GABA, depression and the mechanism of action of antidepressant drugs: a neuroendocrine approach

Assessment of Paroxetine Molecular Interactions with Selected Monoamine and γ-Aminobutyric Acid Transporters

Thus far, many hypotheses have been proposed explaining the cause of depression. Among the most popular of these are: monoamine, neurogenesis, neurobiology, inflammation and stress hypotheses. Many studies have proven that neurogenesis in the brains of adult mammals occurs throughout life. The generation of new neurons persists throughout adulthood in the mammalian brain due to the proliferation and differentiation of adult neural stem cells. For this reason, the search for drugs acting in this mechanism seems to be a priority for modern pharmacotherapy. Paroxetine is one of the most commonly used antidepressants. However, the exact mechanism of its action is not fully understood. The fact that the therapeutic effect after the administration of paroxetine occurs after a few weeks, even if the levels of monoamine are rapidly increased (within a few minutes), allows us to assume a neurogenic mechanism of action. Due to the confirmed dependence of depression on serotonin, norepinephrine, dopamine and γ-aminobutyric acid levels, studies have been undertaken into paroxetine interactions with these primary neurotransmitters using in silico and in vitro methods. We confirmed that paroxetine interacts most strongly with monoamine transporters and shows some interaction with γ-aminobutyric acid transporters. However, studies of the potency inhibitors and binding affinity values indicate that the neurogenic mechanism of paroxetine's action may be determined mainly by its interactions with serotonin transporters.

Brain-derived neurotropic factor and GABAergic transmission in neurodegeneration and neuroregeneration

Neurotoxicity induced by stress, radiation, chemicals, or metabolic diseases, is commonly associated with excitotoxicity, oxidative stress, and neuroinflammation. The pathological process of neurotoxicity induces neuronal death, interrupts synaptic plasticity in the brain, and is similar to that of diverse neurodegenerative diseases. Animal models of neurotoxicity have revealed that clinical symptoms and brain lesions can recover over time via neuroregenerative processes. Specifically, brain-derived neurotropic factor (BDNF) and gamma-aminobutyric acid (GABA)-ergic transmission are related to both neurodegeneration and neuroregeneration. This review summarizes the accumulating evidences that suggest a pathogenic role of BDNF and GABAergic transmission, their underlying mechanisms, and the relationship between BDNF and GABA in neurodegeneration and neuroregeneration. This review will provide a comprehensive overview of the underlying mechanisms of neuroregeneration that may help in developing potential strategies for pharmacotherapeutic approaches to treat neurotoxicity and neurodegenerative disease.

Metabolomic analysis of anti-hypoxia and anti-anxiety effects of Fu Fang Jin Jing Oral Liquid

Background

Herba Rhodiolae is a traditional Chinese medicine used by the Tibetan people for treating hypoxia related diseases such as anxiety. Based on the previous work, we developed and patented an anti-anxiety herbal formula Fu Fang Jin Jing Oral Liquid (FJJOL) with Herba Rhodiolae as a chief ingredient. In this study, the anti-hypoxia and anti-anxiety effects of FJJOL in a high altitude forced-swimming mouse model with anxiety symptoms will be elucidated by NMR-based metabolomics.

Methods

In our experiments, the mice were divided randomly into four groups as flatland group, high altitude saline-treated group, high altitude FJJOL-treated group, and high altitude diazepam-treated group. To cause anxiety effects and hypoxic defects, a combination use of oxygen level decreasing (hypobaric cabin) and oxygen consumption increasing (exhaustive swimming) were applied to mice. After a three-day experimental handling, aqueous metabolites of mouse brain tissues were extracted and then subjected to NMR analysis. The therapeutic effects of FJJOL on the hypobaric hypoxia mice with anxiety symptoms were verified.

Results

Upon hypoxic exposure, both energy metabolism defects and disorders of functional metabolites in brain tissues of mice were observed. PCA, PLS-DA and OPLS-DA scatter plots revealed a clear group clustering for metabolic profiles in the hypoxia versus normoxia samples. After a three-day treatment with FJJOL, significant rescue effects on energy metabolism were detected, and levels of ATP, fumarate, malate and lactate in brain tissues of hypoxic mice recovered. Meanwhile, FJJOL also up-regulated the neurotransmitter GABA, and the improvement of anxiety symptoms was highly related to this effect.

