Evidence for P2-purinoceptor-mediated Inhibition of Noradrenaline Release in Rat Brain Cortex

1. Some postganglionic sympathetic axons possess P2Y-like P2-purinoceptors which, when activated, decrease the release of noradrenaline. We examined the question of whether such receptors also occur at the noradrenergic axons in the rat brain cortex. Slices of the brain cortex were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine (1 microM) and stimulated electrically, in most experiments by trains of 4 pulses/100 Hz. 2. The selective adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; 0.03-3 microM) as well as the non-subtype-selective agonist 5'-N-ethylcarboxamido-adenosine (NECA; 0.3-3 microM) reduced the evoked overflow of tritium, whereas the adenosine A2a-receptor agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.003-30 microM) and the adenosine A3-receptor agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA; 0.03-3 microM) caused no change. Of the nucleotides tested, ATP (30-300 microM), adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; 30-300 microM), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S; 30-300 microM), P1,P4-di(adenosine-5')-tetraphosphate (Ap4A; 30-300 microM) and the preferential P2Y-purinoceptor agonist, 2-methylthio-ATP (300 microM) decreased the evoked overflow of tritium. The P2X-purinoceptor agonist, alpha,beta-methylene-ATP (3-300 microM) caused no change. 3. The A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM) attenuated the effects of the nucleosides CPA (apparent pKB value 9.8) and NECA as well as of the nucleotides ATP (apparent pKB 9.3), ATP gamma S (apparent pKB 9.2) and ADP beta S (apparent pKB 8.7). CGS-21680 and APNEA were ineffective also in the presence of DPCPX. The A2-selective antagonist 1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KF-17837) reduced the effects of CPA, NECA and ATP gamma S only when given at a concentration of 300 nM but not at 1O nM.4. The P2-purinoceptor antagonists, suramin (300 micro M), reactive blue 2 (30 micro M) and cibacron blue 3GA(30 micro M) did not change the effect of CPA. Suramin and cibacron blue 3GA shifted the concentration response curve of ATP gamma S to the right (apparent pKB values 3.7 and 5.0, respectively). Reactive blue 2 also attenuated the effect of ATPyS, and cibacron blue 3GA attenuated the effect of ATP, but in these cases the agonist concentration-response curves were not shifted to the right. There was no antagonistic effect of suramin against ATP and ADP beta S.5. The results indicate that rat cerebrocortical noradrenergic axons possess, in addition to the knownadenosine Al-receptor, a separate purinoceptor for nucleotides (P2) which, in contrast to the Al-receptor,is blocked by suramin, reactive blue 2 and cibacron blue 3GA. Nucleotides such as ATP and ATP gamma S activate both receptors. Inconsistencies in antagonist effects against nucleotides are probably due to this activation of two receptors. The presynaptic P2-purinoceptor is P2Y-like, as it is in the peripheral sympathetic nervous system.