What neurochemistry tells us about the retina

The salvaging of sight in those with visionthreatening retinal disease is one of the last frontiers of modern ophthalmology. Improvements in our understanding of the structure and function of the retina have been exponential over the last 50 years and instrumental in improving our understanding of the mechanisms of disease and treatment. The special article by Kalloniatis and colleagues in this issue of Clinical and Experimental Optometry shows how understanding the neurochemistry of the retina has influenced our knowledge of the structure and function of the retina and how it changes in disease. An e-supplement to this paper is a massive database of amino acid profiles in the retina and will be a very important collation of data for researchers in this field. The e-supplement is open access, as is the Kalloniatis paper, and can be accessed either from the paper or from the home page of Clinical and Experimental Optometry at http:// onlinelibrary.wiley. com/journal/10.1111/ (ISSN)1444-0938 The amino acids glutamate, GABA and glycine are recognised as the main neurotransmitters mediating communication between the various neurons of the retina. Glutamate is known to mediate communication between photoreceptors, bipolar cells and ganglion cells, while GABA and glycine mediate communication between the lateral elements of the retina, amacrine and horizontal cells. By using sophisticated postembedding immunocytochemical methods to label cells that contain glutamate, GABA and/or glycine, one can examine virtually every neuron in the retina. As shown in this article by Kalloniatis and colleagues, there are remarkable similarities in neurochemistry across the animal kingdom. Indeed, the authors show that the neurochemistry of neurons in the primate retina is remarkably similar to those of crocodiles, cats, Port Jackson sharks and even kangaroos, despite these other animals having vastly different visual needs from our own. This neurochemical architecture is conserved through evolution, perhaps indicative of its importance for vision. Aside from the observation that the retinae of our eyes are similar to a plethora of other animals, including a rather large pig found on the menu of some restaurants in South America (the peccary), the localisation of amino acids also provides insights on how neurons change during disease. The e-supplement database (www.aminoacidimmunoreactivity.com) provides a wealth of information on how neurons change in retinal detachment, retinal dystrophy and following metabolic insult. A key finding generated from neurochemical analysis has been the wholesale changes that occur in inner retinal neurons well after the loss of photoreceptors in models of retinal dystrophy. Despite the inner retina looking relatively normal following photoreceptor loss when viewed using nuclear stains such as toluidine blue, immunolabelling with glutamate, GABA and glycine uncovers an array of changes in inner retinal neurons, including areas where neurons migrate in columns from the inner to outer retina, regions of the inner plexiform layer that are displaced to ectopic sites and aberrant connections between some neurons. Glial cells also change, forming large scars that fragment the retina. This information has dramatically improved our understanding of retinal remodelling and plasticity in disease and may have implications for the optimal development of photoreceptor restorative therapies such as retinal implants. Localisation of amino acids provides more information than merely whether a neuronal type is present in the retina or not. The amino acid neurotransmitters glutamate and GABA are linked with metabolism by virtue of their dependence on the normal function of the retinal glia, Müller cells, for uptake, degradation and recycling. Glutamate released from neurons is removed from the synaptic cleft by high affinity uptake into Müller cells and rapidly degraded to glutamine via the enzyme glutamine synthetase. Glutamine is then shuttled from Müller cells back into neurons, to act as a precursor for the formation of both glutamate and GABA. Similarly, GABA turnover is linked with metabolism because of high affinity uptake into glial cells and degradation by one of the main metabolic cycles in cells, the Krebs cycle. The ‘GABA-shunt’ is recognised as a major contributor to the overall energy needs of the central nervous system. What this means is that evaluation of neurochemistry can provide information about the metabolic state of neurons. In situations where metabolism is affected, such as following ischaemia or retinal detachment, neurochemistry is altered. Evaluation of changes in neurochemistry in disease requires a statistically rigorous approach. Pattern recognition has emerged as one such tool that can very effectively quantify neurochemical changes in populations of neurons across the retina. Pattern recognition uses software originally developed for analysing imagery from satellites, and traditionally has been used to ascribe unique identifiers or ‘signatures’ to crops, houses or other ground features in satellite photographs generated by imaging the earth through different filters. In an analogous fashion, retinal neurons that have been labelled by one or more of a series of amino acids can be identified by their unique amino acid signatures. Differences in the numbers of statistically different groups or classes across the retina can uncover alterations in the function of neurons, cell loss or changes in amino acid recycling. Examples of this are provided in the internet-based database. Finally, the nexus between neurochemistry and neuronal function is important. Functional mapping using an organic cation called agmatine is useful for further C L I N I C A L A N D E X P E R I M E N T A L