title: Regulation of muscarinic acetylcholine receptor-mediated synaptic responses by adenosine receptors in the rat hippocampus.
abstract: 1. Intracellular current clamp recordings were made from CA1 pyramidal neurones in rat hippocampal slices. Experiments were performed in the presence of ionotropic glutamate receptor antagonists and gamma-aminobutyric acid (GABA) receptor antagonists to block all fast excitatory and inhibitory synaptic transmission. A single stimulus, delivered extracellularly in the stratum oriens, caused a reduction in spike frequency adaptation in response to a depolarizing current step delivered 2 s after the stimulus. A 2- to 10-fold increase in stimulus intensity evoked a slow excitatory postsynaptic potential (EPSP) which was associated with a small increase in input resistance. The peak amplitude of the EPSP occurred approximately 2.5 s after the stimulus and its magnitude (up to 30 mV) and duration (10-50 s) increased with increasing stimulus intensity. 2. The slow EPSP was unaffected by the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG; 1000 microM) but was greatly enhanced by the acetylcholinesterase inhibitor physostigmine (1-5 microM). Both the slow EPSP and the stimulus-evoked reduction in spike frequency adaptation were inhibited by the muscarinic acetylcholine receptor (mAChR) antagonist atropine (1-5 microM). These results are consistent with these effects being mediated by mAChRs. 3. Both the mAChR-mediated EPSP (EPSPm) and the associated reduction in spike frequency adaptation were reversibly depressed (up to 97%) by either adenosine (100 microM) or its non-hydrolysable analogue 2-chloroadenosine (CADO; 0.1-5.0 microM). These effects were often accompanied by postsynaptic hyperpolarization (up to 8 mV) and a reduction in input resistance (up to 11%). The selective adenosine A1 receptor agonists 2-chloro-N6-cyclopentyladenosine (CCPA; 0.1-0.4 microM) and R(-)N6-(2-phenylisopropyl)-adenosine (R-PIA; 1 microM) both depressed the EPSPm. In contrast, the adenosine A2A receptor agonist 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 0.5-1.0 microM) did not significantly affect the EPSPm. 4. The selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM) fully reversed the depressant effects of both adenosine (100 microM) and CADO (1 microM) on the EPSPm and the stimulus-evoked reductions in spike frequency adaptation. 5. DPCPX (0.2 microM) alone caused a small but variable mean increase in the EPSPm of 22 +/- 19% and enabled activation of an EPSPm by a previously subthreshold stimulus. In contrast, the selective adenosine kinase inhibitor 5-iodotubercidin (5-IT; 10 microM) inhibited the EPSPm by 74 +/- 10%, an effect that was reversed by DPCPX. 6. The concentration-response relationship for the depressant action of CADO on the EPSPm more closely paralleled that for its presynaptic depressant action on glutamate-mediated EPSPs than that for postsynaptic hyperpolarization. The respective mean IC50 and EC50 concentrations for these effects were 0.3, 0.8 and 3.0 microM. 7. CADO (1-5 microM) did not have a significant effect on the postsynaptic depolarization, increase in input resistance and reduction in spike frequency adaptation evoked by carbachol (0.5-3.0 microM). All these effects were abolished by atropine (1 microM). 8. These data provide good evidence for an adenosine A1 receptor-mediated inhibition of mAChR-mediated synaptic responses in hippocampal CA1 pyramidal neurones. This inhibition is mediated predominantly presynaptically, is active tonically and can be enhanced when extracellular levels of endogenous adenosine are raised.
authors.affiliation: Department of Pharmacology, University of Edinburgh, UK. Robin@srv0.phm.ed.ac.uk
authors.forename: R A
authors.initials: RA
authors.lastname: Morton
authors.forename: C H
authors.initials: CH
authors.lastname: Davies
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D000248:Adenosine Kinase / Q000037:antagonists & inhibitors; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D018756:GABA Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D006624:Hippocampus / Q000737:chemistry* / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D010276:Parasympatholytics / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology; D010830:Physostigmine / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D011976:Receptors, Muscarinic / Q000502:physiology*; D018047:Receptors, Purinergic P1 / Q000502:physiology*; D013570:Synaptic Membranes / Q000737:chemistry / Q000187:drug effects / Q000201:enzymology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D014970:Xanthines / Q000494:pharmacology
chemical_list: C106363:1-(5-phosphoribosyl)-2'-phosphoadenosine 5'-phosphate cyclic anhydride; D018691:Excitatory Amino Acid Antagonists; D018756:GABA Antagonists; D010276:Parasympatholytics; D010277:Parasympathomimetics; D011810:Quinoxalines; D011976:Receptors, Muscarinic; D018047:Receptors, Purinergic P1; D014970:Xanthines; C062865:2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline; C058394:2-chloro-N(6)cyclopentyladenosine; D018698:Glutamic Acid; D001285:Atropine; C051360:1,3-dipropyl-8-cyclopentylxanthine; D010830:Physostigmine; D000242:Cyclic AMP; D000248:Adenosine Kinase; D000241:Adenosine