treated mice. In mice receiving the highest dose of TCB there was also a minor increase (5%), although significant, in the density of MAChR in the hippocampus, assayed by using the tritium-labeled muscarinic antagonist, quinuclidinyl benzilate [( 3H]QNB). As previously reported, this part of the brain was affected in the neonatal mouse, which shows that the cholinergic system during rapid development in the neonatal mouse brain is sensitive to disturbance. This may lead to permanent changes in the animals as adults, accompanied by behavioural changes.

Early development of the nucleus basalis-cortical projection but late expression of its cholinergic function

The aim of this study was to examine the development of the basalocortical pathway by using choline acetyltransferase and nerve growth factor receptor immunocytochemistry, acetylcholinesterase histochemistry and retrograde axonal transport. The observations were made in the ferret because in this species brain development occurs over a much more protracted period than in the rat. Staining for choline acetyltransferase immunoreactivity in the brain was minimal before birth. Adult levels of staining for the enzyme were not seen in cell bodies until three weeks after birth and in axons up to six weeks after birth. This, however, did not mean that presumptive cholinergic pathways are absent early in development. There was strong staining for nerve growth factor receptor in basal forebrain neurons from at least two weeks before birth. Positive staining for acetylcholinesterase was found in axons that begin to invade the cerebral cortex a week before birth. The retrograde axonal transport technique showed that the basalocortical pathway has a normal organization in the neonate. The conclusion is that cholinergic pathways form early in the prenatal period in the ferret but express their transmitter function late in postnatal development.

Developmental expression of somatostatin in mouse brain. I. Immunocytochemical studies

The postnatal development of the distribution of somatostatin immunoreactive (SOMLI) neurons and fibers in the forebrain of the Balb/C mouse and their relationship to cholinergic afferents have been examined. SOMLI was first discernable in the hypothalamus on postnatal day (PND) 3 and increased gradually to reach adult levels by PND 30. In the limbic system, SOMLI is detectable at birth. In all other structures of the forebrain, SOMLI could be observed by PND 3 but the distribution, density and morphology of the immunoreactive neurons evolved over the following 2-3 weeks. In general, SOMLI cells and fibers increased for 1-3 weeks after their initial appearance and subsequently declined to achieve adult levels. The distribution pattern of SOMLI elements in adult mouse brain was similar to previous reports in rat with a few notable differences in thalamus, olfactory structures and, to a lesser degree, cortex and hippocampus. The temporal pattern of SOMLI expression in extrahypothalamus forebrain regions, during development, suggests a role of this peptide in differentiation and synapse formation. Such an hypothesis receives further support from neonatal lesions of the basal forebrain which resulted in transient cortical cholinergic deafferentation, a delay of cortical differentiation and a transient increase in the number of SOMLI cells in cortex.

[125I]alpha-bungarotoxin binding marks primary sensory area developing rat neocortex

The postnatal ontogeny of [125I]alpha-bungarotoxin (alpha-Btx) binding distribution in rat neocortex was described and quantified using autoradiography of in vitro labeled brain sections. During the first two weeks, distinctive transitory radial and laminar patterns emerged. Dense columnar bands of alpha-Btx binding extended through the depth of primary sensory cortex, including somatosensory, visual and auditory areas. An association of alpha-Btx binding with thalamic input zones was further demonstrated within developing somatosensory cortex, where discrete radial bands appeared over the whisker barrels around the time that ingrowing thalamocortical fibers segregate as they selectively innervate the barrels. The early laminar distribution of alpha-Btx binding also resembled that of developing thalamocortical afferents. From P12 to P20, alpha-Btx radial distinctions faded and the laminar pattern changed further to achieve the adult distribution. The spatiotemporal ontogeny of alpha-Btx binding suggests a role for alpha-Btx binding sites in the development of cortical connectivity.

The development of the muscarinic acetylcholine receptor in normal and hyperphenylalaninemic rat cerebrum

The effect of hyperphenylalaninemia on the development of the muscarinic acetylcholine receptor in rat cerebrum has been studied. Rats were subjected to the hyperphenylalaninemic regimen as of 5 days of age. A gradual and steady decrease in the number of binding sites for L-[3H]quinuclidinylbenzilate was observed, with the white matter more affected than the gray matter. A return to normal blood phenylalanine levels after the age of 21 days does not lead to an increase in this number of binding sites.

Neonatal lesions of the basal forebrain cholinergic neurons result in abnormal cortical development

The effect of electrolytic lesions of the neonatal forebrain on the morphogenesis of the mouse neocortex has been examined. Balb/C mice were lesioned unilaterally within 24 h of birth. The development of cortical cytoarchitecture was assessed in Nissl-stained sections, and the levels of presynaptic markers for cholinergic, noradrenergic and serotonergic afferents were measured in the fronto-parietal cortex ipsilateral and contralateral to the lesion at various postnatal ages and in adulthood. The basal forebrain (nBM) lesion resulted in a transient but severe reduction of cortical cholinergic markers and in abnormal cortical cytoarchitecture. Cytoarchitectural abnormalities were expressed as delay in the emergence of differentiated cell populations and affected sequentially more superficial layers with maturation following lesion. Furthermore, the location and extent of these morphologic abnormalities appeared to correlate with the degree of cholinergic denervation. Cortical monoamines were also temporarily reduced as a result of the lesion; however, pharmacologic lesions of the monoaminergic projections alone did not result in the abnormal cortical cytoarchitecture. Thus, the basal forebrain cholinergic projection appears to serve a role in regulating cortical differentiation.

