| Extracellular ATP is currently recognized as an important signal molecule in Central Nervous System (CNS). Although purine receptors are widely distributed throughout the CNS, direct ATP neurotransmission has only been reported in a few cases, suggesting that extracellular ATP may mainly function as a modulator rather than direct neurotransmitter. Indeed, both inhibiting and potentiating effects of ATP on synaptic transmission have been frequently reported, although there are still debates as for whether ATP can modulate synaptic transmission directly by activating P2 receptors or indirectly by generating adenosine through its extracellular catabolism by the ecto-nucleotidase cascade.Multiple sources and mechanisms of endogenous ATP release have been suggested. Vesicular co-release of ATP with Ach and NE has been reported in sympathetic neurons, but the clear evidence of such co-release mechanism with glutamate and GABA, the most important excitatory and inhibitory transmitter in CNS, is still missing. Non-vesicular neuronal release of ATP has been also suggested. ATP released from astrocyte has been reported as an important messenger underlying inter-glia communications. Despite the facts that both ATP release and ATP mediated modulation of synaptic transmission have been reported frequently, modulation of synaptic activity by endogenous ATP with clear release source and mechanism has been rarely elucidated.In the present study we showed that in cultured hippocampal neurons, ATP (0.5-100 (M) inhibited both glutamatergic and GABAergic synaptic transmission, and this inhibitory effect was dose-dependent. ATP increased the frequency of mEPSC, but did not change their amplitude. ATP augmented the paired-pulse ratio (I2 / I1) of two successive EPSC, repressed the voltage-gated calcium channel, but did not affect Glu- or GABA-induced current. All these suggest a pre-synaptic site of ATP action. The results of application of ATP and adenosine antagonists indicate that the effects of ATP were mediated by P2Y receptor, adenosine receptor was also involved.Reactive blue 2 (RB-2), a P2Y receptor antagonist, increased the amplitude of eEPSC (117(8.4%), suggesting there was endogenous ATP release.Repeated activation of pre-synaptic neuron with a 10 Hz, 1s pulse train caused a short term depression (I(I1 value decreased) of eEPSC. RB-2 enhanced the depression and prolonged the recovery time constant of eEPSCs from 589 ms to 876 ms, whereas application of ATP reduced the depression. On pairs of neurons which had reciprocal synapses, we found intense activities of glutamatergic synapse caused hetero-synaptic inhibition. This inhibition was blocked by RB-2 or Glu receptor antagonist DNQX.The effects of application of ATP and RB-2 on synaptic activity in pure neuron culture and mixed culture of neurons and glia cells were different, suggesting there was endogenous ATP release from glial cells in the mixed culture.Glu substantially induced ATP release from pure astrocytes, this can be blocked by DNQX. Although Glu induced mild ATP release from neurons, this is insensitive to DNQX. GABA did not induce ATP release from astrocytes. In acutely dissociated hippocampal slices, ATP still had dramatic inhibitory effect on the excitatory synaptic transmission (EPSP). CPT but not RB-2 increased EPSPs in slices, suggesting the effect of endogenous adenosine but not ATP. When the activity of ecto-nucleotidase was inhibited, RB-2 substantially facilitated the EPSP. After the function of glia was blocked by fluoroacetate (FAC), RB-2's facilitating effect on EPSP disappeared, suggesting that ATP comes from glial cells.Heterosynaptic suppression was also observed on Schaffer Collaterals. This phenomenon was mediated by adenosine. The suppression could be mediated by endogenous ATP after the activity of ecto-ATPase had been inhibited by ARL 67156. When glial cells were inhibited by FAC, CPT did not alter the heterosynaptic suppression any longer. These results suggest that endogenous adenosine, w...
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