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

## TL;DR

This study explores the antidepressant and neuroprotective effects of Lippia grata essential oil and thymol in zebrafish, showing potential as safer alternatives to conventional antidepressants.

## Contribution

The study introduces Lippia grata essential oil and thymol as novel natural antidepressants with serotonergic and neuroprotective mechanisms.

## Key findings

- Lippia grata essential oil and thymol showed antidepressant effects in zebrafish without sedation.
- Thymol exhibited strong antioxidant and acetylcholinesterase inhibitory activity in vitro.
- Molecular docking confirmed thymol's affinity for key serotonin receptors.

## Abstract

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-HT1B, 5-HT2A, 5-HT2C, and 5-HT3A receptors. These
findings reinforce the importance of Lippia Grata essential oil as a source of the phytoceutical thymol with neuroprotective
potential in neurological disorders.