EVIDENCE FOR P-2-PURINOCEPTOR-MEDIATED INHIBITION OF NORADRENALINE RELEASE IN RAT-BRAIN CORTEX

1 Some postganglionic sympathetic axons possess P-2y-like P-2-purinoce ptors which, when activated, decrease the release of noradrenaline. We examined the question of whether such receptors also occur at the nor adrenergic axons in the rat brain cortex. Slices of the brain cortex w ere preincubated with [H-3]-noradrenaline, then superfused with medium containing desipramine (1 mu M) and stimulated electrically, in most experiments by trains of 4 pulses/100 Hz. 2 The selective adenosine A( 1)-receptor agonist, N-6-cyclopentyl-adenosine (CPA; 0.03-3 mu M) as w ell as the non-subtype-selective agonist 5'-N-ethylcarboxamido-adenosi ne (NECA; 0.3-3 mu M) reduced the evoked overflow of tritium, whereas the adenosine A(2a)-receptor agonist, yl)-phenethylamino-5'-N-ethylcar boxamido-adenosine (CGS-21680; 0.003-30 mu M) and the adenosine A(3)-r eceptor agonist N-6-2-(4-aminophenyl)ethyl-adenosine (APNEA; 0.03-3 mu M) caused no change. Of the nucleotides tested, ATP (30-300 mu M), ad enosine-5'-O-(3-thiotriphosphate) (ATP gamma S; 30-300 mu M), adenosin e-5'-O-(2-thiodiphosphate) (ADP beta S; 30-300 mu M), P-1,P-4-di(adeno sine-5')-tetraphosphate (Ap(4)A; 30-300 mu M) and the preferential P-2 Y-purinoceptor agonist, 2-methylthio-ATP (300 mu M) decreased the evok ed overflow of tritium. The P-2X-purinoceptor agonist, alpha,beta-meth ylene-ATP (3-300 mu M) caused no change. 3 The A(1)-selective antagoni st, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM) attenuated the e ffects of the nucleosides CPA (apparent pK(B) value 9.8) and NECA as w ell as of the nucleotides ATP (apparent pK(B) 9.3), ATP gamma S (appar ent pK(B) 9.2) and ADP beta S (apparent pK(B) 8.7). CGS-21680 and APNE A were ineffective also in the presence of DPCPX. The A(2)-selective a ntagonist -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 10 nM. 4 The P-2-purinoceptor ant agonists, suramin (300 mu M), reactive blue 2 (30 mu M) and cibacron b lue 3GA (30 mu M) did not change the effect of CPA. Suramin and cibacr on blue 3GA shifted the concentration-response curve of ATP gamma S to the right (apparent pK(B) values 3.7 and 5.0, respectively). Reactive blue 2 also attenuated the effect of ATP gamma S, and cibacron blue 3 GA attenuated the effect of ATP, but in these cases the agonist concen tration-response curves were not shifted to the right. There was no an tagonistic effect of suramin against ATP and ADP beta S. 5 The results indicate that rat cerebrocortical noradrenergic axons possess, in add ition to the known adenosine A(1)-receptor, a separate purinoceptor fo r nucleotides (P-2) which, in contrast to the A(1)-receptor, is blocke d by suramin, reactive blue 2 and cibacron blue 3GA. Nucleotides such as ATP and ATP gamma S activate both receptors. Inconsistencies in ant agonist effects against nucleotides are probably due to this activatio n of two receptors. The presynaptic P-2-purinoceptor is P-2Y-like, as it is in the peripheral sympathetic nervous system.