Volume transmission of dopamine may modulate light-adaptive plasticity of horizontal cell dendrites in the recovery phase following dopamine depletion in goldfish retina

Effect of vitamin D deficiency on spatial contrast sensitivity function

CLINICAL RELEVANCE

Vitamin D has regulatory effects on non-skeletal tissues including neurons. The contrast sensitivity function occurs as a result of interaction between retinal neurons.

BACKGROUND

The association between plasma vitamin D deficiency and contrast sensitivity function was investigated.

METHODS

Forty-one eyes of 41 subjects with vitamin D deficiency with plasma vitamin D level <20 ng/mL (Group 1), and 30 eyes of 30 subjects without vitamin D deficiency with plasma vitamin D level ≥20 ng/mL (Group 2), were included in this prospective study. OPTEC 6500 was used to measure the contrast sensitivity function at all spatial frequencies involving 1.5 cpd, 3 cpd, 6 cpd, 12cpd, and 18 cpd. The average and sectorial retinal nerve fibre layer thickness, the average and minimum ganglion cell-inner plexiform thickness and tear meniscus height were measured by using optical coherence tomography.

RESULTS

A significant difference was present between Group 1 and Group 2 regarding the plasma vitamin D level (12.4 ± 4.7 ng/mL in Group 1 versus 27.1 ± 6.7 ng/mL in Group 2 p < 0.001). All spatial frequencies of contrast sensitivity function were significantly greater in Group 2 than those in Group 1, as follows: 45 ± 22.6 in Group 1 versus 57.5 ± 20.9 in Group 2, p = 0.08 in 1.5cpd; 71.3 ± 31.3 in Group 1 versus 91.8 ± 27.8 in Group 2, p = 0.001 in 3cpd; 77.9 ± 39.9 in Group 1 versus 100.4 ± 38.4 in Group 2, p = 0.013 in 6cpd; 32 ± 17.5 in Group 1 versus 48.8 ± 25.2 in Group 2, p = 0.002 in 12cpd; and 12.1 ± 5 in Group 1 versus 17.5 ± 9.5 in Group 2, p = 0.001 in 18cpd. However, there were no significant difference between two groups in terms of retinal fibre layer thicknesses, ganglion cell-inner plexiform layer thicknesses, and tear meniscus height.

CONCLUSION

Vitamin D deficiency can lead to a decrease in contrast sensitivity function that is an indicator of visual quality. This may be an underlying reason for certain visual complaints.

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.

Schizophrenia and the eye

Although visual processing impairments are common in schizophrenia, it is not clear to what extent these originate in the eye vs. the brain. This review highlights potential contributions, from the retina and other structures of the eye, to visual processing impairments in schizophrenia and high-risk states. A second goal is to evaluate the status of retinal abnormalities as biomarkers for schizophrenia. The review was motivated by known retinal changes in other disorders (e.g., Parkinson’s disease, multiple sclerosis), and their relationships to perceptual and cognitive impairments, and disease progression therein. The evidence reviewed suggests two major conclusions. One is that there are multiple structural and functional disturbances of the eye in schizophrenia, all of which could be factors in the visual disturbances of patients. These include retinal venule widening, retinal nerve fiber layer thinning, dopaminergic abnormalities, abnormal ouput of retinal cells as measured by electroretinography (ERG), maculopathies and retinopathies, cataracts, poor acuity, and strabismus. Some of these are likely to be illness-related, whereas others may be due to medication or comorbid conditions. The second conclusion is that certain retinal findings can serve as biomarkers of neural pathology, and disease progression, in schizophrenia. The strongest evidence for this to date involves findings of widened retinal venules, thinning of the retinal nerve fiber layer, and abnormal ERG amplitudes. These data suggest that a greater understanding of the contribution of retinal and other ocular pathology to the visual and cognitive disturbances of schizophrenia is warranted, and that retinal changes have untapped clinical utility.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Minor retinal degeneration in Parkinson's disease

Parkinson's disease is a neurodegenerative disorder with selective and progressive loss of dopaminergic neurons in substantia nigra. Studies on Parkinson's disease patients and dopamine-depleted animals indicate that dopaminergic neurons in the retina degenerate due to the genetic and environmental factors that cause dopaminergic neuron loss in the substantia nigra. Besides motor and non-motor symptoms, visual symptoms are common in Parkinson's disease patients, ranging from complaints of reading and driving difficulties, to complex visual hallucinations. The delicate network of various neurons in the retina ensures the accuracy of visual signal transmission, and dopamine is primarily a modulator in this complicated process. In retinitis pigmentosa, the gradual loss of photoreceptors causes gross remodeling of the neural retina and eventually loss of visual capacity. We hypothesize that the retina in Parkinson's disease patients undergoes comparatively minor degeneration due to progressive loss of dopaminergic neurons, which are less in amount and auxiliary in function compared to photoreceptors, and thus lead to various visual dysfunctions.

