Inhibition of monoamine oxidase-a increases respiration in isolated mouse cortical mitochondria.

Monoamine oxidase (MAO) is an enzyme located on the outer mitochondrial membrane that metabolizes amine substrates like serotonin, norepinephrine and dopamine. MAO inhibitors (MAOIs) are frequently utilized to treat disorders such as major depression or Parkinson's disease (PD), though their effects on brain mitochondrial bioenergetics are unclear. These studies measured bioenergetic activity in mitochondria isolated from the mouse cortex in the presence of inhibitors of either MAO-A, MAO-B, or both isoforms. We found that only 10 µM clorgyline, the selective inhibitor of MAO-A and not MAO-B, increased mitochondrial oxygen consumption rate in State V(CI) respiration compared to vehicle treatment. We then assessed mitochondrial bioenergetics, reactive oxygen species (ROS) production, and Electron Transport Chain (ETC) complex function in the presence of 0, 5, 10, 20, 40, or 80 µM of clorgyline to determine if this change was dose-dependent. The results showed increased oxygen consumption rates across the majority of respiration states in mitochondria treated with 5, 10, or 20 µM with significant bioenergetic inhibition at 80 µM clorgyline. Next, we assessed mitochondrial ROS production in the presence of the same concentrations of clorgyline in two different states: high mitochondrial membrane potential (ΔΨm) induced by oligomycin and low ΔΨm induced by carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP). There were no changes in ROS production in the presence of 5, 10, 20, or 40 µM clorgyline compared to vehicle after the addition of oligomycin or FCCP. There was a significant increase in mitochondrial ROS in the presence of 80 µM clorgyline after FCCP addition, as well as reduced Complex I and Complex II activities, which are consistent with inhibition of bioenergetics seen at this dose. There were no changes in Complex I, II, or IV activities in mitochondria treated with low doses of clorgyline. These studies shed light on the direct effect of MAO-A inhibition on brain mitochondrial bioenergetic function, which may be a beneficial outcome for those taking these medications.

Effect of Excessive Serotonin on Pharmacokinetics of Cephalexin after Oral Administration: Studies with Serotonin-Excessive Model Rats.

PURPOSE: Serotonin (5-HT) is important for gastrointestinal functions, but its role in drug absorption remains to be clarified. Therefore, the pharmacokinetics and oral absorption of cephalexin (CEX) were examined under 5-HT-excessive condition to understand the role of 5-HT. METHODS: 5-HT-excessive rats were prepared by multiple intraperitoneal dosing of 5-HT and clorgyline, an inhibitor for 5-HT metabolism, and utilized to examine the pharmacokinetics, absorption behavior and the intestinal permeability for CEX. RESULTS: Higher levels of 5-HT in brain, plasma and small intestines were recognized in 5-HT-excessive rats, where the oral bioavailability of CEX was significantly enhanced. The intestinal mucosal transport via passive diffusion of CEX was significantly increased, while its transport via PEPT1 was markedly decreased specifically in the jejunal segment, which was supported by the decrease in PEPT1 expression on brush border membrane (BBM) of intestinal epithelial cells. Since no change in antipyrine permeability and significant increase in FITC dextran-4 permeability were observed in 5-HT-excessive rats, the enhanced permeability for CEX would be attributed to the opening of tight junction, which was supported by the significant decrease in transmucosal electrical resistance. In 5-HT-excessive rats, furthermore, total body clearance of CEX tended to be larger and the decrease in PEPT2 expression on BBM in kidneys was suggested to be one of the reasons for it. CONCLUSIONS: 5-HT-excessive condition enhanced the oral bioavailability of CEX in rats, which would be attributed to the enhanced permeability across the intestinal mucosa via passive diffusion through the paracellular route even though the transport via PEPT1 was decreased.

Ethanol concentration induces production of 3,4-dihydroxyphenylacetic acid and homovanillic acid in mouse brain through activation of monoamine oxidase pathway.

