The fine structural demonstration of monoaminergic synapses in immature rat neocortex

Neural histology and neurogenesis of the human fetal and infant brain

The human brain develops slowly and over a long period of time which lasts for almost three decades. This enables good spatio-temporal resolution of histogenetic and neurogenetic events as well as an appropriate and clinically relevant timing of these events. In order to successfully apply in vivo neuroimaging data, in analyzing both the normal brain development and the neurodevelopmental origin of major neurological and mental disorders, it is important to correlate these neuroimaging data with the existing data on morphogenetic, histogenetic and neurogenetic events. Furthermore, when performing such correlation, the genetic, genomic, and molecular biology data on phenotypic specification of developing brain regions, areas and neurons should also be included. In this review, we focus on early developmental periods (form 8 postconceptional weeks to the second postnatal year) and describe the microstructural organization and neural circuitry elements of the fetal and early postnatal human cerebrum.

The relevance of human fetal subplate zone for developmental neuropathology of neuronal migration disorders and cortical dysplasia

The human fetal cerebral cortex develops through a series of partially overlapping histogenetic events which occur in transient cellular compartments, such as the subplate zone. The subplate serves as waiting compartment for cortical afferent fibers, the major site of early synaptogenesis and neuronal differentiation and the hub of the transient fetal cortical circuitry. Thus, the subplate has an important but hitherto neglected role in the human fetal cortical connectome. The subplate is also an important compartment for radial and tangential migration of future cortical neurons. We review the diversity of subplate neuronal phenotypes and their involvement in cortical circuitry and discuss the complexity of late neuronal migration through the subplate as well as its potential relevance for pathogenesis of migration disorders and cortical dysplasia. While migratory neurons may become misplaced within the subplate, they can easily survive by being involved in early subplate circuitry; this can enhance their subsequent survival even if they have immature or abnormal physiological activity and misrouted connections and thus survive into adulthood. Thus, better understanding of subplate developmental history and various subsets of its neurons may help to elucidate certain types of neuronal disorders, including those accompanied by epilepsy.

Development of axonal pathways in the human fetal fronto-limbic brain: histochemical characterization and diffusion tensor imaging

The development of cortical axonal pathways in the human brain begins during the transition between the embryonic and fetal period, happens in a series of sequential events, and leads to the establishment of major long trajectories by the neonatal period. We have correlated histochemical markers (acetylcholinesterase (AChE) histochemistry, antibody against synaptic protein SNAP-25 (SNAP-25-immunoreactivity) and neurofilament 200) with the diffusion tensor imaging (DTI) database in order to make a reconstruction of the origin, growth pattern and termination of the pathways in the period between 8 and 34 postconceptual weeks (PCW). Histological sections revealed that the initial outgrowth and formation of joined trajectories of subcortico-frontal pathways (external capsule, cerebral stalk-internal capsule) and limbic bundles (fornix, stria terminalis, amygdaloid radiation) occur by 10 PCW. As early as 11 PCW, major afferent fibers invade the corticostriatal junction. At 13-14 PCW, axonal pathways from the thalamus and basal forebrain approach the deep moiety of the cortical plate, causing the first lamination. The period between 15 and 18 PCW is dominated by elaboration of the periventricular crossroads, sagittal strata and spread of fibers in the subplate and marginal zone. Tracing of fibers in the subplate with DTI is unsuccessful due to the isotropy of this zone. Penetration of the cortical plate occurs after 24-26 PCW. In conclusion, frontal axonal pathways form the periventricular crossroads, sagittal strata and 'waiting' compartments during the path-finding and penetration of the cortical plate. Histochemistry is advantageous in the demonstration of a growth pattern, whereas DTI is unique for demonstrating axonal trajectories. The complexity of fibers is the biological substrate of selective vulnerability of the fetal white matter.

Developmental history of the subplate zone, subplate neurons and interstitial white matter neurons: relevance for schizophrenia

The subplate zone is a transient cytoarchitectonic compartment of the fetal telencephalic wall and contains a population of subplate neurons which are the main neurons of the fetal neocortex and play a key role in normal development of cerebral cortical structure and connectivity. While the subplate zone disappears during the perinatal and early postnatal period, numerous subplate neurons survive and remain embedded in the superficial (gyral) white matter of adolescent and adult brain as so-called interstitial neurons. In both fetal and adult brain, subplate/interstitial neurons belong to two major classes of cortical cells: (a) projection (glutamatergic) neurons and (b) local circuit (GABAergic) interneurons. As interstitial neurons remain strategically positioned at the cortical/white matter interface through which various cortical afferent systems enter the deep cortical layers, they probably serve as auxiliary interneurons involved in differential "gating" of cortical input systems. It is widely accepted that prenatal lesions which alter the number of surviving subplate neurons (i.e., the number of interstitial neurons) and/or the nature of their involvement in cortical circuitry represent an important causal factor in pathogenesis of at least some types of schizophrenia--e.g., in the subgroup of patients with cognitive impairment and deficits of frontal lobe functions. The abnormal functioning of cortical circuitry in schizophrenia becomes manifest during the adolescence, when there is an increased demand for proper functioning of the prefrontal cortex. In this review, we describe developmental history of subplate zone, subplate neurons and surviving interstitial neurons, as well as presumed consequences of the increased number of GABAergic interstitial neurons in the prefrontal cortex. We propose that the increased number of GABAergic interstitial neurons leads to the increased inhibition of prefrontal cortical neurons. This inhibitory action of GABAergic interstitial neurons is facilitated by their strategic position at the cortical/white matter interface where limbic and modulatory afferent pathways enter the prefrontal cortex. Thus, enlarged population of inhibitory interstitial neurons (even if they represent a minor fraction of total neuron number, as in the cerebral cortex itself) may alter the differential "gating" of limbic and modulatory inputs (as well as other cortical and subcortical inputs) and cause a functional disconnectivity between the prefrontal and limbic cortex in the adolescent brain. In conclusion, fetal subplate neurons and surviving postnatal interstitial neurons are important modulators of cortical functions in both normal and schizophrenic cerebral cortex.

Role of immediate early gene expression in cortical morphogenesis and plasticity

During the development of the central nervous system, there is a fundamental requirement for synaptic activity in transforming immature neuronal connections into organized functional circuits (Katz 1996). The molecular mechanisms underlying activity-dependent adaptive changes in neurons are believed to involve regulated cascades of gene expression. Immediate early genes (IEGs) comprise the initial cascade of gene expression responsible for initiating the process of stimulus-induced adaptive change, and were identified initially as transcription factors that were regulated in brain by excitatory synaptic activity. More recently, a class of neuronal immediate early genes has been identified that encodes growth factors, signaling molecules, extracellular matrix and adhesion proteins, and cytoskeletal proteins that are rapidly and transiently expressed in response to glutamatergic neurotransmission. This review focuses on the neuronal immediate early gene (nIEG) response, in particular, the class of "effector" immediate early gene proteins that may directly modify neuronal and synaptic function.

