A Golgi study of the early postnatal development of the visual cortex of the hooded rat

Early-onset brain alterations during postnatal development in a mouse model of CDKL5 deficiency disorder

Mutations in the CDKL5 gene are the cause of CDKL5 deficiency disorder (CDD), a rare and severe neurodevelopmental condition characterized by early-onset epilepsy, motor impairment, intellectual disability, and autistic features. A mouse model of CDD, the Cdkl5 KO mouse, that recapitulates several aspects of CDD symptomology, has helped to highlight brain alterations leading to CDD neurological defects. Studies of brain morphogenesis in adult Cdkl5 KO mice showed defects in dendritic arborization of pyramidal neurons and in synaptic connectivity, a hypocellularity of the hippocampal dentate gyrus, and a generalized microglia over-activation. Nevertheless, no studies are available regarding the presence of these brain alterations in Cdkl5 KO pups, and their severity in early stages of life compared to adulthood. A deeper understanding of the CDKL5 deficient brain during an early phase of postnatal development would represent an important milestone for further validation of the CDD mouse model, and for the identification of the optimum time window for treatments that target defects in brain development. In sight of this, we comparatively evaluated the dendritic arborization and spines of cortical pyramidal neurons, cortical excitatory and inhibitory connectivity, microglia activation, and proliferation and survival of granule cells of the hippocampal dentate gyrus in hemizygous Cdkl5 KO male (-/Y) mice aged 7, 14, 21, and 60 days. We found that most of the structural alterations in Cdkl5 -/Y brains are already present in pups aged 7 days and do not worsen with age. In contrast, the difference in the density of excitatory and inhibitory terminals between Cdkl5 -/Y and wild-type mice changes with age, suggesting an age-dependent cortical excitatory/inhibitory synaptic imbalance. Confirming the precocious presence of brain defects, Cdkl5 -/Y pups are characterized by an impairment in neonatal sensory-motor reflexes.

Fatty Acids: A Safe Tool for Improving Neurodevelopmental Alterations in Down Syndrome?

The triplication of chromosome 21 causes Down syndrome (DS), a genetic disorder that is characterized by intellectual disability (ID). The causes of ID start in utero, leading to impairments in neurogenesis, and continue into infancy, leading to impairments in dendritogenesis, spinogenesis, and connectivity. These defects are associated with alterations in mitochondrial and metabolic functions and precocious aging, leading to the early development of Alzheimer’s disease. Intense efforts are currently underway, taking advantage of DS mouse models to discover pharmacotherapies for the neurodevelopmental and cognitive deficits of DS. Many treatments that proved effective in mouse models may raise safety concerns over human use, especially at early life stages. Accumulating evidence shows that fatty acids, which are nutrients present in normal diets, exert numerous positive effects on the brain. Here, we review (i) the knowledge obtained from animal models regarding the effects of fatty acids on the brain, by focusing on alterations that are particularly prominent in DS, and (ii) the progress recently made in a DS mouse model, suggesting that fatty acids may indeed represent a useful treatment for DS. This scenario should prompt the scientific community to further explore the potential benefit of fatty acids for people with DS.

Early appearance of dendritic alterations in neocortical pyramidal neurons of the Ts65Dn model of Down syndrome

Down syndrome (DS), which is due to triplication of chromosome 21, is constantly associated with intellectual disability (ID). ID can be ascribed to both neurogenesis impairment and dendritic pathology. These defects are replicated in the Ts65Dn mouse, a widely used model of DS. While neurogenesis impairment in DS is a fetal event, dendritic pathology occurs after the first postnatal months. Neurogenesis alterations across the lifespan have been extensively studied in the Ts65Dn mouse. In contrast, there is scarce information regarding dendritic alterations at early life stages in this and other models, although there is evidence for dendritic alterations in adult mouse models. Thus, the goal of the current study was to establish whether dendritic alterations are already present in the neonatal period in Ts65Dn mice. In Golgi-stained brains we quantified the dendritic arbors of layer II/III pyramidal neurons in the frontal cortex of Ts65Dn mice aged 2 (P2) and 8 (P8) days and their euploid littermates. In P2 Ts65Dn mice we found a moderate hypotrophy of the apical and collateral dendrites but a patent hypotrophy of the basal dendrites. In P8 Ts65Dn mice the distalmost apical branches were missing or reduced in number but there were no alterations in the collateral and basal dendrites. No genotype effects were detected on either somatic or dendritic spine density. This study shows dendritic branching defects that mainly involve the basal domain in P2 Ts65Dn mice, and the apical but not the other domains in P8 Ts65Dn mice. This suggests that dendritic defects may be related to dendritic compartment and age. The lack of a severe dendritic pathology in Ts65Dn pups is reminiscent of the delayed appearance of patent dendritic alterations in newborns with DS. This similarly highlights the usefulness of the Ts65Dn model for the study of the mechanisms underlying dendritic alterations in DS and the design of possible therapeutic interventions.

Perinatal phthalate exposure increases developmental apoptosis in the rat medial prefrontal cortex

Phthalates are a class of endocrine disruptors found in a variety of consumer goods, and offspring can be exposed to these compounds during gestation and lactation. Our laboratory has found that perinatal exposure to an environmentally relevant mixture of phthalates resulted in a decrease in cognitive flexibility and in neuron number in the adult rat medial prefrontal cortex (mPFC). Here, we examine effects of phthalate treatment on prenatal cellular proliferation and perinatal apoptosis in the mPFC. To examine the phthalate effects on cellular proliferation, dams consumed 0, 1, or 5 mg/kg of the phthalate mixture daily from embryonic day 2 (E2) through the day of birth (P0), and on E16 and E17, they were injected with BrdU. The mPFC of offspring was analyzed on P5 and showed a decrease in labelled cells in the phthalate exposed groups. To examine whether changes in BrdU density observed on P5 were due to altered cell survival, cell death was measured on E18, P0, and P5 using a TUNEL assay in a separate cohort of prenatally exposed offspring. There was an increase in TUNEL labelled cells at E18 in the phthalate exposed groups. In the final experiment, dams consumed the phthalate mixture from E2 through P10, at which time mPFC tissue was stained with TUNEL. Phthalate treated subjects showed a higher density of apoptotic cells at P10. These results indicate both pre- and postnatal phthalate exposure increases apoptosis in the male and female rat mPFC. While the impact of phthalates on proliferation cannot be ruled out, these data do not allow for definitive conclusions.

Development of Auditory Cortex Circuits

The ability to process and perceive sensory stimuli is an essential function for animals. Among the sensory modalities, audition is crucial for communication, pleasure, care for the young, and perceiving threats. The auditory cortex (ACtx) is a key sound processing region that combines ascending signals from the auditory periphery and inputs from other sensory and non-sensory regions. The development of ACtx is a protracted process starting prenatally and requires the complex interplay of molecular programs, spontaneous activity, and sensory experience. Here, we review the development of thalamic and cortical auditory circuits during pre- and early post-natal periods.

