Bupropion: a review of its mechanism of antidepressant activity.

BACKGROUND: The mechanism of action of the novel antidepressant bupropion remains unclear after many years of study. A review of the relevant biochemical, in vivo brain microdialysis, electrophysiologic, behavioral, and clinical data clarifies what is known about this unique compound and suggests possible modes of action. METHOD: A panel of 11 experts was convened for a conference to discuss bupropion's mechanism of antidepressant activity. Four of the panelists presented current research findings, followed by a discussion. RESULTS: (1) Biochemical studies suggest down-regulation of postsynaptic beta-adrenoceptors and desensitization of the norepinephrine-stimulated adenylate cyclase in the rat cortex occur only after chronic administration of very high doses of bupropion. (2) In vivo brain microdialysis studies demonstrate that, after chronic administration, there is an enhancement of bupropion-induced increases in extracellular dopamine in the nucleus accumbens. (3) Electrophysiologic data show that with acute dosing, bupropion reduces the firing rates of noradrenergic neurons in the locus ceruleus. The firing rates of dopaminergic neurons are reduced by bupropion in the A9 and A10 areas of the brain, but only at very high doses, and bupropion does not alter the firing rates of serotonergic neurons in the dorsal raphe. (4) Behavioral studies show that the most active metabolite of bupropion, hydroxybupropion (306U73), appears to be responsible for a large part of the compound's effects in animal models of antidepressant activity. (5) Clinical studies indicate that bupropion enhances noradrenergic functional activity as reflected by an increased excretion of the hydroxy metabolite of melatonin, while at the same time producing a presumably compensatory decrease in norepinephrine turnover. In one study, bupropion elevated plasma levels of the dopamine metabolite homovanillic acid in nonresponders, but not in responders. CONCLUSION: The mechanism of action of bupropion appears to have an unusual, not fully understood, noradrenergic link. The bupropion metabolite hydroxybupropion probably plays a critical role in bupropion's antidepressant activity, which appears to be predominantly associated with long-term noradrenergic effects. The mild central nervous system activating effects of bupropion appear to be due to weak dopaminergic mechanisms. There is some evidence that dopamine may contribute to bupropion's antidepressant properties. Antidepressant effects of bupropion are not serotonergically mediated.

Pharmacological properties of 403U76, a new chemical class of 5-hydroxytryptamine- and noradrenaline-reuptake inhibitor.

403U76 (5-chloro-[[2-[(dimethylamino)methyl]phenyl]thio]benzene- methanol hydrochloride) is a potent, competitive, inhibitor of 5-hydroxytryptamine (5-HT) and noradenaline reuptake into rat brain synaptosomes. Inhibition of 5-HT uptake in-vivo by 403U76 was demonstrated by potentiation of the behavioural effects of 5-hydroxytryptophan in rats and mice and blockade of p-induced depletion of 5-HT in rats. The firing of 5-HT-ergic dorsal raphe neurons in rats was decreased after intravenous administration of low doses of 403U76 as would be predicted for a 5-HT uptake inhibitor. 403U76 antagonized tetrabenazine-induced sedation, an effect associated with inhibitors of noradrenaline uptake, but not with inhibitors of 5-HT uptake. Thus 403U76 affects noradrenergic as well as 5-HT-ergic neurotransmission in-vivo. Potential anxiolytic activity was indicated by reductions in isolation-induced vocalizations in neonates after 403U76 treatment. Low intravenous doses of 403U76 were well tolerated and had no sustained cardiovascular effects. There were no deleterious behavioural side-effects at active doses. Effects observed on isolated tissues or transmitter receptors occurred only at very high concentrations and were pharmacologically unimportant. Thus 403U76 can be considered a potential antidepressant/anxiolytic agent that is a potent, selective inhibitor of 5-HT and noradrenaline reuptake.

Evidence that the acute behavioral and electrophysiological effects of bupropion (Wellbutrin) are mediated by a noradrenergic mechanism.