Differential timing for the appearance of neuronal and astrocytic beta-adrenergic receptors in the developing rat visual cortex as revealed by light and electron-microscopic immunocytochemistry

The developing cerebral cortex is likely to exhibit synaptic circuitries differing from those in adulthood, due to the asynchronous maturation of the various neurotransmitter systems. Two antisera directed against mammalian beta-adrenergic receptors (beta AR), beta AR248 and beta AR404, were used to characterize the laminar, cellular, and subcellular distributions of beta AR in postnatally developing visual cortex of rats. The antigenic sites were the receptor's third intracellular loop for beta AR248 and the C-terminus for beta AR404. During week 1, most of the beta AR404- and beta AR248-immunoreactive sites were dendritic. Morphologically identifiable synapses were rare, even in layer 1: yet, semiquantitative analysis revealed that beta AR404-immunoreactive synapses comprise half of those in layer 1. During week 2, the two antisera began to diverge in their immunoreactivity patterns. With beta AR248, there was an overall decline in immunoreactivity, while with beta AR404, there was an increase in immunoreactive sites, primarily due to labeled astrocytic processes that increased 200-fold in areal density by week 3. In contrast, the areal density of synaptic labeling by beta AR404 barely doubled, in spite of the 30-fold increase in areal density of synapses. These results suggest that beta AR undergo conformational changes during early postnatal periods, causing alterations in their relative antigenicity to the two antisera. Furthermore, the first 2 weeks appear to be characterized by modulation of earliest-formed synapses, and the subsequent phase is marked by addition of astrocytic responses that would be more diffuse temporally and spatially. Activation of beta AR is recognized to increase visually evoked activity relative to spontaneous activity. Moreover, astrocytic beta AR are documented to regulate extracellular concentrations of glutamate, ATP, and neurotrophic factors important for the formation of binocular connections. Thus, neuronal and astrocytic responses may, together and in tandem, facilitate strengthening of intracortical synaptic circuitry during early life.

Postnatal development of adrenergic terminals in rat locus coeruleus, with special reference to growth of noradrenergic neurons

The postnatal development of noradrenergic (NA) neurons and adrenergic (AD) terminals in the rat locus coeruleus (LC) was studied immunohistochemically. Cell body size was measured after staining of NA neurons with anti-tyrosine hydroxylase (TH) serum, and AD terminals were visualized with anti-phenylethanolamine N-methyltransferase serum. NA neurons in the LC were strongly TH-immunoreactive throughout the postnatal period. At birth, their mean cell body volume was 660 +/- 30 microns 3. It reached a maximum of 2580 +/- 230 microns 3 at postnatal day (PD) 14, and decreased thereafter to 930 +/- 50 microns 3 at PD 60. This transient enlargement of NA neurons may be closely related to the development of the cerebral cortex. AD afferents to the LC had terminals forming predominantly asymmetric junctions at birth (about 96% of all junctions). They occasionally made axo-somatic contact, suggesting that AD input already modulated the activity of LC neurons at this stage. AD terminals making axo-spinous synapses increased in number until PD 31, but still represented a minor proportion of these LC terminals, since there were more than 80% in contact with dendritic shafts at all ages examined.

Induction of beta-A activin expression by synaptic activity and during neocortical development

beta-A activin is a member of the transforming growth factor-beta family and has been implicated in nerve cell survival and inhibition of differentiation in vitro [Hashimoto M. et al. (1990) Biochem. biophys. Res. Commun. 173, 193-200; Schubert D. et al. (1990) Nature 344, 868-870]. In our studies to identify genomic mechanisms involved in long-term neuronal responses to synaptic activity, we have determined that beta-A activin messenger RNA is rapidly and transiently induced in neurons of the adult rat brain by excitatory synaptic input. Synaptic mechanisms involved in beta-A activin messenger RNA induction were examined in adult hippocampus and cortex using the long-term potentiation paradigm. beta-A activin messenger RNA is induced in granule cell neurons of the hippocampus by high-frequency synaptic stimuli that produce long-term potentiation, and this induction is blocked by the N-methyl-D-aspartate type glutamate receptor antagonist, dizocilpine. beta-A activin messenger RNA is expressed at basal levels in neurons of layers II/III and V/VI, and this expression rapidly decreases following sensory deafferentation of the visual cortex or systemic administration of dizocilpine, suggesting that beta-A activin expression is regulated by physiological excitatory synaptic activity. In developing brain, beta-A activin is expressed in the neocortex and neostriatum beginning at embryonic day 17. beta-A activin expression in late fetal cortex is enriched in postmitotic neurons at the lower boundary of the dense cortical plate. As development progresses, beta-A activin expression continues to be enriched in neurons at the boundary between the hypercellular cortical plate and the subjacent, more mature deep layers. This inside-out progression of beta-A activin expression follows the well-characterized radial gradient of cortical development. Expression of beta-A activin messenger RNA is rapidly regulated in early postnatal cortex and striatum by GABA and glutamate antagonists, suggesting that beta-A activin is also regulated as a rapid response gene in developing brain, and that the high basal levels reflect a steady-state response to developmental signals. Since activin receptors are enriched in neurons of developing and adult brain [Cameron V. A. et al. (1994) Endocrinology 134, 799-808; Roberts V. J. and Barth S. L. (1994) Endocrinology 134, 914-922], our observations suggest a role for activin signaling in neuronal responses to synaptic and developmental activity. In this study, we analyse the induction of expression of beta-A activin, a member of the transforming growth factor-beta family of secreted peptides, in response to synaptic activity and in the developing brain. The elevated and specific expression of beta-A activin during fetal and early postnatal neocortical development and its later regulation by excitatory activity postnatally and in the adult suggests that the activin signaling pathway functions at multiple developmental stages in the neuroplastic response.

Nerve growth factor receptor-immunoreactive neurons within the developing human cortex

A monoclonal antibody recognizing the p75 receptor for nerve growth factor (NGF) was used to assess the immunohistochemical expression of NGF receptors within the developing human neo-, limbic, and paralimbic cortices as well as the hippocampal complex. Between embryonic weeks 16 and 26, a transient population of neurons located within the upper and lower subplate zones of the neo-, limbic, and paralimbic cortices expressed the receptor for NGF. In contrast, NGF receptor-immunoreactive neurons were only observed in the upper subplate zone of the entorhinal cortex at embryonic week 40 (term), a staining pattern not observed in a 5-year-old specimen. The expression of NGF receptor-immunoreactive neurons within the upper subplate zone between embryonic weeks 16 and 40 was characterized by a dense band of immunoreactive neurons and neuropil. These neurons were bipolar with basal and apically directed neurites. NGF receptor-immunoreactive neurons were also scattered throughout the lower subplate zone and underlying white matter between embryonic weeks 19 and 26. These neurons were multipolar, with less apically directed neurites. NGF receptor-immunoreactive subplate neurons displayed a topographic distribution with the heaviest concentration found within limbic and paralimbic cortices as well as association neocortex. In contrast, light to moderate NGF receptor-immunoreactivity was seen in sensory-motor cortex. Within the hippocampal complex, only a few lightly stained NGF receptor-immunoreactive neurons were seen within the fimbria, hilar region of the dentate gyrus, and subiculum. The expression of NGF receptor-immunoreactivity increased within the subplate zone of the pre- and parasubiculum culminating in intense entorhinal cortex staining. As the entorhinal cortex merged with the developing inferior temporal association cortex, there was a marked reduction in staining intensity. In contrast to those in the subplate zone, neurons within the germinal zone and cortical plate were NGF receptor immunonegative at all times examined. The presence of NGF receptors in the subplate zone suggests that neurotrophins such as NGF play an important role in the transient viability of these neurons as well as in the guidance of cortical afferent inputs into topographically organized regions of the cerebral cortex.

