The rostral migratory stream generates hippocampal CA1 pyramidal-like neurons in a novel organotypic slice co-culture model.

The mouse subventricular zone (SVZ) generates large numbers of neuroblasts, which migrate in a distinct pathway, the rostral migratory stream (RMS), and replace specific interneurons in the olfactory bulb (OB). Here, we introduce an organotypic slice culture model that directly connects the RMS to the hippocampus as a new destination. RMS neuroblasts widely populate the hippocampus and undergo cellular differentiation. We demonstrate that RMS cells give rise to various neuronal subtypes and, surprisingly, to CA1 pyramidal neurons. Pyramidal neurons are typically generated before birth and are lost in various neurological disorders. Hence, this unique slice culture model enables us to investigate their postnatal genesis under defined in vitro conditions from the RMS, an unanticipated source for hippocampal pyramidal neurons.

Long-lived plasma cells and memory B cells produce pathogenic anti-GAD65 autoantibodies in Stiff Person Syndrome.

Stiff person syndrome (SPS) is a rare, neurological disorder characterized by sudden cramps and spasms. High titers of enzyme-inhibiting IgG autoantibodies against the 65 kD isoform of glutamic acid decarboxylase (GAD65) are a hallmark of SPS, implicating an autoimmune component in the pathology of the syndrome. Studying the B cell compartment and the anti-GAD65 B cell response in two monozygotic twins suffering from SPS, who were treated with the B cell-depleting monoclonal anti-CD20 antibody rituximab, we found that the humoral autoimmune response in SPS is composed of a rituximab-sensitive part that is rapidly cleared after treatment, and a rituximab-resistant component, which persists and acts as a reservoir for autoantibodies inhibiting GAD65 enzyme activity. Our data show that these potentially pathogenic anti-GAD65 autoantibodies are secreted by long-lived plasma cells, which may either be persistent or develop from rituximab-resistant memory B lymphocytes. Both subsets represent only a fraction of anti-GAD65 autoantibody secreting cells. Therefore, the identification and targeting of this compartment is a key factor for successful treatment planning of SPS and of similar autoimmune diseases.

Xenobiotics in the limbic system--affecting brain's network function.

Xenobiotic compounds enter the brain through nutrition, environmentals, and drugs. In order to maintain intrinsic homeostasis, the brain has to adapt to xenobiotic influx. Among others, steroid hormones appear as crucial mediators in this process. However, especially in the therapy of neurological diseases or brain tumors, long-term application of neuroactive drugs is advised. Several clinically important malignancies based on hormonal dysbalance rise up after treatment with neuroactive drugs, for example, sexual and mental disorders or severe cognitive changes. A drug-hormone cross talk proceeding over drug-mediated cytochrome P450 induction predominantly in the limbic system and the blood-brain barrier, consequently altered steroid hormone metabolism, and P450-mediated change of steroid hormone receptor expression and signaling may serve as an explanation for such disorders. Especially, the interplay between the expression of AR and P450 at the blood-brain barrier and in structures of the limbic system is of considerable interest in understanding brain's reaction on xenobiotic treatment. This chapter summarizes present models and concepts on brain's reaction after xenobiotics crossing the blood-brain barrier and invading the limbic system.

Murine features of neurogenesis in the human hippocampus across the lifespan from 0 to 100 years.

