Characterization of the monoaminergic innervation of immature rat neocortex: a histofluorescence analysis

Modulators in concert for cognition: modulator interactions in the prefrontal cortex

Research on the regulation and function of ascending noradrenergic, dopaminergic, serotonergic, and cholinergic systems has focused on the organization and function of individual systems. In contrast, evidence describing co-activation and interactions between multiple neuromodulatory systems has remained scarce. However, commonalities in the anatomical organization of these systems and overlapping evidence concerning the post-synaptic effects of neuromodulators strongly suggest that these systems are recruited in concert; they influence each other and simultaneously modulate their target circuits. Therefore, evidence on the regulatory and functional interactions between these systems is considered essential for revealing the role of neuromodulators. This postulate extends to contemporary neurobiological hypotheses of major neuropsychiatric disorders. These hypotheses have focused largely on aberrations in the integrity or regulation of individual ascending modulatory systems, with little regard for the likely possibility that dysregulation in multiple ascending neuromodulatory systems and their interactions contribute essentially to the symptoms of these disorders. This review will paradigmatically focus on neuromodulator interactions in the PFC and be further constrained by an additional focus on their role in cognitive functions. Recent evidence indicates that individual neuromodulators, in addition to their general state-setting or gating functions, encode specific cognitive operations, further substantiating the importance of research concerning the parallel recruitment of neuromodulator systems and interactions between these systems.

Structural Effects and Neurofunctional Sequelae of Developmental Exposure to Psychotherapeutic Drugs: Experimental and Clinical Aspects

The advent of psychotherapeutic drugs has enabled management of mental illness and other neurological problems such as epilepsy in the general population, without requiring hospitalization. The success of these drugs in controlling symptoms has led to their widespread use in the vulnerable population of pregnant women as well, where the potential embryotoxicity of the drugs has to be weighed against the potential problems of the maternal neurological state. This review focuses on the developmental toxicity and neurotoxicity of five broad categories of widely available psychotherapeutic drugs: the neuroleptics, the antiepileptics, the antidepressants, the anxiolytics and mood stabilizers, and a newly emerging class of nonprescription drugs, the herbal remedies. A brief review of nervous system development during gestation and following parturition in mammals is provided, with a description of the development of neurochemical pathways that may be involved in the action of the psychotherapeutic agents. A thorough discussion of animal research and human clinical studies is used to determine the risk associated with the use of each drug category. The potential risks to the fetus, as demonstrated in well described neurotoxicity studies in animals, are contrasted with the often negative findings in the still limited human studies. The potential risk fo the human fetus in the continued use of these chemicals without more adequate research is also addressed. The direction of future research using psychotherapeutic drugs should more closely parallel the methodology developed in the animal laboratories, especially since these models have already been used extremely successfully in specific instances in the investigation of neurotoxic agents.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Noradrenergic innervation of the developing and mature visual and motor cortex of the rat brain: a light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation of the developing and adult visual and motor cortex of the rat was examined with light and electron microscopic immunocytochemistry by using antibodies against dopamine-beta-hydroxylase. At birth, NA fibers were present in both cortical areas, appearing as two tangential streams, one above and the other below the cortical plate. During the subsequent weeks, these two streams arborized gradually innervating all cortical layers. The adult pattern of distribution was attained by postnatal day 14, but the density of innervation, which was higher in the motor than in the visual cortex, appeared similar to the adult by the end of the third postnatal week. Electron microscopic analysis revealed that a low proportion of NA varicosities (the highest value was 12% in the adult motor cortex in single sections) were engaged in synaptic contact, throughout development, in both areas examined. The overwhelming majority of these synapses were symmetrical, involving predominantly small or medium dendrites. This evidence suggests that transmission by diffusion is the major mode of NA action in the developing and adult cerebral cortex. Noradrenaline released in the rare synaptic junctions may act mainly to reduce the activity of its cortical targets. The results altogether provide morphologic evidence for an involvement of noradrenaline in the development of the neocortex and, along with earlier data on the serotonergic system, indicate that the monoaminergic systems are endowed with a specific anatomic organization in various areas of the brain.

Effects of ethanol on development of locus coeruleus brain stem transplants in oculo

In this investigation, we studied the effects of ethanol (EtOH) on the development of noradrenergic (NE) neurons of the locus coeruleus. Fetal brainstem tissue from embryonic days 15-17 was grafted into the anterior chamber of the eye of adult rats. Two different experimental groups were exposed to 16% EtOH in the drinking water during different developmental windows. The first group received EtOH 24 h after transplantation and during the whole experimental period of 7 weeks (continuous EtOH), and the second group only during the last 5 weeks of the experimental period (delayed EtOH). The control group received water ad libitum. After 7 weeks, all the animals were sacrificed and morphological evaluations were performed. Immunohistochemical analysis showed that axon bundle formation and NE fiber outgrowth into the host iris was significantly reduced in the continuous EtOH-treated group compared to controls. We also studied the morphology of TH-positive neurons and processes in the intraocular transplants. A significant decrease in TH-positive staining intensity was observed in the continuous EtOH-treated group compared to controls. Moreover, we found a significant decrease in cell size and neuronal survival in both EtOH-treated groups compared to controls. The present results suggest that chronic EtOH exposure during development leads to an altered axonal outgrowth and decreased cell sizes and number of NE neurons in intraocular brain stem grafts. Furthermore, we found that NE neurons are more sensitive to EtOH exposure during the last prenatal days and the first postnatal week of development, compared to a later developmental period.