Bupropion (BW 323U66) has been considered a dopaminergic antidepressant based on its ability to inhibit the uptake of dopamine (DA) somewhat more selectively than it inhibits uptake of norepinephrine (NE) or serotonin (5-HT). This report describes new evidence that bupropion selectively inhibits firing rates of NE cells in the locus coeruleus (LC) at doses significantly lower than those that inhibit activity of midbrain DA cells or dorsal raphe 5-HT cells. The IC50 dose (13 mg/kg i.p.) for inhibition of LC firing produced plasma concentrations that were not significantly different from those generated by the ED50 in the Porsolt test (10 mg/kg i.p.). The fourfold higher dose needed to inhibit DA cell firing (IC50 = 42 mg/kg i.p.) was similar to the dose associated with locomotor stimulation in freely moving rats. Bupropion did not change the firing rates of 5-HT cells in the dorsal raphe nucleus at any dose. In both in vitro and in vivo tests, the metabolite 306U73 (hydroxybupropion), a weak inhibitor of NE uptake, was approximately equipotent to bupropion with regard to inhibition of LC cells. Another metabolite, 494U73, had no effect on LC firing rates over a wide range of doses. Because of species variation in metabolism, 306U73 was not detected in plasma of rats after i.v. doses of bupropion that inhibited LC firing. Only trace amounts of 306U73 were detected after bupropion dosing for the Porsolt test. Pretreatment with reserpine markedly depleted catecholamines and reduced (by 30-fold) the potency of bupropion to inhibit LC firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and evaluation of the antidepressant activity of the enantiomers of bupropion.

The synthesis of the enantiomers of bupropion, (rac)-2-tert-butylamino-3'-chloropropiophenone 1 (Wellbutrin) is described. The enantiomers were compared with the racemate in both the tetrabenazine-induced sedation model and the inhibition of uptake of biogenic amine assay. No significant differences were found in their potencies to reverse tetrabenazine-induced sedation in mice or in their IC50 values as inhibitors of biogenic amine uptake into nerve endings obtained from mouse brain.

Benzodiazepine receptor binding activity of 9-(1-phenylethyl)purines.

Several alpha-methyl analogues of the 9-benzylpurines that bind to the benzodiazepine receptor (BZR) were synthesized and tested for BZR-binding activity. Although introduction of a m-amino group and an 8-bromo substituent gave an additive increase in BZR affinity with 9-(3-aminobenzyl)-8-bromo-6-(dimethylamino)-9H-purine (4), addition of an alpha-methyl group to 4 resulted in a loss in BZR affinity. This loss in affinity is apparently due to repulsive, steric interactions between the 8-bromo and 9-(1-phenylethyl) substituents, which results in a conformation that is not optimal for interaction with the BZR. Several compounds were tested on a modified Geller-Seifter conflict schedule, but none exhibited significant anxiolytic activity.

Benzodiazepine receptor binding activity of 8-substituted-9-(3-substituted-benzyl)-6-(dimethylamino)-9H-purines.

A series of 8-substituted analogues of 9-(3-aminobenzyl)-6-(dimethylamino)-9H-purine (8) were synthesized and tested for their ability to bind to the benzodiazepine receptor (BZR) in rat brain tissue. The most active compound was the 8-bromo-9-(3-formamidobenzyl) analogue 16 (IC50 = 0.011 microM), which was 1000-fold more active than the parent 9-benzyl-6-(dimethylamino)-9H-purine (1) and nearly as active as diazepam. Although substitution of a m-formamido group and an 8-bromo substituent on 1 imparted potent BZR binding activity, neither 16 nor 11 analogues exhibited significant anxiolytic activity on a modified Geller-Seifter conflict schedule.

Benzodiazepine receptor binding activity of 6,9-disubstituted purines.

