11-cis-retinal is the light-sensitive component in rod and cone photoreceptors, and its isomerization to all-trans retinal in the presence of light initiates the visual response. For photoreceptors to function normally, all-trans retinal must be converted back into 11-cis-retinal through a series of enzymatic steps known as the visual cycle. The interphotoreceptor retinoid-binding protein (IRBP) is a proposed retinoid transporter in the visual cycle, but rods in Irbp−/− mice have a normal visual cycle. While rods are primarily responsible for dim light vision, the ability of cones to function in constant light is essential to human vision and may be facilitated by cone-specific visual cycle pathways. We analyzed the cones in Irbp−/− mice to determine whether IRBP has a cone-specific visual cycle function. Cone electroretinogram (ERG) responses were reduced in Irbp−/− mice, but similar responses from Irbp−/− mice at all ages suggest that degeneration does not underlie cone dysfunction. Furthermore, cone densities and opsin levels in Irbp−/− mice were similar to C57BL/6 (wild-type) mice, and both cone opsins were properly localized to the cone outer segments. To test for retinoid deficiency in Irbp−/− mice, ERGs were analyzed before and after intraperitoneal injections of 9-cis-retinal. Treatment with 9-cis-retinal produced a significant recovery of the cone response in Irbp−/− mice and shows that retinoid deficiency underlies cone dysfunction. These data indicate that IRBP is essential to normal cone function and demonstrate that differences exist in the visual cycle of rods and cones.
Dopamine is a retinal neuromodulator that has been implicated in many aspects of retinal physiology. Photoreceptor cells express dopamine D4 receptors that regulate cAMP metabolism. To assess the effects of dopamine on photoreceptor physiology, we examined the morphology, electrophysiology, and regulation of cAMP metabolism in mice with targeted disruption of the dopamine D4 receptor gene. Photoreceptor morphology and outer segment disc shedding after light onset were normal in D4 knock-out (D4KO) mice. Quinpirole, a dopamine D2/ D3/D4 receptor agonist, decreased cAMP synthesis in retinas of wild-type (WT) mice but not in retinas of D4KO mice. In WT retinas, the photoreceptors of which were functionally isolated by incubation in the presence of exogenous glutamate, light also suppressed cAMP synthesis. Despite the similar inhibition of cAMP synthesis, the effect of light is directly on the photo-receptors and independent of dopamine modulation, because it was unaffected by application of the D4 receptor antagonist L-745,870. Nevertheless, compared with WT retinas, basal cAMP formation was reduced in the photoreceptors of D4KO retinas, and light had no additional inhibitory effect. The results suggest that dopamine, via D4 receptors, normally modulates the cascade that couples light responses to adenylyl cyclase activity in photoreceptor cells, and the absence of this modulation results in dysfunction of the cascade. Dark-adapted electroretinogram (ERG) responses were normal in D4KO mice. However, ERG b-wave responses were greatly suppressed during both light adaptation and early stages of dark adaptation. Thus, the absence of D4 receptors affects adaptation, altering transmission of light responses from photoreceptors to inner retinal neurons. These findings indicate that dopamine D4 receptors normally play a major role in regulating photoreceptor cAMP metabolism and adaptive retinal responses to changing environmental illumination.
A growing body of evidence suggests that plasticity at GABAergic synapses is of critical importance during development and aging. A balance between excitation and inhibition maintains homeostasis at the neuronal and circuit levels, and inhibitory plasticity can function to drive a perturbed system toward homeostasis. Activity-dependent modification of inhibitory synaptic strength must be non-Hebbian, however, because the interaction between an inhibitory neuron and its target prevents them from firing together. Mechanisms that may underlie inhibitory plasticity will be discussed, including the possibility that it is limited to the early period when GABA/glycine release is excitatory (Ben-Ari, 2002) or that corelease of another substance alters synapses that produce inhibition (Gillespie et al., 2005). Alternatively, inhibitory synapses may decline in strength through long-term depression (Kotak et al., 2001; Chang et al., 2003), or an as-yet undiscovered mechanism may be responsible. Whatever the mechanism, it is clear that inhibitory plasticity plays an important role in activity-dependent modification of developing circuits.
by
Nikita Pozdeyev;
Carla Taylor;
Rashidul Haque;
Shyam S. Chaurasia;
Amy Visser;
Aamera Thazyeen;
Yuhong Du;
Haian Fu;
Joan Weller;
David C. Klein;
P Michael Iuvone
14-3-3 proteins are a ubiquitous, highly conserved family of chaperone proteins involved in signal transduction, regulation of cell cycle, intracellular trafficking/targeting, cytoskeletal structure, and transcription. Although 14-3-3 proteins are among the most abundant proteins in the CNS, very little is known about their functional roles in the vertebrate retina. In the present study, we demonstrated that photoreceptors express 14-3-3 protein(s) and identified a 14-3-3 binding partner in photoreceptor cells, the melatonin-synthesizing enzyme arylalkylamine N-acetyltransferase (AANAT). Importantly, our data demonstrate that the binding of 14-3-3 to AANAT is regulated by light, with dramatic functional consequences. During the night in darkness, retinal AANAT is phosphorylated and forms a complex with 14-3-3 proteins with an apparent molecular weight of ∼90 kDa. Phosphorylation of AANAT facilitates the binding of enzyme to 14-3-3 proteins. Within the complex, AANAT is catalytically activated and protected from dephosphorylation and degradation. Light disrupts the AANAT/14-3-3 complex, leading to catalytic inactivation, dephosphorylation, and proteolytic degradation of the enzyme. In the presence of the proteasome inhibitor, lactacystin, light results in the formation of a high molecular weight complex (>150 kDa), which may represent an intermediate in the AANAT degradation process. These findings provide new insight into the roles of 14-3-3 proteins in photoreceptor cells and to the mechanisms controlling melatonin synthesis in the vertebrate retina.
A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod- cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod- dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key rote in modulating sensitivity and rod-cone dominance in the Japanese quail retina.
Dopamine (DA) functions as an essential neuromodulator in the brain and retina such that disruptions in the dopaminergic system are associated with common neurologic disorders such as Parkinson's disease. Although a reduction in DA content has been observed in diabetes, its effects in the development of diabetes-induced neuropathy remains unknown. Because the retina is rich in DA and has a well known diabetes-induced pathology (diabetic retinopathy or DR), this study was designed to examine the role of retinal DA deficiency in early visual defects in DR. Using rodent models of type 1 diabetes mellitus, we investigated whether diabetes caused a reduction in retinal DA content in both rats and mice and determined whether restoring DA levels or activating specific DA receptor pathways could improve visual function (evaluated with optokinetic tracking response) of diabetic mice, potentially via improvement of retinal function (assessed with electroretinography). We found that diabetes significantly reduced DA levels by 4 weeks in rats and by 5 weeks in mice, coincident with the initial detection of visual deficits. Treatment with l-DOPA, a DA precursor, improved overall retinal and visual functions in diabetic mice and acute treatment with DA D1 or D4 receptor agonists improved spatial frequency threshold or contrast sensitivity, respectively. Together, our results indicate that retinal DA deficiency is an underlying mechanism for early, diabetes-induced visual dysfunction and suggest that therapies targeting the retinal dopaminergic system may be beneficial in early-stage DR.