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The in vitro binding of the naturally occurring beta-carbolines harman and norharman in their tritium-labelled forms to cell membranes from the rat brain and liver and from bovine adrenal medulla was investigated. Displacement of the specific [3H]harman binding in bovine adrenal medulla and rat liver by several beta-carbolines and monoamine oxidase (MAO) inhibitors revealed the pharmacological profile of a single, high-affinity binding site (KD 4.92 +/- 0.43 nmol/l, Bmax 8.47 +/- 0.17 pmol/mg protein; adrenal medulla) which corresponded to the active site of MAO type A (MAO-A). Similar characteristics have previously been found for brain tissue from rat, marmoset and pig. In order to determine the temperature dependence of the [3H]harman binding, the KD and Bmax values for rat cerebral cortex were calculated from the results of saturation experiments at 5 temperatures (range: 0 degree C-37 degrees C). Whereas the Bmax values under all conditions were approximately 4 pmol/mg protein, the KD values, with increasing temperature, ranged from approximately 3 nmol/l to 30 nmol/l. The calculated linear van't Hoff plot (-ln KD against 1/T) suggested an enthalpy-driven binding of [3H]harman to MAO-A. At least three different [3H]norharman-binding sites were detected. In the rat forebrain, approximately 85% of the specific binding (at about 2 nmol/l of [3H]norharman) can be attributed to a MAO binding site of type B: the binding is displaceable, in nmol/l concentrations by the potent and selective MAO-B inhibitors MDL 72,974 A, R(-)-deprenyl and pargyline and, in mumol/l concentrations, by S(+)-deprenyl and the potent and selective MAO-A inhibitors clorgyline, harmine, harman, harmaline, brofaromine 5-F-alpha-methyltryptamine. After suppression of the MAO binding sites with 1 mumol/l clorgyline and 1 mumol/l R(-)-deprenyl, a second binding site was found. However, the binding at this site was biphasically displaceable by harman and norharman (Hill-slopes about 0.5 and 0.6, curvilinear Rosenthal plots) suggesting the presence of negative co-operativity or of two binding sites (states). A similar clorgyline/R(-)-deprenyl resistant single (Hill-slopes of displacement by norharman, harman and 6-hydroxy-beta-carboline about unity; linear Rosenthal plots) high affinity binding sites (KD 7.5 +/- 2 nmol/l, Bmax 130+/- 30 fmol/mg protein) was found in bovine adrenal medullary cell membranes. A third quite different clorgyline/R(-)-deprenyl resistant high-affinity (KD approximately 14 nmol/l) and high-density (Bmax 10-30 pmol/mg protein) binding site was detected in the liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Harman occurs in rat brain, with the highest concentration in the cerebellum and the lowest in the striatum. 2 g/kg ethanol were ineffective with respect to the concentration of harman in the brain whereas 5 g/kg ethanol caused a time-dependent increase in the cerebral cortex as well as the cerebellum. A toxic dose (8 g/kg) of ethanol elicited no change of harman in the brain 3 h following the application. The rise in the harman concentration in the brain did not correlate with the increase of acetaldehyde in the blood after treatment with ethanol suggesting that several mechanisms are involved in the changes of the levels of harman. In subchronic experiments rats were treated with ethanol over a period of 5 or 6 days. Harman increased in the brain whereby the effect seemed to be more pronounced in the cerebellum than in the cerebral cortex. The concentration tended to increase over time and reached control levels again during withdrawal. The time course of the excretion of harman into the urine was similar to that of the brain in that it increased continuously during the period of ethanol treatment and reached control levels again during withdrawal.

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