Cell death in the male and female rat medial prefrontal cortex during early postnatal development

Apoptosis, programmed cell death, is a critical component of neurodevelopment occurring in temporal, spatial, and at times, sex-specific, patterns across the cortex during the early postnatal period. During this time, the brain is particularly susceptible to environmental influences that are often used in animal models of neurodevelopmental disorders. In the present study, the timing of peak cell death was assessed by the presence of pyknotic cells in the male and female rat medial prefrontal cortex (mPFC), a cortical region that in humans, is often involved in developmental disorders. One male and one female rat per litter were sacrificed at the following ages: postnatal day (P)2, 4, 6, 8, 10, 12, 14, 16, 18, and 25. The mPFC was Nissl-stained, the densities of pyknotic cells and live neurons were stereologically collected, and the number of pyknotic cells per 100 live neurons, pyknotic cell density, and neuron density were analyzed. Males and females showed a significant peak in the ratio of pyknotic to live neurons on P8, and in females, this elevation persisted through P12. Likewise, the density of pyknotic cells peaked on P8 in both sexes and persisted through P12 in females. The timing of cell death within the rat mPFC will inform study design in experiments that employ early environmental manipulations that might disrupt this process.

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The number of pyknotic cells per live neuron was quantified.

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Postnatal cell death peaked on P8 in the male rat medial prefrontal cortex.

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In females, postnatal cell death peaked from P8 to P12.

Sensorimotor development in neonatal progesterone receptor knockout mice

Early exposure to steroid hormones can permanently and dramatically alter neural development. This is best understood in the organizational effects of hormones during development of brain regions involved in reproductive behaviors or neuroendocrine function. However, recent evidence strongly suggests that steroid hormones play a vital role in shaping brain regions involved in cognitive behavior such as the cerebral cortex. The most abundantly expressed steroid hormone receptor in the developing rodent cortex is the progesterone receptor (PR). In the rat, PR is initially expressed in the developmentally-critical subplate at E18, and subsequently in laminas V and II/III through the first three postnatal weeks (Quadros et al. [2007] J Comp Neurol 504:42-56; Lopez & Wagner [2009]: J Comp Neurol 512:124-139), coinciding with significant periods of dendritic maturation, the arrival of afferents and synaptogenesis. In the present study, we investigated PR expression in the neonatal mouse somatosensory cortex. Additionally, to investigate the potential role of PR in developing cortex, we examined sensorimotor function in the first two postnatal weeks in PR knockout mice and their wildtype (WT) and heterozygous (HZ) counterparts. While the three genotypes were similar in most regards, PRKO and HZ mice lost the rooting reflex 2-3 days earlier than WT mice. These studies represent the first developmental behavioral assessment of PRKO mice and suggest PR expression may play an important role in the maturation of cortical connectivity and sensorimotor integration.

Periadolescent maturation of the prefrontal cortex is sex-specific and is disrupted by prenatal stress

The prefrontal cortex (PFC) undergoes dramatic, sex-specific maturation during adolescence. Adolescence is a vulnerable window for developing mental illnesses that show significant sexual dimorphisms. Gestational stress is associated with increased risk for both schizophrenia, which is more common among men, and cognitive deficits. We have shown that male, but not female, rats exposed to prenatal stress develop postpubertal deficits in cognitive behaviors supported by the prefrontal cortex. Here we tested the hypothesis that repeated variable prenatal stress during the third week of rat gestation disrupts periadolescent development of prefrontal neurons in a sex-specific fashion. Using Golgi-Cox stained tissue, we compared dendritic arborization and spine density of prelimbic layer III neurons in prenatally stressed and control animals at juvenile (day 20), prepubertal (day 30), postpubertal (day 56), and adult (day 90) ages (N = 115). Dendritic ramification followed a sex-specific pattern that was disrupted during adolescence in prenatally stressed males, but not in females. In contrast, the impact of prenatal stress on the female PFC was not evident until adulthood. Prenatal stress also caused reductions in brain and body weights, and the latter effect was more pronounced among males. Additionally, there was a trend toward reduced testosterone levels for adult prenatally stressed males. Our findings indicate that, similarly to humans, the rat PFC undergoes sex-specific development during adolescence and furthermore that this process is disrupted by prenatal stress. These findings may be relevant to both the development of normal sex differences in cognition as well as differential male-female vulnerability to psychiatric conditions.

Increasing our understanding of human cognition through the study of Fragile X Syndrome

Fragile X Syndrome (FXS) is considered the most common form of inherited intellectual disability. It is caused by reductions in the expression level or function of a single protein, the Fragile X Mental Retardation Protein (FMRP), a translational regulator which binds to approximately 4% of brain messenger RNAs. Accumulating evidence suggests that FXS is a complex disorder of cognition, involving interactions between genetic and environmental influences, leading to difficulties in acquiring key life skills including motor skills, language, and proper social behaviors. Since many FXS patients also present with one or more features of autism spectrum disorders (ASDs), insights gained from studying the monogenic basis of FXS could pave the way to a greater understanding of underlying features of multigenic ASDs. Here we present an overview of the FXS and FMRP field with the goal of demonstrating how loss of a single protein involved in translational control affects multiple stages of brain development and leads to debilitating consequences on human cognition. We also focus on studies which have rescued or improved FXS symptoms in mice using genetic or therapeutic approaches to reduce protein expression. We end with a brief description of how deficits in translational control are implicated in FXS and certain cases of ASDs, with many recent studies demonstrating that ASDs are likely caused by increases or decreases in the levels of certain key synaptic proteins. The study of FXS and its underlying single genetic cause offers an invaluable opportunity to study how a single gene influences brain development and behavior.

Synapses on demand require dendrites at the ready: how defining stages of dendritic development in vitro could inform studies of behaviorally driven information storage in the brain

Bill Greenough's work provides a framework for thinking about synaptogenesis not only as a key step in the initial wiring of neural systems according to a species typical plan (i.e., experience-expectant development), but also as a mechanism for storing information based an individual's unique experience over its lifetime (i.e., experience-dependent plasticity). Analysis of synaptic development in vitro brings a new opportunity to test the limits of expectant-expectant development at the level of the individual neuron. We analyzed dendritic growth, synapse formation, and the development of specialized cytoplasmic microdomains during development in cultured hippocampal neurons, to determine if the timing of each of these events is correlated. Taken together, the findings reported here support the hypotheses that (1) dendritic development is rate limiting in synapse formation and (2) synaptic circuits are assembled in a step-wise fashion consistent with a stage-specific shift from genomically pre-programmed to activity-dependent mechanisms.

