Effect of antidepressants on GABA(B) receptor function and subunit expression in rat hippocampus

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

GABAB Receptors: Anxiety and Mood Disorders

Gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, acts at the ionotropic GABA_A and GABA_C receptors, and the metabotropic GABA_B receptor. This chapter summarizes the studies that have investigated the role of the GABA_B receptor in stress-related psychiatric disorders including anxiety and mood disorders. Overall, clinical and preclinical evidences strongly suggest that the GABA_B receptor is a therapeutic candidate for depression and anxiety disorders. However, the clinical development of GABA_B receptor-based drugs to treat these disorders has been hampered by their potential side-effects, particularly those of agonists. Nevertheless, the discovery of novel GABA_B receptor allosteric modulators, and increasing understanding of the influence of specific intracellular GABA_B receptor-associated proteins on GABA_B receptor activity, may now pave the way towards GABA_B receptor therapeutics in the treatment of mood and anxiety disorders.

The Gamma-Aminobutyric Acid B Receptor in Depression and Reward

The metabotropic gamma-aminobutyric acid B (GABA_B) receptor was the first described obligate G protein-coupled receptor heterodimer and continues to set the stage for discoveries in G protein-coupled receptor signaling complexity. In this review, dedicated to the life and work of Athina Markou, we explore the role of GABA_B receptors in depression, reward, and the convergence of these domains in anhedonia, a shared symptom of major depressive disorder and withdrawal from drugs of abuse. GABA_B receptor expression and function are enhanced by antidepressants and reduced in animal models of depression. Generally, GABA_B receptor antagonists are antidepressant-like and agonists are pro-depressive. Exceptions to this rule likely reflect the differential influence of GABA_B1 isoforms in depression-related behavior and neurobiology, including the anhedonic effects of social stress. A wealth of data implicate GABA_B receptors in the rewarding effects of drugs of abuse. We focus on nicotine as an example. GABA_B receptor activation attenuates, and deactivation enhances, nicotine reward and associated neurobiological changes. In nicotine withdrawal, however, GABA_B receptor agonists, antagonists, and positive allosteric modulators enhance anhedonia, perhaps owing to differential effects of GABA_B1 isoforms on the dopaminergic system. Nicotine cue-induced reinstatement is more reliably attenuated by GABA_B receptor activation. Separation of desirable and undesirable side effects of agonists is achievable with positive allosteric modulators, which are poised to enter clinical studies for drug abuse. GABA_B1 isoforms are key to understanding the neurobiology of anhedonia, whereas allosteric modulators may offer a mechanism for targeting specific brain regions and processes associated with reward and depression.

The potential efficacy of GABAB antagonists in depression

Evidence for the potential utility of GABA_B antagonists has been assembled from a variety of sources, including clinical experience with the GABA_B agonist baclofen, murine genetic GABA_B knock-outs, rodent studies of GABA_B receptor expression and function following treatment with antidepressant therapies, animal models of depression, and some functional and post mortem data from human subjects. Definitive testing of GABA_B antagonists in depression, however, still awaits the development of potent, selective and brain-penetrant compounds for human testing.

