Inhibition of monoamine oxidase in 5-hydroxytryptaminergic neurones by substituted p-aminophenylalkylamines.

A series of substituted p-aminophenethylamines and some related compounds were examined with regards to the inhibition of monoamine oxidase (MAO) in vivo inside and outside 5-hydroxytryptaminergic neurones in the rat hypothalamus. This was recorded as the protection against the irreversible inhibition of MAO produced by phenelzine by determining the remaining deaminating activity in the absence and presence of citalopram using a low (0.1 microM) concentration of [14C]-5-hydroxytryptamine (5-HT) as substrate. Some of the phenethylamines were much more potent inside than outside the 5-hydroxytryptaminergic neurones. This neuronal selectivity was antagonized by pretreatment of the rats with norzimeldine, a 5-HT uptake inhibitor, which indicates that these compounds are accumulated in the 5-HT nerve terminals by the 5-HT pump. Selectivity was obtained for compounds with dimethyl, monomethyl or unsubstituted p-amino groups. An isopropyl group appears to substitute for the dimethylamino group but with considerably lower potency. Compounds with 2-substitution showed selectivity for aminergic neurones and this effect decreased with increased size of the substituent. The 2,6-dichloro derivative FLA 365 had, however, no neuronal selective action but was a potent MAO inhibitor. Substitutions in the 3- and 5-positions decreased both potency and selectivity. Prolongation of the side chain with one methylene group abolished the preference for the MAO in 5-hydroxytryptaminergic neurones although the MAO inhibitory potency remained. The selectivity disappeared by increasing the alpha-substituent to an ethyl group but remained for the alpha,alpha-dimethyl substituted derivatives. It is concluded that compounds which are (1) transported by the 5-HT pump and (2) potent reversible MAO-A inhibitors produce pronounced inhibition of MAO in 5-hydroxytryptaminergic neurones.