First, we aimed to investigate ex vivo the effects of ethanol (EtOH) on levels of norepinephrine (NE), dopamine (DA), serotonin (5-HT), and their metabolites in the frontal cortex, hippocampus, and striatum of Aldh2-knockout (Aldh2-KO) and wild-type (WT) mice. Animals were treated intraperitoneally with saline (control) or EtOH (1.0, 2.0, or 3.0 g/kg). Brain samples were collected 60 and 120 min after EtOH injection, and monoamines and their metabolites were measured by HPLC-ECD. We found in both WT and Aldh2-KO mice that 3.0 g/kg EtOH increased the levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and decreased the level of 3-methoxytyramine (3-MT). A 2.0 g/kg dose of EtOH also increased HVA, but there was not a consistent effect within the brain regions of Aldh2-KO and WT mice. There were inconsistent findings of genotype differences in the levels of DA, 5-HT, and their metabolites in the brain regions tested. None of the EtOH doses altered NE, DA, 5-HT, or 5-hydroxyindoleacetic acid contents in any of the brain regions studied. Second, we tested whether EtOH-induced increases in DOPAC and HVA are mediated by increased monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT) activity. To test this, we used the MAO blocker clorgyline (2.0 and 4.0 mg/kg) and the COMT blocker tolcapone (15 and 30 mg/kg) alone or in combination with EtOH (3.0 g/kg). Clorgyline alone increased 3-MT and decreased DOPAC and HVA levels, whereas tolcapone alone increased DOPAC and decreased 3-MT and HVA levels. Surprisingly, the combination of EtOH with clorgyline (4.0 mg/kg) or tolcapone (30 mg/kg) further decreased 3-MT and increased DOPAC and HVA levels, an effect that reversed the inhibitor-induced decreases in HVA. These results suggest that a high concentration of EtOH can accelerate DA metabolism, as evidenced by the increase in DOPAC and HVA, and this effect is likely a consequence of increased degradation of DA by MAO.

Evidence that formulations of the selective MAO-B inhibitor, selegiline, which bypass first-pass metabolism, also inhibit MAO-A in the human brain.

Selegiline (L-deprenyl) is a selective, irreversible inhibitor of monoamine oxidase B (MAO-B) at the conventional dose (10 mg/day oral) that is used in the treatment of Parkinson's disease. However, controlled studies have demonstrated antidepressant activity for high doses of oral selegiline and for transdermal selegiline suggesting that when plasma levels of selegiline are elevated, brain MAO-A might also be inhibited. Zydis selegiline (Zelapar) is an orally disintegrating formulation of selegiline, which is absorbed through the buccal mucosa producing higher plasma levels of selegiline and reduced amphetamine metabolites compared with equal doses of conventional selegiline. Although there is indirect evidence that Zydis selegiline at high doses loses its selectivity for MAO-B, there is no direct evidence that it also inhibits brain MAO-A in humans. We measured brain MAO-A in 18 healthy men after a 28-day treatment with Zydis selegiline (2.5, 5.0, or 10 mg/day) and in 3 subjects receiving the selegiline transdermal system (Emsam patch, 6 mg/day) using positron emission tomography and the MAO-A radiotracer [(11)C]clorgyline. We also measured dopamine transporter (DAT) availability in three subjects from the 10 mg group. The 10 mg Zydis selegiline dose significantly inhibited MAO-A (36.9±19.7%, range 11-70%, p<0.007)) but not DAT; and while Emsam also inhibited MAO-A (33.2±28.9 (range 9-68%) the difference did not reach significance (p=0.10)) presumably because of the small sample size. Our results provide the first direct evidence of brain MAO-A inhibition in humans by formulations of selegiline, which are currently postulated but not verified to target brain MAO-A in addition to MAO-B.

[Liver monoamine oxidase activity of the lamprey Lampetra fluviatilis. the substrate-inhibitory specificity].