Tranylcypromine in mind (Part I): Review of pharmacology

It has been over 50 years since a review has focused exclusively on the monoamine oxidase (MAO) inhibitor tranylcypromine (TCP). A new review has therefore been conducted for TCP in two parts which are written to be read preferably in close conjunction: Part I - pharmacodynamics, pharmacokinetics, drug interactions, toxicology; and Part II - clinical studies with meta-analysis of controlled studies in depression, practice of TCP treatment, place in therapy. Pharmacological data of this review part I characterize TCP as an irreversible and nonselective MAO-A/B inhibitor at low therapeutic doses of 20mg/day with supplementary norepinephrine reuptake inhibition at higher doses of 40-60mg/day. Serotonin, norepinephrine, dopamine, and trace amines, such as the "endogenous amphetamine" phenylethylamine, are increased in brain, which leads to changes in neuroplasticity by e.g. increased neurotrophic growth factors and translates to reduced stress-induced hypersecretion of corticotropin releasing factor (CRF) and positive testing in animal studies of depression. TCP has a pharmacokinetic half-life (t_1/2) of only 2h which is considerably lower than for most other antidepressant drugs. However, a very long pharmacodynamic half-life of about one week is found because of the irreversible MAO inhibition. New studies show that, except for cytochrome P450 (CYP) 2A6, no other drug metabolizing CYP-enzymes are inhibited by TCP at therapeutic doses which defines a low potential of pharmacokinetic interactions in the direction from TCP to other drugs. Insufficient information is available, however, for plasma concentrations of TCP influenced by comedication. More quantitative data are also needed for TCP metabolites such as p-hydroxytranylcypromine and N-acetyltranylcypromine. Pharmacodynamic drug interactions comprise for instance severe serotonin toxicity (SST) with serotonergic drugs and hypertensive crisis with indirect sympathomimetics. Because of the risk of severe food interaction, TCP treatment remains beset with the need for a mandatory tyramine-restricted diet. Toxicity in overdose is similar to amitriptyline and imipramine according to the distance of therapeutic to toxic doses. In conclusion, TCP is characterized by an exceptional pharmacology which is different to most other antidepressant drugs, and a more special evaluation of clinical efficacy and safety may therefore be needed.

Blunted 5-HT1A receptor-mediated responses and antidepressant-like behavior in mice lacking the GABAB1a but not GABAB1b subunit isoforms

RATIONALE

There is accumulating evidence for a role of GABAB receptors in depression. GABAB receptors are heterodimers of GABAB1 and GABAB2 receptor subunits. The predominant GABAB1 subunit isoforms are GABAB1a and GABAB1b. GABAB1 isoforms in mice differentially influence cognition, conditioned fear, and susceptibility to stress, yet their influence in tests of antidepressant-like activity has not been fully investigated.

OBJECTIVES

Given the interactions between GABAB receptors and the serotonergic system and the involvement of 5-HT1A receptors (5-HT1AR) in antidepressant action, we sought to evaluate 5-HT1AR function in GABAB1a-/- and GABAB1b-/- mice.

METHODS

GABAB1a-/- and GABAB1b-/- mice were assessed in the forced swim test (FST), and body temperature and hypothalamic-pituitary-adrenal (HPA) responses to the 5-HT1AR agonist 8-OH-DPAT were determined. Brain 5-HT1AR expression was assessed by [3H]-MPPF and [3H]-8-OH-DPAT autoradiography and 5-HT1AR G-protein coupling by [35S]GTP-γ-S autoradiography.

RESULTS

As previously described, GABAB1a-/- mice showed an antidepressant-like profile in the FST. GABAB1a-/- mice also demonstrated profoundly blunted hypothermic and motoric responses to 8-OH-DPAT. Furthermore, 8-OH-DPAT-induced corticosterone and adrenocorticotropic hormone (ACTH) release were both attenuated in GABAB1a-/- mice. Interestingly, [3H]-MPPF and [3H]-8-OH-DPAT binding was largely unaffected by genotype. [35S]GTP-γ-S autoradiography suggested that altered 5-HT1AR G-protein coupling only partially contributes to the functional presynaptic 5-HT1AR desensitization, and not at all to the blunted postsynaptic 5-HT1AR-mediated responses, seen in GABAB1a-/- mice.

CONCLUSION

These data demonstrate distinct functional links between 5-HT1ARs and the GABAB1a subunit isoform and suggest that the GABAB1a isoform may be implicated in the antidepressant-like effects of GABAB receptor antagonists and in neurobiological mechanisms underlying depression.

Sertraline and venlafaxine improves motor performance and neurobehavioral deficit in quinolinic acid induced Huntington's like symptoms in rats: Possible neurotransmitters modulation

BACKGROUND

Huntington Disease is autosomal, fatal and progressive neurodegenerative disorder for which clinically available drugs offer only symptomatic relief. Emerging strides have indicated that antidepressants improve motor performance, restore neurotransmitters level, ameliorates striatal atrophy, increases BDNF level and may enhance neurogenesis. Therefore, we investigated sertraline and venlafaxine, clinically available drugs for depression with numerous neuroprotective properties, for their beneficial effects, if any, in quinolinic acid induced Huntington's like symptoms in rats.

