A quantitative electron microscopic study of synaptogenesis in the dentate gyrus of the rat

New findings on extinction of conditioned fear early in development: theoretical and clinical implications

Research with adult animals suggests that extinction depends, at least partly, on new inhibitory learning that is specific to the context in which it is learned. However, several recent studies show that extinction processes are dissociated across development. The present article reviews research on the behavioral and neurobiological mechanisms underlying extinction in developing rats. To summarize, postweanling aged rats (i.e., 24-day-olds) display adult-like extinction in that they show renewal, reinstatement, spontaneous recovery, and compound summation of extinguished stimuli. However, preweanling aged rats (i.e., 17-day-olds) do not show any of those behavioral phenomena. Pharmacological studies also show that reducing N-methyl-D-aspartate, gamma-aminobutryic acid, and opioid neurotransmission impairs extinction in 24-day-old rats, but extinction in P17 rats is only affected by the blocking of opioid neurotransmission. Lastly, extinction in 24-day-old rats involves the amygdala and the ventromedial prefrontal cortex (vmPFC), which are critical brain areas in the neural circuitry of extinction in adult rats. Interestingly, extinction in 17-day-old rats involves the amygdala but not the vmPFC. The existing models of extinction cannot account for these developmental differences. These findings showing that distinct processes mediate extinction at different stages of development may have significant clinical implications, which are discussed in this review.

Synaptogenesis of hippocampal neurons in primary cell culture

Hippocampal neurons in dissociated cell culture are one of the most extensively used model systems in the field of molecular and cellular neurobiology. Only limited data are however available on the normal time frame of synaptogenesis, synapse number and ultrastructure of excitatory synapses during early development in culture. Therefore, we analyzed the synaptic ultrastructure and morphology and the localization of presynaptic (Bassoon) and postsynaptic (ProSAP1/Shank2) marker proteins in cultures established from rat embryos at embryonic day 19, after 3, 7, 10, 14, and 21 days in culture. First excitatory synapses were identified at day 7 with a clearly defined postsynaptic density and presynaptically localized synaptic vesicles. Mature synapses on dendritic spines were seen from day 10 onward, and the number of synapses steeply increased in the third week. Fenestrated or multiple synapses were found after 14 or 21 days, respectively. So-called dense-core vesicles, responsible for the transport of proteins to the active zone of the presynaptic specialization, were seen on cultivation day 3 and 7 and could be detected in axons and especially in the presynaptic subcompartments. The expression and localization of the presynaptic protein Bassoon and of the postsynaptic molecule ProSAP1/Shank2 was found to correlate nicely with the ultrastructural results. This regular pattern of development and maturation of excitatory synapses in hippocampal culture starting from day 7 in culture should ease the comparison of synapse number and morphology of synaptic contacts in this widely used model system.

General anesthesia causes long-lasting disturbances in the ultrastructural properties of developing synapses in young rats

Common general anesthetics administered to young rats at the peak of brain development cause widespread apoptotic neurodegeneration in their immature brain. Behavioral studies have shown that this leads to learning and memory deficiencies later in life. The subiculum, a part of the hippocampus proper and Papez's circuit, is involved in cognitive development and is vulnerable to anesthesia-induced developmental neurodegeneration. This degeneration is manifested by acute substantial neuroapoptotic damage and permanent neuronal loss in later stages of synaptogenesis. Since synapse formation is a critical component of brain development, we examined the effects of highly neurotoxic anesthesia combination (isoflurane, nitrous oxide, and midazolam) on ultrastructural development of synapses in the rat subiculum. We found that this anesthesia, when administered at the peak of synaptogenesis, causes long-lasting injury to the subicular neuropil. This is manifested as neuropil scarcity and disarray, morphological changes indicative of mitochondria degeneration, a decrease in the number of neuronal profiles with multiple synaptic boutons and significant decreases in synapse volumetric densities. We believe that observed morphological disturbances of developing synapses may, at least in part, contribute to the learning and memory deficits that occur later in life after exposure of the immature brain to general anesthesia.

Long term synaptic depression that is associated with GluR1 dephosphorylation but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor internalization

Long lasting changes in the strength of synaptic transmission in the hippocampus are thought to underlie certain forms of learning and memory. Accordingly, the molecular mechanisms that account for these changes are heavily studied. Postsynaptically, changes in synaptic strength can occur by altering the amount of neurotransmitter receptors at the synapse or by altering the functional properties of synaptic receptors. In this study, we examined the biochemical changes produced following chemically induced long term depression in acute hippocampal CA1 minislices. Using three independent methods, we found that this treatment did not lead to an internalization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Furthermore, when the plasma membrane was separated into synaptic membrane-enriched and extrasynaptic membrane-enriched fractions, we actually observed a significant increase in the concentration of AMPA receptors at the synapse. However, phosphorylation of Ser-845 on the AMPA receptor subunit GluR1 was significantly decreased throughout the neuron, including in the synaptic membrane-enriched fraction. In addition, phosphorylation of Ser-831 on GluR1 was decreased specifically in the synaptic membrane-enriched fraction. Phosphorylation of these residues has been demonstrated to control AMPA receptor function. From these data, we conclude that the decrease in synaptic strength is likely the result of a change in the functional properties of AMPA receptors at the synapse and not a decrease in the amount of synaptic receptors.