New evidence for neurotransmitter influences on brain development

The early appearance of monoamine systems in the developing mammalian CNS suggests that they play a role in neural development. We review data from two model systems that provide compelling new evidence of this role. In one model system-in utero exposure to cocaine-specific and robust alterations are seen in dopamine-rich areas of the cerebral cortex, such as the anterior cingulate cortex: D1 receptor-G protein coupling is greatly reduced, the GABAergic system is altered and pyramidal dendrites undergo excessive growth. In a second model system-a transgenic mouse line in which the gene that encodes monoamine oxidase A (MAOA) is disrupted, resulting in excessively high 5-HT levels-barrels fail to form in the developing somatosensory cortex. Both models reveal the effects of very early manipulation of monoamines on forebrain development, and the long-term anomalies that persist into adulthood.

Dopamine receptors mediate differential morphological effects on cerebral cortical neurons in vitro

A morphogenic role of neurotransmitters during cellular differentiation in vitro has been demonstrated in recent years. Using in situ hybridization, we confirm the presence of the D1 receptor at E16 and show additionally that the transcript is relatively widespread and present in both proliferative and differentiating areas of the cerebral wall. Because DA receptor expression precedes the arrival of presynaptic terminals during forebrain development, we examined the role of DA in cerebral cortical neuron differentiation in vitro, using immunohistochemical markers of dendrites, microtubule-associated-membrane protein 2 (MAP2) and axons, neurofilament protein (NF-H). Neurite length, cell size, and cell viability in response to D1 and D2 receptor agonists SKF38393 and quinpirole, respectively, and to DA were analyzed in neurons obtained from embryonic (E) day 16 rats. We have shown that 1) paradoxically, DA at different concentrations can either stimulate or inhibit neurite outgrowth; 2) there is a bimodal pattern of DA-induced axonal outgrowth, i.e., at low and high doses; 3) D2 receptor activation induces neurite outgrowth while D1 receptor activation is inhibitory; 4) D2-mediated neurite elongation is preferentially axonal while D1 receptor activation reduces both axonal and dendritic outgrowth; 5) low doses of DA promote the expression of cytoskeletal components of axonal maturation; and 6) D1 receptor activation decreases neuronal size. We suggest that DA may influence cellular differentiation and circuitry formation early in development of the cerebral cortex through receptor-mediated effects on process outgrowth, which could lead to effects on circuit formation.

Postnatal migration of neurons and formation of laminae in rat cerebral cortex

Migration of neurons and formation of laminae in the developing neocortex were studied by means of thymidine autoradiography. Timed pregnant rats received a single pulse injection of [3H]thymidine in the morning of embryonic day (E)13, 14, 15, 16, 17, 18 or 19. Pups were killed on postnatal day (P)0, 1, 2, 3, 4, 6, 10, 30, or 60 and brains were processed for autoradiography. Neurons in posterior (visual) cortical areas labeled by [3H]thymidine administration on E13 or E14 were found predominantly in the cortical subplate; cells labeled on E15 in layer VI; cells labeled on E16 in layers VI and V, cells labeled on E17 in layers V and IV; E18 in layers IV and III; and E19 in layers III and II. By the day of birth (P0), neurons labeled from E13-16 injections were already in their mature laminae in cortex. Many of the cells labeled on E17 were still situated within the cell-dense cortical plate (CP) at P0, and within layer V by P1. Cells labeled on E18 were found in the most superficial part of the CP on P0, in the deep part of the CP on P1, and formed layer IV on P2 and P3. At P0, many E19 labeled cells appeared to be in migration to the cortex and were found in the CP on P1, in layer III by P4, and in layer II by P6. Cells in the auditory cortex labeled by [3H]thymidine injections on a particular day were situated more superficially than comparable labeled cells in the visual cortex, indicating a lateral to medial gradient in which the auditory cortex is formed earlier than the visual cortex. Distributions of labeled cells in the somatosensory cortex were similar to those in the visual cortex. These data provide a detailed and comprehensive description of the position of varied populations of cortical neurons during the early postnatal period, as well as a description of the formation of cortical laminae at times when major systems of afferents are growing into the cortex and making synaptic connections with their target cells.

Differential development of the dual serotoninergic fiber system in the cerebral cortex of the cat

The changes in distribution and density of the serotoninergic innervation of the cerebral cortex were studied in kittens from birth (PO) to 60 days of age (P59). Three cortical areas were sampled: prefrontal, primary auditory, and primary visual areas. Two systems of serotoninergic axons were demonstrated by immunocytochemical techniques: the fine axon system characterized by small fusiform varicosities up to 1 micron in diameter, and the beaded axon system, the fibers of which have round varicosities up to 5 microns in diameter. The density of the two types of fibers across the cortical layers at different ages was measured with a semiautomatic computerized system. In all three areas, the density of fine axons increased steadily from birth, although the pattern of innervation changed from an even distribution at PO to a distinct concentration of the fibers in layers I-III by week 2 in the prefrontal cortex and by week 3 in auditory and visual cortices. By contrast, the beaded axons first appeared in the cortex at week 2 for the prefrontal cortex, at week 3 in auditory and visual areas. Initially, these fibers were distributed throughout all cortical layers and were of much lower density than the fine axons. At later ages the beaded axons became confined to layers I-III where they gradually increased in number, and from week 4, they formed pericellular arrays which were only observed in the prefrontal and auditory cortices, not in visual cortex. These findings provide further evidence for the existence of two parallel subsystems of serotoninergic axons which are different not only in their morphology and nuclear origin, but also in their development. Our finding that the two serotoninergic fiber systems arrive in the cortex in two different stages suggests that they have differential roles in development. The late formation of the pericellular arrays indicates that the formation of the specific connections made by the beaded fibers could be dependent on a certain degree of maturity of the target neurons.

