Inhibition by ATP of Hippocampal Synaptic Transmission Requires Localized Extracellular Catabolism by Ecto-Nucleotidases into Adenosine and Channeling to Adenosine A₁Receptors

Abstract

ATP analogs substituted in the gamma-phosphorus (ATPgammaS, beta, gamma-imido-ATP, and beta,gamma-methylene-ATP) were used to probe the involvement of P2 receptors in the modulation of synaptic transmission in the hippocampus, because their extracellular catabolism was virtually not detected in CA1 slices. ATP and gamma-substituted analogs were equipotent to inhibit synaptic transmission in CA1 pyramid synapses (IC50 of 17-22 microM). The inhibitory effect of ATP and gamma-phosphorus-substituted ATP analogs (30 microM) was not modified by the P2 receptor antagonist suramin (100 microM), was inhibited by 42-49% by the ecto-5'-nucleotidase inhibitor and alpha,beta-methylene ADP (100 microM), was inhibited by 74-85% by 2 U/ml adenosine deaminase (which converts adenosine into its inactive metabolite-inosine), and was nearly prevented by the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (10 nM). Stronger support for the involvement of extracellular adenosine formation as a main requirement for the inhibitory effect of ATP and gamma-substituted ATP analogs was the observation that an inhibitor of adenosine uptake, dipyridamole (20 microM), potentiated by 92-124% the inhibitory effect of ATP and gamma-substituted ATP analogs (10 microM), a potentiation similar to that obtained for 10 microM adenosine (113%). Thus, the present results indicate that inhibition by extracellular ATP of hippocampal synaptic transmission requires localized extracellular catabolism by ecto-nucleotidases and channeling of the generated adenosine to adenosine A1 receptors.

Activity-Dependent Release of Adenosine Contributes to Short-Term Depression at CA3-CA1 Synapses in Rat Hippocampus

“…ATP potently inhibited EPSCs, and this inhibition was prevented by both 500 M ␣,␤-ADP, indicating that ATP is converted to adenosine by a 5Ј-ectonucleotidase, and by DPCPX, indicating that it acted at A 1 receptors ( Fig. 2A), as shown previously (Cunha et al 1998;Dunwiddie et al 1997). ␣,␤-ADP had no significant effect on the depression of EPSCs (control: 22 Ϯ 9% vs. ␣,␤-ADP: 29 Ϯ 11%; n ϭ 7) during 10-Hz stimulation (Fig.…”

Neuroprotection by adenosine in the brain: From A1 receptor activation to A2A receptor blockade

“…In fact, a,b-methylene ADP is a competitive inhibitor of ecto-5 0 -nucleotidase, but does not affect other ecto-enzymes able to metabolise AMP (reviewed in [126]). It should also be kept in mind that the ecto-nucleotidase system is a notable efficient system, probably organised in a channelled manner [127] and able to generate adenosine to act on its receptors in a few milliseconds [128], a time course faster than the K cat of soluble enzymes. Thus, in more integrated preparations, either the concentration of a,bmethylene ADP fails to equilibrate with ecto-5 0 -nucleotidase or enzymes other than ecto-5 0 -nucleotidase are mostly responsible for the formation of extracellular adenosine or there are other (still unknown) pathways of extracellular adenosine formation that are still to be identified.…”

Different cellular sources and different roles of adenosine: A1 receptor-mediated inhibition through astrocytic-driven volume transmission and synapse-restricted A2A receptor-mediated facilitation of plasticity

“…As shall be detailed, there is now compelling evidence supporting an important role for this source of adenosine, at least under physiological conditions (Correia-de-Sá et al, 1996;Cunha et al, 1996a;Dale, 2002;Koizumi et al, 2003;Newman, 2003;Pascual et al, 2005;Zhang et al, 2003). The difficulties in highlighting the role of ecto-nucleotidases in the formation of endogenous extracellular adenosine are probably related to general lack of pharmacological tools to manipulate this large family of enzymes (Zimmermann, 2000) and to the inability to recognise that the ecto-nucleotidase pathway displays an abnormally efficient kinetic profile characteristic of fractal kinetics (Cunha et al, 1998;Cunha, 2001b;Dunwiddie et al, 1997). In fact, we know considerably more about the molecular biology of ecto-nucleotidases than about their localization and kinetic properties in native tissues that ultimately define their physiological role.…”