Amitriptyline, desipramine, cyproheptadine and carbamazepine, in concentrations used therapeutically, reduce kainate- and N-methyl-D-aspartate-induced intracellular Ca2+ levels in neuronal culture

A Brief Review of the Pharmacology of Amitriptyline and Clinical Outcomes in Treating Fibromyalgia

Fibromyalgia is a complex chronic condition characterized by pain, physical fatigue, sleep disorder and cognitive impairment. Evidence-based guidelines recommend antidepressants as treatments of fibromyalgia where tricyclics are often considered to have the greatest efficacy, with amitriptyline often being a first-line treatment. Amitriptyline evokes a preferential reduction in pain and fatigue of fibromyalgia, and in the Fibromyalgia Impact Questionnaire (FIQ) score, which is a quality of life assessment. The multimodal profile of the mechanisms of action of amitriptyline include monoamine reuptake inhibition, receptor modulation and ion channel modulation. Several of the actions of amitriptyline on multiple nociceptive and sensory processes at central and peripheral locations have the potential to act cumulatively to suppress the characteristic symptoms of fibromyalgia. Greater understanding of the role of these mechanisms of action of amitriptyline could provide further clues to the pathophysiology of fibromyalgia and to a preferable pharmacological profile for future drug development.

Fluoxetine reverses the behavioral despair induced by neurogenic stress in mice: role of N-methyl-d-aspartate and opioid receptors

Opioid and N-methyl-d-aspartate (NMDA) receptors mediate different effects of fluoxetine. We investigated whether opioid and NMDA receptors are involved in the protective effect of fluoxetine against the behavioral despair induced by acute physical stress in male mice. We used the forced swimming test (FST), tail suspension test (TST), and open-field test (OFT) for behavioral evaluation. We used fluoxetine, naltrexone (opioid receptor antagonist), MK-801 (NMDA receptor antagonist), morphine (opioid receptor agonist), and NMDA (NMDA receptor agonist). Acute foot-shock stress (FSS) significantly induced behavioral despair (depressive-like) and anxiety-like behaviors in tests. Fluoxetine (5 mg/kg) reversed the depressant-like effect of FSS, but it did not alter the locomotion and anxiety-like behavior in animals. Acute administration of subeffective doses of naltrexone (0.3 mg/kg) or MK-801 (0.01 mg/kg) potentiated the antidepressant-like effect of fluoxetine, while subeffective doses of morphine (1 mg/kg) and NMDA (75 mg/kg) abolished this effect of fluoxetine. Also, co-administration of subeffective doses of naltrexone (0.05 mg/kg) and MK-801 (0.003 mg/kg) with fluoxetine (1 mg/kg) induced a significant decrease in the immobility time in FST and TST. Our results showed that opioid and NMDA receptors (alone or in combination) are involved in the antidepressant-like effect of fluoxetine against physical stress.

Carbamazepine modulates the spatiotemporal activity in the dentate gyrus of rats and pharmacoresistant humans in vitro

INTRODUCTION

Human hippocampal tissue resected from pharmacoresistant epilepsy patients was investigated to study the effect of the antiepileptic drug CBZ (carbamazepine) and was compared to similar experiments in the hippocampus of control rats.

METHODS

The molecular layer of the DG (dentate gyrus) of human epileptic tissue and rat nonepileptic tissue was electrically stimulated and the evoked responses were recorded with voltage-sensitive dye imaging to characterize the spatiotemporal properties.

RESULTS

Bath applied CBZ (100 μmol/L) reduced the amplitude of the evoked responses in the human DG, albeit that no clear use-dependent effects were found at frequencies of 8 or 16 Hz. In nonepileptic control DG from rats, CBZ also reduced the amplitude of the evoked response in the molecular layer of the DG as well as the spatial extent of the response.

CONCLUSIONS

This study demonstrates that CBZ still reduced the activity in the DG, although the patients were clinically diagnosed as pharmacoresistant for CBZ. This suggests that in the human epileptic brain, the targets of CBZ, the voltage-gated Na(+) channels, are still sensitive to CBZ, although we used a relative high concentration and it is not possibility to assess the actual CBZ concentration that reached the target in the patient. We also concluded that the effect of CBZ was found in the activated region of the DG, quite comparable to the observations in the nonepileptic rat.

The role of NMDA receptors in the pathophysiology and treatment of mood disorders

Mood disorders such as major depressive disorder and bipolar disorder are chronic and recurrent illnesses that cause significant disability and affect approximately 350 million people worldwide. Currently available biogenic amine treatments provide relief for many and yet fail to ameliorate symptoms for others, highlighting the need to diversify the search for new therapeutic strategies. Here we present recent evidence implicating the role of N-methyl-D-aspartate receptor (NMDAR) signaling in the pathophysiology of mood disorders. The possible role of NMDARs in mood disorders has been supported by evidence demonstrating that: (i) both BPD and MDD are characterized by altered levels of central excitatory neurotransmitters; (ii) NMDAR expression, distribution, and function are atypical in patients with mood disorders; (iii) NMDAR modulators show positive therapeutic effects in BPD and MDD patients; and (iv) conventional antidepressants/mood stabilizers can modulate NMDAR function. Taken together, this evidence suggests the NMDAR system holds considerable promise as a therapeutic target for developing next generation drugs that may provide more rapid onset relief of symptoms. Identifying the subcircuits involved in mood and elucidating the role of NMDARs subtypes in specific brain circuits would constitute an important step toward the development of more effective therapies with fewer side effects.

The tricyclic antidepressant desipramine inhibited the neurotoxic, kainate-induced [Ca(2+)]i increases in CA1 pyramidal cells in acute hippocampal slices

Kainate (KA), used for modelling neurodegenerative diseases, evokes excitotoxicity. However, the precise mechanism of KA-evoked [Ca(2+)]i increase is unexplored, especially in acute brain slice preparations. We used [Ca(2+)]i imaging and patch clamp electrophysiology to decipher the mechanism of KA-evoked [Ca(2+)]i rise and its inhibition by the tricyclic antidepressant desipramine (DMI) in CA1 pyramidal cells in rat hippocampal slices and in cultured hippocampal cells. The effect of KA was dose-dependent and relied totally on extracellular Ca(2+). The lack of effect of dl-2-amino-5-phosphonopentanoic acid (AP-5) and abolishment of the response by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) suggested the involvement of non-N-methyl-d-aspartate receptors (non-NMDARs). The predominant role of the Ca(2+)-impermeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) in the initiation of the Ca(2+) response was supported by the inhibitory effect of the selective AMPAR antagonist GYKI 53655 and the ineffectiveness of 1-naphthyl acetylspermine (NASPM), an inhibitor of the Ca(2+)-permeable AMPARs. The voltage-gated Ca(2+) channels (VGCC), blocked by ω-Conotoxin MVIIC+nifedipine+NiCl2, contributed to the [Ca(2+)]i rise. VGCCs were also involved, similarly to AMPAR current, in the KA-evoked depolarisation. Inhibition of voltage-gated Na(+) channels (VGSCs; tetrodotoxin, TTX) did not affect the depolarisation of pyramidal cells but blocked the depolarisation-evoked action potential bursts and reduced the Ca(2+) response. The tricyclic antidepressant DMI inhibited the KA-evoked [Ca(2+)]i rise in a dose-dependent manner. It directly attenuated the AMPA-/KAR current, but its more potent inhibition on the Ca(2+) response supports additional effect on VGCCs, VGSCs and Na(+)/Ca(2+) exchangers. The multitarget action on decisive players of excitotoxicity holds out more promise in clinical therapy of neurodegenerative diseases.

