Differential expression of synaptophysin and synaptoporin during pre- and postnatal development of the rat hippocampal network

Synaptoporin and parathyroid hormone 2 as markers of multimodal inputs to the auditory brainstem

The distribution of the synaptic vesicle protein synaptoporin was investigated by immunofluorescence in the central auditory system of the mouse brainstem. Synaptoporin immunostaining displayed region-specific differences. High and moderate accumulations of were seen in the superficial layer of the dorsal cochlear nucleus, dorsal and external regions of the inferior colliculus, the medial and dorsal divisions of the medial geniculate body and in periolivary regions of the superior olivary complex (SOC). Low or absent labeling was observed in the more central parts of these structures such as the principal nuclei of the SOC. It was conspicuous that dense synaptoporin immunoreactivity was detected predominantly in areas, which are known to be synaptic fields of multimodal, extra-auditory inputs. Target neurons of synaptoporin-positive synapses in the SOC were then identified by double-labelling immunofluorescence microscopy. We thereby detected synaptoporin puncta perisomatically at nitrergic, glutamatergic and serotonergic neurons but none next to neurons immunoreactive for choline-acetyltransferase and calcitonin-gene related peptide. These results leave open whether functionally distinct neuronal groups are accessed in the SOC by synaptoporin-containing neurons. The last part of our study sought to find out whether synaptoporin-positive neurons originate in the medial paralemniscal nucleus (MPL), which is characterized by expression of the peptide parathyroid hormone 2 (PTH2). Anterograde neuronal tracing upon injection into the MPL in combination with synaptoporin- and PTH2-immunodetection showed that (1) the MPL projects to the periolivary SOC using PTH2 as transmitter, (2) synaptoporin-positive neurons do not originate in the MPL, and (3) the close juxtaposition of synaptoporin-staining with either the anterograde tracer or PTH2 reflect concerted action of the different inputs to the SOC.

Neuromorphological Atlas of Human Prenatal Brain Development: White Paper

Recent morphological data on human brain development are quite fragmentary. However, they are highly requested for a number of medical practices, educational programs, and fundamental research in the fields of embryology, cytology and histology, neurology, physiology, path anatomy, neonatology, and others. This paper provides the initial information on the new online Human Prenatal Brain Development Atlas (HBDA). The Atlas will start with forebrain annotated hemisphere maps, based on human fetal brain serial sections at the different stages of prenatal ontogenesis. Spatiotemporal changes in the regional-specific immunophenotype profiles will also be demonstrated on virtual serial sections. The HBDA can serve as a reference database for the neurological research, which provides opportunity to compare the data obtained by noninvasive techniques, such as neurosonography, X-ray computed tomography and magnetic resonance imaging, functional magnetic resonance imaging, 3D high-resolution phase-contrast computed tomography visualization techniques, as well as spatial transcriptomics data. It could also become a database for the qualitative and quantitative analysis of individual variability in the human brain. Systemized data on the mechanisms and pathways of prenatal human glio- and neurogenesis could also contribute to the search for new therapy methods for a large spectrum of neurological pathologies, including neurodegenerative and cancer diseases. The preliminary data are now accessible on the special HBDA website.

Primary and secondary olfactory centres in human ontogeny

The olfactory centres are the evolutionary oldest and most conservative area of the telencephalon. Olfactory deficiencies are involved in a large spectrum of neurologic disorders and neurodegenerative diseases. The growing interest in human olfaction has been also been driven by COVID-19-induced transitional anosmia. Nevertheless, recent data on the human olfactory centres concerning normal histology and morphogenesis are rare. Published data in the field are mainly restricted to classic studies with non-uniform nomenclature and varied definitions of certain olfactory areas. While the olfactory system in model animals (rats, mice, and more rarely non-human primates) has been extensively investigated, the developmental timetable of olfactory centres in both human prenatal and postnatal ontogeny are poorly understood and unsystemised, which complicates the process of analysing human material, including medical researches. The main purpose of this review is to provide and discuss relevant morphological data on the normal ontogeny of the human olfactory centres, with a focus on the timetable of maturation and developmental cytoarchitecture, and with special reference to the definitions and terminology of certain olfactory areas.

Adenosine A2A receptors control synaptic remodeling in the adult brain

The molecular mechanisms underlying circuit re-wiring in the mature brain remains ill-defined. An eloquent example of adult circuit remodelling is the hippocampal mossy fiber (MF) sprouting found in diseases such as temporal lobe epilepsy. The molecular determinants underlying this retrograde re-wiring remain unclear. This may involve signaling system(s) controlling axon specification/growth during neurodevelopment reactivated during epileptogenesis. Since adenosine A_2A receptors (A_2AR) control axon formation/outgrowth and synapse stabilization during development, we now examined the contribution of A_2AR to MF sprouting. A_2AR blockade significantly attenuated status epilepticus(SE)-induced MF sprouting in a rat pilocarpine model. This involves A_2AR located in dentate granule cells since their knockdown selectively in dentate granule cells reduced MF sprouting, most likely through the ability of A_2AR to induce the formation/outgrowth of abnormal secondary axons found in rat hippocampal neurons. These A_2AR should be activated by extracellular ATP-derived adenosine since a similar prevention/attenuation of SE-induced hippocampal MF sprouting was observed in CD73 knockout mice. These findings demonstrate that A_2AR contribute to epilepsy-related MF sprouting, most likely through the reactivation of the ability of A_2AR to control axon formation/outgrowth observed during neurodevelopment. These results frame the CD73-A_2AR axis as a regulator of circuit remodeling in the mature brain.

Substantial outcome improvement using a refined pilocarpine mouse model of temporal lobe epilepsy

Systemic pilocarpine treatment is one of the most reliable means of inducing temporal lobe epilepsy (TLE). However, the traditional pilocarpine injection protocol using mice was associated with a high death rate, possibly because of cardiorespiratory collapse following status epilepticus (SE). To prevent this, we developed a modified procedure of pilocarpine SE induction, which included a single injection of a moderate dose of caffeine during the induction phase. That new protocol was based on the use of young male mice as well as on a refined Racine's scale. Using that protocol, we report a substantially increased survival rate, thus enabling the generation of a large cohort of mice that exhibited cardinal histological (e.g., mossy fiber sprouting) and electrophysiological (e.g., chronic interictal events and ictal seizures) characteristics associated with TLE. In conclusion, our refined caffeine- and pilocarpine-based protocol substantially improves the outcome of the reliable pilocarpine mouse model of TLE.

