The new '5-HT' hypothesis of depression: cell-mediated immune activation induces indoleamine 2,3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression

This paper reviews the body of evidence that not only tryptophan and consequent 5-HT depletion, but also induction of indoleamine 2,3-dioxygenase (IDO) and the detrimental effects of tryptophan catabolites (TRYCATs) play a role in the pathophysiology of depression. IDO is induced by interferon (IFN)γ, interleukin-6 and tumor necrosis factor-α, lipopolysaccharides and oxidative stress, factors that play a role in the pathophysiology of depression. TRYCATs, like kynurenine and quinolinic acid, are depressogenic and anxiogenic; activate oxidative pathways; cause mitochondrial dysfunctions; and have neuroexcitatory and neurotoxic effects that may lead to neurodegeneration. The TRYCAT pathway is also activated following induction of tryptophan 2,3-dioxygenase (TDO) by glucocorticoids, which are elevated in depression. There is evidence that activation of IDO reduces plasma tryptophan and increases TRYCAT synthesis in depressive states and that TDO activation may play a role as well. The development of depressive symptoms during IFNα-based immunotherapy is strongly associated with IDO activation, increased production of detrimental TRYCATs and lowered levels of tryptophan. Women show greater IDO activation and TRYCAT production following immune challenge than men. In the early puerperium, IDO activation and TRYCAT production are associated with the development of affective symptoms. Clinical depression is accompanied by lowered levels of neuroprotective TRYCATs or increased levels or neurotoxic TRYCATs, and lowered plasma tryptophan, which is associated with indices of immune activation and glucocorticoid hypersecretion. Lowered tryptophan and increased TRYCATs induce T cell unresponsiveness and therefore may exert a negative feedback on the primary inflammatory response in depression. It is concluded that activation of the TRYCAT pathway by IDO and TDO may be associated with the development of depressive symptoms through tryptophan depletion and the detrimental effects of TRYCATs. Therefore, the TRYCAT pathway should be a new drug target in depression. Direct inhibitors of IDO are less likely to be useful drugs than agents, such as kynurenine hydroxylase inhibitors; drugs which block the primary immune response; compounds that increase the protective effects of kynurenic acid; and specific antioxidants that target IDO activation, the immune and oxidative pathways, and 5-HT as well.

Imbalance between pro- and anti-inflammatory cytokines, and between Th1 and Th2 cytokines in depressed patients: the effect of electroacupuncture or fluoxetine treatment

BACKGROUND

An increase in inflammatory response and an imbalance between T-helper (Th) 1 and 2 functions have been implicated in major depression. The aims of the present study were to 1) study the relationship between pro- and anti-inflammatory cytokines and between Th1 and Th2 produced cytokines in depressed patients and 2) evaluate and compare the effect of treatments with electroacupuncture (EA) and fluoxetine on these cytokines.

METHODS

95 outpatients with major depressive disorder were treated for 6 weeks with EA, fluoxetine or placebo. Hamilton Depression Rating Scale (HDRS) and Clinical Global Impression (CGI) were used to assess severity and therapeutic effects. 30 volunteers served as controls. Serum cytokine concentrations were measured by ELISA.

RESULTS

Increased proinflammatory cytokine interleukin (IL)-1beta and decreased anti-inflammatory cytokine IL-10 were found in the depressed patients. By contract, Th1 produced proinflammatory cytokines, tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma were decreased, and Th2 produced cytokine IL-4 was significantly increased in depressed patients. The ratio of IFN/IL-4 was also increased. Both acupuncture and fluoxetine treatments, but not the placebo, reduced IL-1beta concentrations in responders. However, only acupuncture attenuated TNF-alpha concentration and INF-gamma/IL-4 ratio towards the control level.

DISCUSSION

These results suggest that an imbalance between the pro- and anti-inflammatory cytokines (IL-1 and IL-10), and between Th1 and Th2 cytokines (INF-gamma or TNF-alpha and IL-4) occurred in untreated depressed patients. Both EA and fluoxetine had an anti-inflammatory effect by reducing IL-1beta. EA treatment also restored the balance between Th1 and Th2 systems by increasing TNF-alpha and decreasing IL-4.

Role of paroxetine in interferon-alpha-induced immune and behavioural changes in male Wistar rats

Treatment with pro-inflammatory cytokine, IFNalpha was documented to result in neuropsychiatric complications including depression and treatment with antidepressant, paroxetine could improve the depressive symptoms. Therefore, the effects of IFNalpha on behaviour and cytokine changes in the whole blood culture and in the prefrontal cortex, hypothalamus and hippocampus areas of the brain in wistar rats were investigated with emphasis on the role of paroxetine in the prevention of depressive behaviour induced by pro-inflammatory cytokines. The group of rats treated with IFNalpha (s.c. 50,000 IU/kg for 3 days/week for 5 weeks) was compared with three other groups; 1) saline control group (s.c. normal saline 0.2 ml/kg/day for 7 weeks), 2) paroxetine control group (paroxetine suspension orally 10 mg/kg/day for 7 weeks) and 3) group treated with paroxetine for 2 weeks followed by IFNalpha for 5 weeks. In open filed, the IFNalpha treated rats showed anxiety behaviour compared to the rats from the other groups. There was no significant difference in home cage emergence test, Morris water maze and object recognition test. There is no significant difference in plasma corticosterone between groups. The pro-inflammatory cytokines (TNFalpha, IL1beta and IFNgamma), were significantly higher whereas the anti-inflammatory cytokine, IL10 was lower in the stimulated whole blood culture of IFNalpha treated rats. In the brain, both pro-inflammatory cytokine IL1beta and anti-inflammatory cytokine IL10 were higher in hypothalamus of the IFNalpha treated rats; by contrast the concentration of IL10 was lowest in hippocampus region of this group compared to the other groups. The paroxetine pretreated rats did not show these cytokine changes following IFNalpha treatment. Thus it appears that paroxetine pretreatment prevents the pro-inflammatory changes in blood and brain following IFNalpha treatment in turn prevents the anxiety behaviour.

Inflammation and depression: is there a causal connection with dementia?

Epidemiological studies show that there is a correlation between chronic depression and the likelihood of dementia in later life. There is evidence that inflammatory changes in the brain are pathological features of both depression and dementia. This suggests that an increase in inflammation-induced apoptosis, together with a reduction in the synthesis of neurotrophic factors caused by a rise in brain glucocorticoids, may play a role in the pathology of these disorders. A reduction in the neuroprotective components of the kynurenine pathway, such as kynurenic acid, and an increase in the neurodegenerative components, 3- hydroxykynurenine and quinolinic acid, contribute to the pathological changes. Such changes are postulated to cause neuronal damage and thereby predispose chronically depressed patients to dementia.

The comparison of changes in behavior, neurochemistry, endocrine, and immune functions after different routes, doses and durations of administrations of IL-1beta in rats

BACKGROUND

Increased interleukin-1beta (IL-1) in the brain and periphery has been associated with neurodegenerative and psychiatric disorders. However, results from different IL-1 sources, administrating routes, doses and treatment duration were inconsistent and confused. The neuroendocrine-immune mechanism by which IL-1-induced behavioral changes occur is still unclear.

METHODS

In the present study, the acute and sub-chronic effects of rat recombinant IL-1, following either intraperitoneal (ip) or intracerebroventricular (icv) injection, were studied on the behavior, corticosterone secretion, peripheral inflammatory responses and brain monoamines.

RESULTS

In the open field apparatus, IL-1 (ip) increased locomotor activity but decreased the activity following icv administration. IL-1 had a greater anxiogenic effect in the elevated plus maze after icv than after ip administration. In the Morris water maze spatial memory was only impaired following sub-chronic and icv administration. Both acute and sub-chronic IL-1 increased the serum corticosterone concentration and decreased the release of the anti-inflammatory cytokine IL-10 from whole blood cultures. However, centrally administered IL-1 increased, while peripherally administered decreased, the release of PGE2 from blood cultures. After sub-chronic administration, the noradrenaline concentration was decreased in several limbic regions, while the turnovers of serotonin and dopamine were increased.

DISCUSSION

These results suggest that 1) IL-1 effects depended on the dose, route and duration of administration, and 2) IL-1 enhances the responsiveness of rats to stressful environmental stimuli. In addition, the sub-chronic administration of IL-1 induces behavioral, neurotransmitter, hormonal and immune changes that may be causally implicated in the mechanism of some of psychiatric disorders such as depression.

