Biomedical subjects

Michael Kalloniatis

Publications and source records attributed to Michael Kalloniatis.

6 recordsLinked to original sources

PubMed2003

Correlating retinal function and amino acid immunocytochemistry following post-mortem ischemia.

We wanted to determine the characteristics associated with electrophysiological and neurochemical changes secondary to ischemic insult as well as correlate these electrophysiological and neurochemical changes. A Ganzfeld source was used to elicit electroretinograms in anesthetized adult Sprague-Dawley rats. Following baseline recordings, one eye was removed for control quantitative amino acid immunocytochemistry, and ischemic insult was induced by cervical dislocation. Following the induction of ischemia, a single electroretinogram signal was collected at 1, 2, 4, 6, 8, 16, 32 or 64 min, after which the eye was removed for immunocytochemistry. The post-receptoral b-wave was undetectable after 1 min post-ischemia, whereas phototransduction declined more gradually and persisted for up to 16 min post-mortem. Both phototransduction saturated amplitude and sensitivity decayed with a similar time course (tc=3.06 (2.73, 3.48) versus 3.29 (2.61, 4.62)min). Significant elevation of amino acid neurotransmitter levels was not observed until 6 min post-mortem. Between 8 and 16 min post-ischemia, glutamate and GABA were significantly accumulated in neurons and Müller cells (p<0.05). Beyond 16 min, the neurotransmitter elevation in neurons and Müller cells was relatively attenuated. Aspartate immunoreactivity was significantly elevated at 4 and 6 min post-ischemia in neurons, prior to a change in any other amino acid. Moreover, of the amino acids assessed the post-ischemic change in aspartate immunoreactivity showed the best correlation with phototransduction decay (r2=0.68). Our findings show that complete impairment of phototransduction coincides with the accumulation of amino acid neurotransmitter. The correlation of aspartate immunoreactivity and phototransduction provides evidence of heightened glutamate oxidation during ischemic insult.

Animals↗

PubMed2003

The contribution of glycolytic and oxidative pathways to retinal photoreceptor function.

PURPOSE: To consider how aerobic and anaerobic metabolic processes limit posthypoxemic decay in retinal function, measured by electroretinogram (ERG). METHODS: The hypothesis that lowering metabolic demand would prolong endogenous metabolic stores was tested by comparing the rate of ERG decay in rats in dark- (n = 5) versus light-adapted (15 minutes, 112 cd/m(2), n = 5) conditions and with serial versus single (n = 5 at each of seven time points) light stimulation. Postmortem hypoxemia was induced by cervical dislocation. Glucose (10 and 100 mM) and glutamine or lactate (100 mM) were injected into the vitreous 10 minutes before hypoxemic insult, to consider glycolytic-oxidative versus oxidative metabolism, respectively. RESULTS: Lowering the metabolic drain by light adaptation or serial stimulation significantly improved the photoreceptoral saturated amplitude during the first 5 to 7.5 minutes after postmortem hypoxemia. Increasing substrate availability with exogenous glucose preloading delayed the loss of the photoreceptoral response, thereby extending the delay constant from 4.8 to 10.9 minutes. Postreceptoral amplitudes were not improved by any exogenous substrate. Providing glucose at 5 minutes after hypoxemia provided no benefits. Similar to glucose, glutamine and lactate loading significantly delayed photoreceptoral decay over the first 7.5 minutes, after which time glucose was the more effective substrate. CONCLUSIONS: The postmortem decay of photoreceptoral function reflects depletion of both endogenous oxygen and carbon substrate reserves. The findings provide evidence that a transition between aerobic and anaerobic metabolism occurs after approximately 8 minutes of complete hypoxemia.

Aerobiosis↗

PubMed2002

Cellular dispersion patterns and phenotypes in the developing mouse superior colliculus.

The mammalian superior colliculus is structurally and functionally divided into two entities: superficial visual and deep multimodal motor. To discover the role, if any, of developmental processes in establishing separate tectal compartments, we have used highly unbalanced mouse chimaeras to mark cell dispersion pathways and trace cell lineages. Two forms of cell dispersion were detected: radial and tangential. Neither radial nor tangential forms of cell dispersion were found to exist on their own in any group of labeled cells. Radial cell dispersion was the predominant form of cell movement from the germinal zones and primarily associated with the differentiation of glutamatergic neurons. In contrast, tangential cell dispersion involved a minority of tectal cells, concentrated chiefly in the superficial layers and often associated with the upper aspects of radial columns. More scattered cells expressed gamma-aminobutyric acid (GABA) compared to columnar cells. Taken together, these results indicate separate developmental constraints for the development of glutamatergic and GABAergic neurons in the superior colliculus.