The source of the transient serotoninergic input to the developing visual and somatosensory cortices in rat

For approximately the first two weeks of life, dense serotonin immunoreactivity closely matches the pattern of thalamocortical axons innervating both the granular portion of the primary somatosensory cortex and area 17 in rodents [D'Amato et al. (1987) Proc. natn. Acad. Sci. 84, 4322-4326; Fujimiya et al. (1986) J. comp. Neurol. (1986) 246, 191-201; Rhoades et al. (1990) J. comp. Neurol. 293, 190-207]. This serotonin immunoreactivity is not contained in thalamocortical axons [Rhoades et al. (1990) 293, 190-207] but its source has never been demonstrated. In the present study, a variety of approaches were used to address this issue. The combination of electron microscopy and immunocytochemistry showed that all serotonin immunoreactivity in the developing cerebral cortex was contained in axons and that the terminals of many of these fibers made synapses with the dendrites of cortical cells. Treatment with fluoxetine, a specific inhibitor of serotonin uptake, did not result in a loss of the cortical pattern of serotonin immunoreactivity, indicating that immunoreactive fibers were not labeled solely as a result of serotonin uptake. The combination of retrograde tracing from the primary somatosensory cortex and area 17 with immunocytochemistry demonstrated numerous double-labeled cells in nucleus raphe dorsalis and the median raphe nucleus. Smaller numbers of double-labeled neurons were located in the B9 cell group and the region of the lateral midbrain tegmentum. Large electrolytic lesions that included most of the nucleus raphe dorsalis and median raphe nucleus, but which left the B9 group and more caudal serotoninergic cells undamaged, caused either a substantial reduction in density or complete disappearance of the serotonin pattern in both hemispheres. Unilateral electrolytic lesions of the medial forebrain bundle resulted in a loss of the pattern only on the side of the damage. Injection of the neurotoxin 5,7-dihydroxytryptamine directly into the mesencephalon either abolished or substantially reduced the density of the cortical serotonin immunoreactivity. Injections that produced substantial cell loss in the median raphe nucleus, but only minor cell loss in the nucleus raphe dorsalis had little effect upon the cortical pattern of serotonin immunoreactivity. These results indicate that the dense serotonin immunoreactivity which appears transiently in the visual and somatosensory cortices of perinatal rodents is contained in serotoninergic axons that arise from cells in the nucleus raphe dorsalis and perhaps also the median raphe nucleus.

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.

A fluorescence histochemical method for the demonstration of central catecholamine neurons in young embryonic tissues

A simple fluorescence histochemical technique is described for the production of catecholamine histofluorescence in the developing brain of rat embryos. The procedure combines the magnesium-formaldehyde-glutaraldehyde perfusion technique, with polyethylene glycol dehydration and embedding. Several examples illustrate the sensitivity of the technique and the good preservation of tissue sections. The tissue can also be stored for several weeks without an appreciable loss of fluorescence.

Comparative distributions of dopamine D-1 and D-2 receptors in the cerebral cortex of rats, cats, and monkeys

The distributions and laminar densities of cerebral cortical dopamine D-1 and D-2 receptors were studied in rats, cats, and monkeys. Distributions were determined by using alternate, adjacent tissue sections processed for D-1 and D-2 receptor subtypes and compared to an adjacent, nearly adjacent, or similar sections stained for Nissl substance. [3H]-SCH 23390 and [3H]-spiroperidol (in the presence of 100 nM mianserin) were used to label the D-1 and D-2 receptors, respectively. The regional distribution and laminar density of dopamine receptors were determined by in vitro quantitative autoradiography and video densitometry of selected isocortical and peri-allocortical regions. Granular (prefrontal, primary somatosensory, and primary visual), agranular (primary motor and anterior cingulate), and limbic (entorhinal and perirhinal) cortices were examined. Where possible, homologous areas among the species were compared. The D-1 receptor was present in all regions and laminae of the cerebral cortex of rats, cats, and monkeys. The regional densities for the D-1 receptor were higher in the cat and monkey than in the rat. The rat D-1 receptor displayed a relatively homogeneous laminar pattern in most regions except that the deeper laminae (V and VI) contained more receptors than the superficial layers. The cats and monkeys, however, had distinctly heterogeneous laminar patterns in all regions of cortex that varied from one region to another and were quite different from that seen in the rat. The cats and monkeys had highest densities of the D-1 receptor in layers I and II and lowest densities in layers III and IV, whereas layers V and VI were intermediate. The density of D-1 receptors was greater than the density of D-2 receptors in all regions and laminae of cerebral cortex of the cat and monkey and greater in most regions and laminae of the rat cerebral cortex. The D-2 receptor was also distributed in all regions of the cerebral cortex of rats, cats, and monkeys. The D-2 receptor was very homogeneous in its regional distribution and laminar pattern compared to the D-1 receptor in all 3 species. The D-2 receptor was denser in the superficial layers (I and II) of the cortex than in the deeper layers in the rats, but more homogeneous in the different laminae of the cat and monkey cerebral cortex. The rat cortical D-2 receptor exceeded the D-1 receptor in restricted laminae of selective regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Two types of locus coeruleus neurons born on different embryonic days in the mouse

