Glycoconjugates of the Rat Eyeball Structural Components under Experimental Hypothyroidism according to Lectin Histochemistry Studies

Abstract
Objective: Thyroid disorders are currently among the most widespread endocrine pathologies, influencing multiple organs including the eyeballs. Lectins are used for the histochemical investigation of cell and tissue carbohydrates, which are involved in a wide range of physiological and pathological conditions, e.g. manifestations of cellular metabolism disorders, malignant transformation of cells, agglutination of viruses and microorganisms etc.
Aim: of present investigation was to use a set of lectins with different carbohydrate specificities to study the rearrangement of carbohydrate determinants of rat eye structural components under the influences of experimental mercazolil-induced hypothyroidism.
Methods: Experiments were carried out on 35 adult male Wistar rats weighing 180-240 g. Control group included 10 animals; experimental group of 25 rats during 14 days received 5 mg/kg of mercazolil with daily food allowance. Samples of thyroid glands and eyeballs were fixed in Bouin’s fluid and embedded in paraffin. The lectin panel included PNA, HPA, SNA, LABA, WGA and CNFA, conjugated to horseradish peroxidase. Lectin receptor sites were visualized with 3,3’-diaminobenzidine in the presence of H2O2. For specificity control of histochemical reactions the exclusion of lectin-peroxidase conjugates from staining protocol was used. For routine histological examination sections were stained haematoxylin and eosin.
Results: Morphological investigations revealed negative influence of experimental hypothyroidism on corneal epithelium and retina, as well as on the eyeball associated Garder’s gland. By means of lectin histochemistry methods it was identified increased reactivity of rod and cone layer with WGA, CNFA and LABA, encompassing
the accumulation of DGlcNАc, GalNAc(β1-4)GlcNAc and αLFuc carbohydrate determinants. These changes were accompanied with reduced HPA and SNA reactivity (αDGalNАc and NeuNAc(α2-6) DGal residues). These modifications in rods and cones lectin labeling were supplemented with the increased CNFA binding in the areas of synaptic contacts in the outer and inner plexiform layers, as well as with enhanced exposure of LABA receptor sites (αLFuc) in perikarya of amacrine and ganglion neurons of retina. In the cornea under experimental hypothyroidism it was detected reduction of HPA and CNFA labeling (αDGalNАc and GalNAc (β1-4) GlcNAc determinants) in the anterior epithelium, which was accompanied with the increased reactivity of these same lectins with stromal keratocytes and collagen fibers.
Conclusion: Reported drift in corneal lectin binding apparently indicates negative influence of hypothyroidism on epithelial adhesion to Bowman’s membrane and corneal transparency conditions, the latters depending on crystallins and keratan sulphates produced by keratocytes. Detected rearrangements in carbohydrate determinants of retinal compartments presumably reflect alterations in the functional activity of neurons and subsequent disturbancies in nerve impulses transduction. Lectins WGA, CNFA, LABA and HPA can be recommended for selective histochemical labeling of structural components of the rat eyeball.