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

Neocortical cholinergic pathology after neonatal brain injury is increased by Alzheimer's disease-related genes in mice

Hypoxic-ischemic encephalopathy (HIE) in neonates causes mortality and neurologic morbidity, including poor cognition with a complex neuropathology. Injury to the cholinergic basal forebrain and its rich innervation of cerebral cortex may also drive cognitive pathology. It is uncertain whether genes associated with adult cognition-related neurodegeneration worsen outcomes after neonatal HIE. We hypothesized that neocortical damage caused by neonatal HI in mice is ushered by persistent cholinergic innervation and interneuron (IN) pathology that correlates with cognitive outcome and is exacerbated by genes linked to Alzheimer's disease. We subjected non-transgenic (nTg) C57Bl6 mice and mice transgenically (Tg) expressing human mutant amyloid precursor protein (APP-Swedish variant) and mutant presenilin (PS1-ΔE9) to the Rice-Vannucci HI model on postnatal day 10 (P10). nTg and Tg mice with sham procedure were controls. Visual discrimination (VD) was tested for cognition. Cortical and hippocampal cholinergic axonal and IN pathology and Aβ plaques, identified by immunohistochemistry for choline acetyltransferase (ChAT) and 6E10 antibody respectively, were counted at P210. Simple ChAT+ axonal swellings were present in all sham and HI groups; Tg mice had more than their nTg counterparts, but HI did not affect the number of axonal swellings in APP/PS1 Tg mice. In contrast, complex ChAT+ neuritic clusters (NC) occurred only in Tg mice; HI increased that burden. The abundance of ChAT+ clusters in specific regions correlated with decreased VD. The frequency of attritional ChAT+ INs in the entorhinal cortex (EC) was increased in Tg shams relative to their nTg counterparts, but HI obviated this difference. Cholinergic IN pathology in EC correlated with NC number. The Aβ deposition in APP/PS1 Tg mice was not exacerbated by HI, nor did it correlate with other metrics. Adult APP/PS1 Tg mice have significant cortical cholinergic axon and EC ChAT+ IN pathologies; some pathology was exacerbated by neonatal HI and correlated with VD. Mechanisms of neonatal HI induced cognitive deficits and cortical neuropathology may be modulated by genetic risk, perhaps accounting for some of the variability in outcomes.

A role for neurotrophin-3 in targeting developing cholinergic axon projections to cerebral cortex

This study examined the relationship between expression of neurotrophin-3 (NT-3) and the ingrowth of cholinergic axonal projections in cerebral cortex. Patterns of expression of NT-3 (defined by beta-galactosidase reporter expression in heterozygous offspring of transgenic NT-3(lacZneo/+) mice) revealed that limbic cortical regions (including frontal, cingulate, and insular cortex, as well as the dentate gyrus) express NT-3 and that these cortical regions receive early and relatively dense cholinergic axons (stained for acetylcholinesterase, AChE). Using the dentate gyrus as a model system, studies revealed that expression of the NT-3 reporter parallels, and precedes by approximately 2 days, the ingrowth of AChE positive cholinergic axons. Studies of forebrain organotypic slice cultures demonstrate that basal forebrain-derived cholinergic axons extend into cortical regions in a pattern that mimics the pattern of expression of the NT-3 reporter. Similarly, chimeric co-cultures, combining wild type septum with a slice of hippocampus from heterozygous NT-3(lacZneo/+) mice, demonstrate that cholinergic axons grow into regions of the dentate gyrus that express the NT-3 reporter. Hemisphere slice cultures made from NT-3 knockout mice reveal cholinergic axonal growth into cortex, but these axons do not form the regional pattern characteristic of slice cultures made from wild type or heterozygous NT-3(lacZneo/+) mice. Further, chimeric co-cultures made using slices of wild type septum combined with slices of hippocampus from NT-3 knockout mice demonstrate robust cholinergic axonal growth into the hippocampus, but the cholinergic axons do not form the characteristic preterminal pattern associated with the dentate gyrus. Slice cultures from limbic cortical tissue from the NT-3 null mice do not display exaggerated levels of cell death. In aggregate, these data support the hypothesis that expression of NT-3 by cortical neurons serves to attract basal forebrain cholinergic projections to their target cells in cerebral cortex.

