Increase In Density Of Synapses Research Articles

Neurexin-2 restricts synapse numbers and restrains the presynaptic release probability by an alternative splicing-dependent mechanism

α- and β-neurexins are extensively alternatively spliced, presynaptic cell-adhesion molecules that are thought to organize synapse assembly. However, recent data revealed that, in the hippocampus invivo, the deletion of one neurexin isoform, Nrxn2, surprisingly increased excitatory synapse numbers and enhanced their presynaptic release probability, suggesting that Nrxn2 restricts, instead of enabling, synapse assembly. To delineate the synaptic function and mechanism of action of Nrxn2, we examined cultured hippocampal neurons as a reduced system. In heterologous synapse formation assays, different alternatively spliced Nrxn2β isoforms robustly promoted synapse assembly similar to Nrxn1β and Nrxn3β, consistent with a general synaptogenic function of neurexins. Deletion of Nrxn2 from cultured hippocampal neurons, however, caused a significant increase in synapse density and release probability, replicating the invivo data that suggested a synapse-restricting function. Rescue experiments revealed that two of the four Nrxn2β splice variants (Nrxn2β-SS4+/SS5- and Nrxn2β-SS4+/SS5+) reversed the increase in synapse density in Nrxn2-deficient neurons, whereas only one of the four Nrxn2β splice variants (Nrxn2β-SS4+/SS5+) normalized the increase in release probability in Nrxn2-deficient neurons. Thus, a subset of Nrxn2 splice variants restricts synapse numbers and restrains their release probability in cultured neurons.

Evidence for upregulation of excitatory synaptic transmission in the substantia nigra in Schizophrenia: a postmortem ultrastructural study.

The dopamine hypothesis of schizophrenia suggests that psychotic symptoms originate from dysregulation of dopaminergic activity, which may be controlled by upstream innervation. We hypothesized that we would find anatomical evidence for the hyperexcitability seen in the SN. We examined and quantified synaptic morphology, which correlates with function, in the postmortem substantia nigra (SN) from 15 schizophrenia and 12 normal subjects. Synapses were counted using stereological techniques and classified based on the morphology of the post-synaptic density (PSD) and the presence or absence of a presynaptic density. The density and proportion of excitatory synapses was higher in the schizophrenia group than in controls, while the proportion (but not density) of inhibitory synapses was lower. We also detected in the schizophrenia group an increase in density of synapses with a PSD of intermediate thickness, which may represent excitatory synapses. The density of synapses with presynaptic densities was similar in both groups. The density of synapses with mixed morphologies was higher in the schizophrenia group than in controls. The human SN contains atypical synaptic morphology. We found an excess amount and proportion of excitatory synapses in the SN in schizophrenia that could result in hyperactivity and drive the psychotic symptoms of schizophrenia. The sources of afferent excitatory inputs to the SN arise from the subthalamic nucleus, the pedunculopontine nucleus, and the ventral tegmental area (VTA), areas that could be the source of excess excitation. Synapses with mixed morphologies may represent inputs from the VTA, which release multiple transmitters.

The Jun-dependent axon regeneration gene program: Jun promotes regeneration over plasticity

The regeneration-associated gene (RAG) expression program is activated in injured peripheral neurons after axotomy and enables long-distance axon re-growth. Over 1000 genes are regulated, and many transcription factors are upregulated or activated as part of this response. However, a detailed picture of how RAG expression is regulated is lacking. In particular, the transcriptional targets and specific functions of the various transcription factors are unclear. Jun was the first-regeneration-associated transcription factor identified and the first shown to be functionally important. Here we fully define the role of Jun in the RAG expression program in regenerating facial motor neurons. At 1, 4 and 14 days after axotomy, Jun upregulates 11, 23 and 44% of the RAG program, respectively. Jun functions relevant to regeneration include cytoskeleton production, metabolic functions and cell activation, and the downregulation of neurotransmission machinery. In silico analysis of promoter regions of Jun targets identifies stronger over-representation of AP1-like sites than CRE-like sites, although CRE sites were also over-represented in regions flanking AP1 sites. Strikingly, in motor neurons lacking Jun, an alternative SRF-dependent gene expression program is initiated after axotomy. The promoters of these newly expressed genes exhibit over-representation of CRE sites in regions near to SRF target sites. This alternative gene expression program includes plasticity-associated transcription factors and leads to an aberrant early increase in synapse density on motor neurons. Jun thus has the important function in the early phase after axotomy of pushing the injured neuron away from a plasticity response and towards a regenerative phenotype.

