The effects of chronic regimens of clorgyline and pargyline on monoamine metabolism in the rat brain

Chronic clorgyline treatment enhances release of norepinephrine following sympathetic stimulation in the rat

Plasma catecholamine levels were determined in pithed rats during electrical stimulation of the entire sympathetic nervous system. In animals treated chronically with clorgyline (1 mg/kg daily for 21 days) the increment in plasma norepinephrine concentration during stimulation was greater than in control animals, whereas a single dose of clorgyline (2 mg/kg 2 h before pithing), which produced the same degree of inhibition of arterial MAO type A and a similar increase in arterial norepinephrine content, had no effect on the plasma norepinephrine response to stimulation. Injection of yohimbine (1 mg/kg) produced the same degree of enhancement of plasma norepinephrine response to stimulation in chronically treated and control animals, showing that the overall gain of the alpha 2-adrenoceptor inhibitory loop in vascular sympathetic nerves was not affected. Plasma epinephrine concentration during electrical stimulation was also increased by chronic but not by acute clorgyline treatment. Chronic clorgyline treatment did not significantly affect the total systemic metabolic clearance rate of infused norepinephrine, thus the increased plasma norepinephrine response to stimulation reflects an increased release rate from sympathetic neurons. In rats treated chronically with clorgyline, the pressor response to norepinephrine in the presence of yohimbine (0.3 mg/kg) was significantly reduced, whereas the pressor response to guanabenz was unchanged. There was also no change in the guanabenz-induced inhibition of the tachycardic response to electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the role of alpha 2-adrenoceptors in the control of synaptosomal [3H]5-hydroxytryptamine release: effects of antidepressant drugs

The sensitivity of alpha 2-adrenoceptors on 5-hydroxytryptamine (5-HT) nerve endings obtained from rat cerebral cortex was investigated following treatment with the antidepressant drugs desipramine (10 mg/kg/day for 21-28 days) or clorgyline (1 mg/kg/day for 21-28 days). [3H]5-HT (100 nM) was used to load cortical synaptosomes (P2) after experiments with uptake inhibitors confirmed that this concentration of amine ensured exclusive uptake into 5-HT nerve terminals. The sensitivity of K+-stimulated release of [3H]5-HT to alpha 2-adrenoceptor occupancy was assessed in a superfusion system by means of the dose-dependent inhibition of [3H]5-HT release by clonidine. This is blocked by yohimbine (1 microM), which, when administered alone, enhances release, suggesting that endogenous catecholamines released from other synaptosomes act on these alpha 2-heteroreceptors. The effect of addition of citalopram (1 microM) to superfusates suggests that some reuptake of [3H]5-HT occurs during superfusion. Of the tritium released into superfusates during "background" and K+-stimulated release, 17 and 90%, respectively is [3H]5-HT. The attenuation of K+-stimulated release by clonidine is apparently diminished by the chronic clorgyline regimen but not by desipramine. However, clorgyline elevates catecholamine levels, and this might increase endogenous noradrenaline (NA) efflux, which by competition with clonidine could appear to alter alpha 2-adrenoceptor sensitivity. This possibility was investigated by depleting NA with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4). These studies showed that the apparent effect of chronic clorgyline on alpha 2-adrenoceptor sensitivity to clonidine was due to competition with increased levels of endogenous NA.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic clorgyline and pargyline increase apomorphine-induced stereotypy in the rat

The effects of monoamine oxidase inhibiting antidepressant drugs on behavioral and biochemical measures of dopamine receptor status were measured in the rat. Male Wistar rats received clorgyline (1 mg/kg/day for 21-28 days), pargyline (1 mg/kg/day for 21-28 days) or a combination of these regimens. They were then either tested for stereotypy induced by 1 mg/kg SC injection of apomorphine or were sacrificed and their striata used to measure specific [3H]spiroperidol binding. All three chronic treatment regimens produced statistically significant increases in apomorphine induced stereotypy: there was, however, no significant difference between the three drug regimens. None of the antidepressant drug treatments significantly affected [3H]spiroperidol binding in the corpus striatum. This study demonstrates that behavioral and biochemical measures of dopamine function may not always be closely correlated. It is proposed that the behavioral changes may be related to alterations in other monoaminergic systems, which are known to have fibres running into the nigrostriatal pathway.

