Maintenance treatment with fluoxetine is necessary to sustain normal levels of synaptic markers in an experimental model of depression: correlation with behavioral response

Potentiation of omega-3 fatty acid antidepressant-like effects with low non-antidepressant doses of fluoxetine and mirtazapine

Despite the advances in psychopharmacology, the treatment of depressive disorders is still not satisfactory. Side effects and resistance to antidepressant drugs are the greatest complications during treatment. Based on recent evidence, omega-3 fatty acids may influence vulnerability and outcome in depressive disorders. The aim of this study was to further characterize the omega-3 antidepressant-like effect in rats in terms of its behavioral features in the depression model forced swimming test either alone or in combination with antidepressants fluoxetine or mirtazapine. Ultimately, we prompted to determine the lowest dose at which omega-3 fatty acids and antidepressant drugs may still represent a pharmacological advantage when employed in combined treatments. Chronic diet supplementation with omega-3 fatty acids produced concentration-dependent antidepressant-like effects in the forced swimming test displaying a behavioral profile similar to fluoxetine but different from mirtazapine. Fluoxetine or mirtazapine at antidepressant doses (10 and 20 mg/kg/day, respectively) rendered additive effects in combination with omega-3 fatty acid supplementation (720 mg/kg/day). Beneficial effects of combined treatment were also observed at sub-effective doses (1 mg/kg/day) of fluoxetine or mirtazapine, since in combination with omega-3 fatty acids (720 mg/kg/day), antidepressants potentiated omega-3 antidepressant-like effects. The antidepressant-like effects occurred in the absence of changes in brain phospholipid classes. The therapeutic approach of combining omega-3 fatty acids with low ineffective doses of antidepressants might represent benefits in the treatment of depression, especially in patients with depression resistant to conventional treatments and even may contribute to patient compliance by decreasing the magnitude of some antidepressant dose-dependent side effects.

Regulation of markers of synaptic function in mouse models of depression: chronic mild stress and decreased expression of VGLUT1

Depression has been linked to failure in synaptic plasticity originating from environmental and/or genetic risk factors. The chronic mild stress model regulates the expression of synaptic markers of neurotransmitter function and associated depressive-like behaviour. Moreover, mice heterozygous for the synaptic vesicle protein vesicular glutamate transporter 1 (VGLUT1), have been proposed as a genetic model of deficient glutamate function linked to depressive-like behaviour. Here, we aimed to identify, in these two experimental models, mechanisms of failure in synaptic plasticity, common to stress and impaired glutamate function. First, we show that chronic mild stress induced a transient decrease of different plasticity markers (VGLUT1, synapsin 1, sinaptophysin, rab3A and activity regulated cytoskeletal protein - Arc) but a long-lasting decrease of the brain derived neurotrophic factor as well as depressive-like behaviour. The immediate early gene Arc was also down-regulated in VGLUT1+/- heterozygous mice. In contrast, an opposite regulation of synapsin 1 was observed. Finally, both models showed a marked increase of cortical Arc response to novelty. Increased Arc response to novelty could be suggested as a molecular mechanism underlying failure to adapt to environmental changes, common to chronic stress and altered glutamate function. Further studies should investigate whether these changes are associated to depressive-like behaviour both in animal models and in depressed patients.

Quantitative changes in hippocampal microvasculature of chronically stressed rats: no effect of fluoxetine treatment

Exposure to chronic stress alters the number and morphology of neurons and glia in the hippocampal formation; however, little is known about possible changes in vasculature. Here, we examined the effect of chronic social defeat stress on hippocampal vascular supply in rats. Recent reports document that antidepressant treatment can influence angiogenesis in the hippocampus; therefore, we also studied the effect of antidepressant drug treatment on hippocampal capillarization. Animals were subjected to 5 weeks of daily social defeat by an aggressive conspecific and received concomitant, daily, oral fluoxetine (10 mg/kg) treatment during the last 4 weeks. Rat endothelial cell antigen-1 (RECA-1)-labeling of capillaries and quantitative stereological techniques were used to evaluate the treatment effects on capillary number. Special attention was paid to analysis of the vascular supply of the subgranular zone, which is regarded as an important component of the neurogenic niche for adult hippocampal neurogenesis. Chronic stress significantly decreased the number of microvessels by 30% in all hippocampal subregions, whereas fluoxetine treatment had no influence on capillary number. Furthermore, chronic stress decreased the capillarization of the subgranular zone to a similar extent, indicating that chronic stress affects the vascular niche for adult hippocampal neurogenesis. However, fluoxetine treatment had no impact on capillarization in the subgranular zone. We also detected a decrease in hippocampal volume in the animals as a result of stress, which was mildly altered by fluoxetine treatment. These pronounced changes in vascular supply may explain why the hippocampus is more vulnerable to insults when chronic stress precedes or coincides with other harmful conditions. Reduced microvasculature may also contribute to hippocampal volume decrease in stress-related disorders.