Synapsin-dependent development of glutamatergic synaptic vesicles and presynaptic plasticity in postnatal mouse brain

Inactivation of the genes encoding the neuronal, synaptic vesicle-associated proteins synapsin I and II leads to severe reductions in the number of synaptic vesicles in the CNS. We here define the postnatal developmental period during which the synapsin I and/or II proteins modulate synaptic vesicle number and function in excitatory glutamatergic synapses in mouse brain. In wild-type mice, brain levels of both synapsin I and synapsin IIb showed developmental increases during synaptogenesis from postnatal days 5-20, while synapsin IIa showed a protracted increase during postnatal days 20-30. The vesicular glutamate transporters (VGLUT) 1 and VGLUT2 showed synapsin-independent development during postnatal days 5-10, following which significant reductions were seen when synapsin-deficient brains were compared with wild-type brains following postnatal day 20. A similar, synapsin-dependent developmental profile of vesicular glutamate uptake occurred during the same age periods. Physiological analysis of the development of excitatory glutamatergic synapses, performed in the CA1 stratum radiatum of the hippocampus from the two genotypes, showed that both the synapsin-dependent part of the frequency facilitation and the synapsin-dependent delayed response enhancement were restricted to the period after postnatal day 10. Our data demonstrate that while both synaptic vesicle number and presynaptic short-term plasticity are essentially independent of synapsin I and II prior to postnatal day 10, maturation and function of excitatory synapses appear to be strongly dependent on synapsin I and II from postnatal day 20.

Synaptophysin protein and mRNA expression in the human hippocampal formation from birth to old age

In the human neocortex, progressive synaptogenesis in early postnatal life is followed by a decline in synaptic density, then stability from adolescence until middle age. No comparable data are available in the hippocampus. In this study, the integral synaptic vesicle protein synaptophysin, measured immunoautoradiographically, was used as an index of synaptic terminal abundance in the hippocampal formation of 37 subjects from 5 weeks to 86 yr old, divided into 4 age groups (10 infants, 15 adolescents/young adults, 6 adults, and 6 elderly). In all hippocampal subfields, synaptophysin was lowest in infancy, but did not differ significantly between the older age groups, except in dentate gyrus (DG) where the rise was delayed until adulthood. A similar developmental profile was found in the rat hippocampus. We also measured synaptophysin mRNA in the human subjects and found no age-related changes, except in parahippocampal gyrus wherein the mRNA declined from infancy to adolescence, and again in old age. The synaptophysin protein data demonstrate a significant presynaptic component to human postnatal hippocampal development. In so far as synaptophysin abundance reflects synaptic density, the findings support an increase in hippocampal and parahippocampal synapse formation during early childhood, but provide no evidence for adolescent synaptic pruning. The mRNA data indicate that the maturational increases in synaptophysin protein are either translational rather than transcriptional in origin, or else are secondary to mRNA increases in neurons, the cell bodies of which lie outside the hippocampal formation.

Morphological development and maturation of granule neuron dendrites in the rat dentate gyrus

The first granule neurons in the dentate gyrus are born during late embryogenesis in the rodent, and the primary period of granule cell neurogenesis continues into the second postnatal week. On the day of birth in the rat, the oldest granule neurons are visible in the suprapyramidal blade and exhibit rudimentary dendrites extending into the molecular layer. Here we describe the morphological development of the dendritic trees between birth and day 14, and we then review the process of dendritic remodeling that occurs after the end of the second week. Data indicate that the first adult-like granule neurons are present on day 7, and, furthermore, physiological recordings demonstrate that some granule neurons are functional at this time. Taken together, these results suggest that the dentate gyrus may be incorporated into the hippocampal circuit as early as the end of the first week. The dendritic trees of the granule neurons, however, continue to increase in size until day 14. After that time, the dendritic trees of the oldest granule neurons are sculpted and refined. Some dendrites elongate while others are lost, resulting in a conservation of total dendritic length. We end this chapter with a review of the quantitative aspects of granule cell dendrites in the adult rat and a discussion of the relationship between the morphology of a granule neuron and the location of its cell body within stratum granulosum and along the transverse axis of the dentate gyrus.

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

Neurobiology of infant attachment

A strong attachment to the caregiver is critical for survival in altricial species, including humans. While some behavioral aspects of attachment have been characterized, its neurobiology has only recently received attention. Using a mammalian imprinting model, we are assessing the neural circuitry that enables infant rats to attach quickly to a caregiver, thus enhancing survival in the nest. Specifically, the hyper-functioning noradrenergic locus coeruleus (LC) enables pups to learn rapid, robust preference for the caregiver. Conversely, a hypo-functional amygdala appears to prevent the infant from learning aversions to the caregiver. Adult LC and amygdala functional emergence correlates with sensitive period termination. This study suggests the neonatal brain is not an immature version of the adult brain but is uniquely designed to optimize attachment to the caregiver. Although human attachment may not rely on identical circuitry, the work reviewed here suggests a new conceptual framework in which to explore human attachments, particularly attachments to abusive caregivers.

Age-related changes in Usp9x protein expression and DNA methylation in mouse brain

Usp9x, a ubiquitin-specific protease implicated in synaptic development, was found to be more abundant in adult as compared to newborn mouse brain tissue. The Usp9x gene was less methylated in adults than in newborns in both the promoter and the protein coding region. Compared with newborns, the adult mouse brain also had lower levels of Dnmt1, the enzyme responsible for maintaining DNA methylation state. These age-associated changes in DNA methylation and ubiquitin system protein concentrations potentially contribute to developmental changes in brain structure and function.

Rett syndrome and neuronal development

The clinical signs of Rett syndrome, as well as neuropathology and brain imaging, suggest that the disorder disrupts neuronal circuits. Studies using receptor autoradiography demonstrate abnormalities in the density of excitatory glutamate and inhibitory gamma-aminobutyric acid (GABA) synaptic receptors in postmortem brain from young female subjects with Rett syndrome. MeCP2, the protein that is abnormal in most female individuals with Rett syndrome, is expressed predominantly in neurons and appears during development at the time of synapse formation. Studies of nasal epithelium from patients with Rett syndrome show that the maturation of olfactory receptor neurons is impeded prior to the time of synapse formation. Recent reports indicate that MeCP2 controls the expression of brain-derived neurotrophic factor and the DNA-binding homeobox protein Dlx5. Brain-derived neurotrophic factor enhances glutamate neurotransmission at excitatory synapses, whereas Dlx5 is expressed in most GABAergic neurons and stimulates the synthesis of GABA. Taken together, this information supports the hypothesis that Rett syndrome is a genetic disorder of synapse development, especially synapses that use glutamate and GABA as neurotransmitters.

