# Plant Iridoids Affect Intraocular Pressure and Vascular Flow in the Rabbit Eye
## 1 Introduction  
Iridoids are secondary terpenoid plant metabolites, structurally distinguished by the presence of cyclopentane-pyrans fused with bicyclic ten-carbon cis. A slit in any of the rings builds the subgroup called the seco-iridoids group (from Latin _seco_, \"to cut\"). About 800 iridoids that include 60 non-glycosidic and 30 seco-iridoids are identified [1]. They have been found in plants from many families, mainly _Apvocnace_, _Lamiaceae_, _Lganiaceae_, _Rubiaceae_, _Scrophulariaceae_, _Verbenaceae_, and _Caprifoliaceae_. Many of the herbs used as the main bioactive compounds in well-established traditional medicines possess iridoids, such as _Nepeta cataria_ (nepetalactone), _Olea europea_ (ligstroside and oleuropein), _Harpagophytum procumbens_, known as the devil's claw (harpagoside), as well as _Valleriana officinalis_ (vale-opotriatres). Iridoids, both as constituents of plant extracts and as pure isolated substances,exhibit a wide range of pharmacological activities, such as cardiovascular, hepatoprotective, hypoglycemic, antimutagenic, antispasmodic, anticancer, antiviral, immunomodulatoryulatory, laxative, and anti-inflammatory effects [2]. Infusions and extracts obtained from plants such as chamomile (_Matricaria chamomilla_), ribwort plantain (_Plantago lantecolata_), common mallow (_Malva sylvestris_), witch hazel (_Hamamelis virginiana_), and eyebright (_Euphrasia officinalis_) have all been used to treat eye diseases [3]. Eyebright extracts have been used for hundreds of years in traditional folk medicine to treat inflammation, as well as dry eye syndrome [4], conjunctivitis [5], hordeolum, and ocular allergy symptoms [6]. Well-established compounds of eyebright belong to the iridoid group, with examples including aucubin, catalpol, euphroside, vernicoside, and acteoside. Aucubin, the main component isolated from _Aucuba japonica_ extracts, exhibits well-documented activity in the treatment of dry eye disease. Apoptotic aucubin and extracts rich in iridoid were found to decrease apoptotic cells in the cornea [7]. Iridoid catalpol (Catal) is also known to have pleiotropic protective effects in the treatment of neurodegenerative diseases, ischemic diseases, ischemic stroke, metabolic disorders, and others. Glaucoma affects more than 70 million people worldwide, of whom approximately 10% are bilaterally blind. Glaucoma is a group of diseases with different causes of onset, characterized by neuropathy of the optic nerve caused by progressive damage to the retinal ganglion cells (RGCs). Degeneration of these cells leads to a characteristic appearance of the optic nerve disc, with corresponding loss of the visual field, often accompanied by elevated intraocular pressure. Advanced disease results in permanent, irreversible loss of vision [8]. The most common form of glaucoma in the Western world is primary open-angle glaucoma (POAG) [9; 10]. The pharmacological treatment of glaucoma involves lowering intraocular pressure, improving vascular flow, and neuroprotection. The mainstay of treatment is the reduction of intraocular pressure, most commonly with topical eye drop medications, whose mechanism is to reduce the formation of aqueous fluid, increasing choroidal outflow or conventional outflow through the angle of glaucoma [8]. Glaucoma is not the only eye disease that can be caused by vascular disorders. Among the most serious and common are central retinal vein thrombosis or, more rarely, central retinal artery occlusion [11; 12]. Retinal vein obstruction (RVO) is the second most common retinal vascular condition, affecting 16.4 million people worldwide in 2008 [13]. Risk factors for RVO include hypertension, myocardial infarction, stroke, and higher total cholesterol and creatinine levels [13]. Iridoids as a group of compounds where chosen in our preliminary study, where iridoid- (loganin, loganic acid, secologanin) rich extracts where administrated orally to rabbits. We found decreasing levels of IOP.