Neurobehavioral and molecular changes in a rodent model of ACTH-induced HPA axis dysfunction

Hypothalamic-pituitary-adrenal (HPA) axis dysregulation is linked to the pathophysiology of depression. Although exogenous adrenocorticotropic hormone (ACTH) is associated with a depressive-like phenotype in rodents, comprehensive neurobehavioral and mechanistic evidence to support these findings are limited. Sprague-Dawley rats (male, n = 30; female, n = 10) were randomly assigned to the control (male, n = 10) or ACTH (male, n = 20; female n = 10) groups that received saline (0.1 ml, sc.) or ACTH (100 μg/day, sc.), respectively, for two weeks. Thereafter, rats in the ACTH group were subdivided to receive ACTH plus saline (ACTH_S; male, n = 10; female, n = 5; 0.2 ml, ip.) or ACTH plus imipramine (ACTH_I; male, n = 10; female, n = 5;10 mg/kg, ip.) for a further four weeks. Neurobehavioral changes were assessed using the forced swim test (FST), the sucrose preference test (SPT), and the open field test (OFT). Following termination, the brain regional mRNA expression of BDNF and CREB was determined using RT-PCR. After two-weeks, ACTH administration significantly increased immobility in the FST (p = 0.03), decreased interaction with the center of the OFT (p < 0.01), and increased sucrose consumption (p = 0.03) in male, but not female rats. ACTH administration significantly increased the expression of BDNF in the hippocampus and CREB in all brain regions in males (p < 0.05), but not in female rats. Imipramine treatment did not ameliorate these ACTH-induced neurobehavioral or molecular changes. In conclusion, ACTH administration resulted in a sex-specific onset of depressive-like symptoms and changes in brain regional expression of neurotrophic factors. These results suggest sex-specific mechanisms underlying the development of depressive-like behavior in a model of ACTH-induced HPA axis dysregulation.

Sildenafil, alone and in combination with imipramine or escitalopram, display antidepressant-like effects in an adrenocorticotropic hormone-induced (ACTH) rodent model of treatment-resistant depression

BACKGROUND

Major depressive disorder (MDD) represents a challenge with high prevalence and limited effectiveness of existing treatments, particularly in cases of treatment-resistant depression (TRD). Innovative strategies and alternative drug targets are therefore necessary. Sildenafil, a selective phosphodiesterase type 5 (PDE5) inhibitor, is known to exert neuroplastic, anti-inflammatory, and antioxidant properties, and is a promising antidepressant drug candidate.

AIM

To investigate whether sildenafil monotherapy or in combination with a known antidepressant, can elicit antidepressant-like effects in an adrenocorticotropic hormone (ACTH)-induced rodent model of TRD.

METHODS

ACTH-naïve and ACTH-treated male Sprague-Dawley (SD) rats received various sub-acute drug treatments, followed by behavioural tests and biochemical analyses conversant with antidepressant actions.

RESULTS

Sub-chronic ACTH treatment induced significant depressive-like behaviour in rats, evidenced by increased immobility during the forced swim test (FST). Sub-acute sildenafil (10 mg/kg) (SIL-10) (but not SIL-3), and combinations of imipramine (15 mg/kg) (IMI-15) and sildenafil (3 mg/kg) (SIL-3) or escitalopram (15 mg/kg) (ESC-15) and SIL-3, exhibited significant antidepressant-like effects. ACTH treatment significantly elevated hippocampal levels of brain-derived neurotrophic factor (BDNF), serotonin, norepinephrine, kynurenic acid (KYNUA), quinolinic acid (QUINA), and glutathione. The various mono- and combined treatments significantly reversed some of these changes, whereas IMI-15 + SIL-10 significantly increased glutathione disulfide levels. ESC-15 + SIL-3 significantly reduced plasma corticosterone levels.

CONCLUSION

This study suggests that sildenafil shows promise as a treatment for TRD, either as a stand-alone therapy or in combination with a traditional antidepressant. The neurobiological mechanism underlying the antidepressant-like effects of the different sildenafil mono- and combination therapies reflects a multimodal action and cannot be explained in full by changes in the individually measured biomarker levels.