Spaceflight-induced synaptic modifications within hair cells of the mammalian utricle.

Exposure to the microgravity conditions of spaceflight alleviates the load normally imposed by the Earth's gravitational field on the inner ear utricular epithelia. Previous ultrastructural investigations have shown that spaceflight induces an increase in synapse density within hair cells of the rat utricle. However, the utricle exhibits broad physiological heterogeneity across different epithelial regions, and it is unknown whether capabilities for synaptic plasticity generalize to hair cells across its topography. To achieve systematic and broader sampling of the epithelium than was previously conducted, we used immunohistochemistry and volumetric image analyses to quantify synapse distributions across representative utricular regions in specimens from mice exposed to spaceflight (a 15-day mission of the space shuttle Discovery). These measures were compared with similarly sampled Earth-bound controls. Following paraformaldehyde fixation and microdissection, immunohistochemistry was performed on intact specimens to label presynaptic ribbons (anti-CtBP2) and postsynaptic receptor complexes (anti-Shank1A). Synapses were identified as closely apposed pre- and postsynaptic puncta. Epithelia from horizontal semicircular canal cristae served as "within-specimen" controls, whereas utricles and cristae from Earth-bound cohorts served as experimental controls. We found that synapse densities decreased in the medial extrastriolae of microgravity specimens compared with experimental controls, whereas they were unchanged in the striolae and horizontal cristae from the two conditions. These data demonstrate that structural plasticity was topographically localized to the utricular region that encodes very low frequency and static changes in linear acceleration, and illuminates the remarkable capabilities of utricular hair cells for synaptic plasticity in adapting to novel gravitational environments.NEW & NOTEWORTHY Spaceflight imposes a radically different sensory environment from that in which the inner ear utricle normally operates. We investigated synaptic modifications in utricles from mice flown aboard a space shuttle mission. Structural synaptic plasticity was detected in the medial extrastriola, a region associated with encoding static head position, as decreased synapse density. These results are remarkably congruent with a recent report of decreased utricular function in astronauts immediately after returning from the International Space Station.

Neurotrophin-3 Regulates Synapse Development by Modulating TrkC-PTPσ Synaptic Adhesion and Intracellular Signaling Pathways.

Neurotrophin-3 (NT-3) is a secreted neurotrophic factor that binds neurotrophin receptor tyrosine kinase C (TrkC), which in turn binds to presynaptic protein tyrosine phosphatase σ (PTPσ) to govern excitatory synapse development. However, whether and how NT-3 cooperates with the TrkC-PTPσ synaptic adhesion pathway and TrkC-mediated intracellular signaling pathways in rat cultured neurons has remained unclear. Here, we report that NT-3 enhances TrkC binding affinity for PTPσ. Strikingly, NT-3 treatment bidirectionally regulates the synaptogenic activity of TrkC: at concentrations of 10-25 ng/ml, NT-3 further enhanced the increase in synapse density induced by TrkC overexpression, whereas at higher concentrations, NT-3 abrogated TrkC-induced increases in synapse density. Semiquantitative immunoblotting and optogenetics-based imaging showed that 25 ng/ml NT-3 or light stimulation at a power that produced a comparable level of NT-3 (6.25 μW) activated only extracellular signal-regulated kinase (ERK) and Akt, whereas 100 ng/ml NT-3 (light intensity, 25 μW) further triggered the activation of phospholipase C-γ1 and CREB independently of PTPσ. Notably, disruption of TrkC intracellular signaling pathways, extracellular ligand binding, or kinase activity by point mutations compromised TrkC-induced increases in synapse density. Furthermore, only sparse, but not global, TrkC knock-down in cultured rat neurons significantly decreased synapse density, suggesting that intercellular differences in TrkC expression level are critical for its synapse-promoting action. Together, our data demonstrate that NT-3 is a key factor in excitatory synapse development that may direct higher-order assembly of the TrkC/PTPσ complex and activate distinct intracellular signaling cascades in a concentration-dependent manner to promote competition-based synapse development processes. In this study, we present several lines of experimental evidences to support the conclusion that neurotrophin-3 (NT-3) modulates the synaptic adhesion pathway involving neurotrophin receptor tyrosine kinase C (TrkC) and presynaptic protein tyrosine phosphatase σ (PTPσ) in a bidirectional manner at excitatory synapses. NT-3 acts in concentration-independent manner to facilitate TrkC-mediated presynaptic differentiation, whereas it acts in a concentration-dependent manner to exert differential effects on TrkC-mediated organization of postsynaptic development. We further investigated TrkC extracellular ligand binding, intracellular signaling pathways, and kinase activity in NT-3-induced synapse development. Last, we found that interneuronal differences in TrkC levels regulate the synapse number. Overall, these results suggest that NT-3 functions as a positive modulator of synaptogenesis involving TrkC and PTPσ.