The plasticity of the association between mu-opioid receptor and glutamate ionotropic receptor N in opioid analgesic tolerance and neuropathic pain

Multiple groups have reported the functional cross-regulation between mu-opioid (MOP) receptor and glutamate ionotropic receptor N (GluN), and the post-synaptic association of these receptors has been implicated in the transmission and modulation of nociceptive signals. Opioids, such as morphine, disrupt the MOP receptor-GluN receptor complex to stimulate the activity of GluN receptors via protein kinase C (PKC)/Src. This increased GluN receptor activity opposes MOP receptor signalling, and via neural nitric oxide synthase (nNOS) and calcium and calmodulin regulated kinase II (CaMKII) induces the phosphorylation and uncoupling of the opioid receptor, which results in the development of morphine analgesic tolerance. Both experimental in vivo activation of GluN receptors and neuropathic pain separate the MOP receptor-GluN receptor complex via protein kinase A (PKA) and reduce the analgesic capacity of morphine. The histidine triad nucleotide-binding protein 1 (HINT1) associates with the MOP receptor C-terminus and connects the activities of MOP receptor and GluN receptor. In HINT1⁻/⁻ mice, morphine promotes enhanced analgesia and produces tolerance that is not related to GluN receptor activity. In these mice, the GluN receptor agonist N-methyl-D-aspartate acid (NMDA) does not antagonise the analgesic effects of morphine. Treatments that rescue morphine from analgesic tolerance, such as GluN receptor antagonism or PKC, nNOS and CaMKII inhibitors, all induce MOP receptor-GluN receptor re-association and reduce GluN receptor/CaMKII activity. In mice treated with NMDA or suffering from neuropathic pain (induced by chronic constriction injury, CCI), GluN receptor antagonists, PKA inhibitors or certain antidepressants also diminish CaMKII activity and restore the MOP receptor-GluN receptor association. Thus, the HINT1 protein stabilises the association between MOP receptor and GluN receptor, necessary for the analgesic efficacy of morphine, and this coupling is reduced following the activation of GluN receptors, similar to what is observed in neuropathic pain.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Amitriptyline inhibits the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

Objectives

Evidence suggests that glutamatergic systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. We have investigated the effects of amitriptyline, a tricyclic antidepressant, on the activity of the excitatory amino acid transporter type 3 (EAAT3), a protein that can regulate extracellular glutamate concentrations in the brain.

Methods

EAAT3 was expressed in Xenopus oocytes. Using a two-electrode voltage clamp, membrane currents were recorded after application of 30 μM l-glutamate in the presence or absence of various concentrations of amitriptyline or after application of various concentrations of l-glutamate in the presence or absence of 0.64 μM amitriptyline.

Key findings

Amitriptyline concentration-dependently reduced EAAT3 activity. This inhibition reached statistical significance at 0.38–1.27 μM amitriptyline. Amitriptyline 0.64 μM reduced the pharmacokinetic parameter Vmax, but did not affect the pharmacokinetic parameter Km, of EAAT3 for l-glutamate. The amitriptyline inhibition disappeared after a 4-min washout. Phorbol-12-myristate-13-acetate, a protein kinase C activator, increased EAAT3 activity. However, 0.64 μM amitriptyline induced a similar degree of decrease in EAAT3 activity in the presence or absence of phorbol-12-myristate-13-acetate.

Conclusions

Our results suggested that amitriptyline at clinically relevant concentrations reversibly reduced EAAT3 activity via decreasing its maximal velocity of glutamate transporting function. The effects of amitriptyline on EAAT3 activity may have represented a novel site of action for amitriptyline to increase glutamatergic neuro-transmission. Protein kinase C may not have been involved in the effects of amitriptyline on EAAT3.

Amitriptyline inhibits the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

OBJECTIVES

Evidence suggests that glutamatergic systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. We have investigated the effects of amitriptyline, a tricyclic antidepressant, on the activity of the excitatory amino acid transporter type 3 (EAAT3), a protein that can regulate extracellular glutamate concentrations in the brain.

METHODS

EAAT3 was expressed in Xenopus oocytes. Using a two-electrode voltage clamp, membrane currents were recorded after application of 30 microM L-glutamate in the presence or absence of various concentrations of amitriptyline or after application of various concentrations of L-glutamate in the presence or absence of 0.64 microM amitriptyline.

KEY FINDINGS

Amitriptyline concentration-dependently reduced EAAT3 activity. This inhibition reached statistical significance at 0.38-1.27 microM amitriptyline. Amitriptyline 0.64 microM reduced the pharmacokinetic parameter Vmax, but did not affect the pharmacokinetic parameter Km, of EAAT3 for L-glutamate. The amitriptyline inhibition disappeared after a 4-min washout. Phorbol-12-myristate-13-acetate, a protein kinase C activator, increased EAAT3 activity. However, 0.64 microM amitriptyline induced a similar degree of decrease in EAAT3 activity in the presence or absence of phorbol-12-myristate-13-acetate.

CONCLUSIONS

Our results suggested that amitriptyline at clinically relevant concentrations reversibly reduced EAAT3 activity via decreasing its maximal velocity of glutamate transporting function. The effects of amitriptyline on EAAT3 activity may have represented a novel site of action for amitriptyline to increase glutamatergic neurotransmission. Protein kinase C may not have been involved in the effects of amitriptyline on EAAT3.

