Adenylate cyclase activity in postmortem brain of suicide subjects: reduced response to beta-adrenergic stimulation

Increased α2- and β1-adrenoceptor densities in postmortem brain of subjects with depression: differential effect of antidepressant treatment

BACKGROUND

Brain α2- and β-adrenoceptor alterations have been suggested in suicide and major depressive disorder.

METHODS

The densities of α2-, β1- and β2-adrenoceptors in postmortem prefrontal cortex of 26 subjects with depression were compared with those of age-, gender- and postmortem delay-matched controls. The effect of antidepressant treatment on α2- and β-adrenoceptor densities was also evaluated. α2- and β-adrenoceptor densities were measured by saturation experiments with respective radioligands [(3)H]UK14304 and [(3)H]CGP12177. β1- and β2-adrenoceptor subtype densities were dissected by means of β1-adrenoceptor selective antagonist CGP20712A.

RESULTS

Both, α2- and β1-adrenoceptors densities were higher in antidepressant-free depressed subjects (n=14) than those in matched controls (Δ~24%, p=0.013 and Δ~20%, p=0.044, respectively). In antidepressant-treated subjects (n=12), α2-adrenoceptor density remained increased over that in controls (Δ~20%), suggesting a resistance of α2-adrenoceptors to the down-regulatory effect of antidepressants. By contrast, β1-adrenoceptor density in antidepressant-treated depressed subjects was not different from controls, suggesting a possible down-regulation by antidepressants. The down-regulation of β1-adrenoceptor density in antidepressant-treated depressed subjects differs from the unaltered β1-adrenoceptor density observed in citalopram-treated rats and in a group of non-depressed subjects also treated with antidepressants (n=6). β2-adrenoceptor density was not altered in depressed subjects independently of treatment.

LIMITATIONS

Antidepressant-treated subjects had been treated with a heterogeneous variety of antidepressant drugs. The results should be understood in the context of suicide victims with depression.

CONCLUSIONS

These results show the up-regulation of brain α2- and β1-adrenoceptors in depression and suggest that the regulation induced by chronic antidepressant treatment would be altered in these subjects.

Do certain signal transduction mechanisms explain the comorbidity of epilepsy and mood disorders?

It is well known that mood disorders are highly prevalent in patients with epilepsy. Although several studies have aimed to characterize alterations in different types of receptors associated with both disturbances, there is a lack of studies focused on identifying the causes of this comorbidity. Here, we described some changes at the biochemical level involving serotonin, dopamine, and γ-aminobutyric acid (GABA) receptors as well as signal transduction mechanisms that may explain the coexistence of both epilepsy and mood disorders. Finally, the identification of common pathophysiological mechanisms associated with receptor-receptor interaction (heterodimers) could allow designing new strategies for treatment of patients with epilepsy and comorbid mood disorders.

Neural plasticity and proliferation in the generation of antidepressant effects: hippocampal implication

It is widely accepted that changes underlying depression and antidepressant-like effects involve not only alterations in the levels of neurotransmitters as monoamines and their receptors in the brain, but also structural and functional changes far beyond. During the last two decades, emerging theories are providing new explanations about the neurobiology of depression and the mechanism of action of antidepressant strategies based on cellular changes at the CNS level. The neurotrophic/plasticity hypothesis of depression, proposed more than a decade ago, is now supported by multiple basic and clinical studies focused on the role of intracellular-signalling cascades that govern neural proliferation and plasticity. Herein, we review the state-of-the-art of the changes in these signalling pathways which appear to underlie both depressive disorders and antidepressant actions. We will especially focus on the hippocampal cellularity and plasticity modulation by serotonin, trophic factors as brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) through intracellular signalling pathways—cAMP, Wnt/β-catenin, and mTOR. Connecting the classic monoaminergic hypothesis with proliferation/neuroplasticity-related evidence is an appealing and comprehensive attempt for improving our knowledge about the neurobiological events leading to depression and associated to antidepressant therapies.