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

Selective serotonin reuptake inhibitor treatment of early postnatal mice reverses their prenatal stress-induced brain dysfunction

Prenatal stress has long-lasting effects on cognitive function and on the hypothalamic-pituitary-adrenal response to stress. We previously reported that the serotonin concentration and synaptic density in the hippocampus were reduced following prenatal stress [Int J Dev Neurosci 16 (1998) 209]. Since serotonin plays a role in the formation and maintenance of synapses, we hypothesized that a neonatal reduction in hippocampal serotonin levels may lead to learning disabilities in prenatally stressed mice. To test this hypothesis, we treated prenatally stressed mice with a selective serotonin reuptake inhibitor in order to normalize their postnatal serotonin turnover levels. What we found was that the oral administration of a selective serotonin reuptake inhibitor to prenatally stressed mice during postnatal weeks 1-3 but not 6-8 normalized their corticosterone response to stress, serotonin turnover in the hippocampus, and density of dendritic spines and synapses in the hippocampal CA3 region. Concomitantly, such treatment partially restored their ability to learn spatial information.

Expression of mKirre, a mammalian homolog of Drosophila kirre, in the developing and adult mouse brain

mKirre, a mammalian homolog of the Drosophila kirre, is expressed in bone marrow stromal cells and the brain. Although mKirre has been shown to support the hematopoietic stem cells, little is known about the function of mKirre in the brain. In the present study, to gain insights into the function of mKirre, we investigated the expression pattern of mKirre gene in the developing and adult mouse brain using in situ hybridization. In the adult brain, mKirre mRNA was highly expressed in the olfactory bulb, the piriform cortex, the cochlear nucleus, and the cerebellum. At embryonic day (E) 11.5, we could observe mKirre mRNA in the differentiating zones of various regions, such as the caudate-putamen, the geniculate body, the thalamus, the amygdala, and the brainstem. Its gene expression in these regions at E11.5 also persisted to the adult, in which its expression levels were much less prominent. After birth, we could first observe high expression of mKirre mRNA in the glomerular and mitral layers of the olfactory bulb, the cortical plate of the neocortex, the cochlear nucleus, and the molecular and granule cell layers of the cerebellum. In the hippocampus, its gene expression was first observed in the dentate gyrus at postnatal day 7. The spatiotemporal expression pattern of mKirre mRNA suggests important roles of mKirre in later developmental processes, especially the synapse formation.

Corticosterone controls the developmental emergence of fear and amygdala function to predator odors in infant rat pups

In many altricial species, fear responses such as freezing do not emerge until sometime later in development. In infant rats, fear to natural predator odors emerges around postnatal day (PN) 10 when infant rats begin walking. The behavioral emergence of fear is correlated with two physiological events: functional emergence of the amygdala and increasing corticosterone (CORT) levels. Here, we hypothesize that increasing corticosterone levels influence amygdala activity to permit the emergence of fear expression. We assessed the relationship between fear expression (immobility similar to freezing), amygdala function (c-fos) and the level of corticosterone in pups in response to presentation of novel male odor (predator), littermate odor and no odor. CORT levels were increased in PN8 pups (no fear, normally low CORT) by exogenous CORT (3 mg/kg) and decreased in PN12 pups (express fear, CORT levels higher) through adrenalectomy and CORT replacement. Results showed that PN8 expression of fear to a predator odor and basolateral/lateral amygdala activity could be prematurely evoked with exogenous CORT, while adrenalectomy in PN12 pups prevented both fear expression and amygdala activation. These results suggest that low neonatal CORT level serves to protect pups from responding to fear inducing stimuli and attenuate amygdala activation. This suggests that alteration of the neonatal CORT system by environmental insults such as alcohol, stress and illegal drugs, may also alter the neonatal fear system and its underlying neural control.

Altered localization of choline transporter sites in the mouse hippocampus after prenatal heroin exposure

Prenatal heroin exposure disrupts hippocampal cholinergic synaptic function and related behaviors. Biochemical studies indicate an increase in the number of presynaptic high-affinity choline transporter (HACT) sites, as assessed by [3H]hemicholinium-3 (HC-3) binding. The present study was designed to assess whether this effect involves global upregulation of the transporter, or whether disruption occurs with a specific tempero-spatial distribution. Pregnant mice were given 10mg/kg per day of heroin subcutaneously on gestational days (GD) 9-18. Autoradiographic distribution of HC-3 binding sites was evaluated in the hippocampus of the offspring at postnatal days 15, 25, and 53. These results, suggestive of hippocampal "miswiring," are likely to explain the net impairment of cholinergic synaptic function after prenatal heroin exposure, despite the simultaneous upregulation of both presynaptic cholinergic activity and postsynaptic receptors. Understanding the subregional selectivity of hippocampal defects can lead to the development of strategies that may potentially enable therapeutic interventions to offset or reverse the neurobehavioral defects.

Developmental expression of methyl-CpG binding protein 2 is dynamically regulated in the rodent brain

The gene encoding methyl-CpG binding protein 2 (MeCP2) is mutated in the large majority of girls that have Rett Syndrome (RTT), an X-linked neurodevelopmental disorder. To better understand the developmental role of MeCP2, we studied the ontogeny of MeCP2 expression in rat brain using MeCP2 immunostaining and Western blots. MeCP2 positive neurons were present throughout the brain at all ages examined, although expression varied by region and age. At early postnatal ages, regions having neurons that were generated early and more mature had the strongest MeCP2 expression. Late developing structures including cortex, hippocampus and cerebellum exhibited the most significant changes in MeCP2 expression. Of these regions, the cerebellum showed the most striking cell-specific changes in MeCP2 expression. For example, the early-generated Purkinje cells became MeCP2 positive by P6, while the late-generated granule cells did not express MeCP2 until the fourth postnatal week. The timing of MeCP2 expression in the granule cell layer is coincident with the onset of granule cell synapse formation. Although more subtle, the degree of MeCP2 expression in cortex and hippocampus was most closely correlated with synaptogenesis in both regions. Our finding that MeCP2 expression is correlated with synaptogenesis is consistent with the hypothesis that Rett Syndrome is caused by defects in the formation or maintenance of synapses.