BACKGROUND: Essentially all knowledge about adult hippocampal neurogenesis in humans still comes from one seminal study by Eriksson et al. in 1998, although several others have provided suggestive findings. But only little information has been available in how far the situation in animal models would reflect the conditions in the adult and aging human brain. We therefore here mapped numerous features associated with adult neurogenesis in rodents in samples from human hippocampus across the entire lifespan. Such data would not offer proof of adult neurogenesis in humans, because it is based on the assumption that humans and rodents share marker expression patterns in adult neurogenesis. Nevertheless, together the data provide valuable information at least about the presence of markers, for which a link to adult neurogenesis might more reasonably be assumed than for others, in the adult human brain and their change with increasing age. METHODS AND FINDINGS: In rodents, doublecortin (DCX) is transiently expressed during adult neurogenesis and within the neurogenic niche of the dentate gyrus can serve as a valuable marker. We validated DCX as marker of granule cell development in fetal human tissue and used DCX expression as seed to examine the dentate gyrus for additional neurogenesis-associated features across the lifespan. We studied 54 individuals and detected DCX expression between birth and 100 years of age. Caveats for post-mortem analyses of human tissues apply but all samples were free of signs of ischemia and activated caspase-3. Fourteen markers related to adult hippocampal neurogenesis in rodents were assessed in DCX-positive cells. Total numbers of DCX expressing cells declined exponentially with increasing age, and co-expression of DCX with the other markers decreased. This argued against a non-specific re-appearance of immature markers in specimen from old brains. Early postnatally all 14 markers were co-expressed in DCX-positive cells. Until 30 to 40 years of age, for example, an overlap of DCX with Ki67, Mcm2, Sox2, Nestin, Prox1, PSA-NCAM, Calretinin, NeuN, and others was detected, and some key markers (Nestin, Sox2, Prox1) remained co-expressed into oldest age. CONCLUSIONS: Our data suggest that in the adult human hippocampus neurogenesis-associated features that have been identified in rodents show patterns, as well as qualitative and quantitative age-related changes, that are similar to the course of adult hippocampal neurogenesis in rodents. Consequently, although further validation as well as the application of independent methodology (e.g. electron microscopy and cell culture work) is desirable, our data will help to devise the framework for specific research on cellular plasticity in the aging human hippocampus.

7-Benzyloxyresorufin-O-dealkylase activity as a marker for measuring cytochrome P450 CYP3A induction in mouse liver.

The cytochrome P450 subfamily CYP3A belongs to the most important detoxification enzymes. Because the main CYP3A isoforms are not polymorphic and therefore detract themselves from genetic screening as a potent prediction marker for drug metabolism or induction effects, effective in vitro testing of a putative drug-CYP3A interaction is indicated. We used mouse liver microsomes treated with the model drug phenytoin to set up an effective and reliable in vitro test system. A metabolic assay analyzing 7-alkoxyresorufin-O-dealkylation showed specific CYP3A-dependent 7-benzyloxyresorufin oxidation (BROD). This was confirmed by testing other alkoxyresorufins (7-ethoxy-, 7-methoxy-, and 7-pentoxyresorufin) in mice and correlation of the data with testosterone 6beta-hydroxylation and a plethora of isoform-specific chemical inhibitors (orphenadrine, chloramphenicol, nifedipine, ketoconazole, and sulfaphenazole). Isoform-specific expression and induction of CYP3A11 in mouse liver was tested by RNase protection assay, reverse transcription polymerase chain reaction (RT-PCR), and immunoblot. With the BROD assay, we could clearly dissect CYP3A11 from other P450s induced by phenytoin-like CYP2C29, CYP2B9, CYP1A1, and CYP4A. We conclude that the BROD assay is a specific tool to assign CYP3A induction by drugs or other chemicals, at least in a mouse model system.

Modulation of androgen and estrogen receptor expression by antiepileptic drugs and steroids in hippocampus of patients with temporal lobe epilepsy.

