Adenosine transport inhibitors enhance high K(+)-evoked taurine release from rat hippocampus

Modulation of the ischemia-induced taurine release by adenosine receptors in the developing and adult mouse hippocampus

The release of the inhibitory amino acid taurine is markedly enhanced under ischemic conditions in both adult and developing hippocampus, together with a pronounced increase in the release of excitatory amino acids and the neuromodulator adenosine. We studied the effects of adenosine receptor agonists and antagonists as well as adenosine transport inhibitors on hippocampal [(3)H]taurine release in normoxia and ischemia, using a superfusion system. Under standard conditions the adenosine A(1) receptor agonists N(6)-cyclohexyladenosine and R(-)N(6)-(2-phenylisopropyl)adenosine potentiated basal taurine release in developing mice and depressed the release in adults in a receptor-mediated manner. Adenosine A(2) receptor compounds had only minor effects on the basal release and the K(+)-stimulated release was not affected by these drugs. The adenosine uptake inhibitor dipyridamole enhanced basal taurine release in the developing hippocampus and reduced it in the adult. In ischemia the adenosine compounds had no marked effects on taurine release in immature animals, whereas A(1) receptor activation was still able to evoke taurine release in adults by a receptor-mediated mechanism. The results show that the basal release of taurine is modulated by A(1) receptors in both mature and immature hippocampus, whereas in ischemia these receptors potentiate taurine release only in adults. The elevated taurine levels together with the depression of excitatory amino acid release by adenosine receptor activation could be beneficial under ischemic conditions, protecting neural cells against excitotoxicity and hyperexcitation.

Distribution of equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporters (ENT1) in brain

Nucleoside transport processes may play a role in regulating endogenous levels of the inhibitory neuromodulator adenosine in brain. The cDNAs encoding species homologues of one member of the equilibrative nucleoside transporter (ENT) gene family have recently been isolated from rat (rENT1) and human (hENT1) tissues. The current study used RT-PCR, northern blot, in situ hybridization, and [3H]nitrobenzylthioinosine autoradiography to determine the distribution of mRNA and protein for ENT1 in rat and human brain. Northern blot analysis indicated that hENT1 mRNA is widely distributed in adult human brain. 35S-labeled sense and antisense riboprobes, transcribed from a 153-bp segment of rENT1, were hybridized to fresh frozen coronal sections from adult rat brain and revealed widespread rENT1 mRNA in pyramidal neurons of the hippocampus, granule neurons of the dentate gyrus, Purkinje and granule neurons of the cerebellum, and cortical and striatal neurons. Regional localization in rat brain was confirmed by RT-PCR. Thus, ENT1 mRNA has a wide cellular and regional distribution in brain, indicating that this nucleoside transporter subtype may be important in regulating intra- and extracellular levels of adenosine in brain.

Activation of adenosine A2 receptors enhances high K(+)-evoked taurine release from rat hippocampus: a microdialysis study

The present study was designed to examine which type of adenosine receptors was involved in enhancement of high K(+)-evoked taurine release from in vivo rat hippocampus using microdialysis. Perfusion with 0.5 or 5.0 mM adenosine enhanced high K(+)-evoked taurine release. Perfusion with 2 microM R(-)-N6-2-phenylisopropyladenosine (PIA), a selective adenosine A1 receptor agonist, did not modulate taurine release. Perfusion with 1 microM 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, increased taurine release. On the other hand, perfusion with 20 microM 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethyl-carboxamide-adenos ine (CGS21680), a selective adenosine A2A receptor agonist, enhanced taurine release, while perfusion with 1 mM 3,7-dimethyl-propagylxanthine (DMPX), an adenosine A2 receptor antagonist, did not affect taurine release. These results demonstrate that adenosine enhances high K(+)-evoked taurine release via activation of adenosine A2A receptors from both neurons and glial cells of in vivo rat hippocampus.