Scopolamine prevents aberrant mossy fiber sprouting and facilitates remission of epilepsy after brain injury

Prevention or modification of acquired epilepsy in patients at risk is an urgent, yet unmet, clinical need. Following acute brain insults, there is an increased risk of mesial temporal lobe epilepsy (mTLE), which is often associated with debilitating comorbidities and reduced life expectancy. The latent period between brain injury and the onset of epilepsy may offer a therapeutic window for interfering with epileptogenesis. The pilocarpine model of mTLE is widely used in the search for novel antiepileptogenic treatments. Recent biochemical studies indicated that cholinergic mechanisms play a role in the epileptogenic alterations induced by status epilepticus (SE) in this and other models of mTLE, which prompted us to evaluate whether treatment with the muscarinic antagonist scopolamine during the latent period after SE is capable of preventing or modifying epilepsy and associated behavioral and cognitive alterations in female Sprague-Dawley rats. First, in silico pharmacokinetic modeling was used to select a dosing protocol by which M-receptor inhibitory brain levels of scopolamine are maintained during prolonged treatment. This protocol was verified by drug analysis in vivo. Rats were then treated twice daily with scopolamine over 17 days after SE, followed by drug wash-out and behavioral and video/EEG monitoring up to ~6 months after SE. Compared to vehicle controls, rats that were treated with scopolamine during the latent period exhibited a significantly lower incidence of spontaneous recurrent seizures during periods of intermittent recording in the chronic phase of epilepsy, less behavioral excitability, less cognitive impairment, and significantly reduced aberrant mossy fiber sprouting in the hippocampus. The present data may indicate that scopolamine exerts antiepileptogenic/disease-modifying activity in the lithium-pilocarpine rat model, possibly involving increased remission of epilepsy as a new mechanism of disease-modification. For evaluating the rigor of the present data, we envision a study that more thoroughly addresses the gender bias and video-EEG recording limitations of the present study.

Dendritic and Synaptic Degeneration in Pyramidal Neurons of the Sensorimotor Cortex in Neonatal Mice With Kaolin-Induced Hydrocephalus

Obstructive hydrocephalus is a brain disorder in which the circulation of cerebrospinal fluid (CSF) is altered in a manner that causes expansion of fluid-filled intracranial compartments particularly the ventricles. The pyramidal neurons of the sensorimotor cortex are excitatory in nature and their dendritic spines are targets of excitatory synapses. This study evaluated the effect of hydrocephalus on dendritic arborization and synaptic structure of the pyramidal neurons of the sensorimotor cortex of neonatal hydrocephalic mice. Sterile kaolin suspension (0.01 ml of 250 mg/mL) was injected intracisternally into day old mice. Control animals mice received sham injections. Pups were weighed and sacrificed on postnatal days (PND) 7, 14 and 21. Fixed brain tissue blocks were silver impregnated using a modified Golgi staining technique and immunolabeled with synaptophysin to determine dendritic morphology and synaptic integrity respectively. Data were analyzed using ANOVA at α 0.05. Golgi staining revealed diminished arborization of the basal dendrites and loss of dendritic spines in the pyramidal neurons of hydrocephalic mice. Compared to age-matched controls, there was a significant reduction in the percentage immunoreactivity of anti-synaptophysin in hydrocephalic mice on PND 7 (14.26 ± 1.91% vs. 62.57 ± 9.40%), PND 14 (4.19 ± 1.57% vs. 93.01 ± 1.66%) and PND 21 (17.55 ± 2.76% vs. 99.11 ± 0.63%) respectively. These alterations suggest impaired neuronal connections that are essential for the development of cortical circuits and may be the structural basis of the neurobehavioral deficits observed in neonatal hydrocephalus.

Developmental dynamics of prepiriform cortex in prenatal human ontogenesis

The prepiriform cortex is a part of the phylogenetically oldest pallial division (paleocortex) representing the primary olfactory cortex. While olfactory centers in laboratory animals have been extensively investigated, the developmental timetable of the human prepiriform area is poorly understood. Thus, in the present study we aim to examine the prepiriform cortex in human fetuses from eight postconceptional weeks to birth. Based on cytoarchitecture and immunohistochemistry analysis (NeuN-, SYP-, NSE-, TH-, GFAP-, MBP-) four main periods of the prepiriform cortex fetal development are suggested: the beginning of prefetal stage (the eighth week from conception), the period from the ending of prefetal stage (9-12 postconceptional weeks) to 17 weeks of gestation, 18-27 weeks of gestation and the late fetal period (29-40 gestational weeks). We found that the initial layer differentiation took place before the ninthtenth weeks from conception and by ten weeks the paleocortical plate of the prepiriform cortex was shaped. Both total cell density and NeuN-immunoreactive cell density peaked in the early fetuses and started to decrease after 17 gestational weeks, attaining intermediate values at 18-27 weeks and becoming significantly lower in the late fetuses. In contrast, the NeuN-immunoreactive cell ratio gradually increased over the whole examined period. The prepiriform cortex was defined as approaches the state at birth at 30 gestational weeks. The same developmental periods were observed with SYP- and NSE-assays. No significant distribution of TH immunoreactivity was described in the prepiriform cortex of human fetuses. The prior paleocortex development was demonstrated using glial markers: GFAPimmunoreactivity appeared in the prepiriform cortex at the middle of the early fetal period, ahead of the neocortex and insular cortex. The earlier rates of GFAP-immunoreactivity expansion in the prepiriform cortex, as compared to other pallial regions, persisted in the later fetuses. The first MBP-immunoreactive fibres within pallium were detected in the lateral olfactory tract at 30 weeks. Therefore, the prepiriform cortex approaches a level of maturation similar to that at birth already at the beginning of the late fetal period and matures prior to other pallial regions.

Study of the chlorpyrifos neurotoxicity using neural differentiation of adipose tissue-derived stem cells

Chlorpyrifos (CPF) is the most commonly used organophosphorus insecticide which causes neurodevelopmental toxicity. So far, animals have been used as ideal models for neurotoxicity studies, but working with animals is very expensive, laborious, and ethically challenging. This has encouraged researchers to seek alternatives. During recent years, several studies have reported successful differentiation of embryonic and adult stem cells to neurons. This has provided an excellent model for neurotoxicologic studies. In this study, neural differentiation of mouse adipose tissue-derived stem cells (ADSCs) was used as an in vitro model for investigation of CPF neurotoxicity. For this purpose, mouse ADSCs were cultured in a medium containing knockout serum replacement and were treated with different concentrations of CPF at several stages of differentiation. Cytotoxic effect of CPF and the expression of neuron-specific genes and proteins were studied in the differentiating ADSCs. Furthermore, the activity of acetylcholinesterase was assessed by Ellman assay at different stages of differentiation. This study showed that up to 500 μM CPF did not alter viability of the undifferentiated ADSCs, whereas viability of the differentiating cells decreased with 500 μM CPF. CPF upregulated the expression of some neuron-specific genes and seemed to decrease the number of β-tubulin III and MAP2 proteins-expressing cells. There was no detectable acetylcholine esterase activity in differentiated ADSCs. In summary, it was shown that CPF treatment can decrease the viability of ADSC-derived neurons and dysregulate the expression of some neuronal markers through acetylcholinesterase-independent mechanisms. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1510-1519, 2016.

Neuritin Mediates Activity-Dependent Axonal Branch Formation in Part via FGF Signaling

Aberrant branch formation of granule cell axons (mossy fiber sprouting) is observed in the dentate gyrus of many patients with temporal lobe epilepsy and in animal models of epilepsy. However, the mechanisms underlying mossy fiber sprouting remain elusive. Based on the hypothesis that seizure-mediated gene expression induces abnormal mossy fiber growth, we screened activity-regulated genes in the hippocampus and found that neuritin, an extracellular protein anchored to the cell surface, was rapidly upregulated after electroconvulsive seizures. Overexpression of neuritin in the cultured rat granule cells promoted their axonal branching. Also, kainic acid-dependent axonal branching was abolished in the cultured granule cells fromneuritinknock-out mice, suggesting that neuritin may be involved in activity-dependent axonal branching. Moreover,neuritinknock-out mice showed less-severe seizures in chemical kindling probably by reduced mossy fiber sprouting and/or increased seizure resistance. We found that inhibition of the fibroblast growth factor (FGF) receptor attenuated the neuritin-dependent axonal branching. FGF administration also increased branching in granule neurons, whereasneuritinknock-out mice did not show FGF-dependent axonal branching. In addition, FGF and neuritin treatment enhanced the recruitment of FGF receptors to the cell surface. These findings suggest that neuritin and FGF cooperate in inducing mossy fiber sprouting through FGF signaling. Together, these results suggest that FGF and neuritin-mediated axonal branch induction are involved in the aggravation of epilepsy.