Differential effect of carbamazepine and oxcarbazepine on excitatory synaptic transmission in rat hippocampus

In this study, we have compared the effects of two structurally related compounds carbamazepine (CBZ) and oxcarbazepine (OXC), both in current use for the treatment of epilepsy and bipolar disorder, on fast excitatory transmission in rat hippocampal slices. Using electrophysiological recordings, we have investigated the effects of CBZ and OXC on repetitive action potential discharge of CA1 pyramidal neurons demonstrating that both compounds produced firing inhibition with similar IC(50) values. Moreover, we show that bath applied CBZ (0.01-1 mM) exerted a concentration-dependent decrease in the amplitude of the field excitatory postsynaptic potentials with an IC(50) of approximately 194.3 microM. When OXC was used at the same concentrations, the concentration-response curve was shifted to the right (IC(50) of approximately 711.07 microM). In addition, we demonstrated that CBZ and OXC reduced, to a different extent, both evoked excitatory postsynaptic currents and NMDA-, AMPA-, and KA-mediated inward currents, CBZ being more potent than OXC. These data highlight distinct presynaptic and postsynaptic sites of action for both compounds and suggest that CBZ, by markedly depressing postsynaptic ionotropic glutamate receptors-mediated responses, may produce more severe cognitive and memory impairment. Thus, we assume that relatively high doses of OXC could be better tolerated than therapeutically equivalent doses of CBZ, justifying the preferential use of OXC as first-line treatment in the therapy of neurological and psychiatric disorders, particularly when compared with CBZ.

Desipramine reduces stress-activated dynorphin expression and CREB phosphorylation in NAc tissue

The nucleus accumbens (NAc) is a critical brain area for reward and motivated behavior. Accumulating evidence suggests that altered function of the transcription factor cAMP response element binding protein (CREB) within the NAc is involved in depressive behavior. In rats, stress activates CREB within the NAc, and elevation of CREB expression in this region produces depressive-like behaviors that are accompanied by activation of CREB-regulated target genes. The depressive-like behaviors seem to be due, at least in part, to CREB-mediated increases in dynorphin function, because they are mimicked by kappa-opioid receptor (KOR) agonists and attenuated by KOR antagonists. We hypothesized that if CREB-mediated dynorphin expression in the NAc contributes to depressive behavior, then antidepressants might reduce dynorphin function in this region. Here, we demonstrate that desipramine (DMI), a norepinephrine reuptake inhibitor that has been used for decades to treat clinical depression, blocks swim stress-induced activation of prodynorphin (encodes dynorphin) in the NAc. In primary cultures of NAc and striatum, DMI decreases basal and stimulated CREB phosphorylation by causing reductions in intracellular calcium (Ca(2+)) availability that are independent of norepinephrine or other monoaminergic inputs, identifying a potential mechanism for alterations in CREB-mediated gene expression. Fluoxetine (FLX), a selective serotonin reuptake inhibitor, has similar effects in culture, suggesting a common intracellular effect of these antidepressants. These findings raise the possibility that a therapeutically relevant mechanism of action of DMI occurs through attenuation of CREB-mediated gene transcription, which is mediated via previously uncharacterized mechanisms that occur directly within the NAc.

Direct inhibitory effect of fluoxetine on N-methyl-D-aspartate receptors in the central nervous system

BACKGROUND

Data accumulated in the last decade indicate that N-methyl-D-aspartate (NMDA) receptors might be involved in the pathophysiology of depression and the mechanism of action of antidepressants, although a direct inhibitory effect has been reported only in connection with tricyclic compounds, which interact with a wide range of receptors.

METHODS

Using whole-cell patch-clamp recording in rat cortical cell cultures, we investigated whether the selective serotonin reuptake inhibitor fluoxetine, which has a much better adverse effect profile, has a direct effect on NMDA receptors, and we compared its action to that of the tricyclic desipramine.

RESULTS

Both desipramine (concentration that causes 50% inhibition (IC(50)) = 3.13 microM) and fluoxetine (IC(50) = 10.51 microM) inhibited NMDA-evoked currents with similar efficacy in the clinically relevant low micromolar concentration range. However, in contrast to desipramine, the inhibition by fluoxetine was not voltage-dependent, and fluoxetine partially preserved its ability to associate with NMDA receptor in the presence of Mg(2+), suggesting different binding sites for the two drugs.

CONCLUSIONS

The fact that different classes of antidepressants were found to be low-affinity NMDA antagonists suggests that direct inhibition of NMDA receptors may contribute to the clinical effects of antidepressants.

Chronic carbamazepine administration reduces N-methyl-D-aspartate receptor-initiated signaling via arachidonic acid in rat brain

BACKGROUND

Lithium and carbamazepine (CBZ) are used to treat mania in bipolar disorder. When given chronically to rats, both agents reduce arachidonic acid (AA) turnover in brain phospholipids and downstream AA metabolism. Lithium in rats also attenuates brain N-methyl-D-aspartic acid receptor (NMDAR) signaling via AA. We hypothesized that, like chronic lithium, chronic CBZ administration to rats would reduce NMDAR-mediated signaling via AA.

METHODS

We used our fatty acid method with quantitative autoradiography to image the regional brain incorporation coefficient k* of AA, a marker of AA signaling, in unanesthetized rats that had been given 25 mg/kg/day I.P. CBZ or vehicle for 30 days, then injected with NMDA (25 mg/kg I.P.) or saline. We also measured brain concentrations of two AA metabolites, prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TXB(2)).

RESULTS

In chronic vehicle-treated rats, NMDA compared with saline increased k* significantly in 69 of 82 brain regions examined, but did not change k* significantly in any region in CBZ-treated rats. In vehicle- but not CBZ-treated rats, NMDA also increased brain concentrations of PGE(2) and TXB(2).

CONCLUSIONS

Chronic CBZ administration to rats blocks increments in the AA signal k*, and in PGE(2) and TXB(2) concentrations that are produced by NMDA in vehicle-treated rats. The clinical action of antimanic drugs might involve inhibition of brain NMDAR-mediated signaling involving AA and its metabolites.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Metabotropic glutamate receptor ligands as possible anxiolytic and antidepressant drugs

Depression and anxiety represent a major problem. However, the current treatment of both groups of diseases is not satisfactory. As the glutamatergic system may play an important role in pathophysiology of both depression and anxiety, we decided to discuss the recent data on possible anxiolytic and/or antidepressant effects of metabotropic glutamate (mGlu) receptor ligands. Preclinical data indicated that antagonists of group I mGlu receptors, particularly antagonists of mGlu5 receptors, produced both anxiolytic-like and antidepressant-like effects. Clinical data also demonstrated that mGlu5 receptor antagonist, fenobam, was an active anxiolytic drug. The anxiolytic effects exerted by mGlu5 receptor antagonists are profound, comparable with or stronger than those of benzodiazepines. However, the problem with the psychotomimetic activity of mGlu5 receptor antagonists and their possible influence on memory has to be further investigated. Among all mGlu receptor ligands, group II mGlu receptor agonists seem to be the drugs with the most promising therapeutic potential and a good safety profile. Animal studies showed anxiolytic-like effects of group II mGlu receptor agonists. Currently, group II mGlu receptor agonists are in phase III clinical trials for potential treatment of anxiety disorders. On the other hand, data has been accumulated, indicating that antagonists of group II mGlu receptors have an antidepressant potential. Group III mGlu receptor ligands represent the least investigated group of mGlu receptors. However, preclinical data also indicates that ligands of these receptors, both agonists and antagonists, may have an anxiolytic-like and antidepressant-like potential.