The HPA and immune axes in stress: the involvement of the serotonergic system

The impact of acute and chronic stress on the hypothalamic-pituitary-adrenal (HPA) axis is reviewed and evidence presented that corticotrophin releasing factor (CRF) is the stress neurotransmitter which plays an important role in the activation of the central sympathetic and serotonergic systems. The activity of CRF is expressed through specific receptors (CRF 1 and 2) that are antagonistic in their actions and widely distributed in the limbic regions of the brain, as well as in the hypothalamus, and on immune cells. The mechanism whereby chronic stress, via the CRF induced activation of the dorsal raphe nucleus, can induce a change in the serotonergic system, involves an increase in the 5HT2A and a decrease in the 5HT1A receptor mediated function. Such changes contribute to the onset of anxiety and depression. In addition, the hypersecretion of glucocorticoids that is associated with chronic stress and depression desensitises the central glucocorticoid receptors to the negative feedback inhibition of the HPA axis. This indirectly results in the further activation of the HPA axis. The rise in pro-inflammatory cytokines that usually accompanies the chronic stress response results in a further stimulation of the HPA axis thereby adding to the stress response. While CRF would appear to play a pivotal role, evidence is provided that simultaneous changes in the serotonergic and noradrenergic systems, combined with the activation of peripheral and central macrophages that increase the pro-inflammatory cytokine concentrations in the brain and blood, also play a critical role in predisposing to anxiety and depression. Neurodegenerative changes in the brain that frequently occur in the elderly patient with major depression, could result from the activation of indoleaminedioxygenase (IDO), a widely distributed enzyme that converts tryptophan via the kynenine pathway to for the neurotoxic end product quinolinic acid.

Ketter's hypothesis of the mood effects of antiepileptic drugs coupled to the mechanism of action of topiramate and levetiracetam

Mood-modulating profiles of antiepileptic drugs (AEDs) have been classified by Ketter, Post, and Theodore [Neurology 1999; 53 (5, Suppl. 2) S53-76] into two classes: the first class is assumed to have deactivating effects related to GABA potentiation, and the second class is assumed to have activating effects that are associated with glutamate attenuation. We tested this hypothesis by reviewing the multiple mechanisms of action of topiramate (TPM) and levetiracetam (LEV) together with clinical behavioral side effects of patients who had been treated with TPM and LEV in a tertiary referral center for epilepsy. We found LEV to manifest activating and deactivating side effects equally and TPM to act as a deactivating AED, with tiredness/sleepiness side effects being predominant. TPM, in comparison to LEV, was found to be associated with a high incidence of side effects. Testing the hypothesis of Ketter et al. (1999) the deactivating effects of TPM may be coupled to a predominance of potentiation of GABA, but the oversimplified basis of the model needs to be acknowledged.

Sigma receptors and sigma ligands: background to a pharmacological enigma

Although it is now well established that the sigma receptors are distinct from neurotransmitter, neuropeptide and steroid receptors, their precise physiological role, and possible involvement in the pathology of neurological and psychiatric disorders, remains an enigma. The purpose of this short review is to provide a basis upon which to explain the pharmacological properties of such a diverse group of drugs as the antidepressants, antipsychotics and neuroprotective agents. However, a greater understanding of the importance of the sigma receptors will depend on the development of more potent and selective sigma agonists and antagonists if the therapeutic potential of the sigma receptors is to be fully realised.

Ethyl-eicosapentaenoic acid ingestion prevents corticosterone-mediated memory impairment induced by central administration of interleukin-1beta in rats

Central or peripheral administration of the proinflammatory cytokine interleukin (IL)-1beta can impair performance on spatial memory tasks and also elevate circulating concentration of corticosterone. The present experiment provides independent confirmation that intracerebroventricular administration of 10 ng IL-1beta in the rat can have a selective effect on the retrieval of trial unique information about the location of food on an eight-arm radial maze. The probable involvement of corticosterone in IL-1beta-induced memory impairment was indicated by elevated corticosterone levels after IL-1beta administration. Further evidence comes from the blockade of the associated impairment in working memory by coadministration of the glucocorticoid receptor antagonist RU486. Ingestion of diet containing omega-3 fatty acid eicosapentaenoic acid (EPA) is known to antagonize the synthesis of prostaglandin (PG) E2 from aracadonic acid, and the present study confirmed that ethyl EPA (1%) reduced IL-1beta-elevated concentrations of PGE2 and corticosterone. Furthermore, rats given the ethyl-EPA diet for 8 weeks were unaffected by the disruptive effects of IL-1beta on working memory. IL-1beta-induced suppression of mitogen-stimulated release of the anti-inflammatory cytokine IL-10 was also blocked by treatment with ethyl-EPA. Collectively, these data demonstrate that IL-1beta can impair memory function by elevating the concentration of corticosterone and that prior consumption of 1% ethyl-EPA can block both the neuroendocrine and cognitive effects of IL-1beta. These findings in turn may indicate beneficial effects of ethyl-EPA in the treatment of cognitive and affective disorders in which inflammation and stress play a critical role.

Effects of acute and chronic administration of selective monoamine re-uptake inhibitors in the rat forced swim test

The rat forced swim test (FST) is a model that is used extensively as a screening test for antidepressant activity. It has previously been reported that thorough analysis of behaviour in this model reveals two distinct types of active response - climbing and swimming - and that these are separately evoked by re-uptake inhibitors selective for noradrenaline (NA) and serotonin (5-HT), respectively. In the present study, utilising re-uptake inhibitors selective for NA, talsupram, and 5-HT, 5-chloro-1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)- phthalan (Lu 10-134-C), we examined if this scoring technique could detect the antidepressant potential of a selective serotonin re-uptake inhibitor (SSRI), and whether re-uptake inhibitors selective for distinct monoamine systems induce exclusive behavioural responses. We also analysed if chronic antidepressant administration for three weeks was more effective than acute treatment. We found Lu 10-134-C (40 mg/kg; PO) to be behaviourally active in this paradigm. Although treatment with talsupram (40 mg/kg; PO) resulted solely in climbing behaviour, Lu 10-134-C induced both climbing and swimming behaviour. However, chronic pre-treatment with either re-uptake inhibitor (20 mg/kg; twice daily; PO) failed to augment the response observed with acute treatment. Similarly, chronic administration of either compound was without effect on the basal, or stress-induced, serum corticosterone concentrations or anterior pituitary (AP) preproopiomelanocorticotropin (POMC) mRNA expression. These results suggest that selective monoamine re-uptake inhibition produces distinct, but not necessarily exclusive, behavioural responses in the forced swim test.

The effect of lofepramine and other related agents on the motility of Tetrahymena pyriformis

Tricyclic antidepressants (TCAs) were introduced almost 50 years ago. Whilst there is no doubt that TCAs are effective in treating depression, they are also more cardiotoxic when taken in overdose than other antidepressant groups. Lofepramine is a more recently introduced modified TCA, which in animals and man has low toxicity when compared to older TCAs. Paradoxically, lofepramine is extensively metabolised to desipramine, which has considerable toxicity, both experimentally and in overdose. The toxicity of such compounds is attributed, in part, to a membrane stabilising effect (MSA) on cell membranes. This MSA causes gross effects to the cell structure and in turn, normal cell activity. The aim of this study was to compare the MSA of lofepramine with that of desipramine and amitriptyline in order to see if this might help to explain the low toxicity of lofepramine. The local anaesthetic agent lignocaine was also studied for comparison. Each compound was enclosed in a beta-cyclodextrin to increase its solubility in aqueous medium. The extent of MSA was determined as a measure of the effect on the swimming speed of the protozoan Tetrahymena pyriformis using a video image analysis system. The IC50s for the various drugs were then correlated with their respective octanol-water partition coefficient values (Pow). Amitriptyline had an IC50 of 1.26+/-0.29 mM, desipramine 75.99+/-14.40 mM, while lofepramine had an IC50 of 357.40+/-25.00 mM. Lignocaine had an IC50 of 85.73+/-18.30 mM. There was also a significant correlation between the IC50 values and the Pow values.