Retrograde axonal tracing studies were performed in combination with tritiated thymidine cell birthday analyses in order to determine whether or not any hodologicotemporal gradients exist in neuron genesis within the murine locus coeruleus. Following injections of retrograde tracers within the forebrain or cerebellum in mice exposed in utero to the radiolabeled nucleoside on embryonic days 9-11 (E9-11), combined histochemical and autoradiographic preparations revealed: 1) Locus coeruleus neurons that give rise to long distance axonal projections to the cortices are born exclusively on E9 (other studies indicate that these cells are noradrenergic); and 2) Locus coeruleus cells born on E10 and E11 are a class of smaller cells which were never observed to project to distant structures. The transmitters of these apparent local circuit neurons have not yet been determined, but gamma aminobutyric acid is one possible candidate. These findings support the interpretation that monoaminergic neurons tend to arise earlier during development than non-monoaminergic neurons within the locus coeruleus, and that distinctly different connectional arrangements exist for these monoaminergic and non-monoaminergic cells.

The development of basal forebrain projections to the rat visual cortex

The development of the basal forebrain projections to the visual cortex of the rat were studied using retrograde tracer techniques. Injections of wheat germ agglutinin-horseradish peroxidase placed in the visual cortex of newborn animals resulted in labelling of neurons throughout the basal forebrain nuclei. Although at this time the overall distribution of retrogradely labelled cells within the basal forebrain appeared similar to that seen in the adult, cells were smaller and weakly stained. It was only at the end of the second postnatal week that the somata of stained neurons showed sizes and staining intensity comparable to the adult. This precedes or coincides with the reported significant increases in cortical and basal forebrain ChAT activity and the first detection of ChAT-labelled fibres in this cortical area. These data suggest an important developmental point around the end of the second postnatal week that may correspond to the time when a significant number of cholinergic axons first appear within the visual cortex. They also suggest that the cholinergic projections to the visual cortex develop late in comparison with the thalamic and other subcortical afferents in this cortical area.

Regional and laminar distribution of the dopamine and serotonin innervation in the macaque cerebral cortex: a radioautographic study

The regional density and laminar distribution of dopamine (DA) and serotonin (5-HT) afferents were investigated in the cerebral cortex of cynomolgus monkeys using a radioautographic technique that is based on the high affinity uptake capacity of these aminergic neurons. Large vibratome sections, 50 micron thick, were incubated with [3H] DA (0.2 microM) and desipramine (5 microM) or with unlabeled norepinephrine (5 microM) and [3H] 5-HT (0.6 microM), which allowed for the specific labeling of the DA and 5-HT innervations, respectively. After fixation, these sections were dried, defatted, and radioautographed by dipping. Semiquantitative data on the DA innervation also were provided by counting [3H] DA-labeled axonal varicosities in radioautographs from 4-micron-thick sections of the slices obtained after epon embedding. The DA innervation was widespread and differed in density and laminar distribution in the agranular and granular cortices. DA afferents were densest in the anterior cingulate (area 24) and the motor areas (areas 4, 6, and supplementary motor area [SMA]). In the latter they displayed a trilaminar pattern of distribution, predominating in layers I, IIIa, and V-VI, with characteristic cluster-like formations in layer IIIa, especially in the medial part of motor areas. In the granular prefrontal (areas 46, 9, 10, 11, 12), parietal (areas 1, 2, 3, 5, 7), temporal (areas 21, 22), and posterior cingulate (area 23) cortices, DA afferents were less dense and showed a bilaminar pattern of distribution, predominating in the depth of layer I and in layers V-VI; density in layers II, III, and IV was only 20% of that in layer I. The lowest density was in the visual cortex, particularly in area 17, where the DA afferents were almost restricted to layer I. The density of 5-HT innervation was generally greater than that of DA except in the motor areas and in the anterior cingulate cortex. Region-specific laminar patterns characterized (1) motor areas where a lower density in layer III contrasted with the clusters of DA axons in the same layer; (2) the primary visual cortex (area 17), where two bands of higher density in layers III-IV and layer V outlined a poorly innervated zone in layer IVc-beta; (3) the peristriate area 18, where the 5-HT network was relatively loose but with a denser band in layer III. Thus, DA innervation of the cerebral cortex displays major differences between rodents and primates, characterized by expanded cortical targets and by a highly differentiated laminar distribution.(ABSTRACT TRUNCATED AT 400 WORDS)

Redistribution of synaptic vesicle antigens is correlated with the disappearance of a transient synaptic zone in the developing cerebral cortex

To examine the distribution of synaptic vesicle antigens during development of the cerebral cortex, antibodies against synapsin I and p65 were used on sections of cat cerebral cortex between E40 and adulthood. In the adult, the layers of the cerebral cortex are immunoreactive for each of these antigens, while the white matter is free of staining. In contrast, the fetal and neonatal pattern of immunostaining is reversed: the cortical plate (future cortical layers) is devoid of immunoreactivity, while the marginal (future layer 1) and the intermediate zones (future white matter) are stained. Electron microscopic immunohistochemistry shows that immunolabeling is associated with presynaptic nerve terminals in the adult and during development. These observations suggest that during development the white matter is a transient synaptic neuropil and that a global redistribution of synapses takes place as the mature pattern of connections within the cerebral cortex emerges.

