GABAB Receptors and Depression: Current Status

Recombinant human growth hormone affects the density and functionality of GABAB receptors in the male rat brain

The beneficial effects of growth hormone (GH) on memory and learning have previously been confirmed in both humans and in animal models. An important role of GABAB receptors for multiple forms of learning and memory has also been reported. In this study, we examined the effect of GH on the density and functionality of the metabotropic GABAB receptors in the rat brain. Male Sprague-Dawley rats (n = 24) divided into 3 groups were injected twice daily with recombinant human GH (0.07 or 0.7 IU/kg) for 7 days. The effects of the hormone were determined by quantitative autoradiography and by GABAB stimulated [(35)S]-GTPγS binding using the selective GABAB receptor agonist baclofen. The results demonstrate moderate but significant alterations in both receptor density and functionality in a number of brain regions. For example, a dose-dependent upregulation of GABAB receptors was found in the cingulate cortex, primary motor cortex and caudate putamen, whereas attenuation in the receptor density was encountered in, for example, the medial geniculate nucleus. Although the GH-induced effects on the GABAB receptor in brain areas associated with cognition were fairly pronounced, they were significant and we propose that the physiological responses observed after GH administration at least partly can be mediated through a mechanism involving GABAB receptors.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

On communication between gut microbes and the brain

PURPOSE OF REVIEW

Interest in the microbiota-gut-brain axis is increasing apace and what was, not so long ago, a hypothetical relationship is emerging as a potentially critical factor in the regulation of intestinal and mental health. Studies are now addressing the neural circuitry and mechanisms underlying the influence of gut bacteria on the central nervous system and behavior.

RECENT FINDINGS

Gut bacteria influence development of the central nervous systems (CNS) and stress responses. In adult animals, the overall composition of the microbiota or exposure to specific bacterial strains can modulate neural function, peripherally and centrally. Gut bacteria can provide protection from the central effects of infection and inflammation as well as modulate normal behavioral responses. Behavioral effects described to date are largely related to stress and anxiety and an altered hypothalamus-pituitary-adrenal axis response is a common observation in many model systems. The vagus nerve has also emerged as an important means of communicating signals from gut microbes to the CNS.

SUMMARY

Studies of microbiota-gut-brain communication are providing us with a deeper understanding of the relationship between the gut bacteria and their hosts while also suggesting the potential for microbial-based therapeutic strategies that may aid in the treatment of mood disorders.

Control of alternative splicing by forskolin through hnRNP K during neuronal differentiation

The molecular basis of cell signal-regulated alternative splicing at the 3′ splice site remains largely unknown. We isolated a protein kinase A-responsive ribonucleic acid (RNA) element from a 3′ splice site of the synaptosomal-associated protein 25 (Snap25) gene for forskolin-inhibited splicing during neuronal differentiation of rat pheochromocytoma PC12 cells. The element binds specifically to heterogeneous nuclear ribonucleo protein (hnRNP) K in a phosphatase-sensitive way, which directly competes with the U2 auxiliary factor U2AF65, an essential component of early spliceosomes. Transcripts with similarly localized hnRNP K target motifs upstream of alternative exons are enriched in genes often associated with neurological diseases. We show that such motifs upstream of the Runx1 exon 6 also bind hnRNP K, and importantly, hnRNP K is required for forskolin-induced repression of the exon. Interestingly, this exon encodes the peptide domain that determines the switch of the transcriptional repressor/activator activity of Runx1, a change known to be critical in specifying neuron lineages. Consistent with an important role of the target genes in neurons, knocking down hnRNP K severely disrupts forskolin-induced neurite growth. Thus, through hnRNP K, the neuronal differentiation stimulus forskolin targets a critical 3′ splice site component of the splicing machinery to control alternative splicing of crucial genes. This also provides a regulated direct competitor of U2AF65 for cell signal control of 3′ splice site usage.

Reconsidering GHB: orphan drug or new model antidepressant?