METHODS

Rats were administered quinolinic acid (QA) (200 nmol/2μl saline) intrastriatal bilaterally on 0day. Sertraline and venlafaxine (10 and 20mg/kg, po) each were administered for 21days once a day. Motor performance was assessed using rotarod test, grip strength test, narrow beam walk test on weekly basis. On day 22, animals were sacrificed and rat striatum was isolated for biochemical (LPO, GSH and Nitrite), neuroinflammation (TNF-α, IL-1β and IL-6) and neurochemical analysis (GABA, glutamate, norepinephrine, dopamine, serotonin, DOPAC, HVA and 5-HIAA).

RESULTS

QA treatment significantly altered body weight, motor performance, oxidative defense (increased LPO, nitrite and decreased GSH), pro-inflammatory cytokines levels (TNF-α, IL-6 and IL-1β), neurochemical level (GABA, glutamate, nor-epinephrine, dopamine, serotonin, HVA, DOPAC, 5-HIAA). Sertraline and venlafaxine at selected doses significantly attenuated QA induced alterations in striatum.

CONCLUSION

The present study suggests that modulation of monoamines level, normalization of GABA and glutamatergic signaling, anti-oxidant and anti-inflammatory properties could underlie the neuroprotective effect of sertraline and venlafaxine in QA induced Huntington's like symptoms.

Identifying the role of pre-and postsynaptic GABA(B) receptors in behavior

Although many reviews exist characterizing the molecular differences of GABAB receptor isoforms, there is no current review of the in vivo effects of these isoforms. The current review focuses on whether the GABAB1a and GABAB1b isoforms contribute differentially to behaviors in isoform knockout mice. The roles of these receptors have primarily been characterized in cognitive, anxiety, and depressive phenotypes. Currently, the field supports a role of GABAB1a in memory maintenance and protection against an anhedonic phenotype, whereas GABAB1b appears to be involved in memory formation and a susceptibility to developing an anhedonic phenotype. Although GABAB receptors have been strongly implicated in drug abuse phenotypes, no isoform-specific work has been done in this field. Future directions include developing site-specific isoform knockdown to identify the role of different brain regions in behavior, as well as identifying how these isoforms are involved in development of behavioral phenotypes.

Chronic SSRI stimulation of astrocytic 5-HT2B receptors change multiple gene expressions/editings and metabolism of glutamate, glucose and glycogen: a potential paradigm shift

It is firmly believed that the mechanism of action of SSRIs in major depression is to inhibit the serotonin transporter, SERT, and increase extracellular concentration of serotonin. However, this undisputed observation does not prove that SERT inhibition is the mechanism, let alone the only mechanism, by which SSRI's exert their therapeutic effects. It has recently been demonstrated that 5-HT2B receptor stimulation is needed for the antidepressant effect of fluoxetine in vivo. The ability of all five currently used SSRIs to stimulate the 5-HT2B receptor equipotentially in cultured astrocytes has been known for several years, and increasing evidence has shown the importance of astrocytes and astrocyte-neuronal interactions for neuroplasticity and complex brain activity. This paper reviews acute and chronic effects of 5-HT2B receptor stimulation in cultured astrocytes and in astrocytes freshly isolated from brains of mice treated with fluoxetine for 14 days together with effects of anti-depressant therapy on turnover of glutamate and GABA and metabolism of glucose and glycogen. It is suggested that these events are causally related to the mechanism of action of SSRIs and of interest for development of newer antidepressant drugs.