Postnatal development of limb motor innervation in the opossum Monodelphis domestica: immunohistochemical localization of acetylcholine

The development of limb motor innervation was studied in the opossum Monodelphis domestica, a marsupial born with immature mobile forelimbs and immobile hindlimbs. Choline acetyltransferase (ChAT), the synthesis enzyme of acetylcholine, was evidenced on sections of the spinal enlargements, and the protein that transports acetylcholine (VAChT) on limb sections. In newborn, ChAT immunolabeling occurred in small, undifferentiated neurons of the ventral horn, presumably motoneurons, and intermediate and dorsal gray matter, and in the presumptive white matter, all less abundant at lumbosacral than brachial levels. Scant immunolabeling for VAChT marked small terminal-looking profiles, presumably growth cones or immature neuromuscular junctions, decreasing proximodistally in each limb and being less abundant in hindlimbs than forelimbs; it was absent distally in the foot where no muscle tissue was formed. ChAT labeling disappeared from the white matter within 1 week while cholinergic neurons increased in number and size. Motoneurons segregated in a medial and lateral group by 4-5 weeks. VAChT-labeled profiles increased in number and size and they flattened along a proximodistal gradient within each limb, but later in the hindlimbs than in the forelimbs. Labeling appeared in distal foot muscle at 1 week. The density, size, and shape of terminals became comparable in all segments of a given limb by 3-4 weeks. Their number and size increased, and by 8 weeks, they clustered in 3 or 4 along muscle fibers. Thus, limb motor innervation develops largely postnatally in the opossum, along rostrocaudal and proximodistal gradients. Its timecourse is compared to the development of motor behaviors.

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.

Arrival of afferents and the differentiation of target neurons: studies of developing cholinergic projections to the dentate gyrus

This study examined the relationship between the development of cholinergic axons originating from the septum and a group of their target cells, the granule cells of the dentate gyrus of the rat. Acetylcholinesterase histochemistry was used to identify septal cholinergic afferents to the dentate gyrus; parallel studies used anterograde movement of a carbocyanine dye to label the septal projections. Septal cholinergic axons are present in the molecular layer of the internal blade of the dentate gyrus shortly after birth, but these axons do not reach the external blade until several days later. Results demonstrate that acetylcholinesterase positive septal axons grow into the external blade of the dentate gyrus only after the recently generated granule cells have coalesced to form a clearly defined layer. Results from studies using in situ hybridization techniques demonstrate that dentate gyrus granule cells express messenger RNAs for brain derived neurotrophic factor and for neurotrophic factor 3 shortly after formation of the granule cell layer. Ingrowth of septal cholinergic axons follows two days after the formation of the external blade of the dentate gyrus and the expression of neurotrophin messenger RNAs by the dentate granule cells. These data support the hypothesis that target cell development is a prerequisite for attracting the ingrowth of septal afferent axons.

Ontogeny of modulatory inputs to motor networks: early established projection and progressive neurotransmitter acquisition

Modulatory information plays a key role in the expression and the ontogeny of motor networks. Many developmental studies suggest that the acquisition of adult properties by immature networks involves their progressive innervation by modulatory input neurons. Using the stomatogastric nervous system of the European lobster Homarus gammarus, we show that contrary to this assumption, the known population of projection neurons to motor networks, as revealed by retrograde dye migration, is established early in embryonic development. Moreover, these neurons display a large heterogeneity in the chronology of acquisition of their full adult neurotransmitter phenotype. We performed retrograde dye migration to compare the neuronal population projecting to motor networks located in the stomatogastric ganglion in the embryo and adult. We show that this neuronal population is quantitatively established at developmental stage 65%, and each identified projection neuron displays the same axon projection pattern in the adult and the embryo. We then combined retrograde dye migration with FLRFamide-like, histamine, and GABA immunocytochemistry to characterize the chronology of neurotransmitter expression in individual identified projection neurons. We show that this early established population of projection neurons gradually acquires its neurotransmitter phenotype complement. This study indicates that (1) the basic architecture of the known population of projection inputs to a target network is established early in development and (2) ontogenetic plasticity may depend on changes in neurotransmitter phenotype expression within preexisting neurons rather than in the addition of new projection neurons or fibers.