Based on data of substrate-inhibitory analysis with use of specific inhibitors--deprenyl, chlorgi-lin--and specific substrates--serotonin, noradrenalin, benzylamine, beta-phenylethylamine, and N-methylhistamine--a suggestion is put forward about the possible existence of one molecular form of monoamine oxidase (MAO) in liver of mature individuals of the European lamprey Lampetra fluviatilis. There are determined kinetic parameters of monoamine oxidase deamination of eight substrates, which indicates the large spectrum of substrate specificity of the lamprey liver MAO. The studied enzyme does not deaminate histamine and putrescine and is not sensitive to 10(-2) M semicarbaside. Results of study of the substrate-inhibitor specificity allow us to suggest some resemblance of catalytic properties of the lamprey liver MAO and the mammalian form A MAO. The revealed low activity of the enzyme at deamination of all used substrates seems to be connected with low detoxational functional of the lamprey liver.

Reversible inhibitors of monoamine oxidase-A (RIMAs): robust, reversible inhibition of human brain MAO-A by CX157.

Reversible inhibitors of monoamine oxidase-A (RIMA) inhibit the breakdown of three major neurotransmitters, serotonin, norepinephrine and dopamine, offering a multi-neurotransmitter strategy for the treatment of depression. CX157 (3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide) is a RIMA, which is currently in development for the treatment of major depressive disorder. We examined the degree and reversibility of the inhibition of brain monoamine oxidase-A (MAO-A) and plasma CX157 levels at different times after oral dosing to establish a dosing paradigm for future clinical efficacy studies, and to determine whether plasma CX157 levels reflect the degree of brain MAO-A inhibition. Brain MAO-A levels were measured with positron emission tomography (PET) imaging and [(11)C]clorgyline in 15 normal men after oral dosing of CX157 (20-80 mg). PET imaging was conducted after single and repeated doses of CX157 over a 24-h time course. We found that 60 and 80 mg doses of CX157 produced a robust dose-related inhibition (47-72%) of [(11)C]clorgyline binding to brain MAO-A at 2 h after administration and that brain MAO-A recovered completely by 24 h post drug. Plasma CX157 concentration was highly correlated with the inhibition of brain MAO-A (EC(50): 19.3 ng/ml). Thus, CX157 is the first agent in the RIMA class with documented reversible inhibition of human brain MAO-A, supporting its classification as a RIMA, and the first RIMA with observed plasma levels that can serve as a biomarker for the degree of brain MAO-A inhibition. These data were used to establish the dosing regimen for a current clinical efficacy trial with CX157.

Termination of lesion-induced plasticity in the mouse barrel cortex in the absence of oligodendrocytes.

Termination of developmental plasticity occurs at specific points in development, and the mechanisms responsible for it are not well understood. One hypothesis that has been proposed is that oligodendrocytes (OLs) play an important role. Consistent with this, we found that OLs appeared in the mouse somatosensory cortex at the end of the critical period for whisker lesion-induced barrel structural plasticity. To test this hypothesis, we used two mouse lines with defective OL differentiation: Olig1-deficient and jimpy. In Olig1-deficient mice, although OLs were totally absent, the termination of lesion-induced plasticity was not delayed. The timing was normal even when the cytoarchitectonic barrel formation was temporarily blocked by pharmacological treatment in Olig1-deficient mice. Furthermore, the termination was not delayed in jimpy mice. These results demonstrate that, even though OLs appear at the end of the critical period, OLs are not intrinsically necessary for the termination of lesion-induced plasticity. Our findings underscore a mechanistic distinction between the termination of thalamocortical axonal plasticity in the barrel cortex and that in the visual cortex, in which OL-derived Nogo-A/B was recently suggested to be essential.

Perinuclear localization of biogenic amines (serotonin and histamine) in rat immune cells.

