Regulation of signal transduction pathways and gene expression by mood stabilizers and antidepressants

Signaling networks in the pathophysiology and treatment of mood disorders

Over the past decade, the focus of research into the pathophysiology of mood disorders (bipolar disorder and unipolar depression in particular) has shifted from an interest in the biogenic amines to an emphasis on second messenger systems within cells. Second messenger systems rely on cell membrane receptors to relay information from the extracellular environment to the interior of the cell. Within the cell, this information is processed and altered, eventually to the point where gene and protein expression patterns are changed. There is a preponderance of evidence implicating second messenger systems and their primary contact with the extracellular environment, G proteins, in the pathophysiology of mood disorders. After an introduction to G proteins and second messenger pathways, this review focuses on the evidence implicating G proteins and two second messenger systems-the adenylate cyclase (cyclic adenosine monophosphate, cAMP) and phosphoinositide (protein kinase C, PKC) intracellular signaling cascades-in the pathophysiology and treatment of bipolar disorder and unipolar depression. Emerging evidence implicates changes in cellular resiliency, neuroplasticity and additional cellular pathways in the pathophysiology of mood disorders. The systems discussed within this review have been implicated in neuroplastic processes and in modulation of many other cellular pathways, making them likely candidates for mediators of these findings.

Association analysis of a polymorphism in the G-protein stimulatory alpha subunit in patients with major depression

Growing evidence suggests that G-proteins may be involved in pathogenesis and treatment of affective disorders. Several studies have reported altered levels and/or activities of stimulatory G-proteins in depression. The aim of this study was to investigate whether a polymorphism in the stimulatory alpha subunit of G-proteins (T/C point mutation in exon 5; ATT --> ATC at codon 131) is associated with major depression or response to antidepressant treatment. Therefore, we performed a case-control association study with 212 depressive patients and 137 healthy, unrelated controls. There was no evidence for an association between the investigated polymorphism in the G(alpha)(s) gene and major depression, as well as to treatment response. The results of our study are in concordance with recently published findings which do not support the hypothesis that the gene for the stimulatory alpha subunit of G-proteins is a major susceptibility factor in the pathophysiology of major depression.

Anticonvulsants: aspects of their mechanisms of action

An ideal anticonvulsant drug would prevent or inhibit excessive pathological neuronal discharge without interfering with physiological neuronal activity and without producing untoward effects. Such an ideal compound is not yet available. However, during the last few years several new anticonvulsants have appeared (e.g. vigabatrine, gabapentin, topiramate, lamotrigine, tiagabine, felbamate and oxcarbazepine) which may challenge the older, more established substances (i.e. phenytoin, benzodiazepines, phenobarbital, valproate, carbamazepine and ethosuximide). Interestingly, several of the old and new anticonvulsants are beneficial in the treatment of various psychiatric conditions (most notably mood disorders) as well as neuropathic pain. The reason these various drugs are effective in the treatment of such disparate clinical conditions is unknown. The answer may be that the neuronal dysfunctions underlying these conditions are similar in a mechanistic sense, but are manifested in different neurons/locations of the nervous system, or that the drugs possess several mechanisms of action that contribute in different ways to the favourable effect depending on the condition studied. Even though all these drugs inhibit excessive neuronal activity, this acute effect appears to be produced by several mechanisms, which fall into three major categories: (1) blockade of voltage-gated sodium channels; (2) indirect or direct enhancement of inhibitory gamma-aminobutyric acid [GABAergic] neurotransmission; or (3) inhibition of excitatory glutamatergic neurotransmission. Moreover, several of these drugs fall into more than one category, and it is often unclear which category is responsible for a given effect of a drug. It is plausible that some of the beneficial effects observed in the clinic can be explained by the secondary neural depressant mechanisms of action of these substances, whereas other benefits may be due to long-term neuroplastic effects, which may either be common or different across the various conditions treated.

Analysis of polymorphisms in the olfactory G-protein Golf in major depression

It is well established that G-proteins represent essential regulatory components in transmembrane signaling. The alpha subunit of the olfactory G-protein Golf (GNAL) maps to a region on chromosome 18 where linkage to affective disorders has been reported, as well as a parent-of-origin effect in affective disorders with some markers near the locus for the alpha subunit of the Golf gene. We investigated whether two polymorphisms in the alpha subunit of the Golf gene (A-->G in intron 3, and T-->G in intron 10) are associated with major depression in 176 major depressive patients compared with 145 healthy control subjects, and additionally tested for a parent-of-origin effect in separated gender groups. In the control group, we found a significant increase in the G-allele frequency of the intron 3 polymorphism in females (P=0.0036, odds ratio=2.13, 95% confidence interval=1.29-3.54, Fisher's Exact Test). In patients, we found a similar tendency for higher G-allele frequencies in females. Concerning the intron 10 polymorphism, no differences in the genotype or allele frequencies were detectable for any of the separated gender groups. Also, the total patient and control groups showed no differences in allele or genotype frequencies for any of the investigated polymorphisms. The results of this study agree with the reported parent-of-origin effects on chromosome 18, but do not support the hypothesis that the Golf gene is a major susceptibility factor for major depression.