Acetylcholinesterase in lumbar and ventricular cerebrospinal fluid

BACKGROUND

Soluble acetylcholinesterase (AChE, E.C. 3.1.1.7.) is released by neurons, glial and meningeal cells into the CSF. AChE activity in cerebrospinal fluid (CSF) is altered in various disorders of the nervous system. The objects of this study are to define a reference range for CSF AChE activity in human lumbar CSF, to prove that the enzyme activity does not depend on the blood/CSF barrier function, and to provide information about AChE in ventricular CSF. In addition, drugs used in neurosurgical care have been examined for their in vitro effects on CSF AChE activity to exclude interference with the test system.

METHODS

We tested the AChE activity in 64 lumbar CSF samples collected from a clinically healthy population and in 169 ventricular CSF samples obtained from 90 neurosurgical patients. AChE activity was assayed with our inhibitor-free test procedure.

RESULTS

The reference range determined for lumbar CSF AChE activity is 9.2-24.4 nmol/min per ml. Lumbar CSF AChE activity does not correlate with parameters characterising the status of the blood/CSF barrier. Ventricular puncture is only justified for underlying pathology making it impossible to provide reference data for ventricular CSF. Most measurements reveal ventricular enzyme activity below 4 nmol/min per ml.

CONCLUSION

The results of this study suggest the utility of lumbar CSF AChE activity as a measure of specific secretory function in enzyme releasing cells of the nervous system.

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.

Sequential developmental acquisition of cotransmitters in identified sensory neurons of the stomatogastric nervous system of the lobsters, Homarus americanus and Homarus gammarus

We studied the developmental acquisition of three of the cotransmitters found in the gastropyloric receptor (GPR) neurons of the stomatogastric nervous systems of the lobsters Homarus americanus and Homarus gammarus. By using wholemount immunocytochemistry and confocal microscopy, we examined the distribution of serotonin-like, allatostatin-like, and FLRF(NH2)-like immunoreactivities within the stomatogastric nervous system of embryonic, larval, juvenile, and adult animals. The GPR neurons are peripheral sensory neurons that send proprioceptive information to the stomatogastric and commissural ganglia. In H. americanus, GPR neurons of the adult contain serotonin-like, allatostatin-like, and Phe-Leu-Arg-Phe-amide (FLRF(NH2))-like immunoreactivities. In the stomatogastric ganglion (STG) of the adult H. americanus and H. gammarus, all of the serotonin-like and allatostatin-like immunoreactivity colocalizes in neuropil processes that are derived exclusively from ramifications of the GPR neurons. In both species, FLRF(NH2)-like immunoreactivity was detected in the STG neuropil by 50% of embryonic development (E50). Allatostatin-like immunoreactivity was visible first in the STG at approximately E70-E80. In contrast, serotonin staining was not clearly visible until larval stage I (LI) in H. gammarus and until LII or LIII in H. americanus. These data indicate that there is a sequential acquisition of the cotransmitters of the GPR neurons.

Growth inhibitory effects of a mu opioid on cultured cholinergic neurons from fetal rat ventral forebrain, brainstem, and spinal cord

Cholinergic pathways play a role in respiration in the mammalian brain, and agents that affect respiratory function such as opioid peptides might have positive or negative neurotrophic effects during the development of these cholinergic connections. Rat fetal nerve cell cultures from developmental stages E14-E18 were established in 96-well plates from ventral forebrain (VFB), an area rich in cholinergic neurons, and from brainstem and rostral spinal cord, areas where respiratory control systems and cholinergic neurons co-exist. High affinity 3H-choline uptake was highest in E14 VFB cultures and decreased to 20% of this value by E16 and E18. Choline uptakes in E14 brainstem and spinal cord were only 20% and 13%, respectively, of E14 VFB uptake. A mu opioid receptor agonist, d-ala2-mePhe4-gly(ol)5]-enkephalin (DAMGO), was tested for its effect on somal area and neurite outgrowth in E16 cultures. Cholinergic neurons were identified by immunostaining with choline acetyltransferase antibody. DAMGO (10(-8) M) significantly decreased somal area in VFB cultures and spinal cord, but had no effect on somal area in brainstem. Naltrexone (10(-6) M) reversed this inhibition. Spinal cord cell neurite outgrowth was inhibited by DAMGO, and this inhibition was reversed by naltrexone. DAMGO had no significant effect on neurite length in VFB. Brainstem neurite length was paradoxically increased by both DAMGO and naltrexone. It was concluded that mu-selective opioid peptides inhibit growth of cultured cholinergic neurons in VFB and spinal cord, but not in the brainstem. There was no evidence for endogenous opioid activity in either VFB or spinal cord cultures.

