Characteristics of A|1 and A|2 adenosine receptors upon the acetylcholine release in the rat hippocampus

ABSTRACT

As it has been reported that the depolarization induced acetylcholine (ACh) release is modulated by activation of presynaptic A1 adenosine heteroreceptor and various lines of evidence suggest the A2 adenosine receptor is present in the hippocampus. The present study was undertaken to delineate the role of adenosine receptors on the hippocampal ACh release. Slices from the rat hippocampus were equilibrated with (3H)choline and then the release amount of the labelled product, (3H)ACh, which was evoked by electrical stimulation (rectangular pulses, 3 Hz, 2 ms, 24 mA, 5 V/cm-1, 2 min), was measured, and the influence of various adenosine receptor-related agents on the evoked tritium outflow was investigated. And also, the drug-receptor binding assay was performed in order to confirm the presence of A1 and A2 adenosine receptors in the rat hippocampus. N-ethylcarboxamidoadenosine (NECA), a potent adenosine receptor agonist with nearly equal affinity at A1 and A2 adenosine receptors, in concentrations ranging from 1apprx30 muM, decreased the electrically-evoked (3H)ACh release in a concentration-dependent manner without affecting the basal rate of release. And the effect of NECA was significantly inhibited by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 2 micrometer), a selective A1 adenosine receptor antagonist, but was not influenced by 3,7-dimethyl-1-propargylxanthine (DMPX, 5 micrometer, a specific A2 adenosine receptor antagonist. N6-Cyclopentyladenosine (CPA), a selective A1 adenosine receptor agonist, in doses ranging from 0.1 to 10 micrometer, reduced evoked (3H)ACh release in a dose-dependent manner without the change of the basal release. And the effect of CPA was significantly inhibited by 2 micrometer DPCPX treatment. 2-P-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680C), a potent A2 adenosine receptor agonist, in concentrations ranging from 0.1 to 10 micrometer, did not alter the evoked ACh release. In the drug-receptor binding assay, the binding of (3H)2-chloro-N6-Cyclopentyladenosine ((3H)CCPA) to the- A1 adenosine receptor of rat hippocampal membranes was inhibited by CPA (Ki = 1.22 nM), NECA (Ki=10.17 nM) and DPCPX (Ki-161.86 nM), but not by CGS-21680C (Ki=2,380 nM) and DMPX (Ki-22,367 nM). However, the specific binding of (3H)CGS-21680C to the A2 adenosine receptor was not observed. These results suggest that the A1 adenosine heteroreceptor play an important role in evoked ACh release, but the presence of A2 adenosine receptor is not confirmed in this study.