PURPOSE: Many of the antiepileptic drugs (AED) used in therapy of temporal lobe epilepsy (TLE) are known as cytochrome P450 (CYP, P450) inducers. These AEDs are thought to modulate androgen and estrogen pathways in hippocampus, and therefore cause mental and reproductive disorders found in TLE patients. In the present study, we analyzed expression of androgen receptor (AR), estrogen receptor alpha (ERalpha), and CYP3A in the hippocampus of TLE patients and in murine hippocampal cell line HN25.1. METHODS: Patients and cell lines had been treated with P450-inducing or noninducing AEDs, or with prednisolone, applied to prevent oedema formation prior to neurosurgical resection of the epileptic hippocampus. Human patient samples were analyzed by immunohistochemical approach, the HN25.1 cell line by quantitative RT-PCR, CAT reporter gene assay, and immunoblot. RESULTS: In both, humans and cell lines, the expression of testosterone metabolising CYP3A4 (human) or CYP3A11 (mouse) and AR was up-regulated when P450-inducing AEDs and/or prednisolone had been applied. AR responsive CAT reporter gene assay indicated an increase of AR-signalling after treatment of the HN25.1 cells with the P450-inducers phenytoin and carbamazepine. ERalpha expression was increased only by the P450-inducing AEDs, but not by prednisolone, which indicates that pathways different from CYP3A4/11 led to ERalpha enhancement. DISCUSSION: We conclude that P450-inducing AEDs influence AR expression and signalling in hippocampus most likely via CYP3A4/11-induction. The HN25.1 cell line holds promise to investigate the correlation between drug application and AR regulation, and to specifically address issues that are relevant to human TLE patients.

Concordant up-regulation of cytochrome P450 Cyp3a11, testosterone oxidation and androgen receptor expression in mouse brain after xenobiotic treatment.

Inactivation of testosterone by specific hydroxylations is a main function of cytochrome P450 (P450, CYP) in the brain. Recent data imply that induction of brain P450s by neuroactive drugs alters steroid hormone levels and endocrine signalling, giving rise to endocrine disorders. In this study, we investigated this drug-hormone crosstalk in mouse brain. Phenytoin led to a significant increase of 2alpha-, 2beta-, 6beta-, 16alpha- and 16beta-hydroxytestosterones, while 6alpha- and 15alpha-hydroxytestosterones showed no significant alteration of their metabolism compared with untreated controls. Inhibition of testosterone hydroxylation using the chemical inhibitors orphenadrine, chloramphenicol, ketoconazole and nifedipine as well as antibodies against CYP3A- and 2B-isoforms pointed to major role of Cyp3a11 and an only minor function of Cyp2b9/10 in mouse brain. Cyp3a11 revealed to be the major isoform affected by phenytoin. There was considerable overlap of Cyp3a11 and AR expression in neuronal structures of the limbic system, namely the hippocampus, amygdala, hypothalamus and thalamus. Phenytoin treatment led to an increase of both, Cyp3a11 and AR expression in the limbic system. Additionally, the coherence between CYP3A and AR expression was analysed in PC-12 cells. Inhibition of phenytoin-induced endogenous CYP3A2 and AR by ketoconazole led a reduction of their expression to basal levels. We conclude that Cyp3a11 plays a crucial role in directing drug action to hormonal response within the limbic system of mouse brain in a so-called drug-hormone crosstalk.

Defining the actual sensitivity and specificity of the neurosphere assay in stem cell biology.

For more than a decade the 'neurosphere assay' has been used to define and measure neural stem cell (NSC) behavior, with similar assays now used in other organ systems and in cancer. We asked whether neurospheres are clonal structures whose diameter, number and composition accurately reflect the proliferation, self-renewal and multipotency of a single founding NSC. Using time-lapse video microscopy, coculture experiments with genetically labeled cells, and analysis of the volume of spheres, we observed that neurospheres are highly motile structures prone to fuse even under ostensibly 'clonal' culture conditions. Chimeric neurospheres were prevalent independent of ages, species and neural structures. Thus, the intrinsic dynamic of neurospheres, as conventionally assayed, introduces confounders. More accurate conditions (for example, plating a single cell per miniwell) will be crucial for assessing clonality, number and fate of stem cells. These cautions probably have implications for the use of 'cytospheres' as an assay in other organ systems and with other cell types, both normal and neoplastic.

ICOS deficiency is associated with a severe reduction of CXCR5+CD4 germinal center Th cells.

