Subchronic Alpha Linolenic Acid Treatment Enhances Brain Plasticity and Exerts an Antidepressant Effect: A Versatile Potential Therapy for Stroke

Omega 3 polyunsaturated fatty acids are known to have therapeutic potential in several neurological and psychiatric disorders. However, the molecular mechanisms of action underlying these effects are not well elucidated. We previously showed that alpha linolenic acid (ALA) reduced ischemic brain damage after a single treatment. To follow up this finding, we investigated whether subchronic ALA treatment promoted neuronal plasticity. Three sequential injections with a neuroprotective dose of ALA increased neurogenesis and expression of key proteins involved in synaptic functions, namely, synaptophysin1, VAMP 2, and SNAP 25, as well as proteins supporting glutamatergic neurotransmission, namely, GLUT1 and GLUT2. These effects were correlated with an increase in brain derived neurotrophic factor (BDNF) protein levels, both in vitro using neural stem cells and hippocampal cultures and in vivo, after subchronic ALA treatment. Given that BDNF has antidepressant activity, this led us to test whether subchronic ALA treatment could produce antidepressant like behavior. ALA treated mice had significantly reduced measures of depressive like behavior compared with vehicletreated animals, suggesting another aspect of ALA treatment that could stimulate functional stroke recovery by potentially combining acute neuroprotection with long term repair per compensatory plasticity. Indeed, three sequential injections of ALA enhanced protection, either as a pretreatment, wherein it reduced post ischemic infarct volume 24 h after a 1 hour occlusion of the middle cerebral artery or as post treatment therapy, wherein it augmented animal survival rates by threefold 10 days after ischemia. (Authors abstract)
Post stroke depression (PSD) represents a large portion of the cost spent on the disabilities associated with stroke in the United States. Stroke and depression are known to be heterogeneous and multifactorial disorders, but they share common pathological substrates accessible to multitarget strategies. 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. Epidemiological and clinical studies suggest that the consumption of omega 3 fatty acids is inversely correlated to the prevalence and the severity of major depressive disorder (MDD), and that an omega 3 supplementary diet induces benefits in the treatment of depressive states. In a mouse model of stroke, it was found that a single injection of ALA, after the onset of stroke, significantly reduced infarct size and improved the neurological score 24 h post ischemia. Although the single injection of ALA did not improve long term survival rate, repeated ALA injections over 3 weeks significantly improved survival, suggesting that subchronic ALA treatment triggers other protective pathways.  The aims of the present study were to determine whether subchronic ALA treatment could (1) specifically increase neuronal plasticity at the molecular and cellular level, (2) produce antidepressant like behavior, and (3) reduce stroke induced mortality. We also sought to determine the effect of ALA pretreatment on neuronal loss in vitro and in vivo. Subchronic ALA treatment significantly increased the number of proliferating immature neurons that survive and, at term, become mature. These results suggest that ALA promotes neurogenesis in the adult brain that could have a role in motor and cognitive functional recovery of neurological disorders.  This work shows that subchronic ALA treatment promotes NSC proliferation. It is worth noting that NSC can induce the transcription of protective factors, such as BDNF, which would be expected to modify the ischemic environment and promote neuroprotection.  In vitro and in vivo studies show that the expression of BDNF levels was increased after ALA treatment, both in NSC, hippocampal mature neurons and in cortical per hippocampal tissue. BDNF is an immediate early gene that responds rapidly after membrane depolarization, and is involved in the maintenance and support of neurons. The results suggest that ALA treatment induces a strong neurogenesis related to a direct regulation of enhanced neuronal BDNF expression on immature NSCs and mature hippocampal neurons in culture. Furthermore, the neurogenesis and the upregulation of mature BDNF triggered by ALA treatment, which occur together in neurons, can be considered as additional mechanisms that lead to increased brain protection and an environmental change favorabletoward recovery of functional network circuitry.  The significant increase of synaptophysin induced by repeated ALA injections indicates that subchronic ALA treatment promotes presynaptic plasticity and synaptogenesis. The parallel increase of synaptophysin 1 and VAMP 2, two vesicle associated synaptic proteins (SV proteins) and their membrane associated synaptic protein (PM protein), SNAP 25, presents a strong argument in favor of an ALA induced increase in synaptogenesis of the cortex.
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.  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. 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. 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 per or synergic interactions with serotonin, norepinephrine, or dopamine pathways.  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, subchronic ALA treatment enhances endogenous neurogenesis and synaptogenesis by a neuronal pathway perhaps through 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 per 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. (Editors comments)