P2-RECEPTOR-MEDIATED INHIBITION OF NORADRENALINE RELEASE IN THE RAT HIPPOCAMPUS

Experiments on hippocampal slices were carried out in order to find ou t whether the release of noradrenaline in the hippocampus can be modul ated through P2-receptors. The slices were preincubated with [H-3]-nor adrenaline, superfused with medium containing desipramine (1 mu M), an d stimulated electrically, in most experiments by 4 pulses/100 Hz. The adenosine A(1)-receptor agonist N-6-cyclopentyladenosine (CPA) and th e nucleotides ATP, adenosine-5'O-(2-thiotriphosphate) (ATP gamma S) an d adenosine-5'-O-(2-thiodiphosphate) (ADP beta S) decreased the evoked overflow of tritium by up to 55 %. The adenosine A(2a)-agonist 2-p-(2 -carboxyethyl)-phenethylamino-5 '-N-ethylcarbox-amido-adenosine (CGS 2 1680; 0.003-0.3 mu M) caused no change. The concentration-response cur ve of CPA was shifted to the right by the A(1)-antagonist 8-cyclopenty l-1,3-dipropylxanthine (DPCPX; 3 nM) but not by the P2-receptor antago nists cibacron blue 3GA (30 mu M) and reactive blue 2 (30 mu M); the a pparent pK(B) value of DPCPX against CPA was 9.0. In contrast, the con centration-response curve of ATP was shifted to the right by DPCPX (3 nM), apparent pK(B) 8.7, as well as by cibacron blue 3GA (30 mu M), ap parent pK(B) 5.2, and reactive blue 2 (30 mu M), apparent pK(B) 5.6; t he antagonist effects of DPCPX and cibacron blue 3GA were additive in a manner compatible with the blockade of two separate receptors for AT P. The same pattern was obtained with ATP gamma S: its concentration-r esponse curve was shifted to the right by DPCPX as well as by cibacron blue 3GA and reactive blue 2. Suramin (300 mu M) antagonized neither the effect of ATP nor that of ATP gamma S. The 5'-nucleotidase inhibit or alpha,beta-methylene-ADP (100 mu M) did not change the effect of AT P. Only cibacron blue 3GA (30 mu M) but not reactive blue 2 (30 mu M), given alone, consistently caused a small increase of the evoked overf low of tritium. Hippocampal slices degraded exogenous ATP, and this de gradation was reduced by cibacron blue 3GA (30 mu M), reactive blue 2 (30 mu M) and suramin (300 mu M).The results indicate that the noradre nergic terminal axons of the rat hippocampus possess P2-receptors in a ddition to the known A(1)-adenosine receptors. The presynaptic P2-rece ptors mediate an inhibition of noradrenaline release, are activated by nucleotides but not nucleosides, and are blocked by cibacron blue 3GA and reactive blue 2. ATP and ATP gamma S act at both the A(1)- and th e P2-receptors. An autoreceptor function of cerebral presynaptic P2-re ceptors remains doubtful.