Repeated administration of desipramine and a GABAB receptor antagonist, CGP 36742, discretely up-regulates GABAB receptor binding sites in rat frontal cortex

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.

GABAB receptor activation attenuates inflammatory orofacial pain by modulating interleukin-1β in satellite glial cells: Role of NF-κB and MAPK signaling pathways

Orofacial inflammation could activate satellite glial cells (SGCs) in the trigeminal ganglion (TG) to produce interleukin 1β (IL-1β) which plays crucial roles in the development of inflammatory pain. Recent studies have shown that gamma-amino butyric acid-B (GABA_B) receptor could modulate the expression of inflammatory cytokines in microglia and astrocytes in the spinal cord. The objective of this study was to investigate whether GABA_B receptors in TG SGCs attenuate inflammatory facial pain via mediating IL-1β following inflammation and its mechanisms. Complete Freund's adjuvant (CFA) was injected into the whisker pad of rats to induce inflammation in vivo. Lipopolysaccharide (LPS) was added to culture medium to activate SGCs in vitro. Behavioral measures showed that microinjection of baclofen (a selective GABA_B receptor agonist) into the TG ameliorated the mechanical allodynia of CFA-treated rats. Interestingly, baclofen pretreatment inhibited SGC activation and IL-1β production, however, preserved the decreased expression of GABA_B receptors in SGCs activated by CFA in vivo and LPS in vitro. In addition, baclofen suppressed the increased expression of p-NF- κ B p65, p-I κ Bα, and p-p38 MAPK, while reversed the decreased production of I κ Bα, and further enhanced the increased expression of p-ERK(1/2) in LPS-treated SGCs in vitro. Finally, those effects of baclofen were abolished by saclofen (a specific GABA_B receptor antagonist) co-administration. Altogether, these results demonstrated for the first time that activation of GABA_B receptor might inhibit IL-1β production by suppressing NF- κ B and p38 MAPK signaling pathway activation and restore GABA_B receptor expression in SGCs to attenuate inflammatory facial pain.

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.

Cortical adrenoceptor expression, function and adaptation under conditions of cannabinoid receptor deletion

A neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. Acute and repeated administration of a cannabinoid receptor synthetic agonist is capable of increasing multiple indices of noradrenergic activity. This includes cannabinoid-induced 1) increases in norepinephrine (NE) release in the medial prefrontal cortex (mPFC); 2) desensitization of cortical α2-adrenoceptor-mediated effects; 3) activation of c-Fos in brainstem locus coeruleus (LC) noradrenergic neurons; and 4) increases in anxiety-like behaviors. In the present study, we sought to examine adaptations in adrenoceptor expression and function under conditions of cannabinoid receptor type 1 (CB1r) deletion using knockout (KO) mice and compare these to wild type (WT) controls. Electrophysiological analysis of α2-adrenoceptor-mediated responses in mPFC slices in WT mice showed a clonidine-induced α2-adrenoceptor-mediated increase in mPFC cell excitability coupled with an increase in input resistance. In contrast, CB1r KO mice showed an α2-adrenoceptor-mediated decrease in mPFC cell excitability. We then examined protein expression levels of α2- and β1-adrenoceptor subtypes in the mPFC as well as TH expression in the locus coeruleus (LC) of mice deficient in CB1r. Both α2- and β1-adrenoceptors exhibited a significant decrease in expression levels in CB1r KO mice when compared to WT in the mPFC, while a significant increase in TH was observed in the LC. To better define whether the same cortical neurons express α2A-adrenoceptor and CB1r in mPFC, we utilized high-resolution immunoelectron microscopy. We localized α2A-adrenoceptors in a knock-in mouse that expressed a hemoagglutinin (HA) tag downstream of the α2A-adrenoceptor promoter. Although the α2A-adrenoceptor was often identified pre-synaptically, we observed co-localization of CB1r with α2-adrenoceptors post-synaptically in the same mPFC neurons. Finally, using receptor binding, we confirmed prior results showing that α2A-adrenoceptor is unchanged in mPFC following acute or chronic exposure to the synthetic cannabinoid receptor agonist, WIN 55,212-2, but is increased, following chronic treatment followed by a period of abstinence. Taken together, these data provide convergent lines of evidence indicating cannabinoid regulation of the cortical adrenergic system.

Anxiety in major depression and cerebrospinal fluid free gamma-aminobutyric acid

BACKGROUND

Low gamma-aminobutyric acid (GABA) is implicated in both anxiety and depression pathophysiology. They are often comorbid, but most clinical studies have not examined these relationships separately. We investigated the relationship of cerebrospinal fluid (CSF) free GABA to the anxiety and depression components of a major depressive episode (MDE) and to monoamine systems.

METHODS AND MATERIALS

Patients with a DSM-IV major depressive episode (N = 167: 130 major depressive disorder; 37 bipolar disorder) and healthy volunteers (N = 38) had CSF free GABA measured by gas chromatography mass spectroscopy. Monoamine metabolites were assayed by high performance liquid chromatography. Symptomatology was assessed by Hamilton depression rating scale.

RESULTS

Psychic anxiety severity increased with age and correlated with lower CSF free GABA, controlling for age. CSF free GABA declined with age but was not related to depression severity. Other monoamine metabolites correlated positively with CSF GABA but not with psychic anxiety or depression severity. CSF free GABA was lower in MDD compared with bipolar disorder and healthy volunteers. GABA levels did not differ based on a suicide attempt history in mood disorders. Recent exposure to benzodiazepines, but not alcohol or past alcoholism, was associated with a statistical trend for more severe anxiety and lower CSF GABA.

CONCLUSIONS

Lower CSF GABA may explain increasing severity of psychic anxiety in major depression with increasing age. This relationship is not seen with monoamine metabolites, suggesting treatments targeting the GABAergic system should be evaluated in treatment-resistant anxious major depression and in older patients.

