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 greater than NECA much greater than CGS 21680, CV 1808) indicates that the adenosine receptor responsible is of the A1 subtype. Adenosine (10 microM) and R-PIA (0.1 microM) 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 microM). Stimulation of protein kinase C by the phorbol ester 4 beta-phorbol 12,13-dibutyrate caused a concentration-dependent increase in stimulation-evoked 3H overflow, but did not antagonize the presynaptic inhibitory effect of R-PIA or carbachol (0.01-1 microM). Staurosporine (0.1 microM), 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 microM for 10 min), which inactivates G-proteins. The evoked transmitter release was unaffected by nifedipine (1 microM) 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.
Fredholm
B B
BB
Department of Pharmacology, Karolinska Institutet, Stockholm, Sweden.
D000109:Acetylcholine / Q000378:metabolism*; D000241:Adenosine / Q000031:analogs & derivatives; D000818:Animals; D001094:Arachidonate 5-Lipoxygenase / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D004558:Electric Stimulation; D005033:Ethylmaleimide / Q000494:pharmacology; D006624:Hippocampus / Q000378:metabolism*; D008297:Male; D009637:Masoprocol / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology; D011983:Receptors, Purinergic / Q000502:physiology*
D011976:Receptors, Muscarinic; D011983:Receptors, Purinergic; D015240:Phorbol 12,13-Dibutyrate; D009637:Masoprocol; D002217:Carbachol; D001094:Arachidonate 5-Lipoxygenase; D011493:Protein Kinase C; D009543:Nifedipine; D000241:Adenosine; D000109:Acetylcholine; D005033:Ethylmaleimide