Corticosterone influences on Mammalian neonatal sensitive-period learning

Infant rats exhibit sensitive-period odor learning characterized by olfactory bulb neural changes and odor preference acquisitions critical for survival. This sensitive period is coincident with low endogenous corticosterone (CORT) levels and stress hyporesponsivity. The authors hypothesized that low corticosterone levels modulate sensitive-period learning. They assessed the effects of manipulating CORT levels by increasing and removing CORT during (Postnatal Day 8) and after (Postnatal Day 12) the sensitive period. Results show that (a) exogenous CORT prematurely ends sensitive-period odor-shock-induced preferences; (b) adrenalectomy developmentally extends the sensitive period as indicated by odor-shock-induced odor-preference learning in older pups, whereas CORT replacement can reinstate fear learning; and (c) CORT manipulation modulates olfactory bulb correlates of sensitive-period odor learning in a manner consistent with behavior.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Cycles of aberrant synaptic sprouting and neurodegeneration in Alzheimer's and dementia with Lewy bodies

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) are the most common neurodegenerative disorders affecting the elderly. The cognitive and motor deficits in these diseases are associated with the disruption of neuritic substructure, loss of synaptic contacts in selectively vulnerable circuitries, and aberrant sprouting. Where as in AD, accumulation of misfolded forms of Abeta triggers neurodegeneration, in DLB accumulation of alpha-synuclein might play a central role. The mechanisms by which oligomeric forms of these proteins might lead to cycles of synapse loss and aberrant sprouting are currently under investigation. Several possibilities are being considered, including mitochondrial damage, caspase activation, lysosomal leakage, fragmentation of the Golgi apparatus, interference with synaptic vesicle transport and function, and interference with gene transcription and signaling. Among them, recent lines of research support the possibility that alterations in signaling pathways such extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 relevant to synaptic plasticity and cell survival might play a pivotal role. A wide range of cellular functions are affected by the accumulation of misfolded Abeta and alpha-synuclein; thus it is possible that a more fundamental cellular alteration may underlie the mechanisms of synaptic pathology in these disorders. Among them, one possibility is that scaffold proteins, such as caveolin and JNK-interacting protein (JIP), which are necessary to integrate signaling pathways, are affected, leading to cycles of synapse loss and aberrant sprouting. This is significant because both caveolar dysfunction and altered axonal plasticity might be universally important in the pathogenesis of various neurodegenerative disorders, and therefore these signaling pathways might be common therapeutic targets for these devastating diseases.

Early eyelid opening enhances spontaneous alternation and accelerates the development of perforant path synaptic strength in the hippocampus of juvenile rats

Development of the hippocampus is not entirely preprogrammed; its structure and function are sensitive to postnatal experience. For instance, neonatal handling/exposure to novelty and peripubertal environmental enrichment enhance hippocampal function and related memory abilities. However, these complex environmental manipulations make it difficult to deduce the primary stimuli that drive more rapid hippocampal maturation, and few experiments have studied the neural mechanisms that support the behavioral modifications. To address these issues, I performed early eyelid opening in rat pups and examined developmental alterations in exploration of a Y-maze and in synaptic transmission measured in hippocampal slices. Early eyelid opening accelerated development of spontaneous alternation. Additionally, early eyelid opening promoted more rapid remodeling of afferent input to the dentate gyrus and area CA1 as well as earlier maturation of perforant path synaptic physiology. These findings implicate visual input as an extrinsic factor that drives hippocampal development and the emergence of hippocampal-dependent behavior.

Maturation of granule cell dendrites after mossy fiber arrival in hippocampal field CA3

Most granule neurons in the rat dentate gyrus are born over the course of the first 2 postnatal weeks. The resulting heterogeneity has made it difficult to define the relationship between dendritic and axonal maturation and to delineate a time course for the morphological development of the oldest granule neurons. By depositing crystals of the fluorescent label Dil in hippocampal field CA3, we retrogradely labeled granule neurons in fixed tissue slices from rats aged 2-9 days. The results showed that all labeled granule cells, regardless of the age of the animal, exhibited apical dendrites. On day 2, every labeled neuron had rudimentary apical dendrites, and a few dendrites on each cell displayed immature features such as growth cones, varicosities, and filopodia. Some cells displayed basal dendrites. By day 4, the most mature granule neurons had longer and more numerous apical branches, as well as various immature features. Most had basal dendrites. On days 5 and 6, the immature features and the basal dendrites had begun to regress on the oldest cells, and varying numbers of spines were present. On day 7, the first few adult-like neurons were seen: immature features and basal dendrites had disappeared, all dendrites reached the top of the molecular layer, and the entire dendritic tree was covered with spines. These data show that dendritic outgrowth occurs before, or concurrent with, axon arrival in the CA3 target region, and that adult-like granule neurons are present by the end of the first week.

Maternal hypothalamic-pituitary-adrenal disregulation during the third trimester influences human fetal responses

Maternal peptides from the hypothalamic-pituitary-adrenal (HPA) axis rise during human pregnancy. The effects of circulating maternal adrenocorticotropin (ACTH) and beta-endorphin (BE) on human fetal behavior was determined in 135 women during their 32nd week of gestation. Fetal behavior was measured by assessing heart rate habituation to a series of repeated vibroacoustic stimuli. Individual differences in habituation were determined by computing the number of consecutive responses above the standard deviation during a control period. There was no significant relation between levels of ACTH, BE and the rate of fetal heart rate habituation. However, an index of HPA disregulation (uncoupling of ACTH and BE) was related significantly to fetal behavior. Fetal exposure to high levels of maternal BE relative to ACTH was associated with significantly lower rates of habituation. Results indicate that maternal stress and stress-related peptides influence fetal response patterns. It is possible that this influence persists over the life span.