Retinal influences induce bidirectional changes in the kinetics of N-methyl-D-aspartate receptor-mediated responses in striate cortex cells during postnatal development

Development of the visual callosal projection in rodents goes through an early critical period, from postnatal day (P) 4 to P6, during which retinal input specifies the blueprint for normal topographic connections, and a subsequent period of progressive pathway maturation that is largely complete by the time the eyes open, around P13. This study tests the hypothesis that these developmental stages correlate with age-related changes in the kinetics of synaptic responses mediated by the N-methyl-D-aspartate subclass of glutamate receptors (NMDARs). We used an in vitro slice preparation to perform whole-cell recordings from retrogradely-labeled visual callosal cells, as well from cortical cells with unknown projections. We analyzed age-related changes in the decay time constant of evoked as well as spontaneous excitatory postsynaptic currents mediated by N-methyl-D-aspartate subclass of glutamate receptors (NMDAR-EPSCs) in slices from normal pups and pups enucleated at different postnatal ages. In normal pups we found that the decay time constant of NMDAR-EPSCs increases starting at about P6 and decreases by about P13. In contrast, these changes were not observed in rats enucleated at birth. However, by delaying the age at which enucleation was performed we found that the presence of the eyes until P6, but not until P4, is sufficient for inducing slow NMDAR-EPSC kinetics during the second postnatal week, as observed in normal pups. These results provide evidence that the eyes exert a bidirectional effect on the kinetics of NMDARs: during a P4-P6 critical period, retinal influences induce processes that slow down the kinetics of NMDAR-EPSCs, while, near the age of eye opening, retinal input induces a sudden acceleration of NMDAR-EPSC kinetics. These findings suggest that the retinally-driven processes that specify normal callosal topography during the P4-P6 time window also induce an increase in the decay time constant of NMDAR-EPSCs. This increase in response kinetics may play an important role in the maturation of cortical topographic maps after P6. Using ifenprodil, a noncompetitive NR2B-selective blocker, we obtained evidence that although NR1/NR2B diheteromeric receptors contribute to evoked synaptic responses in both normal and enucleated animals, they are not primarily responsible for either the age-related changes in the kinetics of NMDAR-mediated responses, or the effects that bilateral enucleation has on the kinetics of NMDAR-EPSCs.

BDNF regulates primary dendrite formation in cortical neurons via the PI3-kinase and MAP kinase signaling pathways

Neurotrophins are known to regulate dendritic development, but the mechanisms that mediate neurotrophin-dependent dendrite formation are largely unknown. Here we show that brain-derived neurotrophic factor (BDNF) induces the formation of primary dendrites in cortical neurons by a protein synthesis-independent mechanism. BDNF leads to the rapid activation of PI3-kinase, MAP kinase, and PLC-gamma in cortical neurons, and pharmacological inhibition of PI3-kinase and MAP kinase in dissociated cell cultures and cortical slice cultures suppresses the ability of BDNF to induce dendrite formation. A constitutively active form of PI3-kinase, but not MEK, is sufficient to induce primary dendrite formation in cortical neurons. These observations indicate that BDNF induces primary dendrite formation via activation of the PI3-kinase and MAP kinase pathways and provide insight into the mechanisms that mediate the morphological effects of neurotrophin signaling.

Enhanced Ras activity in pyramidal neurons induces cellular hypertrophy and changes in afferent and intrinsic connectivity in synRas mice

Neurotrophic actions are critically controlled and transmitted to cellular responses by the small G protein Ras which is therefore essential for normal functioning and plasticity of the nervous system. The present study summarises findings of recent studies on morphological changes in the neocortex of synRas mice expressing Val12-Ha-Ras in vivo under the control of the rat synapsin I promoter. In the here reported model (introduced by Heumann et al. [J. Cell Biol. 151 (2000) 1537]), transgenic Val12-Ha-Ras expression is confined to the pyramidal cell population and starts postnatally at a time, when neurons are postmitotic and their developmental maturation has been basically completed. Expression of Val12-Ha-Ras results in a significant enlargement of pyramidal neurons. Size, complexity and spine density of dendritic trees are increased, which leads, finally, to cortical expansion. However, the main morphological design principles of 'transgenic' pyramidal cells remain preserved. In addition to somato-dendritic changes, expression of Val12-Ha-Ras in pyramidal cells induces augmented axon calibres and upregulates the establishment of efferent boutons. Despite the enlargement of cortical size, the overall density of terminals representing intra- or interhemispheric, specific and non-specific afferents is unchanged or even higher in transgenic mice suggesting a significant increase in the total afferent input to the neocortex. Although interneurons do not express the transgene and are therefore excluded from direct, intrinsic Val12-Ha-Ras effects, they respond with morphological adaptations to structural changes. Thus, dendritic arbours of interneurons are extended to follow the cortical expansion and basket cells establish a denser inhibitory innervation of 'transgenic' pyramidal cells perikarya. It is concluded that expression of Val12-Ha-Ras in pyramidal neurons results in remodelling of neocortical structuring which strongly implicates a crucial involvement of Ras in cortical plasticity.

Expression of constitutively active p21H-rasval12 in postmitotic pyramidal neurons results in increased dendritic size and complexity

The small G protein p21Ras is a critical molecular switch for relaying neurotrophic actions and is essential for normal functioning and plasticity of the nervous system. In this study, the morphogenetic effects of p21Ras were investigated on neurons in vivo. Morphological changes of layers II/III and Vb commissural pyramidal neurons of the primary somatosensory cortex were analyzed in transgenic mice expressing permanently active p21H-RasVal12 in postmitotic neurons. Pyramidal cells were retrogradely labelled with biotinylated dextran amine and subsequently traced using Neurolucida. Compared with wild-type mice, transgenic animals showed a significant increase in the surface area and volume of basal dendrites on the proximal and intermediate segments in layers II/III and on further distal segments in layer V. In addition, the surface area and volume of the trunk and of the proximal segments of oblique branches of apical dendrites were enlarged in both layers. Sholl analyses of basal and apical dendrites showed a significant increase in dendritic complexity of layer V neurons. A positive correlation was observed between the size of the basal dendrite and the neuronal soma size in the transgenic population, indicating that growth-promoting effects of p21H-RasVal12 affect both cellular compartments in parallel. However, the dendritic surface correlated with the number of tips and dendritic stem diameter in both wild-type and transgenic populations, demonstrating that these relations represent rather conservative design principles in dendritic morphology. The data presented here suggest an important role of p21Ras-dependent signaling in the final differentiation and maintenance of dendritic morphology.

Developmentally regulated expression of Neuro-p24 and its possible function in neurite extension

Process extension is a most marked and characteristic neuronal feature that is observed during the development, regeneration and plasticity of nervous system tissues. Neuro-p24, a novel membranous protein with a molecular weight of 24 kDa, is specifically localized in neurons, particularly in the neurites. Based on its molecular structure and distribution pattern in the brain we proposed that Neuro-p24 plays a role in neurite extension. In the present study we have made several findings that support this hypothesis; first, Neuro-p24 was abundant in motor axonal fibers, neurites of dorsal root ganglia neurons and apical dendrites of cerebral cortex neurons when their extension or arborization was proceeding very actively. Secondly, when COS-7 epithelial cells were transfected with either wild-type or deletion-mutated Neuro-p24 cDNAs, ectopic expression of wild-type cDNA caused morphological alterations resulting in a neuron-like appearance. These observations firmly support our proposal and indicate that Neuro-p24 plays an important role in the nervous tissue.