A series of 6,9-disubstituted purines were tested for their ability to bind to the benzodiazepine receptor in rat brain tissue. One of the most active compounds was 9-(3-aminobenzyl)-6-(dimethylamino)-9H-purine (44) with an IC50 = 0.9 microM, which was only 4.5-fold higher than the IC50 for chlordiazepoxide. Substitution of a 3-aminobenzyl or 3-hydroxybenzyl group at the 9-position of 6-(dimethylamino)purine led to over a 50-fold increase in receptor affinity. Compound 44 did not exhibit significant anxiolytic activity, nor did anticonvulsant activity correlate with relative receptor binding affinity.

Pharmacology of the enantiomers of threo-methylphenidate.

The pharmacology of the enantiomers of threo-methylphenidate (MPH) was evaluated in the rat to assess the relative contribution of each isomer to central and peripheral actions of the racemic drug. Fractional recrystallization of binaphthyl phosphate salts of dl-threo-MPH allowed resolution of d-threo-MPH and 92% enrichment of l-threo-MPH. The enantiomeric disposition was monitored using gas chromatographic separation of trifluoroacetylprolyl diastereomeric derivatives. The activity of the d-isomer was greater than the l-isomer in the induction of locomotor activity and the inhibition of tritiated dopamine and l-norepinephrine uptake into striatal and hypothalamic synaptosomes, respectively. Neither isomer produced a significant change in the spontaneous release of tritiated catecholamines from synaptosomes. Destruction of catecholaminergic neurons by 6-hydroxydopamine pretreatment attenuated the locomotor response to d-threo-MPH, indicating the involvement of catecholaminergic neural pathways in the locomotor response. Only the d-enantiomer significantly potentiated the pressor responses to i.v. l-norepinephrine. Receptor site stereoselectively for threo- vs. erythro-MPH is discussed in terms of isomer conformational preferences. These results suggest that synaptic inhibition of catecholamine uptake by d-threo-MPH may be involved fundamentally in behavioral and pressor effects of the racemic drug.

Evidence that the potential antipsychotic agent rimcazole (BW 234U) is a specific, competitive antagonist of sigma sites in brain.

Rimcazole (BW 234U) is a potential antipsychotic agent which in open-clinical trials appears to be effective in acute schizophrenic patients. In the present study, rimcazole was found to block the specific binding of [3H]-(+)-SKF 10,047 to sigma sites in rat and guinea pig brain (IC50 = 5.0 X 10(-7) M). The compound was 100 times weaker as a blocker of phencyclidine sites (IC50 = 4.3 X 10(-5) M). At 1 X 10(-5) M, rimcazole had only weak effects on mu, delta, kappa and epsilon opioid receptors. Scatchard analysis of the binding data from guinea pig brain revealed an apparent KD for [3H]-(+)-SKF 10,047 of 85 +/- 5 nM and a Bmax of 824 +/- 27 fmole/mg protein. In the presence of 5 X 10(-7) M BW 234U, the apparent KD was 165 +/- 35 nM, but the Bmax (892 +/- 146 fmoles/mg protein) was not affected. This suggests that rimcazole is a competitive inhibitor of sigma sites. The agent was also capable of blocking sigma sites in vivo (ID50 = 6 mg/kg i.p., mice) as judged by an in vivo sigma receptor binding assay. Thus, if the antipsychotic activity of rimcazole is confirmed in double-blind, placebo-controlled trials, it would be the first compound whose mechanism of antipsychotic activity may best be explained by a direct blockade of sigma sites and not by a direct blockade of dopamine (D2) receptors in brain.

Bupropion: a new antidepressant drug, the mechanism of action of which is not associated with down-regulation of postsynaptic beta-adrenergic, serotonergic (5-HT2), alpha 2-adrenergic, imipramine and dopaminergic receptors in brain.