Differentiating antidepressants of the future: Efficacy and safety

There have been significant advances in the treatment of depression since the serendipitous discovery that modulating monoaminergic neurotransmission may be a pathological underpinning of the disease. Despite these advances, particularly over the last 15years with the introduction of selective serotonin and/or norepinephrine reuptake inhibitors (SNRI), there still remain multiple unmet clinical needs that would represent substantial improvements to current treatment regimens. In terms of efficacy there have been improvements in the percentage of patients achieving remission but this can still be dramatically improved and, in fact, issues still remain with relapse. Furthermore, advances are still required in terms of improving the onset of efficacy as well as addressing the large proportion of patients who remain treatment resistant. While this is not well understood, collective research in the area suggests the disease is heterogeneous in terms of the multiple parameters related to etiology, pathology and response to pharmacological agents. In addition to efficacy further therapeutic advances will also need to address such issues as cognitive impairment, pain, sexual dysfunction, nausea and emesis, weight gain and potential cardiovascular effects. With these unmet needs in mind, the next generation of antidepressants will need to differentiate themselves from the current array of therapeutics for depression. There are multiple strategies for addressing unmet needs that are currently being investigated. These range from combination monoaminergic approaches to subtype selective agents to novel targets that include mechanisms to modulate neuropeptides and excitatory amino acids (EAA). This review will discuss the many facets of differentiation and potential strategies for the development of novel antidepressants.

Tricyclic antidepressants, but not the selective serotonin reuptake inhibitor fluoxetine, bind to the S1S2 domain of AMPA receptors

The hypothesis that depression is caused solely by a decrease in synaptic availability of monoaminergic neurotransmitters has been questioned over the past two decades. Based on accumulating data, it seems more plausible that cross-talk exists between neurotransmitters in the CNS, including the glutamatergic system. Glutamate, the major fast excitatory neurotransmitter in the CNS, is the natural agonist for the ionotropic glutamate receptors, a family of ligand-gated ion channels including NMDA (N-methyl-D-aspartate), AMPA (amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), and kainate receptors. In this work, we show that five tricyclic antidepressants bind to the S1S2 domain of the GluR2 subunit of the AMPA receptor. A combination of fluorescence quenching, Stern-Volmer analyses, and protease protection assays differentiate the binding of each antidepressant. These analyses provide no evidence for the binding of the selective serotonin reuptake inhibitor, fluoxetine, to this domain. The data presented provides further support for a role of the glutamatergic system in antidepressant activity.

Functional abdominal pain

Functional abdominal pain or functional abdominal pain syndrome (FAPS) is an uncommon functional gut disorder characterised by chronic or recurrent abdominal pain attributed to the gut but poorly related to gut function. It is associated with abnormal illness behaviour and patients show psychological morbidity that is often minimised or denied in an attempt to discover an organic cause for symptoms. Thus the conventional biomedical approach to the management of such patients is unhelpful and a person's symptom experience is more usefully investigated using a biopsychosocial evaluation, which necessarily entails a multidisciplinary system of healthcare provision. Currently the pathophysiology of the disorder is poorly understood but is most likely to involve a dysfunction of central pain mechanisms either in terms of attentional bias, for example, hypervigilance or a failure of central pain modulation/inhibition. Although modern neurophysiological investigation of patients is promising and may provide important insights into the pathophysiology of FAPS, current clinical management relies on an effective physician-patient relationship in which limits on clinical investigation are set and achievable treatment goals tailored to the patient's needs are pursued.

Antidepressants in the treatment of neuropathic pain

Neuropathic pain is due to lesion or dysfunction of the peripheral or central nervous system. Tricyclic antidepressants and anticonvulsants have long been the mainstay of treatment of this type of pain. Tricyclic antidepressants may relieve neuropathic pain by their unique ability to inhibit presynaptic reuptake of the biogenic amines serotonin and noradrenaline, but other mechanisms such as N-methyl-D-aspartate receptor and ion channel blockade probably also play a role in their pain-relieving effect. The effect of tricyclic antidepressants in neuropathic pain in man has been demonstrated in numerous randomised, controlled trials, and a few trials have shown that serotonin noradrenaline and selective serotonin reuptake inhibitor antidepressants also relieve neuropathic pain although with lower efficacy. Tricyclic antidepressants will relieve one in every 2-3 patients with peripheral neuropathic pain, serotonin noradrenaline reuptake inhibitors one in every 4-5 and selective serotonin reuptake inhibitors one in every 7 patients. Thus, based on efficacy measures such as numbers needed to treat, tricyclic antidepressants tend to work better than the anticonvulsant gabapentin and treatment options such as tramadol and oxycodone, whereas the serotonin noradrenaline reuptake inhibitor venlafaxine appears to be equally effective with these drugs and selective serotonin reuptake inhibitors apparently have lower efficacy. Head-to-head comparisons between antidepressants and the other analgesics are lacking. Contraindications towards the use of tricyclic antidepressants and low tolerability in general of this drug class--may among the antidepressants--favour the use of the serotonin noradrenaline reuptake inhibitors. A recent study on bupropion, which is a noradrenaline and dopamine uptake inhibitor, indicated a surprisingly high efficacy of this drug in peripheral neuropathic pain. In conclusion, antidepressants represent useful tools in neuropathic pain treatment and must still be considered as first line treatments of neuropathic pain. However, without head-to-head comparisons between antidepressants and other analgesics, it is not possible to provide real evidence-based treatment algorithms for neuropathic pain.

1-Methyl-1,2,3,4-tetrahydroisoquinoline enhances the anticonvulsant action of carbamazepine and valproate in the mouse maximal electroshock seizure model