The neurobiology of suicide - A Review of post-mortem studies

The neurobiology of suicidal behaviour, which constitutes one of the most serious problems both in psychiatry and general medical practice, still remains to a large degree unclear. As a result, scientists constantly look for new opportunities of explaining the causes underlying suicidality. In order to elucidate the biological changes occurring in the brains of the suicide victims, studies based on post-mortem brain tissue samples are increasingly being used. These studies employ different research methods to provide an insight into abnormalities in brain functioning on various levels, including gene and protein expression, neuroplasticity and neurotransmission, as well as many other areas. The aim of this paper to summarize the available data on the post-mortem studies, to provide an overview of main research directions and the most up-to-date findings, and to indicate the possibilities of further research in this field.

Neurobiology of chronic mild stress: parallels to major depression

The chronic mild (or unpredictable/variable) stress (CMS) model was developed as an animal model of depression more than 20 years ago. The foundation of this model was that following long-term exposure to a series of mild, but unpredictable stressors, animals would develop a state of impaired reward salience that was akin to the anhedonia observed in major depressive disorder. In the time since its inception, this model has also been used for a variety of studies examining neurobiological variables that are associated with depression, despite the fact that this model has never been critically examined to validate that the neurobiological changes induced by CMS are parallel to those documented in depressive disorder. The aim of the current review is to summarize the current state of knowledge regarding the effects of chronic mild stress on neurobiological variables, such as neurochemistry, neurochemical receptor expression and functionality, neurotrophin expression and cellular plasticity. These findings are then compared to those of clinical research examining common variables in populations with depressive disorders to determine if the changes observed following chronic mild stress are in fact consistent with those observed in major depression. We conclude that the chronic mild stress paradigm: (1) evokes an array of neurobiological changes that mirror those seen in depressive disorders and (2) may be a suitable tool to investigate novel systems that could be disturbed in depression, and thus aid in the development of novel targets for the treatment of depression.

Reduced signal transduction by 5-HT4 receptors after long-term venlafaxine treatment in rats

BACKGROUND AND PURPOSE

The 5-HT(4) receptor may be a target for antidepressant drugs. Here we have examined the effects of the dual antidepressant, venlafaxine, on 5-HT(4) receptor-mediated signalling events.

EXPERIMENTAL APPROACH

The effects of 21 days treatment (p.o.) with high (40 mg·kg(-1)) and low (10 mg·kg(-1)) doses of venlafaxine, were evaluated at different levels of 5-HT(4) receptor-mediated neurotransmission by using in situ hybridization, receptor autoradiography, adenylate cyclase assays and electrophysiological recordings in rat brain. The selective noradrenaline reuptake inhibitor, reboxetine (10 mg·kg(-1), 21 days) was also evaluated on 5-HT(4) receptor density.

KEY RESULTS

Treatment with a high dose (40 mg·kg(-1)) of venlafaxine did not alter 5-HT(4) mRNA expression, but decreased the density of 5-HT(4) receptors in caudate-putamen (% reduction = 26 ± 6), hippocampus (% reduction = 39 ± 7 and 39 ± 8 for CA1 and CA3 respectively) and substantia nigra (% reduction = 49 ± 5). Zacopride-stimulated adenylate cyclase activation was unaltered following low-dose treatment (10 mg·kg(-1)) while it was attenuated in rats treated with 40 mg·kg(-1) of venlafaxine (% reduction = 51 ± 2). Furthermore, the amplitude of population spike in pyramidal cells of CA1 of hippocampus induced by zacopride was significantly attenuated in rats receiving either dose of venlafaxine. Chronic reboxetine did not modify 5-HT(4) receptor density.

CONCLUSIONS AND IMPLICATIONS

Our data indicate a functional desensitization of 5-HT(4) receptors after chronic venlafaxine, similar to that observed after treatment with the classical selective inhibitors of 5-HT reuptake.

α₂-Adrenoceptor functionality in postmortem frontal cortex of depressed suicide victims

BACKGROUND

Alterations in brain density and signaling associated with monoamine receptors are believed to play a role in depressive disorders. This study evaluates the functional status of α₂(A)-adrenoceptors in postmortem frontal cortex of depressed subjects.

METHODS

G-protein activation and inhibition of adenylyl cyclase (AC) activity induced by the α₂-adrenoceptor agonist UK14304 were measured in triplicate in samples from 15 suicide victims with an antemortem diagnosis of major depression and 15 matched control subjects.