Serotoninergic but not noradrenergic neurons in rat central nervous system adapt to long-term treatment with monoamine oxidase inhibitors

Repeated administration of monoamine oxidase inhibitors induces a transient decrease in the firing rate of serotoninergic neurons followed by complete recovery, whereas it results in a persistent reduction of the firing rate of noradrenergic neurons. Under these conditions, serotoninergic, but not noradrenergic, neurons undergo a desensitization of their somatic autoreceptors. Serotoninergic neurons therefore show the capacity to free themselves from their autoregulatory control, a property which noradrenergic neurons appear to be lacking. The time course of the recovery in the firing rate of the serotoninergic neurons is consistent with the delayed antidepressant effect of monoamine oxidase inhibitors.

Operantly conditioned running: effects on brain catecholamine concentrations and receptor densities in the rat

It was hypothesized that endurance exercise results in an alteration in the brain monoamine systems. Rats were trained to run for food reinforcement on a variable ratio schedule in running wheels. Yoked control rats were also allowed to run but were not specifically reinforced for running. The animals ran 5 days per week for 8 weeks and were sacrificed 48 hours after the last endurance training session. The brains were assayed for norepinephrine and dopamine concentrations and beta-adrenergic (3H-dihydroalprenolol binding) and dopaminergic (3H-spiroperidol binding) receptor densities. Changes in norepinephrine concentration and beta-adrenergic receptor densities were not significantly different between reinforced running and yoked control groups. Dopamine concentrations were significantly higher while dopamine receptor densities were significantly lower in the reinforced running group. These results suggest that chronic running elevates dopamine secretion and consequently produces a compensatory down-regulation of dopaminergic receptor sites. The relationship of these changes to motor activity and to the antidepressant effects of exercise are discussed.

Epinephrine accumulation in rat brain after chronic administration of pargyline and LY 51641--comparison with other brain amines

Concentrations of biogenic amines and metabolites were measured in regions of rat brain following administration of monoamine oxidase (MAO) inhibitors for 21 days. Epinephrine concentrations were increased from 350 to 500% following chronic administration of LY 51641, a selective inhibitor of MAO type A. Norepinephrine, dopamine and serotonin showed much less relative accumulation. The marked relative accumulation of epinephrine may be related to the efficacy of inhibitors of MAO type A in the treatment of depression.

Electrophysiological and receptor studies in rat brain: effects of clorgyline

Acute high doses of clorgyline produce a rapid inhibition of monoamine oxidase type A (MAO A) in the rat brain, together with an increase in norepinephrine and a decrease in the firing rate of locus coeruleus (LC) neurones: this decrease is reversed by piperoxane, an alpha 2 antagonist. In control animals, piperoxane increases LC neuronal firing showing that these noradrenergic neurones are under alpha 2-adrenoceptor-mediated tonic inhibition. Chronic administration of clorgyline, like acute doses of this MAO A inhibitor, significantly decreases cell firing in the LC and the effect is partially reversed by piperoxane. Chronic clorgyline treatment also produces significant decreases in [3H]clonidine and [3H]dihydroalprenolol binding in cerebral cortex, receptor changes which are slightly greater in animals showing greater inhibition of LC neuronal firing: such receptor changes do not occur following a single exposure to clorgyline. Electrophysiological studies in hippocampal pyramidal cells show that the chronic clorgyline treatment does not significantly induced subsensitivity to NE in these adrenoreceptive cells.

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Effects of acute and repeated administration of amiflamine on monoamine oxidase inhibition in the rat

The inhibitory effect on monoamine oxidase (MAO) of the reversible MAO-A inhibitor (+)-4-dimethylamino-2,alpha-dimethylphenethylamine [amiflamine, FLA 336(+)] was evaluated in the rat after acute and repeated (twice daily for two weeks) oral treatment. MAO activity was measured ex vivo in slices from the hypothalamus and the duodenum for both MAO-A and MAO-B. Amiflamine selectively inhibited the A form of MAO after repeated as well as after acute treatment (ED50 approximately 7 mumoles/kg both acute and repeated). In the brain slices this inhibition corresponded to a decrease in the concentration of 5-hydroxyindoleacetic acid (5-HIAA) and to an increase in the concentration of 5-HT in the hypothalamus, the hippocampus and the striatum. The concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) was decreased in the striatum to the same extent as the decrease in the 5-HIAA concentrations. The effect on the homovanillic acid (HVA) concentration was somewhat weaker as was the increase in the concentration of dopamine. No essential difference was found after acute and chronic treatment on the amine and metabolite levels. The MAO activity returned to normal 24 hours after final dosing. A large difference between the neuronal and the extraneuronal protection against the phenelzine-induced irreversible MAO inhibition in the hypothalamus was found after both acute and repeated treatment. The ED50 of the protection within the serotonergic neurons was 1.3 mumoles/kg p.o. (acute) and 0.75 mumoles/kg p.o. (repeated). Amiflamine was 3 times less potent within noradrenergic neurons than within serotonergic neurons. A brain to plasma ratio of about 20:1 was found for amiflamine and its metabolites. The plasma and the brain concentrations of the N-demethylated metabolite [FLA 788(+)] exceeded that of amiflamine after a single dose, whereas the N,N-demethylated [FLA 668(+)] was found in low concentrations. The effect on MAO-A correlated significantly with the plasma and the brain concentration of FLA 788(+).

