Adenosine: an Endogenous Modulator of Hippocampal Noradrenaline Release

In slices of hippocampus from the rabbit, preincubated with [3H]noradrenaline and then continuously superfused, the modulation of the release of noradrenaline by adenosine receptors was studied. Electrical field stimulation of the slices elicited a release of [3H]noradrenaline which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. From the order of potency: cyclohexyladenosine greater than (-)phenylisopropyladenosine [(-)PIA] greater than 5'-N-ethylcarboxamide-adenosine (NECA) greater than 2-chloro-adenosine greater than adenosine (+)phenylisopropyladenosine greater than ATP, the inhibitory adenosine receptor was classified as A1- (Ri-) receptor. The effect of the agonist was strongly reduced by adenosine receptor antagonists, the methylxanthines. A role for endogenous adenosine in the modulation of hippocampal noradrenaline release is supported by these findings: (1) that blockade of adenosine receptors by methylxanthines, especially by 8-phenyltheophylline, increased, whereas (2) inhibition of the uptake of adenosine decreased the evoked release of noradrenaline and (3) that deamination of endogenous extracellular adenosine by addition of adenosine deaminase to the medium enhanced the evoked transmitter release. Inhibitors of endogenous adenosine deaminase and 5'-nucleotidase were without effect. It is concluded that release of noradrenaline in the hippocampus is inhibited at the level of the noradrenergic nerve terminals by endogenous adenosine via A1 (or Ri) receptors.