Stimulus pattern related plasticity of synapses between cones and horizontal cells in carp retina

Form-deprivation myopia downregulates calcium levels in retinal horizontal cells in mice

The process of eye axis lengthening in myopic eyes is regulated by multiple mechanisms in the retina, and horizontal cells (HCs) are an essential interneuron in the visual regulatory system. Wherein intracellular Ca²⁺ plays an important role in the events involved in the regulatory role of HCs in the retinal neural network. It is unknown if intracellular Ca²⁺ regulation in HCs mediates changes in the retinal neural network during myopia progression. We describe here a novel calcium fluorescence indicator system that monitors HCs' intracellular Ca²⁺ levels during form-deprivation myopia (FDM) in mice. AAV injection of GCaMP6s, as a protein calcium sensor, into a Gja10-Cre mouse monitored the changes in Ca²⁺signaling in HC that accompany FDM progression in mice. An alternative Gja10-Cre/Ai96-GCaMP6s mouse model was created by cross mating Gja10-Cre with Ai96 mice. Immunofluorescence imaging and live imaging of the retinal cells verified the identity of these animal models. Changes in retinal horizontal cellular Ca²⁺ levels were resolved during FDM development. The numbers of GCaMP6s and the proportion of HCs were tracked based on profiling changes in GCaMP6s⁺calbindin⁺/calbindin⁺ coimmunostaining patterns. They significantly decreased more after either two days (P < 0.01) or two weeks (P < 0.001) in form deprived eyes than in the untreated fellow eyes. These decreases in their proportion reached significance only in the retinal central region rather than also in the retinal periphery. A novel approach employing a GCaMP6s mouse model was developed that may ultimately clarify if HCs mediate Ca²⁺ signals that contribute to controlling FDM progression in mice. The results indicate so far that FDM progression is associated with declines in HC Ca²⁺ signaling activity.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Modeling the pre- and post-synaptic components involved in the synaptic modification between cones and horizontal cells in carp retina

In retinal synapses between cones and luminosity type horizontal cells (LHC), it was previously found in this laboratory that repetitive red flashes progressively strengthened the LHC's response to red flash, whereas weakened the LHC's response to green flash; repetitive green flash remarkably depressed the LHC's red response, but caused little changes in the cell's green response. However, the detailed mechanisms underlying these phenomena are not entirely clear. In the present study, based on an ion-channel model described mainly in the form of Hodgkin-Huxley equations, possible mechanisms of the short-term synaptic modification are investigated. The simulation results suggest that: (1) the auto-enhancement effect might be induced by the Ca2+-dependent process on the post-synaptic AMPA receptors, which could lead to changes of the ionic channel's properties; (2) the asymmetric response to red- and green-flashes and the mutual-chromatic suppression effects might be attributed to the regulatory effects on the presynaptic glutamate release.

Mechanically-evoked C-fiber activity in painful alcohol and AIDS therapy neuropathy in the rat

While altered activities in sensory neurons were noticed in neuropathic pain, caused by highly diverse insults to the peripheral nervous system, such as diabetes, alcohol ingestion, cancer chemotherapy and drugs used to treat AIDS, other infections and autoimmune diseases, as well as trauma, our understanding of how these various peripheral neuropathies manifest as altered neuronal activity is still rudimentary. The recent development of models of several of those neuropathies has, however, now made it possible to address their impact on primary afferent nociceptor function. We compared changes in mechanically-evoked C-fiber activity, in models of painful peripheral neuropathy induced by drinking ethanol (alcohol) or administering 2',3'-dideoxycytidine (ddC), a nucleoside reverse transcriptase inhibitor for AIDS therapy, two co-morbid conditions in which pain is thought to be mediated by different second messenger signaling pathways. In C-fiber afferents, ddC decreased conduction velocity. In contrast, alcohol but not ddC caused enhanced response to mechanical stimulation (i.e., decrease in threshold and increase in response to sustained threshold and supra-threshold stimulation) and changes in pattern of evoked activity (interspike interval and action potential variability analyses). These marked differences in primary afferent nociceptor function, in two different forms of neuropathy that produce mechanical hyperalgesia of similar magnitude, suggest that optimal treatment of neuropathic pain may differ depending on the nature of the causative insult to the peripheral nervous system, and emphasize the value of studying co-morbid conditions that produce painful peripheral neuropathy by different mechanisms.