Influence of 5-HT2A receptor function on anxiety-like behavior induced by a combination treatment with doxorubicin and cyclophosphamide in rats

Anxiety-like behavior induced by a combination of doxorubicin and cyclophosphamide may be mediated by serotonin (5-HT)_2A receptor hyperactivity. The anxiolytic effects of fluoxetine may be inhibited by this combination. The present study examined the mechanisms underlying anxiety-like behavior induced by the combination doxorubicin and cyclophosphamide in rats. Anxiety-like behavior was induced during a light-dark test by the doxorubicin and cyclophosphamide treatment (once a week for 2 weeks). 5-HT_2A receptor and 5-HT_2A receptor-mediated extracellular signal-related kinase (ERK)1/2 levels were measured using Western blotting. 5-HT reuptake activity in fluoxetine-treated rats was also examined using microdialysis. ( ±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane, a 5-HT_2A receptor agonist, induced anxiety-like behavior. The fluoxetine treatment increased extracellular 5-HT concentrations in the hippocampus of vehicle- and doxorubicin and cyclophosphamide-treated rats. 5-HT transporter levels in the hippocampus were not affected by chemotherapy. The doxorubicin and cyclophosphamide treatment did not alter 5-HT_2A receptor levels in the frontal cortex. However, chemotherapy increased 5-HT_2A receptor-mediated ERK1/2 phosphorylation levels significantly more than the vehicle treatment. The present results suggest that anxiety-like behavior induced by the combination of doxorubicin and cyclophosphamide is mediated by 5-HT_2A receptor hyperactivity without an increase in 5-HT_2A receptor levels in rats.

Energizing effects of bupropion on effortful behaviors in mice under positive and negative test conditions: modulation of DARPP-32 phosphorylation patterns

Motivational symptoms such as anergia, fatigue, and reduced exertion of effort are seen in depressed people. To model this, nucleus accumbens (Nacb) dopamine (DA) depletions are used to induce a low-effort bias in rodents tested on effort-based decision-making. We evaluated the effect of the catecholamine uptake blocker bupropion on its own, and after administration of tetrabenazine (TBZ), which blocks vesicular storage, depletes DA, and induces depressive symptoms in humans. Male CD1 mice were tested on a 3-choice-T-maze task that assessed preference between a reinforcer involving voluntary physical activity (running wheel, RW) vs. sedentary activities (sweet food pellet intake or a neutral non-social odor). Mice also were tested on the forced swim test (FST), two anxiety-related measures (dark-light box (DL), and elevated plus maze (EPM)). Expression of phosphorylated DARPP-32 (Thr34 and Thr75) was evaluated by immunohistochemistry as a marker of DA-related signal transduction. Bupropion increased selection of RW activity on the T-maze. TBZ reduced time running, but increased time-consuming sucrose, indicating an induction of a low-effort bias, but not an effect on primary sucrose motivation. In the FST, bupropion reduced immobility, increasing swimming and climbing, and TBZ produced the opposite effects. Bupropion reversed the effects of TBZ on the T-maze and the FST, and also on pDARPP32-Thr34 expression in Nacb core. None of these manipulations affected anxiety-related parameters. Thus, bupropion improved active behaviors, which were negatively motivated in the FST, and active behaviors that were positively motivated in the T-maze task, which has implications for using catecholamine uptake inhibitors for treating anergia and fatigue-like symptoms.

Esketamine and rapastinel, but not imipramine, have antidepressant-like effect in a treatment-resistant animal model of depression

OBJECTIVES

Treatment-resistance to antidepressants is a major problem in the pharmacotherapy of major depressive disorder (MDD). Unfortunately, only a few animal models are suitable for studying treatment-resistant depression, among them repeated treatment with Adrenocorticotropic hormone (ACTH) appears to be useful to mimic treatment-resistance to monoaminergic antidepressants. Therefore, the present work aimed to investigate the effectiveness of s-ketamine and rapastinel (formerly GLYX13), modulators of the glutamatergic N-methyl-D-aspartate receptor in ACTH-treated animals.