EDAC fixation, which polymerizes biogenic amines, and clorgyline, which blocks monoamine oxydase, were used to demonstrate the perinuclear and nuclear localization of histamine and serotonin (5-HT) of immune cells in rat peritoneal fluid and thymus. For detection, an immunocytochemical method combined with confocal microscopy was used. In peritoneal cells (lymphocytes) 5-HT formed spots around the nucleus, while histamine was localized diffusely. In thymocytes, 5-HT was distributed in patches, while histamine formed a garland around the nuclear envelope. Clorgyline enhanced the fluorescence and increased the number of fluorescent positive cells only for histamine, and then histamine-positive nuclei were also present. The results show that: (1) EDAC fixation demonstrates the presence of biogenic amines in immune cells, (2) using this fixation, the perinuclear localization of biogenic amines can be demonstrated, (3) the localization of the two amines is different and (4) the nuclear localization of histamine in thymocytes can be surmised and in mast cells (described previously) is proven again. The potential importance of the nuclear and near-nuclear localization of biogenic amines is discussed.

Effects of monoamine oxidase inhibitors on methamphetamine-induced stereotypy in mice and rats.

In male ICR mice, a single intraperitoneal administration of methamphetamine (METH) (10 mg/kg) induced stereotyped behavior such as continuous sniffing, circling, and nail biting, reaching a plateau level 20 min after the injection. Subcutaneous pretreatment with clorgyline, a monoamine oxidase (MAO)-A inhibitor, at a dose of 0.1 mg/kg 2 h prior to the drug challenge significantly decreased the initial (first 20 min) intensity of stereotypies and increased the latency to onset. The effect was not observed with either higher doses of clorgyline (1 and 10 mg/kg) or l-deprenyl, a MAO-B inhibitor, at doses of 0.1-10 mg/kg. In male Wistar rats, the inhibitory effect of clorgyline on METH-induced stereotypy was not observed. Pretreatment of the mice with clorgyline (0.1 mg/kg) had no effect on apparent serotonin and dopamine turnover in the striatum, although the higher doses of clorgyline (1 and 10 mg/kg) significantly decreased the turnover. These results suggest that a low dose of clorgyline tends to increase the latency and decrease the intensity of stereotypies induced by METH in a dopamine metabolism-independent manner in mice.

Evidence for reduced arterial plasma input, prolonged lung retention and reduced lung monoamine oxidase in smokers.

UNLABELLED: We have previously found that smokers have reduced brain monoamine oxidase (MAO) A and B using positron emission tomography (PET) and the irreversible mechanism-based radiotracers [(11)C]-labeled clorgyline (CLG) and deprenyl (DEP) and their deuterated analogs (D CLG, D DEP). More recently, we have estimated MAO A and B activity in other organs using the deuterium isotope effect to determine binding specificity for MAO and a three-compartment model to estimate k(3), the model term proportional to MAO A activity. Here, we have investigated the robustness of the model term k(3) for estimating lung MAO A and B in light of our unexpected finding that lung MAO activity (k(3)) was reduced for smokers relative to nonsmokers, although radiotracer uptake in the lungs was similar at peak and plateau for the two groups. METHODS: Time-activity data from lung and arterial plasma were used from seven nonsmokers and seven smokers scanned previously with CLG and D CLG, and five nonsmokers and nine smokers scanned previously with DEP and D DEP. The measured time-activity curves for lung and plasma and the integrals for the arterial plasma time-activity curves were compared at an early time point (2.5 min) and at the end of the study (55 min). A three-compartment irreversible model was used to estimate the differences between smokers and nonsmokers, and the stability of the parameter (k(3)) while varying model assumptions for the relative fractions of lung tissue, blood and air in the PET voxel. RESULTS: The peak in the arterial plasma input function and the integral of the arterial plasma time-activity curve over the first 2.5 min after radiotracer injection were significantly lower for smokers relative to nonsmokers for all four tracers. However, although the peak and plateau of the lung time-activity curves were similar for smokers and nonsmokers, the decline in radioactivity from peak to plateau was slower for smokers for all tracers. Using a three-compartment irreversible model, we estimated the ratio of MAO subtypes A and B in normal lung tissue to be on the order of 3 to 1 (MAO A to B) and that smokers have reduced MAO levels for both subtypes as measured by the model parameter, k(3). The values of k(3) are insensitive to model assumptions of variations in air and tissue fraction in the PET voxel. Most of the effects of changes in these fractions are absorbed into the parameter K(1), which governs the plasma-to-tissue transfer of tracer and is a function of blood flow. K(1) was found to be larger in smokers, although the values depend upon model assumptions of air and tissue fractions. k(3) was found to be significantly lower in smokers; for CLG, a 50% reduction in MAO A for both CLG and D CLG was observed. For DEP, k(3) was also significantly lower in smokers with a reduction of approximately 80% in lung MAO B, although there was a very large coefficient of variation in the smoker's k(3). We also found larger values of lambda (K(1)/k(2)) for smokers relative to nonsmokers for all tracers consistent with a longer lung retention of the nonenzyme-bound tracer, which explains the slower decline in uptake from peak radioactivity for smokers. CONCLUSIONS: The measured arterial input function values for smokers and nonsmokers are significantly different for these two tracer pairs for nonsmokers and smokers particularly for the first few minutes after radiotracer injection. Model estimates of k(3) that indicate that smokers have lower lung MAO A and B activity than nonsmokers are robust and insensitive to variations in model assumptions for relative fractions of lung tissue, blood and air in the PET voxel. Although we have only investigated the behavior of [(11)C]clorgyline and [(11)C]l-deprenyl and their deuterium-substituted analogs in this report, the extent to which reduced arterial input and longer lung retention also hold for other tracers for subjects who smoke merits investigation.

