Observations on the retina and 'optical fold' of a mesopelagic sabretooth fish, Evermanella balbo.

The 'optical fold' of Evermanella balbo covers the ventro-lateral cornea and is presumed to capture illumination that would otherwise remain undetected by the tubular eye of this mesopelagic teleost. It contains alternating bands of cellular and acellular material, running approximately perpendicular to the lateral surface of the eye. Only parts of this lamellar body lie within the eyelid-like structure. The cellular lamellae are 2-2.5 µm thick centrally and composed of fibroblast-like cells. The extracellular bands (4.5-5 µm thick) contain regular arrays of collagen fibrils, with layers of thin fibrils sandwiching a region of thicker fibrils. The thin fibrils are organised in alternating sheets where fibrils, although all parallel, change their orientation by 90° between each sheet. All thick fibrils are oriented parallel to the lateral surface of the 'optical fold'. In the main retina, small bundles of rod inner/outer segments are separated by the processes of the retinal pigment epithelium (rpe) laterally. Centrally, the length of tightly packed rods increases, but rpe processes no longer divide them into bundles. Medially, rod length increases further, but packing is less dense. The accessory retina is significantly thinner, and less well-developed than the main retina. Ventrally, the rods show no regular arrangement and are not grouped. Dorsally, however, rods are arranged into bundles, separated by melanosome-filled rpe processes. The thickness of the retina increases as it approaches the crystalline lens. It is on this dorsal accessory retina that light traversing the 'optical fold' most likely falls, facilitating the detection of moving objects in the ventro-lateral field of view.

The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals.

Although ultraviolet (UV) sensitivity is widespread among animals it is considered rare in mammals, being restricted to the few species that have a visual pigment maximally sensitive (λmax) below 400 nm. However, even animals without such a pigment will be UV-sensitive if they have ocular media that transmit these wavelengths, as all visual pigments absorb significant amounts of UV if the energy level is sufficient. Although it is known that lenses of diurnal sciurid rodents, tree shrews and primates prevent UV from reaching the retina, the degree of UV transmission by ocular media of most other mammals without a visual pigment with λmax in the UV is unknown. We examined lenses of 38 mammalian species from 25 families in nine orders and observed large diversity in the degree of short-wavelength transmission. All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity. Other mammals, however, such as hedgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315-400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.

Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis).

We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.

Retinal repair by transplantation of photoreceptor precursors.

Photoreceptor loss causes irreversible blindness in many retinal diseases. Repair of such damage by cell transplantation is one of the most feasible types of central nervous system repair; photoreceptor degeneration initially leaves the inner retinal circuitry intact and new photoreceptors need only make single, short synaptic connections to contribute to the retinotopic map. So far, brain- and retina-derived stem cells transplanted into adult retina have shown little evidence of being able to integrate into the outer nuclear layer and differentiate into new photoreceptors. Furthermore, there has been no demonstration that transplanted cells form functional synaptic connections with other neurons in the recipient retina or restore visual function. This might be because the mature mammalian retina lacks the ability to accept and incorporate stem cells or to promote photoreceptor differentiation. We hypothesized that committed progenitor or precursor cells at later ontogenetic stages might have a higher probability of success upon transplantation. Here we show that donor cells can integrate into the adult or degenerating retina if they are taken from the developing retina at a time coincident with the peak of rod genesis. These transplanted cells integrate, differentiate into rod photoreceptors, form synaptic connections and improve visual function. Furthermore, we use genetically tagged post-mitotic rod precursors expressing the transcription factor Nrl (ref. 6) (neural retina leucine zipper) to show that successfully integrated rod photoreceptors are derived only from immature post-mitotic rod precursors and not from proliferating progenitor or stem cells. These findings define the ontogenetic stage of donor cells for successful rod photoreceptor transplantation.

Progesterone production in mares and echographic evaluation of the corpora lutea formed after follicular aspiration.