METHODS

Naïve male Sprague Dawley rats were subjected to repeated subcutaneous injections with ACTH (100 µg/0.1 ml/rat/day) for 14 days and drug treatment on the test day (open field and forced swim test) with imipramine, s-ketamine or rapastinel. In addition, assessment of plasma levels of corticosterone and ACTH was carried out.

RESULTS

We found that rats repeatedly treated with ACTH for 14 days responded to single injections with s-ketamine (15 mg/kg) and rapastinel (10 mg/kg), but failed to respond to imipramine (15 mg/kg). In the plasma, the levels of corticosterone and ACTH were increased after 14 days of daily treatment with ACTH, independently of the treatment.

CONCLUSION

The present data confirm development of a resistance to treatment following chronic ACTH administration. In addition, the study confirms the possible effectiveness of s-ketamine and rapastinel as treatment options in treatment-resistant depression. Moreover, it highlights the importance of the glutamatergic system in the neurobiology of depression. Further studies are necessary to evaluate how repeated treatment with ACTH leads to a depressed condition resistant to monoaminergic antidepressants.

The α2C-adrenoceptor antagonist, ORM-10921, exerts antidepressant-like effects in the Flinders Sensitive Line rat

Depression involves deficits in monoaminergic neurotransmission. Differential roles for α2A, B and C subtypes of the α2-adrenoceptor (AR) are evident, with selective α2C-AR antagonists purported to have antidepressant and procognitive properties. However, this has not been demonstrated in a genetic animal model of depression. The role of the α2C-AR in modulating two key depression-related behaviours in the Flinders Sensitive Line (FSL) rat was studied using a dose-response analysis following subcutaneous administration with the selective α2C-AR antagonist ORM-10921 (0.03; 0.3 mg/kg), the nonselective α2-AR antagonist idazoxan (3 mg/kg), or vehicle once daily for 14 days. Behaviour in the novel object recognition test, forced swim test (FST) and locomotor activity test was assessed. To ratify the validity of the FSL model, the reference tricyclic antidepressant imipramine (15 mg/kg, intraperitoneally) was used as a comparator drug in the FST. FSL rats demonstrated significantly increased immobility and recognition memory deficits versus Flinders Resistant Line controls, with imipramine significantly reversing said immobility. Similarly, ORM-10921 at both doses but not idazoxan significantly reversed immobility in the FST as well as attenuated cognitive deficits in FSL animals. We conclude that selective α2C-AR antagonism has potential as a novel therapeutic strategy in the treatment of depression and cognitive dysfunction.

A urinary metabolomics (GC-MS) strategy to evaluate the antidepressant-like effect of chlorogenic acid in adrenocorticotropic hormone-treated rats

Major depressive disorder (MDD) is a chronic recurring illness that seriously affects human health. Chlorogenic acid (CGA), an important polyphenol extracted from Eucommia ulmoides Oliver bark, has been reported to have anti-depression, neuroprotection, memory improvement and other pharmacological effects. However, little is known about the underlying mechanisms of CGA on the treatment of depression. Here, we investigated the antidepressant-like effects of CGA on an adrenocorticotropic hormone (ACTH)-treated rat model. Thirty-two male Wistar rats were randomly divided into four groups: normal diet group (N), ACTH-treated model group (M), memantine positive control group (M + Mem) and CGA intervened group (M + CGA). Sucrose preference tests (SPTs) and open-field tests (OFTs) were performed to evaluate depressive-like behaviors. Memantine (30 mg kg⁻¹) and CGA (500 mg kg⁻¹) administration dramatically increased hedonic behaviors of the rats in SPT. The scores of crossing and rearing were significantly increased in the M + Mem group and M + CGA group. These results of the behaviour tests might be suggestive of antidepressant-like effects. Moreover, memantine and CGA reversed the levels of serum 5-hydroxytryptamine (5-HT), ACTH, corticotropin-releasing hormone (CRH), and dopamine (DA) that were altered in ACTH-treated rats. Based on a GC-MS metabolomic approach, significant differences in the metabolic profile were observed in ACTH-treated rats compared with the control group, as well as the M + CGA group and M + Mem group compared with the ACTH-treated group. A total of 19 metabolites were identified for the discrimination of normal rats and ACTH-treated rats, and 12 out of 19 differential metabolites were reversed with CGA intervention. Combined with pattern recognition and bioinformatics, nine perturbed metabolic pathways, including energy metabolism, neurotransmitter metabolism, and amino acid metabolism, were identified based on these metabolites. These integrative studies might give a holistic insight into the pathophysiological mechanism of the ACTH-treated depressive rat model, and also showed that CGA has antidepressant-like activities in ACTH-treated rats, providing an important drug candidate for the prevention and treatment of tricyclic anti-depressant treatment-resistant depression.