A fibronectin-like molecule is present in the developing cat cerebral cortex and is correlated with subplate neurons

The subplate is a transient zone of the developing cerebral cortex through which postmitotic neurons migrate and growing axons elongate en route to their adult positions within the cortical plate. To learn more about the cellular interactions that occur in this zone, we have examined whether fibronectins (FNs), a family of molecules known to promote migration and elongation in other systems, are present during the fetal and postnatal development of the cat's cerebral cortex. Three different anti-FN antisera recognized a single broad band with an apparent molecular mass of 200-250 kD in antigen-transfer analyses (reducing conditions) of plasma-depleted (perfused) whole fetal brain or synaptosome preparations, indicating that FNs are present at these ages. This band can be detected as early as 1 mo before birth at embryonic day 39. Immunohistochemical examination of the developing cerebral cortex from animals between embryonic day 46 and postnatal day 7 using any of the three antisera revealed that FN-like immunoreactivity is restricted to the subplate and the marginal zones, and is not found in the cortical plate. As these zones mature into their adult counterparts (the white matter and layer 1 of the cerebral cortex), immunostaining gradually disappears and is not detectable by postnatal day 70. Previous studies have shown that the subplate and marginal zones contain a special, transient population of neurons (Chun, J. J. M., M. J. Nakamura, and C. J. Shatz. 1987. Nature (Lond.). 325:617-620). The FN-like immunostaining in the subplate and marginal zone is closely associated with these neurons, and some of the immunostaining delineates them. Moreover, the postnatal disappearance of FN-like immunostaining from the subplate is correlated spatially and temporally with the disappearance of the subplate neurons. When subplate neurons are killed by neurotoxins, FN-like immunostaining is depleted in the lesioned area. These observations show that an FN-like molecule is present transiently in the subplate of the developing cerebral cortex and, further, is spatially and temporally correlated with the transient subplate neurons. The presence of FNs within this zone, but not in the cortical plate, suggests that the extracellular milieu of the subplate mediates a unique set of interactions required for the development of the cerebral cortex.

Noradrenaline neuron plasticity in developing rat brain: effects of neonatal 6-hydroxydopamine demonstrated by dopamine-beta-hydroxylase immunocytochemistry

The present study was conducted to assess the morphological changes produced by neonatal administration of 6-hydroxydopamine (6-OHDA) in the noradrenergic innervation of the developing and adult rat brain. As demonstrated by dopamine-beta-hydroxylase (DBH) immunohistochemistry, the major alterations are the following. First, neocortical and hippocampal noradrenergic innervation is permanently eliminated by the treatment, with lesser effects on other telencephalic structures. These changes appear by postnatal day 5 and are permanent in nature. In adult treated animals, most thalamic nuclei are hyperinnervated by DBH-immunoreactive axons as are the cerebellum and a number of brainstem nuclei. The hyperinnervation of these structures occurs after postnatal day 20, and is extremely specific, with the pattern of organization and distribution of noradrenergic axons in treated animals identical to that of controls. In contrast, the noradrenergic innervation of the hypothalamus is relatively unaffected by 6-OHDA treatment. The principal exception is the development of an anomalous plexus of DBH immunoreactive axons in the lateral hypothalamus. The timing and organization of the changes produced by neonatal 6-OHDA administration are consistent with the hypothesis that noradrenergic neurons, and particularly those of the locus coeruleus, are programmed to produce a defined amount of axon and terminal field, with any developmental loss resulting in a 'pruning effect' such that the total terminal field appears conserved. Given the specificity of the hyperinnervation, inductive influences from the target nuclei probably play a major role in determining the pattern of the noradrenergic innervation.

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)

Postnatal development of serotonin nerve fibers in the somatosensory cortex of mice studied by immunohistochemistry

Postnatal serotonin (5HT) innervation in the cerebral cortex of mice has been studied by 5HT immunohistochemistry. 5HT-like immunoreactive (5HT-LI) nerve fibers and terminals appeared to increase transiently, particularly in the somatosensory (Sm) cortex during early postnatal days. As pups grow, 5HT afferent inputs decreased rapidly to reach a similar pattern of distribution to that in adult animals. Since the transient increase was seen at a critical period (seventh postnatal day) for the differentiation of layer IV, it is suggested that increased 5HT concentrations might have an effect on thalamocortical inputs and/or cortical lamination of the developing brains.

Events governing organization of postmigratory neurons: studies on brain development in normal and reeler mice