Effects of estradiol on learned helplessness and associated remodeling of hippocampal spine synapses in female rats

BACKGROUND

Despite the fact that women are twice as likely to develop depression as men, our understanding of depression neurobiology in female subjects is limited. We have recently reported in male rats that development of helpless behavior is associated with a severe loss of hippocampal spine synapses, which is reversed by treatment with the antidepressant desipramine. Considering that estradiol has a hippocampal synaptogenic effect similar to those of antidepressants, the presence of estradiol during the female reproductive life might influence behavioral and synaptic responses to stress and depression.

METHODS

With electron microscopic stereology, we analyzed hippocampal spine synapses in association with helpless behavior in ovariectomized female rats (n = 70), under different conditions of estradiol exposure.

RESULTS

Stress induced an acute and persistent loss of hippocampal spine synapses, whereas subchronic treatment with desipramine reversed the stress-induced synaptic loss. Estradiol supplementation given either before stress or before escape testing of nonstressed animals increased the number of hippocampal spine synapses. Correlation analysis demonstrated a statistically significant negative correlation between the severity of helpless behavior and hippocampal spine synapse numbers.

CONCLUSIONS

These findings suggest that hippocampal spine synapse remodeling might be a critical factor underlying learned helplessness and, possibly, the neurobiology of depression.

An enriched environment restores normal behavior while providing cytoskeletal restoration and synaptic changes in the hippocampus of rats exposed to an experimental model of depression

The exposure of rats to an enriched environment (EE) has several effects in common with the administration of antidepressants. However, there is still little information about the molecular underpinnings of these effects on rats subjected to experimental models of depression. The aim of this research was to evaluate the effects of EE on rats exposed to the learned helplessness paradigm (LH), a well-known model of the disease. We found that a 21 day exposure to EE reverts helplessness behavior to normal in LH animals. Inmunohistochemical labeling showed that this effect was accompanied by normalization of two structural proteins of hippocampal neurons to control values: the light neurofilament subunit (NFL) and the postsynaptic density 95 (PSD-95) protein, which were decreased in LH animals housed in standard cages. The decrease in the presynaptic protein synaptophysin (SYN) observed in LH animals remained unchanged after exposure to EE. There was no increase in neurogenesis as measured by quantification of double-labeled cells with 5-bromo-2'-deoxyuridine (BrdU) and the neuronal marker beta-tubulin class III. These results show that EE may have behavioral and synaptic effects on animals exposed to an experimental model of depression, and that such actions seem to be independent from neurogenesis.

Early parental deprivation in the marmoset monkey produces long-term changes in hippocampal expression of genes involved in synaptic plasticity and implicated in mood disorder

In mood disorder, early stressors including parental separation are vulnerability factors, and hippocampal involvement is prominent. In common marmoset monkeys, daily parental deprivation during infancy produces a prodepressive state of increased basal activity and reactivity in stress systems and mild anhedonia that persists at least to adolescence. Here we examined the expression of eight genes, each implicated in neural plasticity and in the pathophysiology of mood disorder, in the hippocampus of these same adolescent marmosets, relative to their normally reared sibling controls. We also measured hippocampal volume. Early deprivation led to decreases in hippocampal growth-associated protein-43 (GAP-43) mRNA, serotonin 1A receptor (5-HT(1A)R) mRNA and binding ([3H]WAY100635), and to increased vesicular GABA transporter mRNA. Brain-derived neurotrophic factor (BDNF), synaptophysin, vesicular glutamate transporter 1 (VGluT1), microtubule-associated protein-2, and spinophilin transcripts were unchanged. There were some correlations with in vivo biochemical and behavioral indices, including VGluT1 mRNA with reward-seeking behavior, and serotonin 1A receptor mRNA with CSF cortisol. Early deprivation did not affect hippocampal volume. We conclude that early deprivation in a nonhuman primate, in the absence of subsequent stressors, has a long-term effect on the hippocampal expression of genes implicated in synaptic function and plasticity. The reductions in GAP-43 and serotonin 1A receptor expressions are comparable with findings in mood disorder, supporting the possibility that the latter reflect an early developmental contribution to disease vulnerability. Equally, the negative results suggest that other features of mood disorder, such as decreased hippocampal volume and BDNF expression, are related to different aspects of the pathophysiological process.