Distinct intracellular signaling mediates C-MET regulation of dendritic growth and synaptogenesis.

Hepatocyte growth factor (HGF) activation of the MET receptor tyrosine kinase influences multiple neurodevelopmental processes. Evidence from human imaging and mouse models shows that, in the forebrain, disruptions in MET signaling alter circuit formation and function. One likely means of modulation is by controlling neuron maturation. Here, we examined the signaling mechanisms through which MET exerts developmental effects in the neocortex. In situ hybridization revealed that hgf is located near MET-expressing neurons, including deep neocortical layers and periventricular zones. Western blot analyses of neocortical crude membranes demonstrated that HGF-induced MET autophosphorylation peaks during synaptogenesis, with a striking reduction in activation between P14 and P17 just before pruning. In vitro analysis of postnatal neocortical neurons assessed the roles of intracellular signaling following MET activation. There is rapid, HGF-induced phosphorylation of MET, ERK1/2, and Akt that is accompanied by two major morphological changes: increases in total dendritic growth and synapse density. Selective inhibition of each signaling pathway altered only one of the two distinct events. MAPK/ERK pathway inhibition significantly reduced the HGF-induced increase in dendritic length, but had no effect on synapse density. In contrast, inhibition of the PI3K/Akt pathway reduced HGF-induced increases in synapse density, with no effect on dendritic length. The data reveal a key role for MET activation during the period of neocortical neuron growth and synaptogenesis, with distinct biological outcomes mediated via discrete MET-linked intracellular signaling pathways in the same neurons. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1160-1181, 2016.

Unbiased, High-Throughput Electron Microscopy Analysis of Experience-Dependent Synaptic Changes in the Neocortex.

Development and sensory experience can change synapse properties in the neocortex. Here we use a semiautomated analysis of electron microscopy images for an unbiased, column-wide analysis of synapse changes. This analysis reveals new loci for synaptic change that can be verified by targeted electrophysiological investigation. This method can be used as a platform for generating new hypotheses about synaptic changes across different brain areas and experimental conditions.

Regional dissociation of paradigm-specific synapse remodeling during memory consolidation in the adult rat dentate gyrus

Arising from studies on the amnesia that follows site-specific physical or chemical lesions, the acquisition and consolidation of certain behavioral tasks has been demonstrated to be associated with different hippocampal subregions. Although not absolute, spatial learning is reliant on the dorsal region of the hippocampus, whereas avoidance- and fear-conditioning tasks appear to be dependent on its more ventral aspects. Thus, if learning-associated synapse remodeling is a true feature of memory consolidation it must also follow these regional dissociations. We therefore determined if the learning-associated increases in synapse density that occur in the mid-molecular layer of the dentate gyrus at the 6-h post-training time and the frequency of polysialylated cells at the infragranular zone that occur at the 12-h post-training time were dissociated to specific hippocampal subregions following training in either a massed water maze task or light–dark passive avoidance response. Synapse remodeling was found to occur only in the dorsal hippocampus following spatial learning. We could not, however, discern any regional dissociation of neural remodeling following avoidance conditioning. These results point to strong associations between learning and specific groups of novel synapses during consolidation of spatial learning and avoidance conditioning paradigms.

Estradiol Targets Synaptic Proteins to Induce Glutamatergic Synapse Formation in Cultured Hippocampal Neurons: Critical Role of Estrogen Receptor-