SIGNIFICANCE STATEMENT

This study reveals the molecular mechanism underlying mossy fiber sprouting. Mossy fiber sprouting is the aberrant axonal branching of granule neurons in the hippocampus, which is observed in patients with epilepsy. Excess amounts of neuritin, a protein upregulated by neural activity, promoted axonal branching in granule neurons. A deficiency of neuritin suppressed mossy fiber sprouting and resulted in mitigation of seizure severity. Neuritin and fibroblast growth factor (FGF) cooperated in stimulating FGF signaling and enhancing axonal branching. Neuritin is necessary for FGF-mediated recruitment of FGF receptors to the cell surface. The recruitment of FGF receptors would promote axonal branching. The discovery of this new mechanism should contribute to the development of novel antiepileptic drugs to inhibit axonal branching via neuritin-FGF signaling.

Contribution of aberrant GluK2-containing kainate receptors to chronic seizures in temporal lobe epilepsy

Kainate is a potent neurotoxin known to induce acute seizures. However, whether kainate receptors (KARs) play any role in the pathophysiology of temporal lobe epilepsy (TLE) is not known. In TLE, recurrent mossy fiber (rMF) axons form abnormal excitatory synapses onto other dentate granule cells that operate via KARs. The present study explores the pathophysiological implications of KARs in generating recurrent seizures in chronic epilepsy. In an in vitro model of TLE, seizure-like activity was minimized in mice lacking the GluK2 subunit, which is a main component of aberrant synaptic KARs at rMF synapses. In vivo, the frequency of interictal spikes and ictal discharges was strongly reduced in GluK2(-/-) mice or in the presence of a GluK2/GluK5 receptor antagonist. Our data show that aberrant GluK2-containing KARs play a major role in the chronic seizures that characterize TLE and thus constitute a promising antiepileptic target.

Optic nerve crush induces spatial and temporal gene expression patterns in retina and optic nerve of BALB/cJ mice

Background

Central nervous system (CNS) trauma and neurodegenerative disorders trigger a cascade of cellular and molecular events resulting in neuronal apoptosis and regenerative failure. The pathogenic mechanisms and gene expression changes associated with these detrimental events can be effectively studied using a rodent optic nerve crush (ONC) model. The purpose of this study was to use a mouse ONC model to: (a) evaluate changes in retina and optic nerve (ON) gene expression, (b) identify neurodegenerative pathogenic pathways and (c) discover potential new therapeutic targets.

Results

Only 54% of total neurons survived in the ganglion cell layer (GCL) 28 days post crush. Using Bayesian Estimation of Temporal Regulation (BETR) gene expression analysis, we identified significantly altered expression of 1,723 and 2,110 genes in the retina and ON, respectively. Meta-analysis of altered gene expression (≥1.5, ≤-1.5, p < 0.05) using Partek and DAVID demonstrated 28 up and 20 down-regulated retinal gene clusters and 57 up and 41 down-regulated optic nerve clusters. Regulated gene clusters included regenerative change, synaptic plasticity, axonogenesis, neuron projection, and neuron differentiation. Expression of selected genes (Vsnl1, Syt1, Synpr and Nrn1) from retinal and ON neuronal clusters were quantitatively and qualitatively examined for their relation to axonal neurodegeneration by immunohistochemistry and qRT-PCR.

Conclusion

A number of detrimental gene expression changes occur that contribute to trauma-induced neurodegeneration after injury to ON axons. Nrn1 (synaptic plasticity gene), Synpr and Syt1 (synaptic vesicle fusion genes), and Vsnl1 (neuron differentiation associated gene) were a few of the potentially unique genes identified that were down-regulated spatially and temporally in our rodent ONC model. Bioinformatic meta-analysis identified significant tissue-specific and time-dependent gene clusters associated with regenerative changes, synaptic plasticity, axonogenesis, neuron projection, and neuron differentiation. These ONC induced neuronal loss and regenerative failure associated clusters can be extrapolated to changes occurring in other forms of CNS trauma or in clinical neurodegenerative pathological settings. In conclusion, this study identified potential therapeutic targets to address two key mechanisms of CNS trauma and neurodegeneration: neuronal loss and regenerative failure.

Prenatal loud music and noise: differential impact on physiological arousal, hippocampal synaptogenesis and spatial behavior in one day-old chicks

Prenatal auditory stimulation in chicks with species-specific sound and music at 65 dB facilitates spatial orientation and learning and is associated with significant morphological and biochemical changes in the hippocampus and brainstem auditory nuclei. Increased noradrenaline level due to physiological arousal is suggested as a possible mediator for the observed beneficial effects following patterned and rhythmic sound exposure. However, studies regarding the effects of prenatal high decibel sound (110 dB; music and noise) exposure on the plasma noradrenaline level, synaptic protein expression in the hippocampus and spatial behavior of neonatal chicks remained unexplored. Here, we report that high decibel music stimulation moderately increases plasma noradrenaline level and positively modulates spatial orientation, learning and memory of one day-old chicks. In contrast, noise at the same sound pressure level results in excessive increase of plasma noradrenaline level and impairs the spatial behavior. Further, to assess the changes at the molecular level, we have quantified the expression of functional synapse markers: synaptophysin and PSD-95 in the hippocampus. Compared to the controls, both proteins show significantly increased expressions in the music stimulated group but decrease in expressions in the noise group. We propose that the differential increase of plasma noradrenaline level and altered expression of synaptic proteins in the hippocampus are responsible for the observed behavioral consequences following prenatal 110 dB music and noise stimulation.

Differential expression of SLC9A9 and interacting molecules in the hippocampus of rat models for attention deficit/hyperactivity disorder

SLC9A9 [solute carrier family 9, member 9, also known as Na(+)/H(+) exchanger member 9 (NHE9)], has been implicated in human attention deficit/hyperactivity disorder (ADHD), autism, and rat studies of hyperactivity and inattentiveness. SLC9A9 is a membrane protein that regulates the luminal pH of the recycling endosome. We recently reported the interactions of SLC9A9 with two molecules: calcineurin homologous protein (CHP) and receptor for activated C-kinase 1 (RACK1). We also reported two novel SLC9A9 mutations and abnormal gene expression profiles in the brains of an inattentive type rat model of ADHD (WKY/NCrl rat). In this study, we further examined the expression and relationship of SLC9A9 and 9 additional genes (CHP, RACK1, CaM, PPP3R1, PPP1R10, PKCm, CaMKI, NR2B, PLCb1) that may directly or indirectly interact with SLC9A9 in the hippocampus of the WKY/NCrl rat and the spontaneously hypertensive rat (SHR) model of the combined type of ADHD. We found that the expression levels of these genes were significantly correlated, suggesting that they may be coregulated. Principal component analysis identified two main factors that accounted for 94% of the expression variance of the 10 genes. Significant differences were found for both factors across the 3 different rat strains. The two ADHD rat models (WKY/NCrl and SHR), although different from each other in adulthood, showed similar profiles in adolescence. Both models were significantly different from WKY/NHsd control rats at both ages. The expression abnormalities of each gene were evaluated and their roles in cell signaling processes such as calcium signaling and protein phosphorylation are discussed. Our results suggest that abnormalities in SLC9A9-mediated signaling pathways could contribute to the ADHD phenotype of two rat models (WKY/NCrl and SHR/NCrl), and that the perturbation of the SLC9A9 network is age-dependent.