The purpose of the present work is to examine some of the mechanisms responsible for the early architectonic differentiation of the central nervous system, as well as for the abnormal development which occurs in certain hereditary malformations. In order to approach these questions, the embryonic development of the cerebral cortex, the cerebellum, the inferior olivary complex and the facial nerve nucleus has been studied in normal and reeler mutant mice, using morphological methods. The adult reeler phenotype is characterized not only by extreme laminar abnormalities of cell positioning in the telencephalic and cerebellar cortices, but also by relatively less extreme, though distinct abnormal architectonics in non-cortical structures such as the inferior olive and the facial nerve nucleus. Study of the embryonic development of these structures reveals that neurons are generated at the normal time and migrate along normal pathways. Moreover, the processes of directional axonal growth, differentiation of class specific features of neurons and glia, and synaptogenesis appear similar in both genotypes and are probably not directly affected by the reeler mutation. However, in all instances, the early architectonic organization achieved by reeler cortical, Purkinje, olivary or facial neurons at the end of their migration is consistently less regular than in normal embryos. In addition, these anomalies become amplified during the later developmental period. This evidence for the early appearance of abnormalities in reeler embryos indicates that the disposition of neurons at maturity cannot be exclusively regarded as secondary to the maturation of cells, neurites and connections, but is contingent upon a specific mechanism. One may infer that the presence of a normal allele at the reeler locus is necessary for the normal completion of this histogenetic step, which consequently is submitted to genetic control. Although the factor(s) responsible for the stable configuration of the early architectonics is unknown, various hypotheses are considered. Several lines of evidence are presented which argue against a major role being played by diffusible factors, mesodermal components and afferent fiber systems. Two mechanisms are considered particularly worth evaluating: (1) a diminution of relative adhesivity between neurons and radial glial fibers at the end of migration, and (2) a stabilization of neuronal configuration by selective recognition-adhesion among postmigratory neurons. The reeler gene could, directly or indirectly, affect these cell-cell interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Norepinephrine levels in developing pigeon brain: effect of monocular deprivation on the Wulst noradrenergic system

The endogenous level of norepinephrine (NE) was measured in discrete brain areas of the pigeon during post-hatching development. The pontine tegmentum showed the highest NE content, which remained constant during the post-hatching period. On the contrary, the NE content in the Wulst and cerebellum gradually decreased from hatching to 6 days. After this period, the Wulst NE level did not change significantly. In fact, there was no significant difference between NE values at 6 days and those at 6 months of age. In contrast, the difference between the cerebellar NE level at 6 days and that at the adult stage was highly significant. The NE content in the Wulst could be related to noradrenergic afferents originating in the ipsilateral locus coeruleus and substantia grisea centralis, since an electrolytic lesion of the pontine tegmentum caused a 60% reduction in the NE level in the ipsilateral Wulst. In line with the hypothesis that NE plays an important role in cortical plasticity, effects of early monocular deprivation on the Wulst NE content were also observed. After monocular deprivation during the first 6 months of life, the NE level increased by 40% in the Wulst ipsilateral to the deprived eye in comparison to the other side, where the NE level was normal. Monocular deprivation performed in adult animals did not affect the NE content in the Wulst. These results indicate that noradrenergic systems in the Wulst are affected by early, but not late visual deprivation.

Mass fragmentographic analysis of monoamine metabolites in the spinal cord of rat after the administration of morphine

A mass fragmentographic method was used in which homovanillic acid (HVA), methoxyhydroxyphenylglycol (MHPG), and 5-hydroxyindoleacetic acid (5-HIAA) were measured from a single sample. The results describe the effect of morphine on the metabolism of the major monoamines, dopamine (DA), noradrenaline (NA), and 5-hydroxytryptamine (5-HT) in the spinal cord. Morphine has very little effect on the metabolism of DA and NA in the spinal cord. However, morphine causes a significant increase in the metabolism of spinal 5-HT. The increase in 5-HIAA induced by morphine is not restricted to the dorsal horn. The three main functional regions of the cord--dorsal horn (sensory), zona intermedia (autonomic), and ventral horn (somatic motor)--are affected to the same degree. The results indicate that morphine causes a generalized activation of serotonin neurons in the spinal cord. There appears to be little or no selectivity for those serotonergic neurons that innervate the dorsal horn. The results are discussed with reference to current data, which indicate a fairly strong link between descending serotonergic nerves and the mechanism of action of morphine-induced analgesia.

Structural organization of the human cerebral cortex prior to the appearance of the cortical plate

The early development and the structural organization of the human cerebral cortex, prior to the appearance of the cortical plate (Carnegie stage 22, ca. 54 days), was studied in two embryos: 43 (stage 18) and 50 day old (stage 20), respectively. It has been shown that the human cerebral cortex begins its ontogenetic development around the sixth rather than around the eighth week of gestation as it has been previously assumed. The human cerebral cortex starts to develop soon after the cerebral vesicles have been formed (stage 15) and a primitive internal capsule has been established (stage 17, ca. 41 days). By stage 18 of human development fibres from this primitive internal capsule have reached and probably have penetrated into the developing cerebral vesicle, through its more superficial zone. Fibres from this primitive internal capsule have been traced backward through the ventral thalamus to the mesencephalic tegmentum. The possible existence of primitive ascending fibres from the mid-brain which terminate in the superficial zone of the developing cerebral cortex (tegmento-thalamostriato-cortical tract) is suggested. The arrival of these primitive corticipetal fibres establishes in the outer zone of the cerebral cortex a primordial plexiform lamina or an external white matter. Horizontal-bipolar cells (embryonic Cajal-Retzius neurons) begin to differentiate by stage 18 of human development (43 days in our case). By stage 20 (50 days in our case), the primordial plexiform lamina is well established, extends throughout the entire surface of the developing cerebral cortex, and is considered to be functionally active. It is, by this age, a superficial, 40 micrometers thick, complex fibrillar neuronal organization composed of numerous horizontal corticipetal fibres (demonstrable with silver methods), horizontal-bipolar Cajal-Retzius neurons and a few other, less defined, cellular elements. This primordial plexiform lamina is considered to represent a primitive "premammalian" cortical organization. The next event in cortical ontogenesis is the appearance of the cortical plate or the mammalian neocortical grey at stage 22 (ca. 54 days). Migrating neuroblasts attracted toward the preexisting primordial plexiform lamina and guided by glial fibres start to accumulate within it.(ABSTRACT TRUNCATED AT 400 WORDS)