Neonatal brain dopamine depletion and the cortical and behavioral consequences of enriched postweaning environment

This study investigated the effects of neonatal intraventricular administration of 6-hydroxydopamine (6-OHDA, 15 micrograms total with and without desmethylimipramine pretreatment) on the cortical thickening and behavioral effects of 35 days of enriched postweaning housing (ENR) in the rat. The 6-OHDA treatment depleted cortical dopamine (DA) to about 40% of control. It did not affect the thickness of the cerebral cortex nor did it affect the capacity for the cortex to be thickened by ENR. In addition, it did not alter the superior performance on two spatial water maze tasks that was caused by ENR. Thus, the potential for neurobehavioral plasticity was not changed by neonatal DA depletion. ENR eliminated the spatial learning/memory deficits that were caused by neonatal DA depletion and that were manifested when the rat was raised in standard (impoverished) laboratory conditions. Hence, environmental factors can modulate the cognitive effects of neonatal DA depletion. ENR did not attenuate the hyperactivity of the neonatal DA-depleted rat. This may reflect the subcortical mediation of this behavioral abnormality.

Effects of congenital hydrocephalus on serotonergic input and barrel cytoarchitecture in the developing somatosensory cortex of rats

The effects of progressive ventricular dilation on the development of the somatosensory cortex (SmI) were studied in congenital hydrocephalic rats, with regard to early serotonergic innervation and formation of functional cellular columns. In hydrocephalic rats, the time course, immunoreactivity, and patterns of formation and synaptogenesis of serotonin immunoreactive (5-HT-IR) terminal aggregations, which characterize the development of the SmI, were preserved. After disappearance of 5-HT-IR terminals, characteristic barrel cytoarchitecture formed normally at the site where 5-HT-IR terminal aggregations had been present. With the progression of hydrocephalus, the cerebral cortex became extremely thin and its total surface area was greatly increased, while barrels were preserved and their areas did not enlarge. These findings suggest that the basic development and the fundamental cytoarchitecture of the cortex are resistant to adverse effects of hydrocephalus.

Fine structure of noradrenergic terminals and their synapses in the rat spinal dorsal horn: an immunohistochemical study

Noradrenergic fibers in the spinal dorsal horn originate from neurons in the A5-7 cell groups, and may participate in the modulation of pain. Here we studied the fine structure of noradrenergic terminals in the rat by immunohistochemistry using antiserum against dopamine-beta-hydroxylase (DBH). We also investigated the relationship between such terminals and primary afferent terminals. DBH-like immunoreactive terminals were found in lamina I and the outer layer of lamina II of the dorsal horn and they contained many clear round vesicles and some large granular vesicles. More than half of these terminals made synaptic contact with other neuronal elements with membrane specialization. Most of the postsynaptic structures of these terminals were small dendrites (69%); 28% were spines, and no synaptic contact was made with primary afferent terminals. These findings suggest that noradrenaline acts on the spinal dorsal horn neurons postsynaptically mainly via a direct synaptic mechanism.

Developmental history of the transient subplate zone in the visual and somatosensory cortex of the macaque monkey and human brain

The cytological organization and the timetable of emergence and dissolution of the transient subplate zone subjacent to the developing visual and somatosensory cortex were studied in a series of human and monkey fetal brains. Cerebral walls processed with Nissl, Golgi, electron-microscopic, and histochemical methods show that this zone consists of migratory and postmigratory neurons, growth cones, loosely arranged axons, dendrites, synapses, and glial cells. In both species the subplate zone becomes visible at the beginning of the mid-third of gestation as a cell-poor/fiber-rich layer situated between the intermediate zone and the developing cortical plate. The subplate zone appears earlier in the somatosensory than in the visual area and reaches maximal width at the beginning of the last third of gestation in both regions. At the peak of its size the ratio between the width of the subplate zone and cortical plate in the somatosensory cortex is 2:1 in monkey and 4:1 in man while in the occipital lobe these structures have about equal width in both species. The dissolution of the subplate zone begins during the last third of gestation with degeneration of some subplate neurons and the relocation of fiber terminals into the cortex. The subplate zone disappears faster in the visual than in the somatosensory area. The present results together with our previous findings support the hypothesis that the subplate zone may serve as a "waiting" compartment for transient cellular interactions and a substrate for competition, segregation, and growth of afferents originated sequentially from the brain stem, basal forebrain, thalamus, and from the ipsi- and contralateral cerebral hemisphere. After a variable and partially overlapping time period, these fibers enter the cortical plate while the subplate zone disappears leaving only a vestige of cells scattered throughout the subcortical white matter. A comparison between species indicates that the size and duration of the subplate zone increases during mammalian evolution and culminates in human fetuses concomitantly with an enlargement of cortico-cortical fiber systems. The regional difference in the size, pattern, and resolution of the subplate zone correlates also with the pattern of cerebral convolutions. Our findings indicate that, contrary to prevailing notions, the subplate may not be a vestige of the phylogenetically old network but a transient embryonic structure that expanded during evolution to subserve the increasing number of its connections.

Development of layer I and the subplate in the rat neocortex

Development of layer I and the subplate of the rat neocortex was examined with [3H]thymidine autoradiography. The experimental animals used for neurogenesis were the offspring of pregnant females injected with [3H]thymidine on 2 consecutive days: Embryonic Day (E) 13-E14, E14-E15, . . . E21-E22, respectively. On Postnatal Day 5, the proportion of layer I and subplate cells originating during 24-h periods were quantified at three anteroposterior levels. Presumptive Cajal-Retzius cells (large horizontal cells) are generated mainly on E14 and subplate cells on E14 and E15 ("outside-in" gradient). Both populations are generated earlier than cells in the cortical plate, which has an "inside-out" gradient. The subplate also has a ventrolateral/older to dorsomedial/younger neurogenetic gradient. The small- to medium-sized horizontal cells in layer I have an extensive period of neurogenesis with an "outside-in" gradient. To study morphogenesis, pregnant females were given single injections of [3H]-thymidine during gestation and embryos were removed in successive 24-h intervals (sequential-survival). On E15 and E16, cells accumulate outside the neuroepithelium in the primordial plexiform layer with older presumptive Cajal-Retzius cells superficial and younger presumptive subplate cells deep. The Cajal-Retzius cells permanently settle superficially among a first system of extracellular channels that appears on E14. Before reaching their final settling sites, subplate cells form the incipient cortical plate in the ventrolateral neocortex on E16. On E17, a seocnd system of extracellular channels appears below the cortical plate. On E18 and E19, subplate cells leave the cortical plate and permanently settle among the deep extracellular channels in a separate layer.

The monoaminergic innervation of the cerebral cortex is not diffuse and nonspecific

It is generally thought that monoamines play a nonspecific role in modulating cortical activity. However, several lines of evidence now indicate that cortical monoaminergic afferents show remarkable anatomical specificity. In particular, there is unequivocal evidence for regional, laminar and intracortical specificity, and for action of monoamines through conventional synapses.

Light and electron microscopic immunocytochemical analysis of the noradrenaline innervation of the rat visual cortex

Immunocytochemistry with an antiserum against noradrenaline was used to examine the organization and morphology of noradrenergic axons in the rat visual cortex. Observations with the light microscope confirmed earlier reports concerning the distribution pattern of noradrenergic fibres, and provided some further clues about their intracortical organization. Particularly striking was the finding of fibres which followed an oscillating course within the boundaries of layers II-IV as they ran in the mediolateral direction. Examination of the morphological characteristics of noradrenaline-containing axon terminals in serial ultrathin sections has provided further evidence that the vast majority (87.6%) form conventional synapses in the visual and frontoparietal cortex, and has given clues about the postsynaptic elements involved in these synaptic contacts; they are, in decreasing frequency, spines, dendritic shafts of various diameters, and pyramidal and non-pyramidal somata. In addition, a few labelled terminals were visualized in close association with intracerebral capillaries.