Lipopolysaccharide administration produces time-dependent and region-specific alterations in tryptophan and tyrosine hydroxylase activities in rat brain

The present study examined the effect of systemic administration of lipopolysaccharide (LPS; 100 and 250 microg/kg, i.p.) on tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) activities in frontal cortex, striatum and midbrain of the rat. Enzyme activities were determined by measuring accumulation of the transient intermediates 5-hydroxytrptophan (5-HTP) and L-dihydroxyphenylalanine (L-DOPA) following in vivo administration of the decarboxylase inhibitor, NSD 1015. TPH activity was increased 2 hours after administration of LPS (100 and 250 microg/kg) in both frontal cortex and midbrain, and a secondary increase was seen in the midbrain 12 hours after challenge. LPS provoked an increase in TH activity in the midbrain only, and this was evident for up to 24 hours after LPS administration. Thus in addition to previous studies demonstrating that LPS increases in vivo NA, DA and 5-HT release, this study shows that LPS increases the activity of the rate-limiting enzymes responsible for their synthesis.

Beta-amyloid(1-42)-induced cholinergic lesions in rat nucleus basalis bidirectionally modulate serotonergic innervation of the basal forebrain and cerebral cortex

Ample experimental evidence suggests that beta-amyloid (A beta), when injected into the rat magnocellular nucleus basalis (MBN), impels excitotoxic injury of cholinergic projection neurons. Whereas learning and memory dysfunction is a hallmark of A beta-induced cholinergic deficits, anxiety, or hypoactivity under novel conditions cannot be attributed to the loss of cholinergic MBN neurons. As mood-related behavioral parameters are primarily influenced by the central serotonergic system, in the present study we investigated whether A beta(1-42) toxicity in the rat MBN leads to an altered serotonergic innervation pattern in the rat basal forebrain and cerebral cortex 7 days postsurgery. A beta infusion into the MBN elicited significant anxiety in the elevated plus maze. A beta toxicity on cholinergic MBN neurons, expressed as the loss of acetylcholinesterase-positive cortical projections, was accompanied by sprouting of serotonergic projection fibers in the MBN. In contrast, the loss of serotonin-positive fiber projections, decreased concentrations of both serotonin and 5-hydroxyindoleacetic acid, and decline of cortical 5-HT(1A) receptor binding sites indicated reduced serotonergic activity in the somatosensory cortex. In conclusion, the A beta-induced primary cholinergic deficit in the MBN and subsequent cortical cholinergic denervation bidirectionally modulate serotonergic parameters in the rat basal forebrain and cerebral cortex. We assume that enhanced serotonin immunoreactivity in the damaged MBN indicates intrinsic processes facilitating neuronal recovery and cellular repair mechanisms, while diminished cortical serotonergic activity correlates with the loss of the subcortical cholinergic input, thereby maintaining the balance of neurotransmitter concentrations in the cerebral cortex.

Changes in the immune system in depression and dementia: causal or co-incidental effects?

It is now widely accepted that psychological stress and psychiatric illness can compromise immune function. Furthermore the mechanisms whereby such changes occur are probably associated with the activities of the cytokines and other inflammatory mediators of the immune system which are known to initiate changes in behaviour. This review aims to summarise the experimental and clinical evidence that implicates the pro-inflammatory cytokines in the pathological changes seen in major depression and in Alzheimer's disease (AD). In major depression, evidence is provided to show that both activation (e.g., macrophage activity, acute phase proteins) and inhibition (e.g., natural killer cell activity) of the immune system occur. Many of the behavioural changes seen in depression are simulated by three pro-inflammatory cytokines (IL-1, IL-6 and TNF-alpha), which may produce their impact on the brain by activating cyclooxygenase, nitric acid synthase and corticotrophin releasing factor. Effective antidepressant treatments largely attenuate the immune changes thereby raising the possibility that the normalisation of central biogenic amine function that are conventionally implicated in the cause of depression may be secondary to those of the pro-inflammatory cytokines. With respect to AD, while the cause(s) are unknown, there is both experimental and clinical evidence to suggest that inflammatory processes in the brain caused in particular by TNF-alpha together with the subsequent rise in free radicals, are instrumental in causing the pathological changes which underlie the disease. Evidence in favour of the inflammatory hypothesis is supported by the finding that nonsteroidal anti-inflammatory drugs slow down the progression of the disease.Although, more research is needed into the inter-relationships between the various pro-inflammatory cytokines and the behavioural changes invoked in major depression and AD, the immunological hypothesis has been important in stimulating new concepts regarding the causes of the pathological changes in these diseases and how effective drug treatments may attenuate them.

The immune system, depression and the action of antidepressants

It is well established that the hypothalamic-pituitary-adrenal axis (HPA) is activated by both external and internal stressors which result in the hypersecretion of adrenal glucocorticoids. In major depression the prolonged elevation of the glucocorticoid concentration leads to a desensitisation of the central glucocorticoid receptors and probably those receptors located on macrophages. These changes may account for the observation that many aspects of cellular immunity are activated in depression (for example, the increased release of pro-inflammatory cytokines from activated macrophages in the periphery and brain, and the increased release of acute phase proteins from the liver) even though other aspects of immunity (for example, natural killer cell activity and T-cell replication) are depressed. It is also known that some of the pro-inflammatory cytokines are potent activators of the HPA axis. Evidence is provided that the consequences of the hypersecretion of glucocorticoids and pro-inflammatory cytokines result in the malfunctioning of noradrenergic and serotonergic neurotransmission in the brain, changes which are reflected in the major symptoms of depression. Support for this view is provided by observations of the effects of some of these cytokines in non-depressed individuals being treated with pro-inflammatory and related cytokines for cancer. This has led to the hypothesis that depression is a form of sickness behaviour which forms the basis of the macrophage theory of depression. The review concludes with a discussion of the role of antidepressants in attenuating the adverse effects of glucocorticoids and pro-inflammatory cytokines on central neurotransmission. Although the precise mechanisms whereby antidepressants these changes is uncertain, there is evidence that they reduce the release of pro-inflammatory cytokines from activated macrophages and thereby facilitate the feedback inhibition of the HPA axis; this results in a reduction in the release of glucocorticoids from the adrenal glands. In addition, many antidepressants have been shown to increase the release of endogenous cytokine antagonists such as interleukin-1 receptor antagonist and interleukin-10. Evidence is also presented to show that different classes of antidepressants act as cyclooxygenase inhibitors which, by lowering the concentration of inflammatory prostaglandins in the brain, reduce the detrimental impact of the inflammatory changes on neurotransmitter function. An advantage of the macrophage hypothesis is that it extends the biogenic amine hypothesis of depression to take account of changes in the endocrine and immune systems which also play a crucial role in the aetiology of depression. In addition, the macrophage hypothesis may broaden the basis of understanding the mechanism of action of antidepressants.

Effects of reboxetine and sertraline treatments alone and in combination on the binding properties of cortical NMDA and beta1-adrenergic receptors in an animal model of depression

Changes to the binding properties of cortical N-methyl-D-aspartic acid (NMDA) and beta-adrenergic receptors have both been reported as potential indicators of antidepressant activity. In the present investigation we examined the effects of the noradrenaline reuptake inhibitor, reboxetine, the serotonin reuptake inhibitor, sertraline, alone and in combination on the binding properties of cortical NMDA receptors and cortical beta1-adrenoceptors following 14 days of treatment in the olfactory bulbectomized rat model of depression. A decrease in the potency of glycine to displace the strychnine insensitive glycine antagonist [3H] 5,7 dichlorokynurenic acid (5,7 DCKA) was observed in cortical homogenates of OB rats when compared to sham-operated controls. Similarly, treatment with the combination of reboxetine and sertraline for 14 days produced a decrease in the potency of glycine when compared to vehicle treated controls. By contrast neither olfactory bulbectomy or drug treatment significantly altered basal or glycine enhanced binding of the non-competitive NMDA antagonist [3H] MK-801 in cortical homogenates. Reboxetine alone, and in combination with sertraline, down-regulated [3H]-CGP 12177 (a selective beta-adrenoceptor antagonist) binding in both OB and sham-operated animals. The lack of a bulbectomy effect in the [3H] CGP-12177 binding assay, and the fact that olfactory bulbectomy and antidepressant treatments produce a similar change to the potency of glycine at the NMDA receptor, suggests that these tests do not provide a neurochemical marker for either the behavioral hyperactivity deficit or antidepressant response in the model.

Modulation of MK-801-induced behaviour by noradrenergic agents in mice

RATIONALE

Inhibition of glutamatergic N-methyl-D-aspartate (NMDA) receptors following the administration of NMDA receptor antagonists results in psychotic-like behaviour. Whereas it is known that pharmacological manipulation of dopaminergic and serotonergic pathways affect this drug-induced psychosis, a role for noradrenaline has not yet been clearly defined.