Trophic effects of brain areas on the developing cerebral cortex: I. Growth and histological organization of intraocular grafts

Trophic interactions during development of brain regions were examined in rats using intraocular grafts of central nervous tissue. The increase in volume of transplanted fetal parietal cerebral cortex, as measured through the cornea, was markedly augmented by the presence of several different previously grafted CNS areas such as locus coeruleus, tectum, or cerebral cortex. DNA measurements and histological examinations suggested that this increased volume was due both to hyperplasia and hypertrophy. Previous grafts of iris, in contrast, did not significantly alter the final size of subsequently grafted cortex pieces. Contact between the two transplants was found to be critical in eliciting the trophic response. Growth-stimulated cortical grafts had a better organized cyto-architecture with larger neurons, including typical pyramidal cells, more neuropil, a lower cell density, and a more organotypic distribution of the cell bodies than non-stimulated controls. The experiments thus demonstrate a profound effect of adjacent neural tissue on development of neocortex. It is concluded that trophic interactions upon brain development can be revealed by sequential intraocular grafting.

The development of beta-adrenergic receptors in the visual cortex of the rat

The development of beta-adrenergic receptors in the rat visual cortex was examined and the density of beta-receptors and associated subtypes (beta 1 and beta 2) was compared between visual and non-visual or whole cortical tissues using radioreceptor assays employing [125I]iodohydroxybenzylpindolol and [125I]iodocyanopindolol as ligands. Saturation assays revealed not only similar affinities of beta-receptors for [125I]iodohydroxybenzylpindolol in visual cortical samples at 10, 24 and 160 days after birth but also practically identical saturation curves for visual and non-visual cortical samples at 160 days of age. Displacement of [125I]iodohydroxybenzylpindolol with propranolol in visual cortical membranes at various postnatal ages showed a gradual increase in receptor density from day 4 to day 24 with no change thereafter. No significant differences were observed in the overall density of beta-receptors or in the distribution and density of beta 1 and beta 2-receptors between visual and non-visual or whole cortical samples; however, there was a definite decline in the density of beta-receptors in these samples between 40 and 160 days of age. The results indicate that the developmental pattern of beta-receptor density and the distribution of beta 1 and beta 2-receptors are similar between visual and whole cortical tissues. In addition, the results emphasize the importance of maintaining the dissociation constant at a fixed value when comparing receptor densities between experiments, and also show the utility of employing the high-affinity ligand, [125I]iodocyanopindolol, with a combination of serotoninergic, dopaminergic and alpha-adrenergic antagonists to examine beta-adrenergic receptors in a specific region of the brain. Study of beta-receptors in the visual cortex may be beneficial in elucidating the role of norepinephrine in this region.

Prefrontal cortex: dense dopaminergic input in the newborn rat

Using a recently introduced modification of the aluminum-formaldehyde histofluorescence method, in combination with exogeneous administration of alpha-methylnoradrenaline and biochemical analyses, a remarkably advanced development of the dopaminergic and noradrenergic afferent to the frontal lobe has been demonstrated in neonatal rat. At birth, the density and general distributional pattern of the catecholamine innervation was similar to that found in the fully developed prefrontal cortex. The previously not recognized, early and extensive maturation of the mesocortical dopamine projection suggests a functional role of dopamine already in the early postnatal period.

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.

Improved catecholamine histofluorescence in the developing brain based on the magnesium and aluminum (ALFA) perfusion techniques: methodology and anatomical observations

Detailed protocols for the application of two different metal salt perfusion procedures are described for the production of superior catecholamine histofluorescence in the brains of immature rats up to 2 weeks of age. As in the adult, both magnesium and aluminum salts are highly advantageous for catecholamine histofluorescence in developing animals, and yield marked increases in sensitivity. In the magnesium-perfusion technique, animals are perfused in a simple one-step process using a hand-held syringe with cold buffer containing magnesium sulphate, formaldehyde and glyoxylic acid. The aluminum-perfusion (ALFA) technique provides even greater sensitivity and richness of detail, but requires a controlled-pressure perfusion system and a two-step perfusion process. Animals are first perfused with a room-temperature buffer containing magnesium sulphate and procain (to prevent vasoconstriction) followed by cold buffer containing aluminum sulphate and formaldehyde. In both methods, tissue pieces are subsequently freeze-dried, reacted with formaldehyde vapour and paraffin-sectioned according to the standard Falck-Hillarp procedure. Tissue pieces can also be taken from aluminum-perfused brains for simultaneous catecholamine assay using radioenzymatic methods, thereby permitting correlated histochemical and neurochemical analyses on the same brains. Many catecholamine terminal systems can be visualized in the rat brain even at birth with the ALFA procedure following pargyline pretreatment. However, the endogenous intraaxonal catecholamine concentration is so low in immature brains that the full anatomical extent of these systems cannot be reliably seen without recourse to pre-loading with an exogenously administered amine. For this purpose systemic injections of alpha-methyl-noradrenaline were extensively investigated. In combination with the ALFA procedure, such pretreatment was found to cause a dramatic increase in both the intensity and number of terminal and preterminal fibers throughout the brain. Control experiments with 6-hydroxydopamine and the catecholamine uptake blocker, nomifensine, indicate that this loading is specific for catecholamine systems. This approach has indicated that certain of the forebrain noradrenergic and dopaminergic systems are very extensive at birth, and in some regions an intermediate stage of hyperinnervation is a normal feature of ontogeny. Some of these findings are illustrated here and will also be presented in greater detail in further reports.