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTHIQ - an endogenous parkinsonism-preventing substance) administered intraperitoneally at a dose of 20 mg/kg considerably elevated the threshold for electroconvulsions in mice from 6.4 to 8.4 mA (P < 0.05). In contrast, the agent administered at 5 and 10 mg/kg had no significant impact on the electroconvulsive threshold in mice. Moreover, 1-MeTHIQ (at a subthreshold dose of 10 mg/kg) potentiated the anticonvulsant action of valproate (VPA) against maximal electroshock (MES)-induced seizures in mice, reducing its median effective dose (ED50) from 232 to 170 mg/kg (P < 0.001). Similarly, 1-MeTHIQ (at 10 mg/kg) enhanced the antielectroshock activity of carbamazepine (CBZ) in mice, decreasing its ED50 from 10.8 to 7.8 mg/kg (P < 0.05). In contrast, 1-MeTHIQ (at 10 mg/kg) did not affect the anticonvulsant action of phenytoin and phenobarbital against MES-induced seizures in mice. The evaluation of acute neurotoxic effects of the studied antiepileptic drugs (AEDs) in combination with 1-MeTHIQ, as regards motor coordination impairment in the chimney test, revealed no significant changes in median toxic doses (TD50) of conventional AEDs after systemic administration of 1-MeTHIQ (up to 10 mg/kg). Pharmacokinetic characterization of interactions between 1-MeTHIQ (10 mg/kg) and VPA (170 mg/kg) or CBZ (7.8 mg/kg) revealed no significant changes in total brain concentrations of CBZ and VPA, indicating that the observed enhancement of antiseizure effects of CBZ and VPA by 1-MeTHIQ was pharmacodynamic in nature. Based on our preclinical study, it may be concluded that 1-MeTHIQ exerts the anticonvulsant effects increasing the threshold for electroconvulsions and potentiating the antiseizure action of CBZ and VPA against maximal electroshock. The antiseizure properties of 1-MeTHIQ (an endogenous parkinsonism-preventing substance) and its exact physiological role in the brain need extensive examination in further neuropharmacological studies.

Evaluation of the effects of lamotrigine, valproate and carbamazepine in a rodent model of mania

Bipolar disorder is a psychiatric condition characterised by episodes of mania, depression, and underlying mood instability. Anticonvulsant drugs have an established place in the treatment of the disorder, but identifying novel drugs in this class is complicated by the absence of validated animal models. We have evaluated the efficacy of three anticonvulsant mood stabilising drugs (lamotrigine, valproate, and carbamazepine) in a model of mania, in which hyperactivity is induced by the combination of D-amphetamine and chlordiazepoxide. All three drugs were effective at preventing the hyperactivity. Lower doses of valproate and carbamazepine were required to prevent hyperactivity compared to doses required to block tonic-clonic seizures induced by pentylenetetrazole. Lamotrigine was equipotent in the two models. However, the complex pharmacology of the D-amphetamine/chlordiazepoxide model means that there may be several mechanisms by which hyperactivity can be reduced, and these may have more or less relevance to the treatment of bipolar disorder. To address this issue, we also evaluated effects of the three anticonvulsants on baseline locomotion, on activity in the presence of chlordiazepoxide alone, or on activity induced by D-amphetamine alone. Based on the results, we propose that hyperactivity induced by D-amphetamine/chlordiazepoxide may arise through dopaminergic drive coupled with disinhibition caused by low doses of the benzodiazepine. The efficacy of lamotrigine may then arise through a reduction in neuronal excitability or increased glutamate transmission, these latter a consequence of the disinhibition. Carbamazepine may also reduce excitability and glutamate release, but its broader pharmacology, manifested by sedation at higher doses complicates interpretation of its efficacy and reflects its poorer tolerability in the clinic. Valproate may be effective, at least in part, through an enhancement of GABAergic transmission. The predictive validity of the D-amphetamine/chlordiazepoxide model for efficacy in bipolar disorder remains to be established, and research with a wider range of clinically tested drugs is warranted to help validate the model further. In the meantime, the model may be useful for distinguishing novel anticonvulsant drugs with different mechanisms of action.

Intrathecal administration of doxepin attenuated development of formalin-induced pain in rats

The aim of the present research was to assess the influence of a tricyclic antidepressant doxepin administered intrathecally (i.t.) on the pain behavior in the formalin test (100 microl of 12% formalin was injected into the dorsal part of the hind paw under halotane anesthesia) in male Wistar rats. The influence of doxepin (62.5 microg i.t.) on the pain threshold and number of formalin-induced pain behaviors, as well as antinociceptive effect of morphine was studied. Doxepin significantly increased the nociceptive threshold in the paw pressure test, reduced formalin-induced pain behavior and potentiated morphine antinociceptive effect in formalin test. The obtained results indicate that analgesic effect of doxepin used before the injury is observable at the spinal level after intrathecal treatment, but not only after peripheral administration, which was shown in our previous study. The results of the present research demonstrated a possibility to modify the spinal nociceptive process by administration of doxepin before the formalin injection.

Emerging drugs in migraine treatment

Ergot alkaloids have been the mainstay of acute migraine therapy for most of the 20th century. They have been supplanted by sumatriptan-like drugs ('triptans'), which, while keeping some of the ergotś mechanisms of action, show improved safety profiles due to their increased receptor selectivity. However, triptans are still far from being perfect drugs: they can constrict human coronary arteries at therapeutic doses and, therefore, are contra-indicated in the presence of cardiovascular disease. Another problem with these agents is recurrence of moderate-to-severe pain within 24 h of initial headache relief. While mechanism-driven drug design has led to the development of various novel, albeit still imperfect, acute antimigraine medications, only a few new prophylactic agents have been made available to migraine clinicians. The efficacy of most, if not all of them has been discovered serendipitously. This is probably due to the fact that, while the pathophysiology of a migraine attack is now reasonably understood, the mechanisms leading to an attack are still mostly unknown. This update analyses the profile of some antimigraine drugs in clinical trials, their mode of action and their potential advantages or drawbacks over already available agents.

Glutamate and Depression

The past decade has seen a steady accumulation of evidence supporting a role for the excitatory amino acid (EAA) neurotransmitter, glutamate, and its receptors in depression and antidepressant activity. To date, evidence has emerged indicating that N-methyl-d-aspartate (NMDA) receptor antagonists, group I metabotropic glutamate receptor (mGluR1 and mGluR5) antagonists, as well as positive modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have antidepressant-like activity in a variety of preclinical models. Moreover, antidepressant-like activity can be produced not only by drugs modulating the glutamatergic synapse, but also by agents that affect subcellular signaling systems linked to EAA receptors (e.g., nitric oxide synthase). In view of the extensive colocalization of EAA and monoamine markers in nuclei such as the locus coeruleus and dorsal raphe, it is likely that an intimate relationship exists between regulation of monoaminergic and EAA neurotransmission and antidepressant effects. Further, there is also evidence implicating disturbances in glutamate metabolism, NMDA, and mGluR1,5 receptors in depression and suicidality. Finally, recent data indicate that a single intravenous dose of an NMDA receptor antagonist is sufficient to produce sustained relief from depressive symptoms. Taken together with the proposed role of neurotrophic factors in the neuroplastic responses to stressors and antidepressant treatments, these findings represent exciting and novel avenues to both understand depressive symptomatology and develop more effective antidepressants.