RESULTS

Basal [³⁵S] guanosine γ thio-phosphate (GTPγS) binding and cyclic adenosine monophosphate accumulation did not differ between groups. In depressed victims, an increase in [³⁵S] GTPγS binding potency (EC₅₀ = .58 μmol/L vs. EC₅₀ = 3.31 μmol/L; p < .01; depressed vs. control) and a significant reduction in the maximal inhibition of AC activity (I(max) = 27 ± 4% vs. I(max) = 47 ± 5%; p < .01) were observed after incubation with the α(2)-adrenoceptor agonist UK14304. No differences were found between antidepressant-free and antidepressant-treated subjects. A significant relationship between EC₅₀ values for [³⁵S] GTPγS and I(max) values for AC assay was found (n = 30; r = -.43; p < .05).

CONCLUSIONS

The dual regulation of α(2A)-adrenoceptor signaling pathways raises the possibility that factors affecting the G-protein cycle and/or selective access of Gα(i/o)-protein to AC might be relevant to receptor abnormalities in depression, providing further support for the involvement of α₂(A)-adrenoceptors in the pathogenesis of depression.

Getting the balance right: Established and emerging therapies for major depressive disorders

Major depressive disorder (MDD) is a common and serious illness of our times, associated with monoamine deficiency in the brain. Moreover, increased levels of cortisol, possibly caused by stress, may be related to depression. In the treatment of MDD, the use of older antidepressants such as monoamine oxidase inhibitors and tricyclic antidepressants is decreasing rapidly, mainly due to their adverse effect profiles. In contrast, the use of serotonin reuptake inhibitors and newer antidepressants, which have dual modes of action such as inhibition of the serotonin and noradrenaline or dopamine reuptake, is increasing. Novel antidepressants have additive modes of action such as agomelatine, a potent agonist of melatonin receptors. Drugs in development for treatment of MDD include triple reuptake inhibitors, dual-acting serotonin reuptake inhibitors and histamine antagonists, and many more. Newer antidepressants have similar efficacy and in general good tolerability profiles. Nevertheless, compliance with treatment for MDD is poor and may contribute to treatment failure. Despite the broad spectrum of available antidepressants, there are still at least 30% of depressive patients who do not benefit from treatment. Therefore, new approaches in drug development are necessary and, according to current research developments, the future of antidepressant treatment may be promising.

Signalling pathways associated with 5-HT6 receptors: relevance for cognitive effects

A growing body of evidence supports the use of serotonin 5-HT6 receptor antagonists as a promising mechanism for treating cognitive dysfunction. We evaluated 5-HT6 receptor expression and associated biochemical mechanisms in the hippocampus of rats that had been trained in the Morris water maze (MWM), a spatial learning task. Training in the MWM induces a down-regulation of 5-HT6 receptor protein and mRNA receptor expression. The learning procedure or the administration of the selective 5-HT6 receptor antagonist SB-271046 induced an increase in pCREB1 levels while CREB2 levels were significantly reduced. However, although SB-271046 was able to improve retention in the MWM, no further changes in pCREB1 or CREB2 levels were found to be associated with the presence of the 5-HT6 receptor antagonist during the learning procedure. The MWM procedure significantly increased pERK1/2 levels and interestingly, further increases were seen when treating with SB-271046 during the MWM. These results suggest that, in the hippocampus, biochemical pathways associated with pERK1/2 expression, and not with the CREB family of transcription factors, seem to be related to the cognitive-enhancing properties of 5-HT6 receptor antagonists.

Early-life blockade of 5-HT(1A) receptors alters adult anxiety behavior and benzodiazepine sensitivity

BACKGROUND

Early-life stress may affect 5-HT(1A) receptor circuitry, which could result in increased anxiety in later life. An increased anxiety phenotype in 5-HT(1A) receptor KO mice (1AKO) mice has been ascribed to 5-HT(1A) receptor absence during the early postnatal period. Thus, subtle and transient serotonergic changes during the early postnatal period may lead to an increased risk for developing stress-related disorders during adulthood.

METHODS

Wildtype and 1AKO mice on a Swiss-Webster (SW) background were treated during the early postnatal period with vehicle or the 5-HT(1A) receptor antagonist WAY-100,635.

