Excitatory and Inhibitory Effects of A₁ and A_2A Adenosine Receptor Activation on the Electrically Evoked [³H]Acetylcholine Release from Different Areas of the Rat Hippocampus

Abstract

The modulation by adenosine analogues and endogenous adenosine of the electrically evoked release of [3H]acetylcholine ([3H]ACh) was compared in subslices of the three areas of the rat hippocampus (CA1, CA3, and dentate gyrus). The mixed A1/A2 agonist 2‐chloroadenosine (CADO; 2–10 µM) inhibited, in a concentration‐dependent manner, the release of [3H]ACh from the three hippocampal areas, being more potent in the CA1 and CA3 areas than in the dentate gyrus. The inhibitory effect of CADO (5 µM) on [3H]ACh release was prevented by the A1 antagonist 1,3‐dipropyl‐8‐cyclopentylxanthine (DPCPX; 50 nM) in the three hippocampal areas and was converted in an excitatory effect in the CA3 and dentate gyrus areas. The A2A agonist CGS‐21680 (30 nM) produced a greater increase of the evoked release of [3H]ACh in the CA3 than in the dentate gyrus areas, whereas no consistent effect was found in the CA1 area or in the whole hippocampal slice. The excitatory effect of CGS‐21680 (30 nM) in the CA3 area was prevented by the adenosine receptor antagonist 3,7‐dimethyl‐1‐propargylxanthine (10 µM). Both adenosine deaminase (2 U/ml) and DPCPX (250 nM) increased the evoked release of [3H]ACh in the CA1 and CA3 areas but not in the dentate gyrus. The amplitude of the effect of DPCPX and adenosine deaminase was similar in the CA1 area, but in the CA3 area DPCPX produced a greater effect than adenosine deaminase. It is concluded that the electrically evoked release of [3H]ACh in the three areas of the rat hippocampus can be differentially modulated by adenosine. In the CA1 area, only A1 inhibitory receptors modulate ACh release, whereas in the CA3 area, both A2A excitatory and A1 inhibitory adenosine receptors modulate ACh release. In the dentate gyrus, both A1 inhibitory and A2A excitatory adenosine receptors are present, but endogenous adenosine does not activate them.

Transducing system operated by adenosine A2A receptors to facilitate acetylcholine release in the rat hippocampus

“…As illustrated in Fig. 2A, 2-chloroadenosine (10 AM) inhibited the evoked release of acetylcholine, in accordance with previous observations in hippocampal slices (Cunha et al, 1994a). This inhibitory effect was abolished by the adenosine A 1 receptor selective antagonist, DPCPX (20 nM), indicating that it is an adenosine A 1 receptor-mediated effect.…”

“…3 H]actylcholine in the total tritium outflow, [ 3 H]acetylcholine was separated using a cation exchanger, tetraphenylboron, after phosphorylation of [ 3 H]choline, as previously described (Cunha et al, 1994a).…”

“…As occurs for the modulation of glutamate (Ambrósio et al, 1997;Lopes et al, 2002) and serotonin release (Okada et al, 2001), the modulation of acetylcholine release by adenosine in the hippocampus is also controlled by both adenosine A 1 and A 2A receptors (Cunha et al, 1994a;Jin and Fredholm, 1997). The presence of adenosine A 2A receptors in the hippocampus has been confirmed using molecular biology studies, like polymerase chain reaction (PCR), in situ hybridization (Cunha et al, 1994b) and Northern blot analysis (Peterfreund et al, 1996), but binding studies have revealed an atypical binding pattern (Cunha et al, 1996) and G protein coupling (Cunha et al, 1999) of these putative adenosine A 2A receptors.…”

Homo‐ and heteroexchange of adenine nucleotides and nucleosides in rat hippocampal slices by the nucleoside transport system

“…Extracellular ATP -activating P2X receptors located in the postsynaptic sites (Rubio & Soto, 2001) -takes part in the excitatory transmission and synaptic plasticity in CA1 (Pankratov et al, 1998(Pankratov et al, , 2002 and CA3 regions (Mori et al, 2001) and regulates the release of glutamate and GABA via P2X 7 receptors expressed on the presynaptic sites (Sperla´gh et al, 2002). Extraneuronal ATP is inactivated by the ectonucleotidase enzyme cascade which in turn gives rise to the formation of extracellular adenosine acting as a neuroprotective modulator, inhibiting neuronal discharge and modulating the release of various transmitters via A 1 and A 2A receptors respectively (Cunha et al, 1994;Cunha & Ribeiro, 2000;Cunha, 2001a;Masino et al, 2002). In line with this concept, the frequency-dependent contribution of ATP to extracellular adenosine levels (Cunha et al, 1996) and the rapid and highly effective in situ formation of adenosine from pressure-ejected ATP has been shown in earlier studies (Dunwiddie et al, 1997).…”

Input-Specific Modulation of Neurotransmitter Release in the Lateral Horn of the Spinal Cord via Adenosine Receptors

“…Other studies have reported that A1 and A2 A receptors are often expressed in the same regions [e.g., hippocampus (Cunha et al, 1994) and neuromuscular junction ] and indeed on the same nerve terminals at the neuromuscular junction . The coexistence of both subtypes of high-affinity adenosine receptors leads to the question of which receptor adenosine preferentially activates.…”