The role of ATP and adenosine in the brain under normoxic and ischemic conditions

Abstract

By taking advantage of some recently synthesized compounds that are able to block ecto-ATPase activity, we demonstrated that adenosine triphosphate (ATP) in the hippocampus exerts an inhibitory action independent of its degradation to adenosine. In addition, tonic activation of P2 receptors contributes to the normally recorded excitatory neurotransmission. The role of P2 receptors becomes critical during ischemia when extracellular ATP concentrations increase. Under such conditions, P2 antagonism is protective. Although ATP exerts a detrimental role under ischemia, it also exerts a trophic role in terms of cell division and differentiation. We recently reported that ATP is spontaneously released from human mesenchymal stem cells (hMSCs) in culture. Moreover, it decreases hMSC proliferation rate at early stages of culture. Increased hMSC differentiation could account for an ATP-induced decrease in cell proliferation. ATP as a homeostatic regulator might exert a different effect on cell trophism according to the rate of its efflux and receptor expression during the cell life cycle. During ischemia, adenosine formed by intracellular ATP escapes from cells through the equilibrative transporter. The protective role of adenosine A(1) receptors during ischemia is well accepted. However, the use of selective A(1) agonists is hampered by unwanted peripheral effects, thus attention has been focused on A(2A) and A(3) receptors. The protective effects of A(2A) antagonists in brain ischemia may be largely due to reduced glutamate outflow from neurones and glial cells. Reduced activation of p38 mitogen-activated protein kinases that are involved in neuronal death through transcriptional mechanisms may also contribute to protection by A(2A) antagonism. Evidence that A(3) receptor antagonism may be protective after ischemia is also reported.

Fig. 1a, b

The inhibitory effect induced by ATP on fEPSP amplitude is potentiated in the presence of different NTPDase inhibitors. a Time-course of fEPSP amplitude before, during and after the application of ATP in the absence or in the presence of the NTPDase1,2,3 inhibitor PV4. Each point in the graph represents the mean ± SE of fEPSP value measured as percent of baseline, pre-drug level. b Columns in the graph summarize the average amplitude (mean ± SE) of evoked fEPSP recorded from CA1 hippocampal region in control conditions, 5 min after superfusion of ATP alone and 5 min after ATP superfusion in the presence of different ecto-ATPases inhibitors. Note that the inhibitory effect of ATP on fEPSP amplitude is potentiated by BGO 136, PV4 and ARL 67156. *P < 0.05 one-way ANOVA, Newman-Keuls multiple comparison post-hoc test versus pre-drug value. §P < 0.05, one-way ANOVA, Newman-Keuls multiple comparison post-hoc test versus 10 μM ATP treated slices. (Modified from [15])

Fig. 2

Inhibitory and excitatory effects of the stable ATP analogue ATPγS. Averaged time-course (n = 4) of PS amplitude before, during and after the application of different concentrations of ATPγS. PS amplitude (mean ± SE) is measured as percent of baseline level. Upper panels represent single traces recorded in a typical experiment before, during and after ATPγS application at different concentrations

Fig. 3

Excitatory effects of endogenous ATP. Bars in the graphs represent the average of fEPSP amplitude in the presence of P2 antagonists: PPADS (30 μM) and MRS 2179 (10 μM). *P < 0.05, paired Student’s t-test. (Modified from [15])

Fig. 4a, b

Human mesenchymal stem cells in culture spontaneously release ATP that modulates cell proliferation. a Extracellular concentrations of ATP were measured in the medium containing hMSCs and in control medium not containing cells. Data are expressed as mean ± SE, n = 11, unpaired Student’s t-test: *P < 0.0001 vs medium alone. b Effect on hMSC proliferation after daily application of ATP (10 μM) and P2 antagonists, PPADS (30 μM) and MRS 2179 (10 μM). Data are expressed as percentage of proliferation. Proliferation of untreated cells was assumed as 100%. The cell number was determined after 5 days of culture by a culture counter. Each column bar represents the mean ± SE of n = 4 for each experimental condition. Paired Student’s t-test: *P < 0.05 vs respective control; one-way ANOVA, Newman-Keuls post-test: §P < 0.05 vs 10 μM ATP-treated cells. (Modified from [46])

Fig. 5

Schematic drawing of intracellular and extracellular adenosine formation. In the extracellular space, adenosine and ATP act on own purinergic receptor subtypes: P1 and P2 receptors, respectively. ADP adenosine diphosphate, AMP adenosine monophosphate, ATP adenosine triphosphate, e5′-NT ecto-5′-nucleotidase, 5′-NT 5′-nucleotidase, NTDPase ecto-nucleoside triphosphate diphosphohydrolases, P1 adenosine receptor, P2 ATP receptor, SAH S-adenosylhomocysteine, T bidirectional nucleoside transporter. (Modified from [142])