RESULTS

Pharmacologic 5-HT(1A) receptor blockade during the early postnatal period induced long-lasting effects on anxiety and benzodiazepine sensitivity in adolescent and adult mice on a Swiss-Webster background and resembles the SW 1AKO phenotype. Furthermore, WAY-100,635-treated mice had increased cortical gamma-aminobutyric acid-A receptor (GABA(A)R) alpha(1) and alpha(3) subunit levels and increased hippocampal GABA(A)R alpha(2) subunit levels.

CONCLUSIONS

Absence of 5-HT(1A)R signaling during early stages of brain maturation predisposes an organism to affective dysfunction later in life. Because early-life treatment with WAY-100,635 in Swiss-Webster mice reduced diazepam sensitivity and increased GABA(A)R alpha subunit levels in the prefrontal cortex and hippocampus, our data suggest a putative link between early-life disruption of the serotonergic system and the emergence of increased anxiety and decreased benzodiazepine responsivity at adult age. Moreover, early-life 5-HT(1A) receptor functionality appears to be essential for the development of normal GABA(A)R functionality. This study may have clinical implications for psychoactive drug use during pregnancy and for the pharmacogenetic background of benzodiazepine sensitivity.

Brain creatine kinase activity is increased by chronic administration of paroxetine

Major depression is a serious and recurrent disorder often manifested with symptoms at the psychological, behavioral, and physiological levels. In addition, several works also suggest brain metabolism impairment as a mechanism underlying depression. Creatine kinase (CK) plays a central role in the metabolism of high-energy consuming tissues such as brain, where it functions as an effective buffering system of cellular ATP levels. Considering that CK plays an important role in brain energy homeostasis and that some antidepressants may modulate energy metabolism, we decided to investigate CK activity from rat brain after chronic administration of paroxetine (selective serotonin reuptake inhibitor), nortriptiline (tricyclic antidepressant) and venlafaxine (selective serotonin-norepinephrine reuptake inhibitor). Adult male Wistar rats received daily injections of paroxetine (10 mg/kg), nortriptiline (15 mg/kg), venlafaxine (10 mg/kg) or saline in 1.0 mL/kg volume for 15 days. Twelve hours after the last administration, the rats were killed by decapitation, the hippocampus, striatum and prefrontal cortex were immediately removed, and activity of CK was measured. Our results demonstrated that chronic administration of paroxetine increased CK activity in the prefrontal cortex, hippocampus and striatum of adult rats. On the other hand, nortriptiline and venlafaxine chronic administration did not affect CK activity in these brain areas. In order to verify whether the effect of paroxetine on CK is direct or indirect, we also measured the in vitro effect of this drug on the activity of the enzyme. We verified that paroxetine did not affect CK activity in vitro. Considering that metabolism impairment is probably involved in the pathophysiology of depressive disorders, an increase in CK activity by antidepressants may be an important mechanism of action of these drugs.

Long-term treatment with fluoxetine induces desensitization of 5-HT4 receptor-dependent signalling and functionality in rat brain

The mode of action of antidepressant drugs may be related to mechanisms of monoamines receptor adaptation, including serotonin 5-HT(4) receptor subtypes. Here we investigated the effects of repeated treatment with the selective serotonin reuptake inhibitor fluoxetine for 21 days (5 and 10 mg/kg, p.o., once daily) on the sensitivity of 5-HT(4) receptors by using receptor autoradiography, adenylate cyclase assays and extracellular recording techniques in rat brain. Fluoxetine treatment decreased the density of 5-HT(4) receptor binding in the CA1 field of hippocampus as well as in several areas of the striatum over the doses of 5-10 mg/kg. In a similar way, we found a significant lower response to zacopride-stimulated adenylate cyclase activity in the fluoxetine 10 mg/kg/day treated group. Furthermore, post-synaptic 5-HT(4) receptor activity in hippocampus-measured as the excitatory action of zacopride in the pyramidal cells of CA1 evoked by Schaffer collateral stimulation was attenuated in rats treated with both doses of fluoxetine. Taken together, these results support the concept that a net decrease in the signalization pathway of 5-HT(4) receptors occurs after chronic selective serotonin reuptake inhibitor treatment: this effect may underlie the therapeutic efficacy of these drugs.