Estradiol mediates structural changes at synapses of the hippocampus, an area in the brain important for learning and memory. This study was designed to test the hypothesis that estradiol mediates subcellular changes of synaptic proteins to induce new synapses via an estrogen receptor (ER)-mediated process. To elucidate the mechanisms involved in glutamatergic synapse formation, we investigated effects of estradiol on synaptic proteins in cultured hippocampal neurons using immunocytochemistry and confocal microscopy. Synaptic protein distribution and size were identified with antibodies to the presynaptic vesicular glutamate transporter protein (vGlut1) and postsynaptic NMDA receptor (NR1 subunit). We observed an increase in synapse density, as detected by NR1 and vGlut1 colocalization, along dendrites of neurons cultured in steroid-stripped media and exposed to estradiol (10 nM) for 48 h. Additionally, the NR1 subunit was enriched at synaptic clusters. Immunocytochemistry and confocal imaging revealed punctate staining of extranuclear ERs along dendrites of hippocampal neurons expressing NR1. Estradiol increased the density of both ER-alpha and ER-beta protein clusters along dendrites. To test whether ERs play an important functional role in the estradiol-induced synaptogenesis, we used the ER antagonist [7alpha,17beta-[9[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]estra-1,3,5(10)-triene-3,17-diol (ICI 182,780)] and the ER-alpha- and ER-beta-specific agonists [1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) and 2,3-bis(4-hydroxyphenyl) propionitrile (DPN), respectively]. ICI 182,780 blocked the increase in synapse density. Treatment with PPT, but not DPN, induced significant increases in synapse density that mimicked treatment with estradiol. Together, our results demonstrate that estradiol stimulates glutamatergic synapse formation in the developing hippocampus through an ER-alpha-dependent mechanism. These findings carry profound implications regarding the potential of estrogen to influence learning, memory, and possibly hormone-modulated neurodegeneration.

Synaptic differences in the postmortem striatum of subjects with schizophrenia: A stereological ultrastructural analysis

The striatum processes motor, cognitive, and limbic function, all of which are perturbed in schizophrenia. The present study examined the synaptic organization of the caudate and putamen in schizophrenia. Postmortem striatum was obtained from 10 normal controls (NC) and 17 subjects with schizophrenia (SZ), prepared for electron microscopy, and analyzed using stereological principles. The densities of total synapses, asymmetric synapses (characteristic of excitatory inputs), and asymmetric axospinous synapses (characteristic of cortical input) were higher in the caudate of the SZs vs. NCs. These changes were most profound in the off-drug SZ cases and were also elevated in subjects on antipsychotic drugs (APDs). In comparison to NCs, there were no significant differences in the putamen of the SZ cohort as a whole group; however, there were more asymmetric axospinous synapses in the off-drug subgroup. The increase in density of synapses in the SZs does not appear to be caused by antipsychotic medication and may represent failure of normal synaptic pruning or abnormal sprouting. Higher density of cortical-type synapses in SZs vs. NCs may reflect adaptation of corticostriatal circuitry or hyperstimulation of striatal projection neurons. The abnormal synaptic organization could have several important and different downstream effects depending on the precise circuitry involved and may be related to limbic or cognitive dysfunction in schizophrenia.

Androgens increase spine synapse density in the CA1 hippocampal subfield of ovariectomized female rats.

The effects of androgen on the density of spine synapses on pyramidal neurons in the CA1 area of the hippocampus were studied in ovariectomized (OVX) adult female rats. Treatment of OVX rats with testosterone propionate (TP; 500 microg/d, s.c., 2 d) significantly increased spine synapse density (from 0.661 +/- 0.016 spine synapse/microm3 in OVX rats to 1.081 +/- 0.018 spine synapse/microm3 after TP treatment). A smaller, but still statistically significant, increase in synapse density (0.955 +/- 0.029 spine synapse/microm3) was observed in OVX animals after treatment with the nonaromatizable androgen dihydrotestosterone (DHT; 500 microg/d, s.c., 2 d). Administration of 1 mg of letrozole, a powerful nonsteroidal aromatase inhibitor, 1 hr before the steroid injections almost completely blocked the synaptic response to testosterone, resulting in a mean synapse density (0.723 +/- 0.003 spine synapse/microm3) only slightly higher than in OVX control rats. By contrast, the response to DHT was unaffected by letrozole pretreatment. These data suggest that androgen secretion during the female reproductive cycle may contribute to cyclical changes in hippocampal synaptic density. They also indicate that androgen treatment may be as effective as estrogen replacement in reversing the decline in hippocampal CA1 spine synapses that follows loss of ovarian function. Induction of hippocampal synapse formation by androgen is not mediated entirely via intracerebral estrogen biosynthesis, however, because aromatase-independent mechanisms also significantly affect CA1 spine synapse density.

