Sociodemographic and clinical correlates of the frequently hospitalized African American patients with severe and persistent mental illness.

BACKGROUND: Many researchers and clinicians are becoming increasingly concerned about the phenomenon of frequent psychiatric inpatient hospitalization in those with severe and persistent mental illness. This study aims to shed light on this occurrence in the African American psychiatric inpatient population by examining their sociodemographic and clinical correlates. METHODS: We retrospectively reviewed the medical charts of 39 African American patients who have had ≥3 inpatient psychiatric admissions in a year at Howard University Hospital, an urban, academically-affiliated hospital serving a predominantly African American population in Washington, DC. RESULTS: Most frequently readmitted African American inpatients were male (62%), unmarried (95%), homeless (62%), intoxicated at admission (90%), unemployed (97%), and age ≥35 (87%); expressed suicidal ideations (85%); had a DSM-5 diagnosis of a psychotic spectrum disorder (59%) and less than fair insight into their illness (56%); and stayed in the hospital for ≤4 days (82%). CONCLUSIONS: Many lessons can be learned from this study on African American psychiatric inpatient treatment recidivists, despite the work's limitations. Among these lessons are the need for strong case management, creative aftercare planning, and well-orchestrated, multifaceted services focused on these sociodemographic and clinical correlates- especially homelessness, unemployment, substance use, mood dysregulation, and psychosis-to successfully meet this patient populations' clinical needs.

Visualizing synaptic vesicle turnover and pool refilling driven by calcium nanodomains at presynaptic active zones of ribbon synapses.

Ribbon synapses of photoreceptor cells and second-order bipolar neurons in the retina are specialized to transmit graded signals that encode light intensity. Neurotransmitter release at ribbon synapses exhibits two kinetically distinct components, which serve different sensory functions. The faster component is depleted within milliseconds and generates transient postsynaptic responses that emphasize changes in light intensity. Despite the importance of this fast release for processing temporal and spatial contrast in visual signals, the physiological basis for this component is not precisely known. By imaging synaptic vesicle turnover and Ca(2+) signals at single ribbons in zebrafish bipolar neurons, we determined the locus of fast release, the speed and site of Ca(2+) influx driving rapid release, and the location where new vesicles are recruited to replenish the fast pool after it is depleted. At ribbons, Ca(2+) near the membrane rose rapidly during depolarization to levels >10 µM, whereas Ca(2+) at nonribbon locations rose more slowly to the lower level observed globally, consistent with selective positioning of Ca(2+) channels near ribbons. The local Ca(2+) domain drove rapid exocytosis of ribbon-associated synaptic vesicles nearest the plasma membrane, accounting for the fast component of neurotransmitter release. However, new vesicles replacing those lost arrived selectively at the opposite pole of the ribbon, distal to the membrane. Overall, the results suggest a model for fast release in which nanodomain Ca(2+) triggers exocytosis of docked vesicles, which are then replaced by more distant ribbon-attached vesicles, creating opportunities for new vesicles to associate with the ribbon at membrane-distal sites.

Functional roles of complexin in neurotransmitter release at ribbon synapses of mouse retinal bipolar neurons.

Ribbon synapses of photoreceptor cells and bipolar neurons in the retina signal graded changes in light intensity via sustained release of neurotransmitter. One molecular specialization of retinal ribbon synapses is the expression of complexin protein subtypes Cplx3 and Cplx4, whereas conventional synapses express Cplx1 and Cplx2. Because complexins bind to the molecular machinery for synaptic vesicle fusion (the SNARE complex) and modulate transmitter release at conventional synapses, we examined the roles of ribbon-specific complexin in regulating release at ribbon synapses of ON bipolar neurons from mouse retina. To interfere acutely with the interaction of native complexins with the SNARE complex, a peptide consisting of the highly conserved SNARE-binding domain of Cplx3 was introduced via a whole-cell patch pipette placed directly on the synaptic terminal, and vesicle fusion was monitored using capacitance measurements and FM-dye destaining. The inhibitory peptide, but not control peptides, increased spontaneous synaptic vesicle fusion, partially depleted reserve synaptic vesicles, and reduced fusion triggered by opening voltage-gated calcium channels under voltage clamp, without affecting the number of synaptic vesicles associated with ribbons, as revealed by electron microscopy of recorded terminals. The results are consistent with a dual role for ribbon-specific complexin, acting as a brake on the SNARE complex to prevent spontaneous fusion in the absence of calcium influx, while at the same time facilitating release evoked by depolarization.