Conclusions

FJJOL ameliorated hypobaric hypoxia effects by regulating energy metabolism, choline metabolism, and improving the symptoms of anxiety. The anti-anxiety therapeutic effects of FJJOL were comparable to the conventional anti-anxiety drug diazepam on the hypobaric hypoxia mice. FJJOL might serve as an alternative therapy for the hypoxia and anxiety disorders.

Growth hormone responses to GABAB receptor challenge with baclofen and impulsivity in healthy control and personality disorder subjects

BACKGROUND

The role of abnormal GABAergic neural transmission in impulsive aggression is not well understood. We have previously shown that central levels of GABA are positively correlated with impulsivity in adult humans with and without personality disorder. An important regulator of GABAergic function is the GABA(B) receptor, a presynaptic autoreceptor and heteroreceptor. GABA(B) receptor sensitivity may be tested by measuring the growth hormone response to the receptor-agonist baclofen. The purpose of this investigation is to test the hypothesis that dimensional measures of impulsivity and impulsive aggression are negatively correlated with growth hormone response.

METHODS

Twenty healthy volunteers (without Axis I or II disorder) and 20 personality-disordered subjects (meeting DSM-IV general criteria for personality disorder) underwent challenge with 20 mg baclofen administered orally, followed by a time series of blood samples for measure of growth hormone response analyzed by repeated measures ANOVA.

RESULTS

An expected significant effect for drug and drug × time interaction verified that baclofen caused a surge in growth hormone level. There was no effect of group (healthy volunteer or personality disorder) or interaction with group on the time series or peak growth hormone response. As hypothesized, peak growth hormone response was negatively correlated with impulsivity as measured by the Barratt Impulsivity Scale (BIS-11; r = -0.39, n = 37, p < 0.02). The relationship remained significant when examining the healthy volunteer and personality disorder groups separately, indicating that the relationship with impulsivity was not merely due to the presence or absence of personality disorder. The relationship with impulsive aggression was only at a trend level of significance.

CONCLUSION

The magnitude of growth hormone response to baclofen, an index of GABA(B) receptor function, was negatively correlated with a dimensional measure impulsivity, but not related to the categorical diagnosis of personality disorder. Further work is necessary to understand how GABAergic dysfunction may play a role in impulsive aggression.

The effects of electroconvulsive therapy on GABAergic function in major depressive patients

OBJECTIVES

It has been proposed that major depression is associated with a dysfunction of the gamma-aminobutyric acid (GABA) system. This study was planned to investigate whether there are any alterations in GABAergic activities in major depressive patients and, if there are, whether electroconvulsive therapy (ECT) has any effect on these changes.

METHODS

Twenty-five depressed inpatients who responded to a course of ECT and 23 healthy subjects were included in the study. Serum GABA levels were measured 2 days before and 10 minutes after the first ECT and 3 days after the last ECT, and a baclofen challenge test was performed 2 days before the first ECT and 3 days after the last ECT in the patients. The same tests were carried out only once in the control group.

RESULTS

Depressive patients had lower serum GABA levels compared with healthy individuals, and ECT caused a significant increase in these levels. The acute effect of the one-ECT procedure was a huge increase in the baseline GABA levels. Although there was no difference in the maximum alteration in growth hormone with baclofen between the patients and controls before the therapeutic ECT course, it became significantly higher in the depressive patients than in the controls after the treatment.

CONCLUSIONS

The findings of this study support the GABA deficit hypothesis of major depression because major depressive patients have lower levels of serum GABA that are increased by a completed ECT course. ECT seems to increase brain GABA levels as well as GABAB activity, and these effects may contribute to its mechanism of therapeutic effect.

Reduced 5-HT1A- and GABAB receptor function in dorsal raphé neurons upon chronic fluoxetine treatment of socially stressed rats

Autoinhibitory serotonin 1A receptors (5-HT(1A)) in dorsal raphé nucleus (DRN) have been implicated in chronic depression and in actions of selective serotonin reuptake inhibitors (SSRI). Due to experimental limitations, it was never studied at single-cell level whether changes in 5-HT(1A) receptor functionality occur in depression and during SSRI treatment. Here we address this question in a social stress paradigm in rats that mimics anhedonia, a core symptom of depression. We used whole cell patch-clamp recordings of 5-HT- and baclophen-induced G-protein-coupled inwardly rectifying potassium (GIRK) currents as a measure of 5-HT(1A)- and GABA(B) receptor functionality. 5-HT(1A)- and GABA(B) receptor-mediated GIRK-currents were not affected in socially stressed rats, suggesting that there was no abnormal (auto)inhibition in the DRN on social stress. However, chronic fluoxetine treatment of socially stressed rats restored anticipatory behavior and reduced the responsiveness of 5-HT(1A) receptor-mediated GIRK currents. Because GABA(B) receptor-induced GIRK responses were also suppressed, fluoxetine does not appear to desensitize 5-HT(1A) receptors but rather one of the downstream components shared with GABA(B) receptors. This fluoxetine effect on GIRK currents was also present in healthy animals and was independent of the animal's "depressed" state. Thus our data show that symptoms of depression after social stress are not paralleled by changes in 5-HT(1A) receptor signaling in DRN neurons, but SSRI treatment can alleviate these behavioral symptoms while acting strongly on the 5-HT(1A) receptor signaling pathway.