Possible mechanism of flicking-induced short-term plasticity in retinal cone-LHC synapse: a computational study

In retinal cone-HC synapse, it has been found that repetitive stimulation could induce postsynaptic short-term responsiveness enhancement. However, the detailed mechanism underlying this short-term plasticity in the retinal graded neurons remains unclear. In this study, based on an ion-channel model described using Hodgkin--Huxley equations, the possible mechanism of repetitive-stimulation-induced short-term plasticity in the synapse between retinal cones and horizontal cells was investigated. The computational simulation results, together with evidence from experimental observations, suggest that the short-term modification of signal transmission between the retinal graded neurons is likely to be attributed to the regulatory effects that calcium-dependent process exerts on the single-channel properties of the postsynaptic AMPA receptors.

Altered temporal pattern of evoked afferent activity in a rat model of vincristine-induced painful peripheral neuropathy

It is known that the level of activity in nociceptive primary afferent nerve fibers increases in neuropathic conditions that produce pain, but changes in the temporal patterning of action potentials have not been analyzed in any detail. Because the patterning of action potentials in sensory nerve fibers might play a role in the development of pathological pain states, we studied patterning of mechanical stimulus-evoked action potential trains in nociceptive primary afferents in a rat model of vincristine-induced painful peripheral neuropathy. Systemic administration of vincristine (100 microg/kg) caused approximately half the C-fiber nociceptors to become markedly hyperresponsive to mechanical stimulation. Instantaneous frequency plots showed that vincristine induced an irregular pattern of action-potential firing in hyperresponsive C-fibers, characterized by interspersed occurrences of high- and low-frequency firing. This pattern was associated with an increase in the percentage of interspike intervals 100-199 ms in duration compared with that in C-fibers from control rats and vincristine-treated C-fibers that did not become hyperresponsive. Variability in the temporal pattern of action potential firing was quantified by determining the coefficient of variability (CV2) for adjacent interspike intervals. This analysis revealed that vincristine altered the pattern of action-potential timing, so that combinations of higher firing frequency and higher variability occurred that were not observed in control fibers. The abnormal temporal structure of nociceptor responses induced by vincristine in some C-fiber nociceptors could contribute to the pathogenesis of chemotherapy-induced neuropathic pain, perhaps by inducing activity-dependent post-synaptic effects in sensory pathways.

Dopamine effect on the stimulus pattern related changes in response characteristics of R/G horizontal cells in carp retina

Repetitive red flashes increased the R/G horizontal cells' red response amplitude and induced a hyperpolarization of the cells' dark membrane potential. These phenomena were eliminated in 6-OHDA pretreated retinas and restored by exogenous dopamine, which suggests the involvement of dopamine receptor activity changes instead of dopamine release changes. Furthermore, the phenomena persisted on D(1) receptor antagonist (SKF-83566) application, whereas they diminished on D(2) receptor antagonist (eticlopride) application, indicating that the mechanism is related to a D(2) receptor, possibly located on photoreceptors.

Metabotropic glutamate receptor-mediated hetero-synaptic interaction of red- and green-cone inputs to LHC of carp retina

The role of presynaptic metabotropic glutamate receptor (mGluR) on the interaction of red- and green-cone signals was investigated in luminosity-type horizontal cell (LHC) of isolated carp retina. It was found that a dim red background could enhance LHC's light response to green stimulus, and a dim green background was also able to increase the cell's response to red flash. Such mutual color enhancement was eliminated by application of groups II and III mGluR antagonist (S)-methyl-4-carboxyphenyl-glycine (MCPG). Furthermore, inhibition of glutamate uptake by using D-aspartate (D-Asp) or DL-threo-beta-hydroxy-aspartic acid (THA) completely blocked the mutual enhancement of color signals in LHC. However, the GABAergic feedback pathway in the outer retina was unnecessarily involved.