Postnatal development of interstitial (subplate) cells in the white matter of the temporal cortex of kittens: a correlated Golgi and electron microscopic study

The early postnatal development of interstitial cells (IC) in the white matter of the temporal cortex in kittens was studied. Counts in Nissl-stained preparations show that the number of IC diminishes by about 60% during the second postnatal week. In Golgi preparations, IC are bipolar or bitufted with long, beaded dendrites coursing in the white matter toward the ventricular surface. Ascending, shorter dendrites are thinner, often branch in a short bush, and possess long spines resembling filopodia. The majority of their axons descend in the white matter, emitting numerous recurrent collaterals that become ascending fibers reaching various cortical layers. Most IC resemble inverted pyramidal cells. They appear well developed at the time of birth and continue to develop elaborate axonal complexes in the white matter of older animals. Electron microscopic observations of degenerating IC were detected in all cases studied and their presence was related to the existence of cell death responsible for elimination of a fraction of IC. They were recognized by their dark aspect and by dilations of the endoplasmic reticulum. Synapses contacting degenerating profiles were also observed. It is concluded that IC belong to the population of early generated subplate cells which may have a transient function involved in certain morphogenetic events during the development of the cortical plate. Some persist in the adult where they can be recognized as IC of the white matter.

Noradrenaline- and vasoactive intestinal peptide-containing neuronal systems in neocortex: functional convergence with contrasting morphology

Neurotransmitter-specific anatomical techniques have provided a tool to define the morphological constraints within which a given neurotransmitter will exert its cellular actions. Biochemical and electrophysiological approaches have revealed the nature of these cellular actions for several neurotransmitters. Furthermore, by using purified preparations and tissue cultures a certain degree of resolution has been achieved by which the cell type, where a neurotransmitter's effect takes place, can be determined. In this article we review these aspects for noradrenaline and vasoactive intestinal peptide, two neurotransmitters of the cerebral cortex contained within neuronal systems that present strikingly different morphologies. Nevertheless, noradrenaline and vasoactive intestinal peptide share certain cellular actions and can interact synergistically. The experimental evidence accumulated to date indicates that noradrenaline- and vasoactive intestinal peptide-containing neurons can influence three general cell types of the cerebral cortex, i.e. (i) other neurons, (ii) astrocytes and (iii) cells of the vasculature. This diversity in cellular partners supports the notion that noradrenaline and vasoactive intestinal peptide can be released from neurons at conventional synapses as well as at extrasynaptic sites, thus suggesting the co-existence of two modes of release within the same neuron.

Monoaminergic fibers form conventional synapses in the cerebral cortex

The morphological characteristics of the monoaminergic axon-terminals in the mammalian cerebral cortex have been the subject of controversy in recent years. Systematic analysis of serial ultrathin sections, immunocytochemically stained with antibodies against noradrenaline and serotonin, has shown that nearly all stained terminals form synapses characterized by specialized junctional appositions. These results suggest that, contrary to the widely held view, monoamines in the cortex are released at specialized junctions.

6-Hydroxydopamine induces serotonergic axon sprouting in cerebral cortex of newborn rat

Newborn rats were administered the neurotoxin 6-hydroxydopamine (6-OHDA) to determine whether neonatal ablation of the noradrenergic (NE) innervation produces augmented growth (i.e., sprouting) of serotonergic (5-HT) raphe-cortical axons. Following NE denervation at birth, the density of 5-HT axons in motor cortex (AG1) was determined at 4 days postnatal. Using a computer microscope system, the positions of all 5-HT-positive axons were mapped in radial strips of cortex from treated and control rats. Cumulative axon length, expressed as a function of area inspected, was used as a parameter of innervation density. Following 6-hydroxydopamine, the cumulative length of 5-HT axons in motor cortex increases by 32% (P less than 0.05) while cortical serotonin levels measured by HPLC concomitantly increase by 29% (P less than 0.005). The combined increases in 5-HT axon density and in neurotransmitter levels indicate that NE denervation produces increased growth of the cortical 5-HT innervation by the 4th postnatal day. The amount of transmitter stored per unit length of 5-HT axons appears unchanged. In 6-OHDA-treated rats, 5-HT axons exhibit augmented growth in all layers of motor cortex. In the treated rats, the relative density of 5-HT axons in each cortical layer is roughly proportional to the normal innervation density. Accordingly, in motor cortex, the magnitude of 5-HT axon sprouting is greatest in layer VI, which normally receives a dense 5-HT innervation, and is less in layer V, which has a lower innervation density. Qualitative assessment of other cortical areas following 6-OHDA reveals that 5-HT axon density appears increased in cortical zones that normally receive a dense 5-HT innervation, while the density remains low in zones with sparse innervation. The absence of axonal sprouting is particularly striking in those zones which receive a dense NE innervation but are sparsely innervated by 5-HT axons. Thus, while 5-HT axons undergo sprouting, they do not appear to replace ablated NE terminals in areas with a sparse 5-HT innervation. Hence, normal laminar and regional specificity of 5-HT axons is preserved despite ablation of NE afferents. These data indicate that, while NE denervation may trigger serotonergic sprouting, competition between NE and 5-HT fibers for the same postsynaptic sites is not the main factor that regulates postnatal growth of these axonal projections. The present findings demonstrate that the early development of raphe-cortical projections is influenced by NE cortical innervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar and synaptic organization of the projection from the thalamic nucleus centralis to primary visual cortex in the cat

The projection from the nucleus centralis (an intralaminar thalamic nucleus) to the primary visual cortex was examined with anterograde and retrograde tracing techniques. After large injections of horseradish peroxidase into areas 17 and 18 almost one-half of the neurons in the nucleus centralis were retrogradely labeled. An injection of 3H-proline into the nucleus centralis led to sparse anterograde labeling in layers 5 and 6 of areas 17 and 18. Large injections of peroxidase-conjugated wheat germ agglutinin (WGA) into the nucleus centralis led to similar anterograde labeling of layers 5 and 6 and, in addition, to a band in layer 1. No retrogradely labeled cells were seen in areas 17 or 18. The WGA-labeled terminals in area 17 were examined in the electron microscope: they formed type 1 (asymmetric) synapses on dendritic spines. These observations suggest that the afferents from the nucleus centralis primarily contact pyramidal cells that project to subcortical targets. The findings are consistent with physiological studies suggesting that the nucleus centralis is involved in the modulation of cortical outflow with varying levels of arousal.

