CHARACTERIZATION OF THE ADENOSINE RECEPTORS OF THE RAT SUPERIOR CERVICAL-GANGLION

1 Adenosine analogues caused hyperpolarization and inhibition of the depolarizing response to muscarine of the rat isolated superior cervical ganglion (SCG) measured by a 'grease gap' recording technique. The receptors mediating these responses have been characterized by use of a range of selective adenosine analogues and adenosine receptor antagonists. 2 In decreasing order of potency N6-cyclopentyladenosine (CPA), 2-chloroadenosine (2CA), adenosine, 2-phenylaminoadenosine (PAA), caused concentration-dependent hyperpolarizations whilst N6-(9-fluorenylmethyl)adenosine (PD 117,413) was inactive at up to 100 muM. 3 The order of potency of adenosine analogues in depressing depolarization caused by a submaximal concentration of muscarine (100 nM) was: CPA>R-PIA = 2CA>NECA>S-PIA>BZA>adenosine>PAA, where R- and S-PIA = R(-)- and S(+)-N6-(2-phenylisopropyl)adenosine, NECA = 5'N-ethylcarboxamidoadenosine and BZA = N6-benzyladenosine. PD 117,413 was inactive at concentrations up to 100 muM. The maximum inhibitions of the muscarine-induced depolarization by CPA, 2CA, NECA and BZA were similar. R-PIA, S-PIA and PAA produced similar maximal inhibitions which were significantly smaller than those produced by CPA. 4 Hyperpolarizations caused by adenosine were antagonized by the P1-purinoceptor selective antagonist 1,3-dimethy1-8-phenylxanthine (8PT) and by the selective A1-adenosine receptor antagonist, 1,3-dipropyl-8-(4-((2aminoethyl)amino)carbonylmethyloxyphenyl)xanthine (XAC). Hyperpolarizations caused by CPA, adenosine and PAA were antagonized by the A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2-selective antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX). 5 Inhibition of the muscarinic-induced depolarization by CPA was antagonized by 8PT and DPCPX but not by DMPX. 6 It is concluded that the neurones of the rat SCG possess P1-purinoceptors of the A1-adenosine receptor subtype which mediate hyperpolarization and inhibition of depolarization caused by muscarine.