N-methyl--aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

N-methyl- d-aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

Gonadal Hormones Affect Spine Synaptic Density in the CA1 Hippocampal Subfield of Male Rats

The effects of androgen on the density of spine synapses on pyramidal neurons in the CA1 area of the hippocampus were studied in male rats. Gonadectomy (GDNX) had no significant effect on the number of CA1 pyramidal cells but reduced CA1 spine synapse density by almost 50% (to 0.468 +/- 0.018 spine synapses/microm(3)) compared with sham-operated controls (0.917 +/- 0.06 spine synapses/microm(3)). Treatment of GDNX rats with testosterone propionate (500 microg/d, s.c., 2 d) increased spine synapse density to levels (1.01 +/- 0.026 spine synapses/microm(3)) comparable with intact males. A similar increase in synapse density (1.013 +/- 0.05 spine synapses/microm(3)) was observed in GDNX animals after treatment with dihydrotestosterone (DHT) (500 microg/d, s.c., 2 d) but not after estradiol (10 microg/d, s.c., 2 d; 0.455 +/- 0.02 spine synapse/microm(3)). These data indicate that testosterone is important for maintenance of normal spine synapse density in the CA1 region of the male rat hippocampus. The comparable responses to testosterone and the non-aromatizable androgen DHT, coupled with the lack of response to estradiol, suggest that testosterone acts directly on hippocampal androgen receptors rather than indirectly via local estrogen biosynthesis.

Effects of age and insulin-like growth factor-1 on neuron and synapse numbers in area CA3 of hippocampus

Age-related effects associated with the hippocampus include declines in numbers of neurons and synapses in the dentate gyrus and area CA1, and decreased cognitive ability as assessed with the Morris water maze. The present study quantified both neuron and synapse number in the same tissue block of area CA3 of the hippocampus. No investigations of both density of neurons and synapses together in area CA3 of hippocampus have been performed previously, despite its importance as the terminal field of dentate gyrus mossy fibers, the second synapse in the trisynaptic circuit in the hippocampus. Numerical density of neurons and synapses were assessed in 4-, 18-, and 29-month-old rats receiving infusions of saline into the lateral ventricle and in 29-month-old rats receiving infusions of insulin-like growth factor-1 (IGF-1). Numerical density of neurons of the stratum pyramidale of CA3 of hippocampus remained constant across the life span as did the numerical density of synapses in stratum lucidum of area CA3. Despite the reported role of IGF-1 in synaptogenesis and improvements in behavior with age, ventricular infusion of this growth factor did not affect the numerical density of neurons or synapses in 29-month-old rats when compared to saline-infused old rats. Further, reported effects of IGF-1 on adult neurogenesis in the dentate gyrus are not reflected in an IGF-1-related increase in synapse density in this region.

Estradiol increases spine density and NMDA-dependent Ca2+ transients in spines of CA1 pyramidal neurons from hippocampal slices.

To investigate the physiological consequences of the increase in spine density induced by estradiol in pyramidal neurons of the hippocampus, we performed simultaneous whole cell recordings and Ca2+ imaging in CA1 neuron spines and dendrites in hippocampal slices. Four- to eight-days in vitro slice cultures were exposed to 17beta-estradiol (EST) for an additional 4- to 8-day period, and spine density was assessed by confocal microscopy of DiI-labeled CA1 pyramidal neurons. Spine density was doubled in both apical and basal dendrites of the CA1 region in EST-treated slices; consistently, a reduction in cell input resistance was observed in EST-treated CA1 neurons. Double immunofluorescence staining of presynaptic (synaptophysin) and postsynaptic (alpha-subunit of CaMKII) proteins showed an increase in synaptic density after EST treatment. The slopes of the input/output curves of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) postsynaptic currents were steeper in EST-treated CA1 neurons, consistent with the observed increase in synapse density. To characterize NMDA-dependent synaptic currents and dendritic Ca2+ transients during Schaffer collaterals stimulation, neurons were maintained at +40 mV in the presence of nimodipine, picrotoxin, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). No differences in resting spine or dendritic Ca2+ levels were observed between control and EST-treated CA1 neurons. Intracellular Ca2+ transients during afferent stimulation exhibited a faster slope and reached higher levels in spines than in adjacent dendrites. Peak Ca2+ levels were larger in both spines and dendrites of EST-treated CA1 neurons. Ca2+ gradients between spine heads and dendrites during afferent stimulation were also larger in EST-treated neurons. Both spine and dendritic Ca2+ transients during afferent stimulation were reversibly blocked by D, L-2-amino-5-phosphonovaleric acid (D,L-APV). The increase in spine density and the enhanced NMDA-dependent Ca2+ signals in spines and dendrites induced by EST may underlie a threshold reduction for induction of NMDA-dependent synaptic plasticity in the hippocampus.