Chlorogenic acid showed antidepressant-like activity in chronic ACTH-treated rats, providing a potential drug candidate for prevention and treatment of tricyclic antidepressant treatment-resistant depression. Related metabolic pathways were shown.

Characterization of the adrenocorticotrophic hormone - induced mouse model of resistance to antidepressant drug treatment

Approximately 30-60% of patients treated with existing antidepressants fail to achieve remission of depressive symptoms leading to Treatment Resistant Depression (TRD). There is an urgent need to develop novel medications, which is highly limited by the non-availability of relevant animal models with good predictive validity. ACTH administration has been shown to result in the resistance to acute and chronic effects of imipramine. However, the pharmacology of the model and the mechanisms contributing to the resistance are not completely understood. Furthermore, it is not known whether the ACTH administered animals show signs of depression-like behavior. Accordingly, we characterized the behavioral profile and sensitivity to antidepressants in BALB/c mice treated with ACTH and to evaluate some of the mechanisms responsible for the behavioral effects. Daily treatment with ACTH for 14, 21 or 28days failed to produce a depression-like phenotype in the sucrose preference test, voluntary wheel running or FST. In contrast, the acute antidepressant response in the FST was no longer observed in ACTH mice treated with fluoxetine, imipramine, duloxetine or bupropion. Interestingly, the combination of fluoxetine and a low dose of olanzapine, or the combination of fluoxetine and bupropion was efficacious in ACTH treated mice. Further, the sensitivity to a GluN2B receptor antagonist, radiprodil was retained in the ACTH model. To understand the mechanism responsible for the diminished response in these mice, we evaluated p11 (S100A10) mRNA expression and 5-HT2A protein expression. p11 expression was decreased and 5-HT2A protein content increased in ACTH treated mice. In summary, this model may have utility for the identification of novel treatments for TRD.

Antidepressant actions of lateral habenula deep brain stimulation differentially correlate with CaMKII/GSK3/AMPK signaling locally and in the infralimbic cortex

High frequency deep brain stimulation (DBS) of the lateral habenula (LHb) reduces symptoms of depression in severely treatment-resistant individuals. Despite the observed therapeutic effects, the molecular underpinnings of DBS are poorly understood. This study investigated the efficacy of high frequency LHb DBS (130Hz; 200μA; 90μs) in an animal model of tricyclic antidepressant resistance. Further, we reported DBS mediated changes in Ca(2+)/calmodulin-dependent protein kinase (CaMKIIα/β), glycogen synthase kinase 3 (GSK3α/β) and AMP-activated protein kinase (AMPK) both locally and in the infralimbic cortex (IL). Protein expressions were then correlated to immobility time during the forced swim test (FST). Antidepressant actions were quantified via FST. Treatment groups comprised of animals treated with adrenocorticotropic hormone alone (ACTH; 100μg/day, 14days, n=7), ACTH with active DBS (n=7), sham DBS (n=8), surgery only (n=8) or control (n=8). Active DBS significantly reduced immobility in ACTH-treated animals (p<0.05). For this group, western blot results demonstrated phosphorylation status of LHb CaMKIIα/β and GSK3α/β significantly correlated to immobility time in the FST. Concurrently, we observed phosphorylation status of CaMKIIα/β, GSK3α/β, and AMPK in the IL to be negatively correlated with antidepressant actions of DBS. These findings suggest that activity dependent phosphorylation of CaMKIIα/β, and GSK3α/β in the LHb together with the downregulation of CaMKIIα/β, GSK3α/β, and AMPK in the IL, contribute to the antidepressant actions of DBS.