The diverse roles of ribbon synapses in sensory neurotransmission.

Sensory synapses of the visual and auditory systems must faithfully encode a wide dynamic range of graded signals, and must be capable of sustained transmitter release over long periods of time. Functionally and morphologically, these sensory synapses are unique: their active zones are specialized in several ways for sustained, rapid vesicle exocytosis, but their most striking feature is an organelle called the synaptic ribbon, which is a proteinaceous structure that extends into the cytoplasm at the active zone and tethers a large pool of releasable vesicles. But precisely how does the ribbon function to support tonic release at these synapses? Recent genetic and biophysical advances have begun to open the 'black box' of the synaptic ribbon with some surprising findings and promise to resolve its function in vision and hearing.

Stabilization of spontaneous neurotransmitter release at ribbon synapses by ribbon-specific subtypes of complexin.

Ribbon synapses of tonically releasing sensory neurons must provide a large pool of releasable vesicles for sustained release, while minimizing spontaneous release in the absence of stimulation. Complexins are presynaptic proteins that may accomplish this dual task at conventional synapses by interacting with the molecular machinery of synaptic vesicle fusion at the active zone to retard spontaneous vesicle exocytosis yet facilitate release evoked by depolarization. However, ribbon synapses of photoreceptor cells and bipolar neurons in the retina express distinct complexin subtypes, perhaps reflecting the special requirements of these synapses for tonic release. To investigate the role of ribbon-specific complexins in transmitter release, we combined presynaptic voltage clamp, fluorescence imaging, electron microscopy, and behavioral assays of photoreceptive function in zebrafish. Acute interference with complexin function using a peptide derived from the SNARE-binding domain increased spontaneous synaptic vesicle fusion at ribbon synapses of retinal bipolar neurons without affecting release triggered by depolarization. Knockdown of complexin by injection of an antisense morpholino into zebrafish embryos prevented photoreceptor-driven migration of pigment in skin melanophores and caused the pigment distribution to remain in the dark-adapted state even when embryos were exposed to light. This suggests that loss of complexin function elevated spontaneous release in illuminated photoreceptors sufficiently to mimic the higher release rate normally associated with darkness, thus interfering with visual signaling. We conclude that visual system-specific complexins are required for proper illumination-dependent modulation of the rate of neurotransmitter release at visual system ribbon synapses.

The ribbon-associated protein C-terminal-binding protein 1 is not essential for the structure and function of retinal ribbon synapses.

PURPOSE: Synaptic ribbons are organelles found at presynaptic active zones of sensory neurons that generate sustained graded electrical signals in response to stimuli, including retinal photoreceptor cells and bipolar neurons. RIBEYE is the major and specific protein constituent of ribbons; however, over the past decade an increasing number of other proteins have been identified at ribbon active zones, including C-terminal-binding protein 1 (CtBP1; a regulator of transcription and membrane trafficking that might bind to the B domain of RIBEYE). The appearance of CtBP1 together with RIBEYE suggests that it may contribute to ribbon function, but the possible role of CtBP1 at ribbon synapses has not yet been examined. Using CtBP1-knockout mice, we tested for functional effects of absence of CtBP1 protein. METHODS: Confocal microscopy, electrophysiology, and electron microscopy were used to examine the structure and function of ribbon synapses in the retina and in isolated bipolar neurons from CtBP1 null mice compared with their wild-type littermates. RESULTS: Expression of ribbons appeared to be normal in CtBP1 null mouse retina as revealed by immunofluorescence with an antibody to the B domain of RIBEYE and by binding studies using a fluorescent peptide that binds to RIBEYE in ribbons of living bipolar cells. Electron microscopy also showed grossly normal pre- and postsynaptic organization of ribbon synapses in both photoreceptors and bipolar cells. Synaptic vesicles were normal in size, but the overall density of reserve vesicles was reduced by ~20% in the cytoplasm of CtBP1 null ribbon synaptic terminals. However, the reduced vesicle density did not detectably alter synaptic function of bipolar neurons as revealed by activity-dependent loading of synaptic vesicles with FM4-64, presynaptic calcium current, capacitance measurements of synaptic exocytosis, and destaining of FM dye upon stimulation. CONCLUSIONS: Overall the results suggest that CtBP1 protein is not essential for the formation of functional ribbon synapses in the retina.

Role of Ribeye PXDLS/T-binding cleft in normal synaptic ribbon function.

