Occupancy of the serotonin transporter by fluoxetine,paroxetine, and sertraline: In vivo studies with [125i]rti-55

[¹²⁵I]RTI-55 was used in tracer doses to label serotonin (5-HT) transporters in vivo in the mouse brain. Fluoxetine, paroxetine, and sertraline, potent antidepressants and selective inhibitors of serotonin transporter sites, were administered in various doses and at various times. The doses and times that result in significant binding of the drugs to transporters correspond to doses and times where they are reported to have physiological effects. Estimates of occupancy rate and duration of binding to serotonin transporters were made. The rate of occupancy of the 5-HT transporter site was fastest for sertraline, intermediate for paroxetine and slowest for fluoxetine. Similarly, the duration of occupancy was significantly shorter for sertraline and paroxetine (approximately 10 h) than for fluoxetine (approximately 50 h). The results indicate that in competition studies, [¹²⁵I]RTI-55 can be used to identify doses of drugs that are physiologically effective, to determine their relative rate of occupancy, and most importantly, to measure the residency time on the central serotonin transporter in vivo. © 1994 Wiley-Liss, Inc.     

Effects of chronic antidepressant treatments on serotonin transporter function, density, and mRNA level.

To investigate functional changes in the brain serotonin transporter (SERT) after chronic antidepressant treatment, several techniques were used to assess SERT activity, density, or its mRNA content. Rats were treated by osmotic minipump for 21 d with the selective serotonin reuptake inhibitors (SSRIs) paroxetine or sertraline, the selective norepinephrine reuptake inhibitor desipramine (DMI), or the monoamine oxidase inhibitor phenelzine. High-speed in vivo electrochemical recordings were used to assess the ability of the SSRI fluvoxamine to modulate the clearance of locally applied serotonin in the CA3 region of hippocampus in drug- or vehicle-treated rats. Fluvoxamine decreased the clearance of serotonin in rats treated with vehicle, DMI, or phenelzine but had no effect on the clearance of serotonin in SSRI-treated rats. SERT density in the CA3 region of the hippocampus of the same rats, assessed by quantitative autoradiography with tritiated cyanoimipramine ([(3)H]CN-IMI), was decreased by 80-90% in SSRI-treated rats but not in those treated with phenelzine or DMI. The serotonin content of the hippocampus was unaffected by paroxetine or sertraline treatment, ruling out neurotoxicity as a possible explanation for the SSRI-induced decrease in SERT binding and alteration in 5-HT clearance. Levels of mRNA for the SERT in the raphe nucleus were also unaltered by chronic paroxetine treatment. Based on these results, it appears that the SERT is downregulated by chronic administration of SSRIs but not other types of antidepressants; furthermore, the downregulation is not caused by decreases in SERT gene expression.     

Differential in vivo clearance of serotonin in rat dorsal raphe nucleus and CA3 region

In vivo chronoamperometric recordings were used to determine if the majority of serotonin transporters (SERTs) in the dorsal raphe nucleus (DRN) are functionally active. This was achieved by comparing the clearance of exogenously applied serotonin (5-HT) from the extracellular fluid (ECF) of the DRN to that in the CA3 region of the hippocampus, an area with lower SERT density. Serotonin was pressure ejected into these regions in anesthetized rats and reproducible electrochemical signals measured by carbon fiber microelectrodes were recorded. Consistent with SERT density as measured by [3H]cyanoimipramine binding in these brain regions (DRN>CA3), clearance of 5-HT was significantly faster in DRN compared to that in the CA3 region. The selective serotonin reuptake inhibitor, fluvoxamine, prolonged 5-HT clearance in both CA3 and DRN. It is known that the norepinephrine transporter (NET) contributes to clearance of 5-HT in the dentate gyrus (DG) but not in CA3. Given that the DRN receives noradrenergic innervation, it was also determined if the NET contributes to 5-HT clearance in the DRN. Destruction of the NET with the neurotoxin 6-hydroxydopamine failed to alter 5-HT clearance parameters in the DRN. These data support the hypothesis that serotonin transporters are functionally active in the DRN, that serotonin clearance is mediated primarily by the SERT in the DRN and that the faster clearance of 5-HT from this region is related to its greater density of functional SERTs.     