Ultrasound-guided follicular aspiration was performed in 26 Criollo crossbred mares, followed by the evaluation of ultrasonographic images of the Corpus luteum (CL) that was formed after puncture of follicles of different diameters (Group 25-29 mm; Group 30-35 mm and Group >35 mm). Serum progesterone (P(4) ) concentrations were measured to determine CL function. The size of the CL was measured and the CL was classified based on the following echoscore: 1- anechoic tissue; 2- poorly defined luteal structure with low echogenicity; 3- echogenicity analogous to a luteal structure. The proportion of aspirated follicles that formed a functional CL (based on P(4) concentration) 8 days after aspiration was 57.1% (4/7; CL size 25-29 mm), 75.0% (6/8; CL size 30-35 mm) and 72.7% (8/11; CL size >35 mm), respectively (p > 0.05). The echographic scores of aspirated follicles (indicating the presence or absence of a CL) were consistent with serum P(4) concentrations (p < 0.0001). Of 26 aspirations, 18 resulted in luteal function confirmed by increased progesterone concentrations ([P(4) ] > 1.0 ng/ml); 17 of these mares (94.4%) had an echoscore (2-3) compatible with luteinization (p = 0.0372). Eight days after aspiration, serum [P(4) ] > 2.0 ng/ml was associated with high (p = 0.0056) CL echoscore (3) in 15 of 17 mares (88.2%). The echoscore used in this study was valuable as a screening test to detect the presence of a functional CL after aspiration. An echoscore of 3 served as a practical and efficient method to confirm luteinization.

Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice.

In the mammalian retina, besides the conventional rod-cone system, a melanopsin-associated photoreceptive system exists that conveys photic information for accessory visual functions such as pupillary light reflex and circadian photo-entrainment. On ablation of the melanopsin gene, retinal ganglion cells that normally express melanopsin are no longer intrinsically photosensitive. Furthermore, pupil reflex, light-induced phase delays of the circadian clock and period lengthening of the circadian rhythm in constant light are all partially impaired. Here, we investigated whether additional photoreceptive systems participate in these responses. Using mice lacking rods and cones, we measured the action spectrum for phase-shifting the circadian rhythm of locomotor behaviour. This spectrum matches that for the pupillary light reflex in mice of the same genotype, and that for the intrinsic photosensitivity of the melanopsin-expressing retinal ganglion cells. We have also generated mice lacking melanopsin coupled with disabled rod and cone phototransduction mechanisms. These animals have an intact retina but fail to show any significant pupil reflex, to entrain to light/dark cycles, and to show any masking response to light. Thus, the rod-cone and melanopsin systems together seem to provide all of the photic input for these accessory visual functions.

Characterization of an ocular photopigment capable of driving pupillary constriction in mice.

This work demonstrates that transgenic mice lacking both rod and cone photoreceptors (rd/rd cl) retain a pupillary light reflex (PLR) that does not rely on local iris photoreceptors. These data, combined with previous reports that rodless and coneless mice show circadian and pineal responses to light, suggest that multiple non-image-forming light responses use non-rod, non-cone ocular photoreceptors in mice. An action spectrum for the PLR in rd/rd cl mice demonstrates that over the range 420-625 nm, this response is driven by a single opsin/vitamin A-based photopigment with peak sensitivity around 479 nm (opsin photopigment/OP479). These data represent the first functional characterization of a non-rod, non-cone photoreceptive system in the mammalian CNS.

Genome-wide association analyses identify 44 risk variants and refine the genetic architecture of major depression.

Major depressive disorder (MDD) is a common illness accompanied by considerable morbidity, mortality, costs, and heightened risk of suicide. We conducted a genome-wide association meta-analysis based in 135,458 cases and 344,901 controls and identified 44 independent and significant loci. The genetic findings were associated with clinical features of major depression and implicated brain regions exhibiting anatomical differences in cases. Targets of antidepressant medications and genes involved in gene splicing were enriched for smaller association signal. We found important relationships of genetic risk for major depression with educational attainment, body mass, and schizophrenia: lower educational attainment and higher body mass were putatively causal, whereas major depression and schizophrenia reflected a partly shared biological etiology. All humans carry lesser or greater numbers of genetic risk factors for major depression. These findings help refine the basis of major depression and imply that a continuous measure of risk underlies the clinical phenotype.