Peripheral antihyperalgesic and analgesic actions of ketamine and amitriptyline in a model of mild thermal injury in the rat

In this study, we examined antihyperalgesic and analgesic actions after local peripheral administration of ketamine and amitriptyline in a rat model of mild thermal injury. Exposure of the hindpaw to 52 degrees C for 45 s under anesthesia produced a subsequent thermal hyperalgesia lasting at least 2 h. The local peripheral administration of ketamine (100-1000 nmol) 15 min before the thermal injury produced an antihyperalgesic effect when injected into the ipsilateral paw, whereas amitriptyline produced both antihyperalgesic (300 nmol) and analgesic (1000 nmol) effects. Administered after the thermal injury, ketamine had no effect, whereas amitriptyline retained its analgesic but not its antihyperalgesic effect. Amitriptyline (300 and 1000 nmol) produced an analgesic action when administered into the normal nonsensitized hindpaw. Both drugs increase paw volume, particularly at larger doses; biogenic amines are not involved in the action of amitriptyline, as was shown previously for ketamine. These results indicate that (a) ketamine produces antihyperalgesia, but not analgesia, when administered locally with a mild thermal injury model; (b) amitriptyline produces both antihyperalgesia and analgesia when administered locally; and (c) the increase in paw volume produced by these drugs occurs by different mechanisms.

IMPLICATIONS

This study examines the pain-relieving properties of the local peripheral administration of ketamine and amitriptyline, two drugs in current clinical use, in a thermal injury model of hyperalgesia and demonstrates both antihyperalgesic and analgesic properties. These observations provide support for their potential use as local (e.g., topical) analgesics.

Peripheral interactions between dextromethorphan, ketamine and amitriptyline on formalin-evoked behaviors and paw edema in rats

The local, peripheral administration of antidepressants and excitatory amino acid receptor antagonists can cause analgesia in a number of conditions. The present study examined the effects of combinations of dextromethorphan and ketamine, two clinically used N-methyl-D-aspartate (NMDA) receptor antagonists, with amitriptyline on formalin-evoked behaviors and paw edema. Pretreatment with amitriptyline or dextromethorphan (10-300 nmol) resulted in suppression of flinching behaviors induced by 2.5% formalin, but ketamine had no intrinsic effect. Combination of an inactive dose of dextromethorphan with amitriptyline, and vice versa, resulted in an increase of analgesia so that previously inactive doses now caused significant analgesia. Combinations of multiple doses of ketamine with amitriptyline did not modify the response to amitriptyline. Both dextromethorphan and ketamine increased the paw edema induced by formalin, and this was blocked by low doses of amitriptyline. In the absence of formalin, amitriptyline (1-100 nmol) caused a dose-related suppression of the paw edema produced by dextromethorphan and ketamine. Amitriptyline also blocked paw edema produced by 5-hydroxytryptamine and compound 48/80. Each of the drugs used in this study exerts multiple pharmacological effects. Increased analgesia by drug combinations (amitriptyline/dextromethorphan) could show the involvement of a number of these mechanisms (e.g. NMDA receptor blockade, blockage of sodium channels, blockage of biogenic amine receptors), while a lack of intensification (amitriptyline/ketamine) could reflect occluded actions due to expression of similar actions by the other drug. Paw edema induced by dextromethorphan and ketamine involves inhibition of biogenic amine reuptake, and the ability of amitriptyline to block biogenic amine receptors likely accounts for its inhibiton of these actions. Combinations of these particular agents could represent a method for augmented analgesia and minimization of local adverse reactions.

Antidepressants for chronic neuropathic pain

Tricyclic antidepressants have been used to manage pain for several decades, and are superior treatments for some patients suffering from neuropathic pain. Unfortunately, older antidepressants have dose-limiting side effects that can lead to drug intolerance. The most common are anticholinergic side effects, although some patients experience sexual dysfunction. Cognitive impairment, sedation, and orthostatic hypotension also are relatively common. Taking an overdose of tricyclic antidepressants can be lethal in overdose. Several weeks of therapy may be required before antinociception occurs, but tricyclic antidepressants in optimal doses appear to be the most effective treatment for neuropathic pain; this is supported by systematic reviews comparing them with other agents. Newer medications such as atypical antidepressants and anticonvulsants may be overtaking older antidepressants, but they should not be overlooked as important options for the management of pain.

Amitriptyline has a dual effect on the conductive properties of the epithelial Na channel

This study was undertaken with the aim of testing the action of amitriptyline on the epithelial Na channel (ENaC), which belongs to the same family (Deg/ENaC) as ASICs (acid-sensing ion channels) and many other putative members in the brain. We assumed that, having a common protein structure, characterization of the amitriptyline-ENaC interaction could help to elucidate the analgesic mechanism of this tricyclic antidepressant. Na-channel characteristics were derived from the analysis of blocker-induced lorentzian noise produced by amiloride. The effect of amitriptyline, present in the mucosal bathing solution, on the transepithelial short-circuit current (I(sc)) and conductance (G(t)), and on the blocker-induced noise of apical Na channels, was studied on isolated ventral skin of the frog Rana ridibunda. Amitriptyline exerted a dual effect on the macroscopic short-circuit current and conductance of the epithelia, increasing these two parameters in the concentration range 0.1-50 microM, while at higher concentrations (100-1000 microM) it showed an inhibitory action. The decrease in the association rate (k(01)) of amiloride to the apical Na channels from 15.6+/-4.2 microM(-1) s(-1) in control Cl-Ringer to 7.4+/-1.7 microM(-1) s(-1) at 200 microM amitriptyline in a concentration-dependent manner suggests a competitive binding of amitriptyline to the pyrazine ring binding site for amiloride.

Involvement of adenosine in the anti-allodynic effect of amitriptyline in streptozotocin-induced diabetic rats

Recent observations suggest the involvement of adenosine in the peripheral antinociceptive effect of amitriptyline in nerve-injury-induced neuropathic pain. The aim of the present investigation was to evaluate, firstly, the peripheral and systemic effects of amitriptyline on tactile allodynia in the streptozotocin (STZ)-induced diabetic rat model of neuropathic pain and, secondly, whether caffeine coadministration affects the actions of amitriptyline. Diabetes was induced by a single intraperitoneal (i.p.) injection of STZ (50 mg/kg), and tactile allodynia was detected by application of von Frey filaments to the ventral surface of the hindpaw. Both systemic (0.5-2.0 mg/kg, i.p.) and peripheral (10-100 nmol, subcutaneously (s.c.)) administration of amitriptyline were found to produce increases in paw withdrawal thresholds, at higher doses. Coadministration of caffeine (5 mg/kg, i.p.; 1500 nmol, s.c.), at doses which produced no effect on its own, partially reversed systemic and local anti-allodynic effects of amitriptyline. These results indicate an anti-allodynic effect of both peripheral and systemic amitriptyline, and suggest the involvement of endogenous adenosine in the action of amitriptyline in this rat model of painful diabetic neuropathy. These data also suggest that topical application of tricyclic antidepressants may be useful in treating neuropathic pain in diabetics.