Effects of GABAB receptor agonists and antagonists on glycemia regulation in mice

γ-Aminobutyric acid (GABA) inhibits insulin secretion through GABA(B) receptors in pancreatic β-cells. We investigated whether GABA(B) receptors participated in the regulation of glucose homeostasis in vivo. BALB/c mice acutely pre-injected with the GABA(B) receptor agonist baclofen (7.5mg/kg, i.p.) presented glucose intolerance and diminished insulin secretion during a glucose tolerance test (GTT, 2g/kg body weight, i.p.). The GABA(B) receptor antagonist 2-hydroxysaclofen (15 mg/kg, i.p.) improved the GTT and reversed the baclofen effect. Also a slight increase in insulin secretion was observed with 2-hydroxysaclofen. In incubated islets 1.10(-5)M baclofen inhibited 20mM glucose-induced insulin secretion and this effect was reversed by coincubation with 1.10(-5)M 2-hydroxysaclofen. In chronically-treated animals (18 days) both the receptor agonist (5mg/kg/day i.p.) and the receptor antagonist (10mg/kg/day i.p.) induced impaired GTTs; the receptor antagonist, but not the agonist, also induced a decrease in insulin secretion. No alterations in insulin tolerance tests, body weight and food intake were observed with the treatments. In addition glucagon, insulin-like growth factor I, prolactin, corticosterone and growth hormone, other hormones involved in glucose metabolism regulation, were not affected by chronic baclofen or 2-hydroxysaclofen. In islets obtained from chronically injected animals with baclofen, 2-hydroxysaclofen or saline (as above), GABA(B2) mRNA expression was not altered. Results demonstrate that GABA(B) receptors are involved in the regulation of glucose homeostasis in vivo. Treatment with receptor agonists or antagonists, given acutely or chronically, altered glucose homeostasis and insulin secretion alerting to the need to evaluate glucose metabolism during the clinical use of these drugs.

Deficits in GABA(B) receptor system in schizophrenia and mood disorders: a postmortem study

Postmortem and genetic studies have clearly demonstrated changes in GABA(B) receptors in neuropsychiatric disorders such as autism, bipolar disorder, major depression, and schizophrenia. Moreover, a number of recent studies have stressed the importance of cerebellar dysfunction in these same disorders. In the current study, we examined protein levels of the two GABA(B) receptor subunits GABBR1 and GABBR2 in lateral cerebella from a well-characterized cohort of subjects with schizophrenia (n=15), bipolar disorder (n=14), major depression (n=13) and healthy controls (n=12). We found significant reductions in protein for both GABBR1 and GABBR2 in lateral cerebella from subjects with schizophrenia, bipolar disorder and major depression when compared with controls. These results provide further evidence of GABAergic dysfunction in these three disorders as well as identify potential targets for therapeutic intervention.

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.

Pharmacological modulation of brain levels of glutamate and GABA in rats exposed to total sleep deprivation

Modulation of gamma-aminobutyric acid (GABA) and glutamate by selected antidepressants and anticonvulsants could play a beneficial role in total sleep deprivation (TSD) caused by depressed mood. In the present study, albino rats were exposed to TSD for five days. On the sixth day, the brains were removed, and GABA and glutamate levels were measured in the prefrontal cortex and thalamus to identify TSD-induced changes in untreated rats and in rats treated with carbamazepine 40 mg/kg intraperitoneally (IP), fluoxetine 20 mg/kg IP, or desipramine 10 mg/kg IP. Carbamazepine and fluoxetine significantly increased GABA and reduced glutamate levels in both brain areas. Desipramine administration did not affect GABA or glutamate concentrations in the tested brain areas; levels were comparable with those induced by TSD without treatment. These results suggest that administration of carbamazepine or fluoxetine could have a beneficial effect by increasing GABA levels during TSD.

Cannabinoid modulation of cortical adrenergic receptors and transporters

We previously reported that administration of the synthetic cannabinoid agonist WIN 55,212-2 causes an increase in norepinephrine (NE) efflux in the frontal cortex (FC). The present study examined the expression levels of alpha2- and beta1-adrenergic receptors (ARs) as well as the norepinephrine transporter (NET) in the FC of rats following exposure to WIN 55,212-2. Rats received systemic injection of WIN 55,212-2 (3 mg/kg) acutely or for 7 days. Another group of rats received repeated WIN 55,212-2 treatment followed by a period of abstinence. Control rats received vehicle injections. Rats were euthanized 30 min after the last WIN 55,212-2 injection, the FC was microdissected, and protein extracts were probed for alpha2-AR, beta1-AR, and NET. Results showed that beta1-AR expression was significantly decreased following repeated WIN 55,212-2 treatment but significantly increased following a period of abstinence. alpha2-AR expression showed no significant change in all groups examined. NET expression was significantly decreased following acute WIN 55,212-2 treatment, with no changes following chronic administration or a period of abstinence. Alterations in NET may arise from modulation of cannabinoid receptors (CB1) that are localized to noradrenergic axon terminals as we demonstrate colocalization of CB1 receptor and NET in the same cortical axonal processes. The present findings support significant alterations in adrenergic receptor and NET expression in the FC after WIN 55,212 exposure that may underlie the reported changes in attention, cognition, and anxiety commonly observed after cannabinoid exposure.