Differential modulation of rod and cone calcium currents in tiger salamander retina by D2 dopamine receptors and cAMP

Synaptic transmission from vertebrate photoreceptors involves activation of L-type calcium currents (ICa). Dopamine is an important circadian neuromodulator in the retina and photoreceptors possess D2 dopamine receptors. We examined modulation of ICa by dopamine and cAMP in retinal slices and isolated cells of larval tiger salamander. Results show that dopamine and a D2 agonist, quinpirole, enhanced ICa in rods and red-, blue- and UV-sensitive small single cones but inhibited ICa in red-sensitive large single cones. A D1 agonist, SKF-38393, was without effect. Quinpirole effects were blocked by pertussis toxin (PTx) pretreatment indicating involvement of PTx-sensitive G-proteins. Like dopamine, inhibition of cAMP-dependent protein kinase (PKA) by Rp-cAMPS enhanced ICa in rods and small single cones, but inhibited ICa in large single cones. In contrast, forskolin and Sp-cAMPS, which stimulate PKA, inhibited ICa in rods and small single cones but enhanced ICa in large single cones. Sp-cAMPS also occluded effects of quinpirole. These results suggest that D2 receptors modulate ICa via inhibition of cAMP. Differences among the responses of photoreceptors to cAMP are consistent with the possibility that small single cones and rods may possess different Ca2+ channel subtypes than large single cones. The results with dopamine and quinpirole showing inhibition of ICa in large single cones and enhancement of rod ICa were unexpected because previous studies have shown that dopamine suppresses rod inputs and enhances cone inputs into second-order neurons. The present results therefore indicate that the dopaminergic enhancement of cone inputs does not arise from modulation of photoreceptor ICa.

Dopamine D1-receptor immunolocalization in goldfish retina

Dopamine, a neuromodulator in the vertebrate retina, is involved in numerous functions related to light adaptation. However, unlike in mammals, localization of retinal D1-dopamine receptors in nonmammalian vertebrates has been hampered due to a lack of antisera. To address this problem, an antiserum against the 18 C-terminal amino acids of the goldfish D1 receptor (gfD1r) was generated in chicken eggs and tested in retinae of goldfish and rat, and rat caudate putamen, by using immunoblots and light microscopic immunocytochemistry. No labeling was observed in any tissue or immunoblots with preabsorbed gfD1r antiserum. Immunoblot analysis of goldfish retina revealed a single band at about 101 kDa. The patterns of gfD1r immunoreactivity (gfD1r-IR), found in rat caudate putamen and rat retina were virtually identical to that previously reported with other D1-receptor ligands and antisera. In goldfish retina, gfD1r-IR was most intense over cell bodies in the ganglion cell layer, amacrine cells in the proximal inner nuclear layer (INL), and bipolar cells in the distal INL. Weaker gfD1r-IR was observed over horizontal cell bodies and both plexiform layers. Müller cells and axons of cone photoreceptors were labeled as well. Double labeling showed that all protein kinase C-immunoreactive bipolar cells (ON type) were gfD1r-IR on the soma, axon terminal, and dendrites. All glutamate decarboxylase-immunoreactive (i.e., gamma-aminobutyric acid utilizing) amacrine cells and horizontal cells were gfD1r-IR. Retinal D1r distribution is more extensive than dopamine neuron innervation, but is consistent with physiologic estimates of dopamine function, suggestive of both wiring and volume transmission of dopamine in the retina. The gfD1r antiserum displays cross-reactivity to dopamine receptors in a mammal and a nonmammal and should prove useful in future studies of dopaminergic systems.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Levels of dopamine and noradrenaline in the developing of retina--effect of light deprivation

The effect of light deprivation on the levels of dopamine and noradrenaline was studied in the developing rat retina. These transmitters were estimated in three groups of rats: (i) cycling light reared; (ii) dark reared since birth; and (iii) dark reared since birth, but exposed to cycling light for 1 day prior to the estimation of catecholamines. Our results show that (1) there is a progressive decrease in the levels of dopamine and noradrenaline in the cycling light and dark reared rats during postnatal development; (2) dark rearing further reduces the content of dopamine and noradrenaline; and (3) restoration of physiological (light) stimulus in the dark-reared rats during the early postnatal period results in the recovery of noradrenaline to a greater extent than that of dopamine. This study demonstrates a progressive decrease in the plasticity of dopaminergic system during retinal development, while such a decrease is not apparent in the noradrenergic system.