Monoamine oxidase A imaging in peripheral organs in healthy human subjects.

Monoamine oxidase (MAO) catalyzes the oxidative deamination of many biogenic and dietary amines. Though studies of MAO have focused mainly on its regulatory role in the brain, MAO in peripheral organs also represents a vast mechanism for detoxifying vasoactive compounds as well as for terminating the action of physiologically active amines, which can cross the blood brain barrier. Indeed, robust central and peripheral MAO activity is a major requirement in the safe use of many CNS drugs, particularly antidepressants, and thus an awareness of the MAO inhibitory potential of drugs is essential in therapeutics. In this study, we examined the feasibility of quantifying MAO A in peripheral organs in healthy human subjects using comparative positron emission tomography (PET) imaging with carbon-11 (t(1/2): 20.4 min) labeled clorgyline ([(11)C]clorgyline) a suicide inactivator of MAO A and its deuterium labeled counterpart ([(11)C]clorgyline-D2). Heart, lungs, kidneys, thyroid, and spleen showed a robust deuterium isotope effect characteristic of MAO and the magnitude of the effect was similar to that of trancylcypromine, an irreversible MAO inhibitor used in the treatment of depression. Liver time-activity curves were not affected by deuterium substitution precluding the estimation of liver MAO in vivo. In organs showing an isotope effect, MAO A is greatest in the lungs and kidneys followed by the thyroid and heart. This method, which has been previously applied in the human brain, opens the possibility to also directly assess the effects of different variables including smoking, dietary substances, drugs, disease, and genetics on peripheral MAO A in humans.

Non-MAO A binding of clorgyline in white matter in human brain.

Clorgyline is an irreversible inhibitor of monoamine oxidase (MAO A) which has been labeled with carbon-11 (C-11) and used to measure human brain MAO A with positron emission tomography (PET). In this study we compared [11C]clorgyline and deuterium-substituted [11C]clorgyline ([11C]clorgyline-D2) to better understand the molecular link between [11C]clorgyline binding and MAO A. In PET studies of five normal healthy volunteers scanned with [11C]clorgyline and [11C]clorgyline-D2 2 h apart, deuterium substitution generally produced the expected reductions in the brain uptake of [11C]clorgyline. However, the reduction was not uniform with the C-11 binding in white matter being significantly less sensitive to deuterium substitution than other brain regions. The percentages of the total binding attributable to MAO A is largest for the thalamus and smallest for the white matter and this is clearly seen in PET images with [11C]clorgyline-D2. Thus deuterium-substituted [11C]clorgyline selectively reduces the MAO A binding component of clorgyline in the human brain revealing non-MAO A binding which is most apparent in the white matter. The characterization of the non-MAO A binding component of this widely used MAO A inhibitor merits further investigation.