Early disruption of the mother-infant relationship: effects on brain plasticity and implications for psychopathology

Early environmental manipulations can impact on the developing nervous system, contributing to shape individual differences in physiological and behavioral responses to environmental challenges. In particular, it has been shown that disruptions in the mother-infant relationship result in neuroendocrine, neurochemical and behavioural changes in the adult organism, although the basic mechanisms underlying such changes have not been completely elucidated. Recent data suggest that neurotrophins might be among the mediators capable of transducing the effects of external manipulations on brain development. Nerve growth factor and brain-derived neurotrophic factor are known to play a major role during brain development, while in the adult animal they are mainly responsible for the maintenance of neuronal function and structural integrity. Changes in the levels of neurotrophic factors during critical developmental stages might result in long-term changes in neuronal plasticity and lead to increased vulnerability to aging and to psychopathology.

Neonatal maternal separation reduces hippocampal mossy fiber density in adult Long Evans rats

Neonatal maternal separation of rat pups leads to a stable stress hyper-responsive phenotype characterized by increased basal levels of corticotropin releasing factor (CRF) mRNA in the hypothalamic and extra-hypothalamic nuclei, increased hypothalamic CRF release, and enhanced adrenocorticotrophin hormone (ACTH) and corticosterone (CORT) responses to psychological stressors. Stress and exposure to glucocorticoids either early in life or in adulthood have been associated with hippocampal atrophy and impairments in learning and memory. In this study, male Long Evans rat pups were exposed to daily 3-h (HMS180) or 15-min (HMS15) periods of maternal separation on postnatal days (PND) 2-14 or normal animal facility rearing. Maternal separation and subsequent reunion with the dam resulted in elevated plasma CORT levels versus HMS15 animals at PND7, a time when rat pups are normally hyporesponsive to stressors and show limited pituitary-adrenal responses. As adults, HMS180 rats exhibited elevated indices of anxiety, startle-induced pituitary-adrenal hyper-responsiveness, and slight, but significant impairment on acquisition in the Morris water maze task. In addition, HMS180 rats exhibited decreased mossy fiber density in the stratum oriens region of the hippocampus as measured by Timm's staining, but no change in volume of the dentate gyrus. These changes may be the result of neonatal exposure to elevated glucocorticoids and/or changes in other signaling systems in response to maternal separation. Overall the results suggest that repeated, daily, 3-h maternal separations during critical periods of hippocampal development can disrupt hippocampal cytoarchitecture in a stable manner. The resulting change in morphology may contribute to the subtle, but consistent learning deficit and overall stress hyper-responsive phenotype observed in these animals.

Functional neurogenesis in the adult hippocampus

There is extensive evidence indicating that new neurons are generated in the dentate gyrus of the adult mammalian hippocampus, a region of the brain that is important for learning and memory. However, it is not known whether these new neurons become functional, as the methods used to study adult neurogenesis are limited to fixed tissue. We use here a retroviral vector expressing green fluorescent protein that only labels dividing cells, and that can be visualized in live hippocampal slices. We report that newly generated cells in the adult mouse hippocampus have neuronal morphology and can display passive membrane properties, action potentials and functional synaptic inputs similar to those found in mature dentate granule cells. Our findings demonstrate that newly generated cells mature into functional neurons in the adult mammalian brain.

Mechanisms of postnatal neurobiological development: implications for human development

This review focuses on the postnatal neuroanatomical changes that arise during the first years of human life. Development is characterized by 2 major organizational periods. The first period begins at conception and includes the major histogenetic events such as neurulation, proliferation, migration, and differentiation. It has been proposed that these events may be controlled by genetic and epigenetic events, which give rise to neural structures that are amenable to external influence. The second period is a time of reorganization in the human cortex. These events occur during gestation and continue postnatally, possibly through the 2nd decade of life. This stage is characterized by dendritic and axonal growth, synapse production, neuronal and synaptic pruning, and changes in neurotransmitter sensitivity. Although the initiation of these events is influenced by endogenous signals, further neural maturation is primarily influenced by exogenous signals. To illustrate both the progressive and regressive events during the postnatal period, we use examples from the development of the human cortex.

Unique Characteristics of Neonatal Classical Conditioning: The Role of the Amygdala and Locus Coeruleus

The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. During the first postnatal days, infant rats show a sensitive period for learning and are particularly susceptible to learning an attraction to their mother's odor. Classical conditioning appears to underlie this learning that is expressed behaviorally as an increased ability to acquire odor preferences and a decreased ability to acquire odor aversions. Specifically, in neonatal rats, pairing an odor with moderately painful shock (0.5mA) or milk produces a subsequent relative preference for that odor. The neural circuitry supporting the increased ability to acquire odor preferences appears to be the heightened functioning of the noradrenergic pontine nucleus locus coeruleus. Indeed, norepinephrine from the locus coeruleus appears to be both necessary and sufficient for learning during the sensitive period. On the other hand, the decreased ability to acquire odor aversions seems to be due to the lack of participation of the amygdala in at least some aversive learning situations. The site of plasticity in the pup's brain appears to be limited to the olfactory bulb. This neonatal sensitive period for learning ends around postnatal day 9-10, at which time pups make the transition from crawling to walking and classical conditioning becomes "adultlike." The neonatal behavioral and neural induced changes are retained into adulthood where it modifies sexual behavior.

Developmental expression of the novel voltage-gated sodium channel auxiliary subunit beta3, in rat CNS

1. We have compared the mRNA distribution of sodium channel alpha subunits known to be expressed during development with the known auxiliary subunits Nabeta1.1 and Nabeta2.1 and the novel, recently cloned subunit, beta3. 2. In situ hybridisation studies demonstrated high levels of Nav1.2, Nav1.3, Nav1.6 and beta3 mRNA at embryonic stages whilst Nabeta1.1 and Nabeta2.1 mRNA was absent throughout this period. 3. Nabeta1.1 and Nabeta2.1 expression occurred after postnatal day 3 (P3), increasing steadily in most brain regions until adulthood. beta3 expression differentially decreased after P3 in certain areas but remained high in the hippocampus and striatum. 4. Emulsion-dipped slides showed co-localisation of beta3 with Nav1.3 mRNA in areas of the CNS suggesting that these subunits may be capable of functional interaction. 5. Co-expression in Xenopus oocytes revealed that beta3 could modify the properties of Nav1.3; beta3 changed the equilibrium of Nav1.3 between the fast and slow gating modes and caused a negative shift in the voltage dependence of activation and inactivation. 6. In conclusion, beta3 is shown to be the predominant beta subunit expressed during development and is capable of modulating the kinetic properties of the embryonic Nav1.3 subunit. These findings provide new information regarding the nature and properties of voltage-gated sodium channels during development.