OBJECTIVES

Thus, in the present study, we assessed a possible role for noradrenaline in the behavioural response to the non-competitive NMDA receptor anatgonist, MK-801, in male CD-I mice.

RESULTS

MK-801 (0.02-1.28 mg/kg; ED50 0.2 mg/kg; s.c.) induced a dose-dependent increase in locomotor, stereotypic and ataxic behaviours. Pre-treatment with the noradrenaline re-uptake inhibitors, desipramine (10 mg/kg; i.p.) and reboxetine (20 mg/kg; i.p.), attenuated the locomotor, stereotypic and ataxic response to MK-801 (0.2 mg/kg; s.c.). The noradrenergic system was lesioned with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4, 50 mg/kg; i.p., 7 and 4 days prior to challenge) to reduce noradrenaline concentrations in the cortex by 70%-80%. Whereas DSP-4 lesioning had little effect on the response to MK-801, it completely reversed the attenuating effects of reboxetine. Pre-treatment with the alpha2 adrenoceptor agonist, clonidine (0.2 mg/kg; i.p.), and the antagonist, yohimbine (2 mg/kg; i.p.), attenuated and potentiated the response to MK-801, respectively. Pre-treatment with the alpha1 adrenoceptor antagonist, prazosin (2 mg/kg; i.p.), reduced the MK-801-induced response.

CONCLUSIONS

It therefore appears that presynaptic noradrenergic alpha2 and postsynaptic alpha1 adrenoceptor stimulation exert opposing effects on the behavioural expression of MK-801 in mice.

Prior exposure to methylenedioxyamphetamine (MDA) induces serotonergic loss and changes in spontaneous exploratory and amphetamine-induced behaviors in rats

The substituted amphetamine 3,4 methylenedioxyamphetamine (MDA) is a popular recreational drug of abuse. Administration of MDA to experimental animals has been shown to induce damage to serotonergic axons and nerve terminals. However, there is a lack of information on whether these treatments can produce any long-term changes in behavioural performance particularly under stressful conditions. In the present study, MDA (7.5 mg/kg; i.p.) was administered twice daily to adult male Sprague Dawley rats for four days. Four weeks following the last dose, spontaneous behaviors of these animals were tracked and scored in a novel "open field" environment using an automated video registration and computer interpretation system. Changes in behavior were observed in MDA treated animals including reductions in exploratory oriented behaviors (locomotion and rearing) and increases in grooming behavior when compared to vehicle treated controls. MDA-treated animals also displayed an enhanced locomotor and stereotyped response to d-amphetamine (12 mg/kg; i.p.). Significant reductions in 5-HT concentrations (20-30%) were observed in the frontal cortex, amygdala, striatum, and hypothalamus as a result of MDA treatment. In addition, [3H] paroxetine binding was reduced by (40%) in the cortex of MDA treated rats indicating that the decrease in 5-HT concentrations were accompanied by a reduction in intact presynaptic 5-HT nerve terminals. Changes in behavioural performance in a novel "open field" environment and following d-amphetamine challenge might be considered as a behavioural model of serotonergic deficit induced by MDA. The findings of this study also suggest that MDA use may increase both the abuse potential, and the propensity to develop psychosis as a result of abusing other psychostimulants such as d-amphetamine.

The toxicity profile of a single dose of paroxetine: an alternative approach to acute toxicity testing in the rat

In this study we have examined the effect of a single administration of the selective serotonin reuptake inhibitor, paroxetine (120-300 mg kg(-1), orally) in a recently developed rodent model of acute toxicity testing. Reduced body-weight, food consumption, water consumption and body temperature were observed in all paroxetine-treated groups, which were reversible within 7 days. Five days after administration, a dose-dependent increase in red blood cells, haemoglobin and haematocrit was observed with the 3 higher dose levels of paroxetine, which was significant in the 240 and 300 mg kg(-1) treatment groups (P < 0.05). Hyperactivity was apparent in the first 24 hr following treatment, as was evidence of the serotonin syndrome. When the animals were sacrificed (11 days after drug administration), an increase in liver weight was observed in the highest dose. These results are in agreement with those previously observed with paroxetine at the preclinical and clinical levels. They demonstrate that this rodent model, because of its multi-parameter nature, is a useful method for examining the consequences of a single high dose of an antidepressant drug.

Stress, norepinephrine and depression

Stress is an important precipitant factor in depression, and the changes in various body systems that occur in depression are similar to those observed in response to stress. This paper discusses the interactions among the immune, endocrine and norepinephrine systems that are evident in patients with depression, as well as those affected by stress. Many of the stress-induced changes can be reversed by antidepressants, particularly norepinephrine reuptake inhibitors.

A double-blind, placebo-controlled, efficacy, safety, and pharmacokinetic study of INN 00835, a novel antidepressant peptide, in the treatment of major depression

BACKGROUND

INN 00835 is a synthetic pentapeptide with a potential for rapid onset of action as an antidepressant. Its efficacy was investigated in a pilot study in patients diagnosed with major depression.

METHODS

Fifty two patients received either active drug - INN 00835 (26 patients) - or placebo (26 patients), subcutaneously at 0.2 mg/kg for 5 consecutive days. The patients were evaluated for an additional 4 weeks after treatment. Efficacy was evaluated by the following psychiatric rating scales: HAMD, MADRS, CSRS, CGI, and VAS. The effect of treatment was also evaluated by using a biochemical marker: changes in blood platelet serotonin (5HT) uptake rates in drug-treated patients compared to those in the placebo group. Plasma concentrations of INN 00835 were measured by LC/MS.

RESULTS

Statistical analysis indicated a strong pharmacodynamic correlation between plasma drug concentrations at 1 h after dosing and the reduction in the severity of depression as measured by the psychiatric rating scales. A minimum effective plasma concentration (MEC) of INN 00835 was 5 ng/ml. Statistically significant differences in response to treatment (P<0.05) were found between patients with plasma concentrations above MEC and those in the placebo group, as well as between subjects with plasma concentrations above and below the MEC. The peak effect was observed after the 5-day treatment and the response to treatment persisted during the 4-week follow-up period. The change of 5HT uptake rates after treatment was significantly larger in the drug-treated group than in the placebo group.

LIMITATIONS

This was a pilot study conducted in a relatively small population (52 patients) and the limited number of blood sampling times did not allow a comprehensive pharmacokinetic analysis. There was a relatively large placebo response. The results have to be confirmed in future, large scale studies.

CONCLUSIONS

INN 00835 appears to be a promising drug for the treatment of major depression.

An in-vivo magnetic resonance imaging study of the olfactory bulbectomized rat model of depression

The olfactory bulbectomized (OB) rat is a well-accepted animal model of depression. The present magnetic resonance imaging (MRI) investigation demonstrates alterations in signal intensities in cortical, hippocampal, caudate and amygdaloid regions in OB animals, but not in sham operated controls. Ventricular enlargement was also evident in OB animals. These alterations have implications with regard to the face and construct validity of this model.

Methylenedioxymethamphetamine (MDMA; Ecstasy) suppresses IL-1beta and TNF-alpha secretion following an in vivo lipopolysaccharide challenge

In this study we examined the effects of methylenedioxymethamphetamine (MDMA) administration on responsiveness to an in vivo immune challenge with lipopolysaccharide (LPS; 100 microg/kg; i.p.). LPS produced an increase in circulating IL-1beta and TNF-alpha in control animals. MDMA (20 mg/kg; i.p.) significantly impaired LPS-induced IL-1beta and TNF-alpha secretion. The suppressive effect of MDMA on IL-1beta secretion was transient and returned to control levels within 3 hours of administration. In contrast, the MDMA-induced suppression of TNF-alpha secretion was evident for up to 12 hours following administration. In a second study we examined the effect of co-administration of MDMA (5, 10 and 20 mg/kg; i.p.) on LPS-induced IL-1beta and TNF-alpha secretion, and demonstrated that all three doses potently suppressed LPS-induced TNF-alpha secretion, but only MDMA 10 and 20 mg/kg suppressed LPS-induced IL-1beta secretion. In addition, serum MDMA concentrations displayed a dose-dependent increase, with the concentrations achieved following administration of 5 and 10 mg/kg being in the range reported in human MDMA abusers. In order to examine the possibility that the suppressive effect of MDMA on IL-1beta and TNF-alpha could be due to a direct effect of the drug on immune cells, the effect of in vitro exposure to MDMA on IL-1beta and TNF-alpha production in LPS-stimulated diluted whole blood was evaluated. However IL-1beta or TNF-alpha production were not altered by in vitro exposure to MDMA. In conclusion, these data demonstrate that acute MDMA administration impairs IL-1beta and TNF-alpha secretion following an in vivo LPS challenge, and that TNF-alpha is more sensitive to the suppressive effects of MDMA than is IL-1beta. However the suppressive effect of MDMA on IL-1beta and TNF-alpha could not be attributed to a direct effect on immune cells. The relevance of these findings to MDMA-induced immunomodulation is discussed.

