Regulation of muscarinic acetylcholine receptor‐mediated synaptic responses by adenosine receptors in the rat hippocampus

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 γ‐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 (+)‐α‐methyl‐4‐carboxyphenylglycine ((+)‐MCPG; 1000μ m ) but was greatly enhanced by the acetyl‐cholinesterase inhibitor physostigmine (1–5 μ m ). 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 μ m ). These results are consistent with these effects being mediated by mAChRs. 3 Both the mAChR‐mediated EPSP (EPSP m ) and the associated reduction in spike frequency adaptation were reversibly depressed (up to 97%) by either adenosine (100 μ m ) or its non‐hydrolysable analogue 2‐chloroadenosine (CADO; 0.1–5.0 μ m ). These effects were often accompanied by postsynaptic hyperpolarization (up to 8 mV) and a reduction in input resistance (up to 11%). The selective adenosine A 1 receptor agonists 2‐chloro‐ N 6 ‐cyclopentyl‐adenosine (CCPA; 0.1–0.4 μ m ) and R (−) N 6 ‐(2‐phenylisopropyl)‐adenosine (R‐PIA; 1 μ m ) both depressed the EPSP m . In contrast, the adenosine A 2A receptor agonist 2‐ p ‐(2‐carboxyethyl)‐phenethylammo‐5′‐ N ‐ethylcarboxamidoadenosine (CGS 21680; 0.5–1.0 μ m ) did not significantly affect the EPSP m . 4 The selective adenosine A 1 receptor antagonist 8‐cyclopentyl‐l,3‐dipropylxanthine (DPCPX; 0.2 μ m ) fully reversed the depressant effects of both adenosine (100 μ m ) and CADO (1 μ m ) on the EPSP m and the stimulus‐evoked reductions in spike frequency adaptation. 5 DPCPX (0.2 μ m ) alone caused a small but variable mean increase in the EPSP m of 22 ± 19% and enabled activation of an EPSP m by a previously subthreshold stimulus. In contrast, the selective adenosine kinase inhibitor 5‐iodotubercidin (5‐IT; 10 μ m ) inhibited the EPSP m by 74 ± 10%, an effect that was reversed by DPCPX. 6 The concentration–response relationship for the depressant action of CADO on the EPSP m more closely paralleled that for its presynaptic depressant action on glutamate‐mediated EPSPs than that for postsynaptic hyperpolarization. The respective mean IC 50 and EC 50 concentrations for these effects were 0.3, 0.8 and 3.0 μ m . 7 CADO (1–5 μ m ) 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 μ m ). All these effects were abolished by atropine (1 μ m ). 8 These data provide good evidence for an adenosine A 1 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.