St. John's wort extract Ze 117 (Hypericum perforatum) inhibits norepinephrine and serotonin uptake into rat brain slices and reduces 3-adrenoceptor numbers on cultured rat brain cells

Despite almost forty years of widespread use, the mode of action of antidepressant drugs is still largely unknown. There is agreement that these drugs interact with central neurotransmission. Common findings are acute inhibitory actions on reuptake mechanisms for norepinephrine (NE) and for serotonin (5-HT) at presynaptic axons and chronic adaptive effects on neurotransmitter receptors on postsynaptic membranes. In particular, beta-adrenoceptor downregulation has been observed after chronic treatment with most antidepressants in vivo and in cell culture systems. We studied the effectiveness of Ze 117 (St. John's wort) extract (Hypericum perforatum) on NE- and 5-HT-uptake into rat brain slices. Potency and efficacy of the Ze 117 extract were compared with those of tricyclic (TCA) and selective serotonin reuptake inhibitor (SSRI)-type antidepressants. A dose-dependent inhibition was seen on NE and 5-HT uptake into brain slices. The Ze 117 extract was more selective for the uptake of NE than for that of 5-HT. The maximal extent of uptake inhibition by Ze 117 extract was comparable to that of imipramine (IMI), desipramine (DMI) or fluvoxamine for 5-HT, but lower for NE transport, than that of the synthetic antidepressants. Chronic exposure (8 days) of confluent C6-cell cultures to Ze 117 extract resulted in a dose-dependent beta-adrenoceptor downregulation equal to that induced by DMI, a potent TCA. None of these effects could be achieved with either hypericin or hyperforin alone in a relevant dose range. Our results indicate that the St. John's wort extract Ze 117 contains active, but as yet unknown antidepressant principles with effects comparable to those of TCAs.     

Similar properties of fetal and adult amine transporters in the rat brain

A variety of drug classes, including psychomotor stimulants and antidepressants, interact with monoamine transporters in order to exert their effects. Although these transporters have been extensively characterized in the adult brain, little is known about uptake mechanisms in the fetal system. High affinity dopamine (DA) and serotonin (5-HT) uptake in the striatum and frontal cortex, respectively, were examined in rat fetuses (embryonic day 20; E-20). These results were then compared to uptake in adult rat synaptosomal preparations of the same regions. The data indicate that the fetal (E-20) uptake mechanism is sodium-dependent. Furthermore, the potency of various agents to inhibit transporter function was assessed. These drugs produced a concentration-dependent inhibition of uptake, and the resulting IC₅₀ values were not significantly different from those obtained in the adult preparations. Our results provide evidence that the affinity of monoamine uptake inhibitors for fetal (E-20) DA and 5-HT transporters is similar to that observed with adult transporters. This observation has broad implications when considering neuronal development and in utero exposure to drugs that exert their effects through these transporters.     

The effect of isolation rearing on volitional ethanol consumption and central CCK/dopamine systems in Fawn-Hooded rats

Numerous studies have demonstrated that socially isolating rats (from weaning) produces a sustained anxious phenotype and an enhanced response to psychostimulant drugs such as amphetamine and cocaine. In addition, isolation rearing has been shown to induce significant changes in the mesolimbic dopamine system. These data indicate that isolation rearing not only induces an anxiogenic phenotype but also induces neurochemical changes in reward nuclei of the brain, which is correlated with an enhanced response to psychostimulants. For these reasons, the effect of isolation rearing on volitional ethanol consumption was examined in Fawn-Hooded (FH) rats and correlated with neurochemical changes in central dopamine and cholecystokinin systems. Social isolation from weaning produced an anxiogenic phenotype as measured by a decreased time spent on the open arms of an elevated plus-maze. Interestingly, isolation-rearing induced a greater proportion of FH rats to acquire preference for ethanol while having no effect on the amount of ethanol consumed by alcohol-preferring rats. In addition, isolation rearing induced a number of changes in central CCK/dopamine systems.     

Possible alteration of tryptophan metabolism following repeated administration of sertraline in the rat brain

The levels of tryptophan and the serotonin (5-HT) turnover were examined in various brain regions of rats after single or repeated treatment with the selective 5-HT uptake inhibitor, sertraline. A single administration of sertraline (10mg/kg, i.p.) increased tryptophan and 5-HT levels in all the brain regions investigated. The levels of 5-hydroxyindolacetic acid (5-HIAA) decreased in various brain regions. The 5-HIAA/5-HT ratio as turnover index was significantly decreased by a single administration of sertraline in all the brain regions investigated. Daily treatment with sertraline (10mg/kg) for 21 days did not affect tryptophan and 5-HT levels in various brain regions 1h after last injection. The 5-HT turnover was not changed in any of the brain regions investigated by a repeated administration of sertraline. In conclusion, the data show that the increase in tryptophan levels and the decrease in 5-HT turnover in rat brain are attenuated by repeated treatment of sertraline.     