Prenatal development of nucleus basalis complex and related fiber systems in man: a histochemical study

To provide parameters for study of the "cholinergic" innervation of a human fetal cerebrum, we have analyzed the prenatal development of histochemical reactivity in the nucleus basalis complex (a magnocellular complex known to contain a high concentration of cholinergic perikarya). Brains from fetuses and premature infants ranging between 8 and 35 weeks of gestation were frozen cut and processed by the thiocholine method for the demonstration of acetylcholinesterase activity. Since no consistent results were obtained with inhibitors on the material younger than 15 weeks, the histochemical reactivity for early stages was expressed as the total cholinesterase reactivity. The first sign of histochemical differentiation of the basal telencephalon is the appearance of a dark cholinesterase reactive "spot" situated between the developing lenticular nucleus and basal telencephalon surface as early as 9 weeks of gestation. The first cholinesterase reactive bundle connects this reactive area (nucleus basalis complex anlage) with the strongly reactive fiber system situated along the dorsal side of the optic tract. During the next "stage" (10.5 weeks), there is a significant increase in the size of the nucleus basalis complex and strongly cholinesterase reactive neuropil occupies the sublenticular, diagonal and septal areas. At this stage we have seen two new cholinesterase-reactive bundles: one well developed cholinesterase reactive fiber stratum approaching (but not penetrating) the neocortical anlage through the external capsule and another minute bundle running towards the medial limbic cortex through the precommissural septum. The supraoptic fiber system can be traced now to the pregeniculate area and the tegmentum. At 15 weeks, the first acetylcholinesterase reactive perikarya appear and the nucleus basalis complex anlage becomes segregated into several strongly reactive territories, corresponding in position to the medial septal, diagonal and basal nuclei as defined on adjacent Nissl stained sections. At this stage, fibers from the nucleus basalis complex enter the "white" matter of frontal, temporal, parietal and occipital parts of the cerebral hemisphere via the external capsule. Between 15 and 18 weeks, acetylcholinesterase fibers spread throughout the "white" matter of the cerebral hemisphere. In the next "stage" (18-22 weeks), strongly reactive fibers can be followed from the nucleus basalis below the putamen and through the external capsule to the transient, synapse-rich subplate zone of frontal, temporal, parietal and occipital cortices.(ABSTRACT TRUNCATED AT 400 WORDS)

The formation and maturation of synapses in the visual cortex of the rat. II. Quantitative analysis

We have examined quantitatively the formation and maturation of synapses in the visual cortex of the rat. The density of the total number of synapses (synapses per 100 micron2 neuropil) as well as the densities of Gray's type I and type II contacts were estimated from photographic montages of coronal strips of visual cortex from rats of various postnatal ages. Histograms of synaptic density as a function of depth were prepared, and the mean values of the postsynaptic density length and vesicle number per terminal were estimated for the two synapse types at each age examined. During the first few days of life, synapses were concentrated in the subplate region. By the latter part of the second postnatal week they were present throughout the cortex and an adult-like distribution, in which the highest densities were present in the superficial layers, was achieved by day 14. The postsynaptic density length of the type I synapses remained relatively unchanged during development but that of the type II synapses was more variable. Specifically, it was significantly longer during the second and third postnatal weeks compared to earlier ages and to adult values. The mean number of vesicles per terminal for the two synapse types increased with age until day 28. Subsequently, it only increased slightly between days 28 and 90 for the type I synapses but decreased significantly for the type II synaptic contacts. At all ages examined, type I synapses formed the majority of synaptic contacts. The developmental pattern appeared to differ for the two synapse types. The density of type I synapses increased continuously during the first three weeks and achieved a mean value close to that of adult animals by day 20. In contrast, the density of type II synapses did not increase significantly until day 6, increased dramatically in the second and third postnatal weeks, and then declined markedly between days 20 and 90. The observed decrease in the density of type II synaptic contacts is a clear example of synapse elimination in the visual cortex.

Gradients of cellular maturation and synaptogenesis in the superior colliculus of the fetal rhesus monkey

Light (LM) and electron microscopic (EM) qualitative and quantitative analyses were employed to determine the tempo and spatial gradients of synaptogenesis and cellular differentiation in the superficial superior colliculus (SC) of the rhesus monkey between embryonic (E) days E47 and E84. By E47, a majority of the neurons of the prospective superficial gray layer (SGS) have arrived at their final positions and contribute to a uniform band of small, darkly Nissl-stained neurons at the outer surface of the SC. By E54, cells in the middle of the rostral pole of the superficial SC become considerably larger, paler staining, and less densely packed than the more medially or laterally located cells. These regional differences, which extend posteriorly through about the middle of the SC at this age, are evident on both the LM and EM levels and were confirmed by a quantitative EM analysis of the cytodifferentiation and synaptogenesis in the SGS. Several overlapping EM probes made across the medial, middle, and lateral regions of the SGS at each of three coronal levels reveal consistently more developed neuropil and smaller amounts of extracellular space in the middle region than in the medial and lateral portions of the more anterior SC. Further, the densities of synapses, both in terms of synapses/micron2 of total cross-sectional area and synapses/micron2 of neuropil alone, are also higher in the middle than the peripheral regions. Most of the middle-peripheral differences found in the mid-E50s are still evident by the early E60s, but have disappeared by midgestation (E80s). The present results are interpreted to indicate that the middle region of the SGS at a given transverse level begins to mature significantly earlier than the medial or lateral areas. Since our previous 3H-thymidine analysis (Cooper and Rakic, '81a) failed to reveal significant regional variation in the time of neuron origin in the superficial SC, the observed spatiotemporal gradients of neuronal maturation in the primate SGS probably do not arise from underlying gradients of cellular proliferation.

Development of the dopaminergic innervation of the rat cerebral cortex. A light microscopic immunocytochemical study using anti-tyrosine hydroxylase antibodies

A precise topographical analysis of the distribution of tyrosine hydroxylase-like immunoreactive processes was performed in the frontal, cingular and parietal cortex of the rat during late embryonic and early postnatal life. Until birth, labeled processes were only observed in the restricted cortical areas known to receive a dopaminergic innervation in the adult brain. Their distribution differed markedly from that of noradrenergic fibers as identified by their dopamine-beta-hydroxylase-like immunoreactivity. Thus we considered TH-like immunoreactivity to be a selective marker of the cortical dopaminergic innervation during late fetal life, at least with the antibody we used. With this marker, dopaminergic fibers were first detected in the anterior frontal cortex at day 16 of embryonic life (E16). They developed as two bundles passing medially and laterally to the ventricular layer without penetrating it. From E20 on, the terminal fields extended to the cingular and rhinal cortex, still being restricted to the intermediate zone. No fibers were visible in the lateral and dorsal frontal cortex at this time, nor in the cortical plate and molecular layer in any cortical area. At E21, rare labeled fibers were seen in the molecular layer of the medial frontal and cingular cortex. After birth, the terminal fields of the TH-containing fibers extended further caudally in the cingular cortex and also superficially in the cortical plate. Moreover, labeled axons now also appeared in the lateral frontal and parietal cortex where their density gradually increased. At P14, two different patterns of distribution were observed: a high density of TH-positive fibers in the cortical areas known to receive a dopaminergic innervation; a low density of fibers in the other cortical areas which represented noradrenergic fibers. Indeed these TH-containing presumed noradrenergic fibers were absent at P14 following a bilateral destruction of the locus coeruleus in 4-day-old pups.

Characterization of monoaminergic terminals in the neostriatum of neonatal rats: an electron microscopic morphometric analysis

Monoaminergic (MA) boutons in the neostriatum of neonatal rats were studied by using 5-hydroxydopamine. We found: (a) bouton size averaged 0.6 micron in diameter; (b) terminal density was 68 boutons per 7200 micron2 area; (c) synaptic frequency was 33.2%; and (d) MA neurons constituted 9.2% of all boutons. This population was much lower than that reported in immature neocortex where MA neurons are a major innervation.