Evidence that L-deprenyl treatment for one week does not inhibit MAO A or the dopamine transporter in the human brain.

In this study, we investigated whether treatment with L-deprenyl, a selective monoamine oxidase B (MAO B) inhibitor, also inhibits MAO A or the dopamine transporter in the human brain. Six normal volunteers (age 46+/-6 yrs) had two PET sessions, one at baseline and one following L-deprenyl (10 mg/day) for 1 week. Each session included one scan with [11C]clorgyline (to assess MAO A) and one scan 2 hours later with [11C]cocaine (to assess dopamine transporter availability). A 3-compartment model was used to compare the plasma-to-brain transfer constant, K1 (a function of blood flow) and lambdak3 (a kinetic term proportional to brain MAO A) before and after treatment. Dopamine transporter availability was measured as the ratio of distribution volumes of the striatum to cerebellum (DVR) which is equal to Bmax/KD +1. L-Deprenyl treatment for 1 week did not affect either brain MAO A activity or dopamine transporter availability. There was a non-significant trend for an increase in K1 after L-deprenyl. These results confirm that L-deprenyl after one week of treatment at doses typically used clinically is selective for MAO B and that it does not produce a measurable affect on the dopamine transporter, suggesting that MAO A inhibition and dopamine transporter blockade do not contribute to its pharmacological effects.

Monoamine oxidase A inhibition by fluoxetine: an in vitro and in vivo study.

Monoamine oxidase A (MAO-A) inhibition was investigated both in vitro and in vivo in rat brains by using the radioligand, 18F-fluoroclorgyline (N-[3-(2',4'-dichlorophenoxy)-2-18F-fluoropropyl]-N-methylpropa rgylamine). In vitro binding affinities of six compounds, clorgyline, Ro 41-1049, deprenyl, fluoxetine, norfluoxetine and citalopram, were studied. Fluoxetine and norfluoxetine showed in vitro affinities of 36.5 and 68 microM for MAO-A, respectively. Fluoxetine and norfluoxetine also significantly inhibited (more than 20%) the binding of the radioligand in vivo while citalopram and deprenyl showed very poor affinities in vitro for MAO-A and had no effect in vivo. The in vivo effects of the various drugs were directly comparable to their in vitro affinities for binding to MAO-A as seen in the correlation plot of percent control in vivo binding of 18F-fluoroclorgyline and binding affinity, -log IC50 (R2 = 0.979). An acute dose of 20 mg/kg of fluoxetine inhibited binding of 18F-fluoroclorgyline by more than 20%, while lower doses had some significant effects. These results provide evidence on the in vitro and in vivo inhibition of monoamine oxidase A by fluoxetine.

Brain monoamine oxidase A inhibition in cigarette smokers.

Several studies have documented a strong association between smoking and depression. Because cigarette smoke has been reported to inhibit monoamine oxidase (MAO) A in vitro and in animals and because MAO A inhibitors are effective antidepressants, we tested the hypothesis that MAO A would be reduced in the brain of cigarette smokers. We compared brain MAO A in 15 nonsmokers and 16 current smokers with [11C]clorgyline and positron emission tomography (PET). Four of the nonsmokers were also treated with the antidepressant MAO inhibitor drug, tranylcypromine (10 mg/day for 3 days) after the baseline PET scan and then rescanned to assess the sensitivity of [11C]clorgyline binding to MAO inhibition. MAO A levels were quantified by using the model term lambda k3 which is a function of brain MAO A concentration. Smokers had significantly lower brain MAO A than nonsmokers in all brain regions examined (average reduction, 28%). The mean lambda k3 values for the whole brain were 0.18 +/- 0.04 and 0.13 +/- 0.03 ccbrain (mlplasma)-1 min-1 for nonsmokers and smokers, respectively; P < 0.0003). Tranyl-cypromine treatment reduced lambda k3 by an average of 58% for the different brain regions. Our results show that tobacco smoke exposure is associated with a marked reduction in brain MAO A, and this reduction is about half of that produced by a brief treatment with tranylcypromine. This suggests that MAO A inhibition needs to be considered as a potential contributing variable in the high rate of smoking in depression and in the development of more effective strategies for smoking cessation.