The present experiments were undertaken to determine: (1) whether bupropion had any direct effects on receptors present in rat brain; (2) whether the drug could down-regulate postsynaptic beta-adrenergic, alpha 2-adrenergic, serotonergic, imipramine and dopaminergic receptors after chronic administration, as had been demonstrated for tricyclic antidepressants, monoamine oxidase (MAO) inhibitors, electroconvulsive therapy (ECT) and "atypical" antidepressants. Bupropion was found to be weak or inactive when its affinity for 14 different receptors present in brain was assessed by binding assays. The drug failed to desensitize beta-adrenergic receptors in the cerebral cortex of the rat as determined by [3H]dihydroalprenolol binding, after being administered at 25 mg/kg (i.p.) once a day for 6 weeks, or after being administered by the intraperitoneal route to rats at doses as large as 150 mg/kg per day for 4 days. When administered at doses of 37.5, 75 and 150 mg/kg per day for 21 days, the drug had no effect on beta-adrenergic, alpha 2-adrenergic, imipramine or serotonergic (5-HT2) receptors in the brain of the rat as determined by Scatchard analysis of the binding data. These data show that the antidepressant activity of bupropion is not associated with a down-regulation of receptors in the CNS commonly implicated in the mechanism of action of antidepressant drugs. Bupropion also produced a dose-dependent tendency to decrease the activity of norepinephrine-stimulated adenylate cyclase in slices of cerebral cortex obtained from rats treated chronically with the drug. However, the decrease was highly variable, was most obvious in tissues obtained from rats receiving large, non-pharmacologically relevant doses (150 mg/kg per day) of the drug and was statistically significant at only one of three concentrations of the agonist that produced maximal stimulation of the enzyme.

Studies of bupropion's mechanism of antidepressant activity.

The data obtained in these studies show that the antidepressant activity of bupropion cannot be explained by its ability to inhibit MAO present in brain or to increase the release of biogenic amines from nerve endings, since the drug possesses neither of these properties. It is also unlikely that the weak properties of the drug as an inhibitor of dopamine uptake in brain can explain its antidepressant activity. It is clear, however, that dopamine neurons must be present for the CNS properties of bupropion to be manifested in animal models; at antidepressant doses of the drug, dopamine turnover is reduced in brain. Finally, the antidepressant properties of bupropion have been dissociated from down-regulation of postsynaptic beta-receptors. To our knowledge, bupropion is the first clinically effective antidepressant whose mechanism of action cannot be explained on the basis of alterations in either presynaptic events or postsynaptic receptor-mediated events in catecholamine or serotonin pathways. Thus, bupropion is a novel antidepressant whose mechanism of action must still be elucidated.

Effects of in vivo beta-adrenoceptor down-regulation on cardiac responses to prenalterol and pirbuterol.

The effects of beta-adrenoceptor down-regulation on rat left atrial responses to the full agonist isoproterenol and the partial agonists prenalterol and pirbuterol were studied. Atria from rats implanted with a mini-osmotic-pump subcutaneously delivering isoproterenol at 400 micrograms X kg-1 X h-1 for 4 days were 12-16 times less sensitive to isoproterenol than normal atria. Scatchard analyses indicated that the implantation of these mini-osmotic-pumps produced a 50% decrease in the number of beta-adrenoceptors in the ventricles with no alteration in the binding constant of [3H]dihydroalprenolol. Receptor down-regulation produced a parallel shift to the right of concentration-response curves to isoproterenol, and a complete suppression of responses to both partial agonists. The affinities of the partial agonists for the receptors were not altered; in desensitized atria the partial agonists were functional beta-antagonists blocking the responses to isoproterenol. The data are modeled by equations from classical receptor theory which predict the depression of responses to partial agonists with receptor down-regulation. These results are discussed with respect to the utility of prenalterol and pirbuterol in congestive heart failure since possible tachyphylaxis to these drugs with chronic usage could limit their value.

A structure-activity study of the transport sites for the hypothalamic and striatal catecholamine uptake systems. Similarities and differences.

