[³H]Tryptamine Binding Sites Are Not Identical to Monoamine Oxidase in Rat Brain

Abstract

Competition binding studies, subcellular distribution, and in vitro autoradiography were employed to compare the binding in rat brain of [3H]tryptamine with two radioligands for monoamine oxidase (MAO), [3H]pargyline, and [3H]1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine ([3H]MPTP). The MAO inhibitors pargyline, clorgyline, and deprenyl all yielded biphasic competition curves versus [3H]tryptamine. At low concentrations, these drugs stimulated binding by protecting the radioligand from MAO oxidation; at considerably higher concentrations, they inhibited binding by direct competition at the [3H]tryptamine binding site. In subcellular distribution studies, [3H]tryptamine was localized preferentially to the synaptosomal fraction, whereas [3H]pargyline showed greater binding to the mitochondrial fraction. Equilibrium binding studies revealed that the potencies of a series of seven compounds at inhibiting [3H]tryptamine binding were completely different from their potencies at inhibiting [3H]MPTP binding. Finally, the autoradiographic distribution of [3H]tryptamine binding in rat brain was different from that of [3H]MPTP and [3H]pargyline. We conclude that the [3H]tryptamine binding site in rat brain is not equivalent to MAO.

[3H]‐idazoxan binding to rabbit cerebral cortex recognises multiple imidazoline I2‐type receptors: pharmacological characterization and relationship to monoamine oxidase

“…This suggestion is strengthened by the report of a high affinity of clorgyline for rat brain I2 sites and the relative abundance of 12 sites in tissues rich in MAO activity, such as placenta, brain, liver and kidney (Weyler & Salach, 1985;Perry et al, 1988;Tesson et al, 1991;Diamant et al, 1992 Lowry et al (1951) with bovine serum albumin as a standard.…”

Non‐adrenergic binding of [3H]atipamezole in rat kidney–regional distribution and comparison to α2‐adrenoceptors

“…The MAO‐A inhibitor clorgyline has high affinity for rat brain I 2 ‐imidazoline receptors. Tissues rich in MAO activity such as placenta, brain, liver and kidney have also high densities of I 2 ‐imidazoline receptors (Weyler & Salach, 1985; Perry et al ., 1988; Tesson et al ., 1991; Diamant et al ., 1992). In recombinant yeast cells expressing human MAO‐A or MAO‐B, [ 3 H]idazoxan binding is inhibited by the I 2 ‐imidazoline receptor ligands cirazoline and guanabenz (Tesson et al ., 1995).…”

Identification of tryptamine and tryptamine‐serotonin neurons in the rat dorsal raphe nuclei using specific antibodies

“…Thus identification of T-only-containing neurons seems important in so far as T presents specific pharmaco-physiological activities, such as thermoregulation, pain (Larson and Dalo, 1986), locomotor activity (Marien et al, 19871, or spinal reflexes (Martin et al, 1988) and may produce opposite, antagonistic, or potentiating effects to 5-HT action (Baker et al, 1980;Jones, 1982;Jones and Boulton, 1980;Marien et al, 1987;Marsden and Curzon, 1974). In addition, T probably acts through a specific receptor, distinct from 5-HT binding sites and monoamine oxidase (Altar et al, 1986;Greenshaw and Dewhurst, 1987;Perry et al, 1988;Nguyen et al, 1989).…”