Selective inhibition by amiflamine of monoamine oxidase type A in rat brain, liver and duodenum

Amiflamine (FLA 336(+)), N-desmethylamiflamine (FLA 788(+)) and N,N-didesmethylamiflamine (FLA 668(+)) were examined for their monoamine oxidase (MAO) inhibitory effects in rat brain, liver and duodenum and were compared with the irreversible inhibitors clorgyline and (-)-deprenyl. The potency of each FLA compound was the same in each tissue both in vitro and after oral administration with either serotonin or tyramine as substrate. The in vitro effect of FLA 788(+) was 2-6 times stronger than that of amiflamine although the compounds were equipotent after oral administration. FLA 668(+) was 2-3 times less potent than amiflamine in vitro and had very poor activity after oral administration. The deamination of phenethylamine was weakly affected by the three FLA compounds. Clorgyline inhibited strongly the deamination of serotonin and tyramine in the duodenum after oral administration, being 1,000 times more potent than in the brain and the liver. Similar results were obtained for (-)-deprenyl which, however, was more potent in inhibiting the deamination of phenethylamine than that of serotonin and tyramine. Amiflamine was a reversible MAO inhibitor with no MAO inhibitory capacity 24 h after a single oral dose. On the other hand the irreversible inhibitor clorgyline had a maximal effect on brain MAO 48 h after a single dose while the inhibitory effect in the duodenum had almost disappeared. The influence of amiflamine on the excretion of acid and basic metabolites of orally administered 14C-tyramine (58 mumol/kg) in rat was examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of clorgyline on catecholamine and indoleamine metabolites in the cerebrospinal fluid of rhesus monkeys

The effects of acute and chronic administration of clorgyline, an irreversible inhibitor of monoamine oxidase type A (MAO-A), on the deaminated metabolites of norepinephrine, dopamine and serotonin were examined in rhesus monkey cerebrospinal fluid (CSF). Acute clorgyline treatment resulted in highly significant, dose-dependent reductions in 3-methoxy-4-hydroxyphenylglycol (MHPG) of 50% (1 mg/kg) and 68% (2 mg/kg) compared to pretreatment values. Chronic clorgyline administration (0.25 to 0.5 mg/kg X 24 days) resulted in a 67% reduction in CSF MHPG. In contrast, the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) were less affected by acute clorgyline administration, being reduced significantly only after the 2 mg/kg dose, which lowered 5-HIAA 27% and HVA 48%. Chronic clorgyline treatment had no significant effect on the CSF concentrations of HVA and 5-HIAA. These data, which suggest that MAO-A inhibition by clorgyline in vivo is more closely associated with changes in the noradrenergic than the serotonergic or dopaminergic systems in nonhuman primates, are in general agreement with the effects of clorgyline on CSF and urinary biogenic amine metabolites in man. They differ from several in vitro studies which indicate a primary role of MAO-A in the metabolism of serotonin and of MAO-B in norepinephrine degradation in primate brain. The discrepancies may reflect modulating effects of synaptic feedback mechanisms on the actions of clorgyline in vivo or perhaps a failure of CSF metabolites to adequately reflect brain amine metabolism changes. The lack of change in platelet MAO-B activity during clorgyline treatment together with the minimal changes in HVA concentrations indicate that the selective inhibitory effects of clorgyline on MAO-A were maintained during chronic administration of low drug doses.

