Docosahexaenoic acid (DHA, 22:6n‐3), the major polyunsaturated fatty acid accumulated in the brain during development, has been implicated in learning and memory, but underlying cellular mechanisms are not clearly understood. Here, we demonstrate that DHA significantly affects hippocampal neuronal development and synaptic function in developing hippocampi. In embryonic neuronal cultures, DHA supplementation uniquely promoted neurite growth, synapsin puncta formation and synaptic protein expression, particularly synapsins and glutamate receptors. In DHA‐supplemented neurons, spontaneous synaptic activity was significantly increased, mostly because of enhanced glutamatergic synaptic activity. Conversely, hippocampal neurons from DHA‐depleted fetuses showed inhibited neurite growth and synaptogenesis. Furthermore, n‐3 fatty acid deprivation during development resulted in marked decreases of synapsins and glutamate receptor subunits in the hippocampi of 18‐day‐old pups with concomitant impairment of long‐term potentiation, a cellular mechanism underlying learning and memory. While levels of synapsins and NMDA receptor subunit NR2A were decreased in most hippocampal regions, NR2A expression was particularly reduced in CA3, suggesting possible role of DHA in CA3‐NMDA receptor‐dependent learning and memory processes. The DHA‐induced neurite growth, synaptogenesis, synapsin, and glutamate receptor expression, and glutamatergic synaptic function may represent important cellular aspects supporting the hippocampus‐related cognitive function improved by DHA.

Docosahexanoic acid (22:6n‐3; DHA) deficiency during development is associated with impairment in learning and memory, suggesting an important role of DHA in neuronal development. Here we provide evidence that DHA promotes neuronal differentiation in rat embryonic hippocampal primary cultures. DHA deficiency in vitro was spontaneously induced by culturing hippocampal cells in chemically defined medium. DHA supplementation improved DHA levels to values observed in freshly isolated hippocampus. We found that DHA supplementation in culture increased the population of neurons with longer neurite length per neuron and with higher number of branches. However, supplementation with arachidonic, oleic or docosapentaenoic acid did not have any effect, indicating specificity of the DHA action on neurite growth. Furthermore, hippocampal cultures obtained from n‐3 fatty acid deficient animals contained a lower DHA level and a neuronal population with shorter neurite length per neuron in comparison to those obtained from animals with adequate n‐3 fatty acids. DHA supplementation to the deficient group recovered the neurite length to the level similar to n‐3 fatty acid adequate cultures. Our data demonstrates that DHA uniquely promotes neurite growth in hippocampal neurons. Inadequate neurite development due to DHA deficiency may contribute to the cognitive impairment associated with n‐3 fatty acid deficiency.

Enrichment of Neuro 2A cells with docosahexaenoic acid (22:6n-3) decreased apoptotic cell death induced by serum starvation as evidenced by the reduced DNA fragmentation and caspase-3 activity. The protective effect of 22:6n-3 became evident only after at least 24 h of enrichment before serum starvation and was potentiated as a function of the enrichment period. During enrichment 22:6n-3 incorporated into phosphatidylserine (PS) steadily, resulting in a significant increase in the total PS content. Similar treatment with oleic acid (18:1n-9) neither altered PS content nor resulted in protective effect. Hindering PS accumulation by enriching cells in a serine-free medium diminished the protective effect of 22:6n-3. Membrane translocation of Raf-1 was significantly enhanced by 22:6n-3 enrichment in Neuro 2A cells. Consistently, in vitrobiomolecular interaction between PS/phosphatidylethanolamine /phosphatidylcholine liposomes, and Raf-1 increased in a PS concentration-dependent manner. Collectively, enrichment of neuronal cells with 22:6n-3 increases the PS content and Raf-1 translocation, down-regulates caspase-3 activity, and prevents apoptotic cell death. Both the antiapoptotic effect of 22:6n-3 and Raf-1 translocation are sensitive to 22:6n-3 enrichment-induced PS accumulation, strongly suggesting that the protective effect of 22:6n-3 may be mediated at least in part through the promoted accumulation of PS in neuronal membranes.

Docosahexaenoic acid (DHA, 22:6n-3) is an omega-3 fatty acid essential for proper brain development. N-docosahexaenoylethanolamine (synaptamide), an endogenous metabolite of DHA, potently promotes neurogenesis, neuritogenesis and synaptogenesis; however, the underlying molecular mechanism is not known. Here, we demonstrate orphan G-protein coupled receptor 110 (GPR110, ADGRF1) as the synaptamide receptor, mediating synaptamide-induced bioactivity in a cAMP-dependent manner. Mass spectrometry-based proteomic characterization and cellular fluorescence tracing with chemical analogues of synaptamide reveal specific binding of GPR110 to synaptamide, which triggers cAMP production with low nM potency. Disruption of this binding or GPR110 gene knockout abolishes while GPR110 overexpression enhances synaptamide-induced bioactivity. GPR110 is highly expressed in fetal brains but rapidly decreases after birth. GPR110 knockout mice show significant deficits in object recognition and spatial memory. GPR110 deorphanized as a functional synaptamide receptor provides a novel target for neurodevelopmental control and new insight into mechanisms by which DHA promotes brain development and function.