Basal forebrain lesions facilitate adult host fiber ingrowth into neocortical transplants

The ability of mature host thalamic neurons to innervate embryonic (E19) cortex when implanted into the cortex of adult hosts was compared in normal and basal forebrain lesioned mice. The ingrowth of mature horseradish peroxidase-labeled thalamic axons into the transplants is facilitated by prior basal forebrain lesions. We discuss the possible reasons for the lesion-induced enhancement of axonal ingrowth, including the possibility that the enhanced ingrowth of thalamic fiber systems may be related to the loss of cortical innervation by extrathalamic brainstem inputs, especially cholinergic afferent fibers. The results support the interpretation that extrathalamic inputs to cortex play a modulatory role in regulating the growth and connections of specific sensory fiber systems during brain responses to injury.

Effects of 3,3',4,4'-tetrachlorobiphenyl in the brain of the neonatal mouse

To study the retention of TCB in the brain of the immature mouse and its effects on the muscarinic cholinergic receptors in the cerebral cortex and hippocampus, [14C]TCB and TCB were administered to 10-day-old mice as a single peroral dose of 1.5 MBq/kg body wt and 0.41 mg or 41 mg/kg body wt, respectively. The mice were killed 24 h or 7 days after treatment. The amount of radioactivity in the whole brain decreased by 78% between 24 h and 7 days after administration. The density of muscarinic receptors was measured using the muscarinic antagonist [3H]QNB. A significant decrease (16%) in specific [3H]QNB binding was observed in the hippocampus 7 days after treatment. Despite the relatively rapid elimination of TCB from the immature mouse brain, a single low dose of TCB given during the period of rapid development of the cholinergic system appeared to affect the muscarinic cholinergic receptors in the immature mice.

Time of origin of cholinergic neurons in the rat basal forebrain

The timing of the final mitotic division of basal forebrain cholinergic neurons was studied by injecting [3H]thymidine into timed pregnant rats and processing the brains of their progeny as young adults for immunohistochemistry with a monoclonal antibody to choline acetyltransferase (ChAT) followed by autoradiography. ChAT-positive neurons located caudally in the basal forebrain were found to become postmitotic mostly on embryonic (E) days 12 and 13, whereas the peak final mitosis of more rostrally located ChAT-positive neurons occurred increasingly later, with the most rostral ChAT-immunoreactive neurons leaving their final mitotic cycles on E15 and E16. In all basal forebrain regions, cholinergic neurogenesis was complete by E17. These results indicate that the cholinergic neurons in the basal forebrain become postmitotic in a caudal-to-rostral gradient over about 5 days. The continuity of the gradient suggests that these cholinergic neurons may derive from the same germinal source.

The cholinergic nuclei of the basal forebrain of the rat: normal structure, development and experimentally induced degeneration

The normal morphology and distribution of the cholinergic neurones of the basal forebrain of the rat have been studied qualitatively and quantitatively after staining immunohistochemically with a monoclonal antibody to choline acetyl transferase (ChAT). This was done in order to provide an adequate control for the changes found in these cells on both sides of the brain in the experimental investigation of the reaction of the cells to damage of their axons. The cholinergic cells form a more or less continuous anteroposterior band, but they can be subdivided into distinct nuclear groups on the basis of the size and form of the cell bodies and dendrites, their position and arrangement. these nuclei conform closely to previous descriptions of Nissl-stained material: the medial septal nucleus, the vertical and horizontal nuclei of the diagonal band and the basal nucleus. Quantitative measurements of the cross-sectional areas of the cells in the different nuclei confirmed the conclusions drawn from the qualitative examination. Measurements of the ChAT cells at different ages showed that in all nuclei they are significantly larger in size in infancy than in the adult, and they shrink to the mature size by 46 days. The cells in the various cholinergic nuclei show distinctly different reactions to damage of their terminal axonal fields. After removal of a large part of the neocortex by removal of the overlying pia-arachnoid mater the cells in the basal nucleus in the operated hemisphere underwent retrograde cellular degeneration, being swollen and paler-staining up to 14 days, and thereafter shrinking by 20-30% (as compared with those in the brains of age- and sex-matched littermate controls). The degree of shrinkage was appreciably greater when the animals were operated upon at the neonate stage. No cell loss was found, qualitatively or quantitatively, in the basal nucleus. After removal of the hippocampus there is marked loss of cholinergic neurones in the medial septal nucleus and in the vertical nucleus of the diagonal band, and with severe shrinkage of the remaining cells. Removal of the olfactory bulb results in only slight shrinkage of the cells, and no cell loss, in the horizontal nucleus of the diagonal band.(ABSTRACT TRUNCATED AT 400 WORDS)