Fluorescence microscopy used in conjunction with horseradish peroxidase localization and electron microscopy for studying sympathetic nuclei of the rat spinal cord

After application of horseradish peroxidase (HRP) to the transected cervical sympathetic trunk, labeled sympathetic neurons were seen in the intermediolateral nucleus (IML), central autonomic nucleus (CAN) and intercalated nucleus of rat spinal cords. As studied by glyoxylic acid-induced histofluorescence microscopy (FM), catecholaminergic (CA) terminals were most densely packed in the IML. There were 60 +/- 2 CA varicosities per 2,200 mu2 area in 20 mu thick sections through the IML. A combined FM/HRP method confirmed that preganglionic sympathetic neurons are heavily innervated by CA terminals. CA boutons were tagged with 5-hydroxydopamine for EM identification, and in the IML, 56% of CA boutons were seen to make synaptic contacts as compared to 60% in the CAN. It seems likely that virtually all CA boutons may form synapses but serial sections through boutons were not studied. It is inferred that preganglionic sympathetic neurons of both IML and CAN are served by CA inputs, and electron microscopy has revealed that most or all of these CA terminals transmit their signals via synapses; which may permit more precise structural sorting than non-synaptic transmission.

Radioautographic investigation of monoaminergic neurons: an evaluation

This review outlines the most relevant contributions currently available on the detection and ultrastructural characterization of monoaminergic neurons by radioautography after administration of radiolabeled monoamines. It includes methodological considerations and then a critical analysis of the diagnostic value of the radioautographic method for catecholaminergic and serotoninergic neurons, emphasizing in particular its recent applications to the visualization of dopaminergic axon terminals. An attempt is then made to evaluate the method in terms of specificity, sensitivity and resolution and its possibilities with regard to quantitative analysis. Lastly, its value for approaching the dynamic and metabolic properties of monoaminergic neurons is stressed.

Effects of chronic reserpine treatment on development of maturity of the putamen in fetal rabbits

Developing nigrostriatal axons and their perikarya have substantial quantities of dopamine (DA) before the axons reach their postsynaptic target. In order to investigate possible developmental effects of these stores of DA, we have depleted DA chronically during critical periods in the ontogeny of the nigrostriatal system. Reserpine (0.04-0.14 mg/kg/day) was given repeatedly to maternal rabbits for various periods starting before neuroblasts of the substantia nigra first exhibit fluorescence until 2 days before term when the fetuses were sacrificed. Reserpine crossed the placenta and depleted DA in the fetal putamens. Control fetuses had widespread fluorescent axons and terminals. Counts of mature axonal boutons in electron micrographs of the putamen of reserpine-treated fetuses showed that there were 4.3 +/- 0.6 SE/100 microns2, which is less than 1/2 the control value of 10.2 +/- 0.6 SE/100 microns2 (p less than 0.001). The neuropil of the putamen of the reserpine-treated fetus was also less mature; the relative volume occupied by growth cones (40.5% +/- 5.7 SE) was twice that of controls (20.6% +/- 2.4 SE) (p less than 0.005). Although it remains to be shown that the delayed development of both pre- and postsynaptic elements of the nigrostriatal system is specifically related to the known ability of reserpine to deplete DA, the results are consistent with the hypothesis that early stores of DA may be important in developing dopaminergic systems.

Postnatal development of monoamine content and synthesis in the cerebral cortex of rhesus monkeys

The concentration and rates of synthesis of norepinephrine, dopamine and serotonin were determined by spectrophotofluorometric methods in various cytoarchitectonic areas of the cerebral cortex in 54 rhesus monkeys ranging in age from 1 day to 36 months. For most regions studied, norepinephrine levels exhibit steady increases from birth through 36 months while over the same period changes in dopamine concentration are more complex and variable, particularly in the frontal lobe. Among the 3 monoamines examined, endogenous serotonin content shows the least dramatic and most rapid development, reaching adult values between 2 and 5 months of age in most cortical regions. As a consequence of these developmental shifts, the relationship of monoamine levels in various cortical areas also changes with age. At maturity, however, norepinephrine concentration exceeds that of dopamine and serotonin in the cortex of the frontal and parietal lobes whereas serotonin levels are higher than norepinephrine in the occipital cortex. Changes in rates of synthesis of the catecholamines and serotonin generally parallel developmental changes in concentrations. The greatest increments in catecholamine synthesis occur in prefrontal and posterior association cortices. Smaller but significant increases in serotonin metabolism were measured in the parietal and visual cortex between birth and 36 months while in other areas of the cortex, age-related changes in serotonin synthesis were negligible. A consistent finding at all ages is that the distribution of catecholaminergic synthesis varies inversely with that of serotonergic synthesis, indicating substantial interaction in the regulation of the two cortical systems. The present findings demonstrate that in the rhesus monkey development of monoaminergic storage capacity and synthetic processes: (1) continues over a period of months and years; (2) is generally more rapid for serotonin than for catecholamines; and (3) varies greatly in different cytoarchitectonic regions of the cerebral cortex.

An immunohistochemical study of serotonin neuron development in the rat: ascending pathways and terminal fields

The ontogeny of the serotonergic axonal projections may be divided into three periods: one of initial axon elongation (E12-E16), the development of selective pathways (E15-E19) and terminal field development (E19-E21). All serotonergic axons that enter the prosencephalon ascend in the medial forebrain bundle From this bundle fascicles of immunoreactive axons enter several well-defined fiber tracts: specifically, the fasciculus retroflexus, stria medullaris, external capsule, fornix, and supracallosal stria. Axons from these pathways form terminal arborizations in the thalamus, hypothalamus, basal and limbic forebrain, and cerebral cortex. Serotonergic axons appear to be guided by pre-existing non-serotonergic tracts in reaching targets in the forebrain. Innervation of the cerebral cortex is a prolonged process extending from E19 through PND21. Axons enter directly into the marginal and intermediate zones of the immature cortex, at the medial, frontal and lateral edges of the hemisphere, and subsequently spread tangentially to cover the hemispheres. Terminal ramifications then arise from the bilaminar axons and fill in the middle cortical layers. This growth pattern gives rise to tangential and radial gradients in innervation density. While the growth of serotonin axons across the forebrain appears to be a continuous, sequential process, the development of terminal innervation is highly heterogeneous, occurring at different times and at different rates from region to region. Serotonergic axons do not innervate immature, primarily proliferative neuronal populations. The delay in serotonin innervation of the suprachiasmatic nucleus, striatum, and middle cortical layers long after the axons have reached these structures suggests that the formation of serotonin axon terminals is dependent on maturation of other elements in local neuronal circuitry.

The effect of neonatal 6-hydroxydopamine treatment on synaptogenesis in the visual cortex of the rat

It has been proposed repeatedly that the noradrenergic (NE) system may exert an influence on cortical development. We have tested this proposition by examining synaptogenesis in the visual cortex of rats whose NE afferents were selectively lesioned by injections of the neurotoxin 6-hydroxydopamine (6-OHDA). Control littermates were injected with equal volumes of vehicle. Montages of electron micrographs covering approximately 50 micrometers-wide strips of cortex were assembled from both groups of animals at 2,4,6,8,14, and 90 days of age. Synapse counts revealed a significantly higher density of synapses in the cortex of 6-OHDA-treated rats during the first week of postnatal life. The difference between the experimental and control rats was less apparent during the second postnatal week, and at day 90 the densities of synapses were similar for the two groups of animals. The enhanced density, which was the result of the increased number of Gray's type I synapses, was confined to the subplate region at day 2 but became more widespread in the cortex at subsequent stages of development. From these observations it would appear that the NE system exerts an inhibitory influence on synapse formation in the visual cortex in early postnatal life.