Role of environmental factors on brain development and nerve growth factor expression

Numerous evidences suggest that early life events can affect the development of the nervous system, contributing in shaping interindividual differences in vulnerability to stress or psychopathology. A number of studies have shown that mothering style in rodents can produce neuroendocrine, neurochemical, and behavioral changes in the adult, although the basic mechanisms initiating this cascade of events still need to be investigated. This paper reviews research performed in our and other laboratories investigating some of the features characterizing hypothalamic--pituitary--adrenal (HPA) axis activity of rodents during early development, with a special emphasis on extrinsic, social regulatory factors, such as the mother and the siblings. In addition, a possible role for neurotrophins as mediators of the effects of external manipulations on brain development is suggested.

Ontogeny of synaptic transmission in the rat hippocampus

Electrophysiological characteristics of the hippocampal slices of juvenile (14-27 days) or young (36-40 days) Wistar rats have been compared. In the juvenile rats measurements were taken daily, from postnatal day (PN) 14 to PN27. Input-output curves were used to quantify the ontogeny of excitatory processes. The dynamic of population spike (PS) maturation was not even during the investigated postnatal period. After day 19 transient decrease of PS amplitude was observed until day 22. There were also some differences between the shape of input-output curves from the slices of rats of different ages. In general, PS was saturated at lower intensities in younger animals. The slices from 19-day-old rats did not display saturated input-output curve with 2-20 V stimuli intensities. But input-output curves on PN20-22 were rather similar to that obtained before PN19. The periods of gradual increase and subsequent decrease of PS amplitudes during early ontogeny correlate with the appearance of certain forms of behaviour. This fact suggests that hippocampal PS amplitude depression may be relevant functionally.

Insulin-like growth factor-I promotes neurogenesis and synaptogenesis in the hippocampal dentate gyrus during postnatal development

The in vivo actions of insulin-like growth factor-I (IGF-I) on the growth and development of the hippocampal dentate gyrus were investigated in transgenic mice that overexpress IGF-I postnatally in the brain and in normal nontransgenic littermate controls. Stereological analyses of the dentate gyrus were performed by light and electron microscopy on days 7, 14, 21, 28, 35, and 130 to determine postnatal changes in the numerical density and total number of neurons and synapses. The volumes of both the granule cell layer and the molecular layer of the dentate gyrus were significantly increased by 27-69% in transgenic mice after day 7, with the greatest relative increases occurring by day 35. Although the numerical density of neurons in the granule cell layer did not differ significantly between transgenic and control mice at any age studied, the total number of neurons was significantly greater in transgenic mice by 29-61% beginning on day 14. The total number of synapses in the molecular layer was significantly increased by 42-105% in transgenic mice from day 14 to day 130. A transient increase in the synapse-to-neuron ratio was found in transgenic mice at postnatal days 28 and 35 but not at day 130. This finding indicates a disproportionate increase in synaptogenesis, exceeding that expected for the observed increase in neuron number. Our results demonstrate that IGF-I overexpression produces persistent increases in the total number of neurons and synapses in the dentate gyrus, indicating that IGF-I promotes both neurogenesis and synaptogenesis in the developing hippocampus in vivo.

Enhanced CREB phosphorylation in immature dentate gyrus granule cells precedes neurotrophin expression and indicates a specific role of CREB in granule cell differentiation

Differentiation and maturation of dentate gyrus granule cells requires coordinated interactions of numerous processes. These must be regulated by protein factors capable of integrating signals mediated through diverse signalling pathways. Such integrators of inter and intracellular physiological stimuli include the cAMP-response element binding protein (CREB), a leucine-zipper class transcription factor that is activated through phosphorylation. Neuronal activity and neurotrophic factors, known to be involved in granule cell differentiation, are major physiologic regulators of CREB function. To examine whether CREB may play a role in governing coordinated gene transcription during granule cell differentiation, we determined the spatial and temporal profiles of phosphorylated (activated) CREB throughout postnatal development in immature rat hippocampus. We demonstrate that CREB activation is confined to discrete, early stages of granule cell differentiation. In addition, CREB phosphorylation occurs prior to expression of the neurotrophins BDNF and NT-3. These data indicate that in a signal transduction cascade connecting CREB and neurotrophins in the process of granule cell maturation, CREB is located upstream of neurotrophins. Importantly, CREB may be a critical component of the machinery regulating the coordinated transcription of genes contributing to the differentiation of granule cells and their integration into the dentate gyrus network.

NGF expression in the developing rat brain: effects of maternal separation

A number of studies have shown that mothering style in rodents can produce neuroendocrine, neurochemical and behavioural changes in the adult, although the basic mechanisms initiating this cascade of events still need to be investigated. Long term changes in neuronal function might be due to alterations in the expression of neurotrophins which have been shown to promote neuronal survival, differentiation and function during development, such as Nerve Growth Factor (NGF). NGF is essential for proper development of sympathetic and neural crest-derived sensory neurons of the peripheral nervous system as well as of central cholinergic neurons. In previous studies, using a maternal separation paradigm, we have shown that NGF expression is increased in the dentate gyrus and the hilus of the hippocampus as a result of brief (45 min) maternal separations. In the present study neonatal rats were separated for longer periods of time (up to 3 h) and at different ages during development (9 and 16 days postnatally). Results indicate that the effects of maternal separation on NGF expression are stronger with longer separations and are not restricted to the hippocampal region but can be seen also in other brain areas. Overall these results indicate that external factors, such as the presence/absence of the mother, can modify neurotrophic factor's availability in the brain, thus indicating NGF as a potential player in environmentally-mediated brain plasticity during development.