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.

Motor cortical excitability in depressive patients after electroconvulsive therapy and repetitive transcranial magnetic stimulation

Electroconvulsive therapy (ECT) and repetitive transcranial magnetic stimulation (rTMS) of the dorsolateral prefrontal cortex are brain stimulation techniques that are used as therapeutic interventions in major depression. However, the exact therapeutic mode of action needs further clarification. In this case report, we describe the impact of these stimulation techniques on motor cortical excitability, as revealed by transcranial magnetic stimulation-elicited motor-evoked potentials in 2 patients who received consecutively both rTMS and ECT. Both patients showed a decrease in motor cortical excitability after response to antidepressant brain stimulation, whereas parameters of motor cortical excitability remained unchanged after the first non-successful intervention. These results suggest that both ECT and rTMS may have an impact on parameters of motor cortical neuronal excitability. Furthermore, measurement of motor cortical excitability may be a useful tool for investigating and monitoring inhibitory brain effects of different antidepressant stimulation techniques.

Clinical studies implementing glutamate neurotransmission in mood disorders

Emerging evidence suggests that the amino acid neurotransmitter systems are associated with the pathophysiology and treatment of mood disorders. Recent advances in the areas of molecular neurobiology, pharmacology, and magnetic resonance spectroscopy (MRS) now provide better tools to probe the function of the amino acid neurotransmitter systems and are affording us the opportunity to better investigate the relationship of these systems to mood disorders. Here we review the available literature in the field and suggest a possible pathophysiological model that may account for the many of the findings.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

A preliminary study of baclofen-induced growth hormone release in generalised social phobia

OBJECTIVE

The authors sought to examine central GABA(B) receptor responses in patients with generalised social phobia using the growth hormone (GH) response to baclofen.

METHOD

Baclofen 20 mg was administered to 15 patients and 15 healthy matched controls and plasma growth hormone was monitored over a 3 h period.

RESULTS

The GH response in patients was significantly reduced in comparison to that of controls when calculated as both area under the curve (AUC) and delta (Delta) response.

CONCLUSIONS

The results suggest an abnormality of central GABA(B) receptor function in this disorder.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Growth hormone response to baclofen in patients with seasonal affective disorder: effects of light therapy

There is evidence for gamma-aminobutyric acid (GABA) dysfunction in the pathophysiology and treatment response of patients with major depression, but this has not been studied in seasonal affective disorder (SAD). Growth hormone (GH) response to a challenge with a GABAB receptor agonist, baclofen, is considered an in vivo index of hypothalamic GABAB receptor function in humans. To explore the role of GABAB receptor function in SAD, we compared the GH response to baclofen challenge in 15 patients with SAD and 20 matched healthy controls. Of the 15 patients with SAD, 14 had repeat baclofen challenge following 2-week treatment with light therapy. The results showed that baclofen administration led to a significant increase in GH release both in patients with SAD and normal controls. There was no significant difference in the GH response to baclofen between the two groups. Furthermore, 2-week treatment with light therapy did not significantly alter the baclofen-induced GH response in patients with SAD, in spite of a clear therapeutic effect. The results of this study suggest that hypothalamic GABAB receptor function, as measured by baclofen induced GH release, is not altered in patients with SAD or by light therapy.

Mechanisms of action and clinical characteristics of three atypical antidepressants: venlafaxine, nefazodone, bupropion

Tricyclic antidepressants have multiple sites of pharmacological actions which are responsible for their tolerability and toxicological problems as well as their efficacy. That fact has prompted the search for antidepressants with fewer sites of action. That search resulted in the serotonin selective reuptake inhibitors (SSRIs), with presumably only one site of action. Although the SSRIs are safer and better tolerated than the TCAs, a significant percentage of patients do not benefit from SSRIs. A group of "atypical antidepressants" including bupropion, nefazodone, and venlafaxine are known to have multiple sites of antidepressant action but do not interact at sites associated with side effects or tolerance. Thus this group of antidepressants present an important alternative to the SSRIs in the pharmacological therapy of depression. The basic pharmacological properties of bupropion, nefazodone, and venlafaxine are presented along with clinical profiles and the role of these three antidepressants in the pharmacotherapy of depression is discussed.