ICOS is expressed on activated T cells and particularly on CXCR5+ follicular Th cells in germinal centers (GC). Its deletion leads to a profound deficiency in memory B cell formation and switched Ab response in humans. Here, we show that in ICOS-deficient patients the generation of GCs is severely disturbed, and the numbers of circulating CXCR5+CD45RO+ memory CD4 T cells are significantly reduced, indicating an essential role of ICOS in the differentiation of CXCR5+CD4 T cells. The GC-specific CD57+CXCR5+ subpopulation is virtually absent. In ICOS-/- mice, the decrease of circulating CXCR5+CD4 T cells reflects the reduction of CXCR5+ follicular Th cells in lymph nodes and spleen. Therefore, in concurrence with the absence of CXCR5+ T cells in the blood of CD40L-deficient patients, these data support the hypothesis that circulating CD57+CXCR5+ T cells are GC derived and thus may serve as a surrogate marker for the presence of functional GCs in humans.

Human ICOS deficiency abrogates the germinal center reaction and provides a monogenic model for common variable immunodeficiency.

The homozygous deletion of the inducible costimulator (ICOS), an activation-induced member of the CD28 family on T cells, causes an antibody deficiency syndrome in affected humans. The identification of a total of 9 ICOS-deficient patients revealed that this monogenic disease comprises the full clinical phenotype described for common variable immunodeficiency (CVID), including recurrent bacterial infections, adult as well as childhood onset, splenomegaly, autoimmune phenomena (autoimmune neutropenia), intestinal lymphoid hyperplasia, and malignancy (carcinoma of the vulva). All patients exhibited a profound hypogammaglobulinemia and a disturbed B-cell homeostasis. The severe reduction of class-switched memory B cells resulted from poor germinal center formation in the absence of ICOS. The additional decrease of naive B cells was associated with a partial inhibition of the early B-cell development at the pre-B-I stage. T-cell homeostasis seemed not to be affected, but low IL-10 production by ICOS-deficient T cells may contribute to the disturbed germinal center reaction. Human ICOS deficiency is indistinguishable from CVID and thus serves as a monogenic model for this complex syndrome.

Anti-epileptic drug phenytoin enhances androgen metabolism and androgen receptor expression in murine hippocampus.

Epilepsy is very often related to strong impairment of neuronal networks, particularly in the hippocampus. Previous studies of brain tissue have demonstrated that long-term administration of the anti-epileptic drug (AED) phenytoin leads to enhanced metabolism of testosterone mediated by cytochrome P450 (CYP) isoforms. Thus, we speculate that AEDs affect androgen signalling in the hippocampus. In the present study, we investigated how the AED phenytoin influences the levels of testosterone, 17beta-oestradiol, and androgen receptor (AR) in the hippocampus of male C57Bl/6J mice. Phenytoin administration led to a 61.24% decreased hippocampal testosterone level as compared with controls, while serum levels were slightly enhanced. 17beta-Oestradiol serum level was elevated 2.6-fold. Concomitantly, the testosterone metabolizing CYP isoforms CYP3A11 and CYP19 (aromatase) have been found to be induced 2.4- and 4.2-fold, respectively. CYP3A-mediated depletion of testosterone-forming 2beta-, and 6beta-hydroxytestosterone was significantly enhanced. Additionally, AR expression was increased 2-fold (mRNA) and 1.8-fold (protein), predominantly in the CA1 region. AR was shown to concentrate in nuclei of CA1 pyramidal neurons. We conclude that phenytoin affects testosterone metabolism via induction of CYP isoforms. The increased metabolism of testosterone leading to augmented androgen metabolite formation most likely led to enhanced expression of CYP19 and AR in hippocampus. Phenytoin obviously modulates the androgen signalling in the hippocampus.

Endothelial cell transplantation for gene therapy in experimental gliomas.

