The adenosine receptor-mediated inhibition of noradrenaline release possibly involves an N-protein and is increased by alpha 2-autoreceptor blockade.

The stimulation-evoked overflow of [3H]-noradrenaline from slices of the rabbit hippocampus is inhibited by alpha 2-autoreceptors as well as by adenosine (A1)-receptors. Slices of rabbit hippocampus were labelled with [3H]-noradrenaline, superfused continuously and stimulated twice electrically (rectangular pulses; 2 ms, 3 Hz, 24 mA, 5 V cm-1). Treatment of hippocampal slices with N-ethylmaleimide (NEM, 30 microM; 30 min), which functionally disturbs certain N-proteins, decreased the inhibitory action of adenosine receptor agonists like (-)-N6-(R-phenylisopropyl)-adenosine ((-)-PIA) and adenosine on noradrenaline release. Release inhibition caused by (-)-PIA (0.03-1 microM) was antagonized by NEM in a non-competitive manner in the absence and in the presence of the alpha 2-adrenoceptor antagonist yohimbine. The adenosine receptor antagonist 8-phenyltheophylline significantly increased the evoked noradrenaline release by about 15% in control slices by diminishing the inhibitory action of endogenous adenosine. In NEM-treated slices this effect of 8-phenyltheophylline was not seen. In the presence of (-)-PIA (0.1 microM), i.e. under conditions of an increased inhibitory tone, release facilitation by 8-phenyltheophylline was decreased by NEM compared to that in the respective controls. Occupation of the A1-receptor with (-)-PIA prior to and during the NEM treatment did not protect the A1-receptor-coupled signal transduction system from being affected by NEM. In the presence of the alpha 2-adrenoceptor antagonist yohimbine, the inhibitory action of (-)-PIA was strongly increased. The above results suggest the involvement of a regulatory N-protein in the A1-receptor-mediated inhibition of noradrenaline release and an interaction between the alpha 2-autoreceptor and the A1-receptor-coupled signal transduction system, possibly at the level of a N-protein.