For six decades, the principal mode of action of antidepressant drugs is the inhibition of monoamine re-uptake from the synaptic cleft. Tricyclic antidepressants, selective serotonin re-uptake inhibitors (SSRIs) and the new generation of dual antidepressants all exert their antidepressant effects by this mechanism. In the early days of the monoaminergic era, other efforts have been made to ameliorate the symptoms of depression by pharmacological means. The gamma-aminobutyric acid (GABA) system was and possibly still is one of the main alternative drug targets. Gammahydroxybutyrate (GHB) was developed as an orally active GABA analogue. It was tested in animal models of depression and human studies. The effects on sleep, agitation, anhedonia and depression were promising. However, the rise of benzodiazepines and tricyclic antidepressants brought GHB out of the scope of possible treatment alternatives. GHB is a GABA(B) and GHB receptor agonist with a unique spectrum of behavioural, neuroendocrine and sleep effects, and improves daytime sleepiness in various disorders such as narcolepsy, Parkinson's disease and fibromyalgia. Although it was banned from the US market at the end of the 1990s because of its abuse and overdose potential, it later was approved for the treatment of narcolepsy. New research methods and an extended view on other neurotransmitter systems as possible treatment targets of antidepressant treatment brought GHB back to the scene. This article discusses the unique neurobiological effects of GHB, its misuse potential and possible role as a model substance for the development of novel pharmacological treatment strategies in depressive disorders.

Behavioural consequences of two chronic psychosocial stress paradigms: anxiety without depression

Chronic stress, in particular chronic psychosocial stress, is a risk factor in the aetiology of various psychopathologies including anxiety- and depression-related disorders. Therefore, recent studies have focussed on the development of social-stress paradigms, which are believed to be more relevant to the human situation than non-social-stress paradigms. The majority of these paradigms have been reported to increase both anxiety- and depression-related behaviour in rats or mice. However, in order to dissect the mechanisms underlying anxiety or depression, animal models are needed, which specifically induce one, or the other, phenotype. Here, we study both short- (1d after stressor termination) and long-term (4d or 7d after stressor termination) behavioural and physiological consequences of two well-validated chronic psychosocial stress models: social-defeat/overcrowding (SD/OC) and chronic subordinate colony housing (CSC). We demonstrate that SD/OC and CSC result in different physiological alterations: SD/OC more strongly affecting body-weight development, whereas CSC more strongly affects adrenal and pituitary morphology. Both stressors were shown to flatten circadian locomotor activity immediately after stress termination, which normalized 7d later in SD/OC group but reversed to hyperactivity during the dark phase in the CSC group. Importantly, neither stress paradigm resulted in an increase in depression-related behaviour as assessed using the forced swim test, tail suspension test and saccharin preference test at any time-point. However, both stress paradigms lead to an anxiogenic phenotype; albeit with different temporal profiles and not towards a novel con-specific (social anxiety). CSC exposure elevates anxiety-related behaviour immediately after stressor termination, which lasts for at least 1 wk. In contrast, the anxiogenic phenotype only develops 1 wk after SD/OC termination. In conclusion, both models are unique for uncovering the molecular underpinnings of anxiety-related behaviour without conflicting depression-based alterations.

Epigenetics and brain evolution

Fundamental aspects of mammalian brain evolution occurred in the context of viviparity and placentation brought about by the epigenetic regulation of imprinted genes. Since the fetal placenta hormonally primes the maternal brain, two genomes in one individual are transgenerationally co-adapted to ensure maternal care and nurturing. Advanced aspects of neocortical brain evolution has shown very few genetic changes between monkeys and humans. Although these lineages diverged at approximately the same time as the rat and mouse (20 million years ago), synonymous sequence divergence between the rat and mouse is double that when comparing monkey with human sequences. Paradoxically, encephalization of rat and mouse are remarkably similar, while comparison of the human and monkey shows the human cortex to be three times the size of the monkey. This suggests an element of genetic stability between the brains of monkey and man with a greater emphasis on epigenetics providing adaptable variability.