NeuroD2 regulates the development of hippocampal mossy fiber synapses

BACKGROUND

The assembly of neural circuits requires the concerted action of both genetically determined and activity-dependent mechanisms. Calcium-regulated transcription may link these processes, but the influence of specific transcription factors on the differentiation of synapse-specific properties is poorly understood. Here we characterize the influence of NeuroD2, a calcium-dependent transcription factor, in regulating the structural and functional maturation of the hippocampal mossy fiber (MF) synapse.

RESULTS

Using NeuroD2 null mice and in vivo lentivirus-mediated gene knockdown, we demonstrate a critical role for NeuroD2 in the formation of CA3 dendritic spines receiving MF inputs. We also use electrophysiological recordings from CA3 neurons while stimulating MF axons to show that NeuroD2 regulates the differentiation of functional properties at the MF synapse. Finally, we find that NeuroD2 regulates PSD95 expression in hippocampal neurons and that PSD95 loss of function in vivo reproduces CA3 neuron spine defects observed in NeuroD2 null mice.

CONCLUSION

These experiments identify NeuroD2 as a key transcription factor that regulates the structural and functional differentiation of MF synapses in vivo.

SLC9A9 mutations, gene expression, and protein-protein interactions in rat models of attention-deficit/hyperactivity disorder

SLC9A9 (solute carrier family 9, member 9, also known as Na+/H+ exchanger member (NHE9)) is a membrane protein that regulates the luminal pH of the recycling endosome, an essential organelle for synaptic transmission and plasticity. SLC9A9 has been implicated in human attention deficit hyperactivity disorder (ADHD) and in rat studies of hyperactivity. We examined the SLC9A9 gene sequence and expression profile in prefrontal cortex, dorsal striatum and hippocampus in two genetic rat models of ADHD. We report two mutations in a rat model of inattentive ADHD, the WKY/NCrl rat, which affect the interaction of SLC9A9 with calcineurin homologous protein (CHP). We observed an age-dependent abnormal expression of SLC9A9 in brains of this inattentive model and in the Spontaneous Hypertensive Rat (SHR) model of ADHD. Our data suggest a novel mechanism whereby SLC9A9 sequence variants and abnormalities in gene expression could contribute to the ADHD-like symptoms of rat models and possibly the pathophysiology of ADHD in humans.

Cadherin-9 regulates synapse-specific differentiation in the developing hippocampus

Our understanding of mechanisms that regulate the differentiation of specific classes of synapses is limited. Here, we investigate the formation of synapses between hippocampal dentate gyrus (DG) neurons and their target CA3 neurons and find that DG neurons preferentially form synapses with CA3 rather than DG or CA1 neurons in culture, suggesting that specific interactions between DG and CA3 neurons drive synapse formation. Cadherin-9 is expressed selectively in DG and CA3 neurons, and downregulation of cadherin-9 in CA3 neurons leads to a selective decrease in the number and size of DG synapses onto CA3 neurons. In addition, loss of cadherin-9 from DG or CA3 neurons in vivo leads to striking defects in the formation and differentiation of the DG-CA3 mossy fiber synapse. These observations indicate that cadherin-9 bidirectionally regulates DG-CA3 synapse development and highlight the critical role of differentially expressed molecular cues in establishing specific connections in the mammalian brain.

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

PURPOSE

Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion.

METHODS

Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin.

KEY FINDINGS

p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate-injected hippocampus, there was an 80% and 85% reduction of GluR2/3- and GluR2/3/calretinin-positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3-, but only a 20% loss of GluR2/3-calretinin-immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed.

SIGNIFICANCE

These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.

Autaptic cultures of single hippocampal granule cells of mice and rats

When a single neuron is grown on a small island of glial cells, the neuron forms synapses onto itself. The so-called autaptic culture systems have proven extremely valuable in elucidating basic mechanisms of synaptic transmission, as they allow application of technical approaches that cannot be used in slice preparations. However, this method has been almost exclusively used for pyramidal cells and interneurons. In this study, we generated autaptic cultures from granule cells isolated from the dentate gyrus of rodent hippocampi. Our subsequent morphological and functional characterisation of these cells confirms that this culture model is suitable for investigating basic mechanisms of granule cell synaptic transmission. Importantly, the autosynaptic connectivity allows recordings of pure mossy fibre miniature EPSCs, which are not possible in slice preparations. Further, by fast application of hypertonic sucrose solutions it is possible to directly measure the readily releasable pool and to calculate the probability of vesicular release.

Selective apoptosis induction in the hippocampal mossy fiber pathway by exposure to CT105, the C-terminal fragment of Alzheimer's amyloid precursor protein

Beta-amyloid protein (Abeta), a proteolytic byproduct of Alzheimer's amyloid precursor protein (APP), has been shown to play a central role in the development of Alzheimer's disease (AD). In addition, recent studies strongly suggest that other byproducts of proteolysis, such as C-terminal fragments of APP (APP-CTF), are also critically involved in the AD pathology. To explore this possibility, we investigated the histopathological changes induced by repeated low-dose intrahippocampal injection of a recombinant 105 amino acid C-terminal fragment of APP (CT105). First, we carried out a behavioral analysis by using the three-panel runway task, and found that the working memory was significantly impaired by CT105 exposure. Then, via propidium iodide staining, we encountered a number of cells exhibiting fragmented or shrank nuclei in the mossy fiber pathway (stratum lucidum and dentate hilus) in CT105-treated rats. These cells were positive for single-stranded DNA (ssDNA), an apoptosis-specific marker, and thus were considered to be apoptotic. Some of the ssDNA-positive cells were also positive for somatostatin. But neither ionized calcium-binding adapter molecule 1 (Iba1) nor S100beta occurred in ssDNA-positive cells. These findings suggest that CT105 induces apoptotic changes in cells of neuronal origin. Quantitative analysis showed that the densities of ssDNA-positive cells in the mossy fiber pathway were significantly higher in CT105-treated rats than in control animals. The present results suggest that CT105 causes dysfunction in the hippocampal mossy fiber system, and also provide some key to understand the relationship between APP-CTF and glutamatergic synaptic dysregulation in AD.

Differential expression of synaptoporin and synaptophysin in primary sensory neurons and up-regulation of synaptoporin after peripheral nerve injury

Synaptoporin and synaptophysin are integral membrane components of synaptic vesicles. The distribution of synaptoporin and its relationship with synaptophysin in sensory afferent fibers remain unclear. In the present study, we showed that in the rat dorsal root ganglia synaptoporin was expressed in subsets of small neurons that contain either calcitonin gene-related peptide or isolectin B4, and was distributed in their afferent terminals in laminae I-II of the spinal cord. Synaptophysin was expressed in 57% of synaptoporin-containing small dorsal root ganglion neurons and in large dorsal root ganglion neurons. In the spinal dorsal horn, synaptophysin-immunolabeling was weak in the afferent fibers in lamina I, outer lamina II and the dorsal part of inner lamina II, but strong in the afferent fibers in laminae III-IV. However, a subpopulation of isolectin B4-positive small dorsal root ganglion neurons expressed both synaptoporin and synaptophysin, and their afferent fibers were mainly distributed in the ventral part of inner lamina II. After peripheral nerve injury, synaptoporin expression was up-regulated in small dorsal root ganglion neurons, and synaptoporin level was increased in their afferent terminals. Thus, synaptoporin and synaptophysin have topographically distinct distributions in afferent fibers. Synaptoporin is a major synaptic vesicle protein in Adelta- and C-fibers in both physiological and neuropathic pain states.