The effects of antidepressant drug treatments on activator protein-1 binding activity in the rat brain

Since a long-term administration of antidepressant drugs and mood stabilizers is required in the treatment of mood disorders, the regulation of gene expression by these drugs that is mediated by transcription factors, such as activator protein-1 (AP-1) complex, may play an important role in the therapeutic action. In this study, the authors investigated the influence of lithium, antidepressant drugs and stress on AP-1 binding activity in the rat brain. In addition, we examined pretreatment with these drugs on the expression of AP-1 binding activity in response to stress. A gel shift assay was used to measure the levels of AP-1 binding activity. Our results indicate that neither acute nor chronic treatment with antidepressant drugs affects in AP-1 binding activity in the rat frontal cortex or hippocampus. However, the authors found that acute restraint stress for 90 min upregulated the induction of AP-1 binding activity in the rat frontal cortex. In addition, chronic pretreatment with imipramine, but not lithium or paroxetine, downregulated the induction of AP-1 binding activity in response to acute restraint stress in the frontal cortex. The functional classification of antidepressant drugs based on the downregulation of restraint stress-induced AP-1 binding activity may contribute to the advances in our understanding of the pathogenesis of depression.

Psychopharmacogenetics--a challenge for pharmacotherapy in psychiatry

Differences in response to treatment or the incidence of adverse drug effects are quite common in clinical psychopharmacotherapy. Although several factors may account for these discrepancies, there is increasing knowledge that genetic factors play a major role. The aim of pharmacogenetics, a new and rapidly growing field in research, is to elucidate the variability in drug response and metabolism due to hereditary differences. According to the hypotheses on the mechanisms of drug action, several mutations in genes coding for neurotransmitter receptors, degrading enzymes, transport proteins or enzymes of the drug metabolizing system (P-450 isoenzymes) have been identified and investigated in psychiatric disorders over the last years. Although some controversy exists among the results, many studies are supportive of the hypothesis that psychopharmacogenetics will be helpful in predicting an individual patient's drug response while minimising the rate of side effects.

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Clinical and preclinical evidence for the neurotrophic effects of mood stabilizers: implications for the pathophysiology and treatment of manic-depressive illness

Recent neuroimaging studies have demonstrated regional central nervous system volume reductions in mood disorders, findings that are complemented by postmortem observations of cell atrophy and loss. It is thus noteworthy that lithium and valproate have recently been demonstrated to robustly increase the expression of the cytoprotective protein bcl-2 in the central nervous system. Chronic lithium not only exerts neuroprotective effects in several preclinical paradigms but also enhances hippocampal neurogenesis. Valproate robustly promotes neurite outgrowth and activates the ERK mitogen-activated protein kinase pathway, a signaling pathway utilized by many endogenous neurotrophic factors. Consistent with its preclinical neurotrophic/neuroprotective effects, chronic lithium treatment of patients with manic-depressive illness increases brain N-acetylaspartate (a putative marker of neuronal viability and function) levels, an effect that is localized almost exclusively to gray matter. To determine if lithium was producing neuropil increases, quantitative three-dimensional magnetic resonance imaging studies were undertaken, which revealed that chronic lithium significantly increases total gray matter volume in the human brain of patients with manic-depressive illness. Together, these results suggest that a reconceptualization about the optimal long-term treatment of recurrent mood disorders is warranted. Optimal long-term treatment for these severe illnesses may only be achieved by the early use of agents with neurotrophic/neuroprotective effects, irrespective of the primary, symptomatic treatment.

Signaling: cellular insights into the pathophysiology of bipolar disorder

Clinical studies over the years have provided evidence that monoamine signaling and hypothalamic-pituitary-adrenal axis disruption are integral to the pathophysiology of bipolar disorder. A full understanding of the pathophysiology from a molecular to a systems level must await the identification of the susceptibility and protective genes driving the underlying neurobiology of bipolar disorder. Furthermore, the complexity of the unique biology of this affective disorder, which includes the predisposition to episodic and often progressive mood disturbance, and the dynamic nature of compensatory processes in the brain, coupled with limitations in experimental design, have hindered our progress to date. Imaging studies in patient populations have provided evidence of a role for anterior cingulate, amygdala, and prefrontal cortex in the pathophysiology of bipolar disorder. More recent research strategies designed to uncover the molecular mechanisms underlying our pharmacologic treatments and their interaction in the regulation of signal transduction as well as more advanced brain imaging studies remain promising approaches. This experimental strategy provides data derived from the physiologic response of the system in affected individuals and addresses the critical dynamic interaction with pharmacologic agents that effectively modify the clinical expression of the pathophysiology.