Effect of olfactory bulbectomy on adenylyl cyclase activity in the limbic system

Monoaminergic neurotransmission is a key element in the physiopathology of depressive disorders, but information is still sparse on animal models of this disease. Here, we used the olfactory bulbectomy (OBX) model of depression to characterize cAMP-second messenger signaling pathways, i.e., adenylyl cyclase activity (basal, sodium fluoride (NaF)- and forskolin-stimulated conditions) as well as Gi and Gs protein levels in different regions of the limbic system. Two weeks after surgery and compared to sham controls, OBX rats displayed reduced NaF-stimulated adenylyl cyclase activity and increased Gi/Gs ratios in the hypothalamus, pre-frontal and cingulate cortices but not in the amygdala, hippocampus and caudate nucleus. No differences were found in basal or forskolin-stimulated conditions. The observed reduction of adenylyl cyclase activity induced by NaF and the increase in the Gi/Gs ratio could explain the changes in neurotransmission in OBX rats as well as in humans with depression.

Lithium: bipolar disorder and neurodegenerative diseases Possible cellular mechanisms of the therapeutic effects of lithium

Bipolar illness is a major psychiatric disorder that affects 1-3% of the worldwide population. Epidemiological studies have demonstrated that this illness is substantially heritable. However, the genetic characteristics remain unknown and a clear personality has not been identified for these patients. The clinical history of lithium began in mid-19th century when it was used to treat gout. In 1940, it was used as a substitute for sodium chloride in hypertensive patients. However, it was then banned, as it had major side effects. In 1949, Cade reported that lithium could be used as an effective treatment for bipolar disorder and subsequent studies confirmed this effect. Over the years, different authors have proposed many biochemical and biological effects of lithium in the brain. In this review, the main mechanisms of lithium action are summarised, including ion dysregulation; effects on neurotransmitter signalling; the interaction of lithium with the adenylyl cyclase system; inositol phosphate and protein kinase C signalling; and possible effects on arachidonic acid metabolism. However, none of the above mechanisms are definitive, and sometimes results have been contradictory. Recent advances in cellular and molecular biology have reported that lithium may represent an effective therapeutic strategy for treating neurodegenerative disorders like Alzheimer's disease, due to its effects on neuroprotective proteins like Bcl-2 and its actions on regulators of apoptosis and cellular resilience, such as GSK-3. However, results are contradictory and more specific studies into the use of lithium in therapeutic approaches for neurodegenerative diseases are required.

Long-term upregulation of protein kinase A and adenylate cyclase levels in human smokers

Repeated injections of cocaine and morphine in laboratory rats cause a variety of molecular neuroadaptations in the cAMP signaling pathway in nucleus accumbens and ventral tegmental area. Here we report similar neuroadaptations in postmortem tissue from the brains of human smokers and former smokers. Activity levels of two major components of cAMP signaling, cAMP-dependent protein kinase A (PKA) and adenylate cyclase, were abnormally elevated in nucleus accumbens of smokers and in ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of the catalytic subunit of PKA were correspondingly higher in the ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of other candidate neuroadaptations, including glutamate receptor subunits, tyrosine hydroxylase, and other protein kinases, were within normal range. These findings extend our understanding of addiction-related neuroadaptations of cAMP signaling to tobacco smoking in human subjects and suggest that smoking-induced brain neuroadaptations can persist for significant periods in former smokers.

Effects of venlafaxine on p90Rsk activity in rat C6-gliomas and brain

The intracellular actions of the antidepressant, venlafaxine, were studied in C6-gliomas using a phosphoproteomics approach. Long-term pre-treatment of C6-gliomas with venlafaxine followed by an acute challenge with isoproterenol (a beta-adrenoceptor agonist), resulted in increased p90Rsk phosphorylation (three-fold) versus control levels (isoproterenol alone). The effect of venlafaxine pre-treatment on p90Rsk activity was dose-dependent (EC(50)=3.75nM) in C6 gliomas. In rat brain sections, intense immunoreactive phospho-p90Rsk labeling was observed for both neurons and glia, especially in cortical layers II/III and hippocampal formations. In vivo studies demonstrated an intense but similar distribution pattern of phospho-p90Rsk staining after chronic venlafaxine dosing of rats compared to naives and no region-specific drug effect was observed in vivo. In conclusion, our findings suggest that some of the centrally-mediated benefits of venlafaxine in depression may be due to its intracellular properties especially on the neuro-glial circuitry and MAPK/p90Rsk-dependent pathways at an early stage.