Ascorbic acid prevents nonreceptor specific binding of (/sup 3/H)-5-hydroxytryptamine to bovine cerebral cortex membranes
(/sup 3/H)-5-Hydroxytryptamine ((/sup 3/H)-5-HT) decomposes rapidly when exposed to air in solution at physiological pH if antioxidants are not present. The decomposition products appear to bind to two saturable sites on brain membranes (apparent Kd values = 1-2 and 100-1000 nM). This binding mimics ''specific'' ligand/receptor binding in that it is inhibited by 10 microM unlabeled 5-HT. This inhibition is not competitive, but rather is due to the prevention of (/sup 3/H)-5-HT breakdown by excess unlabeled 5-HT. Unlike genuine ligand/receptor binding, the binding of (/sup 3/H)-5-HT breakdown products is essentially irreversible and does not display a tissue distribution consistent with binding to authentic 5-HT receptors. (/sup 3/H)-5-HT decomposition can be eliminated by the inclusion of 0.05 to 5 mM ascorbic acid. At these concentrations ascorbic acid is not deleterious to reversible (/sup 3/H)-5-HT binding. When (/sup 3/H) 5-HT exposure to air occurs in the presence of brain membranes, the apparent antioxidant activity of brain membranes themselves affords protection against (/sup 3/H)-5-HT degradation equal to ascorbic acid. This protection is effective below final (/sup 3/H)-5-HT concentrations of 10 nM. Above 10 nM (/sup 3/H)-5-HT, addition of ascorbic acid or other antioxidants is necessary to avoid the occurrence of additional low affinity (apparent Kd = 15-2000 nM) binding sites that are specific but nonetheless irreversible. When care is taken to limit (/sup 3/H)-5-HT oxidation, the only reversible and saturable specific binding sites observed are of the 5-HT1 high affinity (Kd = 1-2 nM) type. Radioligand oxidation artifacts may be involved in previous reports of low affinity (Kd = 15-250 nM) (/sup 3/H)-5-HT binding sites in brain membrane preparations.
- Research Organization:
- Stanford Univ. School of Medicine, CA
- OSTI ID:
- 6793191
- Journal Information:
- J. Pharmacol. Exp. Ther.; (United States), Journal Name: J. Pharmacol. Exp. Ther.; (United States) Vol. 3; ISSN JPETA
- Country of Publication:
- United States
- Language:
- English
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59 BASIC BIOLOGICAL SCIENCES
AMINES
ANIMALS
AROMATICS
ASCORBIC ACID
AUTONOMIC NERVOUS SYSTEM AGENTS
AZAARENES
AZOLES
BIOCHEMICAL REACTION KINETICS
BODY
BRAIN
CATTLE
CELL CONSTITUENTS
CELL MEMBRANES
CENTRAL NERVOUS SYSTEM
CEREBRAL CORTEX
CEREBRUM
DOMESTIC ANIMALS
DRUGS
HETEROCYCLIC COMPOUNDS
HYDROXY COMPOUNDS
INDOLES
KINETICS
LABELLED COMPOUNDS
MAMMALS
MEMBRANE PROTEINS
MEMBRANES
METABOLISM
MUSCLES
NERVOUS SYSTEM
NEUROREGULATORS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANS
PROTEINS
PYRROLES
RADIOPROTECTIVE SUBSTANCES
REACTION KINETICS
RECEPTORS
RESPONSE MODIFYING FACTORS
RUMINANTS
SEROTONIN
SYMPATHOMIMETICS
TRITIUM COMPOUNDS
TRYPTAMINES
VERTEBRATES
VITAMINS