Goldfish Retina: A Correlate Between Cone Activity and Morphology of the Horizontal Cell in Clone Pedicules

Dopamine causes a significant retraction of neurites of bull-head catfish horizontal cells maintained in culture. The effects of dopamine are blocked by haloperidol and SCH 23390, a D1 antagonist, but not by sulpiride, a D2 antagonist. The dopamine-induced morphological changes were mimicked by SKF 38393, a D1 agonist, but not by quinpirole, a D2 agonist. Kainate also caused process retraction, but other neuroactive substances tested including glutamate, 5-hydroxytryptamine, N-methyl-D-aspartate, y-aminobutyric acid, and glycine caused only minor changes in neurite length. Cyclic AMP analogues do not induce neurite retraction in horizontal cells, indicating that this effect of dopamine is not mediated by cyclic AMP. However, a protein kinase C activator (phorbol 12-myristate 13-acetate) and synthetic diacylglycerol analogs (1-oleoyl-2-acetyl-sn-glycerol and dioctanoglycerol) caused marked neurite retraction. Their effects, as well as the dopamine-induced changes, were blocked by staurosporine, a potent protein kinase antagonist. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol. Dopamine exerts multiple effects on teleost horizontal cells (reviewed in ref. 1). It decreases light responsiveness of the cells by modifying a glutamate conductance (2); it reduces electrical coupling between the cells by decreasing gap junctional conductance (3), and it depresses 'y-aminobutyric acid (GABA) release from the cells by an unknown mechanism (4). All of these effects are mediated by cyclic AMP, which activates cyclic AMP-dependent kinases (protein kinase A) in teleost horizontal cells.We have been studying the effects of neuroactive substances on process outgrowth of teleost horizontal cells maintained in culture and have found that dopamine causes a significant and reversible retraction of their neurites. Cyclic AMP analogues do not affect neurite retraction in these cells, indicating that this effect of dopamine is not mediated by cyclic AMP. Following on a recent report that phorbol esters stimulate spinule formation in teleost horizontal cells (5), we have studied the effects of a phorbol ester, diacylglycerol analogues, and protein kinase inhibitors on process outgrowth in cultured catfish horizontal cells. The results suggest that dopamine causes retraction of processes in horizontal cells by activating phospholipase C, resulting in the formation of diacylglycerol and the activation of protein kinase C (PKC). MATERIALS AND METHODSCell Culture. Methods for the isolation and culture of teleost horizontal cells have been described (6). Darkadapted eyes from young bull-head catfish (Ictalurus nebulosus) were dissected and quickly rinsed in ethanol (70%).The retinas were isolated under red dim light and incubated for 40 min at room temperature (20'C) in Leibowitz's medium (L-15) (GIBCO) containing 60-70 units of papain per ml (Worthington) activated with L-cysteine. The retinas were washed with fresh L-15 and triturated with disposable glass pipett...

Section: Discussionmentioning

confidence: 99%

Dopamine induces neurite retraction in retinal horizontal cells via diacylglycerol and protein kinase C.

Rodrigues

Dowling

1990

“…Network adaptation likely underlies various reported changes in the light responses of post-receptor neurons that depend on the ambient illumination, including changes in the center-surround receptive field organization of ganglion cells (5,6) and in the light responsiveness of horizontal cells, a type of second order cell (7). In nonmammalian vertebrates dark adaptation increases rod, and decreases cone, input to horizontal cells and light adaptation has the opposite effects (8,9).…”

mentioning

confidence: 99%

A circadian clock regulates rod and cone input to fish retinal cone horizontal cells.