Evaluation of radioiodinated iodoclorgyline as a SPECT radiopharmaceutical for MAO-A in the brain.

An in vivo estimation of the newly synthesized MAO-A specific inhibitor, [125I]-labeled N-[3(2,4-dichloro-6-iodophenoxy)propyl]-N-methyl-2- propynylamine ([125I]-iodoclorgyline), was performed. Retention of the radioactivity of this radioligand was observed in the brain from 1 h post-injection. Pretreatments with clorgyline and l-deprenyl showed selective binding of [125I]-iodoclorgyline to MAO-A in the brain at 24 h post-injection. Moreover, a good correlation (r = 0.907) between the uptake of [125I]-iodoclorgyline and MAO-A enzyme activity in the cortex was observed in the pretreatment study with several doses of clorgyline. Although improvement to increase the brain/blood ratio is desirable because of slow blood clearance of the radioactivity, radioiodinated iodoclorgyline may serve as a useful SPECT radiopharmaceutical for quantitative analysis of MAO-A in the brain.

Modification of the N-methyl-D-aspartate response by antidepressant sigma receptor ligands.

Sertraline, a selective serotonin reuptake inhibitor, and clorgyline, a monoamine oxidase inhibitor, both of which have high affinity for sigma receptors, were assessed in an electrophysiological model. In keeping with previous data obtained with other sigma receptor ligands, low doses of sertraline and of clorgyline potentiated selectively with a bell-shaped dose-response curve the effect of N-methyl-D-aspartate (NMDA) on pyramidal neurons in the CA3 region of the rat dorsal hippocampus. This potentiation was reversed by the sigma receptor ligands haloperidol and BMY-14802. The selective serotonin reuptake inhibitor paroxetine and the monoamine oxidase inhibitor tranylcypromine, both devoid of affinity for sigma receptors, had no effects on the NMDA response. These data suggest that the effects of sertraline and clorgyline on the NMDA response are due to their affinity for sigma receptors.

Binding of [3H]brofaromine to monoamine oxidase A in vivo: displacement by clorgyline and moclobemide.

Short duration of action, displaceability by endogenously released monoamines, and absence of cumulation of effect of the selective inhibitor of monoamine oxidase type A (MAO-A), brofaromine (CGP 11305 A), indicate reversibility of its interaction with the enzyme in vivo. However, its in vitro interaction with the enzyme showed features commonly associated with irreversible inhibition. To clarify this issue, the in vivo binding of [3H]brofaromine to MAO-A in areas of the rat brain, and in rat heart and liver was investigated. Specific binding, defined by pretreatment with the irreversible inhibitor, clorgyline, was between 15 and 75% of total binding depending ont eh tissue and the time elapsed after injection of radioactivity. In brain and heart tissue, unlabelled brofaromine, another reversible inhibitor of MAO-A, moclobemide and clorgyline were able to displace [3H]brofaromine when administered after the labelled compound with ED50s of 1-3 mg/kg p.o., 3 mg/kg p.o. and 0.3-1 mg/kg s.c., respectively. In the liver, brofaromine and moclobemide and the inhibitor of drug-metabolizing enzymes, proadifen (SK&F 525 A), were able to significantly inhibit [3H]brofaromine binding. Clorgyline was only marginally effective, suggesting that, in this organ, [3H]brofaromine binds predominantly to such enzymes. In conclusion, the binding of [3H]brofaromine to MAO-A in rat brain and heart in vivo was found to be displaceable by other MAO inhibitors and is therefore reversible. In the liver, the compound bound predominantly to other sites, probably microsomal drug-metabolizing enzymes.