Non-spiking sensory hair cells of the auditory and vestibular systems encode a dynamic range of graded signals with high fidelity by vesicle exocytosis at ribbon synapses. Ribeye, the most abundant protein in the synaptic ribbon, is composed of a unique A domain specific for ribbons and a B-domain nearly identical to the transcriptional corepressor CtBP2. CTBP2 and the B-domain of Ribeye contain a surface cleft that binds to proteins harboring a PXDLS/T peptide motif. Little is known about the importance of this binding site in synaptic function. Piccolo has a well-conserved PVDLT motif and we find that overexpressed Ribeye exhibits striking co-localization with Piccolo in INS-cells, while two separate mutants containing mutations in PXDLS/T-binding region, fail to co-localize with Piccolo. Similarly, co-transfected Ribeye and a piccolo fragment containing the PVDLT region co-localize in HEK cells. Expression of wild-type Ribeye-YFP in zebrafish neuromast hair cells returns electron densities to ribbon structures and mostly rescued normal synaptic transmission and morphological phenotypes in a mutant zebrafish lacking most Ribeye. By contrast, Ribeye-YFP harboring a mutation in the PXDLS/T-binding cleft resulted in ectopic electron dense aggregates that did not collect vesicles and the persistence of ribbons lacking electron densities. Furthermore, overexpression failed to return capacitance responses to normal levels. These results point toward a role for the PXDLS/T-binding cleft in the recruitment of Ribeye to ribbons and in normal synaptic function.

Expression and distribution of synaptotagmin family members in the zebrafish retina.

Synaptotagmins belong to a large family of proteins. Although various synaptotagmins have been implicated as Ca2+ sensors for vesicle replenishment and release at conventional synapses, their roles at retinal ribbon synapses remain incompletely understood. Zebrafish is a widely used experimental model for retinal research. We therefore investigated the homology between human, rat, mouse, and zebrafish synaptotagmins 1-10 using a bioinformatics approach. We also characterized the expression and distribution of various synaptotagmin (syt) genes in the zebrafish retina using RT-PCR, qPCR, and in situhybridization, focusing on the family members whose products likely underlie Ca2+ -dependent exocytosis in the central nervous system (synaptotagmins 1, 2, 5, and 7). Most zebrafish synaptotagmins are well conserved and can be grouped in the same classes as mammalian synaptotagmins, based on crucial amino acid residues needed for coordinating Ca2+ binding and determining phospholipid binding affinity. The only exception is synaptotagmin 1b, which lacks 34 amino acid residues in the C2B domain and is therefore unlikely to bind Ca2+ there. Additionally, the products of zebrafish syt5a and syt5b genes share identity with mammalian class 1 and 5 synaptotagmins. Zebrafish syt1, syt2, syt5, and syt7 paralogues are found in the zebrafish brain, eye, and retina, excepting syt1b, which is only present in the brain. The complementary expression pattern of the remaining paralogues in the retina suggests that syt1a and syt5a may underlie synchronous release and syt7a and syt7b may mediate asynchronous release or other Ca2+ -dependent processes in different retinal neurons.

A doubleheader in endocytosis.

Synaptojanin1 degrades the signaling lipid phosphatidylinositol-4,5-bisphosphate and facilitates compensatory endocytosis, clathrin-coat disassembly, and vesicle reavailability at active synapses. In this issue of Neuron, Mani et al. provide new information about the separate roles of synaptojanin's two phosphatase domains and its interactions with endophilin in regulating these important aspects of the vesicle cycle.

Fibroblast growth factor homologous factors control neuronal excitability through modulation of voltage-gated sodium channels.

Neurons integrate and encode complex synaptic inputs into action potential outputs through a process termed "intrinsic excitability." Here, we report the essential contribution of fibroblast growth factor homologous factors (FHFs), a family of voltage-gated sodium channel binding proteins, to this process. Fhf1-/-Fhf4-/- mice suffer from severe ataxia and other neurological deficits. In mouse cerebellar slice recordings, WT granule neurons can be induced to fire action potentials repetitively (approximately 60 Hz), whereas Fhf1-/-Fhf4-/- neurons often fire only once and at an elevated voltage spike threshold. Sodium channels in Fhf1-/-Fhf4-/- granule neurons inactivate at more negative membrane potential, inactivate more rapidly, and are slower to recover from the inactivated state. Altered sodium channel physiology is sufficient to explain excitation deficits, as tested in a granule cell computer model. These findings offer a physiological mechanism underlying human spinocerebellar ataxia induced by Fhf4 mutation and suggest a broad role for FHFs in the control of excitability throughout the CNS.