Desensitization of GABA(B) receptor signaling by formation of protein complexes of GABA(B2) subunit with GRK4 or GRK5

We investigated the role of G protein coupled-receptor kinases (GRKs) in the desensitization of GABA(B) receptor-mediated signaling using Xenopus oocytes and baby hamster kidney (BHK) cells. Baclofen elicited inward K(+) currents in oocytes coexpressing heterodimeric GABA(B) receptor, GABA(B1a) subunit (GB(1a)R) and GABA(B2) subunit (GB(2)R), together with G protein-activated inwardly rectifying K(+) channels (GIRKs), in a concentration-dependent manner. Repetitive application of baclofen to oocytes coexpressing GABA(B)R and GIRKs did not change peak K(+) currents in the first and second responses, but the latter responses were significantly attenuated by coexpression of either GRK4 or GRK5 with attenuation efficacy of GRK4 > GRK5. Coexpression of other GRKs including GRK2, GRK3, and GRK6 had no effect on GABA(B) receptor-mediated desensitization processes. In BHK cells coexpressing GRK4 fused to Venus (brighter variant of yellow fluorescent protein, GRK4-Venus) with GB(1a)R and GB(2)R, GRK4-Venus was expressed in the cytosol but was translocated to the plasma membranes by GABA(B)R activation. In BHK cells coexpressing GRK4 fused to Cerulean (brighter variant of cyan fluorescent protein, GRK4-Cerulean) with GB(1a)R and GB(2)R-Venus, fluorescence resonance energy transfer (FRET) analysis demonstrated that GRK4-Cerulean formed a protein complex with GB(2)R-Venus. Immunoprecipitation and Western blot analysis confirmed GB(2)R-GRK4 complex formation. GRK5 also formed a complex with GB(2)R on the plasma membranes as determined by FRET and Western blotting but not GRK2, GRK3, and GRK6. Our results indicate that GRK4 and GRK5 desensitize GABA(B) receptor-mediated responses by forming protein complexes with GB(2)R subunit of GABA(B)R at the plasma membranes.

Receptor autoradiographic correlates of deafferentation-induced reorganization in adult primate somatosensory cortex

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. We have previously shown that transection of the median and ulnar nerves is followed by an expansion in the representation of radial nerve skin, and that this plasticity proceeds in stages. Immediately following nerve injury, new receptive fields are "unmasked" in a fraction of the affected cortex. The remaining deprived cortex regains responsiveness to tactile stimulation over the following days to weeks. Given these progressive changes, it has been suggested that different mechanisms might account for the earlier and later phases of reorganization. In the present experiments, we quantified receptor autoradiographic binding data for GABAA and GABAB, AMPA, and NMDA receptors in the primary somatosensory cortices of adult squirrel monkeys at four postnerve injury survival durations: immediately (1-3 hours), 3 days, 1 month, and 2 months. We found dramatic reductions in GABAA binding in layer IV within hours following nerve injury, and this reduction was maintained across all survival durations. This finding is consistent with the idea that the earliest reorganizational changes are due to a relaxation in tonic inhibitory mechanisms permitting the expression of formerly subthreshold receptive fields. GABAB receptor binding is decreased in layer IV by 1 month after nerve injury, while binding for AMPA receptors is increased in layer IV by this time. These findings are consistent with our previous suggestion that the second stage of reorganization proceeds via mechanisms comparable to those revealed to account for NMDA-dependent long-term potentiation in the hippocampus.

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.

Development of baclofen tolerance in a rat model of chronic spasticity and rigidity

Systemic or spinal treatment with baclofen has been associated with the development of tolerance in patients with chronic spasticity. In the present study, we used a rat model of spinal ischemia-induced spasticity to characterize the development of baclofen tolerance after chronic intrathecal (i.t.) baclofen infusion. Following the induction of spinal ischemia and the development of behavioral spasticity, animals were implanted with i.t. catheters connected to osmotic pumps to continuously infuse baclofen (1.0 microg/0.5 microl/h). Hindleg peripheral muscle resistance (PMR) was measured periodically after initiation of chronic infusion and after bolus i.t. baclofen injection (1.0 microg). Peripheral muscle resistance was significantly decreased at the onset of baclofen infusion, however, after 5-7 days of infusion a progressive return of spasticity was noted, where baseline PMR values returned to preinfusion levels. At the same time, the efficacy of bolus i.t. baclofen treatment also decreased, where after 5 days of baclofen infusion 1.0 microg (i.t.) baclofen only reduced PMR by 10% (compared to 40-50% preinfusion). Baclofen efficacy progressively returned once continuous infusion was stopped. These data demonstrate that transient spinal ischemia leads to the development of spasticity which is sensitive to spinal baclofen. Chronic i.t. infusion leads to a progressive development of tolerance. This model offers potential to study tolerance mechanisms after spinal injury, and aid in drug discovery for use in baclofen-tolerant patients.

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.

Magnetic resonance spectroscopic measurement of cerebral gamma-aminobutyric acid concentrations in patients with bipolar disorders

BACKGROUND

Animal models of depression and psychopharmacological mechanisms of action suggest the importance of the gamma-amino butyric acid (GABA) system in the pathophysiology of mood disorders. Mood stabilizers have overlapping effects on GABAergic neurotransmission, and antidepressant use has been associated with alterations in GABAB receptor function. Magnetic resonance spectroscopy (MRS) provides an opportunity to noninvasively assess cerebral GABA concentrations in anterior paralimbic circuits that have been implicated in mood disorders.

METHODS

In bipolar disorder patients and healthy control subjects, we used MRS with a modified GABA-edited point resolved spectroscopy sequence (TE 68 ms, TR 1500 ms, 512 averages, total scan time 26 min) to assess GABA in an 18-cm3 occipital voxel. In addition, in another cohort of bipolar disorder patients and healthy control subjects, we similarly assessed GABA in a 12.5-cm3 medial prefrontal/anterior cingulate (MPF/AC) voxel. The concentration of GABA was referenced to creatine (Cr) from unedited spectra.

RESULTS

In bipolar patients and controls, we consistently detected 3.0 p.p.m. GABA peaks in occipital lobe and MPF/AC. In 16 bipolar (nine bipolar I and seven bipolar II) disorder patients, compared with six healthy control subjects, mean occipital GABA/Cr concentration was 61% higher. In addition, in 15 bipolar (five bipolar I, nine bipolar II, and one bipolar not otherwise specified) disorder patients, compared with six healthy control subjects, mean MPF/AC GABA/Cr concentration tended to be 41% higher.

