INTRODUCTION

Post-stroke depression (PSD) represents a large portion of the cost spent on the disabilities associated with stroke in the United States (US Department of Health and Human Services, 1999). The estimated prevalence of PSD ranges from 25 to 79% (Gordon and Hibbard, 1997) and is often not detected or inadequately treated (Jia et al, 2006). Stroke patients with PSD suffer higher mortality rates and show only minor improvement in rehabilitation programs, resulting in worse functional (motor and cognitive) outcomes and poorer quality of life (Jorge et al, 2003). Stroke and depression are known to be heterogeneous and multifactorial disorders, but they share common pathological substrates accessible to multitarget strategies. For example, there is increasing evidence that neural plasticity has a key role in both pathologies. Thus, substances that enhance plasticity in the brain could attenuate or reverse their adverse effects and improve the outcome.

In a related disorder, that is, major depressive disorder (MDD), epidemiological and clinical studies suggest that the consumption of omega-3 fatty acids is inversely correlated to the prevalence and the severity of MDD, and that an omega-3 supplementary diet induces benefits in the treatment of depressive states (Hibbeln, 1998; Peet et al, 1998). Thus, omega-3 polyunsaturated fatty acids (PUFA) are good candidates for neuronal protective and restorative strategies against depressive states. In normal mice, chronic administration of a PUFA-supplemented diet consisting of alpha-linolenic acid (ALA), linoleic acid, and oleic acid exerted antidepressant-like effects. These behavioral effects were associated with increases in synaptogenesis, cell number, brain-derived neurotrophic factor (BDNF) gene expression, and hippocampal volume (Venna et al, 2009).

In the domain of stroke disorders, there is growing evidence that the omega-3 precursor, ALA, is a potent protector in several models of global and focal ischemia (McAllister et al, 1995, 1996; Tyler and Pozzo-Miller, 2001; Alonso et al, 2004; Ji et al, 2005), explaining the intense immunostaining for SNAP-25 of the cortical circuitry that we find in the ALA500-treated animals.

The effects of subchronic ALA treatment on increased synaptogenesis and proteins levels involved in exocytosis are in good accordance with recent studies showing that interactions of SNARE proteins with PUFA are of critical importance in regulation of vesicle fusion (Darios et al, 2007; Davletov et al, 2007). As ALA treatment promotes synaptogenesis by modulating the level of SNARE complex, we finally analyzed whether ALA is able to increase the protein levels involved neurotransmitter release. We focused on glutamatergic neurotransmission, whose efficiency is reflected by the expression levels of vesicular glutamate transporters, V-GLUT1 and V-GLUT2. Subchronic ALA treatment induces the upregulation of both V-GLUTs in cortex and hippocampus, suggesting that the omega-3 precursor ALA may promote the neurotransmitter glutamate release. As the cortex and hippocampus are two brain regions known to be crucially involved in synaptic plasticity, our findings suggest that subchronic ALA enhances plastic responses in these two brain regions.

It is plausible that an increase in the number of synapses and/or the upregulation of presynaptic machinery results in ALA–BDNF-potentiation of the neurotransmitter release that could have antidepressant effects. However, while the time course of the various markers of synaptogenesis after ALA treatment are supportive of a functional–anatomical relationship, it is also possible that the behavioral modifications might be due to the production of new neurons rather than modification of existing synapses. As we also showed that ALA might also induce BDNF synthesis by the NSCs, positive synergetic or additive effects have to be considered.

Another part of the present work is regarding the effect of repeated ALA injections in depression. Here, we show that subchronic ALA treatment induces an increase in BDNF levels, and improves neurogenesis and synaptic plasticity in specific brain regions, properties well known for the efficiency of antidepressant drugs (Castren et al, 2007). Using behavioral tests, FST and TST, this work shows that a single ALA injection does not induce antidepressant-like effects, whereas subchronic ALA treatment shows beneficial effects on preventing the development of depression-like behavior in mice in FST and TST paradigms. These results are in good accordance with the antidepressant-like effects of omega 3-enriched diet (Carlezon et al, 2005; Huang et al, 2008; Venna et al, 2009). Furthermore, the subchronic ALA treatment offers a shorter duration of treatment, as compared with diet and a long-lasting antidepressant-like effect, which is still observed 3 days after the final administered dose. It is known that antidepressant drugs have a positive effect on PSD by mediating the signaling pathways of brain plasticity (Santarelli et al, 2003; Dranovsky and Hen, 2006). Neurogenesis and synaptogenesis associated with subchronic ALA treatment are strong arguments in favor of such mechanisms involved in the ALA-antidepressant effect, which can also be related to additive/or synergic interactions with serotonin, norepinephrine, or dopamine pathways (Delion et al, 1994, 1996).

With regard to stroke and long-lasting efficiency of ALA on antidepressant behavior and neuroplasticity, we have independently tested whether a protection against ischemia will be observed 3 days after the final administered dose. Our results show that the subchronic ALA treatment is efficient as preventive treatment. The several protective effects of ALA on neurons (including reduction of excitotoxicity, neurotrophic factor induction, and microenvironmental-associated plasticity changes) may be difficult to interpret how neurogenesis, produced by subchronic ALA treatment before MCAO, reduces infarct volume. In contrast, it seems intuitive that post-MCAO, subchronic ALA treatment may restore neuronal loss, in part, by promoting neurogenesis. In addition, neurogenesis could affect the post-stroke recovery period, which represents yet another target for more efficient therapeutic effects. ALA treatment can be perceived as a ‘restorative’ intervention, which can explain the observed reduced mortality.

In this work, we show that subchronic ALA treatment enhances endogenous neurogenesis and synaptogenesis by a neuronal pathway. Given our previous findings of vasoactive ALA-properties (Blondeau et al, 2007) and increases in neurogenesis being associated with vascular changes (Palmer et al, 2000), we cannot exclude a facilitation of neurogenesis and synaptogenesis related to a vasculature-mediated effect of subchronic ALA treatment.

In summary, the present work shows that a subchronic treatment with ALA injections promotes several features associated with neurogenesis/synaptogenesis and possesses antidepressant-like qualities that may be beneficial for stroke recovery. This functional benefit may be related to acute increased vasodilation and excitotoxicity prevention, as well as long-term enhancement of neurogenesis, synaptogenesis, and neurotransmitter transmission. From a clinical point of view, this ‘multitarget’ effect of subchronic ALA treatment may represent a novel approach to stroke and depression. This multitargeted strategy may extend to other neurodegenerative and psychiatric disorders.