Differential appearance of dynamin in constitutive and regulated exo-endocytosis: a single-cell multiplex RT-PCR study

Neurons in the central nervous system establish, via their axons and dendrites, an extended network that allows synaptic transmission. During developmental maturation and process outgrowth, membrane turnover is necessary for the enlargement and subsequent growth of axons and dendrites from the perikarya to the target cell (constitutive exocytosis/endocytosis). After targeting and synapse formation, small synaptic vesicles are needed for the quantal release of neurotransmitters from the presynaptic terminal with subsequent recycling by regulated exocytosis/endocytosis. An investigation of the onset of the appearance of mRNA and protein in dissociated cultures of neurons from mouse hippocampus or from chick retina has shown an early abundance of proteins involved in exocytosis, such as syntaxin 1, SNAP-25, and synaptotagmin 1, whereas dynamin 1, a protein necessary for clathrin-mediated endocytosis, can be detected only after neurons have established contacts with neighboring cells. The results reveal that constitutive membrane incorporation and regulated synaptic transmitter release is mediated by the same neuronal proteins. Moreover, the data exclude that dynamin 1 takes part in constitutive recycling before synapse formation, but dynamin 2 is present at this stage. Thus, dynamin 2 may be the constitutive counterpart of dynamin 1 in growing neurons. Synapse establishment is linked to an upregulation of dynamin 1 and thereby represents the beginning of the regulated recycling of membranes back into the presynaptic terminal.

A role for caveolin-1 in post-injury reactive neuronal plasticity

Remodeling and plasticity in the adult brain require cholesterol redistribution and synthesis for the formation of new membrane components. Caveolin-1 is a cholesterol-binding membrane protein involved in cellular cholesterol transport and homeostasis. Evidence presented here demonstrates an up-regulation of caveolin-1 in the hippocampus, which was temporally correlated with an increase in synaptophysin during the reinnervation phase in a mouse model of hippocampal deafferentation. Using an in vitro model of neuronal reactive plasticity, we examined the effect of virally mediated overexpression of caveolin-1 on injured differentiated PC12 cells undergoing terminal remodeling. Three days post lesion, caveolin-1-overexpressing cells revealed increases in synaptophysin and GAP-43, two markers of neurite sprouting and synaptogenesis. Morphologically, caveolin-1-overexpressing cells showed a decrease in primary neurite outgrowth and branching as well as an increase in neurite density. Caveolin-1-overexpressing cells also revealed the presence of terminal swelling and beading along processes, consistent with a possible alteration of microtubules stability. Moreover, a focal enrichment of caveolin-1 immunofluorescence was observed at the bases of axonal and dendritic terminals of mouse primary hippocampal neurons. Altogether, these results indicate that caveolin-1 plays an active role in the regulation of injury-induced synaptic and terminal remodeling in the adult CNS.

Chronic stress alters amphetamine effects on behavior and synaptophysin levels in female rats

Previous studies show that stress cross-sensitizes with or alters amphetamine (AMPH) effects in male rats; however, few studies include females. We investigated combining daily restraint stress (21 days for 6 h/day) with chronic AMPH (10 injections every other day) on locomotor activity, exploratory activity in an open field and object recognition, a memory task, in female rats. A synaptic protein, synaptophysin, was also quantified by radioimmunocytochemistry (RICC) in brain to determine possible mechanisms for behavioral changes. Beginning at 5 days after cessation of treatments, AMPH increased locomotion, modified exploration, impaired object recognition, and increased serum corticosterone (CORT) levels. Stress did not alter these parameters but blocked AMPH effects on exploration and object recognition, potentiated AMPH-dependent locomotor effects, and did not alter increased CORT levels. AMPH treatment decreased synatophysin expression in the hippocampus. In the caudate nucleus, the AMPH group showed increased synaptophysin expression which was reversed by stress. These results in females corroborate previously shown cross-sensitizations between stress and AMPH for locomotion in males and demonstrate that chronic stress counteracts AMPH-dependent impairments in recognition memory. Stress may counteract AMPH effects on the memory task by blocking both the induction of AMPH anxiety-like effects and neuroplastic changes in the caudate nucleus of female rats.

Heterogeneous expression of the cholecystokinin-like immunoreactivity in the mouse hippocampus, with special reference to the dorsoventral difference

The neuropeptide cholecystokinin (CCK) is widely distributed in the CNS. We herein investigated the immunocytochemical localization of CCK in the glutamatergic excitatory pathways in the mouse hippocampus, with particular reference to the dorsoventral difference. The intense CCK-like immunoreactivity (CCK-LI) was found in the mossy fiber pathway (stratum lucidum and dentate hilus) and in the inner molecular layer of the dentate gyrus. In the mossy fiber pathway, the CCK-LI was more intense at the ventral level than at the dorsal level. On the other hand, the CCK-LI in the stratum lucidum was more intense in the distal portion than in the proximal portion, both at the dorsal and ventral levels. High-resolution three-dimensional image analysis revealed the coexpression of CCK and synaptoporin (SPO) in the single mossy terminal, where they were spatially segregated but adjacent to each other. Quantitative image analysis indicated the difference in the amount of CCK within the mossy terminals along the dorsoventral and transverse axes of the hippocampus. On the other hand, in the inner molecular layer, CCK- and SPO-positive elements appeared to have little relation to each other. We also examined the postnatal development of the CCK-LI in the mouse hippocampus. The CCK-LI was detected in the inner molecular layer of the ventral dentate gyrus at postnatal day (P) 7. In the mossy fiber pathway, the CCK-LI was first evident at P 14, but it was restricted to the distal portion of the stratum lucidum in the ventral hippocampus. Interestingly, the distributions of the SPO immunoreactivity at P 7 were already similar to those of adult mice. The patterns of expression of CCK-LI at P 28 were almost similar to those of adult mice. The present data demonstrate the heterogeneous expression of CCK-LI in the mouse hippocampus, and provide a baseline to understand the role of CCK in the mouse brain.

Activity-dependent changes in synaptophysin immunoreactivity in hippocampus, piriform cortex, and entorhinal cortex of the rat

Synaptophysin, an integral membrane glycoprotein of synaptic vesicles, has been widely used to investigate synaptogenesis in both animal models and human patients. Kindling is an experimental model of complex partial seizures with secondary generalization, and a useful model for studying activation-induced neural growth in adult systems. Many studies using Timm staining have shown that kindling promotes sprouting in the mossy fiber pathway of the dentate gyrus. In the present study, we used synaptophysin immunohistochemistry to demonstrate activation-induced neural sprouting in non-mossy fiber cortical pathways in the adult rat. We found a significant kindling-induced increase in synaptophysin immunoreactivity in the stratum radiatum of CA1 and stratum lucidum/radiatum of CA3, the hilus, the inner molecular layer of the dentate gyrus, and layer II/III of the piriform cortex, but no significant change in layer II/III of the entorhinal cortex, 4 weeks after the last kindling stimulation. We also found that synaptophysin immunoreactivity was lowest in CA3 near the hilus and increased with increasing distance from the hilus, a reverse pattern to that seen with Timm stains in stratum oriens following kindling. Furthermore, synaptophysin immunoreactivity was lowest in dorsal and greatest in ventral sections of both CA3 and dentate gyrus in both kindled and non-kindled animals. This demonstrates that different populations of sprouting axons are labeled by these two techniques, and suggests that activation-induced sprouting extends well beyond the hippocampal mossy fiber system.

The expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina

In the present study, we generated a systematic overview of the expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina. Using indirect immunofluorescence microscopy, we analyzed the spatial and temporal distribution of 11 important membrane and membrane-associated synaptic proteins (syntaxin 1/3, SNAP-25, synaptobrevin 2, synaptogyrin, synaptotagmin I, SV2A, SV2B, Rab3A, clathrin light chains, CSP and neuroligin I) during synaptogenesis. The temporospatial distribution of these synaptic proteins was "normalized" by the simultaneous visualization of the synaptic vesicle protein synaptophysin, which served as an internal reference protein. We found that expression of various synaptic membrane proteins started at different time points and changed progressively during development. At early stages of development synaptic vesicle membrane proteins at extrasynaptic locations did not always colocalize with synaptophysin, indicating that these proteins probably do not reside in the same transport vesicles. Despite a non-synchronized onset of protein expression, clustering and colocalization of all synaptic membrane proteins at ribbon synapses roughly occurred in the same time window (between day 4 after birth, P4, and P5). Thus, the basic synaptic membrane machinery is already present in ribbon synapses before the well-known complete morphological maturation of ribbon synapses between P7 and P12. We conclude that ribbon synapse formation is a multistep process in which the concerted recruitment of synaptic membrane proteins is a relatively early event and clearly not the final step.

Tetraspan vesicle membrane proteins: synthesis, subcellular localization, and functional properties

Tetraspan vesicle membrane proteins (TVPs) are characterized by four transmembrane regions and cytoplasmically located end domains. They are ubiquitous and abundant components of vesicles in most, if not all, cells of multicellular organisms. TVP-containing vesicles shuttle between various membranous compartments and are localized in biosynthetic and endocytotic pathways. Based on gene organization and amino acid sequence similarities TVPs can be grouped into three distinct families that are referred to as physins, gyrins, and secretory carrier-associated membrane proteins (SCAMPs). In mammals synaptophysin, synaptoporin, pantophysin, and mitsugumin29 constitute the physins, synaptogyrin 1-4 the gyrins, and SCAMP1-5 the SCAMPs. Members of each family are cell-type-specifically synthesized resulting in unique patterns of TVP coexpression and subcellular colocalization. TVP orthologs have been identified in most multicellular organisms, including diverse animal and plant species, but have not been detected in unicellular organisms. They are subject to protein modification, most notably to phosphorylation, and are part of multimeric complexes. Experimental evidence is reviewed showing that TVPs contribute to vesicle trafficking and membrane morphogenesis.

Synaptic vesicle protein synaptoporin is differently expressed by subpopulations of mouse hippocampal neurons

In the hippocampus, the synaptic vesicle protein synaptoporin (SPO) has been reported to be exclusively enriched in the granule cell axons, the mossy fibers. In this study, we show that in adult rats and mice SPO immunoreactivity (IR) is also detectable in strata oriens, radiatum, and lacunosum-moleculare of CA1-CA3, as well as perisomatically in the hippocampus proper and fascia dentata. In situ hybridization confirmed that SPO mRNA was present in granule cells and CA3 pyramidal cells but not in CA1 pyramidal cells. Importantly, cells scattered throughout the hippocampal layers resembling the distribution of interneurons were found to synthesize high amounts of SPO mRNA, too. Thus, these findings indicate that SPO expression in the hippocampus was underestimated until now. Moreover, double-labeling immunohistochemistry and confocal microscopy revealed selective colocalization of SPO and glutamate decarboxylase (GAD 65), a marker for gamma-aminobutyric acid (GABA)ergic terminals. To identify SPO expressing interneurons, in situ hybridization was combined with immunocytochemistry against parvalbumin (PV), calbindin (CB), calretinin (CR), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP). We found that SPO transcripts were differentially expressed by various interneuron subpopulations in the hippocampus of C57Bl/6 mice (PV 44.2%, CB 46.3%, CR 19.3%, CCK 38.6%, VIP 59.9%). Immunoelectron microscopy for SPO labeled synaptic vesicle profiles in distinct symmetric and asymmetric synapses. In conclusion, our data demonstrate that hippocampal principal cells and interneurons display a variety of synaptic vesicles that are likely to contribute to the functional characteristics of their output synapses.

Differential expression of neuronal calcium sensor-1 in the developing chick retina

Neuronal calcium sensor-1 (NCS-1) is a Ca(2+) binding protein that has been implicated in the regulation of neurotransmission and synaptogenesis. In this study we investigated the developmental expression and localization of NCS-1 in the chick retina. Single- and double-labeling experiments with three-dimensional reconstruction as well as ultrastructural data of the distribution of NCS-1 suggest that this protein is also involved in axonal process outgrowth. We found an early expression of NCS-1 in ganglion cells and their axons, in amacrine, and in horizontal cells, whereas photoreceptors were immunonegative at embryonic stages. In the early posthatching days we found strong immunostaining for NCS-1 in horizontal cells and their processes in the outer plexiform layer. In contrast, synaptic vesicle protein 2 (SV2) was prominent only in photoreceptor synaptic terminals. Ultrastructural analysis confirmed that NCS-1 was localized postsynaptically in horizontal cell processes, whereas presynaptic terminals were immunonegative. However, at late posthatching days we observed that photoreceptor ribbon synapses (from rods and/or cones) also expressed NCS-1. Thus the results support the notion that NCS-1 is involved in neuronal process outgrowth and is localized in pre- and postsynaptic compartments including mature photoreceptor synapses.

Developmental expression of the cellular prion protein in elongating axons

PrPc, a sialoglycoprotein present in the normal adult hamster brain, is particularly abundant in plastic brain regions but little is known about the level of expression and the localization of the protein during development. Western blot analysis of whole brain homogenates with mab3F4 show very low levels of the three main molecular weight forms of the protein at birth, in contrast to the strong and wide expression of mRNA transcripts. The PrPc levels increase sharply through P14 and are diminished somewhat in the adult. Regional analysis showed that in structures with ongoing growth or plasticity such as the olfactory bulb and hippocampus, PrPc remains high in the adult, while in areas where structural and functional relationships stabilize during development, such as the cortex and the thalamus, PrPc levels decline after the third postnatal week. In the neonate brain PrPc was prominent along fiber tracts similar to markers of axon elongation and in vitro experiments showed that the protein was present on the surface of elongating axons. PrPc is then localized to the synaptic neuropil in close spatio-temporal association with synapse formation. The localization of PrPc on elongating axons suggests a role for the protein in axon growth. In addition, the relative abundance of the protein in developing axon pathways and during synaptogenesis may provide a basis for the age-dependent susceptibility to transmissible spongiform encephalopathies.

