Characterization of the adenosine receptors of the rat superior cervical ganglion

Abstract

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 microM. 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 microM. 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-dimethyl-8-phenylxanthine (8PT) and by the selective A1-adenosine receptor antagonist, 1,3-dipropyl-8-(4-((2-aminoethyl)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 DPCPXbut not by DMPX.6. It is concluded that the neurones of the rat SCG possess P1-purinoceptors of the Al-adenosine receptor subtype which mediate hyperpolarization and inhibition of depolarization caused by muscarine.