Alterations of action potentials and the localization of Nav1.6 sodium channels in spared axons after hemisection injury of the spinal cord in adult rats.

Previously, we reported a pronounced reduction in transmission through surviving axons contralateral to chronic hemisection (HX) of adult rat spinal cord. To examine the cellular and molecular mechanisms responsible for this diminished transmission, we recorded intracellularly from lumbar lateral white matter axons in deeply anesthetized adult rats in vivo and measured the propagation of action potentials (APs) through rubrospinal/reticulospinal tract (RST/RtST) axons contralateral to chronic HX at T10. We found decreased excitability in these axons, manifested by an increased rheobase to trigger APs and longer latency for AP propagation passing the injury level, without significant differences in axonal resting membrane potential and input resistance. These electrophysiological changes were associated with altered spatial localization of Nav1.6 sodium channels along axons: a subset of axons contralateral to the injury exhibited a diffuse localization (>10 µm spread) of Nav1.6 channels, a pattern characteristic of demyelinated axons (Craner MJ, Newcombe J, Black JA, Hartle C, Cuzner ML, Waxman SG. Proc Natl Acad Sci USA 101: 8168-8173, 2004b). This result was substantiated by ultrastructural changes seen with electron microscopy, in which an increased number of large-caliber, demyelinated RST axons were found contralateral to the chronic HX. Therefore, an increased rheobase, pathological changes in the distribution of Nav1.6 sodium channels, and the demyelination of contralateral RST axons are likely responsible for their decreased conduction chronically after HX and thus may provide novel targets for strategies to improve function following incomplete spinal cord injury.

The synaptic vesicle cycle: is kissing overrated?

In this issue of Neuron, Granseth et al. re-examine the mechanism of endocytosis at hippocampal synapses using a new optical reporter, sypHy. They conclude that only a single slow mode of endocytosis operates at this synapse and that retrieval after physiological stimuli is largely, if not solely, dominated by the clathrin-mediated pathway. These conclusions dispute previous assertions that "kiss-and-run" is a major mechanism of vesicle recycling at hippocampal synapses.

Unexpected shock in a fallen older adult: a case report.

INTRODUCTION: Falls are common in older adults and frequently require ambulance service assistance. They are the most frequent cause of injury and associated morbidity and mortality in older adults. In recent years, the typical major trauma patient has changed from being young and male to being older in age, with falls of < 2 metres being the most common mechanism of injury. We present a case of an 84-year-old male who had fallen in his home. This case highlights the complex nature of a relatively common incident. CASE PRESENTATION: The patient was laid on the floor in the prone position unable to move for 12 hours. He did not complain of any pain in his neck, back, hips or legs, and wished to be lifted off the floor promptly. On examination, he had bruising to his chest and abdomen and had suffered a suspected cervical spine injury due to a step-like protrusion around C5-C6. Distal sensory and motor function was intact. While in the ambulance his blood pressure dropped from 154/119 mmHg to 49/28 mmHg unexpectedly. We successfully reversed the shock using the modified Trendelenburg position and intravenous fluids. On follow-up he was diagnosed with dislocated C3, C6 and C7 vertebrae. CONCLUSION: The unexpected episode of shock witnessed in this patient may have been caused by a number of phenomena, including but not limited to crush syndrome, spinal cord concussion and orthostatic hypotension. We recommend that clinicians anticipate sudden shock in older adult patients who have fallen and a) have remained static on the floor for an extended period of time or b) are suspected of a spinal injury. We recommend assertive management of these patients to mitigate the impact of shock through postural positioning and consideration of early cannulation.

Evidence that vesicles undergo compound fusion on the synaptic ribbon.

