Evidence that prejunctional adenosine receptors regulating acetylcholine release from rat hippocampal slices are linked to an N-ethylmaleimide-sensitive G-protein, but not to adenylate cyclase or dihydropyridine-sensitive Ca2+-channeIs

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Abstract
Electrically evoked [3H]acetylcholine ([3H]ACh) release from slices of the rat hippocampus was reduced in a dose-dependent manner by the adenosine A1-receptor agonist R-phenylisopropyladenosine (R-PIA) in the concentration range 0.1-10 microM. The maximal effect was observed with 1 microM R-PIA. Treatment with N-ethylmaleimide (NEM, 100 microM, 10 min), which inactivates nucleotide-binding proteins (G-proteins), caused a slight increase in the basal overflow (0.17 +/- 0.01% v. 0.10 +/- 0.003% in the control slices), but did not affect stimulated release (0.73 +/- 0.05% vs. 0.74 +/- 0.03% in the control slices). N-ethylmaleimide pretreatment significantly reduced the prejunctional inhibitory effect of R-PIA on [3H]ACh release in a non-competitive manner. The S2/S1 ratio was 0.92 +/- 0.03 in controls and was reduced to 0.32 +/- 0.02 by 1 microM R-PIA in the control slices and to 0.57 +/- 0.03 after NEM pretreatment. Stimulation of cyclic AMP-accumulation by forskolin (1 microM) and rolipram (30 microM) before the second stimulation (S2) enhanced the S2/S1 ratio by about 30% to 1.26 +/- 0.12, but did not reduce the inhibitory effect of R-PIA (1 microM). The Ca2+-channel agonist Bay K 86(44) (1 microM), a concentration that increases K+-evoked noradrenaline release, did not affect the basal or electrically evoked [3H]ACh overflow, or the prejunctional effects of R-PIA (0.1 and 1 microM) on [3H]ACh release. Our results suggest that the presynaptic inhibitory effects of A1-receptor agonists on [3H]ACh release are exerted via a nucleotide-binding protein that can be inhibited by NEM.(ABSTRACT TRUNCATED AT 250 WORDS)

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