Dopaminergic control of light-adaptive synaptic plasticity and role in goldfish visual behavior

Dopamine has been implicated in processes of retinal light and dark adaptation. In goldfish retina, horizontal cell dendrites elaborate neurite processes (spinules) into cone terminals, in a light- and dopamine-dependent manner. However, the functions of retinal dopamine and the horizontal cell spinules in visual behavior are unknown. These issues were addressed in behavioral, electroretinographic, and anatomical studies of normal fish and those with unilateral depletion of retinal dopamine induced by intraocular (i.o.) injections with 6-hydroxydopamine (6-OHDA). Dopamine interplexiform cells (DA-IPC) disappear within 2 weeks after 6-OHDA injection; cell bodies appear at the marginal zone within 6 weeks at which time neurites slowly reinnervate the retina with a sparse plexus over the next 12 months. We found that dopamine depletion increased light sensitivity at photopic but not scotopic backgrounds by 2.5 log units, an effect mimicked by i.o. injections of dopamine D1 and D2 antagonists. The ERG b-wave increment thresholds were the same for control and dopamine depleted eyes, indicating a normal transition from rod to cone systems in the ON pathway. Light-dependent spinule formation was reduced by about 60% in dopamine-depleted retinas, but returned to normal by 3 months and 9 months after injection in the entire retina, even areas not directly innervated with DA-IPC processes. Spinule formation in vivo was inhibited 50% with i.o. injection of SCH 23390 in control retinas as well as throughout 3 month 6-OHDA injected retinas, including DA-IPC free areas. This latter result indicates a volume effect of dopamine, diffusing laterally through the retina over several millimeters, in regulating spinules. We conclude that DA-IPCs regulate sensitivity to background at photopic levels not via the ON pathway, but perhaps the OFF pathway. Goldfish display both increased sensitivity to light and a normal Purkinje shift in the ERG b-wave whether or not horizontal cell spinules are present, indicating that dopamine control of photopic vision in fish is not mediated through light-induced spinule formation of horizontal cell dendrites.

Reduction of red-green discrimination by dopamine D1 receptor antagonists and retinal dopamine depletion

Reduction of wavelength discrimination ability in the 560-640 nm range, but not in the 404-540 nm range, has been demonstrated in goldfish after intravitreal injection of D1-dopamine receptor antagonists. Intravitreal injection of the dopaminergic neurotoxin 6-OH-dopamine severely reduced wavelength discrimination ability in the 540-661 nm range within 3 days. Discrimination ability could be reconstituted by the D1-agonist SKF 38393. Animals recovered from injection of 6-OH-dopamine within 14-16 days. No change of wavelength discrimination was induced by 6-OH-dopamine in the 461-540 nm range. We conclude that under photopic conditions dopamine modulates retinal mechanisms involved in red-green colour coding via D1-dopamine receptor-like binding sites.

Immunohistochemical localization of dopamine D1 receptors in rat retina

We have localized the dopamine D1 receptor in rat retina using a subtype-specific monoclonal antibody. Immunolabelling can be detected in the inner and outer plexiform layers and in a number of cells in the inner nuclear layer. In the inner plexiform layer, labelled processes form four distinct horizontal bands and a series of patches. In order further to characterize the labelling pattern of the D1 receptor antibody, double-labelling experiments were performed with antibodies against population-specific neuronal markers in the retina. Antibodies against tyrosine hydroxylase, choline acetyltransferase, calretinin, calbindin, the glutamate transporter GLT-1, protein kinase C, recoverin and parvalbumin were co-applied with the D1 receptor antibody. With these cell markers we demonstrate that horizontal cells, at least three types of cone bipolar cells and a small number of amacrine cells are immunolabelled for the D1 receptor. In the inner plexiform layer, processes labelled by the D1 receptor antibody are co-stratified with processes labelled by the GLT-1 antibody. D1 receptor-labelled processes are not co-localized with the processes of amacrine cells and ganglion cells labelled by antibodies against tyrosine hydroxylase, choline acetyltransferase or calretinin. Our results indicate that dopamine D1 receptors are localized predominantly to horizontal cells and cone bipolar cells. Furthermore, the spatial disparity between dopaminergic processes and the site of the majority of D1 receptors supports the idea that in the retina dopamine acts as a neuromodulator that diffuses through extracellular space. The localization of D1 receptors to a number of identified cell types enables future physiological work to be directed towards specific synaptic circuits within the retina.