A series of eight substrate molecules (substituted phenethylamines, guanethidine, and bretylium) had slightly less affinity for striatal than for hypothalamic synaptosomal uptake receptors as judged by ratios of striatal (s) to hypothalamic (h) IC50 values (s/h average = 3.9; range 2.0--6.0). Catecholamine uptake in striatum was very insensitive to tricyclic antidepressant inhibitors, whereas catecholamine uptake in hypothalamus was very sensitive to these agents (s/h average = 233; range 24--570). By way of contrast with both the substrates and the tricyclic inhibitors, the inhibitors with less rigidly fixed rings or analogous groups (deoxypipradrol, methylphenidate, cocaine) were potent in both brain preparations (s/h average = 1.2; range 0.6--2.3). It is concluded that the rings of nontricyclic inhibitors are able to bind to appropriate hydrophobic binding groups in both receptors, that these receptive groups have different topography in striatum and in hypothalamus, and that the topography in the striatum is incompatible with binding tricyclic systems. The data also indicate that there is great similarity, if not identity, in the receptive area for substrates in striatum and hypothalamus. Although the substrates and inhibitors bind to some groups in common in this substrate receptive area, it is the surrounding hydrophobic molecular environment that is clearly different and permits the phenomenon of selective blockade with drugs.

Neurochemical and neuropharmacological investigations into the mechanisms of action of bupropion . HCl--a new atypical antidepressant agent.

In the present study, bupropion has been shown to be effective in several behavioral models predictive of antidepressant activity suggesting that it should be an effective antidepressant in man. Furthermore, the data also show that the antidepressant activity of the drug cannot be due to its ability to inhibit MAO present in brain or to increase the release of biogenic amines from nerve endings. It also appears unlikely that the weak properties of the drug as an inhibitor of catecholaminergic pumps in brain csn explain its antidepressant activity. However, the weak but selective block of dopaminergic pumps observed in vivo can be correlated with the mild CNS stimulant properties observed in rodents. Bupropion, failed to desensitize beta-adrenergic receptors in rat cerebral cortex in chronic studies and exhibited equivocal results in acute studies. These neurochemical properties of bupropion serve to distinguish it from typical antidepressants of the MAOI and tricyclic classes and suggest that it should be classified as an atypical antidepressant, whose mechanism of action must still be elucidated.

Erythrosine (Red No. 3) and its nonspecific biochemical actions: what relation to behavioral changes?

Biochemical studies have shown that the ability of erythrosine to inhibit dopamine uptake into brain synaptosomal preparations is dependent on the concentration of tissue present in the assay mixture. Thus, the finding that erythrosine inhibits dopamine uptake (which, if true, would provide a plausible explanation of the Feingold hypothesis of childhood hyperactivity) may simply be an artifact that results from nonspecific interactions with brain membranes. In addition, although erythrosine given parenterally (50 milligrams per kilogram) did not alter locomotor activity of control of 6-hydroxydopamine-treated rats, erythrosine (50 to 300 milligrams per kilogram) attenuated the effect of punishment in a "conflict" paradigm.

Comparison of the effects of the isomers of amphetamine, methylphenidate and deoxypipradrol on the uptake of l-[3H]norepinephrine and [3H]dopamine by synaptic vesicles from rat whole brain, striatum and hypothalamus.

The ATP-Mg++-dependent uptake of [3H]dopamine and l-[3H]norepinephrine into purified synaptic vesicles of whole rat brain, rat striatum and rat hypothalamus was inhibited 10-fold more effectively by S-(+)-amphetamine as compared to its corresponding (R-(-)-enantiomer. In contrast, S-(+)-deoxypipradrol and its R-(-)-enantiomer were approximately equipotent inhibitors of 3H-amine uptake into these synaptic vesicular preparations. The 1R:2R-methylphenidate was twice as potent as its 1R:2S-enantiomer as an inhibitor of 3H-catecholamine uptake. These data suggest that the receptor sites on the amine pumps present in the membranes of all three vesicular preparations are similar in so far as they are all sensitive to the stereochemical configuration around the alpha-carbon of amphetamine but are not sensitive to the stereochemical configuration around the analogous carbon of deoxypipradrol and methylphenidate. These observations are the reverse of those previously observed for the phenethylamine pumps present in peripheral and central neuronal membranes.