OBJECTIVE: Malignant gliomas are prominent targets for cancer gene therapy approaches because of their poor prognosis despite all available therapies. Endothelial cells (ECs) are considered attractive vehicles for cell-based gene therapy because of their tropism to the tumor vasculature. In this study, we investigated the potential of ECs to incorporate into glioma vessels after intra-arterial or local application to establish whether ECs can be used as cellular vectors for gene therapy in gliomas. METHODS: Immortalized rat brain endothelial cells (BECs) were modified to express either beta-galactosidase or green fluorescent protein (GFP). The ability of transduced BECs to integrate into tumor vessels after interstitial implantation was evaluated in C6 and 9L glioma models. The fate of GFP-BECs was investigated after selective intracarotid injection into C6 tumor-bearing animals. RESULTS: The interstitially grafted BECs organized themselves into vascular-like structures and integrated into the tumor vasculature. Transgene expression was limited to 10 days after injection. After selective intra-arterial injection, numerous GFP-BECs were adherent to the vascular lumen at least 7 days after injection. These cells were evenly distributed within small vessels and capillaries of the injected hemisphere and did not home selectively to the tumor vessels. CONCLUSION: Cell-based therapy approaches to brain tumor treatment using BECs as cellular vectors might be hampered by the rapid downregulation of transgene expression and by the fact that these cells do not home specifically to tumor vessels after intra-arterial injection. Nevertheless, locoregional administration of BECs might be an interesting approach for delivering molecules to brain tumors when short-term expression of transgene in the perivascular space is desirable.

DNA topoisomerase IIalpha and Her-2/neu gene dosages in pediatric malignant gliomas.

Pediatric malignant non-brainstem glioma (PMNBG) is a rare tumor that accounts for only about five percent of childhood intracranial neoplasms. DNA topoisomerase IIalpha (TIIalpha) is a novel marker of cell-cycle turnover and a target of high-risk chemotherapy in PMNBG. We have shown that TIIalpha protein expression strongly correlates with event-free and overall survival in these malignancies. The molecular mechanism causing the varying TIIalpha protein expression in PMNBG remains unknown. Utilizing a combined approach of immunocytochemistry-based morphology guidance, laser-assisted microdissection and quantitative real-time PCR, we report a low-level co-amplification of the neighboring TIIalpha and Her-2/neu gene loci on chromosome 17q11-q22 in one of seventeen examined PMNBGs. Analysis of both genes by real-time PCR in the crude tumor samples without prior tissue heterogeneity reduction via laser microdissection, resulted in loss of detection of amplification of the syngeneic Her-2/neu locus. Gene dosage assessment in a microscopically distant tumor area revealed no amplification of either gene. Our results suggest that low-level amplification of the TIIalpha gene locus may be a sporadic mechanism of increased TIIalpha protein expression in PMNBG, which can coincide with low-level amplification of Her-2/neu. The observed intratumor genetic heterogeneity for TIIalpha in PMNBGs may have an impact on the relevance of TIIalpha as a biological constituent of outcome in these neoplasms.

Angiopoietin-1 promotes tumor angiogenesis in a rat glioma model.

Angiopoietins have been implicated in playing an important role in blood vessel formation, remodeling, maturation, and maintenance. However, the role of angiopoietins in tumor angiogenesis remains uncertain. In this study, expression of human angiopoietin-1 (hAng-1) and angiopoietin (hAng-2) was amplified in the rat glioma cell line GS9L by stable transfection using an inducible tet-off system. Transfected cells were implanted intracerebrally into syngenic Fischer 344 rats. We demonstrated by means of magnetic resonance imaging that increased hAng-1 expression promoted a significant in vivo growth of intracerebral gliomas in rats. Overexpression of hAng-1 resulted in more numerous, more highly branched vessels, which were covered by pericytes. On the other hand, tumors derived from hAng-2-overexpressing cells were smaller than empty-plasmid control tumors. The tumor vasculature in these tumors was composed of aberrant small vascular cords, which were associated with few mural cells. Our results indicate that in the presence of hAng-1, tumors induce a more functional vascular network, which led to better tumor perfusion and growth. On the other hand, overexpression of hAng-2 led to less intact tumor vessels, inhibited capillary sprouting, and impaired tumor growth.

LE-PAS, a novel Arnt-dependent HLH-PAS protein, is expressed in limbic tissues and transactivates the CNS midline enhancer element.