Regional changes in brain dopamine and serotonin metabolism induced by conditioned circling in rats: effects of water deprivation, learning and individual differences in asymmetry

Rats were trained, using water reinforcement, to turn in circles (rotation) during 1 h daily test sessions. After achieving criterion performance (100 full turns per hour) for at least 10 consecutive sessions, rats were sacrificed 20 min after starting a session and levels of dopamine, DOPAC (3,4-dihydroxyphenylacetic acid), serotonin, and 5-HIAA (5-hydroxyindoleacetic acid) were assayed in nigrostriatal (corpus striatum), mesolimbic (nucleus accumbens) and mesocortical (medial prefrontal cortex) brain regions. Other control groups of rats were comparably water deprived or satiated at the time of sacrifice. Although, as previously reported, evidence of 'two populations' of rats was again apparent with respect to the relationship between direction of spontaneous turning and asymmetry in striatal dopamine levels, there were no lateralized effects of operant rotational training on striatal dopamine and DOPAC levels nor on the DOPAC/dopamine ratio. There were, however, bilateral neurochemical effects of both rotational training and water deprivation in striatum: an increase in the 5-HIAA/serotonin ratio in both sexes was attributable to learning whereas an increase in the DOPAC/dopamine ratio in males was attributed to water deprivation. A bilateral decrease in the DOPAC/dopamine ratio in the mesolimbic and mesocortical regions of both sexes was also induced by water deprivation. The only lateralized neurochemical changes associated with learning to rotate in the operant task occurred in the medial prefrontal cortex: in both sexes, dopamine levels were higher in the ipsilateral than in the contralateral cortex and the DOPAC/dopamine ratio was greater in the contralateral than in the ipsilateral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Opposite modulatory effects of ovarian hormones on rat brain dopamine and serotonin transporters

The present study was designed to investigate the modulatory effect of gonadal steroids on brain dopamine (DA) and serotonin (5-HT) presynaptic transporters in female and male rats. Female and male rats were castrated and treated with either vehicle or gonadal hormones. The pharmacodynamic characteristics of the DA and 5-HT transporters were analyzed by [³H]BTCP and [³H]imipramine binding respectively. Ovariectomy (OVX) resulted in an upregulation of the striatal DA transporter and this alteration was prevented by estradiol (E₂) or E₂+progesterone (P) treatment but not by P alone. In contrast to the DA transporter, the hypothalamic 5-HT transporter was down-regulated by OVX in female rats and this decrease was reversed by the administration of E₂, P or their combination. The striatal DA transporter and the hypothalamic 5-HT transporter in male rat were not affected by orchidectomy or by administration of testicular hormone. Our findings indicate that ovarian, but not testicular, steroid hormones may play an important role in the regulation of brain DA and 5-HT transporters. It appears that ovarian hormones modulate rat brain 5-HT and DA transporters in opposite directions. These interactions between ovarian steroids and presynaptic transporters may be relevant to DA- and 5-HT-related neuropsychiatric disorders.     

In vivo biogenic amine efflux in medial prefiontal cortex with imipramine,fluoxetine, and fluvoxamine

In vivo brain microdialysis was used to determine the effects of the standard tricyclic antidepressant imipramine and the two selective serotonin reuptake inhibitors (SSRIs) antidepressants, fluoxetine and fluvoxamine, on extracellular levels of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) in rat medial prefrontal cortex. When given intraperitoneally (IP), imipramine increased NE in the microdialysis perfusate, and elevated DA and 5-HT to a lesser extent. Similar dose-dependent increases in DA and 5-HT were detected after IP fluoxetine, although NE was less affected. In contrast, IP fluvoxamine produced no change in basal NE nor DA, although a large increase in 5-HT occurred at an intermediate dose. When administered directly into cortex, all three antidepressants increased 5-HT by the same amount in a dose-dependent fashion. Intracortical imipramine and fluoxetine increased NE, and fluoxetine and fluvoxamine both increased DA, with fluoxetine doing so at a lower concentration. These data suggest that the SSRIs are not entirely selective for serotonin in vivo. © 1994 Wiley-Liss, Inc.     