Effects of the 5-HT(1A) Receptor Agonist Tandospirone on ACTH-Induced Sleep Disturbance in Rats

The aim of this study was to compare the effect of the serotonin (5-HT)1A receptor agonist tandospirone versus that of the benzodiazepine hypnotic flunitrazepam in a rat model of long-term adrenocorticotropic hormone (ACTH)-induced sleep disturbance. Rats implanted with electrodes for recording electroencephalogram and electromyogram were injected with ACTH once daily at a dose of 100 µg/rat. Administration of ACTH for 10 d caused a significant increase in sleep latency, decrease in non-rapid eye movement (non-REM) sleep time, and increase in wake time. Tandospirone caused a significant decrease in sleep latency and increase in non-REM sleep time in rats treated with ACTH. The effect of tandospirone on sleep patterns was antagonized by the 5-HT1A receptor antagonist WAY-100635. In contrast, flunitrazepam had no significant effect on sleep parameters in ACTH-treated rats. These results clearly indicate that long-term administration of ACTH causes sleep disturbance, and stimulating the 5-HT1A receptor by tandospirone may be efficacious for improving sleep in cases in which benzodiazepine hypnotics are ineffective.

Analysis of 34 candidate genes in bupropion and placebo remission

There is considerable variability in the rate of response and remission following treatment with antidepressant drugs or placebo in depression patients. No pharmacogenetic studies of bupropion response have been done. We investigated 532 tagging single nucleotide polymorphisms (SNPs) in 34 candidate genes for association with remission and response to either bupropion (n=319) or placebo (n=257) in patients with major depressive disorder. Analyses were performed using conditional logistic regression. Significant association (gene-wide correction) was observed for remission following treatment with bupropion for a SNP within the serotonin receptor 2A gene (HTR2A rs2770296, p(corrected)=0.02). Response to bupropion treatment was significantly associated with a SNP in the dopamine transporter gene (rs6347, p(corrected)=0.013). Among the patients who received placebo, marginal association for remission was observed between a SNP in HTR2A (rs2296972, p(corrected)=0.055) as well as in the serotonin transporter gene (5-HTT or SLC6A4 rs4251417, p(corrected)=0.050). Placebo response was associated with SNPs in the glucocorticoid receptor gene (NR3C1; rs1048261, p(corrected)=0.040) and monoamine oxidase A gene (MAOA; rs6609257, p corrected=0.046). Although the above observations were significant after gene-wide corrections, none of these would be significant after a more conservative study-wide correction for multiple tests. These results suggest a possible role for HTR2A in remission to bupropion treatment. In accordance with bupropion pharmacology, dopamine transporter may play a role in response. The MAOA gene may be involved in placebo response.

Depressive-like behavior in adrenocorticotropic hormone-treated rats blocked by memantine

Hyperactivity of the hypothalamic pituitary-adrenal (HPA) axis plays a role in the pathophysiology of major depressive disorder (MDD). Recent studies suggest the role of the glutamatergic system in the pathophysiology of MDD, and N-methyl-D-aspartate (NMDA) receptor antagonists have shown antidepressant effects in both preclinical and clinical studies. However, little is known about the role of adrenocorticotropic hormone (ACTH) specifically in the glutamatergic response to HPA axis activation. Glutamate is an NMDA receptor agonist, and glycine and D-serine act as co-agonists. Here, we measured brain concentrations of these amino acids in rats given repeated administration of ACTH (100 μg/rat/day, sc, for 14 days). Further, we also evaluated behavioral effects of memantine, a non-competitive NMDA antagonist, on immobility time in the forced swimming test and on locomotor activity in ACTH-treated rats. Compared with control rats, glutamine, glycine, L-serine, and D-serine levels were increased in the hippocampus of ACTH-treated rats; glutamate, glutamine, glycine, L-serine, and D-serine were increased in the cerebellum; and glutamine and glycine were increased in the frontal cortex and striatum, all with statistical significance. Remarkably, these increases in agonists and co-agonists might have led to the augmentation of NMDA receptor activity. ACTH treatment increased immobility time in the forced swimming test and decreased locomotor activity in rats. On the contrary, memantine (10 mg/kg, ip) significantly decreased immobility time in the forced swimming test and increased locomotor activity in ACTH-treated rats. Furthermore, imipramine (15 mg/kg, ip) did not alter immobility time in the forced swimming test whereas this drug significantly decreased locomotor activity in ACTH-treated rats. These results suggest that depressive-like behaviors by chronic ACTH treatment could be blocked by memantine.