Varying responses to the rat forced-swim test under diurnal and nocturnal conditions

The paradox that experiments in behavioural pharmacology employing nocturnal rodent species are carried out almost exclusively in the resting phase of the animals' circadian cycle has remained largely unexamined and unquestioned. This is despite the fact that all major physiological systems in the body are intrinsically aligned with its natural circadian rhythm. The forced-swim test (FST) is a rodent model that is used extensively as a screening test for antidepressant activity. The objectives of the present study were to examine the behaviour of rats in the FST under diurnal and nocturnal conditions and, in addition, to profile the response of neurochemical, neuroendocrine, and cellular indices of stress at time points up to 120 min following exposure to the FST. The time spent in escape-oriented activity was significantly less when animals were tested in the dark phase. The profile of serum corticosterone and adrenal ascorbic acid concentrations indicates that the animals were less stressed by the test situation during the active (i.e., dark) phase of their circadian cycle. Similarly, increases in blood enzymatic markers of stress-induced cellular damage were less marked following FST exposure in the nocturnal period. Characteristic stress-induced increases in 5-HT turnover in the frontal cortex and amygdala observed in the diurnal phase were reversed in the nocturnal period. In conclusion, circadian differences in behaviour in the FST may be related to parallel alterations in the ability of animals to adapt to exposure to stress.

Evidence for a biochemical lesion in depression

The monoamine hypothesis of depression predicts an impairment in central monoaminergic function. The lesion may comprise deficiencies in the absolute concentrations of norepinephrine and/or serotonin (5-HT). Depletion studies have shown a correlation between such deficiencies and depressive symptoms. Measurement of the concentrations of the neurotransmitters and their metabolites in cerebrospinal fluid, urine, and plasma of patients with depression has yielded equivocal results regarding the possibility of altered metabolism of these neurotransmitters. Other studies have investigated the possibility of altered numbers and/or affinities of the serotonin and norepinephrine receptors and uptake sites. For example, there is evidence for a reduction in the activity of the serotonin reuptake transporter in patients with depression and an increase in the density of 5-HT2 receptors in the brains of suicide victims. Similarly, in the noradrenergic system, up-regulation of beta-adrenoceptors is consistently observed. Most recently, attention has focused on the possibility that a lesion may occur in the postreceptor, subcellular components of the monoamine systems, such as the second messenger processes. Also, experimental evidence has shown "cross-talk" between the noradrenergic and serotonergic systems. There is therefore substantial clinical and experimental evidence that lesions in the serotonergic and noradrenergic systems are responsible for depression and that antidepressant treatment can reverse these alterations.

Effect of subchronic antidepressant treatments on behavioral, neurochemical, and endocrine changes in the forced-swim test

The purpose of the present study was to examine the effect of subchronic treatment (24 days) with antidepressants displaying differential effects on noradrenaline and serotonin reuptake, on behavior, neurochemistry, and hypothalamic-pituitary-adrenal (HPA) axis activity following FST exposure in the rat. Desipramine (7.5 mg/kg, IP) significantly decreased immobility in the FST, whilst paroxetine (7.5 mg/kg IP) and venlafaxine (10 mg/kg, IP) were without effect. Nonetheless, treatment with all three antidepressants significantly attenuated stress-related increases in amygdaloid and cortical serotonin turnover. Of the three antidepressants examined, only desipramine attenuated the stress-associated elevation in serum corticosterone. In conclusion, although FST-induced increases in serotonin turnover in the frontal cortex and amygdala were attenuated following treatment with all three antidepressants, FST-induced behavioral changes and increased HPA axis activity were normalized only following desipramine treatment. In addition, these results suggest that neurochemical mechanisms independent of increased serotonergic activity subserve the normalization of behavior and HPA axis responses in the FST. These data also add to our understanding of the interactions between antidepressants and stress-induced behavioral, neurochemical, and endocrine alterations, and illustrates important differences between classes of antidepressants.

Prolonged c-Jun expression in the basolateral amygdala following bulbectomy: possible implications for antidepressant activity and time of onset

Olfactory bulbectomy is a well established animal model of depression. Neurochemical and behavioral alterations observed following olfactory bulbectomy, are due, in part, to the neurodegeneration of specific brain structures. Amygdaloid dysfunction in particular, is known to play a substantial role in the syndrome of the olfactory bulbectomized rat. The present study examined both short- and long-term alterations in immediate early gene expression, tyrosine hydroxylase and serotonin immunoreactivity, and classical silver staining, following olfactory bulbectomy in the basolateral amygdala. The results indicated no consistent change in Fos expression observed over the experimental period. Following bulbectomy, long term (up to 64 days post-lesion) Jun expression, not coincident with silver staining, was observed in the basolateral nucleus. The basolateral nucleus was also intensely immunoreactive for serotonin at this timepoint post-bulbectomy. Thus, following bulbectomy long term alterations in Jun expression occurs in the serotonin rich basolateral amygdala. As a site of action for antidepressant compounds, alterations at the immediate early gene level in this region may have implications both for the model, and antidepressant drug action therein.

Characterization of D-fenfluramine-induced hypothermia: evidence for multiple sites of action

The effects of D-fenfluramine on core body temperature has been largely investigated under conditions of either high or low ambient temperature, whereas little research has focused on this response under normal environmental conditions. Moreover, there has been neglect in research on the mechanisms underlying changes in body temperature. In this study, we demonstrate that D-fenfluramine (5 and 10 mg/kg) induces a sustained decrease in body temperature in the rat under normal ambient temperatures. Pre-treatment with the selective serotonin reuptake inhibitor sertraline (5 mg/kg), the full 5-HT(1A) receptor antagonist 4-fluoro-N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-2-pyridinyl benzamide], WAY 100635 (0.15 mg/kg) and the 5-HT(2C) receptor antagonist benzofuran-2-carboxamidine, RO 43-0440 (2.5 mg/kg) blocked D-fenfluramine-induced hypothermia. Depletion of 5-hydroxytryptamine (5-HT) stores following treatment with the serotonergic neurotoxin parachlorophenylalanine reversed the initial hypothermic effects of D-fenfluramine but not the later effects, as D120 min post-challenge) in animals pre-treated with parachlorophenylalanine. Such findings are consistent with a requirement for D-fenfluramine uptake into 5-HT neurons followed by release of 5-HT from intracellular stores and stimulation of post-synaptic 5-HT receptors to reduce body temperature. The hypothermic response to D-fenfluramine was potentiated by ketanserin pre-treatment 30 min post-challenge but then antagonized at later time intervals. Pre-treatment with the dopamine, D(2) antagonist, haloperidol (1 mg/kg) and sulpiride (30 mg/kg) had a similar effect in blocking the hypothermia as WAY 100635, suggesting a role for dopamine D(2) receptors in the response. Pre-treatment with the alpha(2)-adrenoceptor antagonist yohimbine failed to block the hypothermic response. These results suggest multiple sites of action mediating D-fenfluramine-induced hypothermia and may be the result of a combined effect of D-fenfluramine and its active metabolite norfenfluramine affecting not only the release of 5-HT but also stimulation of post-synaptic receptors.

Test conditions influence the response to a drug challenge in rodents

These studies were conducted to examine the differential response to a drug challenge under varied experimental test conditions routinely employed to study drug-induced behavioral and neurophysiological responses in rodents. Apomorphine, a nonselective dopamine agonist, was selected due to its biphasic behavioral effects, its ability to induce hypothermia, and to produce distinct changes to dopamine turnover in the rodent brain. From such experiments there is evidence that characterization and detection of apomorphine-induced activity in rodents critically depends upon the test conditions employed. In rats, detection of apomorphine-induced hyperactivity was facilitated by a period of acclimatization to the test conditions. Moreover, test conditions can impact upon other physiological responses to apomorphine such as drug-induced hypothermia. In mice, apomorphine produced qualitatively different responses under novel conditions when compared to those behaviors elicited in the home test cage. Drug-induced gross activity counts were increased in the novel exploratory box only, while measures of stereotypic behavior were similar in both. By contrast, apomorphine-induced locomotion was more prominent in the novel exploratory box. Dopamine turnover ratios (DOPAC:DA and HVA:DA) were found to be lower in those animals exposed to the exploratory box when compared to their home cage counterparts. However, apomorphine-induced reductions in striatal dopamine turnover were detected in both novel and home cage environments. The implications of these findings are discussed with particular emphasis upon conducting psychopharmacological challenge tests in rodents.