Alterations in alcohol consumption, withdrawal seizures, and monoamine transmission in rats treated with phentermine and 5-hydroxy-L-tryptophan

We have previously shown that coadministration of the dopamine (DA) agonist phentermine plus the serotonergic agonist fenfluramine suppresses alcohol intake and withdrawal seizures in rats. In the present study, phentermine and the serotonin (5-HT) precursor, 5-hydroxy-L-tryptophan (5-HTP), were administered alone, or in combination, to rats fed on a 6% alcohol-containing diet or an isocaloric control diet. Following a 9-h withdrawal period from the alcohol-containing diet, phentermine enhanced the effects of 5-HTP on both reduction of alcohol withdrawal seizures as well as changes in striatal serotonin. Food intake was monitored for 24 h after drug treatment, and neurochemical measures were examined at various time points. Phentermine alone reduced food intake in all diet conditions, but this anorectic effect was followed by hyperphagia in control rats. Phentermine plus 5-HTP reduced the consumption of the alcohol-containing diet, while its effects on consumption of control diets were mixed. In vivo microdialysis in rat nucleus accumbens revealed that phentermine increased extracellular DA, whereas 5-HTP caused marked elevations in extracellular 5-HT. Coadministration of phentermine and 5-HTP evoked simultaneous elevations in extracellular DA and 5-HT that mirrored the effects of each drug alone. Collectively, these findings show that coadministered phentermine plus 5-HTP is effective in reducing alcohol intake and suppressing alcohol withdrawal seizures. These therapeutic actions may be related to elevations in synaptic DA and 5-HT in critical brain regions.     

Effects of genetic background on neonatal Borna disease virus infection-induced neurodevelopmental damage. II. Neurochemical alterations and responses to pharmacological treatments

The gene-environment interplay is thought to determine variability in clinical conditions and responses to therapy in human neurodevelopmental disorders. Studying abnormal brain and behavior development in inbred strains of rodents can help in the identification of the complex pathogenic mechanisms of the host-environment interaction. This paper is the second one in a series of the two reports of the use of the Borna disease virus (BDV) infection model of neurodevelopmental damage to characterize effects of genetic background on virus-induced neurodevelopmental damage in inbred rat strains, Lewis and Fisher344. The present data demonstrate that neonatal BDV infection produced regional and strain-related alterations in levels of serotonin, norepinephrine and in levels of serotonin turnover at postnatal day 120. Neonatal BDV infection also induced upregulation of hippocampal 5-HT(1a) and cortical 5-HT(2a) receptors in Lewis rats and downregulation of cortical 5-HT(2a) receptors in Fisher344 rats. BDV-associated regional downregulation of D(2) receptors and dopamine transporter sites were noted in Fisher344 rats. In addition to the neurochemical disturbances, neonatal BDV infection induced differential responses to serotonin compounds. While 8-OH-DPAT suppressed virus-enhanced ambulation in BDV-infected Fisher344, fluoxetine inhibited virus-induced hyperactivity in BDV-infected Lewis rats only. The present data provide new insights into the pathogenic events that lead to differential responses to pharmacological treatments in genetically different animals following exposure to the same environmental challenge.     

Neurochemical changes in brain serotonin neurons in immature and adult offspring prenatally exposed to cocaine

The present study investigates the age-dependent effects of prenatal cocaine exposure on changes in the neurochemical and functional status of brain serotonin neurons. Pregnant rats were administered either saline or (−)cocaine HCl (15 mg/kg, subcutaneously), twice daily from gestational days 13 through 20. Neurochemical changes in frontal cortex, hypothalamus, hippocampus, striatum and midbrain of prepubescent and adult offspring were determined by measuring: (1) the content of serotonin (5-HT) and its major metabolite 5-hydroxyindolacetic acid (5-HIAA), and (2) the ability of the serotonin releasing drug p-chloroamphetamine (PCA) to reduce brain serotonin levels. Brain catecholamine content was determined in progeny for comparative purposes. Prior to maturation, prenatal exposure to cocaine did not alter basal levels of brain 5-HT or 5-HIAA in any brain region examined. However, in adult progeny prenatally exposed to cocaine, basal 5-HT content was significantly reduced in the frontal cortex (−32%) and hippocampus (−40%), suggesting maturation-dependent effects of prenatal cocaine exposure on brain 5-HT neurons. Consistent with the maturational onset of changes in 5-HT, striatal dopamine was significantly reduced (−10%) by prenatal exposure to cocaine only in adult offspring. Reductions in 5-HT in most brain regions, produced by pharmacological challenge with p-chloroamphetamine (PCA), were comparable in prenatal saline versus cocaine offspring. One notable exception was the markedly greater reduction (−40%) in 5-HT in the midbrain of immature offspring prenatally exposed to cocaine, suggesting alterations in midbrain 5-HT neurons prior to maturation. Overall, these data demonstrate prenatal cocaine exposure produces region-specific changes in 5-HT neurons in offspring with some deficits occurring only following maturation.     