Chronic fluoxetine treatment increases expression of synaptic proteins in the hippocampus of the ovariectomized rat: role of BDNF signalling

Antidepressant drugs have been suggested to regulate synaptic transmission and structure. We hypothesised that antidepressant-induced changes in synapses and their associated proteins might become more apparent if they were measured under conditions of reduced synapse density. Therefore, in the present study, we examined whether chronic treatment with the antidepressant, fluoxetine alters expression of synaptic proteins in the hippocampus of rodents that underwent ovariectomy, a procedure which reportedly decreases synapse density in the CA1 region of the rat hippocampus. Using Western blotting, we measured changes in hippocampal expression of proteins associated with synapse structure, strength and activity namely, postsynaptic density protein 95 (PSD-95), the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) subunit GluR1 and phosphosynapsin (Ser9), respectively. We found that fluoxetine treatment increased expression of phosphosynapsin, PSD-95 and synaptic GluR1 (but not total GluR1) in the hippocampus of ovariectomized but not sham rats. Since BDNF and signalling at its receptor, TrkB, can mediate behavioural responses to antidepressants and induce neuronal plasticity, we investigated the contribution of TrkB signalling to fluoxetine-induced changes in synaptic protein expression by using a transgenic mouse model overexpressing a truncated form of the TrkB receptor (TrkB.T1). Fluoxetine produced a small but significant increase in hippocampal PSD-95 in ovariectomized wildtype mice but not in ovariectomized TrkB.T1 mice or sham mice. In contrast to rats, fluoxetine did not alter expression of synaptic GluR1 and did not reverse ovariectomy-induced decreases in hippocampal phosphosynapsin in either genotype. Taken together, these results suggest that chronic fluoxetine treatment can increase synaptic protein expression in the hippocampus and at least some of these effects require TrkB signalling. Moreover, these effects were only observed in ovariectomized animals, thus suggesting that fluoxetine-induced increases in synaptic protein levels might only occur or become detectable when hippocampal synaptic connectivity is perturbed.

Remodeling of hippocampal spine synapses in the rat learned helplessness model of depression

BACKGROUND

Although it has been postulated for many years that depression is associated with loss of synapses, primarily in the hippocampus, and that antidepressants facilitate synapse growth, we still lack ultrastructural evidence that changes in depressive behavior are indeed correlated with structural synaptic modifications.

METHODS

We analyzed hippocampal spine synapses of male rats (n=127) with electron microscopic stereology in association with performance in the learned helplessness paradigm.

RESULTS

Inescapable footshock (IES) caused an acute and persistent loss of spine synapses in each of CA1, CA3, and dentate gyrus, which was associated with a severe escape deficit in learned helplessness. On the other hand, IES elicited no significant synaptic alterations in motor cortex. A single injection of corticosterone reproduced both the hippocampal synaptic changes and the behavioral responses induced by IES. Treatment of IES-exposed animals for 6 days with desipramine reversed both the hippocampal spine synapse loss and the escape deficit in learned helplessness. We noted, however, that desipramine failed to restore the number of CA1 spine synapses to nonstressed levels, which was associated with a minor escape deficit compared with nonstressed control rats. Shorter, 1-day or 3-day desipramine treatments, however, had neither synaptic nor behavioral effects.

CONCLUSIONS

These results indicate that changes in depressive behavior are associated with remarkable remodeling of hippocampal spine synapses at the ultrastructural level. Because spine synapse loss contributes to hippocampal dysfunction, this cellular mechanism may be an important component in the neurobiology of stress-related disorders such as depression.