[³H]Harman Binding Experiments. I: A Reversible and Selective Radioligand for Monoamine Oxidase Subtype A in the CNS of the Rat

Abstract

Harman (1-methyl-beta-carboline) is an endogenous compound with neurotropic properties in rats and humans. In a novel in vitro binding assay, the binding site of [3H]harman has been characterized in the rat crude mitochondrial (P2) fraction. The binding was saturable and reversible. Only a single high-affinity binding site was detected by kinetic, saturation, and displacement analyses in the cerebral cortex of the rat. The linear Scatchard plots revealed equilibrium dissociation constant (KD) values of approximately 2.5 nM at 0 degrees C, approximately 9 nM at 23 degrees C, and approximately 30 nM at 37 degrees C. Among six CNS regions (hypothalamus, hippocampus, cerebral cortex, striatum, cerebellum, and spinal cord), the highest density of binding sites (Bmax) was determined in the hypothalamus (approximately 5.5 pmol/mg of protein) and the lowest in the spinal cord (approximately 2.0 pmol/mg of protein). Several drugs known to affect serotonergic, adrenergic, dopaminergic, cholinergic, or GABAergic neurotransmission inhibited specific binding at best in the micromolar range. In contrast, potent and selective inhibitors of monoamine oxidase subtype A were active in the lower and middle nanomolar range. The displacing potency (apparent Ki) of substrates and inhibitors of monoamine oxidase correlated positively and highly significantly with the corresponding values of the inhibition of monoamine oxidase activity of subtype A (r = 0.92, p less than 0.001, n = 17) but not of subtype B (r = -0.47, p greater than 0.05, n = 15). In conclusion, [3H]harman was identified as a specific ligand of the active site of the A subtype of monoamine oxidase in rat brain.

The Harman and Norharman Reduced Dopamine Content and Induced Cytotoxicity in PC12 Cells

“…Norharman inhibits preferentially MAO-B (May et al, 1991) and is found in substantia nigra from humans (Matsubara et al, 1993). The plasma levels of harman and norharman are also increased in parkinsonian patients (Kuhn et al, 1995).…”

“…Harman, a 1-methylated derivative of norharman, has been reported to have a strong inhibitory action of monoamine oxidase type A (MAO-A, EC 1.4.3.4) (May et al, 1991). Norharman inhibits preferentially MAO-B (May et al, 1991) and is found in substantia nigra from humans (Matsubara et al, 1993).…”

Influence of Additives on Cigarette Related Health Risks

“…It had been shown that acetaldehyde and nicotine when applied together exhibited synergistic positive reinforcing effects in self-administering rats (152). It was further speculated that, by reacting with certain aromatic amines, acetaldehyde may give rise to β-carbolines -a group of compounds possibly inhibiting the activity of monoamino oxidase (430,431). With these notions in mind it was assumed that acetaldehyde generated from sugars might enhance nicotine addictiveness (92,429).…”

β-Carboline Alkaloid Constituents from aThermoactinomyces SP.Strain Isolated from Livingston Island, Antarctica

“…rice, wheat, barley, rye, corn, soybeans, mushrooms, grapes, vinegar, wine and beer. These alkaloids are considered to possess pharmacological properties related to the central nervous system -tremorogenesis (12), hypothermia (1), hallucinogenesis (8) and monoamine oxidase inhibition (13,15). In experimental animals (guinea-pigs), harmane, harmine and harmaline have also been demonstrated to cause spasmolytic effects on isolated trachea (17).…”