The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments.

Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.

Morphologic changes in teleost primary and secondary retinal cells following brief exposure to light.

Light adaptive morphologic changes in the teleost retina, such as movements of rods, cones, and epithelial pigment and spinule formation on horizontal cell terminals, are normally associated with continual exposure to light. Depending on a variety of factors these processes are generally completed within 30-60 min. In this report we show that a brief exposure to light (1-2 min) preceded and followed by darkness is sufficient to elicit these changes in four species of teleost; the trout (Salmo gairdneri), the tench (Tinca tinca), the carp (Cyprinus carpio), and the goldfish (Carassius auratus). A brief pulse is as effective in causing cone migration and an increase in the number of spinules as continual exposure; however, it is sometimes less effective in causing pigment migration. The photomechanical changes following a brief period of light are always completed more quickly and show greater species variability than the formation of spinules. These results show that the various morphologic manifestations of light adaptation are autonomous processes that need only a short pulse of light to trigger the whole sequence of events. This is of interest when considering their control mechanisms and may have consequences for physiologic work involving experiments on dark-adapted eyes.

Size constancy in goldfish (Carassius auratus).

This study examines whether fish are able to judge the real size of objects taking distance into account, or if size judgements are based purely on the angle objects subtend on the retina. Goldfish were trained to discriminate between 2 targets which differed from one another only in their diameters (5 cm and 10 cm), presented initially equidistantly from the fish. A choice of the correct target was rewarded either by food or by access to other fish. Following successful training, the targets were shifted so that they both subtended the same visual angle. The fish continued to choose the target to which they had been trained, showing that they display size constancy. This ability was present in both monocular and binocular animals.

Gonadotropin-releasing hormone, a neuropeptide of efferent projections to the teleost retina induces light-adaptive spinule formation on horizontal cell dendrites in dark-adapted preparations kept in vitro.

The teleost retina receives efferent projections from neurons of the nucleus olfactoretinalis at the base of the olfactory bulbs. These fibres contain gonadotropin-releasing hormone (GnRH) immunoreactive material and are presynaptic to retinal dopaminergic interplexiform cells. We have incubated isolated dark-adapted retinae and eyecup preparations of roach with salmon-GnRH and found an increase in horizontal cell spinule numbers to 70% light-adaptive levels. This effect was blocked by addition of haloperidol to the incubation medium suggesting that GnRH acts via stimulation of the dopaminergic interplexiform cells. We conclude that GnRH containing efferent fibres are capable of inducing light-adaptive changes in the retina and discuss their implication in the control of endogenous rhythms.

Brain hydration during alcohol withdrawal in alcoholics measured by magnetic resonance imaging.

Twenty-seven patients had a first Magnetic Resonance Imaging (MRI) scan 1-3 days after stopping drinking and a second approximately 2 weeks later with no change in whole brain T1 or T1 in selected brain areas. Six patients whose first scan was over 36 h after the last drink underwent an increase in whole brain T1 in the interval to the second scan. The later the first scan was performed the greater was the increase in T1. These results are compatible with a very early fall in brain water immediately on cessation of drinking (perhaps due to a rebound increase of vasopressin activity) with a return to 'baseline' after two weeks. A third scan after discharge from hospital in 23 individuals who had abstained from alcohol or drank very little did not reveal any further significant change in brain T1.

The spectral transmission of the lens and cornea of the brown trout (Salmo trutta) and goldfish (Carassius auratus)--effect of age and implications for ultraviolet vision.

This study reports spectral transmission curves (250-700 nm) for the lenses and corneas of two teleost species known to be u.v. sensitive; brown trout (Salmo trutta) aged 0-3 yr, and goldfish (Carassius auratus) aged 0-4 yr. In both cases there is a decrease in lenticular short-wave transmission with increased age which cannot be explained by an increase in lens size alone. Corneas of goldfish go through a similar ageing process while those of the trout do not. An orange pigmentation located in the dorsal cornea of goldfish is also described.