Chronic clorgyline dampens rat retinal rhythms

In the rat retina, the rhythm of disk-shedding from visual cell outer segments is circadian in nature, whereas autophagic degradation of organelles in inner segments follows a circadian rhythm that persist in constant light but not in constant darkness. The monoamineoxidase inhibitor clorgyline can phase delay circadian rhythms in hamster and rat. However, clorgyline does not phase delay the plasma corticosterone rhythm nor does it delay the phase position of the retinal disk-shedding rhythm in a light-dark cycle or in constant darkness: only the amplitude of the circadian rhythm is reduced. A marked reduction in rhythm amplitude is also found in the number of autophagic vacuoles after chronic clorgyline treatment. Thus the circadian rhythm of disk-shedding appears to be primarily driven by an oscillator within the eye, and clorgyline modulates the light response of retinal parameters but not their timing.

Effects of reserpine on recovery rates after inhibition of monoamine oxidase in different regions of mouse brain

Adult mice were injected once with the monoamine oxidase (MAO) inhibitor pargyline, every 3 days with the amine storage depletor reserpine, or with both drugs. Serotonin content of brainstem and cerebrum was increased after pargyline, decreased after reserpine, and showed intermediate values following administration of both drugs. MAO activity after pargyline recovered in a time-dependent fashion, with recovery rates of hypothalamus greater than cerebellum greater than cerebrum greater than brainstem. After reserpine, recovery rates of MAO were less in hypothalamus and cerebellum, suggesting that the amount of MAO synthesized may be related to the level of substrates in these regions.

Rat brain concentrations of 5-hydroxytryptamine following acute and chronic administration of MAO-inhibiting antidepressants

Groups of male Sprague-Dawley rats were injected once daily with phenelzine (15 mg/kg) or tranylcypromine (10 mg/kg) and killed 6 h after drug administration on days 1, 2, 8 and 19. Brains were analyzed for MAO activity by a radiochemical procedure and for 5-HT concentrations by an electron-capture gas chromatographic procedure. Both drugs affected 5-HT concentrations in a similar manner--a significant increase of 5-HT over control levels by day 1, with levels still increasing between days 8 and 19, having attained concentrations approximately 5 times control values by day 19.

Long-term clorgyline treatment antagonizes the eating and motor function responses to m-chlorophenylpiperazine

Treatment for 21 days but not 3 days with clorgyline, a selective monoamine oxidase type A inhibitor with antidepressant effects, causes significant escape from m-chlorophenylpiperazine's effects on food intake, sedation level and induction of limb movements, but sensitizes rats to ejaculation. These findings support the prior reports of functional serotonin pathway adaptational changes in the motor system in response to antidepressant treatment and extend these findings to serotonin pathways involved in eating behavior.

Drug-induced changes in motor activity after selective MAO inhibition

The increase in motor activity produced in mice by phenylethylamine (PEA), L-DOPA and amphetamine was evaluated after selective inhibition of MAO Type A (by clorgyline) or Type B (by low doses of pargyline). PEA-induced motor stimulation was intensified in the presence of MAO-B inhibition, but not when MAO-A was inhibited. This was paralleled by higher concentrations of brain and plasma PEA (after injection) in mice in which there was inhibition of MAO-B compared with control or MAO-A inhibition. Conversely, L-DOPA produced significant stimulation only when MAO-A was inhibited. The clorgyline pretreatment resulted in larger increases in brain dopamine concentrations (in the striatum, olfactory tubercles and in the area containing the substantia nigra) than did MAO-B inhibition. This effect occurred both in mice receiving L-DOPA + inhibitor and in mice receiving the inhibitor alone. Amphetamine-induced stimulation was increased following the inhibition of either form of MAO, and this was not the result of changes in the distribution or metabolism of amphetamine. These results support the concept that MAO-A and MAO-B deaminate different substrates in the rodent CNS and that amphetamine may utilize either dopamine or PEA in producing its stimulant effects.

The metabolism of dopamine by both forms of monoamine oxidase in the rat brain and its inhibition by cimoxatone

In the rat brain, dopamine is metabolised by both A and B forms of monoamine oxidase (MAO), although the A form of the enzyme is the major component. The Km of MAO-A toward dopamine (120 microM) is lower than the Km of MAO-B toward this substrate (340 microM). The activity of MAO-A was lower in old rats than in young rats, and the same degree of decrease was found for 5-hydroxytryptamine as for dopamine as substrates for this enzyme form. The activity of MAO-B was higher in the old rats, the degree of increase being the same for dopamine as for beta-phenethylamine as substrates for this enzyme form. The Ki values of the inhibition of MAO-A by cimoxatone and MD770222 (the principal plasma metabolite of cimoxatone) were independent of the substrate used to assay for activity, but were lower than the Ki values for the inhibition of MAO-B by these compounds.