Bihemispheric Axonal Bifurcation of the Afferents to the Visual Cortical Areas during Postnatal Development in the Rat

Numerous cortical neurons in the juvenile and adult rat project to visual areas of both hemispheres whereas the vast majority of subcortical structures projecting to the visual cortex send strictly ipsilateral projections (Dreher et al., 1990). In the present study, the authors have sought to determine whether this pattern of axonal bifurcation in the connectivity of the visual areas undergoes a change during postnatal development. Two retrograde fluorescent dyes were used, fast blue (FB) and diamidino yellow (DY). Large multiple injections of one of the dyes were placed in all visual areas of one hemisphere and a small injection of the other dye was placed in area 17 of the opposite hemisphere. Labelled neurons were observed in subcortical and cortical structures on the side of the small injection. The experiments were performed on ten neonatal albino rat pups aged between 3 and 12 postnatal days (p.n.d.) at the time of injection and the results were compared with those obtained in the juvenile and adult animals, as reported in the preceding paper. In the thalamus of newborn animals, neurons belonging to nuclei located away from the midline send strictly ipsilateral cortical projections. However, in the midline nuclei of the intralaminar thalamic complex, a small region of overlap was observed between neurons projecting ipsilaterally and neurons projecting contralaterally in animals aged less than 9 postnatal days. In addition, in these neonatal animals a small number of bilaterally projecting neurons was detected in this region of overlap. In all other subcortical structures examined (ventral tegmental area, diagonal band of Broca, claustrum), the laterality of the projection was the same in the newborn and the adult animals. In particular, in the claustrum of neonatal animals, as in adult animals, there was a large contingent of contralaterally projecting neurons and only a very small number of bilaterally projecting neurons. The results in the cortex contrast with those observed in subcortical structures. Whereas ipsilaterally projecting neurons were distributed in a broadly similar way in newborn and adult animals, the laminar and areal distribution of contralaterally projecting neurons in newborn animals clearly differed from those observed in the adult animals. Furthermore, double labelled neurons were more numerous in animals aged less than 12 days than in adults. The proportions of such bilaterally projecting neurons were computed with respect to the numbers of neurons sending ipsilateral projections to area 17. These proportions are constant at all ages in the claustrum and cortical area 8. In areas 18a, 29 and 35 on the other hand, the proportions of bilaterally projecting neurons increase after 5 days and reach a peak in the period extending from 9 to 11 days of age when more than half of the neurons projecting ipsilaterally also send an axonal branch to the contralateral cortex. In cortical areas 29 and 35, this peak is followed by a sudden drop to the adult level at 12 postnatal days, whereas the return to the adult level is gradual in area 18a. These results demonstrate that, in subcortical structures and in cortical area 8, the laterality of the afferent connections to the visual cortex does not change during postnatal development. By contrast, cortical areas 18a, 29 and 35 go through a stage when numerous cells send bifurcating connections to both hemispheres. The timing of the decrease in proportions of bilaterally projecting neurons in these areas suggests that numerous neurons retract their callosal axonal branch when the adult pattern of callosal connectivity is established at 9 - 11 days of age.

Reversal of neuronal polarity characterized by conversion of dendrites into axons in neonatal rat cortical neurons in vitro

The mechanisms for the establishment and maintenance of cell polarity in neurons are not well understood. Axon regeneration from dendrites has been reported after axotomy near the cell body in vivo. We report here in vitro a reversal of neuronal polarity characterized by the conversion of dendrites into axons. We isolated neurons from the neonatal rat cerebral cortex. Neurons that exhibited an apical dendrite with a length of >100 microm were monitored for 3 days in culture. In 66% of neurons examined, a new axon, as identified by reactivity with an antibody to dephosphorylated tau or by lack of reactivity with an antibody to the a and b isoforms of microtubule-associated protein 2, appeared to form from the tip of the original dendrite. Further analysis of such neurons revealed that the distal half of the original dendrite became positive for dephosphorylated tau or negative for microtubule-associated protein 2. Time-lapse video microscopy demonstrated the conversion of the original dendrite into an axon without dendritic retraction. Axon regeneration from dendritic tips required a significantly longer time than axon regeneration from minor processes. Our observations thus demonstrate in vitro a time-consuming reversal of neuronal polarity and the conversion of a dendritic cytoskeleton into an axonal one.

Abnormal development of dendritic spines in FMR1 knock-out mice

Fragile X syndrome is caused by a mutation in the FMR1 gene leading to absence of the fragile X mental retardation protein (FMRP). Reports that patients and adult FMR1 knock-out mice have abnormally long dendritic spines of increased density suggested that the disorder might involve abnormal spine development. Because spine length, density, and motility change dramatically in the first postnatal weeks, we analyzed these properties in mutant mice and littermate controls at 1, 2, and 4 weeks of age. To label neurons, a viral vector carrying the enhanced green fluorescent protein gene was injected into the barrel cortex. Layer V neurons were imaged on a two-photon laser scanning microscope in fixed tissue sections. Analysis of >16,000 spines showed clear developmental patterns. Between 1 and 4 weeks of age, spine density increased 2.5-fold, and mean spine length decreased by 17% in normal animals. Early during cortical synaptogenesis, pyramidal cells in mutant mice had longer spines than controls. At 1 week, spine length was 28% greater in mutants than in controls. At 2 weeks, this difference was 10%, and at 4 weeks only 3%. Similarly, spine density was 33% greater in mutants than in controls at 1 week of age. At 2 or 4 weeks of age, differences were not detectable. The spine abnormality was not detected in neocortical organotypic cultures. The transient nature of the spine abnormality in the intact animal suggests that FMRP might play a role in the normal process of dendritic spine growth in coordination with the experience-dependent development of cortical circuits.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Organizational and activational effects of gonadal steroid hormones on the EEG of male and female rats

To analyze organizational and activational effects of sex steroids on adult rat electroencephalographic activity (recorded at postnatal day 100), seven groups were included: males (48)-intact, neonatally or adult castrated; females (64)-intact, ovariectomized and exposed pre- or neonatally to testosterone propionate. In males, neonatal orchidectomy increased beta relative power, whereas both neonatal and adult castration reduced interparietal correlation. In females, prenatal testosterone administration produced higher theta absolute power; theta relative power was higher in all experimental groups, whereas beta1 and beta2 were decreased by prenatal and increased by neonatal virilization; prenatal virilization enhanced, while neonatal virilization and adult ovariectomy decreased interparietal correlation. These data indicate that females are more sensitive to early prenatal than to neonatal organizational effects of sex steroids, and some electroencephalographic features are feminized in castrated males and virilized in perinatally androgenized females.

Biology and pathobiology of neuronal development

Differentiation of neurons within the central nervous system occurs by the combined effects of intrinsic genetic programs and epigenetic stimuli. Disorders causing mental retardation and other abnormalities of higher cortical function arise by disturbances of the normal developmental sequence. MRDD Research Reviews 6:41-46, 2000.