GABAB(1) receptor subunit isoforms differentially regulate stress resilience

Stressful life events increase the susceptibility to developing psychiatric disorders such as depression; however, many individuals are resilient to such negative effects of stress. Determining the neurobiology underlying this resilience is instrumental to the development of novel and more effective treatments for stress-related psychiatric disorders. GABAB receptors are emerging therapeutic targets for the treatment of stress-related disorders such as depression. These receptors are predominantly expressed as heterodimers of a GABAB(2) subunit with either a GABAB(1a) or a GABAB(1b) subunit. Here we show that mice lacking the GABAB(1b) receptor isoform are more resilient to both early-life stress and chronic psychosocial stress in adulthood, whereas mice lacking GABAB(1a) receptors are more susceptible to stress-induced anhedonia and social avoidance compared with wild-type mice. In addition, increased hippocampal expression of the GABAB(1b) receptor subunit is associated with a depression-like phenotype in the helpless H/Rouen genetic mouse model of depression. Stress resilience in GABAB(1b)(-/-) mice is coupled with increased proliferation and survival of newly born cells in the adult ventral hippocampus and increased stress-induced c-Fos activation in the hippocampus following early-life stress. Taken together, the data suggest that GABAB(1) receptor subunit isoforms differentially regulate the deleterious effects of stress and, thus, may be important therapeutic targets for the treatment of depression.

Marked reductions in visual evoked responses but not γ-aminobutyric acid concentrations or γ-band measures in remitted depression

BACKGROUND

Magnetic resonance spectroscopy (MRS) studies have consistently demonstrated reduced cortical γ-aminobutyric acid (GABA) concentrations in individuals with major depression. However, evidence for a persistent deficit during remission, which would suggest that GABA dysfunction is a possible trait marker of depression, is equivocal. Although MRS measures total concentration of GABA, magneto-encephalography provides direct measures of neural activity, with cortical γ oscillations shaped by the activity of GABAergic inhibitory interneurons. In this study we investigated whether γ oscillations and GABA concentrations would differ in individuals with remitted depression (RD) compared with never depressed control subjects (ND).

METHODS

Thirty-seven healthy, unmedicated female volunteers (n = 19 RD, and n = 18 ND) were recruited. The γ oscillation frequencies and amplitudes in the visual cortex, induced by simple grating stimuli, were quantified with time-frequency analyses. Distinct GABA/glutamate + glutamine MRS peaks were resolved from MEGA-PRESS difference spectra in prefrontal, occipital, and subcortical volumes.

RESULTS

The RD and ND individuals did not differ in the frequency of subclinical depressive symptoms. The ND were slightly older (mean = 23 years vs. 21 years), but age did not correlate with dependent measures. There were no group differences in GABA levels or induced cortical γ measures, but RD individuals had markedly reduced M80 (C1) components of the pattern-onset evoked response (46% reduction, Cohen's d = 1.01, p = .006).

CONCLUSIONS

Both MRS and magneto-encephalography measures of the GABA system are normal in RD. However, the early visual evoked response is a potential trait marker of the disorder.

Analgesic antidepressants promote the responsiveness of locus coeruleus neurons to noxious stimulation: implications for neuropathic pain

Antidepressants that block the reuptake of noradrenaline and/or serotonin are among the first-line treatments for neuropathic pain, although the mechanisms underlying this analgesia remain unclear. The noradrenergic locus coeruleus is an essential element of both the ascending and descending pain modulator systems regulated by these antidepressants. Hence, we investigated the effect of analgesic antidepressants on locus coeruleus activity in Sprague-Dawley rats subjected to chronic constriction injury (CCI), a model of neuropathic pain. In vivo extracellular recordings of locus coeruleus revealed that CCI did not modify the basal tonic activity of this nucleus, although its sensory-evoked response to noxious stimuli was significantly altered. Under normal conditions, noxious stimulation evokes an early response, corresponding to the activation of myelinated A fibers, which is followed by an inhibitory period and a subsequent late capsaicin-sensitive response, consistent with the activation of unmyelinated C fibers. CCI provokes an enhanced excitatory early response in the animals and the loss of the late response. Antidepressant administration over 7 days (desipramine, 10mg/kg/day or duloxetine, 5mg/kg/day, delivered by osmotic minipumps) decreased the excitatory firing rate of the early response in the CCI group. Moreover, in all animals, these antidepressants reduced the inhibitory period and augmented the late response. We propose that N-methyl-d-aspartate and alpha-2-adrenoceptors are involved in the analgesic effect of antidepressants. Antidepressant-mediated changes were correlated with behavioral effects indicative of analgesia in healthy and neuropathic rats.

