The monoamine oxidase inhibitors (MAOIs) consist of a structurally diverse group of compounds whose antidepressant effects have been attributed to their common ability to inhibit monoamine oxidase (MAO) thus increasing the synaptic availability of one or more of the monoamine neurotransmitters. Surprisingly, we have shown that a variety of non-dopaminergic compounds, most-notably MAOIs, inhibit equilibrium [3H]quinpirole binding in rat striatal membranes in vitro (Table 1). In fact, clorgyline, Ro 41-1049, and phenelzine exhibited >1000-fold higher affinity in competition with [3H]quinpirole than [3H]spiperone. These observations suggest that MAOIs interact with the site labeled by [3H]quinpirole in brain. However, in keeping with the presumed lack of dopaminergic activity of most MAOIs, these compounds do not inhibit binding of other D2 receptor agonists or antagonists, such as [3H](-)-propylnorapomorphine and [3H]spiperone. This indicates that interaction with MAOIs is not simply a property of agonist binding at D2-like receptors and suggests that MQB may be unique to [3H]quinpirole, or perhaps the dopaminergic ergot derivatives.

Table 1. Potencies of monoamine oxidase-inhibitors at [3H]quinpirole-labeled
and [3H]spiperone-labeled sites in rat striatal membranes.

Compound/Ligand	[3H]Quinpirole Ki	[3H]Spiperone (nM)
Clorgyline	21 ± 8	>50,000
Ro 41-1049	33 ± 12	>100,000
Phenelzine	95 ± 15	>100,000
Pargyline	85 ± 6	>100,000
Tranylcypromine	269 ± 45	6,633 ± 1,293
(-)-Deprenyl	227 ± 60	10,235 ± 679
(+)-Deprenyl	589 ± 56	>50,000
Isocarboxazid	2,160 ± 241	>100,000
Nialamide	4,486 ± 273	>100,000
Ro 16-6491	3,648 ± 1,013	>100,000
Iproniazid	15,405 ± 3,536	>100,000
Moclobemide	18,700 ± 2,255	>100,000

Data represent the mean ± S.E. of 3 to 5 independent determinations as analyzed by LIGAND.

Although these structurally diverse compounds possess the common ability to inhibit the activity of MAO, several lines of evidence suggest that the enzymatic activity of MAO is not involved in this observation. First, the rank order of potencies of the MAOIs in competition for [3H]quinpirole binding does not correspond with the selectivities of these compounds for the MAO subtypes, A and B. For example, the MAO-A-selective drugs clorgyline and Ro 41-1049 were more than 400-fold more potent in inhibiting [3H]quinpirole binding than iproniazid which inhibits both MAO subtypes. Second, (+)-deprenyl, which is inactive at MAO, has nearly equal affinity to its active isomer (-)-deprenyl. Likewise, the interaction of these compounds with [3H]quinpirole binding is reversible while many of them, such as clorgyline, are irreversible inhibitors of MAO. In addition, quinpirole does not appear to be a substrate for MAO. Finally, these compounds inhibit [3H]quinpirole binding after steady-state has been reached. With the elimination of the involvement of MAO, interactions at the receptor level (e.g. an MQB site) must be considered.

Although some MAOIs have been shown to inhibit binding of sigma, imidazoline, and dopamine uptake site ligands, the potency of MAOIs in competition for [3H]quinpirole binding correlates poorly with their potencies at these sites. Taken together, these data suggest that MAOIs may interact with a novel binding site in rat brain which is either labeled by [3H]quinpirole in addition to dopamine receptors or which modulates [3H]quinpirole binding at D2-like receptors. Subsequent study demonstrated that MQB in striatal membranes is not dependent on specific in vitro assay conditions, such as membrane concentration, the presence or absence of various ions, or incubation time or temperature . Finally, the distribution of MQB in rat brain regions and subcellular fractions parallels that of D2-like dopamine receptors. In fact, no data obtained to date suggests the binding of [3H]quinpirole to any site other than D2-like receptors. Thus, these observations suggest that MQB is associated with D2-like receptors.Further study must confirm whether MQB involves D2-like receptors. Even so, this observation has significant implications for the understanding of the pharmacology of MAOIs and quinpirole, and the nature of ligand-receptor interactions at D2-like receptors.Supported by NARSAD.

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