Effect of an intracellular calcium chelator on the regulation of electrically evoked [3H]-noradrenaline release from rat hippocampal slices.
1. The electrically (3 Hz, 5 min) evoked [3H]-noradrenaline ([3H]-NA) release from rat hippocampal slices was reduced by prior treatment of the slices with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetomethylester (BAPTA/AM) in a concentration-(10 to 500 microM) dependent manner (40% at 30 microM). Reduction of medium calcium from 1.3 to 0.5 mM caused a larger (70%) decrease. BAPTA free acid (100 mM), a non-permeable Ca(2+)-chelator had no significant effect. 2. Basal [3H]-noradrenaline release was reduced by BAPTA/AM in a concentration-dependent manner (50% at 30 microM), but reduction of external Ca2+ from 1.3 to 0.5 mM did not alter basal release. 3. About 10% of total [3H]-NA in the slices was released at 3 Hz stimulation in 1.3 mM Ca2+ buffer. Addition of the alpha 2-adrenoceptor antagonist, idazoxan (1 microM), increased electrically evoked [3H]-NA release to 26% but stimulated release was not altered by the adenosine A1-receptor antagonist, 8-cyclopentyl theophylline (8-CPT) (1 microM). 4. Evoked release was reduced by the alpha 2-receptor agonist, UK 14,304, in a concentration-dependent manner in the presence of 8-CPT (1 microM). The magnitude of this effect was not altered by the treatment of slices with 30 microM BAPTA/AM. 5. The adenosine A1 receptor agonist, N6-cyclohexyl adenosine (CHA) (1 microM) inhibited electrically evoked [3H]-NA release by about 40% in the presence of idazoxan (1 microM). The effect of CHA was not significantly altered by treatment of slices with BAPTA/AM. 7. The present results show that spontaneous [3H]-NA release is affected by reduction of intracellular Ca2+, but not by reduction of extracellular Ca2+ or by the presynaptic agonists or w-conotoxin. By contrast, electrically evoked release was affected more strongly by alterations of extracellular Ca2+ than by buffering intracellular Ca2+. The reduction of electrically evoked [3H]-NA release by agonists at the adenosine Al-receptor and a2-adrenoceptor is probably mediated through the control of Ca2+ entry via membrane ion channels or at a low affinity Ca2'-site governing evoked release.
Fredholm
B B
BB
Department of Pharmacology, Karolinska Institutet, Stockholm, Sweden.
Hu
P S
PS
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D000068438:Brimonidine Tartrate; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology*; D004146:Dioxanes / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004558:Electric Stimulation; D006624:Hippocampus / Q000187:drug effects* / Q000378:metabolism; D019329:Idazoxan; D066298:In Vitro Techniques; D008297:Male; D009638:Norepinephrine / Q000378:metabolism*; D010456:Peptides, Cyclic / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D020960:omega-Conotoxins
D000316:Adrenergic alpha-Agonists; D002121:Calcium Channel Blockers; D002614:Chelating Agents; D004146:Dioxanes; D010456:Peptides, Cyclic; D011810:Quinoxalines; D020960:omega-Conotoxins; C053423:Conus magus toxin; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; C027513:N(6)-cyclohexyladenosine; D000068438:Brimonidine Tartrate; D004533:Egtazic Acid; D000241:Adenosine; D002118:Calcium; D009638:Norepinephrine; D019329:Idazoxan