Brain energy metabolism is increased by chronic administration of bupropion

OBJECTIVES

Based on the hypothesis that energy impairment may be involved in the pathophysiology of depression, we evaluated the activities of citrate synthase, malate dehydrogenase, succinate dehydrogenase (SDH), mitochondrial respiratory chain complexes I, II, II-III, IV and creatine kinase (CK) in the brain of rats submitted to chronic administration of bupropion.

METHODS

Animals received daily administration of bupropion dissolved in saline (10 mg/kg, intraperitoneal) at 1.0 ml/kg body weight. The rats received injections once a day for 14 days; control rats received an equivalent volume of saline. Twelve hours after the last administration, the rats were killed by decapitation and brain was rapidly removed and kept on an ice plate. The activities of the enzymes were measured in different brain areas.

RESULTS

We observed that the activities of citrate synthase and malate dehydrogenase, mithocondrial respiratory chain complexes I, II-III and IV and CK were not altered after chronic administration of bupropion. However, SDH activity was increased in the prefrontal cortex and cerebellum. In the hippocampus, cerebellum and striatum the activity of complex II was increased after chronic administration of bupropion.

CONCLUSIONS

Our results demonstrated that bupropion increased some enzymes of brain energy metabolism. These findings are in accordance with other studies which showed that some antidepressants may improve energy metabolism. The present results reinforce the hypothesis that antidepressants modulate brain energy metabolism.

Inhibition of glutamate release by bupropion in rat cerebral cortex nerve terminals

Central glutamate neurotransmission has been postulated to play a role in pathophysiology of depression and in the mechanism of antidepressants. The present study was undertaken to elucidate the effect and the possible mechanism of bupropion, an atypical antidepressant, on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Result showed that bupropion exhibited a dose-dependent inhibition of 4-aminopyridine (4-AP)-evoked release of glutamate. The effect of bupropion on the evoked glutamate release was prevented by the chelating the intrasynaptosomal Ca(2+) ions, and by the vesicular transporter inhibitor, but was insensitive to the glutamate transporter inhibitor. Bupropion decreased depolarization-induced increase in [Ca(2+)](C), whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The effect of bupropion on evoked glutamate release was abolished by the N-, P- and Q-type Ca(2+) channel blocker, but not by the ryanodine receptor blocker, or the mitochondrial Na(+)/Ca(2+) exchanger blocker. In addition, the inhibitory effect of bupropion on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase kinase (MEK) inhibitors. Western blot analyses showed that bupropion significantly decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2), and this effect also was blocked by MEK inhibitor. These results are the first to suggest that, in rat cerebrocortical nerve terminals, bupropion suppresses voltage-dependent Ca(2+) channel and MEK/ERK activity and in so doing inhibits evoked glutamate release. This finding may provide important information regarding the beneficial effects of bupropion in the brain.

Chronic imipramine but not bupropion increases arachidonic acid signaling in rat brain: is this related to 'switching' in bipolar disorder?