The MAO inhibitor phenelzine improves functional outcomes in mice with experimental autoimmune encephalomyelitis (EAE)

Multiple sclerosis (MS) and the animal model, experimental autoimmune encephalomyelitis (EAE), are both accompanied by motor and non-motor symptoms. Pathological changes in the activities of key neurotransmitters likely underlie many of these symptoms. We have previously described disturbances in the levels of 5-hydroxytryptamine (5-HT/serotonin), noradrenaline (NE) and γ-aminobutyric acid (GABA) in a mouse model of EAE. The potential therapeutic effect of a drug that targets these three neurotransmitters, the antidepressant and anti-panic drug phenelzine (PLZ), was assessed in mice with MOG(35-55) induced EAE. The neurotransmitter content of EAE and control tissue after PLZ administration was first evaluated by HPLC. The ability of PLZ treatment to modulate EAE disease course and clinical signs was then assessed. Daily PLZ treatment, starting seven days after disease induction, delayed EAE onset, reduced disease severity in the chronic phase and was associated with substantial improvements in exploratory behavior and a novel measure of sickness and/or depression. Upon completion of the experiment, PLZ's effects on histopathological markers of the disease were examined. No differences were observed in T cell infiltration, microglia/macrophage reactivity, demyelination or axonal injury in PLZ-treated spinal cords. However, EAE mice treated with PLZ showed a normalization of 5-HT levels in the ventral horn of the spinal cord that might account for the improvements in behavioral outcomes. These results demonstrate the therapeutic potential of MAO inhibitors such as PLZ in MS. Additionally, the behavioral changes observed in EAE mice indicate that alterations in non-motor or 'affective' measures may be valuable to consider in addition to traditional measures of gross locomotor function.

The GABAβ receptor as a target for antidepressant drug action

Preclinical and clinical data suggest that a modification in GABA(B) receptor expression and function may contribute to the symptoms of major depression and the response to antidepressants. This includes laboratory animal experiments demonstrating that antidepressants modify brain GABA(B) receptor expression and function and that GABA(B) receptor antagonists display antidepressant potential in animal models of this condition. Clinical and post-mortem studies reveal changes in GABAergic transmission associated with depression as well as depression-related changes in GABA(B) subunit expression that are localized to the cortical depression network. Detailed in this review are the preclinical and clinical data implicating a role for the GABA(B) receptor system in mediating symptoms of this disorder and its possible involvement in the response to antidepressants. Particular emphasis is placed on clinical and post-mortem studies, including previously unpublished work demonstrating regionally-selective modifications in GABA(B) receptor subunit expression in brain samples obtained from depressed subjects. Together with the earlier preclinical studies, these new data point to a role for the GABA(B) system in major depression and support the antidepressant potential of GABA(B) receptor antagonists.

A legacy of discovery: from monoamines to GABA

Seldom does a single individual have such a profound effect on the development of a scientific discipline as Erminio Costa had on neuropharmacology. During nearly sixty years of research, Costa and his collaborators helped established many of the basic principles of the pharmacodynamic actions of psychotherapeutics. His contributions range from defining basic neurochemical, physiological and behavioral properties of neurotransmitters and their receptors, to the development of novel theories for drug discovery. Outlined in this report is a portion of his work relating to the involvement of monoamines and GABA in mediating the symptoms of neuropsychiatric disorders and as targets for drug therapies. These studies were selected for review because of their influence on my own work and as an illustration of his logical and insightful approach to research and his clever use of techniques and technologies. Given the significance of his work, the legions of scientist who collaborated with him, and those inspired by his reports, his research will continue to have an impact as long as there is a search for new therapeutics to alleviate the pain and suffering associated with neurological and psychiatric disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Down-regulated GABAergic expression in the olfactory bulb layers of the mouse deficient in monoamine oxidase B and administered with amphetamine