The spectral transmission of freshwater teleost ocular media--an interspecific comparison and a guide to potential ultraviolet sensitivity.

Based on their spectral transmission, the lenses of 50 teleost species can be classified into three categories: "type 1" colourless lenses with 50% transmission points between 315 and 354 nm, "type 2" lenses which also appear colourless but have 50% cut-off points around 362-405 nm, and "type 3" lenses which are visibly yellow (50% transmission 425-450 nm). Most corneas transmit all wavelengths down to around 300 nm, with only 3 species showing a distinct yellow colouration. This distribution of ocular media transmission is related both to the phylogenetic group of the fish and to their photic environment.

Spectral transmission and short-wave absorbing pigments in the fish lens--II. Effects of age.

Examination of the spectral transmission and pigments present in lenses of sixteen species of fish revealed that changes in lens pigment type and/or concentration often occur with age. Age-related changes in lens transmission for all species could be fitted to a common framework composed of three stages: an initial rapid accumulation of lens pigments producing a large increase in the wavelength of 50% transmission, followed by a reduction in the rate of pigment deposition which results in a levelling of the 50% transmission and a final stage after pigment accumulation ceases producing a drop in the wavelength of 50% transmission.

Ultraviolet-sensitive cones in the goldfish.

Goldfish have been shown behaviourally to have ultraviolet sensitivity. We have now identified by microspectrophotometry a population of small single cones in the goldfish retina that contain a visual pigment maximally sensitive around 355-360 nm.

Rods trigger light adaptive retinomotor movements in all spectral cone types of a teleost fish.

Action spectra for light induced cone contraction are described for the two spectral cone types of a dichromatic (red/green) cichlid species (Aequidens pulcher). Criterion response thresholds (50% of maximal response amplitude) were determined for seven wavelengths. After correcting for absorption by cornea and lens, the resulting action spectra were compared with the absorption spectra of the rod and cone visual pigments. We find (i) that the action spectra of red and green sensitive cones are almost identical and (ii) matched most closely the absorption spectrum of the rod visual pigment. We therefore conclude that light adaptive cone contraction is triggered by light absorption in rods, which in the dark adapted state are located next to the external limiting membrane and are therefore in an optimal position to capture the incident light during early dawn. Possible mechanisms of signal transfer from rods to cones are discussed.

Spectral transmission and short-wave absorbing pigments in the fish lens--I. Phylogenetic distribution and identity.

Fish lens transmission was found to vary depending on the type and concentration of short-wave absorbing compounds present within the lens. Pigments extracted from lenses of ten species were identified as mycosporine-like amino acids (mainly palythine, palythene and asterina-330, lambda maxs around 320-360 nm) which are also thought to be present in the majority of the 120 species examined here. A novel mycosporine-like pigment with lambda max 385 nm was isolated from the lens of the flying fish, Exocoetus obtusirostris, while lenses of several closely related tropical freshwater species were found to have high concentrations of the tryptophan catabolite 3-hydroxykynurenine (lambda max 370 nm). The type of lens pigment a species possesses and its concentration depends upon both the animal's phylogenetic group and its "optical niche".

The pupil response of a teleost fish, Porichthys notatus: description and comparison to other species.

The pupil response of Porichthys notatus to different intensities of illumination is described and compared to that of P. myriaster, Cephaloscyllium ventroisum, and a human. While the fully dark adapted pupil is round, at the highest light intensities it consists of only two small, almost independent, apertures with a total area 4.9% of that observed in the fully dilated animal. The response is at least partially consensual and occurs, albeit at a much reduced rate, in isolated eyes. P. notatus also displays retinomotor movements comparable to those seen in most teleosts, suggesting that, contrary to most previous assumptions, pupillary responses and retinomotor migrations are not mutually exclusive.