Laminar distribution of isocortical neurons projecting to occipital grafts in neonate and adult rats

Physiologically responsive grafts of embryonic (E16) occipital neurons placed into the visual cortex of adult rats were shown previously (Gaillard et al., 1998, Restor. Neurol. Neurosci. 12: 13-25) to receive a predominant (93-97%) cortical input from the infragranular layers V-VI. The present paper examines whether this specific pattern of connections is related to some process of maturation of the host cortex. Pieces of embryonic (E16) occipital cortical tissue were grafted into the visual cortex of neonate (P0), 1-week-old (P7), and adult (P120) subjects. Four months later, graft responsiveness was assessed through field potential recordings and host-to-graft afferents were labeled with a retrograde tracer (cholera toxin subunit B). The data show first that afferents to physiologically active grafts originate about equally from both supra- and infragranular cortical layers in newborn subjects and second that supragranular neurons contribute only 20 and 1.5% of these inputs in P7 and P120 recipients, respectively. This strong upside-down laminar shift of afferents may correlate with the layout of subsets of host neurons that at a given developmental stage would have the intrinsic capacity to regrow an axon. Substantial axogenesis and synaptic stabilization of host-to-graft cortical afferents appear possible only within the critical period for the supragranular neurons but may occur throughout life for the infragranular neurons.

Developmental changes in purinergic calcium signalling in rat neocortical neurones

The changes in cytoplasm free calcium concentration ([Ca2+]i transients) were measured in Fura-2/AM loaded pyramidal neurones of sensorimotor cortex in acutely prepared slices isolated from 14 days (P14) and 30 (P30) days old rats. Ni2+ (50 microM) diminished the [Ca2+]i transients evoked by 50 mM KCl bath application by 47%+/-8% in neurones of the P14 group and only by 15%+/-6% in those of P30 group (P<0.002). Nifedipine and verapamil in concentration 100 microM reduced the calcium transients amplitude triggered by depolarization to about the same extent in both groups of neurons-on average by 50% and 35%, respectively. The amplitude of [Ca2+]i transients induced by application of 100 microM ATP reached 103+/-6 nM in P14 neurones and 72+/-8 nM in the P30 ones. The ATP-[Ca2+]i induced transient could be evoked in Ca2+-free external solution, indicating the presence of metabotropic (P2y) purinoreceptors. Almost all (90%) P14 neurones were endowed with such receptors. At the same time only 1/3 of the tested (n=42) P30 neurones presented responses to ATP applications in Ca2+-free solution. The share of ionotropic (P2x) purinoreceptors in generation of calcium signal was the same in both groups of neurons. No caffeine-induced Ca2+-release has been observed in the P14 neurons. To the contrary, in 28 cells from 42 investigated neurones of the P30 group application of 40 mM caffeine for 10 s induced considerable [Ca2+]i transients, which did not disappear in calcium-free solution. A conclusion is made about substantial changes in the expression of Ca2+-handling mechanisms which are occurring in neocortical neurones at the third-fourth week of postnatal development.

Compound synapses within the GABAergic innervation of the auditory inner hair cells in the adolescent mouse

Ultrastructural investigation of the gamma-aminobutyric acid (GABA) component of the inner spiral bundle in adolescent mice revealed a pathway of glutamic acid decarboxylase (GAD)-positive and -negative fibers and vesiculated endings that contact inner hair cells and their afferents through a complex of axosomatic and axodendritic synapses. Ultrastructural details were investigated by using conventional electron microscopy. Several synaptic arrangements were observed: Main axosomatic synapses form between vesiculated endings and individual or adjoining inner hair cells (interreceptor synapses). Spinous synapses form on long, spinelike processes that protrude from inner hair cells to reach distant efferent endings. The efferent endings associate with inner hair cells and their synaptic afferents through compound synapses-serial, "converging," and triadic-otherwise characteristic of sensory relay nuclei. Serial synapses form by the sequential presynaptic alignment of the efferent-->receptor-->afferent components. Converging synapses result from the simultaneous apposition of a receptor ribbon synapse and a presynaptic efferent terminal on a recipient afferent dendrite. Triadic synapses comprise a vesiculated efferent ending in contact with an inner hair cell and with its synaptic afferent. Additionally, efferent endings may form simple axodendritic and axoaxonal synapses with GAD-negative vesiculated endings. The combination of different synaptic arrangements leads to short chains of compound synapses. It is assumed that these synaptic patterns seen in the adolescent mouse represent adult synaptology. The patterns of synaptic connectivity suggest an integrative role for the GABA/GAD lateral efferent system, and imply its involvement in the pre- and postsynaptic modulation of auditory signals.

Cocultured, but not isolated, cortical explants display normal dendritic development: a long-term quantitative study

Dendritic growth has been studied in long-term organotypic neonatal rat occipital neocortex grown either apart as isolated explants or in tandem as cocultures. Quantitative light microscopic measurement of dendritic and axonal branching patterns within the cortical slice was accomplished using rapid Golgi stained materials. In both isolates and cocultures the overall cellular organization of the slice was maintained over 4 weeks in vitro with morphologically distinguishable pyramidal and nonpyramidal neurons located within the same layers and with the same orientations as observed in situ. Long-term increases in the total length of basal dendrites, apical dendrite and axons were observed only in cocultures and were similar to growth patterns reported for in situ materials. Dendritic growth was mainly due to elongation of terminal dendritic segments. Surprisingly, isolated explants showed no long-term increases in total (basal) dendrites, apical dendrites or axons with time in vitro. A transient decrease in the number of basal dendritic segments and increase in terminal segment lengths at the end of the first week in vitro, however, was observed in nonpyramidal neurons. It is hypothesized that (i) afferent inputs and/or efferent targets develop only in cocultures and provide a crucial conditions for the continued growth of dendritic/axonal arborization for neocortical neurons in vitro, (ii) intrinsic interconnectivity within isolated explants is not sufficient to maintain long-term growth of neuritic arbors, and (iii) remodelling of dendritic arbors within isolated explants occurs at the same time as these explants are showing noticeable increases in the level of spontaneous bioelectric activity, which suggests that dendritic growth and network formation may be function dependent.

Inhibition of nitric oxide synthase does not alter ocular dominance shifts in kitten visual cortex

1. Since nitric oxide has been proposed as a feedback factor in plasticity in the hippocampus, we tested whether it might also be a feedback factor in sensory-dependent plasticity in the cat visual cortex. 2. The effects of monocular deprivation were compared between eight hemispheres with infusion of a nitric oxide synthase inhibitor, and eight control hemispheres with either infusion of the inactive isomer, or no infusion. Although nitric oxide synthase activity was reduced significantly, the ocular dominance histograms were not substantially different in the two groups of animals. We conclude that the feedback factor for sensory-dependent plasticity in the visual cortex is likely to be some factor other than nitric oxide.

Evidence for a role of dendritic filopodia in synaptogenesis and spine formation

Axo-dendritic synaptogenesis was examined in live hippocampal cell cultures using the fluorescent dyes DiO to label dendrites and FM 4-64 to label functional presynaptic boutons. As the first functional synaptic boutons appeared in these cultures, numerous filopodia (up to 10 micron long) were observed to extend transiently (mean lifetime 9.5 min) from dendritic shafts. With progressively increasing numbers of boutons, there were coincident decreases in numbers of transient filopodia and increases in numbers of stable dendritic spines. Dendritic filopodia were observed to initiate physical contacts with nearby axons. This sometimes resulted in filopodial stabilization and formation of functional presynaptic boutons. These findings suggest that dendritic filopodia may actively initiate synaptogenic contacts with nearby (5-10 micron) axons and thereafter evolve into dendritic spines.