An assessment of the acute effects of the serotonin releasers methylenedioxymethamphetamine, methylenedioxyamphetamine and fenfluramine on immunity in rats

The purpose of the present study was to examine the effect of the serotonin releasing amphetamine derivatives methylenedioxymethamphetamine (MDMA), methylenedioxyamphetamine (MDA) and fenfluramine (FEN) on immunity in rats. Similar to MDA and MDMA, FEN reduced the number of circulating lymphocytes, provoked a suppression of Con A-stimulated lymphocyte proliferation and total IFN-gamma and IL-10 production in diluted whole blood cultures. Thus the non-psychostimulant amphetamine derivative FEN, shares the ability of the psychostimulant methylenedioxy-substituted amphetamine derivatives to alter these indices of immune function in the rat. However, when Con A-stimulated cytokine production was normalised for the number of lymphocytes in culture in order to examine cytokine production at a cellular level, the effect of the amphetamine derivatives begins to diverge. FEN shares with MDMA and MDA the ability to suppress production of the Th2 type cytokine IL-10. However the effect of these drugs on Th1 type cytokine secretion was much more complex. While the methylendioxy-substituted amphetamines increases the secretion of the Th1 type cytokine IL-2 without altering the related Th1 type cytokine IFN-gamma, FEN did not alter IL-2 secretion, but suppressed IFN-gamma secretion. In addition to these effects on T-cell responses, all three drugs inhibited LPS-induced TNF-alpha secretion from diluted whole blood cultures suggesting that macrophage activity is impaired following treatment. In all, these data extend our previous findings concerning the effects of MDMA on the immune system and demonstrate that the related serotonin releasers MDA and FEN also provoke immunological changes in rats.

Olfactory bulbectomy provokes a suppression of interleukin-1beta and tumour necrosis factor-alpha production in response to an in vivo challenge with lipopolysaccharide: effect of chronic desipramine treatment

The olfactory bulbectomized (OB) rat has been developed as an animal model of depression and exhibits several behavioural and neurochemical characteristics that are qualitatively similar to those found in clinically depressed patients. In addition to the behavioural and neurochemical abnormalities seen in OB rats, it has been reported that these animals have alterations in a number ex vivo measures of immune function many of which are reversed following chronic antidepressant treatment. In the present study we sought to examine the effects of olfactory bulbectomy on responsiveness to an in vivo immune challenge with bacterial lipopolysaccharide (LPS; 100 microg/kg, i.p.). In addition, the effect of chronic treatment with the tricyclic antidepressant desipramine (7.5 mg/kg, i.p.) on bulbectomy related behavioural changes, hypothalamic-pituitary-adrenal axis activity and immune responsiveness was evaluated. To our knowledge this is the first time that in vivo immunological responsiveness has been examined in the OB rat model of depression. OB rats exhibited a characteristic hyperactive response in a novel 'open field' environment, which was attenuated following chronic desipramine treatment. LPS provoked a large increase in circulating interleukin (IL)-1beta and tumour necrosis factor (TNF)-alpha in vehicle treated sham operated animals. Vehicle treated OB rats displayed a significant impairment in LPS-induced IL-1beta (54%) and TNF-alpha (70%) secretion compared to their sham operated controls, an effect that was potentiated following chronic desipramine treatment. Furthermore, sham animals that were chronically treated with desipramine displayed decreases in LPS-provoked IL-1beta (51%) and TNF-alpha (49%) secretion compared to vehicle treated counterparts. In addition, LPS-induced alterations in corticosterone and adrenal ascorbic acid concentrations were also attenuated by bulbectomy, an effect that was further enhanced following chronic desipramine treatment. In conclusion, these data provide evidence that olfactory bulbectomy in the rat impairs the ability of macrophages to produce the proinflammatory cytokines IL-1beta and TNF-alpha following an in vivo challenge with bacterial LPS. Whilst chronic treatment with desipramine normalized the behavioural hyperactivity observed in OB rats, such treatment further impaired LPS-induced IL-1beta and TNF-alpha secretion in bulbectomized rats.

Acute toxicity profile of maprotiline in the rat

The aim of the present study was to examine the toxic effects of single oral administrations of the antidepressant maprotiline at 150 mg/kg or 300 mg/kg using female Sprague-Dawley rats. Body-weight gain was significantly reduced in the group receiving 300 mg/kg on days 1-5 of the study (P<0.01). A significant reduction in food and water intake was observed on days 1 and 2 of the study (P<0.01) in the 300 mg/kg group and on day 1 in the 150 mg/kg group (P<0.05). There was a significant decrease in nocturnal home cage activity over the first five days of the study in the 300 mg/kg group (P<0.01). A significant hypothermic response was observed in both 150 and 300 mg/kg groups at 1, 2 and 4 hr after dosing (P<0.01), that had returned to control values within 8 hr following administration. This study demonstrates that a multi-parameter approach is appropriate for the investigation of high doses of antidepressants in rodents.

Differential effect of chronic antidepressant treatments on lipopolysaccharide-induced depressive-like behavioural symptoms in the rat

In the present study we observed that lipopolysaccharide (LPS) administration provoked a characteristic reduction in body weight gain, food consumption, saccharin (but not water) consumption and nocturnal locomotor activity. It has been previously suggested that the ability of LPS to suppress the consumption of, and preference for, a palatable solution such as saccharin without altering water consumption, may represent an anhedonic response. The results of the present study demonstrate that chronic treatment with the tricyclic antidepressant (TCA) desipramine (7.5 mg/kg; i.p.) prevented LPS-induced anorexia, loss of body weight, the antidipsogenic effect and hypoactivity. In contrast, chronic treatment with the antidepressants paroxetine (7.5 mg/kg; i.p.) and venlafaxine (10 mg/kg; i.p.) failed to alter any of the LPS-induced behavioural responses. Furthermore, chronic treatment with desipramine (and to a lesser extent paroxetine) reduced the consumption of, and preference for, saccharin suggesting that these antidepressant treatments induce an "anhedonic" response in their own right. In conclusion, chronic desipramine treatment attenuated LPS-induced depressive-like behavioural symptoms in the rat. However, chronic treatment with paroxetine and venlafaxine did not significantly alter LPS-induced behavioural responses. The results of the present study support the hypothesis that TCA's may exert part of their anti-depressive efficacy through their effects on the immune system. However, this property does not appear to be shared by newer antidepressants which possess a better side effect profile than the TCA's. The suppressive effect of TCA's on proinflammatory cytokine secretion is discussed as a mechanism by which these agents alter LPS-induced behavioural responses.

Comparative effects of serotonergic agonists with varying efficacy at the 5-HT(1A) receptor on core body temperature: modification by the selective 5-HT(1A) receptor antagonist WAY 100635

A reduction in core body temperature is one of the characteristic consequences of 5-HT1A receptor activation in rodents. In this study, we characterized the hypothermic effects of four 5-HT1A receptor ligands with varying affinity and selectivity at the 5-HT1A receptor. 8-OH-DPAT and flesinoxan (full agonists); ipsapirone (selective partial agonist) and eltoprazine (non selective partial agonist), all induced a dose-dependent reduction in core body temperature, which was maximal 30 min subsequent to administration. This response differed quantitatively between the agonists, in both the extent and the duration of its effects. The selective 5-HT1A receptor antagonist WAY 100635 (0.15 mg/kg), attenuated the hypothermia induced by the partial agonists, ipsapirone (10 mg/kg) and eltoprazine (10 mg/kg). In contrast, the higher dose of WAY 100635 (1 mg/kg) antagonized the effects of all agonists. This study therefore further confirms the utility of hypothermia as a simple, robust in-vivo probe of 5-HT1A receptor function. This paradigm, which was enhanced by use of specific antagonists such as WAY 100635, may prove useful for the detection and characterization of novel 5-HT1A receptor ligands.