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.

Effects of carbamazepine and novel 10,11-dihydro-5H-dibenz[b,f]azepine-5-carboxamide derivatives on synaptic transmission in rat hippocampal slices

The effects of carbamazepine on synaptic transmission in rat hippocampal slices were compared with those of two novel analogues (BIA2-093 and BIA2-024) with equivalent anticonvulsant efficacy but with fewer side effects. Carbamazepine (10-1000 microM) inhibited in a concentration-dependent manner the field excitatory postsynaptic potential (fPSP) response, with an EC50 of 263 microM, and also attenuated the presynaptic volley with a similar EC50 value. Carbamazepine was more potent to inhibit the NMDA receptor component of the fPSP (fPSPNMDA), with an EC50 of 160 microM. BIA2-093 and BIA2-024 were nearly equipotent with carbamazepine to inhibit synaptic transmission, and displayed similar potency to inhibit the fPSP (EC50 of 145 microM and 205 microM) and fPSPNMDA responses (EC50 of 198 microM and 206 microM). As with carbamazepine, BIA2-093 and BIA2-024 also attenuated the presynaptic volley with EC50 values ranging from 142 to 322 microM. These results indicate that carbamazepine and its analogues mostly inhibit synaptic transmission through inhibition of conduction, although carbamazepine, but not BIA2-093 and BIA2-024, may also depress NMDA receptor-mediated responses.

Mechanisms of action of carbamazepine and its derivatives, oxcarbazepine, BIA 2-093, and BIA 2-024

Carbamazepine (CBZ) has been extensively used in the treatment of epilepsy, as well as in the treatment of neuropathic pain and affective disorders. However, the mechanisms of action of this drug are not completely elucidated and are still a matter of debate. Since CBZ is not very effective in some epileptic patients and may cause several adverse effects, several antiepileptic drugs have been developed by structural variation of CBZ, such as oxcarbazepine (OXC), which is used in the treatment of epilepsy since 1990. (S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz [b,f]azepine-5-carboxamide (BIA 2-093) and 10,11-dihydro-10-hydroxyimino-5H-dibenz[b,f] azepine-5-carboxamide (BIA 2-024), which were recently developed by BIAL, are new putative antiepileptic drugs, with some improved properties. In this review, we will focus on the mechanisms of action of CBZ and its derivatives, OXC, BIA 2-093 and BIA 2-024. The available data indicate that the anticonvulsant efficacy of these AEDs is mainly due to the inhibition of sodium channel activity.

A study of mechanisms underlying amitriptyline-induced acute lung function impairment

In this study possible mechanisms underlying the vaso- and bronchoconstriction caused by the tricyclic antidepressant drug amitriptyline in isolated rat lungs were investigated. Some features here are similar to those apparent in adult respiratory distress syndrome and acute lung injury. Amitriptyline exposure (50 and 100 microM) caused a dose-related, pronounced, and rapid vaso- (50 microM, 30 min, p < 0.001 and 100 microM, 30 min, p < 0.001) and bronchoconstriction (50 microM, 30 min, p = 0.01 and 100 microM, 30 min, p < 0.001). The maximal noted decrease in perfusion flow was 28 +/- 2.9% at 25 min and 80 +/- 4.5% at 30 min for 50 and 100 microM amitriptyline, respectively. The maximal noted decrease in airway conductance was 29 +/- 4.7% at 25 min and 68 +/- 5.0% at 30 min. To investigate mechanisms thought to be involved in amitriptyline-induced lung function impairment, lungs were treated with several different substances including antiinflammatory agents, antioxidants, inhibitors of enzymes involved in the arachidonic acid cascade, physiological antagonists, and neurogenic antagonists. A significant reduction of amitriptyline-induced vasoconstriction was observed when lungs were treated with the protein kinase inhibitor staurosporine (3 microM, 30 min, p < 0.001), the NO-donor S-nitrosoglutathione (100 microM, 30 min, p < 0.001) and the combined endothelin A/endothelin B receptor antagonist PD 145065. This latter inhibitor caused a significant attenuation of late vasoconstriction (1 microM, 60 min, p = 0.03). The amitriptyline-induced bronchoconstriction was attenuated by the beta(2)-agonist salbutamol (1 microM, 30 min, p = 0.03) and the platelet-activating factor antagonist WEB2086 (10 microM, 30 min, p = 0.03). Staurosporine had an initial protective effect on bronchoconstriction (3 microM, 5 min, p = 0.003), while PD145065 significantly decreased bronchoconstriction 60 min after start of amitriptyline exposure (1 microM, 30 min, p = 0.003). This indicates that endothelin as well as platelet activating factor and protein kinase activation are important in mediating amitriptyline-induced lung function impairment in our experimental model and perhaps also in acute lung injury.

Effects of antidepressants on gamma-aminobutyric acid- and N-methyl-D-aspartate-induced intracellular Ca(2+) concentration increases in primary cultured rat cortical neurons

We investigated the effects of antidepressants on the intracellular Ca2+ concentration ([Ca2+]i) increases induced by gamma-aminobutyric acid (GABA) or N-methyl-D-aspartate (NMDA) in primary cultured rat cortical neurons using fluorescence imaging. Acute treatment with imipramine inhibited GABA- and NMDA-induced increases in [Ca2+]i in a concentration-dependent manner. Doses of 30 microM clomipramine, desipramine, amoxapine and maprotiline also inhibited both the GABA- and NMDA-induced [Ca2+]i increases significantly. Both inhibitory effects of the five major antidepressants on the GABA- or the NMDA-induced [Ca2+]i increases were well-correlated. Imipramine could inhibit significantly high-K+-induced [Ca2+]i increases. Our previous study has already shown that the GABA-induced [Ca2+]i increase involves a similar pathway to high-K+-induced Ca2+ influx. In conclusion, imipramine and several other antidepressants have acute inhibitory effects on the GABA-, NMDA- and high-K+-induced [Ca2+]i increases, suggesting that these inhibitory effects are not related to specific receptors. One possibility is that these effects may be commonly mediated via part of the high-K+-induced [Ca2+]i pathway.