Maternal corticotropin-releasing hormone and habituation in the human fetus

Elevated concentrations of maternal corticotrophin-releasing hormone (CRH) during the 2nd and early 3rd trimester of human pregnancy are associated with spontaneous preterm birth, but the effects of maternal CRH on the fetus are unknown. Maternal plasma was collected for analysis of CRH concentration, m = 156.24 +/- 130.91 pg/ml, from 33 pregnant women during Weeks 31-33 of gestation. Immediately after collection of plasma, fetal heart rate (FHR) measures were obtained in response to a challenge with a series of vibroacoustic stimuli. Fetuses of mothers with highly elevated CRH did not respond significantly to the presence of a novel stimulus in a repeated series, p = 0.016. These effects on the FHR response were not related to parity, fetal gender, medical (antepartum) risk, or eventual birth outcomes. Impaired dishabituation in these fetuses of mothers with high concentrations of CRH suggests that neurological systems rich with CRH receptors that support learning and memory, such as parahippocampal regions, may be targets for maternal/placental CRH, with implications for fetal neurological development.

Changes in mRNA levels for group I metabotropic glutamate receptors following in utero hypoxia-ischemia

The expression of group I metabotropic glutamate receptors (mGluR1 and mGluR5) and inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) mRNA was studied by in situ hybridization in the developing rat hippocampus after in utero hypoxia-ischemia. In utero hypoxia-ischemia was induced by clamping the uterine blood vessels of near-term fetuses for 10 min. Fetuses were delivered surgically, resuscitated and raised by foster mothers until postnatal day 7 and 14. Results indicated a temporal delay in the expression of mGluR1 mRNA in the dentate gyrus of the ischemic animals. The mGluR1 mRNA level was significantly lower in the ischemic animals at postnatal day 7, but reached a similar level as that of controls at postnatal day 14. In utero hypoxia-ischemia did not change the temporal-spatial expression pattern of either mGluR5 or IP3R1 mRNA in the hippocampus. Between postnatal day 7 and 14, mGluR5 mRNA showed a high and relatively constant expression, whereas IP3R1 mRNA levels were increased in all regions examined. The differences in the expressions of group I mGluRs indicate that these receptors may have different functions during hippocampal development and may play different roles in excitotoxicity.

Changes in NADPH-diaphorase positivity induced by status epilepticus in allocortical structures of the immature rat brain

The distribution and time course of changes of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) positivity were studied in immature rats (12 and 25 days old) surviving motor status epilepticus (SE) induced by a high dose of pilocarpine. Motor SE characterized by continuous convulsions was interrupted after 2 h by an injection of clonazepam (0.5 mg/kg or 1 mg/kg in 12- and 25-day-old rats, respectively) in order to reduce mortality. Correlation between electroencephalographic and behavioral seizure activity was confirmed using animals with electrodes implanted bilaterally in the hippocampus and sensorimotor cortex. Brains were examined 2, 6, 13, and 21 days after motor SE using NADPH-diaphorase histochemistry. Two types of changes were found in both age groups: (a) decrease of NADPH-d positivity occurred in both neuropil and cell bodies in piriform, periamygdalar, and entorhinal cortices; and (b) NADPH-d positivity was induced in the cell bodies in the hippocampal fields CA1/2, CA3, and dentate gyrus. These changes were more intense in animals surviving SE at postnatal day 25 than in younger age group, and they peaked 2 days after SE. The changes observed after SE disappeared quickly in 12-day-old rat pups, where only moderate changes could be observed in piriform, periamygdalar, and entorhinal cortices 6 days after SE, whereas the changes in the histochemical positivity persisted in older animals even 21 days after SE.

Signals in stochastically generated neurons

To incorporate variation of neuron shape in neural models, we developed a method of generating a population of realistically shaped neurons. Parameters that characterize a neuron include soma diameters, distances to branch points, fiber diameters, and overall dendritic tree shape and size. Experimentally measured distributions provide a means of treating these morphological parameters as stochastic variables in an algorithm for production of neurons. Stochastically generated neurons shapes were used in a model of hippocampal dentate gyrus granule cells. A large part of the variation of whole neuron input resistance R(N) is due to variation in shape. Membrane resistivity Rm computed from R(N) varies accordingly. Statistics of responses to synaptic activation were computed for different dendritic shapes. Magnitude of response variation depended on synapse location, measurement site, and attribute of response.

Traumatic brain injury in the developing rat: effects of maturation on Morris water maze acquisition

Previous work has demonstrated that postnatal and adult rats show different physiological responses to lateral fluid percussion (FP) brain injury. Compared to adult animals, the younger rats showed longer apnea and shorter unconsciousness, and sustained hypotension at all injury severities, with higher mortality following severe traumatic brain injury (TBI). To determine if these younger rats exhibit differential cognitive impairments, the Morris water maze (MWM) was used to compare the degree of spatial learning deficits between moderately injured postnatal day 17 (P17), P28, and adult rats, as well as their age-matched controls. Comparisons between shams of different ages showed a maturational time course for MWM acquisition, where adult rats learned the task 34-58% faster than younger age groups. Injured adults showed escape latency deficits throughout the entire training period, took 39% fewer direct paths to the platform during training, took 24% longer to reach criterion performance, and showed poor probe trial performance than adult shams. Injured P28s exhibited escape latency deficits during the first week, with 23% more trials to criterion and 24% fewer direct paths compared to P28 shams. In contrast, injured P17 rats showed no significant difference from age-matched controls in terms of escape latency, number of direct paths taken, or time to criterion performance. This work suggests that, upon surviving the insult, P17 injured rats show remarkable sparing compared to P28 and adult injured animals.

