Lippia grata Schauer: Essential Oil and Phytoceutical Thymol Antioxidants and Neuroprotectors with Inhibition of Acetylcholinesterase and Depressive Behaviors in Adult Zebrafish (D. rerio)

Depression, a growing mental disorder, affects millions of people globally and faces treatment challenges due to the low efficacy and adverse effects of conventional antidepressants. In this context, medicinal plants such as Lippia grata Schauer, endemic to Brazil and recognized for their therapeutic properties, stand out as promising alternatives for developing more effective and safe treatments. Therefore, this work reports the standardization of the depression model in adult zebrafish (aZF), in addition to evaluating the antidepressant effect of Lippia grata essential oil (EOLg) and the phytoceutical thymol, as well as their potential neuromodulatory mechanisms and in vitro antioxidant and anticholinesterase (AChE) activities. Initially, aZF were treated with fluoxetine (Flx) or EOLg or thymol or vehicle and subjected to Toxicity and Open Field tests. After 1 h of the same treatments, in other aZF groups, the animals were individually immersed in EtOH for 30 min, with the exception of the naïve group. Subsequently, the aZF were subjected to the Zebrafish Tail Immobilization Test, and the antidepressant effect was characterized by an increase in Mobility Time (s), MT. The possible mechanisms of action were investigated through the administration of antagonists of the serotonergic system. The antioxidant capacity and acetylcholinesterase (AChE) inhibitory effect were assessed in vitro, including the determination of IC₅₀ values for the DPPH and ABTS radicals and the AChE enzyme. Furthermore, molecular docking simulations of thymol with 5-HT receptors were investigated. The toxicological results indicated that the samples are safe against aZF. Flx presented an antidepressant effect, but with a sedative effect, while EOLg and thymol exhibited an antidepressant effect, without a sedative effect and via serotonergic systems. In vitro tests showed antioxidant and neuroprotective potential against AChE in the samples analyzed. Furthermore, in silico tests confirmed the affinity of thymol for the 5-HT_1B, 5-HT_2A, 5-HT_2C, and 5-HT_3A receptors. These findings reinforce the importance of Lippia Grata essential oil as a source of the phytoceutical thymol with neuroprotective potential in neurological disorders.