Selective changes in the contents of noradrenaline, dopamine and serotonin in rat brain areas during aging

This study examines the age-associated changes in noradrenaline (NA), dopamine (DA), 3,4-dihydroxyphenyl-acetic acid (DOPAC), serotonin (5-HT) and 5-hydroxy-3-indoleacetic acid (5-HIAA) in different brain areas of rats. DA and DOPAC concentrations in striatum increased at third month of age, remaining without significant variations until 12th month of age, and decreasing in 24-month-old rats. DA concentration dropped in hippocampus, amygdala and brainstem of 24-month-old-rats, whereas DOPAC levels decreased only in hippocampus. These changes suggest an age-dependent deficit of the dopaminergic system, presumably related to a reduced number/activity of DA nigrostriatal and mesolimbic neurons. An age-induced decline in NA content was found in the pons-medulla, the area containing NA neuronal bodies. Concentrations of 5-HT were reduced with aging in frontal cortex, showing a tendency to decrease in all brain areas examined. The increased 5-HIAA/5-HT ratio found in frontal cortex, amygdala and striatum suggests an age-related decreased synthesis and an accelerated 5-HT metabolism. The 5-HIAA content decreased in brainstem of the oldest rats. These findings point to a selective impairment of nigrostriatal and mesolimbic DA in aging rats, whereas reductions in NA were restricted to cell bodies region and 5-HT showed changes of different extent in areas of terminals and neuronal cell bodies.     

Zinc modulation of serotonin uptake in the adult rat corpus callosum

Antidepressants partially inhibit the uptake of 5-hydroxytryptamine (5-HT; serotonin) in the rat corpus callosum (CC), a white matter commissure involved in interhemispheric brain communication. It is also known that zinc modulates many proteins, including neurotransmitter transporters. We examined the effects of zinc on the uptake of 5-HT into slices of the adult rat CC, in the absence or presence of some antidepressants. Zinc increased 5-HT uptake in a concentration-dependent manner when the CC slices were incubated in a solution buffered with sodium bicarbonate; however, zinc exerted no effect on 5-HT transport when HEPES was the buffer. Potentiation of 5-HT uptake by zinc was maximal with 1 microM (45% over the control uptake). Moreover, 1 microM zinc potentiated 5-HT uptake in the cingulate cortex by 58% and in the Raphe nucleus by 65%. The antidepressants fluoxetine and imipramine inhibited 5-HT uptake in the CC by approximately 50%, whereas 6-nitroquipazine, a potent 5-HT uptake blocker, inhibited uptake by only 23%. Interestingly, inhibition of 5-HT uptake by all three substances, fluoxetine, imipramine, and 6-nitroquipazine, was counteracted by the presence of 1 microM zinc. Free zinc may thus contribute to modulation of extracellular levels of 5-HT and its removal. These actions should be considered in the treatment of mental depression with antidepressants.     

Changes in the serotonergic, noradrenergic and dopaminergic levels in the brain of scrapie-infected rats

Levels of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethyleneglycol (DHPG) were determined by high-performance liquid chromatography in different brain areas of scrapie-infected rats, 8.5 months after intracerebral inoculation of a rat-adapted strain from mice brain (C 506). At this time, rats developed early clinical signs of the disease. Scrapie-infected rats showed a reduction in the levels of 5-HT and 5-HIAA (frontal cortex, hippocampus, mesolimbic structure). Concentrations of DHPG decreased in the frontal and parietal cortices but remained unchanged in the hippocampus. DOPAC levels decreased in the striatum but not in the mesolimbic structure. These results confirm that the serotonergic, noradrenergic and dopaminergic systems are altered in the brain of scrapie-infected rats. They can partly account for clinical signs of scrapie and are in agreement with the scarce data provided by the postmortem analysis of Creutzfeldt-Jakob disease brains.     