This study explores primarily the role of the activity of monoamine oxidase B (MAOB) in the regulation of glutamic acid decarboxylase(67) (GAD(67)) expression in distinct layers of main olfactory bulb (OlfB), which links the limbic system. Moreover, the response of GAD(67) was investigated to amphetamine perturbation in the absence of MAOB activity. Immunocytochemical analysis was performed on OlfB sections prepared from the adult wild type (WT) and the MAOB gene-knocked-out (KO) mice after receiving repeated intraperitoneal injections (two doses per day, total seven doses) of saline or amphetamine, 5 mg/kg. The levels of the GAD(67) immunoreactivity were approximate 25 and 38% lower in respective glomerular (GloL) and mitral cell layers (ML) of saline-treated KO mice than that of WT, whereas similar in the external plexiform or granule cell layers (GraL) of the KO and WT. In the GloL, the level of tyrosine hydroxylase was 39% lower in the KO mice than WT, implicating different dopamine content in the KO from WT. The amphetamine exposure down-regulated the levels of GAD(67) in the WT layers by 46 to 52%, and in KO layers 65 to 71%, except ML. The GraL GAD(67) level may be regulated by the activation of CREB, as the phosphorylated (p) CREB coexisted with GAD(67), and the percentage of GAD(67)-expressing pCREB neurons was decreased by the amphetamine exposure. The data indicate that the activity of MAOB could modulate the regular and amphetamine-perturbed expression of GAD(67) and pCREB. Thus, interactions are suggested among the MAOB activity, GABA content of OlfB, and olfaction.

Role of GABA in anxiety and depression

This review assesses the parallel data on the role of gamma-aminobutyric acid (GABA) in depression and anxiety. We review historical and new data from both animal and human experimentation which have helped define the key role for this transmitter in both these mental pathologies. By exploring the overlap in these conditions in terms of GABAergic neurochemistry, neurogenetics, brain circuitry, and pharmacology, we develop a theory that the two conditions are intrinsically interrelated. The role of GABAergic agents in demonstrating this interrelationship and in pointing the way to future research is discussed.

Differential effects of chronic amphetamine and baclofen administration on cAMP levels and phosphorylation of CREB in distinct brain regions of wild type and monoamine oxidase B-deficient mice

Roles of GABA(B) transmission were explored in the action of amphetamine (Amph) on the brain. Adult male wild type (WT) and monoamine oxidase B-knocked out (MAOBKO) mice received i.p. injections of saline, d-Amph (5 mg/kg), plus baclofen (GABA(B) receptor agonist, 10 mg/kg), or baclofen and Amph, twice daily for 3 days and single treatments on day 4, followed by immuno-cyclic-AMP (cAMP) and immunoblotting assays on the brain tissue. The WT mice responded with higher levels of behavioral responses than the KO to the daily Amph injection; however, baclofen blocked the Amph-induced behavioral hyperactivity of both WT and KO mice. After the last treatment, levels of cAMP and phosphorylated (p) cyclic-AMP response element binding protein (CREB) were up-regulated in the striatum and somatosensory cortex of Amph-treated WT mice, while similar to the saline-controls in the baclofen+Amph-treated group, indicating the blockade by baclofen to Amph. Baclofen similarly suppressed the Amph-induced increases in pCREB levels of WT hippocampus and amygdala, and decreases of olfactory bulb and thalamus. For MAOBKO mice, baclofen hindered the Amph-generated increases in motor cortical cAMP and pCREB, and amygdaloid pCREB, and the decrease in olfactory bulb pCREB, whereas did not affect the Amph-raised hippocampal pCREB. Furthermore, the levels of CREB were variably modified in distinct regions by the drug exposures. The data reveal that the GABA(B)-mediated intracellular signaling differentially participates in mechanisms underlying Amph perturbation to various regions, and may thereby contribute explanations to the behavioral consequences. Moreover, MAOB is region-dependently involved in responses of the brain to Amph and baclofen, supporting interactions between GABA and monoamines.

Antidepressant-like activity of CGP 36742 and CGP 51176, selective GABAB receptor antagonists, in rodents

BACKGROUND AND PURPOSE

A crucial role for the GABAB receptor in depression was proposed several years ago, but there are limited data to support this proposition. Therefore we decided to investigate the antidepressant-like activity of the selective GABAB receptor antagonists CGP 36742 and CGP 51176, and a selective agonist CGP 44532 in models of depression in rats and mice.