Modulation of intrinsic circuits by serotonin 5-HT3 receptors in developing ferret visual cortex

Serotonergic projections are widespread in the developing neocortex, but their functions are obscure. The effects of 5-HT3 receptor agonists on cortical circuit response properties were studied in slices of ferret primary visual cortex using high-speed optical imaging of voltage-sensitive dye signals and whole-cell patch-clamp recording. Activation of the 5-HT3 receptor decreased the amplitude and lateral extent of excitation throughout postnatal development. This effect peaks after eye opening, which indicates a function for serotonergic modulation of circuit responses during the period of refinement of cortical connections. Whole-cell patch-clamp recordings from single neurons revealed that synaptic responses evoked by white matter stimulation were reduced by 5-HT3 receptor agonists, whereas the frequency of spontaneous GABAergic synaptic currents was enhanced dramatically. This indicates that the modulation of spontaneous synaptic activity by fast-acting serotonin receptors is reflected in an inhibition of the circuit response, in line with the notion of background synaptic activity altering the spatiotemporal integration properties of cortical cells by changing their membrane potential and their electrotonic structure. These mechanisms may regulate the response properties of intrinsic circuits in both the adult and developing neocortex.

Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex

Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.

Development of basal forebrain projections to visual cortex: DiI studies in rat

We performed experiments using retrograde and anterograde labeling with DiI to examine the development of basal forebrain (BFB) projections to the visual cortex in postnatal rats. DiI placed in occipital cortex led to retrograde labeling of BFB neurons as early as postnatal day 0 (P0); labeled cells were found mainly in the diagonal band complex but also in the medial septum, globus pallidus, and substantia innominata. The retrogradely labeled BFB cells displayed remarkably well-developed dendritic arbors, even in younger animals, and showed increases in soma size, dendritic arbors, and dendritic spines over the first 2 postnatal weeks. DiI placements in the diagonal band led to anterogradely labeled axons in cortex. At early ages (P0-P1), labeled axons were largely confined to white matter. With increasing age, greater numbers of labeled axons were seen in the white matter and in deep cortical layers, and labeled axons extended into superficial layers. The leading edge of labeled fibers reached layer V of visual cortex by P2 and layer IV by P4 and were found throughout the cortical layers by P6. Numbers and densities of labeled axons in visual cortex were greater in older animals, at least through P14. The time of ingrowth of labeled BFB axons into visual cortex indicates that these afferents grow into particular cortical layers after those layers have differentiated from the cortical plate. These data indicate that basal forebrain projections arrive in occipital cortex after cortical lamination is well underway and after the entry of primary thalamocortical projections.

Postnatal development of parvalbumin immunoreactivity in the cerebral cortex of the cat

Parvalbumin immunoreactivity in the developing neocortex of the cat progresses following specific laminar, areal, and, in a particular area, roughly anteroposterior gradients. Parvalbumin immunoreactivity first occurs in basket cells and later in chandelier neurons. Pyramid-like immunoreactive neurons are also transitorily observed from the second to the third week in layer V of the auditory association-related areas. Parvalbumin-immunoreactive neurons first appear in the primary somatosensory cortex and primary auditory and visual areas, followed by the primary motor and polysensory association areas and, finally, the auditory association areas and cortical areas related to the limbic system. In addition to cortical neurons, three fiber systems are immunolabeled with antiparvalbumin antibodies: thalamocortical, callosal, and ipsilateral corticocortical. Parvalbumin-immunoreactive thalamocortical fibers appear during the first month of postnatal life. Parvalbumin-immunoreactive callosal and ipsilateral corticocortical fibers are seen from the fourth postnatal week onward. Because all parvalbumin-immunoreactive cortical neurons in adulthood are nonpyramidal inhibitory cells, the present findings suggest that a number of ipsilateral corticocortical and callosal connections may be inhibitory.