CONCLUSIONS

Patients with bipolar disorders may have increased cerebral GABA concentrations. Although this was more evident in the occipital lobe, MPC/AC GABA disturbance may be of greater potential interest in view the more established role of MPF/AC in affective processing. Additional studies are warranted to assess changes in GABAergic neurotransmission and the influences of diagnosis, mood state, and medication status in bipolar disorder patients.

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.

Ethanol modulates GABA(B) receptor expression in cortex and hippocampus of the adult rat brain

Using in situ hybridization, RNase protection assay and Western blot, we studied the effects of ethanol on the expression levels of GABA(B) receptor mRNA and protein in the cortex and hippocampus from adult rat brain. The results showed that ethanol significantly increased GABA(B1) and GABA(B2) receptor protein expression in the cortex, whereas only GABA(B2) was increased in the hippocampus. GABA(B) receptor agonist baclofen could partially reverse the effect of ethanol. Further studies of the mRNA levels defined that GABA(B1) mRNA levels were significantly increased in the hippocampus, with no significant changes of GABA(B2) mRNA levels. Moreover, GABA(B1) and GABA(B2) receptor mRNA levels were increased on 3-week ethanol treatment. Finally, GABA(B) agonist baclofen and antagonist phaclofen showed significant decreasing effects on GABA(B1) receptor mRNA levels in the cortex, but not in the hippocampus. These results were further confirmed by in situ hybridization. Thus, the present results showed the effects of ethanol on GABA(B) receptors in the cortex and hippocampus, implying the possible role of GABA(B) receptor in ethanol effects. The effects of GABA(B) receptor agonist and antagonist suggested that the possible mechanisms underlying that GABA(B) receptor modulated the behavioral effect induced by ethanol.

Up-regulation of GABA(B) receptors by chronic administration of the GABA(B) receptor antagonist SCH 50,911

Chronic treatment of mice with the specific gamma-aminobutyric acid(B) (GABA(B)) receptor antagonist (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50,911) increased both the number of GABA(B) receptors in the whole brain (measured as [3H]CGP 54626 [S-(R,R)]-3-[[1-(3,4-dichlorophenyl)amino]-2-hydroxypropyl](cyclohexylmethyl)phosphinic acid hydrochloride binding) and the ability of baclofen to activate GABA(B) receptor coupled G-protein (measured as % reduction of the EC50 of baclofen to activate [35S]GTP(gamma)S binding). The results indicate that persistent blockade of GABA(B) receptors leads to their compensatory up-regulation and suggest that GABA(B) receptors are tonically activated by endogenous GABA.

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

Laboratory and clinical studies suggest that depression is associated with changes in the hippocampus and that this brain region is a major target for antidepressant drugs. Given the data suggesting that GABA(B) receptor antagonists display antidepressant properties, the present study was undertaken to assess the effect of antidepressant administration on GABA(B) receptors in the rat hippocampus to determine whether changes in this regional receptor system may play a role in the response to these agents. Rats were administered (i.p.) the monoamine oxidase inhibitors tranylcypromine (10mg/kg) or phenelzine (10mg/kg), the tricyclic antidepressant desipramine (15 mg/kg), or fluoxetine (5mg/kg), a selective serotonin re-uptake inhibitor, once daily for seven consecutive days. Two hours following the last drug treatment the hippocampal tissue was prepared for defining the distribution and quantity of GABA(B) receptor subunits using in situ hybridization and for assessing GABA(B) receptor function by quantifying baclofen-stimulated [(35)S]-GTPgammaS binding. All of these antidepressants selectively increased the expression of the GABA(B(1a)) subunit in hippocampus, having no consistent effect on the expression of GABA(B(1b)) or GABA(B(2)). Moreover, except for fluoxetine, these treatments increased GABA(B) receptor function in this brain region. The results indicate that an enhancement in the production of hippocampal GABA(B(1a)) subunits may be a component of the response to antidepressants, supporting a possible role for this receptor in the symptoms of depression and the treatment of this condition.

GABA(B) receptor alterations as indicators of physiological and pharmacological function

Given the widespread distribution of GABA(B) receptors throughout the central nervous system, and within certain peripheral organs, it is likely their selective pharmacological manipulation could be of benefit in the treatment of a variety of disorders. Studies aimed at defining the clinical potential of GABA(B) receptor agonists and antagonists have included gene deletion experiments, examination of changes in receptor binding, subunit expression and function in diseased tissue, as well as after the chronic administration of drugs. The results indicate that a functional GABA(B) receptor requires the combination of GABA(B(1)) and GABA(B(2)) subunits, that receptor function does not always correlate with subunit expression and receptor binding, and that GABA(B) receptor modifications may be associated with the clinical response to antidepressants, mood stabilizers, and GABA(B) receptor agonists and antagonists. Moreover, changes in GABA(B) binding or expression suggest this receptor may be involved in mediating symptoms associated with chronic pain, epilepsy and schizophrenia. This, together with results from other types of studies, indicates the potential therapeutic value of developing drugs capable of selectively activating, inhibiting, or modulating GABA(B) receptor function.

SGS742: the first GABA(B) receptor antagonist in clinical trials

The GABA(B) receptor antagonist SGS742 (CGP36742) displays pronounced cognition enhancing effects in mice, young and old rats and in Rhesus monkeys in active and passive avoidance paradigms, in an eight-arm radial maze and a Morris water maze and in a social learning task. SGS742 blocks the late inhibitory postsynaptic potential and the paired-pulse inhibition of population spikes recorded from CA1 pyramidal neurons of the hippocampus of rats in vitro and in vivo. SGS742 significantly enhances the release of glutamate, aspartate, glycine and somatostatin in vivo. Chronic administration of SGS742 causes an up-regulation of GABA(B) receptors in the frontal cortex of rats. Single doses cause a significant enhancement of the mRNA and protein levels of NGF and BDNF in the cortex and hippocampus of rats. The observed antidepressant effects of SGS742 in rats may be explained by these findings. SGS742 was well tolerated in experimental animals as well as in young and elderly human volunteers with an absolute bioavailability in humans of 44%. In a Phase II double-blind, placebo-controlled study in 110 patients with mild cognitive impairment (MCI), oral administration of SGS742 at a dose of 600 mg t.i.d. for 8 weeks significantly improved attention, in particular choice reaction time and visual information processing as well as working memory measured as pattern recognition speed. A second Phase II clinical trial in 280 Alzheimer's disease patients is underway.