The dynamics of dendritic structure in developing hippocampal slices

Time-lapse fluorescence confocal microscopy was used to directly visualize the formation and dynamics of postsynaptic target structures (i.e., dendritic branches and spines) on pyramidal neurons within developing tissue slices. Within a 2 week period of time, pyramidal neurons in cultured slices derived from early postnatal rat (postnatal days 2-7) developed complex dendritic arbors bearing numerous postsynaptic spines. At early stages (1-2 d in vitro), many fine filopodial protrusions on dendrite shafts rapidly extended (maximum rate approximately 2.5 microM/minute) and retracted (median filopodial lifetime, 10 min), but some filopodia transformed into growth cones and nascent dendrite branches. As dendritic arbors matured, the population of fleeting lateral filopodia was replaced by spine-like structures having a low rate of turnover. This developmental progression involved a transitional stage in which dendrites were dominated by persistent (up to 22 hr) but dynamic spiny protrusions (i.e., protospines) that showed substantial changes in length and shape on a timescale of minutes. These observations reveal a highly dynamic state of postsynaptic target structures that may actively contribute to the formation and plasticity of synaptic connections during CNS development.

Morphological plasticity in dendritic spines of cultured hippocampal neurons

Rat hippocampal neurons, grown in dissociated culture for about 18 days, were exposed for 6 h to three days to stimuli which cause either an increase (GABAA antagonists, bicuculline or picrotoxin), or decrease (tetrodotoxin) in spontaneous neuronal activity. Individual neurons were stained with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate and visualized with a confocal laser scanning microscope. GABA antagonists caused a marked, up to 60%, increase in spine density on secondary dendrites of cultured hippocampal neurons. This was associated with a small decrease in spine length. The rise in spine density was partially prevented by treatment with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetra-acetate, or by blockade of protein synthesis with cycloheximide. Tetrodotoxin caused a marked elongation of dendritic spines (but did not cause a decrease in spine density comparable to the increase caused by picrotoxin). This effect was seen primarily but not exclusively in spines with no distinct head. Both treatments were most effective within 24 h of exposure. There were no other systematic effects of the drugs on the morphology of the dendritic spines. These results indicate that dendritic spines in cultured neurons depend on ongoing synaptic activity to maintain their shape, and that neurons respond to an increase in synaptic demand by an increase in spine density. Thus, dendritic spines are likely to have a role in short-term synaptic interaction rather than to constitute a long-term memory storage device.

Regressive development in neuronal structure during song learning in birds

We investigated the development of spiny neurons in the lateral magnocellular nucleus of the anterior neostriatum before, during, and after song learning in male zebra finches (Taeniopygia guttata). The frequency of dendritic spines, dendritic field size, and branching characteristics were quantified at different ages in Golgi-stained tissue using a three-dimensional computerized tracing system. During development, overall spine frequencies increase between 3 and 5 weeks and decrease thereafter. In particular, spine frequencies of middle segments decrease significantly by 14% between 5 and 7 weeks posthatching (p = 0.017). A further reduction of 48% occurs between 7 weeks and adulthood (p < 0.001), resulting in a spine reduction of 56% on middle segments between 35 days of age and adulthood. In addition to the reduction of spine frequencies, we find regressive events also on some of the neuronal parameters that we have quantified. In general, dendrites of adult animals terminate closer to the cell body than those of 7-, 5-, or 3-week-old birds. Whereas no changes in segment length of first- and second-order dendrites have been identified, third-order dendrites end 19% closer to the cell body in adults than in younger birds (p < 0.024). Second-order dendrites in adult animals branch less frequently than in 3-week-old animals (35%, p = 0.017). There is also a trend of a smaller number of tertiary branches in adulthood compared with 3-week-old birds (41%, p = 0.060). The morphological changes may be related to the function of this nucleus and the sensitive phase for song acquisition.

Sex differences in the number of synaptic junctions in the binocular area of the rat visual cortex

We had found that the binocular area of the visual cortex is larger in volume and has more neurons in male than in female rats. The present study examined whether the number of synaptic junctions in this area is sexually dimorphic. Ten littermate pairs of 90-day-old (socially housed) Long-Evans hooded rats were used. Synaptic junctions were counted and their lengths were measured on electron micrographs taken from layers II-III of the binocular visual cortex. There were no sex differences in the numerical density of synaptic junctions, the number of synaptic junctions per neuron, or the length of synaptic junctions within any synaptic category or of all synapses combined. Sex differences were found in the total number of synaptic junctions and in several categories (asymmetric synapses, spine synapses, asymmetric spine synapses): male rats had more synaptic junctions than female rats because of the larger volume of layers II-III in the binocular area of male rats. These data indicate that neurons in the binocular visual cortex of both male and female rats receive a characteristic number of synaptic junctions, but the greater number of neurons in the binocular area of male rats results in more synaptic junctions in the area.

Alterations in intracellular calcium chelation reproduce developmental differences in repetitive firing and afterhyperpolarizations in rat neocortical neurons

Many 1-week-old rat sensorimotor cortical neurons exhibit extreme spike-frequency adaptation (neurons only fire for the first 100-250 ms of a 1 s current injection) accompanied by a large, prolonged afterhyperpolarization (AHP). Relatively greater expression of a Ca-dependent K+ current appears to underlie the extreme adaptation observed in immature cells. In the present study, we examined whether altering intracellular Ca2+ buffering by introducing Ca2+ chelators via the recording electrode could reproduce the age-related differences in firing and AHPs. We studied firing behavior and AHPs in 1-week-old and adult neocortical neurons with sharp microelectrodes, under three recording conditions: no chelator, 2 mM BAPTA, or 100-200 mM BAPTA. Our principal findings in regard to firing behavior and AHPs were that (1) adult-low BAPTA neurons mimicked 1 week-control cells, (2) 1 week-high BAPTA neurons were similar to adult-control cells, (3) a greater percentage of 1 week-low BAPTA neurons showed complete adaptation, and (4) adult neurons impaled with high BAPTA electrodes fired in a burst-spiking mode. These data suggest that Ca2+ regulation is qualitatively different in immature and adult neurons.