The structure of cerebral cortex in the rat following prenatal administration of 6-hydroxydopamine

The early, prenatal formation of noradrenergic projections to the forebrain has led to the proposition that these axons exert a trophic influence on cerebral cortex during ontogeny. To test this hypothesis, we have examined a number of different structural features of cortical development following prenatal lesions of the ascending noradrenergic axons. The parameters that were analyzed include cytoarchitecture, dendritic morphology, and the distribution of monoaminergic and nonmonoaminergic cortical afferents. Rat fetuses were administered the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) by transuterine, intraperitoneal injection on embryonic day 17. Vehicle-injected controls and fetuses treated with the catecholamine uptake inhibitor desmethylimipramine (DMI) prior to 6-OHDA were prepared. After reaching maturity (200-300 g), the brain of treated and control rats were examined using Nissl and Golgi preparations (for cytoarchitecture and dendritic morphology), histofluorescence (for monoaminergic afferents, especially dopaminergic axons), and serotonin and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Effective lesioning of the ascending noradrenergic system was confirmed in each case, using DBH immunocytochemistry. Prenatal treatment with 6-OHDA resulted in complete and long-lasting destruction of the noradrenergic innervation of the cerebral cortex, along with hyperinnervation of the diencephalon and brain stem. Despite the widespread denervation of cerebral cortex, no significant alterations in cytoarchitecture, dendritic morphology, or spine counts were found in treated brains. In particular, no abnormalities were observed in the apical dendrites of layer VI pyramidal cells, based on qualitative criteria. The distribution, density and morphology of serotonergic and dopaminergic afferents were unaffected. Thalamocortical afferents had developed normally as reflected by the cortical barrels. In 33% of the 6-OHDA-treated fetuses foci of ectopic neurons were found at the cortical surface. The ectopias contain neuronal processes, somata, and synapses interspersed with collagen and other connective tissue elements. While the ectopias may result from selective damage to the noradrenergic neurons, the finding of similar (but smaller) malformations in DMI-protected animals is equally consistent with a non-specific effect of 6-OHDA upon non-adrenergic cells. The examination of intervening stages will be needed to resolve this question. Based on the parameters of cortical structure analyzed in this study we conclude that the neocortex develops normally even in the absence of the noradrenergic system.

High resolution radioautography of central dopaminergic fibers labelled in vitro with [3H]dopamine or [3H]norepinephrine

An original method is reported for the radioautographic visualizing of dopaminergic axons terminals in the rat central nervous system. This method is based on an in vitro labelling with either tritiated norepinephrine or dopamine under conditions of specificity previously established by fluorescence histochemistry. The immobilization of [3H]catecholamines in their sites of uptake and storage during post-fixation for electron microscopic processing was ensured by the use of osmic acid vapours instead of osmic acid solution. Selective labelling of the dopaminergic fibers may be achieved throughout large brain sections and the technique should be applicable to human biopsy material.

Postnatal development of presynaptic alpha-adrenoceptors in rat cerebral cortex: studies with brain slices and synaptosomes

The development of alpha-adrenoceptor mediated presynaptic modulation of noradrenaline (NA) release in the rat cerebral cortex was studied during the first postnatal month by examining the effects of exogenous NA and phentolamine in K+-induced [3H]NA release from superfused slices or synaptosomes. Exogenous NA strongly inhibited [3H]NA release from cortex slices of newborn rats, indicating that presynaptic alpha-receptors exist already on the day of birth. In fact, at least up to and including the 17th day after birth, Na caused a stronger inhibition of [3H]NA release from neonatal than from adult cortex slices. However, with synaptosomal preparations the inhibitory effect of NA was found to be similar in neonatal and adult rats. Further analysis of the data obtained with brain slices suggested that the differences between neonatal and adult rats observed with regard to the presynaptic modulation of [3H]NA release could be ascribed to differences in the concentrations of released endogenous NA in the extracellular spaces of brain slices. This hypothesis was strengthened by the data obtained in experiments using cortex slices of adult rats in which the endogenous NA level was lowered by pretreatment with alpha-methylparatyrosine. In cortex slices of neonatal (7-day-old) rats the concentration of endogenous NA released upon stimulation with 26 mM K+ in the vicinity of presynaptic alpha-receptors was estimated to be about 10 nM, as compared to approximately 25 nM in adult cortex slices. In both cases NA (i.e. exogenous + endogenous) appeared to cause maximally about 80% inhibition of [3H]NA release from cortex slices.

Synaptic remodeling of serotonin axon terminals in rat agranular cerebellum

In order to assess the influence of the target zone on the synaptic modeling of central serotonin (5-HT) axons, the 5-HT innervation of the posterior vermal cortex was studied by high resolution radioautography in both normal and X-ray-induced agranular rat cerebella, following topical application of [3H]5-HT. Two major systems of 5-HT afferents were identified in normal cerebellar cortex: (1) typical mossy fibers confined to the granular layer and (2) fine beaded axons diffusely distributed through all layers. The density of this innervation was estimated to be approximately 240,000 varicosities/cu.mm of cortex. The labeled mossy terminals all established synaptic contacts with the dendrites of granule cells. In contrast, only 3% of the varicosities belonging to the 'diffuse system' exhibited active zones in single thin sections, implying that less than 9% were actually engaged in junctional synaptic relationships. In the agranular cerebellar cortex, all 5-HT terminals belonging to the so-called 'diffuse system'. Their density was more than 8 times higher than in normal rat (2 million/cu.mm of cortex), an increase accounted for by the smaller volume of the experimental cerebellum. Thirty-five per cent of these 5-HT varicosities were seen in synaptic contact, indicating that all established at least one junctional complex. Most of these synapses were made on the branchlet spines of Purkinje cell dendrites, but some were also observed on the dendritic shafts of Golgi cells. Thus, in the absence of granule cells, the 5-HT innervation of rat cerebellar cortex evolves from a mostly 'non-junctional' into an entirely 'junctional' input. This finding indicates that the territory of innervation can exert a determinant influence on the synaptic modeling of incoming 5-HT afferents.

An ultrastructural and biochemical analysis of norepinephrine-containing varicosities in the cerebral cortex of the turtle Pseudemys