A growing family of helix-loop-helix PAS (HLH-PAS) transcription factors has emerged recently. These proteins form heterodimers and are involved in adaptation to environmental or physiologic stresses as well as in embryonic development. We describe the cloning and characterization of a novel HLH-PAS protein termed LE-PAS (limbic-enriched PAS) in mouse. LE-PAS consists of 802 amino acids and has a predicted molecular mass of 87.4 kDa. In adult mice, it is expressed exclusively in brain. Northern blot and in situ hybridization analysis indicate that LE-PAS is expressed in the limbic system and olfactory bulb. By in vitro overexpression in COS-7, cells we found that LE-PAS is a nuclear protein. Reporter gene analysis indicated that LE-PAS transactivates the CNS midline enhancer (CME) motif but not the xenobiotic response element in an Arnt-dependent fashion and without prior activation of LE-PAS protein. Our observation suggests that LE-PAS shares a similar mode of function with HLH-PAS proteins such as single minded or trachealess indicating that LE-PAS also has constitutive or developmental functions which may be critical for regulating the transcriptional control of limbic patterning and function.

Neurogranin is expressed by principal cells but not interneurons in the rodent and monkey neocortex and hippocampus.

As a substrate of protein kinase C (PKC), neurogranin (NG) is involved in the regulation of calcium signaling and activity-dependent plasticity. Recently, we have shown that, in the rodent cerebellum, NG is exclusively expressed by gamma-aminobutyric acidergic Golgi cells, whereas, in the monkey cerebellum, brush cells were the only neuronal population expressing NG (Singec et al. [2003] J. Comp. Neurol. 459:278-289). In the present study, we analyzed the neocortical and hippocampal expression patterns of NG in adult mouse (C57Bl/6), rat (Wistar), and monkey (Cercopithecus aetiops). By using immunocytochemistry and nonradioactive in situ hybridization, we demonstrate strong NG expression by principal cells in different neocortical layers and in the hippocampus by granule cells of the dentate gyrus and pyramidal neurons of CA1-CA3. In contrast, double-labeling experiments in rodents revealed that neocortical and hippocampal interneurons expressing glutamate decarboxylase 67 (GAD67) were consistently devoid of NG. In addition, by using antibodies against parvalbumin, calbindin, and calretinin, we could demonstrate the absence of NG in interneurons of monkey frontal cortex and hippocampus. Together these findings corroborate the idea of different calcium signaling pathways in excitatory and inhibitory cells that may contribute to different modes of synaptic plasticity in these neurons.

Neurogranin expression by cerebellar neurons in rodents and non-human primates.

Neurogranin (NG) is a brain-specific protein kinase C substrate involved in the regulation of calcium signaling and neuronal plasticity. A rostrocaudal expression profile, with large amounts in telencephalic brain regions and low expression levels in phylogenetically older brain structures, was reported previously. In the cerebellum, expression of NG has not been described. By using immunocytochemistry and in situ hybridization, we found that NG is expressed in the mouse (C57Bl/6), rat (Wistar), and monkey (Cercopithecus aetiops) cerebella. In the mouse cerebellum, Golgi cells were strongly immunoreactive for NG, whereas other cerebellar neurons were devoid of this protein. Cell counts showed 1.6-fold more immunopositive Golgi cells in the hemispheres (61.1 +/- 8.0 cells/mm(2)) than in the vermis (37.5 +/- 3.3 cells/mm(2)). Developmental studies showed detectable NG in the mouse cerebellum as early as on postnatal day 10 (P10). In contrast to the mouse, in the rat cerebellum we found only a few Golgi cells containing NG (hemispheres, 2.4 +/- 0.5 cells/mm(2); vermis, 1.5 +/- 0.3 cells/mm(2)). In the monkey cerebellum, unipolar brush cells, localized in the granular layer, were heavily labeled, whereas Golgi cells were devoid of NG. This study demonstrated that NG is strongly expressed in specific gamma-aminobutyric acidergic neurons in the rodent cerebellum. In addition, NG expression in the primate cerebellum by brush cells, which are excitatory, showed remarkable cell type-specific and species-specific expression patterns of a postsynaptic protein mediating calcium signaling mechanisms.

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.