Wang

Mangel

1996

149

In the vertebrate retina, the light responses of post-receptor neurons depend on the ambient or background illumination. Using intracellular recording, we have found that a circadian clock regulates the light responses of dark-adapted fish cone horizontal cells. Goldfish were maintained on a 12-hr light/12-hr dark cycle. At different times of the day or night, retinas were superfused in darkness for 90 min ("prolonged darkness"), following which horizontal cells were impaled without the aid of any light flashes. The vertebrate retina is able to respond to visual images in starlight, in the midday sun, and at all times in between, during which the ambient or background illumination changes by 6 to 12 orders of magnitude (1, 2). This ability derives from the presence of two kinds of photoreceptor cells, rods and cones, which subserve nighttime and daytime vision, respectively, and from at least two kinds of adaptive mechanisms. The first kind of adaptive mechanism, photoreceptor adaptation, is determined by the relative degree of visual pigment bleached or regenerated in photoreceptors (3) and by the regulatory role of Ca2+ in the phototransduction process (4). The second adaptive mechanism, network or neural adaptation, is determined by post-receptor cellular and synaptic mechanisms of retinal networks or circuits (3).Network adaptation likely underlies various reported changes in the light responses of post-receptor neurons that depend on the ambient illumination, including changes in the center-surround receptive field organization of ganglion cells (5, 6) and in the light responsiveness of horizontal cells, a type of second order cell (7). In nonmammalian vertebrates dark adaptation increases rod, and decreases cone, input to horizontal cells and light adaptation has the opposite effects (8, 9).A circadian clock is a type of biological oscillator that has persistent rhythmicity with a period of approximately 24 hr in the absence of external timing cues (e.g., constant darkness). In addition, a circadian clock can be entrained by cyclic environmental stimuli, such as light (10). In vertebrate retinas, a variety of cellular phenomena are regulated by circadian rhythms, including melatonin production and release (11, 12), tyrosine hydroxylase activity (13) Intact, isolated retinas were superfused at 0.5 ml/min with a Ringer's solution that contained 130 mM NaCl, 2.5 mM KCl, 20 mM NaHCO3, 0.7 mM CaCl2, 1.0 mM MgCl2, and 20 mM glucose, as described (22,23). Oxygenation of the superfusate with a mixture of 95% 02/5% CO2 maintained the superfusate at a pH of 7.4 in the retinal chamber. After surgery, the retinas were superfused in darkness for 90 min ("prolonged darkness"), following which a horizontal cell was impaled without Abbreviation: ZT, Zeitgeber.

“…During dark adaptation, these spinules are retracted and disappear on about the same time scale. The formation of these spinules is paralleled by the appearance of color opponency in horizontal and ganglion cells (8)(9)(10)(11), making it very likely that these spinules are the site of the inhibitory synapses in the negative feedback circuit.…”

mentioning

confidence: 99%

Protein kinase C mediates transient spinule-type neurite outgrowth in the retina during light adaptation.

Weiler

Köhler

Janssen

1991

Light and dark adaptation of the teleost retina is accompanied by a remarkable morphological rearrangement of the synaptic connections between photoreceptors and second-order neurons: during light adaptation, numerous new neurites, the so-called spinules, arise from the terminal dendrites of horizontal cells invaginating the cone pedicle, and during dark adaptation, these spinules are retracted. The formation of these spinules is paralleled by the appearance of color opponency in horizontal and ganglion cells, which led to the suggestion that these spinules are the site of the inhibitory synapses in the negative feedback loop between cones and horizontal cells. The formation of the spinules in the light and their disappearance in darkness have a time course of minutes and are modulated by the neurotransmitters dopamine and glutamate, respectively. Neurotransmitters can modulate neuronal processing through a variety of second messengers that activate protein kinases, resulting most commonly in protein phosphorylation. Herein we report that activation of protein kinase C by phorbol esters promotes the formation of new horizontal-cell spinules in animals kept in the dark. Partial inhibition of protein kinase C activation with sphingosines prevents the formation of new spinules during light adaptation but does not affect established spinules. The spinule-forming effect of phorbol esters is not mediated by dopaminergic neurons, since the effect is also seen in retinas depleted of dopaminergic neurons. Phorbol esters also initiate the formation of spinules in synaptically isolated horizontal cells, demonstrating that they have a direct action on these cells. In addition, isolated horizontal cells have substrate proteins that are phosphorylated in a protein kinase C-dependent manner.