Relationship between the antinociceptive response to desipramine and changes in GABAB receptor function and subunit expression in the dorsal horn of the rat spinal cord

Although tricyclic antidepressants are among the drugs of choice for the treatment of neuropathic pain, their mechanism of action in this regard remains unknown. Because previous reports suggest these agents may influence gamma-aminobutyric acid (GABA) neurotransmission, and GABAB receptors are known to participate in the transmission of pain impulses, the present experiments were undertaken to examine whether the administration of desipramine alters GABAB receptor subunit expression and function in the dorsal horn of the rat spinal cord. For the study, rats were injected (i.p.) once daily with desipramine (15 mg/kg) for 7 consecutive days, during which their thermal withdrawal threshold was monitored, and after which GABAB receptor function, and the levels of GABAB receptor subunit mRNA, were quantified in the spinal cord dorsal horn. The results indicate that 4-7 days of continuous administration of desipramine are necessary to observe a significant increase in the thermal pain threshold. Moreover, it was found that 7 days of treatment with desipramine enhances GABAB receptor function, as measured by baclofen-stimulated [35S]GTPgammaS binding, and increases mRNA expression for the GABAB(1a) and GABAB(2), but not GABAB(1b), subunits. These findings suggest the antinociceptive effect of desipramine is accompanied by a change in spinal cord GABAB receptor sensitivity that could be an important component in the analgesic response to this agent.

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.

Effects of repeated administration of baclofen to rats on GABAB receptor binding sites and subunit expression in the brain

Repeated stimulation of the GABAB receptor with baclofen frequently produces tolerance, the underlying mechanisms of which are poorly understood. The purpose of the present work was to determine whether repeated administration of baclofen to rats is accompanied by changes in cerebral GABAB receptor binding sites, mRNA for the subunits GABAB(1) and GABAB(2), and protein levels for these subunits. Rats were injected with placebo or baclofen (20 micromol/kg subcutaneously) once daily for 14 days. Decreases in body temperature were measured as an index of pharmacological effects of baclofen. Binding of radiolabeled GABA to GABAB receptors was quantitated in brain membranes, mRNA levels were determined using quantitative real-time PCR, and GABAB receptor protein levels were assessed with Western blot analysis. Baclofen caused a decline in temperature amounting to approximately 2.5 degrees C after the first dose. This effect was partly lost after the fifth and abolished after the seventh injection. Despite the complete development of tolerance, there were no significant alterations in GABAB receptor binding sites (number or affinity) or mRNA levels for the subtypes GABAB(1a), GABAB(1b), or GABAB(2). Receptor protein levels were also unchanged. It is concluded that baclofen induces tolerance through mechanisms other than down-regulation of GABAB receptor transcription or translation.

Acute reductions in GABAA receptor binding in layer IV of adult primate somatosensory cortex after peripheral nerve injury

Following peripheral nerve transection, reorganizational plasticity has been reported to occur in two phases, one immediate and one more protracted. GABA (gamma-aminobutyric acid) has been implicated in the immediate "unmasking" phase of reorganization. We have used quantitative autoradiography to assess potential changes in GABA(A) and GABA(B) receptor binding in primate somatosensory cortex following peripheral nerve injury. Here we report reductions in GABA(A) receptor binding in layer IV of primate somatosensory cortex deprived of its normal activating inputs for 2-5 h by peripheral nerve transection.

GABA(B) receptor activation desensitizes postsynaptic GABA(B) and A(1) adenosine responses in rat hippocampal neurones

Whole-cell recordings of EPSCs and G-protein-activated inwardly rectifying (GIRK) currents were made from cultured hippocampal neurones to determine the effect of long-term agonist treatment on the presynaptic and postsynaptic responses mediated by GABA(B) receptors (GABA(B)Rs). GABA(B)R-mediated presynaptic inhibition was unaffected by agonist (baclofen) treatment for up to 48 h, and was desensitized by about one-half after 96 h. In contrast, GABA(B)R-mediated GIRK currents were desensitized by a similar amount after only 2 h of agonist treatment. In addition, presynaptic inhibition mediated by A(1) adenosine receptors (A(1)Rs) was unaffected by prolonged GABA(B)R activation, whereas A(1)R-mediated GIRK currents were desensitized. Desensitization of postsynaptic GABA(B)R and A(1)R responses was blocked by the GABA(B)R antagonist (1-(S)-3,4-dichlorophenylethyl)amino-2-(S) hydroxypropyl-p-benzyl-phosphonic acid (CGP 55845A), but not by the A(1)R antagonist cyclopentyldipropylxanthine (DPCPX). GIRK current amplitude could be partially restored after baclofen treatment by either coapplication of baclofen and adenosine, or intracellular infusion of the non-hydrolysable GTP analog 5'-guanylylimidodiphosphate (Gpp(NH)p). Short-term (4-24 h) baclofen treatment also significantly desensitized the inhibition of postsynaptic voltage-gated calcium channels by activation of GABA(B)Rs or A(1)Rs. These results show that responses mediated by GABA(B)Rs and A(1)Rs desensitize differently in presynaptic and postsynaptic compartments, and demonstrate the heterologous desensitization of postsynaptic A1R responses.