The organization of the embryonic and early postnatal murine hippocampus. II. Development of entorhinal, commissural, and septal connections studied with the lipophilic tracer DiI

We have analyzed the early development of the main hippocampal afferents in the mouse. Following injections of the lipophilic tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in the entorhinal cortex, entorhinal axons were observed for the first time in the hippocampus at E15, in the white matter. At E17, entorhinal fibers arborized within the stratum lacunosum-moleculare. At subsequent stages entorhinal axons formed dense networks that were restricted to their appropriate termination zone in the lacunosum-moleculare. The first axons invading the fascia dentata were noticed at E19, their density increasing at later stages. These axons were mainly present in the outer molecular layer. This onset of entorhinohippocampal projections was corroborated by retrograde labeling data after injections in the hippocampus. Commissural fibers first entered the contralateral hippocampus at E18, their number increasing at the following stages. Commissural axons arborized within the stratum oriens and radiatum in the hippocampus proper. In the fascia dentata, the earliest commissural fibers were seen at P2, terminating in the inner zone of the molecular layer and in the hilus. We conclude that developing entorhinal and commissural axons show a high degree of laminar specificity from the earliest stages of formation, which is compatible with the notion that distinct subsets of early maturing neurons populating the hippocampal plexiform layers may attract particular fiber systems. Hippocamposeptal fibers develop at E15, before the first septal fibers can be detected in the hippocampus. These early hippocamposeptal fibers originated from nonpyramidal neurons and terminated in the medial septal area, which is the main source of septal afferents to the hippocampus. In contrast, septohippocampal fibers were not seen in the hippocampus until E17. At perinatal stages, the hippocamposeptal connection reshapes, sending axons to the dorsolateral septal area as the innervation of the medial septum becomes less conspicuous. This sequence suggests that hippocampal neurons pioneer the formation of septohippocampal connections.

Formation of synapses between basal forebrain afferents and cerebral cortex neurons: an electron microscopic study in organotypic slice cultures

Co-cultures of rat basal forebrain and cerebral cortex were maintained from 1 to 5 weeks in vitro with serum-free defined medium. The formation of synaptic connections between basal forebrain afferent fibres and cortical neurons was studied by specific labelling with three staining techniques, including (i) neuronal tract tracing with the fluorescent dye 1,1'-dioctodecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate, (ii) acetylcholinesterase histochemistry, and (iii) choline acetyltransferase immunocytochemistry. Both basal forebrain and cerebral cortex tissue displayed organotypic characteristics in culture. Cerebral cortex revealed a dense innervation by axonal projections from the basal forebrain. All three labelling techniques produced similar results at the light microscopic level, with densest innervation located in the marginal zone. At the fine structural level, the 1,1'-dioctodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate-, acetylcholinesterase- and choline acetyltransferase-stained basal forebrain afferents all revealed a number of synaptic contacts with cortical neurons. The contacts displayed consistent synaptic features, including presynaptic accumulation of small round vesicles, cleft widening, and postsynaptic densities forming symmetric synapses. These morphological characteristics of connections formed in vitro are similar to basal forebrain cholinergic projections to cerebral cortex in normal brain. Based on these results, this tissue culture model appears to be an useful tool for investigations of the development of cholinergic innervation of cerebral cortex.

Transition from developing to mature patterns of acetylcholinesterase activity in rat visual cortex: implications for the time-course of geniculocortical development

Patterns of acetylcholinesterase (AChE) histochemical staining in cortical area 17 differ in infant and mature rats. In infants, intense AChE activity is seen as a band corresponding to layer IV and deep layer III of the visual cortex, and this staining is associated with terminal fields of geniculocortical neurons. In adult animals, AChE activity is densest in deep layer IV and layer V and is associated with projections originating in the basal forebrain. The present study investigated the transition from developing to mature patterns of AChE staining in visual cortex. Unilateral lesions were placed in either the lateral geniculate body or the basal forebrain of rats postnatal days 8 (P8) to adulthood; the effects of these lesions on patterns of AChE activity in visual cortex were studied with histochemical techniques and optical densitometry. Lesions involving the lateral geniculate body markedly reduce AChE activity in visual cortex of P12 rats, had moderate effects in P20 rats, and had no apparent effect on AChE activity of visual cortex of rats aged P40 and older. Lesions of basal forebrain had little effect on AChE activity in visual cortex of P12 animals, increasing effect in P15-35 rats, and eliminated much of AChE staining in visual cortex of adults. The period of transition from developing to mature patterns of AChE activity in visual cortex of animals bilaterally enucleated at birth was not different from the period of transition in normally sighted animals. These data indicate that mature patterns of AChE activity in visual cortex are not achieved until well into the second month of life. If transient AChE expression is characteristic of geniculocortical neurons during the period of time in which axons are proliferating within visual cortex, then these data indicate that geniculocortical connections may be forming well into the second month of life in the rat.