Evidence that two stereochemically different alpha-2 adrenoceptors modulate norepinephrine release in rat cerebral cortex.
Cerebral cortex slices from the rat were loaded with [3H]norepinephrine ([3H]NE) and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. The (-)-isomer and the (+)-isomer of CH-38083 ([7,8-(methylenedioxy)-14- alpha-hydroxyalloberbane HCl), a selective alpha-2-adrenoceptor antagonist with an alloberbane skeleton, increased the electrically induced release of [3H]NE in a concentration-dependent manner, and a similar effect was observed with racemic CH-38083 and idazoxan. The stereoisomers of CH-38083 applied in a concentration range of 10(-8) to 10(-6) mol/l were equipotent in facilitating stimulation-evoked [3H]NE release: concentrations needed to enhance tritium outflow by 50% were 1.3 X 10(-7) mol/l for (-)-CH-38083 and 1.4 X 10(-7) mol/l for (+)-CH-38083. Exogenous NE decreased the electrically stimulated release of [3H]NE, and the stereoisomers of CH-38083 antagonized this inhibition with different potencies: the dissociation constant (KB) values for (-)-isomer and for (+)-isomer of CH-38083 were 14.29 and 97.18 nmol/l. These data indicate that presynaptic alpha-2 adrenoceptors that are available for NE released from axon terminals do not show stereospecificity toward enantiomers of CH-38083, whereas those that are occupied by exogenous NE are much more sensitive toward (-)-CH-38083. The alpha-1 adrenoceptor antagonist prazosin also differentiated between the alpha-2 adrenoceptor subtypes: prazosin (10(-6) mol/l) did not alter the increase of electrically induced [3H]NE release evoked by (-)- and (+)-CH-38083; however, in its presence, the stereoisomers of CH-38083 failed to antagonize the inhibitory effect of exogenous NE on its own release.(ABSTRACT TRUNCATED AT 250 WORDS)
Harsing
L G
LG
Institute of Experimental Medicine, Hungarian Academy of Sciences, Budapest.
Vizi
E S
ES
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001599:Berberine / Q000031:analogs & derivatives / Q000494:pharmacology; D002540:Cerebral Cortex / Q000033:anatomy & histology / Q000378:metabolism* / Q000648:ultrastructure; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D008297:Male; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000502:physiology; D013237:Stereoisomerism; D014316:Tritium
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D001599:Berberine; D014316:Tritium; C048886:7,8-(methylenedioxy)-14-hydroxyberbane; D009638:Norepinephrine; D011224:Prazosin