Neuropharmacological profile of a selective sigma ligand, igmesine: a potential antidepressant

Igmesine is a selective sigma (sigma(1)) ligand that was reported to exert antidepressant action through an unknown mechanism of action. A number of neurochemical measures were taken in this study in efforts to understand its mode of action. Following 21-day drug treatments, the actions of igmesine on a number of neurochemical measures were investigated. Data obtained showed significant decreases in the densities of beta-adrenergic but not 5-HT(1A), sigma(1) and GABA(B) receptors in fluoxetine (18%), desipramine (DMI, 32%) and igmesine (20%)-treated groups when compared with control. Tyrosine hydroxylase (TH) activity was significantly (30-32%) reduced in all treated groups. Further, fluoxetine and DMI excluding the igmesine-treated groups showed 85 and 40% reductions in serotonin (5-HT) and noradrenaline (NE) neuronal uptake, respectively. Following acute treatment, igmesine lacked activity for monoamine oxidase (MAO) A or B (IC(50)>10 microM). In in vivo studies, at behaviorally active doses, igmesine showed weak effects on the NE uptake but lacked activity in altering 5-HT and DA synthesis or antagonizing selective drug-induced depletion of monoamine neuronal uptake. N-methyl-D-aspartate (NMDA)-induced increases in cGMP was blocked by igmesine indicating that igmesine may interfere with the NMDA receptor/nitric oxide synthase/cGMP pathway. Although it appears that part of the pharmacological actions of igmesine is mediated by the monoaminergic system, there is still need to explore other possible mechanisms of antidepressant action.

Involvement of GABAergic systems in manifestation of pharmacological activity of desipramine

We have conducted this study to elucidate the influence of GABAergic systems on manifestation of pharmacological activity of desipramine using both pharmacological and electrophysiological methods. Desipramine (20 mg/kg, i.p.) significantly blocked the adjuvant-induced thermal hyperalgesia, which was facilitated by treatment with the GABA(A) antagonist picrotoxin (2 mg/kg, i.p.) or the GABA(B) antagonist saclofen (2 mg/kg, i.p.). This analgesic effect of desipramine was antagonized by post-treatment with picrotoxin or saclofen. However, none of these compounds showed any effect in normal animals without adjuvant-induced inflammation. In a slice preparation of the hippocampus, treatment with GABA (10(-5)-5 x 10(-4) M), baclofen (10(-5)-10(-4) M) or muscimol (10(-5)-10(-4) M) inhibited the field potential evoked in pyramidal neurons by Schaffer collateral stimulation. The inhibitory effect of GABA was facilitated by concurrent application of desipramine, carbamazepine or diazepam at a concentration of 5 x 10(-5)-2 x 10(-4) M. The rank of order of facilitation is: desipramine > carbamazepine > diazepam. Desipramine also enhanced the inhibitory effect of baclofen and muscimol. These results suggest that desipramine causes GABAergic systems to activate still more, and this phenomenon appears to be involved in manifestation of the pharmacological activity of desipramine such as antinociception.

Effects of learned helplessness on brain GABA receptors

GABA is involved in both clinical depression and in animal models of depression; however, the roles of GABA(A) and GABA(B) receptors in specific brain regions are not clear. Changes in densities of both GABA(A) and GABA(B) receptors have been reported with the learned helplessness animal model of depression and with chronic antidepressant drug treatment. However, some of these findings are discrepant. Thus, we used quantitative autoradiography to study the GABA(A) and GABA(B) receptors in learned helplessness and we used an experimental paradigm that allows non-specific effects of stress to be differentiated from learned helplessness. Densities of GABA binding were measured in prefrontal cortex, septum, hippocampus, hypothalamus and amygdala. In the septum, learned helpless rats had increased densities of GABA(A) receptors and rats that did not become helpless after inescapable stress had decreased GABA(B) receptor densities. No significant group differences of GABA(A) or GABA(B) receptor densities were observed in any other brain region studied. These results suggest a unique role for the septum in modulating GABA in the learned helplessness animal model of depression.

Effects of repeated administration of baclofen on transient lower esophageal sphincter relaxation in the dog

The metabotropic gamma-aminobutyric acid (GABA) receptor (GABA(B) receptor) agonist baclofen inhibits transient lower esophageal sphincter relaxation in dogs, ferrets, and humans. Since transient lower esophageal sphincter relaxations are the major cause of gastroesophageal reflux, GABA(B) receptor agonists may have a therapeutic value in the treatment of gastroesophageal reflux disease. However, repeated stimulation of the GABA(B) receptor may induce receptor desensitization which, depending on the magnitude, would limit the therapeutic effect. The aim of the present study was to follow the effects of baclofen on transient lower esophageal sphincter relaxation in the dog after repeated administration. The effect of 7 micromol/kg baclofen b.i.d. (given intragastrically) on transient lower esophageal sphincter relaxation and related parameters was determined in four dogs. Transient lower esophageal sphincter relaxations stimulated by infusion of liquid nutrient and insufflation of air were quantified after placebo and then after the 1st, 13th, and 27th dose. Baclofen reduced the number of transient lower esophageal sphincter relaxations without affecting their duration, and the latency to the first transient lower esophageal sphincter relaxation was prolonged. Basal sphincter pressure was unaffected by baclofen, and the number of reflux episodes and esophageal acid exposure decreased. There was a statistically insignificant numerical decrease (approximately 30%) in the effect of baclofen on transient lower esophageal sphincter relaxation after the seventh dose but this was not further accentuated after the 27th dose. The effect on latency was also reduced with repeated dosing, but again, the effects after the 1st, 13th, and 27th doses were not statistically significant. The attenuation of acid exposure and reflux episodes was unaltered after repeated dosing. Three of the dogs greatly reduced their food intake within the first 2-3 days but this side effect was resolved subsequently. It is concluded that repeated dosing of baclofen leads to mild tolerance development in terms of the effects on transient lower esophageal sphincter relaxation, but that the tolerance is much less pronounced than that previously reported in other animal models.

