Alpha-2 adrenergic inhibition of Ca(++)-evoked [3H]norepinephrine release from synaptosomes is blocked by depolarization.
The Ca(++)-evoked release of [3H]norepinephrine was used in these studies to investigate presynaptic regulation of norepinephrine release. In hippocampal synaptosomes, previously unexposed to Ca++ during isolation and superfusion, 1.25 mM Ca++ evoked a modest (4 to 7% of total stores) release of [3H]norepinephrine with 4.5 mM [K+] present. The alpha-2 adrenergic agonist clonidine inhibited 60% of the Ca(++)-evoked [3H]norepinephrine release. The alpha-2 adrenergic antagonists idazoxan and yohimbine reversed clonidine inhibition of release whereas the alpha-1 antagonist prazosin did not. Increasing the [K+] before Ca++ exposure increased [3H]norepinephrine release, and at 20 [K+] the release increased to over 20% of total stores. However, at [K+] above 9 mM, inhibition of Ca(++)-evoked release by clonidine decreased, and by 20 mM [K+] clonidine no longer inhibited release. Release was unaffected by 5 microM idazoxan or the opiate antagonist naloxone at 15 or 20 mM [K+]. The K+ channel blockers tetraethylammonium (5 mM) and 4-aminopyridine (0.1 mM) increased Ca(++)-evoked release almost 4-fold above control (4.5 mM [K+] present). Neither clonidine nor idazoxan affected Ca(++)-evoked release with the K+ channel blockers present. Therefore, even though K+ channel blockers and 20 mM [K+] increase neurotransmitter release, it is not autoreceptor activation by released endogenous norepinephrine that is responsible for blocking alpha-2 inhibition, but the depolarization produced by these treatments. The 20 mM [K+] blockade of alpha-2 inhibition was decreased by lowering the [Ca++] in the superfusion buffer. Therefore, synaptosomal accumulation of Ca++ may partially explain the loss of alpha-2 inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)
Bowyer
J F
JF
Department of Pharmacology, University of Colorado Health Sciences Center, Denver.
Weiner
N
N
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D003000:Clonidine / Q000494:pharmacology; D015259:Dopamine Agents / Q000494:pharmacology; D004347:Drug Interactions; D004873:Ergolines / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D009638:Norepinephrine / Q000032:analysis / Q000378:metabolism*; D010656:Phenylephrine / Q000494:pharmacology; D011188:Potassium / Q000037:antagonists & inhibitors; D019257:Quinpirole; D051381:Rats; D013566:Sympathomimetics / Q000494:pharmacology*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*
D002121:Calcium Channel Blockers; D015259:Dopamine Agents; D004873:Ergolines; D013566:Sympathomimetics; D010656:Phenylephrine; D019257:Quinpirole; D003000:Clonidine; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine