Enhancement of Neuroprotective Effects of <Emphasis Type="Italic">Spirulina maxima</Emphasis> by a Low-temperature Extraction Process with Ultrasonic Pretreatment

17.5 ± 2.41 mg/g and 15.2 ± 2.09 mg/g of total chlorophyll and chlorophyll a, respectively, were obtained from the marine microalga, Spirulina maxima under an optimal extraction condition of 70% ethanol at 65°C for 4 h associated with ultrasonic pretreatment at 40 kHz for 8 h. In comparison, 14.5 ± 1.36 mg/g and 7.1 ± 0.99 mg/g of total chlorophyll and chlorophyll a, respectively, were obtained from a conventional extraction process using 70% ethanol at 80°C for 12 h. The extract from the optimal conditions had the highest ratio of chlorophyll a to total chlorophyll, approximately 3:4, indicating that more intact chlorophyll a was obtained at low temperature. Moreover, the extract obtained using the optimal extraction condition showed substantial neuroprotective effects such as 92.78 ± 0.04% protection against glutamate-induced mouse hippocampal neuronal cells, compared to 81.64 ± 0.07% protection with the extract from the conventional extraction process. Compared to the control, the activities of two key enzymes related to glutathione synthesis, i.e., glutathione reductase and glutathione peroxidase, were also strongly increased, up to 90%, by the extracts from the optimal conditions. Interestingly, the addition of the same concentration of chlorophyll a as in the optimized extract had lower neuroprotective effects than did the extracts. This finding indicates that the extract likely exerted a synergistic effect, showing that the extract had better neuroprotective activity than the single component (chlorophyll a) alone. This work also confirmed the neuroprotective mechanism of the extract mainly due to its high antioxidant activity, allowing it to greatly decrease accumulation of ROS and Ca²⁺ within HT22 cells. The results of this work will have implications for expanding the use of a nonthermal ultrasonic process to extract heat-sensitive bioactive substances from natural resources.