Rapid compensatory changes in GABA receptor efficacy in rat vestibular neurones after unilateral labyrinthectomy

1. The inhibitory effects of the GABAA agonist muscimol and the GABAB agonist baclofen on tonically active medial vestibular nucleus (MVN) neurones were recorded in slices of the rat dorsal brainstem in vitro, to determine whether any changes occurred in the functional efficacy of GABAergic inhibition in these cells during the initial rapid stage of 'vestibular compensation', the behavioural recovery that takes place after unilateral labyrinthectomy (UL). These experiments were carried out in preparations where the midline was cut, severing all commissural connections between the two vestibular nuclei. 2. Slices of the MVN were prepared from normal animals and animals that had been unilaterally labyrinthectomised 4 h earlier. The mean in vitro discharge rate of MVN neurones in the rostral region of the ipsi-lesional nucleus after UL was significantly higher than that in control slices, confirming our earlier reports of an increase in intrinsic excitability of these cells in the early stage of vestibular compensation. The in vitro discharge rates of caudal ipsi-lesional MVN cells, and rostral and caudal contra-lesional MVN cells, were not different from controls. 3. Muscimol and baclofen caused reversible, dose-related inhibition of the tonic discharge rate of MVN cells in control slices. In slices prepared from UL animals, MVN cells in the rostral region of the ipsi-lesional nucleus showed a marked downregulation of their response to both muscimol and baclofen, seen as a rightward shift and a decrease in slope of the dose-response relationships for the two agonists. In the contra-lesional nucleus, there was a small but significant upregulation of the responsiveness of both rostral and caudal MVN cells to baclofen, and a marked upregulation of the responsiveness of caudal MVN cells to muscimol. 4. In slices from animals that had undergone bilateral labyrinthectomy 4 h earlier, the downregulation of the functional efficacy of GABA receptors in the rostral MVN cells did not occur. The changes in GABA receptor efficacy after UL are therefore not due to the vestibular de-afferentation itself, but are instead due to the imbalance in excitability of the vestibular nuclei of the lesioned and intact sides, and the enhanced commissural inhibition of the ipsi-lesional MVN cells that follows UL. 5. The downregulation of GABA receptor efficacy in the ipsi-lesional MVN neurones is functionally compensatory, in that their response to commissural and cerebellar inhibitory drive will be significantly reduced after UL. Their intrinsic membrane conductances, and their remaining excitatory synaptic inputs, will consequently be more effective in causing depolarisation and the restoration of resting activity. Simultaneously the upregulation of GABAergic efficacy in the contra-lesional MVN will tend to reduce the hyperactivity on the contralateral side. These adaptive changes therefore represent a plausible cellular mechanism for the recovery of resting discharge in the ipsi-lesional MVN neurones, and the 're-balancing' of the excitability of the vestibular neurones of the lesioned and intact sides, as occurs after UL in vivo. 6. We propose that the adaptive regulation of the functional efficacy of GABA receptors in the MVN neurones may be an important cellular mechanism for the 'homeostasis of bilateral excitability' of the vestibular nuclei of the two sides.

The GABA(B) receptor antagonist CGP36742 improves learned helplessness in rats

Effects of 3-aminopropyl-n-butyl-phosphinic acid (CGP36742), a GABA(B) receptor antagonist, in the learned helplessness paradigm were examined in rats in comparison with those of imipramine and endo-8-methyl-8-azabicyclo[3,2,1]oct-3-ol indol-3-yl-carboxylate hydrochloride (ICS205-930). Rats were treated with CGP36742, imipramine or ICS205-930 for 14 days. On day 14, the rats were subjected to 90 inescapable shocks. On day 15, the rats received the 40-trial escape test. The inescapable shocks increased escape failures in the escape test. CGP36742, imipramine and ICS205-930 dose-dependently improved the escape failures induced by the inescapable shocks. Baclofen attenuated the escape failures-improving effect of CGP36742, imipramine and ICS205-930. Although the action of imipramine and ICS205-930 was attenuated by 1-(m-chlorophenyl)-biguanide (mCPBG), mCPBG failed to influence the CGP36742 action. Therefore, it is suggested that CGP36742 may have an antidepressant profile and that the mechanisms of CGP36742 in antidepressant action may be different from those of imipramine and ICS205-930.

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.

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.

Effect of a novel cognition enhancer NS-105 on learned helplessness in rats: possible involvement of GABA(B) receptor up-regulation after repeated treatment

We have previously found that a cognition enhancer [(+)-5-oxo-D-prolinepiperidinamide monohydrate] (NS-105) reversed the inhibition of cyclic AMP formation induced by the GABA(B) receptor agonist baclofen. The GABA(B) receptor has been implicated in the pathophysiology of depressive illness. The present experiment was designed to evaluate the antidepressant activity of NS-105 in the forced swimming and learned helplessness tests in rats. NS-105 (1-100 mg/kg, p.o.) significantly decreased immobility time in the forced swimming test, an effect similar to that of desipramine. Repeated administration of NS-105 also reversed the failure to escape in the shuttle-box test of rats previously exposed to inescapable footshock. Biochemical data showed that repeated administration of NS-105 increased the number of GABA(B) receptors in rat cerebral cortex without affecting the binding properties of beta-adrenoceptors and 5-HT2 receptors. In contrast to other antidepressants, NS-105 did not inhibit monoamine uptake in vitro, nor did it change monoamine concentrations in brain tissues or extracellular fluids. These findings suggest that NS-105, which lacks an effect on monoaminergic systems, has potent antidepressant activity, which may involve up-regulation of GABA(B) receptors after repeated administration.

Mechanism of colchicine impairment on learning and memory, and protective effect of CGP36742 in mice

Fourteen days after hippocampal microinfusion with colchicine (COL), learning and memory ability of mice was significantly impaired, while glutamate (Glu), gamma-aminobutyric acid (GABA), Glu/GABAB and GABAB receptor levels in the cortex and/or the hippocampus were significantly changed. After treatment with a GABAB receptor antagonist, CGP36742, learning and memory impairment caused by COL could be significantly improved, and the above indices in brain regions reversed. These results suggest GABAB antagonists may have therapeutic value in the treatment of Alzheimer's disease.