EXPERIMENTAL APPROACH

Effects of CGP 36742 and CGP 51176 as well as the agonist CGP 44532 were assessed in the forced swim test in mice. Both antagonists were also investigated in an olfactory bulbectomy (OB) model of depression in rats, while CGP 51176 was also investigated in the chronic mild stress (CMS) rat model of depression. The density of GABAB receptors in the mouse hippocampus after chronic administration of CGP 51176 was also investigated.

KEY RESULTS

The GABAB receptor antagonists CGP 36742 and CGP 51176 exhibited antidepressant-like activity in the forced swim test in mice. The GABAB receptor agonist CGP 44532 was not effective in this test, however, it counteracted the antidepressant-like effects of CGP 51176. The antagonists CGP 36742 and CGP 51176 were effective in an OB model of depression in rats. CGP 51176 was also effective in the CMS rat model of depression. Administration of CGP 51176 increased the density of GABAB receptors in the mouse hippocampus.

CONCLUSIONS AND IMPLICATIONS

These data suggest that selective GABAB receptor antagonists may be useful in treatment of depression, and support an important role for GABA-ergic transmission in this disorder.

Tranylcypromine: new perspectives on an "old" drug

The irreversible inhibitor of monoamine oxidase, tranylcypromine, is a potent antidepressant, but its use is limited to special indications due to side effects and dietary restrictions. The antidepressant action of tranylcypromine is not completely explainable by its effects on monoamine oxidase. Tranylcypromine also leads to an increase in brain trace amines, which are believed to play a key role in the pathophysiology of depression. It also affects other pathophysiological pathways associated with depression. Tranylcypromine treatment leads to an up-regulation of GABA(B)-receptors and modulates the phospholipid metabolism, which is essential for normal brain function. These findings implicate that the efficacy of tranylcypromine as an antidepressant may be due to its multiple actions within the human brain.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Innovative approaches for the development of antidepressant drugs: current and future strategies

Depression is a highly debilitating disorder that has been estimated to affect up to 21% of the world population. Despite the advances in the treatment of depression with selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs), there continue to be many unmet clinical needs with respect to both efficacy and side effects. These needs range from efficacy in treatment resistant patients, to improved onset, to reductions in side effects such as emesis or sexual dysfunction. To address these needs, there are numerous combination therapies and novel targets that have been identified that may demonstrate improvements in one or more areas. There is tremendous diversity in the types of targets and approaches being taken. At one end of a spectrum is combination therapies that maintain the benefits associated with SSRIs but attempt to either improve efficacy or reduce side effects by adding additional mechanisms (5-HT1A, 5-HT1B, 5-HT1D, 5-HT2C, alpha-2A). At the other end of the spectrum are more novel targets, such as neurotrophins (BDNF, IGF), based on recent findings that antidepressants induce neurogenesis. In between, there are many approaches that range from directly targeting serotonin receptors (5-HT2C, 5-HT6) to targeting the multiplicity of potential mechanisms associated with excitatory (glutamate, NMDA, mGluR2, mGluR5) or inhibitory amino acid systems (GABA) or peptidergic systems (neurokinin 1, corticotropin-releasing factor 1, melanin-concentrating hormone 1, V1b). The present review addresses the most exciting approaches and reviews the localization, neurochemical and behavioral data that provide the supporting rationale for each of these targets or target combinations.