Animals↗

PubMed2002

Left--right word recognition asymmetries in central and peripheral vision.

Word recognition for Western languages shows an increased probability of a correct response when words are presented to the right of fixation. We considered whether this right bias was consistent at eccentricities superior and inferior to fixation and whether this bias can be altered by different presentation strategies. A right bias of up to approximately 0.9 degrees to the right of fixation was found when words were presented along one horizontal meridian. The eccentricities tested extended up to 8 degrees above and below the point of fixation. However, the right bias was reduced for stimulus conditions where the word was randomly presented within a mosaic containing all possible presentation locations. We have therefore demonstrated that reading habit (right bias) can be manipulated based upon experimental paradigm, strongly supporting the proposition that the left-right asymmetry is a consequence of attending to a particular area of visual space as part of the normal reading habit, rather than an innate superiority for word recognition of the right visual field or reduced visual performance.

Fixation, Ocular↗

PubMed2002

Retinal neurochemical changes following application of glutamate as a metablolic substrate.

BACKGROUND: Retinal neural and glial cells share an intricate relationship that includes uptake and recycling of the amino acid neurotransmitters, glutamate and gamma-amino butyric acid (GABA), as well as metabolic links. The aim of this work was to determine the neurochemical and morphological changes induced by the removal of glucose but with the provision of exogenous glutamate in the isolated retinal preparation incubated under aerobic conditions. The carbon skeleton of glutamate can enter the tricarboxylic acid cycle as alpha-ketogluterate, providing an alternative metabolic substrate in cases of glucose deprivation. METHODS: Isolated rat retinas were incubated in physiological media with and without glucose, using a range of glutamate concentrations to provide an alternative source of metabolic substrate. We conducted post-embedding immunocytochemistry and quantified the change in glutamate and GABA immunoreactivity within Müller cells under these different incubation conditions. RESULTS: The provision of glutamate with normal (6 mM) glucose levels resulted in a gradual accumulation of glutamate and GABA in Müller cells, with Müller loading when exogenous glutamate concentrations were above 0.1 mM. However, when these varying levels of glutamate were applied in the absence of glucose, glutamate accumulation in Müller cells was decreased compared to the 6 mM glucose condition and GABA accumulation in Müller cells was at a minimum at moderate (0.5 and 1 mM) glutamate levels. Under hypoglycaemic conditions, exogenous glutamate (0.5 to 1 mM) is rapidly metabolised by Müller cells to the extent that no glial loading is evident, despite the high concentrations. CONCLUSIONS: Normal neurochemical function appears to be maintained secondary to exogenous glutamate provision of 0.5 to 1 mM when glucose is not in the incubation medium, implying that glutamate can be used as an alternative metabolic substrate. We also show that Müller cells possess more rapid glutamate metabolic capabilities compared to the metabolism of GABA.

Animals↗

PubMedDate not supplied

Mapping photoreceptor and postreceptoral labelling patterns using a channel permeable probe (agmatine) during development in the normal and RCS rat retina.

The aim of this study was to determine whether agmatine, a channel permeable probe, can identify photoreceptor dysfunction in the Royal College of Surgeons (RCS) retina at an earlier stage to that shown by apoptosis or anatomical markers, and also characterize the neurochemical development of the inner retina in the normal and degenerating rat. We used isolated retinas at different ages incubated in physiological media containing agmatine. Subsequently, postembedding immunocytochemistry was used to determine the number of labelled photoreceptors and the labelling pattern within postreceptoral neurons. Agmatine labelling patterns revealed a sequential development of retinal neurons beginning at postnatal day (PND) 11/12 with most horizontal cells, a few ganglion and amacrine cells, showing a strong signal. The neurochemical development progressed rapidly, and reflects to a large part the known distribution of glutamate receptors, with inner nuclear labelling being evident by PND14, continuing with the same pattern of labelling in adulthood for the control retina. The RCS retina showed markedly reduced agmatine labelling in the inner retina at PND20. A rapid increase in photoreceptor AGB labelling was evident during the degeneration phase. Multiple samples at PND14 and PND16 confirmed a significant increase of labelled photoreceptors in the RCS retina.

Agmatine↗