Intracellular calcium increase induced by GABA in visual cortex of fetal and neonatal rats and its disappearance with development

To address the question of whether gamma-aminobutyric acid (GABA) induces a change in the concentration of Ca2+ in neurons of the developing visual cortex, and if so, to elucidate a developmental profile of such a GABA-induced change, we measured intracellular Ca2+ signals using microscopic fluorometry in visual cortical slices loaded with rhod-2. The slices were prepared from rat fetuses of embryonic day 18 (E18) and rat pups of postnatal days 0-30 (P0-P30). Application of GABA through the perfusate at 100 microM induced a marked rise in intracellular Ca2+ signals in the cortical plate and subplate at E18 and P0-P2. After P5 the GABA-induced rise in Ca2+ dramatically reduced, and at P20 and thereafter it became undetectable. At E18 and P0-P2 an agonist for GABAA receptor, muscimol, induced a Ca2+ rise in the same way as did GABA, while a GABAB receptor agonist, baclofen, did not induce any significant rise in Ca2+ signals. Also, a GABAA receptor antagonist, bicuculline, blocked the GABA-induced rise in Ca2+ signals. These results indicate that the Ca2+ rise is triggered by activation of GABAA receptors. The application of Ni2+ at a concentration high enough to block all types of voltage-dependent CA2+ channels prevented the Ca2+ signals from increasing in response to GABA application, suggesting that Ca2+ may be influxed through such channels following depolarization evoked by GABA.

Neonatal frontal cortical lesions in rats alter cortical structure and connectivity

Rats were given frontal cortical lesions at day 1 or 10 of life. Later, as adults, they were either: (1) processed with Golgi-Cox in order to analyze cortical dendritic arborization; (2) given injections of True Blue into the parietal or visual cortex, or (3) given injections of [3H]leucine into the substantia nigra. An additional group of normal rats were given injections of fluorescent dyes into the cortex on day 4 or 10 of life. The main findings were that (1) adult hemispheres with day 10 lesions had greater dendritic arbor than normal hemispheres, (2) adult hemispheres with day 1 lesions had reduced dendritic branching relative to normal hemispheres, (3) adult rats with day 10 lesions had no obvious abnormalities in cortical connections, (4) adult rats with day 1 lesions had abnormal thalamo-cortical, amygdalo-cortical, and nigro-cortical connections, and (5) many of these abnormal connections were present in the brains of 4-day-old normal rats. Since the 'abnormal' connections in the very early frontal operates were present in day 4 animals, it appears that they result from the failure of exuberant connections to retract after the lesions. The increased dendritic growth in day 10 operates does not appear related to qualitative changes in cortical afferents or efferents and may related to increased intrinsic cortical connectivity. Since rats with day 10 lesions have previously been shown to exhibit significant recovery of function, it is possible that the increased dendritic arborization is supporting the functional restitution.

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex

The cat's visual cortex is immature at birth and undergoes extensive postnatal development. For example, cells of layers 2 and 3 do not complete migration until about 3 weeks after birth. Despite the importance of dendritic growth for synaptic and functional development, there have been few studies of dendritic development in the cat's visual cortex to correlate with numerous studies of functional and synaptic development. Accordingly, we used the Golgi method to study the development of the dendrites of layer 3 pyramidal cells in the visual cortex of a series of cats ranging in age from 2 days to 3 years. Blocks of visual cortex were impregnated by the Golgi-Kopsch method and sectioned in the tangential plane. Layer 3 pyramidal cells were drawn with a camera lucida and analyzed by Sholl diagrams and vector addition. In kittens < 1 week old, these cells were very immature, with only an apical dendrite and no basal dendrites. Basal dendrites appeared during the second week. By 2 weeks, all of the basal dendrites had emerged from the soma, but they had few branches and were tipped with growth cones. By 4 weeks, they had finished branching but continued to grow in length until, by 5 weeks, they reached their adult size. Examination of the basal dendritic fields in the tangential plane revealed that their dendritic fields were more elongated at 2 weeks than at later ages, perhaps because of their smaller size. The distribution of dendritic field orientations was uniform at all ages except 3 and 4 weeks, when there was a preponderance of fields oriented in the rostrocaudal direction. Because dendritic growth and branching occurred very rapidly over a period that precedes and overlaps with the peak periods of synaptogenesis and of sensitivity to the effects of early visual experience, they may depend on afferent visual activity. The early emergence of primary dendrites, however, suggests that this process is independent of afferent activity. The coincident timing of dendritic branching with the presence of dendritic growth cones suggests that branching may occur at growth cones.

Aspects of early postnatal development of cortical neurons that proceed independently of normally present extrinsic influences

To examine the contribution of local versus extrinsic influences on postnatal development of cortical neurons, we compared the maturation of deep (infragranular) layer neurons in isolated slices of neocortex grown in organotypic culture to a similar population of neurons developing in vivo. All slice cultures were prepared from sensorimotor cortices of newborn mice (P0) and neurons in these cultures were examined at daily intervals during the first 9 days in vitro (DIV). The maturational state of neurons developing in vivo over this same time period was assessed in acute slices prepared from animals of equivalent postnatal age, P1-P9. Electrophysiological recordings were obtained from neurons in both cultured and acute slices, using Lucifer yellow filled whole-cell recording electrodes, enabling subsequent morphometric analysis of the labeled cells. We report significant changes in both cellular morphology and electrical membrane properties of these deep layer cortical neurons during the first week in culture. Morphological maturation over this time period was characterized by a two- to three-fold increase in cell body size and total process length, and an increase in dendritic complexity. In this same population of cells a three-fold decrease in input resistance and changes in the action potential waveform, including a two-fold decrease in the AP duration, also occur. The degree of morphological and electrophysiological differentiation of individual neurons was highly correlated across developmental ages, suggesting that the maturational state of a cell is reflected in both cellular morphology and intrinsic membrane properties. A remarkably similar pattern of neuronal maturation was observed in neurons in layers V, VI/SP examined in acute slices prepared from animals between P1-P9. Because our culture system preserves many aspects of the local cortical environment while eliminating normal extrinsic influences (including thalamic, brainstem, and callosal connections), our findings argue that this early phase of neuronal differentiation, including the rate and extent of dendritic growth and development of AP waveform, results from instructive and/or permissive local influences, and appears to proceed independently of the many normally present extrinsic factors.

Development of geniculocortical projections to visual cortex in rat: evidence early ingrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Astrocytic and synaptic response to kindling in hippocampal subfield CA1. I. Synaptogenesis in response to kindling in vitro

Early morphological events associated with the genesis of epileptiform activity are essentially unknown, despite significant progress on morphological correlates of potentially related plastic neural phenomena. Hippocampal area CA1 shows the capacity to generate epileptiform bursting activity after certain patterns of electrical stimulation. Using an in vitro slice kindling preparation, we found increases in the numbers (areal densities) of shaft and sessile spine synapses in hippocampal subfield CA1 within minutes following the establishment of stable afterdischarges. These data strongly suggest that synaptogenesis is associated with the early stages of epilepsy formation.