Modeling mitochondrial dysfunctions in the brain: from mice to men

The biologist Lewis Thomas once wrote: "my mitochondria comprise a very large proportion of me. I cannot do the calculation, but I suppose there is almost as much of them in sheer dry bulk as there is the rest of me". As humans, or indeed as any mammal, bird, or insect, we contain a specific molecular makeup that is driven by vast numbers of these miniscule powerhouses residing in most of our cells (mature red blood cells notwithstanding), quietly replicating, living independent lives and containing their own DNA. Everything we do, from running a marathon to breathing, is driven by these small batteries, and yet there is evidence that these molecular energy sources were originally bacteria, possibly parasitic, incorporated into our cells through symbiosis. Dysfunctions in these organelles can lead to debilitating, and sometimes fatal, diseases of almost all the bodies' major organs. Mitochondrial dysfunction has been implicated in a wide variety of human disorders either as a primary cause or as a secondary consequence. To better understand the role of mitochondrial dysfunction in human disease, a multitude of pharmacologically induced and genetically manipulated animal models have been developed showing to a greater or lesser extent the clinical symptoms observed in patients with known and unknown causes of the disease. This review will focus on diseases of the brain and spinal cord in which mitochondrial dysfunction has been proven or is suspected and on animal models that are currently used to study the etiology, pathogenesis and treatment of these diseases.

Forward genetic approaches to understanding complex behaviors

Assigning function to genes has long been a focus of biomedical research.Even with complete knowledge of the genomic sequences of humans, mice and other experimental organisms, there is still much to be learned about gene function and control. Ablation or overexpression of single genes using knockout or transgenic technologies has provided functional annotation for many genes, but these technologies do not capture the extensive genetic variation present in existing experimental mouse populations. Researchers have only recently begun to truly appreciate naturally occurring genetic variation resulting from single nucleotide substitutions,insertions, deletions, copy number variation, epigenetic changes (DNA methylation,histone modifications, etc.) and gene expression differences and how this variation contributes to complex phenotypes. In this chapter, we will discuss the benefits and limitations of different forward genetic approaches that capture the genetic variation present in inbred mouse strains and present the utility of these approaches for mapping QTL that influence complex behavioral phenotypes.

Stress and animal models of inflammatory bowel disease--an update on the role of the hypothalamo-pituitary-adrenal axis

Chronic psychosocial stress has been repeatedly shown in humans to be a risk factor for the development of several affective and somatic disorders, including inflammatory bowel diseases (IBD). There is also a large body of evidence from rodent studies indicating a link between stress and gastrointestinal dysfunction, resembling IBD in humans. Despite this knowledge, the detailed underlying neuroendocrine mechanisms are not sufficiently understood. This is due, in part, to a lack of appropriate animal models, as most commonly used rodent stress paradigms do not adequately resemble the human situation and/or do not cause the development of spontaneous colitis. Therefore, our knowledge regarding the link between stress and IBD is largely based on rodent models with low face and predictive validity, investigating the effects of unnatural stressors on chemically induced colitis. These studies have consistently reported that hypothalamo-pituitary-adrenal (HPA) axis activation during stressor exposure has an ameliorating effect on the severity of a chemically induced colitis. However, to show the biological importance of this finding, it needs to be replicated in animal models employing more clinically relevant stressors, themselves triggering the development of spontaneous colitis. Important in view of this, recent studies employing chronic/repeated psychosocial stressors were able to demonstrate that such stressors indeed cause the development of spontaneous colitis and, thus, represent promising tools to uncover the mechanisms underlying stress-induced development of IBD. Interestingly, in these models the development of spontaneous colitis was paralleled by decreased anti-inflammatory glucocorticoid (GC) signaling, whereas adrenalectomy (ADX) prior to stressor exposure prevented its development. These findings suggest a more complex role of the HPA axis in the development of spontaneous colitis. In the present review I summarize the available human and rodent data in order to provide a comprehensive understanding of the biphasic role of the HPA axis and/or the GC signaling during stressor exposure in terms of spontaneous colitis development.

The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?