Antidepressant activity and calcium signaling cascades

Although antidepressant treatments produce clear effects on monoaminergic neuronal function, the link between these effects and therapeutic response to treatment is controversial. Previous studies have demonstrated that antagonists of the NMDA receptor-gated calcium ionophore result in antidepressant-like responses in rodents and humans. Likewise, antidepressant treatments produce regionally selective adaptation of the NMDA receptor suggestive of diminished capacity to gate calcium into receptive neurons. Similarly, voltage-dependent calcium channel antagonists have been reported to produce antidepressant-like effects in rodents. A major target of increases in subcellular calcium concentration is nitric oxide synthase (NOS) which liberates NO in response to stimulation. Recently, we have demonstrated that nitric oxide synthase antagonists produced antidepressant-like response in both in vivo preclinical screening procedures and in post-mortem in vitro studies of beta-adrenoceptor density. We propose: 1) that interruption of the Ca(2+)-calmodulin-NOS-guanylyl cyclase subcellular signaling pathway at any point will produce antidepressant-like effects; 2) that the acute actions of antidepressants in preclinical screening procedures are a consequence of their ability to disrupt Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling; 3) that chronic but, not acute treatment with antidepressants results in adaptation of the Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling pathway; 4) that this adaptation is necessary for the achievement of the therapeutic actions of antidepressants and; 5) that major depression is accompanied by an alteration (hyperactivity?) of subcellular Ca(2+) signaling. Copyright 2001 John Wiley & Sons, Ltd.

Serotonin inhibition of the NMDA receptor/nitric oxide/cyclic GMP pathway in human neocortex slices: involvement of 5-HT(2C) and 5-HT(1A) receptors

The NMDA receptor/nitric oxide (NO)/cyclic GMP pathway and its modulation by 5-hydroxytryptamine (5-HT) was studied in slices of neocortical samples obtained from patients undergoing neurosurgery. The cyclic GMP elevation produced by 100 microM NMDA was blocked by 100 microM of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NOARG) or by 10 microM of the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-alpha] quinoxaline-1-one (ODQ). The NMDA effect was prevented by 5-HT or by the 5-HT(2) agonist (+/-)-1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/-)-DOI; EC(50)=22 nM). The (+/-)-DOI inhibition was insensitive to the 5-HT(2A) receptor antagonist MDL 100907 or the 5-HT(2B) antagonist rauwolscine; it was largely prevented by 1 microM of the non-selective 5-HT(2C) antagonists mesulergine (5-HT(2A,B,C)), ketanserin (5-HT(2A,C)) or SB 200646A (5-HT(2B,C)); it was completely abolished by 0.1 microM of the selective 5-HT(2C) receptor antagonist SB 242084. The NMDA-induced cyclic GMP elevation also was potently inhibited by the selective 5-HT(2C) agonist RO 60-0175 and by the antidepressant trazodone, both added at 1 microM, in an SB 242084-sensitive manner. Finally, the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 1 microM) inhibited the NMDA-evoked cyclic GMP response, an effect blocked by the selective 5-HT(1A) receptor antagonist WAY 100635. In conclusion, the NMDA receptor/NO/cyclic GMP pathway in human neocortex slices can be potently inhibited by activation of 5-HT(2C) or 5-HT(1A) receptors.

The N-methyl-D-aspartate receptor, synaptic plasticity, and depressive disorder. A critical review

The roles of the N-methyl-D-aspartate (NMDA) receptor and NMDA receptor-mediated synaptic plasticity are reviewed in the context of depressive disorder and its treatment. The mode of action of antidepressant treatment is poorly understood. Animal studies have suggested that many antidepressant drugs show activity at the NMDA receptor and that NMDA antagonists have antidepressant profiles in preclinical models of depression. A post-mortem study in humans has suggested that certain binding characteristics of the NMDA receptor may be down-regulated in the brains of suicide victims. "Depressogenic" stressors in animals and chronic administration of antidepressant agents perturb NMDA-dependent synaptic plasticity in the hippocampus.

Hyperforin attenuates various ionic conductance mechanisms in the isolated hippocampal neurons of rat

Effects of hyperforin, an acylphloroglucinol derivative isolated from antidepressive medicinal herb Hypericum perforatum (St. John's Wort), on voltage- and ligand-gated ionic conductances were investigated. Whole-cell patch clamp and concentration clamp techniques on acutely isolated hippocampal pyramidal neurons and on cerebellar Purkinje neurons of rat were used. At concentrations between 3 to 100 microM hyperforin induced a dose and time dependent inward current which completely stabilized within a few seconds. Although 1 microM hyperforin inhibited virtually all investigated conductances (GABA > or = I(Ca(N)) > I(Na) > I(Ca(P) > or = AMPA > or = I(K(A)) > NMDA > I(K(DR))), its effects on several of them could not be reversed by repeated washings. Dose response studies revealed that although AMPA induced current is inhibited by hyperforin in a competitive manner, these responses are not completely blocked by very high concentration of the agent. On the contrary, however, NMDA receptor-activated ionic conductance could be completely and uncompetitively inhibited by the agent. Taken together these observation not only reconfirm that hyperforin is a major neuroactive component of hypericum extracts but also demonstrate that this structurally unique and naturally abundant molecule is a potent modulation of mechanism involved in the control of neuronal ionic conductances. Various observed effects of hyperforin do not, however, seem to be mediated by one single molecular mechanism of action of the agent.

Carbamazepine inhibits L-type Ca2+ channels in cultured rat hippocampal neurons stimulated with glutamate receptor agonists

In order to better understand the mechanism(s) of action of carbamazepine (CBZ), we studied its effects on the increase in [Ca2+]i and [Na+]i stimulated by glutamate ionotropic receptor agonists, in cultured rat hippocampal neurons, as followed by indo- or SBFI fluorescence, respectively. CBZ inhibited the increase in [Ca2+]i stimulated either by glutamate, kainate, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), or N-methyl-D-aspartate (NMDA), in a concentration-dependent manner. In order to discriminate the effects of CBZ on the activation of glutamate receptors from possible effects on Ca2+ channels, we determined the inhibitory effects of Ca2+ channel blockers on [Ca2+]i changes in the absence or in the presence of CBZ. The presence of 1 microM nitrendipine, 0.5 microM omega-conotoxin GVIA (omega-CgTx GVIA), or of both blockers, inhibited the kainate-stimulated increase in [Ca2+]i by 51.6, 32.9 or 68.7%, respectively. In the presence of both 100 microM CBZ and nitrendipine, the inhibition was similar (54.1%) to that obtained with nitrendipine alone, but in the presence of both CBZ and omega-CgTx GVIA, the inhibition was greater (54%) than that caused by omega-CgTx GVIA alone. However, CBZ did not inhibit the increase in [Na+]i stimulated by the glutamate receptor agonists, but inhibited the increase in [Na+]i due to veratridine. Tetrodotoxin, or MK-801, did not inhibit the influx of Na+ stimulated by kainate, indicating that Na+ influx occurs mainly through the glutamate ionotropic non-NMDA receptors. Moreover, LY 303070, a specific AMPA receptor antagonist, inhibited the [Na+]i response to kainate or AMPA by about 70 or 80%, respectively, suggesting that AMPA receptors are mainly involved. Taken together, the results suggest that CBZ inhibits L-type Ca2+ channels and Na+ channels, but does not inhibit activation of glutamate ionotropic receptors.