The effect of treatment with a new antidepressant, INN 00835, on platelet serotonin uptake in depressed patients

BACKGROUND

INN 00835 (4-fluoro-L-phenylalanyl-trans-4-hydroxy-L-prolyl-L-arginyl-glycyl-trypt ophanamide ditrifluoroacetate) is a synthetic pentapeptide antidepressant with a potential for rapid onset of action. We were interested to see if such action could be correlated with serotonin uptake by platelets.

METHODS

In a phase II clinical trial, unipolar depressed patients were administered active drug, INN 00835 or placebo, subcutaneously, at 0.2 mg/kg, once daily for 5 consecutive days. Efficacy of treatment was evaluated by psychometric tests (HAMD, MADS, CSRS, CGI and total VAS). Changes in platelet uptake rates of serotonin (3H-5HT) were measured in plasma from the patients participating in the phase II clinical trial, prior to and immediately after treatment with INN 00835 (19 patients) or placebo (16 patients), to evaluate the effect of treatment with INN 00835 on the rate of platelet 5-HT uptake.

RESULTS

The data evaluated by using the psychometric tests indicated a significant response to treatment with INN 00835 after 5 days of dosing. The rates of platelet 5-HT uptake were lower prior to treatment (baseline), and increased after the 5-day treatment period. The change in the uptake rate (deltaVmax) following treatment was significantly larger in the active group than in the placebo group (P < 0.05). The difference between the placebo group and the patients who responded to treatment was even larger.

LIMITATIONS

Small number of subjects.

CONCLUSION

The data tend to substantiate the use of platelet serotonin uptake as a biochemical marker of effective treatment of depression.

Differential effect of a single high dose of the tricyclic antidepressant imipramine on interleukin-1beta and tumor necrosis factor-alpha secretion following an in vivo lipopolysaccharide challenge in rats

Tricyclic antidepressants (TCAs) of which imipramine is one, are commonly used in the treatment of depressive disorders and other forms of psychiatric illness. There have been many reports regarding the suppressive effects of TCAs on immune function. However, information is still limited regarding the effects of TCAs on the immune system, as many of the studies conducted to date have concentrated on in vitro exposure to such drugs, or ex vivo measures of immunity following drug administration. Thus in the present investigation, an in vivo challenge with bacterial lipopolysaccharide (LPS) (100 microg/kg; i.p.) was used to assess immunocompetence following administration of a single high dose of the TCA, imipramine (100 mg/kg, p.o.). The results demonstrated that imipramine pretreatment inhibits LPS-induced increases in serum concentrations of the proinflammatory cytokine, tumor necrosis factor (TNF)-alpha both 3 and 6 h, following administration. However, LPS-induced interleukin (IL)-1beta secretion was not significantly altered following imipramine treatment at either of the timepoints examined. In addition, serum concentrations of corticosterone and the antiinflammatory cytokine IL-10 were measured, and imipramine treatment failed to alter either basal, or LPS-induced increases in these immunosuppressive agents. In conclusion, although IL-1beta and TNF-alpha are both macrophage-derived proinflammatory cytokines, the present study demonstrates a differential sensitivity of these cytokines to the suppressive effects of the TCA imipramine. Furthermore, the suppressive effects of imipramine on LPS-induced TNF-alpha secretion could not be attributed to either increased glucocorticoid levels, or increased secretion of the antiinflammatory cytokine IL-10. The relevance of these findings to antidepressant-induced immunotoxicity are discussed.

Reboxetine attenuates forced swim test-induced behavioural and neurochemical alterations in the rat

The forced swim test is a behavioural paradigm that is predicative of antidepressant activity in rodents. Until recently, research has focused on the ability of antidepressant drugs to decrease immobility in the forced swim test paradigm, but the neurochemical sequelae induced by swim stress, or the neurochemical basis of antidepressant-induced behavioural changes have received little attention. In this regard, we have recently demonstrated that forced swim test exposure increases serotonergic activity in the amygdala, frontal cortex and hippocampus and dopamine turnover in the striatum. In addition, forced swim test-exposure activates the hypothalamic pituitary adrenal axis. The purpose of the present study was to examine the effect of treatment with the selective noradrenaline reuptake inhibitor reboxetine (3, 10 and 30 mg/kg; i.p.) on immobility and defaecation scores in the forced swim test, and on forced swim test-induced neurochemical and hypothalamic pituitary adrenal axis changes in the rat. Reboxetine treatment (10 and 30 mg/kg) significantly decreased immobility and defaecation in the forced swim test in dose dependent manner. Furthermore, reboxetine produced a dose dependent attenuation of forced swim test-induced increases in serotonin turnover in the amygdala and frontal cortex and dopamine turnover in the striatum. Reboxetine (30 mg/kg) produced a modest, but non-significant, attenuation of forced swim test-induced increases in serum corticosterone concentrations. These data demonstrate that, in addition to the behavioural activity of reboxetine in the rat forced swim test paradigm, a dose-dependent attenuation of swim stress-induced increases in serotonergic and dopaminergic activity occurred in a region specific manner. These are the first data to demonstrate that treatment with the selective noradrenaline reuptake inhibitor, reboxetine can impact on the activity of other neurotransmitter systems in response to stress. Moreover, these data further demonstrate that this paradigm is a valuable tool in studying the effect of antidepressants, on both behaviour and swim stress-related alterations in central neurotransmitter function and hypothalamic pituitary adrenal axis activity in the rat.

Metyrapone displays antidepressant-like properties in preclinical paradigms

A possible involvement of glucocorticoids in the aetiology of depression is suggested by commonly reported hypothalamo-pituitary-adrenocortical (HPA) axis abnormalities in depressed patients, the modulation of the HPA axis by antidepressant drugs and clinical reports of antidepressant efficacy with antiglucocorticoid agents. The effects of treatment with metyrapone, a glucocorticoid synthesis inhibitor, and the tricyclic antidepressant, desipramine, in two rodent models of depression, namely the forced swim test and olfactory bulbectomized (OB) rat, were investigated. In addition, the effect of chronic metyrapone and desipramine treatments on the hypothermic response to a challenge with the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) was assessed. There is experimental evidence to suggest that attenuation of the hypothermic response to this agonist occurs following chronic antidepressant treatment. In the forced swim test, metyrapone (50 mg/kg) and desipramine (10 mg/kg) significantly reduced the immobility time. In the olfactory bulbectomized rat model of depression, chronic administration (14 days) of metyrapone (50 mg/kg b.i.d.) and desipramine (5 mg/kg b.i.d.) attenuated the OB-related hyperactivity in a novel stressful environment. Chronic metyrapone treatment (50 mg/kg b.i.d.) attenuated the hypothermic response to an acute challenge with 8-OH-DPAT (0.05 mg/kg s.c.), indicating a change to the sensitivity of 5-HT1A receptors. These preclinical tests demonstrate an antidepressant-like profile for metyrapone. Further exploration of the therapeutic potential and possible mechanism of action of glucocorticoid antagonism in depression is warranted.

Onset of the effects of the 5-HT1A antagonist, WAY-100635, alone, and in combination with paroxetine, on olfactory bulbectomy and 8-OH-DPAT-induced changes in the rat

5-HT1A receptor antagonists have recently been shown to accelerate the effects of some antidepressant drugs in clinical trials. In this study we investigate the effects of combining a full antagonist at the 5-HT1A receptor, WAY 100635 (0.2 mg/kg, SC) with the selective serotonin reuptake inhibitor (SSRI) paroxetine (5 mg/kg. SC) in the olfactory bulbectomized (OB) rat, an animal model of chronic (but not acute) antidepressant activity. Ambulation scores were measured in the open-field apparatus, following 3, 7, and 14 days of treatment. Further to the OB study, we simultaneously studied adaptive changes in 5-HT1A receptor function, utilizing alterations in the hypothermic response to the 5-HT1A receptor agonist 8-OH-DPAT. Paroxetine, in combination with WAY 100635, attenuated the hypothermic effects of 8-OH-DPAT as early as 3 days, with a full reversal evident following 7 days, whereas paroxetine, although attenuating the hypothermic effects in OB group by day 7, only reversed it fully after 14 days. Paroxetine alone and in combination with the antagonist reversed the olfactory bulbectomy-induced hyperactivity in the open field following 14 days of treatment only, this being the normal time of an "antidepressant" response in this model. However, there was no significant attenuation at any of the earlier time points. This further demonstrates that the reversal of this aspect of the olfactory bulbectomy-induced behavioral syndrome is insensitive to the potential faster onset of antidepressant action induced by 5-HT1A receptor antagonists. Nonetheless, WAY 100635, unlike previous studies with pindolol, did not interfere with the effects of the antidepressant in the model. The ability of the combination group to attenuate the hypothermic effects of 8-OH-DPAT faster than paroxetine alone, further emphasizes the role of the 5-HT1A receptor in the mechanism of action of antidepressants, and as a target for the development of faster acting antidepressants.