Effects of repetitive transcranial magnetic stimulation on behavioral and neurochemical changes in rats during an elevated plus-maze test

In transcranial magnetic stimulation (TMS) the regional electrical activity in the brain is influenced by a pulsed magnetic field. The rapidly changed magnetic field produces electrical currents that activate neurons. Repetitive TMS (rTMS) treatment can cause functional changes in the cortex. The present study clarified the effects of rTMS treatment on behavioral changes in rats, focusing on anxiety by using an elevated plus-maze (plus-maze) test. The effects of rTMS treatment on neurochemical changes during the plus-maze test were investigated by determining the extracellular levels of serotonin (5-HT) and dopamine (DA) in the prefrontal cortex by using in vivo microdialysis. Each rat received rTMS of the frontal brain for 3 days, during which 125 stimuli from five trains in a day were applied at 25 Hz for 1 s with 2-min intervals between trains. Three-day series of (chronic) rTMS treatment caused significant increases in the time spent in open arms and the number of entries into open arms of the plus-maze compared with non-treated and sham-treated rats, which were not observed in 1-day series of (acute) rTMS treatment. Chronic rTMS treatment suppressed the increases in 5-HT levels induced by the plus-maze test, but did not influence the elicited DA levels. These data suggest that chronic rTMS treatment of the frontal brain has anxiolytic effects in rats, which are related to the 5-HTergic neuronal system.     

5-HT6 receptor antagonist SB-271046 enhances extracellular levels of monoamines in the rat medial prefrontal cortex

The present study investigated the neurochemical effects of the selective 5-HT(6) receptor antagonist SB-271046 in the rat medial prefrontal cortex (mPFC). The effect of SB-271046 on extracellular levels of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the mPFC was examined using in vivo microdialysis in the freely moving rat. SB-271046 (10 mg/kg, p.o.) produced a significant increase in extracellular levels of both DA and NE without altering 5-HT neurotransmission. These results further support the rationale for the use of 5-HT(6) receptor antagonists in the treatment of cognitive dysfunction associated with psychiatric diseases.     

Effects of Hypericum perforatum (St. John's wort) on passive avoidance in the rat: evaluation of potential neurochemical mechanisms underlying its antidepressant activity

Along with traditional pharmacotherapies, extracts of Hypericum perforatum L. (St. John's wort) are used in the treatment of mild to moderately severe depression. Hypericum is a nonspecific inhibitor of the neuronal uptake of monoamines (serotonin, 5-HT; noradrenaline, NA; dopamine, DA) as well as GABA and glutamate. Hypericum extracts have been shown to be active in several different "animal models for antidepressant drugs". As one of a large number of chemical constituents, the phoroglucinol derivative hyperforin might be an important "antidepressant component" of hypericum. However, the exact role of neurochemical mechanisms underlying in vivo actions of hypericum and hyperforin are not well defined. In the present study, we compared the effects of hypericum, hyperforin and hyperforin-free hypericum and the three conventional antidepressants paroxetine, imipramine and desipramine using the passive avoidance (PA) task in the rat. The 5-HT-releasing compound p-chloroamphetamine (PCA), which operates through the 5-HT neuronal transporter, was used to reveal the potential in vivo effects on 5-HT uptake mechanisms. To examine the ability of the test-compounds to enhance noradrenaline (NA) transmission in vivo, subeffective doses of scopolamine were used. Taken together, our results suggest that (1) hypericum given at high doses can probably affect the neuronal 5-HT uptake mechanisms in a manner more reminiscent of TCAs than SSRIs; (2) similar to TCAs and SSRIs, hypericum and hyperforin are active in the scopolamine test. Hyperforin appears to play a major role in the action of hypericum in this model. Both 5-HT and NA might concomitantly contribute to the effects of different antidepressants in the "low-dose scopolamine" model; (3) hypericum might enhance both 5-HT and NA transmission in forebrain limbic brain circuits important for mood control, which could underly its antidepressant effects. However, the relative contribution of different constituents and exact mechanisms of action require further evaluation.