Laminar, tangential and regional organization of the noradrenergic innervation of monkey cortex: dopamine-beta-hydroxylase immunohistochemistry

An antiserum directed against human dopamine-beta-hydroxylase purified from pheochromocytoma tissue was employed in an immunohistochemical study of the organization of the noradrenergic innervation of monkey neocortex. A detailed description is given of the laminar pattern of noradrenergic innervation in the dorsolateral prefrontal cortex (Brodmann areas 9 and 10) and the primary somatosensory cortex of the postcentral gyrus (Brodmann areas 3,1,2). The noradrenergic innervation of these two regions is similar in the following respects: (1) fibers are present in all six layers, (2) the innervation is dense and terminal-like in layers IV and V, and (3) layer VI is characterized by fibers oriented parallel to the pial surface which follow the contours of the subcortical white matter. However, these regions differ with respect to specific laminar patterns of fiber distribution and orientation and by virtue of the fact that the primary somatosensory cortex has a very dense noradrenergic innervation, while the density of innervation in dorsolateral prefrontal cortex is low relative to the postcentral gyrus and most other neocortical areas. The laminar pattern of noradrenergic innervation in primary visual cortex differs fundamentally from both prefrontal and primary somatosensory cortices. In a separate series of experiments, dorsolateral frontal cortex lesions were used to investigate the intracortical trajectory of noradrenergic fibers. A discrete aspiration lesion confined to the grey matter of the prefrontal cortex led to a substantial loss of noradrenergic fibers in cortical regions caudal to the lesion. The decrease in density of noradrenergic innervation was particularly pronounced in the pre- and postcentral gyri. These results demonstrate that while the noradrenergic innervation of primate cortex exhibits a far greater degree of regional variation than is present in the rat cortex, the tangential intracortical trajectory that is characteristic of the lissencephalic rat brain is also a dominant feature of the noradrenergic innervation of the gyrencephalic primate brain.

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.

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.

Evidence for a neurotropic role of noradrenaline neurons in the postnatal development of rat cerebral cortex

The effects of neonatal administration of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA; 1-4 doses of 100 mg/kg body weight s.c.) on the postnatal development of pyramidal neurons in several cortical regions of the rat was studied using a Golgi-Cox neuronal impregnation technique. Rats were sacrificed in the adult stage (eight weeks) and the following regions were studied: anterior frontal cortex, posterior frontal cortex (including motor cortex), anterior parietal cortex (including sensory cortex), posterior parieto-occipital cortex and cingulate cortex. Significant alterations were seen in animals which received four doses of 6-OHDA. These alterations can be summarized as follows: (1) a decreased length and branching of basolateral dendrites of pyramidal cells, with loss of dendritic spines, which were found in both the internal pyrimidal layer (layer V) and the external pyramidal layer (layer III), most abundantly in the frontal cortex and cingulate cortex; (2) an increased number of pyramidal cells of layer V with premature apical dendritic termination in layer III rather than the usual termination in layers I and II. This was most abundant in the cingulate cortex; (3) occasional disorientation of pyramidal cell apical dendrites away from the normal vertical plane by 15 or more degrees, seen in frontal, parietal and cingulate cortex; (4) an increased number of pyramidal cells with rounded somatic contours, found in frontal, anterior parietal and cingulate cortex. These phenomena were occasionally seen in normal cortex, but were significantly increased in their occurrence after four doses of 6-OHDA. Such alterations were not significant in rats treated with one or three doses of 6-OHDA. The extent and severity of morphological alterations correlate with reductions in endogenous noradrenaline (NA) in cerebral cortex, which was found to average 50% of control levels after one dose of 6-OHDA, and 80% reduction after three doses, and a 97-98% reduction after four doses, suggesting that the NA denervation must be almost complete to result in readily detectable significant morphological changes in the development of cortical pyramidal cells. No consistent changes in endogenous dopamine (DA) levels were observed, except for an increase in the cingulate cortex. The anatomical alterations in pyramidal cells described in the present study suggest that NA neurons which project into the cerebral cortex have a neurotrophic role in the postnatal development of cortex.

Monoaminergic afferents to the neocortex: a developmental histofluorescence study in normal and Reeler mouse embryos

The patterns of distribution of monoaminergic (MA) afferents during early histogenesis of the neocortex of normal and Reeler mice are studied by histofluorescence microscopy. Fluorescing fibers appear rostrally in the cortex of both genotypes on the 14th embryonic day (E14), which is within 24 h of the development of the cortical plate. They are distributed to all regions of the cortex by the time of birth. Although the patterns of intracortical deployment differ in the two genotypes, the fibers appear to have homologous target structures. These are: (1) the polymorphic cells of the subplate in the depths of the normal and in the superplate near the surface of the mutant cortex; and (2) the zones of consolidation of apical dendrites of pyramidal cells: the external plexiform zone of normal and a series of intracortical plexiform planes in the mutant cortex. By contrast, the axons of this system do not branch significantly among the compactly ordered somata of pyramidal cells within the cortical plate of either genotype.