The ribbon synapse can release a stream of transmitter quanta at very high rates. Although the ribbon tethers numerous vesicles near the presynaptic membrane, most of the tethered vesicles are held at a considerable distance from the plasma membrane. Therefore, it remains unclear how their contents are released. We evoked prolonged bouts of exocytosis from a retinal bipolar cell, fixed within seconds, and then studied the ribbons by electron microscopy. Vesicle density on ribbons was reduced by approximately 50% compared with cells where exocytosis was blocked with intracellular ATP-gammaS. Large, irregularly shaped vesicles appeared on the ribbon in cells fixed during repetitive stimulation of exocytosis, and in some cases the large vesicles could be traced in adjacent sections to cisternae open to the medium. The large cisternal structures were attached to the ribbon by filaments similar to those that tether synaptic vesicles to the ribbon, and they occupied the base of the ribbon near the plasma membrane, where normal synaptic vesicles are found in resting cells. We suggest that the cisternae attached to ribbons represent synaptic vesicles that fused by compound exocytosis during strong repetitive stimulation and, thus, that vesicles tethered to the ribbon can empty their contents by fusing to other vesicles docked at the presynaptic membrane. Such compound fusion could explain the extremely high release rates and the multivesicular release reported for auditory and visual ribbon synapses.

Mobility and turnover of vesicles at the synaptic ribbon.

Ribbon synapses release neurotransmitter continuously at high rates, and the ribbons tether a large pool of synaptic vesicles. To determine whether the tethered vesicles are actually released, we tracked vesicles labeled with styryl dye in mouse retinal bipolar cell terminals whose ribbons had been labeled with a fluorescent peptide. We photobleached vesicles in regions with ribbons and without them and then followed recovery of fluorescence as bleached regions were repopulated by labeled vesicles. In the resting terminal, fluorescence recovered by approximately 50% in non-ribbon regions but by only approximately 20% at ribbons. Thus, at rest, vesicles associated with ribbons cannot exchange freely with cytoplasmic vesicles. Depolarization stimulated vesicle turnover at ribbons as bleached, immobile vesicles were released by exocytosis and were then replaced by fluorescent vesicles from the cytoplasm, producing an additional increase in fluorescence specifically at the ribbon location. We conclude that vesicles immobilized at synaptic ribbons participate in the readily releasable pool that is tapped rapidly during depolarization.

The role of ribbons at sensory synapses.

Synaptic ribbons are organelles that tether vesicles at the presynaptic active zones of sensory neurons in the visual, auditory, and vestibular systems. These neurons generate sustained, graded electrical signals in response to sensory stimuli, and fidelity of transmission therefore requires their synapses to release neurotransmitter continuously at high rates. It has long been thought that the ribbons at the active zones of sensory synapses accomplish this task by enhancing the size and accessibility of the readily releasable pool of synaptic vesicles, which may represent the vesicles attached to the ribbon. Recent evidence suggests that synaptic ribbons immobilize vesicles in the resting cell and coordinate the transient, synchronous release of vesicles in response to stimulation, but it is not yet clear how the ribbon can efficiently mobilize and coordinate multiple vesicles for release. However, detailed anatomical, electrophysiological, and optical studies have begun to reveal the mechanics of release at ribbon synapses, and this multidisciplinary approach promises to reconcile structure, function, and mechanism at these important sensory synapses.

The molecular architecture of ribbon presynaptic terminals.

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Tracking Newly Released Synaptic Vesicle Proteins at Ribbon Active Zones.

Clearance of synaptic vesicle proteins from active zones may be rate limiting for sustained neurotransmission. Issues of clearance are critical at ribbon synapses, which continually release neurotransmitters for prolonged periods of time. We used synaptophysin-pHluorin (SypHy) to visualize protein clearance from active zones in retinal bipolar cell ribbon synapses. Depolarizing voltage steps gave rise to small step-like changes in fluorescence likely indicating release of single SypHy molecules from fused synaptic vesicles near active zones. Temporal and spatial fluorescence profiles of individual responses were highly variable, but ensemble averages were well fit by clearance via free diffusion using Monte Carlo simulations. The rate of fluorescence decay of ensemble averages varied with the time and location of the fusion event, with clearance being most rapid at the onset of a stimulus when release rate is the highest.

Cycling the synapse: scenic versus direct routes for vesicles.

What happens to synaptic vesicles after they release their neurotransmitter content? Recent work on a variety of synaptic systems shows that there is no single answer to this question. Rather, it seems that neurons use a variety of methods to retrieve and reuse synaptic vesicles after they have undergone exocytosis. The challenge now is to establish the molecular mechanisms and to decipher the rules that govern which cycling pathway is used in a given functional context.

Synaptic vesicle exocytosis. Does a lingering kiss lead to fusion?

Direct optical measurements of single synaptic vesicles undergoing exocytosis at a synapse reveal rapid and complete transfer of membrane marker from the vesicle to the plasma membrane (; this issue of Neuron). Contact between the two membranes is consistent with free lipid exchange, such as might result from full fusion of the vesicle and plasma membranes.