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Vol. 303, Issue 1, 356-363, October 2002
Department of Pharmacology and Program in Neuroscience, University
of Colorado Health Sciences Center, Denver, Colorado
Excitatory glutamatergic synapses in the hippocampal CA1 region of rats
are potently inhibited by purines, including adenosine, ATP, and ATP
analogs. Adenosine A1 receptors are known to mediate at
least part of the response to adenine nucleotides, either because adenine nucleotides activate A1 receptors directly, or
activate them secondarily upon the nucleotides' conversion to
adenosine. In the present studies, the inhibitory effects of adenosine,
ATP, the purportedly stable ATP analog
adenosine-5'-O-(3-thio)triphosphate (ATP
S), and
cyclic AMP were examined in mice with a null mutation in the adenosine
A1 receptor gene. ATP
S displaced the binding of
A1-selective ligands to intact brain sections and brain
homogenates from adenosine A1 receptor wild-type animals.
In homogenates, but not in intact brain sections, this displacement was
abolished by adenosine deaminase. In hippocampal slices from wild-type
mice, purines abolished synaptic responses, but slices from mice
lacking functional A1 receptors showed no synaptic
modulation by adenosine, ATP, cAMP, or ATP
S. In slices from
heterozygous mice the dose-response curve for both adenosine and ATP
was shifted to the right. In all cases, inhibition of synaptic
responses by purines could be blocked by prior treatment with the
competitive adenosine A1 receptor antagonist
8-cyclopentyltheophylline. Taken together, these results show that even
supposedly stable adenine nucleotides are rapidly converted to
adenosine at sites close to the A1 receptor, and that
inhibition of synaptic transmission by purine nucleotides is mediated
exclusively by A1 receptors.
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