Characterization of a novel synGAP isoform, synGAP-beta

We cloned a cDNA encoding a novel synGAP, synGAP-d (GenBank(TM) accession number ), from a rat brain cDNA library. The clone consisted of 4801 nucleotides with a coding sequence of 3501 nucleotides, encoded a protein consisting of 1166 amino acids with >99% homology with 1092 amino acid overlaps to synGAP, and contained a 13-nucleotide insertion to the previously reported synGAP mRNAs, which suggested that the clone was a splice variant of synGAP. We also found that there are at least seven variants in the 3' portion of the synGAP mRNA and that they encoded five different protein isoforms. The coding sequence of these C-terminal variants were classified into alpha1, alpha2, beta1, beta2, beta3, beta4, and gamma, and synGAP-d was classified as the beta1 form. The previously reported synGAPs (synGAP-a, -b, and -c and p135synGAP) can be classified as the alpha1 isoform. All isoforms were expressed specifically in the brain. Unexpectedly, the beta isoform, which lacks a C-terminal PSD-95-binding motif ((S/T)XV), was more restricted to the postsynaptic density fraction than the motif-containing alpha1 isoform. The beta isoform did not interact with PSD-95 but specifically interacted with a nonphosphorylated alpha subunit of Ca(2+)/calmodulin-dependent protein kinase II through its unique C-terminal tail.

Developmental expression of amphiphysin in the retinotectal system of the chick: from mRNA to protein

The role of amphiphysin in clathrin-mediated endocytosis of synaptic vesicles is well established. However, it is still uncertain if the protein is also involved in developmental mechanisms, e.g. axon outgrowth and synapse formation. To investigate the developmental changes in the expression of amphiphysin we used the retinotectal system of the chick, a highly ordered and easily accessible primary neuronal pathway. Reverse transcription polymerase chain reaction (RT-PCR) of total RNA from chick retina and tectum revealed first transcripts for amphiphysin, dynamin and synaptotagmin at embryonic day 5 (E5) for both regions. Surprisingly, Western blots of the retina revealed an increase of protein expression for amphiphysin only after E11 in the retina and the tectum. Immunofluorescence for amphiphysin was not detectable before E10 in the developing chick retina, while other presynaptic proteins like synaptotagmin showed already intense signals in the inner and outer plexiform layers. Subsequently, amphiphysin immunoreactivity follows the expression of synaptotagmin and synaptic vesicle protein 2 (SV2) as seen in the retina and the tectum, and exhibits the same staining as the other proteins in the mature chick brain. Ultrastructural data revealed for the first time that amphiphysin is not only limited to conventional synapses but is also abundant in retinal ribbon terminals. Taken together our data reveal that: (i) there is a developmental delay between mRNA transcription and protein expression for key proteins involved in endocytosis; (ii) amphiphysin gets upregulated after synapse formation; and (iii) amphiphysin is present in the synaptic vesicle cycle in retinal ribbon synapses.

Hemisynaptic distribution patterns of presenilins and beta-APP isoforms in the rodent cerebellum and hippocampus

Healthy brain neurons co-express Alzheimer's disease (AD) related proteins presenilins (PS) and beta-amyloid precursor protein (beta-APP). Deposition of beta-amyloid and PS in the senile plaques of AD brain and their ability to interact in vitro suggest that AD pathology could arise from a defect in the physiological interactions between beta-APP and PS within and/or between neurons. The present study compares the immunocytochemical distribution of PS (1 and 2) and beta-APP major isoforms (695 and 751/770) in the synapses of the cerebellum and hippocampus of the adult rat and mouse. In the cerebellar cortex of both species, the four molecules are immunodetected in the presynaptic or the postsynaptic compartments of synapses, suggesting that they are involved in interneuronal relationships. In contrast, PS and beta-APP are postsynaptic in almost all the immunoreactive synapses of the hippocampus. The different distribution patterns of these proteins in cerebellar and hippocampal synapses may reflect specific physiological differences, responsible for differential vulnerability of neurons to AD synaptic pathology. Defective interactions between beta-APP and PS at the synapses could impede the synaptic functions of beta-APP, inducing the selective loss of synapses that accounts for cognitive impairment in AD.

Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined this issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory postsynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMPA receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is also reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duration is increased by bicuculline suggesting the participation of N-methyl-D-aspartate (NMDA) and GABAA (gamma-aminobutyric acid) receptors. Finally, AMPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are smaller and more labile than in the IHF. Our results suggest that in embryonic CA1 neurons, glutamate acting on AMPA receptors already provides a substantial part of the excitatory drive and may play an important role in the activity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.

Developmental expression of dynamin in the chick retinotectal system

Dynamin I, a GTPase involved in the endocytic cycle of synaptic vesicle membranes, is believed to support axonal outgrowth and/or synaptogenesis. To explore the temporal and spatial patterns of dynamin I distribution in neuronal morphogenesis, we compared the developmental expression of dynamin with the expression of presynaptic membrane proteins such as SV2, synaptotagmin, and syntaxin in the chick primary visual pathway. Western blots of retina and tectum revealed a steady increase of synaptotagmin and syntaxin from embryonic Day 7 (E7) to E11, whereas for the same time frame no detectable increase of dynamin was found. Later stages showed increasing amounts of all tested proteins until the first postnatal week. Immunofluorescence revealed that SV2, synaptotagmin, and syntaxin are present in retinal ganglion cell axons from E4 on. In later stages, the staining pattern in the retina and along the visual pathway paralleled the formation and maturation of axons. In contrast, dynamin is not detectable by immunofluorescence in the developing retina and optic tectum before synapse formation. Our data indicate that, in contrast to the early expression of synaptotagmin, SV2, and syntaxin during axonal growth, dynamin is upregulated after synapse formation, suggesting its function predominantly during and after synaptogenesis but not in axonogenesis.(J Histochem Cytochem 47:1297-1306, 1999)

Astrocytes regulate the developmental appearance of GABAergic and glutamatergic postsynaptic currents in cultured embryonic rat spinal neurons

The effects of astrocytes on the emergence of synaptic transients and excitable membrane properties in cultured, embryonic, rat ventral spinal neurons were studied with electrical and optical recording techniques. Neurons on astrocytes had significantly longer neurites and an accelerated rate of growth in surface membrane during the initial 24 h in culture compared to neurons on poly-D-lysine (PDL). GABAergic (GABA, gamma-aminobutyric acid) and glutamatergic transients appeared spontaneously in co-cultured neurons by 24 h. GABAergic quanta did not appear in neurons on PDL until 4 days in culture, and glutamatergic transients did not emerge until 7 days in culture. Astrocyte-conditioned medium (ACM) partially mimicked the effects of direct astrocytic contact. GABAergic transients appeared by 2 days, and glutamatergic signals by 4 days in neurons on PDL exposed to ACM. All of the spontaneous, synaptic-like transients were eliminated by tetrodotoxin or Ca2+o-free saline, implicating voltage-dependent cation channels in their generation. Astrocytes immediately and significantly increased the density of voltage-dependent Na+ currents compared to neurons on PDL, but by the end of 24 h, Na+ current densities were identical. Electrophysiological and optical recording revealed comparable densities of high-voltage-activated (HVA) Ca2+ currents on both co-cultured neurons and neurons on PDL throughout the first week. However, neurons on astrocytes had significantly greater contributions of P/Q-type currents and lesser contributions of L-type currents beginning at 24 h and continuing for 7 days. The contribution of N-type current was significantly more in co-cultured neurons only at 24 h. Thus, in vitro, astrocytes help to differentiate specific excitable membrane properties in spinal neurons, along with GABAergic and glutamatergic forms of synaptic transmission.

