STRUCTURE-ACTIVITY RELATIONSHIP OF A PYRIMIDINE RECEPTOR IN THE RAT ISOLATED SUPERIOR CERVICAL-GANGLION

1 The effects of pyrimidines and purines on the d.c. potential of the rat isolated superior cervical ganglion (SCG) have been examined by a grease-gap technique to determine the structure-activity requirements of the receptor activated by pyrimidines, i.e. a pyrimidinoceptor. 2 5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl (ZTP), the pyrimidines, cytidine 5'-triphosphhate (CTP), uridine 5'-triphosphate (UTP) and thymidine 5'-triphosphate (TTP) and the purines, adenosine 5'-triphosphate (ATP; in the presence of an A(1)-purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1 mu M)), adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), guanosine 5'-triphosphate (GTP), inosine 5'-triphosphate (ITP) depolarized ganglia in a concentration-dependent manner. The relative order of ZTP and purine S-triphosphates in depolarizing ganglia was ZTP greater than or equal to ATP gamma S >> ATP greater than or equal to ITP=GTP, and for the pyrimidine 5'-triphosphates UTP > TTP greater than or equal to CTP. Depolarizations evoked by ATP gamma S were followed by concentration-dependent hyperpolarizations at 100 and 1000 mu M. 3 At concentrations of between 0.1 mu M and 1 mM, uridine 5'-diphosphate (UDP), uridine 5'-diphosphoglucose (UDPG) and uridine 5'-diphosphoglucuronic acid (UDPGA) evoked significant and concentration-dependent depolarizations, whereas uridine 5'-monophosphate (UMP), uridine and uracil were inactive or produced small (<45 mu V) depolarizations. The relative order of potency of uridine analogues in depolarizing ganglia was UDP greater than or equal to UTP > UDPG > UDPGA >> uracil greater than or equal to UMP = pseudouridine greater than or equal to uridine. At 3 and 10 mM, uridine produced concentration-dependent hyperpolarizations. Nikkomycin Z, a nucleoside resembling UTP (viz. the triphosphate chain at the 5'-position on the ribose moiety being replaced by a peptide), was inactive between 1 mu M and 1 mM. Generally, a concentration of 10 mM was required before thymidine, 6-azathymine, 6-azauracil or 6-azauridine depolarized ganglia. 4 Suramin (300 mu M), a P-2-purinoceptor antagonist, significantly depressed depolarizations evoked by alpha,beta-methylene-ATP (alpha,beta-MeATP; 100 mu M), ATP gamma S (100 mu M), CTP (1 mM), GTP (1 mM), ZTP (30 mu M) and ATP (300 mu M) in the presence of DPCPX (1 mu M). Suramin reversed a small depolarization evoked by UMP (1 mM) into a small hyperpolarization. In contrast depolarizations evoked by UDP, UTP, UDPG (all at 100 mu M) and TTP (300 mu M) were unaltered or enhanced by suramin. 5 It is concluded that the rat SCG contains distinct nucleotide receptors including a P-2-purinoceptor (activated by alpha,beta-MeATP, ATP, GTP, ITP and ZTP) and a pyrimidinoceptor (activated by UTP, UDP, UDPG, UDPGA and TTP). The pyrimidinoceptor on rat SCG neurones had specific structure activity requirements with the di- and triphosphates of uridine being the most effective depolarizing agonists examined.