Neuronal death due to excessive activation of N-methyl-d-aspartate (NMDA) receptors is a hallmark of neurodegenerative diseases. The polyamines: putrescine, spermine, and spermidine, bind to specific sites on the NMDA receptor and promote its activation, but their role in NMDA-induced neuronal death is ill defined. In this study, we characterized the role of polyamines in excitotoxic death of retinal ganglion cells (RGCs), a population of central neurons susceptible to NMDA-induced damage. Our data show that endogenous arginase I, the rate limiting enzyme for polyamine biosynthesis, is expressed in the intact, adult retina. Intraocular injection of NMDA visibly increased arginase I expression in Muller cells, the predominant glial cell-type in the mammalian retina. Inhibition of polyamine synthesis using di-fluoro-methyl-ornithine (DFMO) was markedly neuroprotective, while injection of exogenous polyamines in conjunction with NMDA exacerbated RGC death. Blockade of the polyamine binding sites on NMDA receptors using the non-competitive antagonist ifenprodil was neuroprotective, suggesting that polyamines contribute to excitotoxic death, at least partly, by binding to NMDA receptors. Importantly, we also demonstrate that NMDA leads to activation of both the Erk1/2 and PI3 K/Akt pathways, but only the PI3 K/Akt kinase was required for di-fluoro-methyl-ornithine-induced RGC survival. In summary, our study reveals that polyamines modulate neuronal death in the retina via different mechanisms that potentiate NMDA-triggered excitotoxicity.

A role for polyamines in retinal ganglion cell excitotoxic death

Sigma 1 receptor: A novel therapeutic target in retinal disease.

Background. All currently approved glaucoma medications are directed toward lowering intraocular pressure. However, it is apparent that there are pressure-independent mechanisms associated with the development of glaucomatous optic neuropathy. There has been considerable effort to develop therapeutics that rescue the retinal ganglion cells from undergoing secondary degeneration after the original insult has occurred. This therapeutic strategy has been termed neuroprotection. Methods. The literature was reviewed to examine the current knowledge of the degenerative cascade involved in glaucomatous damage, with emphasis on potential therapeutic targets for neuroprotective strategies. Results. There are a number of promising areas of research for new glaucoma therapies including glutamate antagonists, calcium channel blockers, antioxidants, nitric oxide synthase inhibitors, neurotrophins, and anti-apoptotic agents. Conclusions. Glaucoma is a complex disease with a number of risk factors and mechanisms leading to ganglion cell death. Future glaucoma therapy will likely include neuroprotectants that could be used as an adjunct therapy with other medications designed to lessen the initial insult (i.e., intraocular pressure-lowering compounds). As the word neuroprotection becomes more popular, care must be taken in evaluating the research literature for clinically effective therapies.

Beyond intraocular pressure: neuroprotective strategies for future glaucoma therapy.

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery proc...

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

DP-class prostaglandins and prostaglandin analogs (collectively, prostaglandins or PGs) such as PGD2, BW245C, ZK110841, and ZK118182, lower intraocular pressure (IOP) in animal models of ocular hypertension. A new analog of ZK118182 (AL-6556; 13,14-dihydro-ZK118182) was synthesized, and the isopropyl ester of AL-6556 (AL-6598) was shown recently to lower IOP in the ocular hypertensive cynomolgus monkey model of glaucoma and in human subjects. AL-6556 and AL-6598 had an affinity (Ki) of 2.66-4.43 microM for DP receptors but a much lower affinity (K(i)s = 38-103 microM) for EP3, FP, IP, and TP receptors (n = 3-5). In addition, AL-6556 and AL-6598 exhibited K(i)s > 100 microM for 19 nonprostanoid receptors. Both PGs stimulated cAMP production (EC50 = 1.07 +/- 0.1 microM and EC50 = 2.64 +/- 0.84 microM; n = 3) by way of DP receptors in embryonic bovine tracheal fibroblasts. While AL-6556 and AL-6598 were partial agonists (EC(50)s = 0.47-0.69 microM; E(max) = 35%-46%) at EP2 receptors in human nonpigmented epithelial cells, neither had any agonist activity at EP4, IP, or FP receptors. The DP antagonist, BWA868C, effectively antagonized the effects of AL-6556 with a high potency (IC50 = 22.8 +/- 3.9 nM; n = 3). DP receptors radiolabeled with [3H]BWA868C on human eye sections by quantitative autoradiography were highly concentrated in the ciliary process (CP), longitudinal (LCM) and circular (CCM) ciliary muscles, and iris with much lower specific binding in the cornea (CN), lens (LNS), and retina (RET). EP2 receptors labeled with [3H]PGE2 were concentrated in the LCM, CM, RET, and iris. In conclusion, AL-6598 and AL-6556 are relatively DP-receptor-selective PGs with full agonist activity at the DP and partial agonist activity at the EP2 receptor. The IOP-lowering activities of these compounds may involve both the inflow and outflow mechanisms, as DP and EP2 receptors were visualized in human ocular tissues involved in such aqueous humor dynamics.

Molecular pharmacology of the DP/EP2 class prostaglandin AL-6598 and quantitative autoradiographic visualization of DP and EP2 receptor sites in human eyes.

AIMS—This study characterised the pharmacology of [3H]-ifenprodil binding to the polyamine binding sites (PBS) on the N-methyl-D-aspartate (NMDA) receptor channel complex on human retinas These data were correlated with the known neuroprotective effects of ifenprodil and eliprodil
METHODS—Specific binding of [3H]-ifenprodil (under sigma site blockade) was investigated using human retinal homogenates and radioligand binding techniques Scatchard and competition analyses were utilised to define the pharmacology of the [3H]-ifenprodil binding sites
RESULTS—Specific binding of [3H]-ifenprodil comprised 73% (SEM 3%) of total and reflected interaction with two affinity sites (Kds = 039 and 43 µM) of different densities (Bmax = 144 and 105 pmol/ mg protein) (n = 5) The rank order of affinity of compounds competing for [3H]-ifenprodil binding to the high affinity PBS was: ifenprodil > eliprodil > arcaine > spermine > diaminodecane > spermidine > putrescine >> MK-801 (n = 3-7) However, [3H]-ifenprodil binding was minimally inhibited by glutamate, NMDA, and kainate
CONCLUSION—These studies have shown, for the first time, the presence of specific [3H]-ifenprodil binding sites in the human retina with pharmacological characteristics of PBS associated with the NMDA receptor ionophore complex The neuroprotective effects of eliprodil and ifenprodil may, in part, be mediated via these [3H]-ifenprodil labelled sites

Keywords: retina; neuroprotection; eliprodil; NMDA receptors

https://bjo.bmj.com/content/bjophthalmol/83/2/236.full.pdf

N A Sharif

Papers

Human retina contains polyamine sensitive [3H]-ifenprodil binding sites: implications for neuroprotection?