ADENOSINE-A1-RECEPTOR-MEDIATED INHIBITION OF EVOKED ACETYLCHOLINE-RELEASE IN THE RAT HIPPOCAMPUS DOES NOT DEPEND ON PROTEIN-KINASE-C

The possible involvement of protein kinase C and/or a lipoxygenase product in the mechanism by which adenosine inhibits release of [3H]acetylcholine evoked by electrical pulses from [3H]choline-labelled hippocampal slices was examined. For comparison, the muscarinic autoreceptors were examined using carbachol. The order of potency of adenosine analogues (CHA = R-PIA > NECA >> CGS 21680, CV 1808) indicates that the adenosine receptor responsible is of the A1 subtype. Adenosine (10 μM) and R-PIA (0.1 μM) were virtually equiactive as inhibitors and were antagonized to an equal extent by 8-CPT with a potency (IC50 approximately 25 nM) which is also compatible with A1-receptor mediation. The effects of carbachol and of R-PIA were not antagonized by the lipoxygenase inhibitor NDGA (10 or 50 μM). Stimulation of protein kinase C by the phorbol ester 4β-phorbol 12,13-dibutyrate caused a concentration-dependent increase in stimulation-evoked 3H overflow, but not antagonize the presynaptic inhibitory effect of R-PIA or carbachol (0.01-1 μM). Staurosporine (0.1 μM), which inhibited the stimulating effect of phorbol dibutyrate, did not alter the effects of carbachol or R-PIA. The presynaptic effects of phorbol dibutyrate, R-PIA and adenosine were reduced by pretreatment with N-ethylmaleimide (100 μM for 10 min), which inactivates G-proteins. The evoked transmitter release was unaffected by nifedipine (1 μM) in the presence and in the absence of phorbol dibutyrate. These results indicate that adenosine, by acting at presynaptic A1-receptors, reduces transmitter release by a mechanism that involves neither an NDGA-sensitive lipoxygenase nor protein kinase C. The results also indicate that the enhancement of transmitter release by phorbol esters is due to protein kinase C activation and that a G-protein may be involved in the effect but a dihydropyridine-sensitive L-type Ca2+ channel probably is not.