Lyn Beazley

Repeated Prenatal Corticosteroids Delay Myelination in the Ovine Central Nervous System

Maternal administration of corticosteroids is used to promote lung maturation in human infants considered at risk of preterm delivery [1]. Randomised trials of a single course of corticosteroid treatment have indicated no adverse long-term neurological or cognitive sequelae [2-5]. However, the current trend in many obstetric centres is to use repeated courses in cases where preterm birth has not eventuated, but the risk persists 7 days beyond administration of the original course [6-7]. This practice has not yet been subject to randomised trials of outcome. We have examined the effect of repeated injections of corticosteroids on the development of the optic nerve in prenatal fetal sheep and report a significant delay in the myelination of optic axons. Our results, together with those from other animal studies [8], show that repeated courses of corticosteroids may be detrimental to central nervous system (CNS) development. Clinical practice should balance the known beneficial effects on lung maturation of a single course of corticosteroid against the potential damage to the CNS of repeated courses.

USE OF CORTICOSTEROIDS BY AUSTRALIAN OBSTETRICIANS: A SURVEY OF CLINICAL PRACTICE

Summary: All Fellows, Members and trainees of the Royal Australian College of Obstetricians and Gynaecologists resident in Australia (n = 1,281) received a questionnaire relating to their practice of prescribing antenatal corticosteroids. 833 (65%) responded. The key findings were that 97% of Australian obstetricians prescribe antenatal corticosteroids in the classical setting of uncomplicated early preterm labour and 85% prescribe repeated courses in those cases in which the risk of preterm birth persists or recurs; 50% of obstetricians prescribe this agent weekly in cases with persisting risk of preterm birth. Some of the prescribing practices were found to be related to the number of years since obtaining specialist qualification. In view of the widespread clinical use of repeated doses of corticosteroids revealed in this present survey, it is clear that further research is warranted to determine the possible benefits and hazards of repeated exposures of the developing fetus to this therapy.

Trichromacy in Australian Marsupials

Vertebrate color vision is best developed in fish, reptiles, and birds with four distinct cone receptor visual pigments. These pigments, providing sensitivity from ultraviolet to infrared light, are thought to have been present in ancestral vertebrates. When placental mammals adopted nocturnality, they lost two visual pigments, reducing them to dichromacy; primates subsequently reevolved trichromacy. Studies of mammalian color vision have largely overlooked marsupials despite the wide variety of species and ecological niches and, most importantly, their retention of reptilian retinal features such as oil droplets and double cones. Using microspectrophotometry (MSP), we have investigated the spectral sensitivity of the photoreceptors of two Australian marsupials, the crepuscular, nectivorous honey possum (Tarsipes rostratus) and the arhythmic, insectivorous fat-tailed dunnart (Sminthopsis crassicaudata); these species are representatives of the two major taxonomic divisions of marsupials, the diprotodonts and polyprotodonts, respectively. Here, we report the presence of three spectrally distinct cone photoreceptor types in both species. It is the first evidence for the basis of trichromatic color vision in mammals other than primates. We suggest that Australian marsupials have retained an ancestral visual pigment that has been lost from placental mammals.

Redefining the role of metallothionein within the injured brain: extracellular metallothioneins play an important role in the astrocyte-neuron response to injury

A number of intracellular proteins that are protective after brain injury are classically thought to exert their effect within the expressing cell. The astrocytic metallothioneins (MT) are one example and are thought to act via intracellular free radical scavenging and heavy metal regulation, and in particular zinc. Indeed, we have previously established that astrocytic MTs are required for successful brain healing. Here we provide evidence for a fundamentally different mode of action relying upon intercellular transfer from astrocytes to neurons, which in turn leads to uptake-dependent axonal regeneration. First, we show that MT can be detected within the extracellular fluid of the injured brain, and that cultured astrocytes are capable of actively secreting MT in a regulatable manner. Second, we identify a receptor, megalin, that mediates MT transport into neurons. Third, we directly demonstrate for the first time the transfer of MT from astrocytes to neurons over a specific time course in vitro. Finally, we show that MT is rapidly internalized via the cell bodies of retinal ganglion cells in vivo and is a powerful promoter of axonal regeneration through the inhibitory environment of the completely severed mature optic nerve. Our work suggests that the protective functions of MT in the central nervous system should be widened from a purely astrocytic focus to include extracellular and intra-neuronal roles. This unsuspected action of MT represents a novel paradigm of astrocyte-neuronal interaction after injury and may have implications for the development of MT-based therapeutic agents.