Hypotheses are scaffoldings erected in front of a building and then dismantled when the building is finished. They are indispensable for the workman; but you mustn't mistake the scaffolding for the building. Johann Wolfgang von Goethe. The neurogenesis hypothesis of affective disorders - in its simplest form - postulates that the generation of neurons in the postnatal hippocampal dentate gyrus is involved in the etiology and treatment efficacy of major depressive disorder (MDD). The hypothesis was established in the 1990s but was built on a broad foundation of earlier research on the hippocampus, serotonin and MDD. It has gone through several growth phases fueled by discoveries both correlative and causative in nature. Recently, the hypothesis has also been broadened to also include potential relevance for anxiety disorders, like post-traumatic stress disorder (PTSD). As any hypothesis should be, it has been tested and challenged, sometimes vigorously. Here we review the current standing of the neurogenesis hypothesis of affective and anxiety disorders, noting in particular how a central postulate - that decreased neurogenesis results in depression or anxiety - has, in general, been rejected. We also review the controversies on whether treatments for these disorders, like antidepressants, rely on intact neurogenesis for their efficacy, and the existence of neurogenesis-dependent and -independent effects of antidepressants. In addition, we review the implications that the hypothesis has for the response to stress, PTSD, and the neurobiology of resilience, and highlight our own work showing that adult-generated neurons are functionally important for the behavioral response to social stress. We conclude by emphasizing how advancements in transgenic mouse technology, rodent behavioral analyses, and our understanding of the neurogenesis process will allow us to refine our conclusions and perform ever more specific experiments. Such scrutiny is critical, since if we "mistake the scaffolding for the building" we could overlook opportunities for translational impact in the clinic. This article is part of a special Issue entitled 'Anxiety and Depression'.

Late-life depression and Alzheimer's disease: the glutamatergic system inside of this mirror relationship

Late-life depressive syndromes often arise in the context of predementia, dementia syndromes, and Alzheimer's disease (AD). Conversely, patients with a history of mood disorders are at higher risk of developing cognitive impairment. The high rate of co-occurrence of these two disorders is becoming a major health problem in older subjects for both their epidemiological impact and the negative outcomes in terms of disability and increased mortality. In this perspective, it is possible to speculate on the presence of a mirror relationship between depressive and cognitive disorders in late-life. Indeed, although a causal contribution of genetic, environmental, and social factors is widely recognized in these disorders, the neurobiological links still remain largely unknown. l-glutamic acid and γ-aminobutyric acid are the principal excitatory and inhibitory neurotransmitters in the central nervous system, respectively, and increasing evidence suggests that alterations in this neurotransmitter system may contribute to the neurobiology linking depression and cognitive impairment. In the present review article, we examined the neurobiological bases of the relationship between late-life depressive syndromes and AD, with a particular attention to glutamatergic pathway signalling like a bridge connecting these two conditions. In addition, attempts have been made to explain changes in glutamatergic pathway, depression in older age, and dementia through the analysis of signal transduction mechanisms associated with these disabling disorders.

Both GABA(B) receptor activation and blockade exacerbated anhedonic aspects of nicotine withdrawal in rats

Nicotine dependence is maintained by the aversive, depression-like effects of nicotine withdrawal and the rewarding effects of acute nicotine. GABA(B) receptor antagonists exhibit antidepressant-like effects in rodents, whereas GABA(B) receptor agonists attenuate the rewarding effects of nicotine. Recent studies with GABA(B) receptor positive modulators showed that these compounds represent potentially improved medications for the treatment of nicotine dependence because of fewer side-effects than GABA(B) receptor agonists. Thus, GABA(B) receptor agonists and antagonists, and GABA(B) receptor positive modulators may have efficacy as smoking cessation aids by targeting different aspects of nicotine dependence and withdrawal. The present study assessed the effects of the GABA(B) receptor agonist CGP44532, the GABA(B) receptor antagonist CGP56433A, and the GABA(B) receptor positive modulator BHF177 on the anhedonic aspects of nicotine withdrawal. Rats were prepared with stimulating electrodes in the posterior lateral hypothalamus. After establishing stable intracranial self-stimulation (ICSS) thresholds, rats were prepared with subcutaneous osmotic minipumps delivering either nicotine or saline for 7 or 14days. ICSS thresholds were assessed 6h post-pump removal. Thirty hours after pump removal, CGP44532, CGP56433A, and BHF177 were administered 30min prior to ICSS testing. Both GABA(B) receptor activation (CGP44532 and BHF177) and blockade (CGP56433A) elevated ICSS thresholds in all groups, resulting in exacerbated effects of nicotine withdrawal in the nicotine-treated groups. These similar effects of GABA(B) receptor activation and blockade on the anhedonic depression-like aspects of nicotine withdrawal were surprising and perhaps reflect differential efficacy of these compounds at presynaptic hetero- and autoreceptors, as well as postsynaptic, GABA(B) receptors.

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.