The determination of the optimal dose of milnacipran in the olfactory bulbectomized rat model of depression

Olfactory bulbectomy (OB) is associated with a variety of behavioral abnormalities such as hyperactivity in the "open-field" test. Previous studies have shown that chronic administration of antidepressants can reverse this behavioral deficit. The activity of milnacipran (20, 30, and 40 mg/kg, PO bid) administered in two equally divided doses twice daily was assessed in the olfactory bulbectomized rat model of depression. It was found that chronic treatment with milnacipran at the doses of 30 and 40 mg/kg, but not 20 mg/kg, attenuated the lesion-induced hyperactivity of the OB rat in the "open-field" test following 14 days of treatment. In the step-through passive avoidance test, administration of milnacipran at doses of 20, 30, and 40 mg/kg had no effect on the performance deficit associated with olfactory bulbectomy. Olfactory bulbectomy reduced the concentration of noradrenaline (NA) in the frontal cortex. However, chronic milnacipran treatment did not significantly alter this deficit. It is concluded that milnacipran, when administered chronically at doses of 30 and 40 mg/kg, are effective at reversing the "open-field" deficit associated with olfactory bulbectomy, and that a dose of 30 mg/kg is an optimal dose.

Stressor-induced alterations in serotonergic activity in an animal model of depression

The present study examined the effect of two neurogenic stressors (air puff and restraint) and a metabolic stressor (lipopolysaccharide; LPS 100 microg/kg, i.p.) on accumbal serotonergic neurotransmission in the olfactory bulbectomized (OB) rat model of depression. Both air puff and restraint stress caused greater increases in accumbal 5-HIAA in OB than in sham-operated rats. In contrast, bulbectomy resulted in a blunted serotonergic response to a challenge with LPS (a metabolic stressor). In addition, OB rats displayed significantly lower basal levels of 5-HIAA than sham-operated counterparts, a finding consistent with previous reports of the OB rat being a model of hyposerotonergic depression. The relevance of these findings to stressor-provoked depressive-like behaviors in the OB rat are discussed.

Activity and onset of action of reboxetine and effect of combination with sertraline in an animal model of depression

The limitations of antidepressant drugs to treat depression has warranted ongoing research to identify pharmacological agents and strategies which offer a faster onset of action and greater therapeutic efficacy. Noradrenaline and serotonin are widely reported to be involved in the mechanism of action of antidepressants and the recent development of selective reuptake inhibitors of these transmitters has provided the opportunity to determine the effects of targeting these transmitter systems, alone and in combination, in an antidepressant response. The present study investigated the effects of reboxetine, a new antidepressant that selectively inhibits noradrenaline reuptake, sertraline, a selective serotonin reuptake inhibitor and a combination treatment composed of the two drugs in the olfactory bulbectomized (OB) rat model of depression. Sub-acute (2 days) administration of reboxetine (2.5, 5, and 10 mg/kg, i.p.) to sham-operated and OB rats reduced the immobility time in the forced swim test. Repeated (14 days) reboxetine (10 mg/kg) treatment attenuated the OB-related behavioural hyperactivity in the 'open-field' test. Examination of the onset of the antidepressant effect in the 'open-field' test demonstrated that reboxetine (10 mg/kg), sertraline (5 mg/kg) and the combination reduced the behavioural hyperactivity after 14 days but not before this following 3, 7 or 10 days of treatment. Reduced 5-hydroxyindoleacetic acid (5-HIAA) concentrations in amygdaloid cortex of both sham and OB rats following sertraline and combination treatments are likely to be related to acute pharmacological effects on the reuptake of 5-hydroxytryptamine (5-HT). Attenuation of the hypothermia induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.05 mg/kg s.c.) and clonidine (0.1 mg/kg s.c.) occurred in the reboxetine and sertraline combination treated groups following both 7 and 14 days administration indicating changes to 5-HT1A receptor and alpha2-adrenoceptor sensitivity. The results indicate that changes to 8-OH-DPAT and clonidine-induced responses occur quicker with the combination treatment than with either reboxetine or sertraline treatments alone.

An investigation of the antidepressant properties of lofepramine and its desmethylated metabolites in the forced swim and olfactory bulbectomized rat models of depression

The purpose of this study was to examine the metabolites of lofepramine (LOF), namely desipramine (DMI), desmethyl desipramine (DDMI) and desmethyl lofepramine (DML) in the forced swim and olfactory bulbectomized (OB) rat models of depression. In the first study, subacute treatment with DMI (10 mg/kg) and DML (20 mg/kg), but not LOF, reduced the immobility time in the forced swim test. In the "open field", chronic (14 day) treatment with all drugs attenuated the hyperactivity associated with olfactory bulbectomy. In the second experiment, a lower dose of DML (10 mg/kg) demonstrated activity following subacute treatment in the forced swim and following chronic treatment in the OB model. In addition, DDMI (10 mg/kg) was active in both models. From these results, it can be concluded that, at the doses employed, LOF and it's desmethylated metabolites, DMI, DML and DDMI, exhibits activity in the OB model. In contrast, lofepramine but not it's desmethylated derivatives is inactive in the forced swim test, perhaps suggesting the requirement of metabolic conversion of LOF to reveal antidepressant activity in this model.

Effects of acute and chronic antidepressant administration on phencyclidine (PCP) induced locomotor hyperactivity

Previously it was found that both acute and chronic antidepressant pre-treatment enhanced the locomotor hyperactivity induced by a challenge injection of the non-competitive NMDA receptor antagonist, dizocilpine (MK-801). In the present study the effects of acute and chronic antidepressant administration on phencyclidine (PCP)-induced locomotor hyperactivity were examined. Phencyclidine (PCP), a non-competitive NMDA receptor antagonist increased locomotor activity in rats. Fluoxetine given acutely increased and prolonged the PCP-induced locomotor hyperactivity, while citalopram, sertraline and paroxetine had no effect on the PCP-induced behavioural effect. Repeated treatment with fluoxetine, citalopram and paroxetine increased the PCP-induced locomotor hyperactivity. In contrast, chronic sertraline administration attenuated the locomotor response to a PCP challenge. These results indicate that these antidepressants which are presumed to have a similar pharmacological profile, differ in their ability to alter PCP-induced hyperactivity. Whether these differences have any bearing on the therapeutic or adverse effects of these drugs remains to be shown.

Combining pindolol and paroxetine in an animal model of chronic antidepressant action--can early onset of action be detected?

The realisation that pindolol may accelerate the effects of some antidepressant drugs in clinical trials has added extra impetus to the search for faster acting antidepressants. Currently, no animal model of depression can identify potential faster acting antidepressant drugs or drug combinations. In this study, we investigate the effects of combining pindolol (2 mg/kg, s.c., bid) with the antidepressant paroxetine (2.5 mg/kg, i.p., bid) in the olfactory bulbectomised rat, an animal model of chronic (but not acute) antidepressant activity. Ambulation scores were measured in separate groups of rats, following 3, 7 and 14 days of treatment. Further, we simultaneously study adaptive changes in 5-HT1A receptor function, utilising alterations in the hypothermic response to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Pindolol in combination with paroxetine attenuated the hypothermic effects of 8-OH-DPAT as early as 3 days with a full reversal evident following 7 days, whereas paroxetine alone did so after 14 days only. Likewise, paroxetine alone reversed the olfactory bulbectomy-induced hyperactivity in the open field following 14 days of treatment only, this being the normal time of an 'antidepressant' response in this model. However, the group treated with both paroxetine and pindolol failed to reverse the hyperactive response. This suggests that a factor intrinsic to pindolol antagonises the behavioural effects of paroxetine in the olfactory bulbectomised rat. It also demonstrates that the reversal of this aspect of the olfactory bulbectomy-induced behavioural syndrome is insensitive to the potential faster onset of antidepressant action induced by pindolol. The ability of the combination group to attenuate the hypothermic effects of 8-OH-DPAT much faster further emphasises the role of the 5-HT1A receptor in the mechanism of action of antidepressants and as a target for the development of faster acting antidepressants. However, an animal model sensitive to the effects of any such compound and the actions of pindolol remains elusive.