The influence of ontogeny and light environment on the expression of visual pigment opsins in the retina of the black bream, Acanthopagrus butcheri

SUMMARY The correlation between ontogenetic changes in the spectral absorption characteristics of retinal photoreceptors and expression of visual pigment opsins was investigated in the black bream, Acanthopagrus butcheri. To establish whether the spectral qualities of environmental light affected the complement of visual pigments during ontogeny, comparisons were made between fishes reared in: (1) broad spectrum aquarium conditions; (2) short wavelength-reduced conditions similar to the natural environment; or (3) the natural environment (wild-caught). Microspectrophotometry was used to determine the wavelengths of spectral sensitivity of the photoreceptors at four developmental stages: larval, post-settlement, juvenile and adult. The molecular sequences of the rod (Rh1) and six cone (SWS1, SWS2A and B, Rh2Aα and β, and LWS) opsins were obtained and their expression levels in larval and adult stages examined using quantitative RT-PCR. The changes in spectral sensitivity of the cones were related to the differing levels of opsin expression during ontogeny. During the larval stage the predominantly expressed opsin classes were SWS1, SWS2B and Rh2Aα, contrasting with SWS2A, Rh2Aβ and LWS in the adult. An increased proportion of long wavelength-sensitive double cones was found in fishes reared in the short wavelength-reduced conditions and in wild-caught animals, indicating that the expression of cone opsin genes is also regulated by environmental light.

Erythropoietin is both neuroprotective and neuroregenerative following optic nerve transection

The cytokine hormone erythropoietin (EPO) is neuroprotective in models of brain injury and disease, and protects retinal ganglion cells (RGC) from cell death after axotomy. Here, we assessed EPOs neuroprotective properties in vivo by examining RGC survival and axon regeneration at 4 weeks following intraorbital optic nerve transection in adult rat. EPO was administered as a single intravitreal injection at the time of transection (5, 10, 25, 50 units, PBS control). Intravitreal EPO (5, 10 units) significantly increased RGC somata and axon survival between the eye and transection site. Twenty five units did not improve survival of RGC somata but did increase axon survival between the eye and transection site. In addition, a small proportion of axons penetrated the transection site and regenerated up to 1 mm into the distal nerve. In a second series, intravitreal EPO (25 units) doubled the number of RGC axons regenerating along a length of peripheral nerve grafted onto the retrobulbar optic nerve. Our in vivo evidence of both neuroregeneration and neuroprotection, taken together with the natural occurrence of EPO within the body and its ability to cross the blood-brain barrier, suggests that it offers promise as a therapeutic agent for central nerve repair.

A morphometric study of the retinal ganglion cell layer and optic nerve from metamorphosis in Xenopus laevis

The number and distribution of cells in the retinal ganglion layer have been examined in Nissl-stained wholemounts during postmetamorphic life in Xenopus laevis. Total cell number at metamorphic climax, in the 3 and 4 month juveniles and in 5 year adults were 30,000-41,000, 45,000-51,000 and 71,000-82,000 respectively. Optic axon counts of 29,000, 39,000 and 58,000 at these stages suggested that most cells counted were ganglion cells and that their number doubled during this period. Cell distribution changed after metamorphosis producing nasal and temporal regions of higher density in juveniles and adults. These results are discussed in relation to retinal growth and plasticity of retinotectal connections.

Visual pigments of the platypus: A novel route to mammalian colour vision

The ancestral complement of cone visual pigments in vertebrates comprises four classes whose protein components are encoded by opsin genes and whose spectral sensitivities range from ultraviolet to red. This complement has been retained throughout the radiations of teleosts, amphibians, reptiles and birds. However, eutherian mammals have lost the shortwave-sensitive-2 (SWS2) and middlewave-sensitive (Rh2) classes [1] and retain only the longwave-sensitive (LWS) and shortwave-sensitive-1 (SWS1) classes.

Expression of ephrin-A2 in the superior colliculus and EphA5 in the retina following optic nerve section in adult rat

The vertebrate retina projects topographically to visual brain centres. In the developing visual system, gradients of ephrins and Eph receptors play a role in defining topography. At maturity, ephrins but not Ephs are downregulated. Here we show that optic nerve section in adult rat differentially regulates the expression of ephrin‐A2 in the superior colliculus (SC) and of EphA5 in the retina. Expression was quantified immunohistochemically; ephrin‐A2 levels were also estimated by semiquantitative reverse transcriptase polymerase chain reaction. In the normal SC, ephrin‐A2 was expressed at low levels. At 1 month, levels of protein and of mRNA were upregulated across the contralateral SC giving rise to an increasing rostro‐caudal gradient. At 6 months, levels had fallen but a gradient remained. In the retina of normal animals, EphA5 was expressed as an increasing naso‐temporal gradient. By 1 month, expression was decreased in far temporal retina, resulting in a uniform expression across the naso‐temporal axis. We suggest that denervation‐induced plastic changes within the SC modify expression of these molecules.

Transient up-regulation of the rostrocaudal gradient of Ephrin A2 in the tectum coincides with reestablishment of orderly projections during optic nerve regeneration in goldfish

During development, a graded expression of ephrin A2 has been implicated in retinotectal map formation. Here we have examined ephrin A2 expression during optic nerve regeneration in the mature goldfish. In the tecta of normal animals, a gradient of ephrin A2 expression is detected in cell bodies within the stratum fibrosum et griseum superficiale with more immunopositive cells caudally than rostrally. The gradient in the mature animal presumably reflects the plasticity associated with continued retinal and tectal neurogenesis. During optic nerve regeneration, expression throughout the tectum is increased by 1 month as a strong rostrocaudal gradient. The gradient declines to normal by 3 months. The up-regulation of ephrin A2 during optic nerve regeneration is likely to be instrumental in reestablishing the retinotectal map.