GABA(B) receptor function and subunit expression in the rat spinal cord as indicators of stress and the antinociceptive response to antidepressants

Experiments were undertaken to examine whether once daily i.p. administration of either of two antidepressants used for the treatment of neuropathic pain, amitriptyline (10 mg/kg) and fluoxetine (5 mg/kg), to rats for 7 days modifies GABA(B) receptor function and subunit expression in the lumbar spinal cord. The results indicate that, as previously reported for desipramine, both amitriptyline and fluoxetine increase the pain threshold to a thermal stimulus, the expression of GABA(B(1)) subunits, and baclofen-stimulated [35S]GTPgammaS binding, a measure of GABA(B) receptor function. The effects of antidepressant administration on GABA(B(1b)) and GABA(B(2)) subunit expression in spinal cord are more variable than for GABA(B(1a)). It was also discovered that repeated daily exposure to a thermal stimulus or immobilization stress increases GABA(B(1a)) expression in the lumbar spinal cord, with no commensurate change in thermal pain threshold or GABA(B) receptor sensitivity. These results support a relationship between GABA(B) receptors and the action of antidepressants. The findings demonstrate that drug-induced increases in GABA(B) receptor function can occur independently of any change in GABA(B) receptor subunit expression and are consistent with the notion that GABA(B) receptor subunits have multiple functions, only one of which is dimerization to form GABA(B) receptors. The data also suggest that GABA(B) subunit gene expression may serve as a preclinical marker of antidepressant efficacy and of drug- or stress-induced modifications in central nervous system activity.

Amitriptyline prevents thermal hyperalgesia and modifications in rat spinal cord GABAB receptor expression and function in an animal model of neuropathic pain

Using an animal model of neuropathic pain, behavioral and biochemical experiments were performed to assess the effects of this condition on pain threshold and GABA(B) receptor sensitivity and subunit gene expression in the rat lumbar spinal cord. The results indicate that partial sciatic nerve ligation decreases thermal and mechanical pain withdrawal latencies, and increases baclofen-stimulated [35S]GTPgammaS binding and GABA(B) receptor subunit gene expression in the rat lumbar spinal cord, suggesting that neuropathic pain may be due, in part, to a deficiency in GABAergic transmission. The experiments also demonstrate that daily administration (10 mg/kg, i.p.) of amitriptyline, a tricyclic antidepressant used for the treatment of neuropathic pain, for 1 week after surgery prevents the decline in thermal pain threshold, the increase in GABA(B2) gene expression, and development of increased GABA(B) receptor function in spinal cord resulting from nerve damage. These findings indicate that the efficacy of amitriptyline as a treatment for neuropathic pain may be related to an ability to maintain spinal cord GABA(B) receptor activity.

Screening the receptorome reveals molecular targets responsible for drug-induced side effects: focus on ‘fen–phen’

The in vitro pharmacological profiling of drugs using a large panel of cloned receptors (e.g., G protein-coupled receptors, ligand-gated ion channels, Na(+)-dependent monoamine transporters), an approach that has come to be known as 'receptorome screening', has unveiled novel molecular mechanisms responsible for the actions and/or side effects of certain drugs. For instance, receptorome screening has been employed to uncover novel molecular targets involved in the actions of antipsychotic medications and the hallucinogenic mint extract salvinorin A. This review highlights the recent application of receptorome screening to discover why the anorexigen fenfluramine causes serious cardiopulmonary side effects. Receptorome screening has implicated N-deethylation of fenfluramine and serotonin 5-hydroxy-t-ryptamine 2B receptors in the adverse effects of the drug; subsequent studies corroborated this finding. The results discussed highlight the utility of determining the potential activity of drugs -- and, importantly, of their in vivo metabolites -- at as many molecular targets as possible in order to reliably predict side effect profiles. Receptorome screening represents one of the most effective methods for identifying potentially serious drug-related side effects at the preclinical stage, thereby avoiding significant economic and human health consequences.

Role of arginine in immunonutrition

Arginine plays an important role in many physiologic and biologic processes beyond its role as a protein-incorporated amino acid. Dietary supplementation of arginine can enhance wound healing, regulate endocrine activity and potentiate immune activity. Under normal unstressed conditions the arginine requirement of adult humans is fulfilled by endogenous sources, however this is compromised during times of stress, especially in critical illness. These finding have led to use of arginine supplementation as part of an immune-enhancing dietary regimen to help combat the immune suppression seen in such patients. Though the results from studies examining the use of this type of immunonutrition in critically ill patients are far from definitive, they are promising that this mode of therapy may be of some advantage. A better understanding of the in vivo biology of arginine and its metabolism is necessary to truly define a benefit from arginine supplementation.