Sex differences in cortical thickness and the dendritic tree in the monocular and binocular subfields of the rat visual cortex at weaning age

The visual cortex of adult rats is sexually dimorphic at both the gross size and dendritic levels [Brain Res., 295 (1984) 27-34; J. Comp. Neurol., in press]. In addition, sex differences in the dendritic tree are dynamic and can be altered by environmental conditions imposed at weaning [Brain Res., 295 (1984) 27-34]. The present study examines sex differences in cortical thickness and in the dendritic tree of the monocular (Oc1M) and binocular (Oc1B) subfields in littermate male and female pairs of Long-Evans rats at weaning age (25 days). From Nissl-stained tissue, it was found that the whole cortex and layer II-IV of Oc1B was thicker in males than females. No sex differences were found in the thickness of Oc1M. Golgi-Cox-stained pyramidal neurons in layer III from the Oc1M and Oc1B regions were quantified in 7 littermate pairs of weaning-age rats. There were no sex differences in the basilar tree, while the apical oblique branches were sexually dimorphic, especially in the monocular region. Females had greater total dendritic length and longer terminal branches in Oc1M compared to males. Females also had longer bifurcating branches in both Oc1M and Oc1B than males. The present study found that sex differences at weaning age do not completely mirror the dimorphisms found in the visual cortex of the adult rat. This study also indicates that related subfields can differ in their morphology and should be examined separately.

Ultrastructural organization of slice cultures from rat visual cortex

We have been studying the fine structural organization of slice cultures prepared from the visual cortex of 6-day-old rats and cultured for 2 weeks using a roller culture technique. Neurons in culture exhibited the characteristic cytological differences between perikarya, axons and dendrites. Neuronal and glial processes formed a dense neuropil with minimal extracellular spaces, and within the neuropil there were numerous synaptic contacts. Both morphological types of cortical synapses, type I (asymmetrical) and type II (symmetrical) could be readily identified in slice cultures. The pattern of synaptic connections in culture was remarkably similar to that observed in normal cerebral cortex; asymmetrical synapses were usually found in contact with dendritic spines, less frequently with dendritic shafts, and never on perikarya, whereas symmetrical synapses were found mostly on perikarya, occasionally on dendritic shafts but never on dendritic spines. Synaptic morphology appeared mature after 2 weeks in vitro and did not show the immature features observed at the time of culture preparation. Taken together with our previous light microscopic studies, these results indicate that cortical slice cultures are organotypically organized and serve as a useful model to study mechanisms of cortical development and plasticity.

Intracellular recording and injection study of corticospinal neurons in the rat somatosensory cortex: effect of prenatal exposure to ethanol

The effects of prenatal exposure to ethanol on the structure and function of corticospinal neurons was investigated. The subjects were the 3-4-month-old offspring of hooded rats fed a nutritionally balanced liquid diet containing 6.7% (v/v) ethanol (Et), pair-fed a nutritionally matched isocaloric diet (Ct), or fed chow and water (Ch). Corticospinal neurons in primary somatosensory cortex were examined by intracellularly recording and filling cells that were driven by antidromic stimulation of the pyramidal decussation. In the control rats, corticospinal neurons comprised a homogeneous morphophysiological population. Morphologically, all of the antidromically driven cells examined were pyramidal neurons with cell bodies in layer Vb. The dendrites of these neurons were spinous and branched within layers I, IV, and V. Their axons arborized within layers IV, V, and VI and some collaterals extended laterally for distances up to 2.6 mm from the cell body. The mean conduction latency was 3.6 and 3.4 msec for Ch- and Ct-treated rats, respectively. In Et-treated rats, corticospinal neurons constituted a heterogeneous population. The laminar distribution of the corticospinal neurons in Et-treated rats was broad; the cell bodies of labeled neurons were in layers II, IV, V, and VI. The dendrites of layer Vb neurons were spinous; however, many of the spines appeared dysmorphic and the density of spines was significantly greater (32%) in Et-treated rats than in Ct-treated rats. Although the dendritic branching pattern for layer Vb neurons was similar to that described for the controls, a Sholl analysis showed that the complexity and extent of their dendritic trees were significantly greater in Et-treated rats. The axons of all layer Vb neurons in Et-treated rats had long horizontal processes that arborized in layers IV-VI, and some neurons also had an array of collaterals that ascended to layer I. The mean conduction latency for layer Vb neurons was 3.9 msec. The structure and function of ectopic neurons (those in layers II, IV, Va, Vc, and VI) in Et-treated rats differed markedly from those of the layer Vb neurons. Morphologically, the dendritic and axonal fields of these neurons were narrower than for the layer Vb neurons, and the ectopic neurons had a mean conduction latency of 7.1 msec. The heterogeneity of the population of corticospinal neurons in Et-treated rats may result from the effects of ethanol on early events in neuronal development such as neuronal generation and migration.

Cellular organization and development of slice cultures from rat visual cortex

Slice cultures from the visual cortex of young rats were prepared using the roller culture technique (Gähwiler 1984). After 10 days in vitro the cortical cultures flattened to 1-3 cell layers, surviving for up to 12 weeks. The cultures were organotypically organized, the typical layered structure of the cortex was preserved. The neuronal composition of slice cultures was studied using intracellular staining, Golgi impregnation and GABA immunohistochemistry. Both pyramidal cells and several types of nonpyramidal cells were identified in the slice cultures. Electrophysiological recordings showed that the electrical properties of cells in culture were similar to those measured in acute slice preparations; for some cells, however, the spontaneous activity was higher. The maintained activity was strongly increased by application of the GABA antagonist bicuculline and decreased by GABA, suggesting that GABAergic inhibition is present in these preparations. We could observe the postnatal maturation of some characteristic morphological features in culture. For example, pyramidal cells in 6 day-old rats in situ have very short basal dendrites with growth-cones, and the dendrites are free of spines. After 2-3 weeks in culture growth-cones were no longer observed. Instead, the cells had developed a large basal dendritic field and the dendrites were covered with spines. Slice cultures therefore may provide a useful tool for physiological, anatomical, pharmacological and developmental studies of cortical neurons in an organotypical environment.

Endogenous opioid systems and the regulation of dendritic growth and spine formation

The role of endogenous opioid systems (endogenous opioids and opioid receptors) in neuronal development was examined in 10- and 21-day-old rats by utilizing an opioid antagonist (naltrexone) paradigm. Throughout the first 3 weeks of life, Sprague-Dawley rats were given daily subcutaneous injections of either 50 mg/kg naltrexone, a dosage that invoked a complete (24 hours/day) receptor blockade, or 1 mg/kg naltrexone, a dosage which intermittently blocked (4-6 hours/day) opioid receptors and exacerbated opioid action; animals injected with sterile water served as controls. Pyramidal cells from the frontoparietal cortex (layer III) and hippocampal field CA1, and cerebellar Purkinje cells, were impregnated by using the Golgi-Kopsch method; total and mean dendrite segment length, branch frequency, and spine concentration were analyzed morphometrically. Perturbations of endogenous opioid systems caused region-dependent alterations in dendrite complexity and/or spine concentration in all brain areas. Continuous opioid receptor blockade resulted in dramatic increases in dendrite and/or spine elaboration compared to controls at 10 days in all brain regions; however, these increases were only evident in the hippocampus at 21 days. With intermittent blockade, dendrite and/or spine growth were often subnormal, being predominant at day 21. Our results indicate that endogenous opioid systems are critical regulators of neuronal differentiation, and they control growth through an inhibitory mechanism. Considering previous findings demonstrating that neurobehavioral ontogeny is dependent on endogenous opioid-opioid receptor interactions, the present results suggest an opioid-dependent, structure-function relationship between neuronal and behavioral maturation.