The fine structure and norepinephrine content of small granular vesicle-containing profiles were studied in normal and norepinephrine-depleted cerebral cortex of the turtle, Pseudemys. The cortex was fixed for electron microscopy with the KMnO4 procedure of Koda and Bloom ('77), while the norepinephrine content was assayed wit the radioenzymatic method of Coyle and Henry ('73). Green fluorescent fibers have been described by Parent and Poitras ('74) as located almost exclusively in the outer half of the molecular layer in turtle cortex. Small granular vesicle-containing profiles are found down to 100 microns below the pial surface, but over 50% lie within 20 microns of the surface. Within the outer 100 microns of cortex, the frequency of labeled varicosities is 1.39/1,000 microns2. The average area of the norepinephrine-containing varicosities is 0.61 microns2, and there is a mean of 18.4 vesicles per single section. The average number of large plus small vesicles in an entire varicosity was estimated to be 72. Synaptic membranes are not well-preserved with KMnO4 fixation, but good examples were found of small granular vesicle-containing profiles forming both symmetrical and asymmetrical membrane differentiations. Only a small percentage of the small granular vesicle profiles were associated with a synaptic membrane differentiation in single sections. When norepinephrine-fiber synapses are seen, they usually share a postsynaptic element with another unlabeled vesicle-containing profile. Normal turtle cortex contains an average norepinephrine concentration of 1.95 micrograms/gr, which is about eight times higher than in rat cortex. The ratio of norepinephrine to dopamine is about 18 to one, suggesting that dopamine is present predominantly in a precursor pool for norepinephrine. Small granular vesicle-containing profiles were eliminated after treatment with reserpine and 6-hydroxydopamine in concentrations that were shown to reduce norepinephrine concentration by 94% and 86%, respectively. The labeled varicosities were partially depleted by midbrain hemisection and by an inhibitor of dopamine-beta-hydroxylase (FLA-63). The norepinephrine-containing varicosities are remarkably coextensive with the distribution of thalamic fibers, both in the total extent of cortex where they are found and in the depth of cortex where they terminate. The results support the idea that there is a close structural and functional association between locus coeruleus and thalamic fibers in cerebral cortex, and the apparent difference in frequency of synapses suggests that each fiber system exerts its influence on cortical cells in a different way.

Structural and biochemical changes in rat cerebral cortex after neonatal 6-hydroxydopamine administration

Newborn rats received an intracisternal injection of 6-hydroxydopamine (100 micrograms) within 16 h after birth. Treatment effects upon noradrenaline uptake (with or without desmethylimipramine pre-incubation), endogenous noradrenaline, dopamine, and serotonin were biochemically assayed. Noradrenaline uptake and endogenous noradrenaline content were permanently reduced to less than 5% of control values. Reduction of endogenous dopamine content was less marked: at day 60, values were about 40% of controls. Serotonin content remained unaffected. Cell density countings in postnatal day 15 temporal cortex revealed an about 16% reduction in layers II and III of treated animals. These modifications of cortical geometry were discussed with reference to measurements of cortical thickness and ultrastructural observations on postnatal days 2, 5 and 15. Both supranormal involution and growth processes might result from the neurotoxin treatment. Whereas some of the degeneration processes might be due o general cytotoxic effects, this is less likely for the supranormal growth processes.

Cytology and time of origin of interstitial neurons in the white matter in infant and adult human and monkey telencephalon

The fine structure, synaptic relationships, distribution and time of origin of interstitial neurons situated within the white matter subjacent to the visual, somatosensory and motor cortices were studied in the human and monkey telencephalon. The analysis was carried out on Nissl-stained serial sections, rapid Golgi impregnations, by acetylcholinesterase (AChE) histochemistry, electron microscopy and [3H]thymidine ([3H]TdR) autoradiography. Interstitial neurons have a similar distribution, morphology and histochemistry in both human and monkey telencephalon. Their highest density and the most extensive distribution is found in the neonatal period in both species. The number of interstitial neurons decreases during infancy, but numerous cells remain in the adult. Two types of interstitial neuron can be recognized in Golgi preparations: polymorphic cells, usually situated close to the cortex and fusiform cells, located predominantly in the depths of the white matter. The polymorphic cell type is prevalent during neonatal and infant stages, while fusiform cells are relatively more numerous in the adult. Interstitial cells have ultrastructural features and organelles typical of neurons of the central nervous system with well-defined axosomatic and axodendritic synapses of both symmetrical and asymmetrical types. About 20% fo the interstitial cells show strong specific AChE activity. Autoradiographic analysis of postnatal monkeys exposed to [3H]TdR at various embryonic (E) and early postnatal days indicates that interstitial neurons which lie beneath the visual and somatosensory-motor cortices are generated between E38 and E48. Contrary to the prevailing notion that interstitial neurons are the latest generated cells arreste during migration across the maturing white matter, they prove to be produced at the end of the first third of the 165-day gestation in the rhesus monkey concomitantly with the generation of neurons destined for the deep neocortical layers. These findings raise the possibility that interstitial cells represent a vestige of the transient embryonic subplate layer.

Development of neocortical circuitry: quantitative ultrastructural analysis of putative monoaminergic synapses

The distribution, numbers and morphology of presumed monoaminergic (MA) synapses were examined in somatosensory cortex of neonatal rats and mice (newborn to 16 days of age). MA synapses were identified using an ultrastructural cytochemical marker, 5-hydroxydopamine (5-OHDA), which results in the appearance of small granular vesicles (SGV) in their presynaptic terminals. From birth to 7 days of age, 20--30% of all synapses sampled in somatosensory cortex contain SGVs. However, few SGV synapses are seen in 8-day-old cortex and by 12 days of age, SGVs are no longer detectable in cortex. A specific laminar distribution for these SGV synapses -- which is distinct from the overall synaptic distribution -- is first seen at 3 days of age and is essentially unchanged until 7 days postnatally. During this entire period, the SGV synapses predominate in the primordium of layer IV, where they account for 50--70% of all synapses. Morphometric analysis of SGV synapses indicates that there are differences in junctional symmetry, vesicle shape and configuration of the contact zone between SGV and non-SGV synapses, as well as between SGV synapses themselves in the various cortical layers. The laminar distribution and morphological characteristics of SGV synapses suggest that the MA projection to neocortex exhibits a high degree of spatial specificity during its ingrowth. Also, the relatively high proportion of SGV synapses in the first postnatal week may reflect a potent influence exerted by the MA inputs on immature neocortex. The decreased numerical density of SGV synapses after 7 days of age is probably due to the development of the blood-brain barrier to 5-OHDA.

Noradrenergic innervation of cerebral cortex: widespread effects of local cortical lesions

The trajectory of the intracortical noradrenergic fibers has been characterized by histochemical analysis following the production of cortical lesions in the rate. A large group of noradrenergic fibers enters the cortex at the frontal pole and proceeds caudally through the deep layers of dorsolateral cortex. Branches arise from these longitudinally directed fibers and form a uniform pattern of innervation throughout lateral cortex. Because these fibers travel long distances rostrocaudally within the gray matter, a large area of cortex can be deprived of noradrenergic innervation by a relatively small lesion of frontal cortex. The medial and lateral cortex can be selectively and differentially denervated of noradrenergic fibers, and there is a medial to lateral topographic relationship between deep longitudinally running fibers and overlying cortex.

Development of neocortical circuitry: histochemical localization of acetylcholinesterase in relation to the cell layers of rat somatosensory cortex

Acetylcholinesterase (AchE) was histochemically localized in neocortex cerebri of newborn to 1-week-old rats. At birth AchE-dependent staining is limited to scattered somata (Cajal-Retzius cells) in the marginal zone and a few fibers and somata in primordial layer VI. By the end of the first week, neuronal elements are relatively well stained in particular cell laminae, giving the appearance of three horizontal AchE-rich "bands" which alternate with AchE-poor laminae. The subpial band (layer I) is a narrow tangential zone of intensely staining fibers and scattered somata. The mid-cortical band contains an AchE-positive fiber plexus (primordial layer IV) and numerous stained somata (pyramids of primordial layer V). In layer IV of the SmI region, intermittent foci of staining are noted which overlap the barrels' distribution in the barrel field. The deep cortical band (bottom of primordial layer VI) consists of numerous stained somata (Martinotti cells). It is concluded that there is a laminar pattern of acetylcholinesterase-dependent staining in postnatal rat somatosensory cortex, and that the laminar pattern bears a consistent spatial relation both to the cell layers and to the depths of high synapse density in immature cortex.