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

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

Protein-synthetic machinery at postsynaptic sites during synaptogenesis: a quantitative study of the association between polyribosomes and developing synapses

Previous studies have revealed dramatic accumulations of polyribosomes under growing spine synapses, suggesting a critical role for protein synthesis at the postsynaptic site during synaptogenesis. The present study quantitatively analyzes the distribution of polyribosomes under synapses during developmental synaptogenesis in the rat's dentate gyrus. The middle molecular layer of the suprapyramidal blade of the dentate gyrus was examined electron-microscopically at 1, 4, 7, 10, 15, 20, and 28 d of age. At each age, we evaluated (1) synapse density (the number of synapses/100 micron2 of neuropil), (2) the width of the molecular layer, (3) the proportion of spine synapses with underlying polyribosomes, and (4) the number of polyribosome-containing synapses/1000 micron2 of neuropil. From the first two measures, an estimate was obtained of the total extent of synaptogenesis, taking into account both the increase in synapse density and the increase in total area of neuropil. At 1 d of age, very few synapses were found in the molecular layer of the dentate gyrus, and those that were present were quite immature in appearance. Synapse density increased about 140-fold between 1 and 28 d of age, from an average of 0.36 synapses/100 micron2 at 1 d of age to 49 synapses/100 micron2 at 28 d of age. An inverse relationship was found between synapse density and the proportion of synapses with polyribosomes. Between 1 and 7 d of age, about 60% of the spine synapses had one or more polyribosomes under the spine base. Thereafter, the proportion of spines with polyribosomes decreased as synapse density increased. Similarly, the proportion of shaft synapses with underlying polyribosomes was greatest between 1 and 7 d postnatal, and decreased thereafter. While the proportion of synapses with polyribosomes was greatest between 1 and 7 d, the actual number of polyribosome-containing synapses/1000 micron2 of neuropil was negligible at 1 d, increased to a peak at 7 d of age, and then decreased as synapse density increased. Qualitatively, the most dramatic accumulations of polyribosomes were also found at 7 d of age. We conclude that spine-associated polyribosomes represent a structural specialization of dendrites at sites of synapse construction and as such may represent a marker for growing synapses. We propose that these elements produce protein(s) that are critically involved in the formation of the synaptic contact.

The role of support cells in the growth and differentiation of neurones in the abdominal ganglion of Aplysia.

During the late premetamorphic stages of development, the abdominal ganglion of Aplysia is surrounded by a group of support cells which later develop morphological properties characteristic of glial cells. These support cells contain large secretory granules whose contents are released primarily after the onset of the metamorphic phase. The release of the granule contents may signal the burst of neuronal growth and maturation that occurs following metamorphosis. The evidence supporting this idea is the following: (1) The release of the granule material after the onset of metamorphosis coincides with an increase in cell body growth and a more marked increase in the density of synapses within the neuropil. Both release and neuronal maturation can be blocked when metamorphosis is postponed by withholding the appropriate macroalgal substrate. (2) Premature release of the granule contents 2-3 weeks before metamorphosis with artificial sea water containing a high concentration of potassium results in an increase in cell body growth, density of synapses, and the number of spines formed and contacts received by specific identified cells. (3) Artificially inducing the release of the granule material in animals whose metamorphosis has been prevented (by withholding the appropriate substrate) still produces an increase in cell body growth and density of synapses. These results suggest that the release of material from support cell granules provides a general stimulus for neuronal differentiation including cell body growth, spine development, and synapse formation.

Development of neurons in the abdominal ganglion of Aplysia californica: II. Nonneural support cells

The release by nonneural support cells of a diffusable chemical substance into the local environment in which sympathetic neurons develop is thought to play a crucial role in their differentiation. In this paper, we describe a novel class of nonneural support cells associated with a central ganglion of Aplysia californica during the premetamorphic stages of development. These support cells contain secretory granules whose contents are primarily released at metamorphosis. The release of these contents may signal the burst of neuronal growth and maturation that occurs following metamorphosis. The evidence in support of this notion is the following: (1) Spontaneous release of the granule material at metamorphosis coincides with an increase in cell body growth and a more marked increase in the density of synapses of the abdominal ganglion. (2) Premature release of the granule material before metamorphosis with artificial seawater containing a high concentration of potassium results in a burst in cell body growth and a premature increase in synapse density. (3) Premature release of granule material also results in a precocious increase in the number of spines formed and synaptic contacts received by specific identified cells. Based on the findings in this and the preceding paper, we propose a two-stage model of the developmental program for differentiation of neurons in the abdominal ganglion. First, axosomatic contacts trigger axonal outgrowth. Second, material released from the granules of the support cells stimulates further steps in neuronal differentiation, including cell growth, spine development, and synapse formation.