Agents effective against mania in bipolar disorder are reported to decrease turnover of arachidonic acid (AA) in phospholipids and expression of calcium-dependent AA-selective cytosolic phospholipase A(2) (cPLA(2)) in rat brain. In contrast, fluoxetine, an antidepressant that is reported to switch bipolar depressed patients to mania, increases cPLA(2) expression and AA turnover in rat brain. We therefore hypothesized that antidepressants that increase switching to mania generally increase cPLA(2) and AA turnover in brain. To test this hypothesis, adult male CDF-344 rats were administered imipramine and bupropion, with reported high and low switching rates, respectively, at daily doses of 10 and 30 mg kg(-1) i.p., respectively, or i.p. saline (control) for 21 days. Frontal cortex expression of different PLA(2) enzymes and AA turnover rates in brain when the rats were unanesthetized were measured. Compared with chronic saline, chronic imipramine but not bupropion significantly increased cortex cPLA(2) mRNA activity, protein and phosphorylation, expression of the cPLA(2) transcription factor, activator protein-2alpha (AP-2alpha) and AA turnover in phospholipids. Protein levels of secretory phospholipase A(2), calcium-independent phospholipase A(2), cyclooxygenase (COX)-1 and COX-2 were unchanged, and prostaglandin E(2) was unaffected. These results, taken with prior data on chronic fluoxetine in rats, suggest that antidepressants that increase the switching tendency of bipolar depressed patients to mania do so by increasing AA recycling and metabolism in brain. Mania in bipolar disorder thus may involve upregulated brain AA metabolism.

Possible involvement of sigma-1 receptors in the anti-immobility action of bupropion, a dopamine reuptake inhibitor

Sigma receptors particularly, sigma-1 subtype is known to modulate the release of catecholamines in the brain and may participate in the mechanism of action of various antidepressants. The present study investigated the possible involvement of sigma receptors in modulating the anti-immobility-like effect of bupropion (a dopamine reuptake inhibitor) using the forced swim test (FST) in mice. Bupropion produced dose-dependent (10-40 mg/kg, i.p.) reduction in immobility period and the ED(50) value was found to be 18.5 (7.34-46.6) mg/kg, i.p. (+)-Pentazocine (2.5 mg/kg, i.p.), a high-affinity sigma-1 receptor agonist, produced synergistic response when it was co-administered with a subeffective dose of bupropion (10 mg/kg, i.p.). On the contrary, pretreatment with progesterone (10 mg/kg, s.c.), a sigma-1 receptor antagonist neurosteroid, rimcazole (5 mg/kg, i.p.), another sigma-1 receptor antagonist, or BD 1047 (1 mg/kg, i.p.), a novel sigma-1 receptor antagonist, reversed the anti-immobility effects of bupropion (20 mg/kg, i.p.). The various modulators used in the study did not show any effect per se on locomotor activity except bupropion which at a higher dose (15-40 mg/kg, i.p.) significantly increased the locomotor activity. The results for the first time demonstrated the involvement of sigma-1 receptors in the anti-immobility effects of bupropion.

Sustained administration of bupropion alters the neuronal activity of serotonin, norepinephrine but not dopamine neurons in the rat brain

Bupropion is widely used in the treatment of depression. There are, however, limited data on its long-term effects on monoaminergic neurons and therefore the mechanism of its delayed onset of action is at present not well understood. The present study was conducted to examine the effects of prolonged bupropion administration on the firing activity of dorsal raphe nucleus (DRN), locus coeruleus (LC), and ventral tegmental area (VTA) neurons. Spontaneously firing neurons were recorded extracellularly in rats anesthetized with chloral hydrate. Bupropion (30 mg/kg/day) was administered using subcutaneously implanted minipumps. In the DRN, the firing rate of serotonin (5-HT) neurons was significantly increased after 2, 7 and 14 days of administration. The suppressant effect of LSD was significantly diminished after the two-day regimen, indicating a desensitization of 5-HT1A autoreceptors. In the LC, the firing rate of norepinephrine (NE) neurons was significantly attenuated after a 2-day regimen, but recovered progressively over 14 days of administration. The suppressant effect of clonidine on NE neuronal firing was significantly attenuated in rats treated with bupropion for 14 days, indicating a desensitization of alpha2-adrenoceptors. In the VTA, neither 2 nor 14 days of bupropion administration altered the firing and burst activity of dopamine neurons. These results indicate that bupropion, unlike 5-HT reuptake inhibitors, promptly increased 5-HT neuronal activity, due to early desensitization of the 5-HT1A autoreceptor. The gradual recovery of neuronal firing of NE neurons, due to the desensitization of alpha2-adrenoceptors, in the presence of the sustained increase in 5-HT neuronal firing, may explain in part the delayed onset of action of bupropion in major depression.