Early maternal separation increases NGF expression in the developing rat hippocampus

Nerve Growth Factor (NGF) is a neurotrophin involved in growth and differentiation of central cholinergic neurons. In this study a maternal separation paradigm was used to test whether levels of NGF might be affected by brief manipulations of rat pups early during ontogeny. The expression of NGF mRNA was examined in 3-day-old rat pups following 45 min maternal separation using in situ hybridization. Early maternal separation in neonatal rats resulted in increased expression of NGF mRNA in the dentate gyrus and the hilus of the hippocampus. NGF protein levels measured (by means of a sensitive ELISA assay) in the whole hippocampus the day following the separation procedure did not differ in separated vs. nonseparated pups. These data indicate that brief manipulations performed early during development can affect hippocampal NGF expression.

Differential expression of rat brain phospholipase C isozymes in development and aging

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in signal transduction. In the present study we examined developmental and aging changes in three PLC isozymes (beta 1, gamma 1, and delta 1) in the rat brain. Enzyme assays and immunoblot analyses after gel filtration chromatography of brain extracts from embryonic day 19 and postnatal 4- and 48-week rats indicated that gamma-specific activity was highest in fetal brain and decreased with aging, that beta 1-specific activity was high at 4 weeks but essentially undetected in fetal brain, and that delta 1-specific activity was high at both 4 and 48 weeks with faint detection in fetal brain. Our results suggest that the gamma 1 isozyme may be particularly involved in cell division and growth during the histo-genesis of the central nervous system, while beta 1 and delta 1 isozymes may take part in processes of its maturation and maintenance.

Neuronal dye coupling in the developing rat fascia dentata

The present study describes dye coupling among neurons of the developing rat fascia dentata following impalement and intracellular filling with Neurobiotin. The number of neuronal impalements resulting in dye-coupled cells decreases from P14 to P120. The most rapid decline in dye coupling was observed between P14 and P21, the beginning of the most active period of synaptogenesis in the dentate molecular layer. Dye coupling between granule cells and axo-axonic interneurons (chandelier cells) accounts for about 10% of the dye-coupled neuronal population acquired in slices from P14 and P21 rats and declines to less than 5% by P60 and P120. Our data suggest that dye coupling is related reciprocally to the number of synapses formed on granule cells. Thus the relationship of dye coupling to synaptic density in the developing fascia dentata is similar to that reported in studies of the aging fascia dentata. Also the observation of axo-axonic interneurons coupled to granule cells at all ages suggests an interesting neuronal arrangement with the potential of limiting granule cell discharge to discrete neuronal assemblies in response to perforant path input.

Changes in ubiquitin immunoreactivity in developing rat brain: a putative role for ubiquitin and ubiquitin conjugates in dendrite outgrowth and differentiation

The role of ubiquitin in development of the mammalian brain has been studied using a monoclonal antibody, RHUb1, specific for ubiquitin. Immunodevelopment of western blots of homogenate samples of the cerebral cortex, hippocampus and cerebellum prepared from animals of known postnatal age show marked developmental changes in conjugate level. Striking decreases in the level of a prominent conjugate of molecular weight 22,000, which is identified as ubiquitinated histone, are observed during the first postnatal week in the cerebral cortex and hippocampus, but not the cerebellum. A marked overall developmental decrease in the level of high-molecular-weight (> 40,000) ubiquitin conjugates which occurs predominantly during the third, but also the fourth, postnatal week is observed in all three regions. Immunocytochemical data obtained with the RHUb1 antibody show intense staining of neuronal perikarya, nuclei and dendrites in early postnatal cerebral cortex and hippocampus. Staining of pyramidal cell perikarya and dendrites is particularly prominent. The intensity of dendritic staining, particularly for the cerebral cortex, shows a striking decrease after postnatal day 14 and only faint dendritic staining is observed in the adult. In early postnatal cerebellum, immunoreactivity is predominantly nuclear, though some staining of the proximal regions of Purkinje cell dendrites is observed between postnatal days 4 and 19. As with the cerebral cortex and hippocampus, most of the ubiquitin reactivity is lost in adult animals. The loss of dendritic staining, particularly in the cerebral cortex, correlates with the decrease in the level of high-molecular-weight ubiquitin conjugates observed on the western blots. Immunodevelopment of western blots of a range of subcellular fractions prepared from developing rat forebrain shows that the developmental decrease in the level of high-molecular-weight ubiquitin conjugates is not uniform for all fractions. The decrease in conjugate level is most marked for the cell-soluble, mitochondrial and detergent-insoluble cytoskeletal fractions. Taken overall, the data suggest a role for ubiquitin in dendrite outgrowth and arborization, loss of dendritic ubiquitin immunoreactivity correlating with completion of these processes.

Insensitivity of the hippocampus to environmental stimulation during postnatal development

Development of cortical sensory systems is influenced by environmental experience during "sensitive periods," before onset of behavioral function. During these periods, synaptic plasticity is observed, and neuronal function shows increased responsiveness to environmental stimulation. Because the hippocampus is late to develop, and because it demonstrates synaptic plasticity before the onset of behavioral function, this experiment was designed to determine whether, like the sensory cortices, the hippocampus undergoes a period of enhanced responsiveness to the environment. Rats at three ages [postnatal day 16 (P16), P23, and P30] were tested on a hippocampally dependent task, spontaneous alternation, and exposed to a novel environment. They were then killed and processed for immunocytochemistry to Fos or for in vitro electrophysiology in hippocampal area CA1. Age-matched control subjects were killed immediately after removal from the home cage. Spontaneous alternation was only observed in the oldest (P30) animals. In these same animals, the environmental manipulation resulted in an increase in Fos-like immunoreactivity (FL-IR), relative to controls, and a decrease in the ability to induce long-term potentiation (LTP). In P16 and P23 animals, the environmental manipulation resulted in no differences in hippocampal FL-IR or LTP. These results suggest that, rather than showing increased responsiveness to the environment at these ages, the hippocampus is environmentally insensitive and that it is isolated from the effects of environmental stimuli. The hippocampus, a neural region important for higher cognitive function, may develop via a mechanism different from those observed in the primary sensory cortices.