SNAP-25 requirement for dendritic growth of hippocampal neurons

Structure and dimension of the dendritic arbor are important determinants of information processing by the nerve cell, but mechanisms and molecules involved in dendritic growth are essentially unknown. We investigated early mechanisms of dendritic growth using mouse fetal hippocampal neurons in primary culture, which form processes during the first week in vitro. We detected a key component of regulated exocytosis, SNAP-25 (synaptosomal associated protein of 25 kDa), in axons and axonal terminals as well as in dendrites identified by the occurrence of the dendritic markers transferrin receptor and MAP2. Selective inactivation of SNAP-25 by botulinum neurotoxin A (BoNTA) resulted in inhibition of axonal growth and of vesicle recycling in axonal terminals. In addition, dendritic growth of hippocampal pyramidal and granule neurons was significantly inhibited by BoNTA. In contrast, cleavage of synaptobrevin by tetanus toxin had an effect on neither axonal nor dendritic growth. Our observations indicate that SNAP-25, but not synaptobrevin, is involved in constitutive axonal growth and dendrite formation by hippocampal neurons.

Patterns of synaptophysin expression during development of the inner ear in the chick

The onset of active neural connections between the periphery and the central nervous system is integral to the development of sensory systems. This study presents patterns of synaptogenesis in the chick basilar papilla (i.e., cochlea) by examining the immunohistochemical expression of synaptophysin with a specific monoclonal antibody, SBI 20.10. The initial onset of synaptophysin expression occurs in nerve fibers and ganglion cell bodies at a time when neurites reach the basement membrane of the chick cochlea on embryonic day 6-7 (ED 6-7). By ED 8, synaptophysin positive fibers invade the neural side of the entire length of the cochlea, so that by ED 9-10, fibers are forming multiple terminals on the basolateral ends of retracting receptor or hair cells. In contrast, on the abneural side, immunoreactive terminals are seen first as small, punctate contacts and then as large, synaptophysin positive calyceal endings beneath short hair cells. These terminals are sparse during early development, more numerous by ED 17-19, but still incomplete after 2 weeks posthatching. In comparison, hair cells show synaptophysin immunoreactivity in both supra- and infranuclear regions by ED 11-12, a time when efferent innervation is incomplete. Thus, during development, synaptophysin is expressed at both synaptic and nonsynaptic sites, is relatively selective in its regional distribution, and is expressed in hair cells at a time when auditory function begins. Our results present a framework with which to understand the potential role of synaptophysin in early synaptogenesis of the cochlea.

Spinal cord neuronal precursors generate multiple neuronal phenotypes in culture

Neuronal restricted precursors (NRPs) () can generate multiple neurotransmitter phenotypes during maturation in culture. Undifferentiated E-NCAM+ (embryonic neural cell adhesion molecule) immunoreactive NRPs are mitotically active and electrically immature, and they express only a subset of neuronal markers. Fully mature cells are postmitotic, process-bearing cells that are neurofilament-M and synaptophysin immunoreactive, and they synthesize and respond to different subsets of neurotransmitter molecules. Mature neurons that synthesize and respond to glycine, glutamate, GABA, dopamine, and acetylcholine can be identified by immunocytochemistry, RT-PCR, and calcium imaging in mass cultures. Individual NRPs also generate heterogeneous progeny as assessed by neurotransmitter response and synthesis, demonstrating the multipotent nature of the precursor cells. Differentiation can be modulated by sonic hedgehog (Shh) and bone morphogenetic protein (BMP)-2/4 molecules. Shh acts as a mitogen and inhibits differentiation (including cholinergic differentiation). BMP-2 and BMP-4, in contrast, inhibit cell division and promote differentiation (including cholinergic differentiation). Thus, a single neuronal precursor cell can differentiate into multiple classes of neurons, and this differentiation can be modulated by environmental signals.

A sequential view of neurotransmitter release

Only a few years ago it was thought that a single Ca2+-dependent membrane binding protein might control regulated exocytosis, but it is now clear that the coordinated actions of a large number of proteins and lipids are required for the precise targeting, docking and fusion of vesicles to the plasma membrane. Thinking was focused in 1993 by the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) hypothesis, which proposed that certain synaptic vesicle membrane proteins combined specifically with particular proteins in the synaptic membrane active zone to form a complex that interacted with synaptoplasmic proteins, ATP and calcium ions to fuse the vesicles with the presynaptic membrane. Much research that has followed has verified the basic predictions of the SNARE hypothesis. However, recent research indicates that SNARE proteins are more widely distributed in secretory systems and that the sequence in which the proteins function may not occur as was originally proposed. That has recently produced a period of deconstruction and reinterpretation of the SNARE hypothesis. Our present state of knowledge is briefly summarized in this review.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Presynaptic localization of Kv1.4-containing A-type potassium channels near excitatory synapses in the hippocampus

Mammalian Shaker voltage-gated potassium channels that contain the Kv1.4 subunit exhibit rapid activation and prominent inactivation processes, which enable these channels to integrate brief (approximiately milliseconds) depolarizations over time intervals of up to tens of seconds. In the hippocampus, Kv1.4 immunoreactivity is detected at greatest density in two regions: (1) the middle molecular layer (MML), where perforant path axons synapse with dentate granule cells, and (2) the stratum lucidum (SL) of CA3, where the mossy fibers travel in tight fasciculi and form en passante synapses onto CA3 pyramidal cells. We have studied the localization of Kv1.4 within these regions in detail. First, we compared the distribution of Kv1.4 and synaptophysin (a synaptic vesicle protein primarily localized near termini) under confocal immunofluorescence microscopy. In the MML, Kv1.4 and synaptophysin immunofluorescence appeared to overlap. In the SL, however, Kv1.4 and synaptophysin staining was detected in nonoverlapping, irregular patches ( approximately 5-10 micro m in diameter). Ultrastructural studies of these two regions revealed that Kv1.4 immunoreactivity was absent from the surface membranes of cell bodies and dendrites and occurred prominently on axons, including axonal "necks" near termini. Small excitatory synaptic boutons also were labeled in the MML; by contrast, the mossy fiber synaptic expansions in the SL were not stained. These localizations may enable Kv1.4-containing channels to regulate the process of neurotransmitter release at these excitatory synapses.

Synaptophysin immunoreactivity in the aging rat pituitary gland

The density of synaptophysin (SN)-immunoreactivity (IR) was examined in pituitary glands of aging male Sprague-Dawley rats. SN-IR was observed as dense dots among endocrine cells of the intermediate lobe, while the neural lobe contained numerous, highly dense immunopositive regions. Some anterior lobe secretory cells contained SN-IR within the cytoplasm, suggestive of the presence of the protein in secretory granules, but no dot-like staining was observed between endocrine cells of that region. A quantitative analysis of the dot-like SN-immunostaining within the intermediate lobe found that tissue from groups of rats aged 13 months, or 15-17 months, contained significantly fewer SN-immunopositive areas than did tissues from 8-month-old animals. Diminished SN immunostaining is suggestive of reduced numbers of synapses in the intermediate lobe, which may lead to alterations in regulation of pituitary hormone secretion from endocrine cells in the older animals.