GABA function in mood disorders: an update and critical review

Over the past twenty years, several lines of evidence from preclinical and clinical studies has accumulated suggesting that a GABA deficit may be involved in mood disorders, particularly in depression, and that increasing GABAergic neurotransmission may exert an antidepressant effect and perhaps a mood stabilizing effect. Given that GABA has an inhibitory effect on biogenic amine neurotransmitters such as norepinephrine and serotonin and this inhibition may be involved in local circuits and interneurons, it has been suggested that the hypothesis of a GABA deficit in mood disorders does not compete with but complements the well-established hypotheses of alterations in noradrenergic and serotonergic function in mood disorders. In this paper, we systematically reviewed the results from preclinical and clinical studies of GABA function in the pathophysiology of mood disorders and in the mechanism of action of mood stabilizers, antidepressants and electroconvulsive therapy. We also discussed the unifying theory of the neurochemistry of mood disorders, which integrates the GABA hypothesis into the biogenic amine hypotheses, and indicated future directions for research.

Psychoneuroendocrinology of depression. Growth hormone

The release of growth hormone (GH) from the anterior pituitary is regulated by hypothalamic peptides especially GH-releasing hormone (GHRH) and somatostatin, which in turn are controlled by classic neurotransmitters such as noradrenaline, dopamine, and acetylcholine, as well as negative feedback from GH and insulin-like growth factor-1. There has been extensive investigation of this axis in patients with depression. The most consistently reported abnormality is in noradrenergic-mediated GH release, which probably occurs via GHRH containing neurones. ACh-induced GH release through the somatostatin system, GABA, and also GHRH-stimulated release are reported as abnormal by some researchers.

Growth hormone response to the GABAB agonist baclofen in major depressive disorder

Growth hormone (GH) response to the gamma-aminobutyric acid B receptor agonist, baclofen, was measured in 16 male patients with major depressive disorder and in 16 age-matched healthy male controls. No significant differences were found in the GH response to baclofen between the depressed patients and controls. On repeat testing, the GH response to baclofen showed significant retest reliability in both groups. There was no significant correlation between serum baclofen levels and the GH response to baclofen. Age significantly correlated with GH response, with older subjects having lower GH response to baclofen. These data do not suggest that a blunted GH response to baclofen. represents a specific neuroendocrine feature of major depression.

Growth hormone response to baclofen: a comparison of 10-mg and 20-mg doses in healthy men

Growth hormone (GH) response and baclofen levels were measured in seven healthy adult men following a 10-mg and a 20-mg dose of oral baclofen (gamma-aminobutyric acidB agonist) to determine the preferred dose in baclofen challenge studies. Multivariate analysis of variance (ANOVA) with repeated measures revealed no differences between the doses. However, when a univariate ANOVA with repeated measures was performed for each dose, the 10-mg dose showed no significant GH response over time, whereas the 20-mg dose showed a significant GH response over time. The average delta GH (change in GH from baseline) was 7.84 ng/ml (SD = 10.17) for the 10-mg dose and 3.34 ng/ml (SD = 3.64) for the 20-mg dose. The variability in the delta GH response to the 10-mg dose was significantly greater than the response to the 20-mg dose of baclofen. This variance in GH response was not explained by the differences in serum baclofen levels. Thus, a 20-mg baclofen dose appears to be preferable to a 10 mg-dose in baclofen challenge studies.

Baclofen administration for the treatment of affective disorders in alcoholic patients

Ninety alcoholic patients with the secondary affective disorders (anxiety, depression) were divided into four groups. Patients in the first group received GABAB receptor ligands (baclofen), those in the second group, diazepam, those in the third group, amitriptyline and those in the fourth group, placebo. The results of clinical, psychological (tests of Spielberger, Zung and MMPI), and electrophysiological (superslow omega-potential) investigations showed that baclofen is an effective drug for affective disturbances in alcoholic patients, with efficacy superior to placebo and equal to diazepam and amitriptyline. At the same time baclofen does not have the side-effects and complications of the latter. Significant changes in platelet MAOB activity and the dopamine, serotonin and GABA concentrations in blood after treatment were not found in the four patient groups. The peripheral matabolism of GABA and monoamines do not seem to be related to the development of secondary affective disorders in alcoholic patients. This investigation encourages the search for drugs acting on the affective psychopathology of GABAB receptor ligands.