Involvement of GABAB receptor systems in experimental depression: baclofen but not bicuculline exacerbates helplessness in rats

There are two gamma-aminobutyric acid (GABA) hypotheses of the antidepressants action: an increase in GABAA neurotransmission or a decrease in GABAB neurotransmission may contribute to action of antidepressants. In this study, involvement of GABAA and GABAB receptor systems was examined in the learned helplessness paradigm in rats. Rats were injected with bicuculline or baclofen for 14 days. On day 14, the rats were subjected to 15 inescapable shocks. On day 15, they underwent the 40-trial escape test. Baclofen exacerbated the escape failures in the rats subjected to the inescapable shocks, although baclofen had no effects in the animals without shock pre-treatment. Bicuculline failed to influence the escape failures in the rats with the 15-shock pre-treatment. These results suggest that the long-term increase in GABAB neurotransmission but not the long-term attenuation of GABAA neurotransmission may be related to helplessness in rats.

Involvement of GABAB receptor systems in action of antidepressants. II: Baclofen attenuates the effect of desipramine whereas muscimol has no effect in learned helplessness paradigm in rats

Involvement of GABAergic systems in action of antidepressants was examined in the learned helplessness paradigm in rats. Rats were treated with desipramine, baclofen or muscimol for 14 days. On day 14, the rats were subjected to 90 inescapable shocks. On day 15, the rats received the 40-trial escape test. The inescapable shocks induced the subsequent increase in escape failures in the escape test. Desipramine dose-dependently improved the increased escape failures induced by the inescapable shocks. Baclofen attenuated the escape failures-improving effect of desipramine, although baclofen had no effects on the increased escape failures when it was injected alone. Muscimol at any dose failed to influence the increased escape failures. Therefore, it is suggested that the long-term decrease in GABAB neurotransmission may be involved in the action of antidepressants. Our present results do not support the hypothesis that activation of GABAA receptors may contribute to the action of antidepressants.

Involvement of GABA(B) receptor systems in action of antidepressants: baclofen but not bicuculline attenuates the effects of antidepressants on the forced swim test in rats

Involvement of GABAergic systems in action of antidepressants was examined in the forced swim test in rats. Rats were forced to swim in a cylinder for 15 min on day 1 and for 5 min on day 2. Desipramine, mianserin and buspirone, administered after the 15-min swim session on day 1 and before the 5-min swim test on day 2, dose-dependently decreased the duration of immobility in the swim test on day 2. Baclofen attenuated the decreased duration of immobility induced by desipramine, mianserin and buspirone in the swim test, although baclofen did not affect the duration of immobility when it was injected alone. Muscimol dose-dependently decreased the duration of immobility in the swim test on day 2. Bicuculline antagonized the decreased duration of immobility induced by muscimol. However, bicuculline failed to antagonize the decreased duration of immobility induced by desipramine, mianserin and buspirone. These results suggest that GABA(B) but not GABA(A) receptor systems may be involved in action of antidepressants.

Complex changes of GABAA and GABAB receptor binding in the spinal cord dorsal horn following peripheral inflammation or neurectomy

Chronic peripheral inflammation or peripheral neurectomy cause changes in GABA levels and GABA immunoreactivity in the spinal cord dorsal horn. The present study aimed to investigate if such changes are accompanied by alterations in GABA receptor binding. Neurectomy of the sciatic nerve caused an ipsilateral down-regulation of GABAB receptor binding in lamina II of the spinal cord 2-4 weeks after the nerve injury. Since approximately 50% of GABAB receptor binding in that region is located on primary afferent endings, degenerative changes of such endings caused by the nerve lesion can explain the observed reduction. In contrast, GABAA binding was substantially enhanced following neurectomy, which may be due to an up-regulation of the receptors issued by the concomitant decrease of endogenous GABA. In rats bearing unilateral chronic peripheral inflammation induced by intraarticular injection of complete Freund's adjuvant we found a reduction of GABAB binding in the superficial dorsal horn. This effect, which was maximal at 3-4 weeks after adjuvant injection, was attributed to an enhanced release of GABA by spinal interneurons. GABAA receptor binding was not changed in this experimental model. Together, these results suggest that the two receptor types may be located at different loci and are differently affected by variations in sensory input.

Antidepressant-induced modulation of GABAA receptors and beta-adrenoceptors but not GABAB receptors in the frontal cortex of olfactory bulbectomised rats

The effects of prolonged administration of antidepressant drugs, belonging to three different classes, on high-affinity GABAA receptor, GABAB receptor and beta-adrenoceptor binding parameters were determined in the frontal cortex of olfactory bulbectomised rats. Clorgyline (1 mg/kg/day), paroxetine (10 mg/kg/day) or desipramine (10 mg/kg/day) were administered for 21 days via subcutaneous osmotic minipumps implanted in the scapular region 7 days after bulbectomy. Cortical GABAA receptor densities, defined with [3H]gamma-aminobutyric acid ([3H]GABA), were significantly increased following bulbectomy. This effect on Bmax values was reversed by all three antidepressant drugs. GABAB receptor densities decreased slightly after bulbectomy. Chronic antidepressant administration had no effect on GABAB receptor binding parameters. Olfactory bulbectomy did not induce any changes in cortical beta-adrenoceptor binding parameters determined with [3H]CGP-12177 ((-)-4-(3-t- butylamino-2-hydroxypropxy)- [5,7-3H]benzimidazol-2-one). However, prolonged administration of all three antidepressant drugs induced a downregulation of beta-adrenoceptors. The results of the present study confirm the involvement of cortical GABAA rather than GABAB receptors in the olfactory bulbectomy animal model of human depression. Moreover, the data further support the hypothesis that a decrease in function of the GABAA receptor complex could play a role in the therapeutic effects of antidepressant treatments.