adenosine-kinase and 2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline

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 EPS

Topics: Adenosine; Adenosine Kinase; Animals; Atropine; Cyclic AMP; Dose-Response Relationship, Drug; Electrophysiology; Excitatory Amino Acid Antagonists; Female; GABA Antagonists; Glutamic Acid; Hippocampus; Membrane Potentials; Parasympatholytics; Parasympathomimetics; Physostigmine; Quinoxalines; Rats; Rats, Wistar; Receptors, Muscarinic; Receptors, Purinergic P1; Synaptic Membranes; Synaptic Transmission; Xanthines