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.

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca²⁺) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca²⁺ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca²⁺ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Embryonic hyperglycemia negatively impacts retinal development, leading to abnormal visual behavior, altered timing of retinal progenitor differentiation, decreased numbers of retinal ganglion cells and Müller glia, and vascular leakage. Because synaptic disorganization is a prominent feature of many neurological diseases, the goal of the current work was to study the potential impact of hyperglycemia on retinal ribbon synapses during embryonic development. Our approach utilized reverse transcription quantitative PCR (RT-qPCR) and immunofluorescence labeling to compare the transcription of synaptic proteins and their localization in hyperglycemic zebrafish embryos, respectively. Our data revealed that the maturity of synaptic ribbons was compromised in hyperglycemic zebrafish larvae, where altered ribeye expression coincided with the delay in establishing retinal ribbon synapses and an increase in the immature synaptic ribbons. Our results suggested that embryonic hyperglycemia disrupts retinal synapses by altering the development of the synaptic ribbon, which can lead to visual defects. Future studies using zebrafish models of hyperglycemia will allow us to study the underlying mechanisms of retinal synapse development.

Clearance of fused synaptic vesicle components and availability of release sites are important determinants of recovery from short-term synaptic depression. However, the dynamics of release site clearance are not well established. This protocol illustrates single-molecule imaging of an exocytosis reporter, synaptophysin-pHluorin fusion protein (SypHy), by combining two-color laser scanning confocal microscopy with whole-cell patch-clamp recording of retinal bipolar cells from transgenic zebrafish that weakly express SypHy to track the dynamics of newly fused vesicle proteins at the active zone.

The global health concern posed by age-related visual impairment highlights the need for further research focused on the visual changes that occur during the process of aging. To date, multiple sensory alterations related to aging have been identified, including morphological and functional changes in inner hair cochlear cells, photoreceptors, and retinal ganglion cells. While some age-related morphological changes are known to occur in rod bipolar cells in the retina, their effects on these cells and on their connection to other cells via ribbon synapses remain elusive. To investigate the effects of aging on rod bipolar cells and their ribbon synapses, we compared synaptic calcium currents, calcium dynamics, and exocytosis in zebrafish (Danio rerio) that were middle-aged (MA,18 months) or old-aged (OA, 36 months). The bipolar cell terminal in OA zebrafish exhibited a two-fold reduction in number of synaptic ribbons, an increased ribbon length, and a decrease in local Ca²⁺ signals at the tested ribbon location, with little change in the overall magnitude of the calcium current or exocytosis in response to brief pulses. Staining of the synaptic ribbons with antibodies specific for PKCa revealed shortening of the inner nuclear and plexiform layers (INL and IPL). These findings shed light on age-related changes in the retina that are related to synaptic ribbons and calcium signals.

To investigate how dysregulated transient receptor potential canonical channels (TRPCs) are associated with Alzheimer’s disease (AD), we challenged primary neurons with amyloid-β (Aβ). Both the naturally secreted or synthetic Aβ oligomers (AβOs) induced long-lasting increased TRPC3 and downregulated the TRPC6 expression in mature excitatory neurons (CaMKIIα-high) via a Ca²⁺-dependent calcineurin-coupled NFAT transcriptionally and calpain-mediated protein degradation, respectively. The TRPC3 expression was also found to be upregulated in pyramidal neurons of human AD brains. The selective downregulation of the Trpc6 gene induced synaptotoxicity, while no significant effect was observed from the Trpc3-targeting siRNA, suggesting potentially differential roles of TRPC3 and 6 in modulating the synaptic morphology and functions. Electrophysiological recordings of mouse hippocampal slices overexpressing TRPC3 revealed increased neuronal hyperactivity upon the TRPC3 channel activation by its agonist. Furthermore, the AβO-mediated synaptotoxicity appeared to be positively correlated with the degrees of the induced dendritic Ca²⁺ flux in neurons, which was completely prevented by the co-treatment with two pyrazole-based TRPC3-selective antagonists Pyr3 or Pyr10. Taken together, our findings suggest that the aberrantly upregulated TRPC3 is another ion channel critically contributing to the process of AβO-induced Ca²⁺ overload, neuronal hyperexcitation, and synaptotoxicity, thus representing a potential therapeutic target of AD.