The effects of valproate in brain monoamines of juvenile rats after stress

1. Clinical data suggest that valproate (VPA) may be useful in prophylaxis of affective disorders, which show disturbances of the serotoninergic system. On the other hand, chronic stress has an adverse effect on affective disorders, those with disturbances of the serotonergic system, especially. 2. In order to study the effects of VPA on brain monoamines and acute stress, 200 mgr VPA/Kgr was administered intraperitoneal (ip) to juvenile male rats; the control group was treated with NaCL 0.9% ip. After 30 min, all animals were evoked on predictable neurogenic or systemic stress (30 min foot shock, or 15 min ether stress, respectively), and 48 hours later, VPA or NaCL were administered ip again; 30 min afterwards, the rats were decapitated. Rats without stress were also sacrificed 30 min after VPA or NaCL administration. 3. Measurements of brain monoamines noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA), were done in Frontal Cortex (FC), Hypothalamus (HY) and Striatum (S), by High Performance Liquid Chromatography (HPLC). 4. Compared with the control stress group the level of 5-HIAA in the FC was significantly increased (P < 0.01) in VPA stress rats; in the HY and in S the increase of 5-HIAA was not significant. No remarkable differences were observed in NA, DA, 5-HT and DOPAC concentrations, in any of the brain regions. No changes in brain monoamine levels were found in non stress rats, either. 5. The augmentation of 5-HIAA level after VPA administration and after stress, in correlation with the decrease of 5-HIAA that is observed in depression, support the hypothesis that VPA may be effective in affective disorders by influencing the serotoninergic system.

The Flinders sensitive line rats: a genetic animal model of depression

The Flinders Sensitive Line (FSL) rat, selectively bred for increased responses to the anticholinesterase DFP, was originally proposed as an animal model of depression because, like depressed humans, it is supersensitive to the behavioral and hormonal effects of cholinergic (muscarinic) agonists. The present review critically examines earlier and recent data collected on FSL rats to assess whether the model has good face, construct and/or predictive validity. With respect to face validity, FSL rats resemble depressed humans, at least superficially, in that they demonstrate: (a) reduced locomotor activity, (b) reduced body weight, (c) increased REM sleep, and (d) cognitive (learning) difficulties. So far, studies designed to assess the presence of anhedonia, a cardinal symptom of melancholic depression, have been inconclusive, but there are trends for the FSL rats to be more anhedonic than their control counterparts, the Flinders Resistant Line (FRL) rats, when exposed to chronic mild stress. Thus, FSL rats fulfill the criterion of face validity. Because FSL rats also are more sensitive to cholinergic agonists and have phase advanced circadian rhythms, they meet the criteria for the cholinergic and circadian rhythm models of depression and, therefore, have good construct validity. A key behavioral symptom exhibited by the FSL rat is demonstration of an exaggerated immobility when exposed to stressors such as foot shock and forced swimming. This behavioral abnormality has been normalized by a number of well-recognized antidepressant drugs such as imipramine and desipramine, as well as newer generation antidepressants with promising clinical effects such as sertraline and rolipram. However, several treatments that have not been routinely used to treat depression (lithium, exposure to bright light, the anticholinesterase DFP) have been ineffective in reversing the exaggerated immobility. Thus, the evidence in the present review indicates that the FSL rat model of depression fulfills the criteria of face, construct, and predictive validities.

Effects of the antidepressant/antipanic drug phenelzine on GABA concentrations and GABA-transaminase activity in rat brain

The effects of long-term (28-day) administration of several antidepressant/antipanic drugs [imipramine, desipramine, tranylcypromine and phenelzine (PLZ)] on gamma-aminobutyric acid-tranaminase (GABA-T) activity and GABA levels were investigated in rat frontal cortex. Of the drugs investigated, only PLZ inhibited GABA-T and elevated GABA levels. Additional short-term experiments were conducted with PLZ, and they demonstrated a dose-dependent inhibition of GABA-T in rat whole brain. Time-response studies on inhibition of GABA-T in whole brain demonstrated that at a dose of PLZ of 15 mg/kg i.p. inhibition of GABA-T remained relatively constant from 1 to 8 hr and that the enzyme was still inhibited by 23% at 24 hr after PLZ administration.