Regional inhibition of monoamine oxidase activity by administration of clorgyline in rat brain

The inhibiting effect of monoamine oxidase (MAO) activities towards 5-hydroxytryptamine, dopamine and beta-phenylethylamine by an acute and chronic administration of clorgyline was investigated in five locations of the rat brain. Inhibiting the percentage distributed unevenly in each hypothalamic nucleus and in the circumventricular areas, except the median eminence, the percentages were significantly low as compared with the hypothalamic lateral nucleus. A significant percentage of inhibition for type B MAO towards beta-phenylethylamine was noticed in the chronically administered group. In conclusion, the MAO-inhibiting effect of clorgyline administered intraperitoneally was unevenly distributed in the discrete brain nuclei and the regional difference depended upon the dose and the duration of the clorgyline administration.

Immunocytochemical demonstration of monoamine oxidase B in brain astrocytes and serotonergic neurons

An antiserum to monoamine oxidase B (MAO-B) was used to define the distribution of this metabolic enzyme in the adult rat brain immunocytochemically. MAO-B is specifically located in two major central nervous system cell classes, astrocytes and serotonin-containing neurons. Double-immunofluorescence experiments using antisera to glial fibrillary acidic protein and MAO-B showed that both protoplasmic and fibrillary astrocytes throughout the brain contain MAO-B, whereas oligodendrocytes do not contain the enzyme. Areas lacking a blood-brain barrier, such as the specialized circumventricular organs, also contain MAO-B-positive cells. A double-immunofluorescence experiment using antisera to serotonin and MAO-B enabled the positive identification of neurons containing both molecules. The catecholamine-containing neurons of the brain did not contain detectable amounts of MAO-B. The specific distribution of MAO-B in the adult central nervous system indicates that the role of MAO-B in monoamine metabolism may be more specifically defined than previously believed.

Clorgyline delays the phase-position of circadian neurotransmitter receptor rhythms

The number of alpha- and beta-adrenergic, muscarinic cholinergic, opiate, and benzodiazepine receptors in rat forebrain, and dopamine and benzodiazepine receptors in striatum, change throughout the day. The diurnal rhythms of these receptors were altered by treatment with the monoamine-oxidase inhibitor clorgyline: following treatment some or all rhythm characteristics of wave form, amplitude, 24-h mean, and phase, were affected. One common effect of treatment was a delay in phase-position of binding to alpha- and beta-adrenergic, opiate and benzodiazepine receptors. Additionally, the nocturnal elevation in pineal melatonin which normally returns to baseline at light onset, persisted 3 h into the light period after clorgyline administration. These biochemical observations extend behavioural findings that clorgyline can delay the phase-position of rodent nocturnal activity onset, and does so by slowing the central circadian pacemaker.

Localization of monoamine oxidases A and B in primate brains relative to neuron-specific and non-neuronal enolases

Using serotonin and phenylethylamine deamination as measures of MAO A and MAO B activity respectively, positive correlations were observed between the activities of MAO A and MAO B in different areas of rhesus monkey and human brains. When the activities of MAO A and MAO B were compared with those of neuron-specific enolase and nonneuronal enolase (isozymes which are markers for neurons and glia), a slight but non-significant correlation was observed, suggesting that a simple distribution of MAO A in neurons and MAO B in glia is unlikely. This conclusion is supported by studies using synaptosomes, but contrasts with that from investigations of MAO from peripheral tissues, where experiments indicate that MAO A is predominantly localized in neurones.

Cardiovascular changes in response to selective monoamine oxidase inhibition in the rat

Chronic (21 days) treatment with the selective monoamine oxidase (MAO)-A inhibitor clorgyline, but not with the MAO-B inhibitor deprenyl in pithed rats leads to increased blood pressure responses to sympathetic stimulation and intravenous tyramine, and to elevated unstimulated heart rates. No significant changes are observed in plasma catecholamine responses to sympathetic stimulation, nor in beta-adrenoreceptor numbers in heart ventricles. These findings suggest that the hypotensive effects of MAO inhibitors result from central nervous system rather than peripheral nervous system alterations.

Lifetime monoamine oxidase inhibition and sleep

The effects of clorgyline, a type A monoamine oxidase (MAO) inhibitor, on the sleep of the rat were examined after subacute and lifetime administration. When 2 mg/kg 24 hours were given for 60 hours, type A MAO was inhibited by 92% and a significant reduction in REM sleep time was noted. When fetal rats were exposed to maternal dosage of 1 mg/kg/24 hours and then received this dose from one to 6 weeks postnatally, type A MAO was inhibited by 99%, but there were no alterations in the EEG sleep stages. In summary, subacute administration of clorgyline resulted in decreases in both Type A MAO and REM sleep; during chronic administration in a developing animal, Type A MAO was again decreased, but there was no corresponding change in REM sleep.

