Guanine nucleotides protect against kainate toxicity in an ex vivo chick retinal preparation

Abstract

Ex vivo preparations of chick neural retina have been successfully used in the assessment of excitotoxicity and in the evaluation of the protective effects of glutamate antagonists. Using a variation of this approach, and measuring the acute and delayed toxic effects of kainate (KA) in terms of lactate dehydrogenase release, we have shown that guanine nucleotides behave as effective neuroprotecting agents. The anti-excitotoxic potency of guanine nucleotides (in the case of GMP and GDPL LS it is about 100 times lower than that of DNQX, a powerful kainate antagonist) correlates well with their ability to displace KA from retinal KA receptors.z 1998 Federation of European Biochemical Societies.

Guanine Nucleotides, Including GMP, Antagonize Kainate Responses in Xenopus Oocytes Injected with Chick Cerebellar Membranes

“…In some of these cases, GTP and its analogues were ineffective or downright toxic by themselves. Interestingly, in this regard, we have recorded, in one oocyte, currents elicited by GppNHp alone that were antagonized by GMP, in spite of the fact that in the same oocyte the nucleotide did antagonize kainate responses at higher concentrations (data not shown for lack of sufficient confirmation), an ambiguous behavior (partial agonisdantagonist) that we have also observed in the chick retinal model of excitotoxicity/ neuroprotection (Burgos et al, 1998).…”

“…Although displacement experiments do not give a precise clue as to the antagonistic or agonistic nature of the GNs, further experiments using standard patch-clamp techniques have confirmed the antagonistic behavior of guanosine 5'-0-(2-thiodiphosphate) (GDPPS; a nonhydrolyzable GDP analogue) and GTP toward glutamate, kainate, and NMDA (Budson et al, 1991; Paas et al, 1996a), whereas our own antiexcitotoxic neuroprotection experiments show that GMP and, in most cases, GDPPS do indeed behave as efficient glutamate antagonists, acting at both a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (Ah4PA)kainate and NMDA receptors, with 5'-guanylylimidodiphosphate (GppNHp; a nonhydrolyzable GTP analogue) displaying an ambiguous behavior (Malcon et al, 1997;Burgos et al, 1998;Regner et al, 1998). As GMP has the additional advantage over GDPPS of not being directly involved in G-protein function, we have adopted GMP as the reference GN for our neuroprotection experiments (even if it is not the most active displacer in ligand-binding studies) and for…”

“…We were, however, more interested in the results using GMP (Fig. 6) reasons: (a) GMP is not directly involved in the activatiodinactivation cycle of G proteins and related effector systems, being a relatively inert molecule in the context of cell-signaling processes; and (b) GMP has exhibited so far, perhaps together with GDPPS, a consistent neuroprotective activity against excitotoxic aggressions in a number of experimental paradigms, including in vivo, ex vivo, and in vitro systems (Malcon et al, 1997;Burgos et al, 1998;Regner et al, 1998). In some of these cases, GTP and its analogues were ineffective or downright toxic by themselves.…”

Theoretical Survey on Tautomerism of Thioguanine Tautomers by Polarisable Continuum Method (PCM)

“…However, guanine-based purines can also be considered part of the purinergic system and they are released from neurons and/or glia both under basal conditions and after various types of stimulation, including stress conditions [1][2][3] . Thus, although traditionally guaninebased purines have been studied as modulators of intracellular processes, they can exert extracellular effects not related to their direct modulation of G proteins, including modulation of the glutamatergic activity [4][5][6][7][8][9][10][11][12][13] , behavioural effects 14,15 , and trophic effects on neural cells 16 . Some of the actions of the guanosine may be mediated intracellularly after its uptake, even if many trophic effects of guanine-based purines are not substantially affected by the nucleoside uptake inhibitors indicating that they are independent of intracellular mechanisms 17 .…”

The Role of the Guanine-Based Purinergic System in Seizures and Epilepsy

“…Traditionally, guanine-based purines have been studied as intracellular modulators of signal transduction processes, modulating the activity of G-proteins [2]. Nevertheless, more recently, guanine-based purines have been shown to exert relevant extracellular effects, including those related to the modulation of the glutamatergic system [3][4][5][6][7][8][9][10][11][12]. Although the exact mechanism underlying these extracellular effects remains unclear, it does not seem to involve a direct modulation of G-proteins [12].…”