REM sleep suppression induced by selective monoamine oxidase inhibitors

The effects of 4 weeks of treatment with the selective monoamine oxidase (MAO) inhibiting antidepressant clorgyline and pargyline on the sleep of affectively disordered patients were studied. Both inhibitors resulted in near total suppression of REM sleep, a decrease in total sleep time, and an increase in the percent of stage 2 sleep. Clorgyline also increased awake time and decreased total recording period and sleep latency. In general, changes were greater for clorgyline than for pargyline and were about 50% slower to return to baseline after clorgyline compared to pargyline discontinuation. The results were consistent with the hypothesis that selective inhibition of the MAO type A, as produced by clorgyline, is sufficient to induce marked sleep changes. MAO inhibitor-induced receptor changes are proposed to account for the time course of the REM suppression and the REM rebound observed upon withdrawal.

Alteration of dopamine synthesis in rat striatum subsequent to selective type A monoamine oxidase inhibition

Pretreatment of rat striatal slices with the selective type A monoamine oxidase (MAO) inhibitor clorgyline was found to produce significant inhibition of dopamine (DA) synthesis. DA synthesis was reduced by nearly 50%, but not until more than 90% of the type A enzyme was inhibited. In contrast, complete inhibition of the type B MAO following deprenyl treatment had no effect. It is suggested that interneuronal accumulation of DA following inhibition of type A MAO leads to feedback inhibition at the rate-limiting step in DA biosynthesis, tyrosine hydroxylation. These results are also consistent with the presence of a type A MAO within DA-containing neurons and provide evidence of a regulatory role for type A MAO in the synthesis of brain DA.

Tyramine infusions and selective monoamine oxidase inhibitor treatment. II. Interrelationships among pressor sensitivity changes, platelet MAO inhibition, and plasma MHPG reduction

The relationship between changes in IV tyramine pressor sensitivity accompanying selective monoamine oxidase (MAO) inhibitor treatment and estimates of MAO-A and MAO-B inhibition in vivo were studied. Reductions in platelet MAO activity provided an index of MAO-B inhibition, while changes in plasma 3-methoxy-4-hydroxyphenethylene glycol (MHPG) were used as an hypothesized reflection of MAO-A inhibition. Chronic treatment with the MAO-A inhibitor clorgyline and the MAO-B inhibitor pargyline showed significant inhibition of the alternate MAO enzyme as well, although this crossover effect was greater for pargyline than clorgyline. The MAO-B inhibitor deprenyl appeared to maintain the greatest degree of MAO inhibition selectivity in vivo. Tyramine pressor sensitivity changes accompanying administration of the MAO inhibitors were highly correlated with decreases in plasma MHPG (r = 0.92), supporting our previous data indicating the rank order of clorgyline greater than pargyline greater than deprenyl for enhancement of tyramine pressor sensitivity and, thus, suggesting that tyramine potentiation is primarily a function of MAO-A rather than MAO-B inhibition. Changes in plasma MHPG are suggested to provide a potentially useful clinical index of in vivo MAO-A inhibition.

Depletion of epinephrine in rat hypothalamus by Ro 4-1284: influence of pargyline and harmaline

Ro 4-1284, 2-ethyl-2,3,4,6,7,11b-hexahydro-3 isobutyl-9,10-dimethoxy-2H-benzo[a]quinolizin-2-ol hydrochloride, depleted epinephrine in rat hypothalamus; the depletion occurred within 1 hr and persisted at 24 hr. The rapid lowering of epinephrine by Ro 4-1284 was antagonized by pargyline, a monoamine oxidase inhibitor, which alone elevated epinephrine concentration. The ability of pargyline to antagonize epinephrine depletion by Ro 4-1284 was prevented by co-treatment with harmaline, which protects aginst type A but not type B monoamine oxidase inhibition by pargyline. Pargyline also antagonized the Ro 4-1284-induced depletion of norepinephrine in hypothalamus and of norepinephrine and dopamine in cerebral hemispheres, and these effects of pargyline were also prevented by co-treatment with harmaline. The results suggest that epinephrine in rat brain, like norepinephrine and dopamine, is destroyed primarily by type A monoamine oxidase.