1. Introduction
Anxiety is considered pathological when it causes psychological and social harm to the individual [
1]. In recent years, anxiety has been one of the most prevalent neuropsychiatric disorders in the world [
2]. According to the World Health Organization – WHO, around 3.6 % of the world population is affected by the disease, which in Brazil affects approximately 9.3 % of the population, ranking among the countries with the highest rates [
3].
The etiology of anxiety is multifactorial and is mainly associated with interactions of genetic, neurobiological, psychological variables and environmental influence [
4]. In the central nervous system (CNS), several neurotransmission pathways contribute to the mechanisms that mediate anxiety, especially changes in the serotonergic and GABAergic systems [
1]. Furthermore, high levels of polyunsaturated acids and low reserves of antioxidant species in the CNS contribute to making neurons more vulnerable to oxidative stress and triggering neuroinflammation, associated with the development of pathological anxiety [
5].
Benzodiazepines (BZDs), γ-aminobutyric acid (GABA) receptor agonists, and serotonin reuptake inhibitors (SSRIs) are commonly used anxiolytic drugs in the treatment of anxiety [
3]. However, chronic use of BZD produces tolerance, and abrupt cessation of treatment can lead to a withdrawal syndrome [
6]. From another perspective, the chronic use of SSRIs can reduce motor coordination, drowsiness, sedation and, when used for a prolonged period, they can also lead to dependence and withdrawal syndromes, and in high doses they can be fatal [
7].
The adverse effects of these medications are the main reason why there is intensified research into the anxiolytic effects of products of natural origin. In general, plants synthesize a diversity of substances derived from their secondary metabolism, which often present satisfactory results in therapy against various psychological disorders [
8].
Species from the Rhamnaceae family are commonly used in therapy against neurological diseases [
9]. Among these,
Sarcomphalus joazeiro (Mart.) Hauenschild, previously classified as
Ziziphus joazeiro Mart [
10], can be considered an alternative for pre-clinical investigations. Popularly known as “juá”, “joazeiro”, it is native to the Brazilian Northeast and endemic to the caatinga. In traditional medicine, it is used in the therapy of dermatological, respiratory, and digestive problems, in addition, it exhibits antioxidant, anti-inflammatory and antimicrobial properties [
11]. Chemical data indicate the presence of flavonoid and saponin derivatives, which have reported and well-established anxiolytic activities [
12], [
13].
Animal research is being carried out as a model to screen natural products capable of combating anxiety [
3].
Zebrafish (
Danio rerio) have been a useful model as they possess key neuromediation systems, such as neurotransmitter receptors, transporters, sensitivity to anxiolytics, and behavioral paradigms similar to mammals [
14]. Furthermore, they have a signaling system developed with high sensitivity to GABAergic sedatives [
15].
Considering the importance of finding new active substances for use as an anxiolytic, this study aimed to identify the chemical components through hyphenated analytical methods and evaluate the antioxidant and anxiolytic capacity of the S. joazeiro leaf extract.
3. Discussion
In phytochemical screening studies
, S. joazeiro extract exhibited classes of secondary metabolites, such as flavonoids, saponins and tannins, which are associated with different biological activities [
12]. Previous studies performed UPLC-ESI-QTOF-MS analysis and showed the presence of rutin (2), kaempferol (3), isorhamnetin (4, 5 and 7), quercetin (6) and saponin derivatives (8, 9, 10 and 11), corroborating this study [
16], [
23].
Compounds 4 and 7 showed the same peak of the deprotonated molecular ion [M−H]
− m/z 477 (C
29H
17O
7). Previous research suggests that both are isomers of isorhamnetin-3-
O-hexoside, due to the detection of fragment ions m/z 315 and m/z 162 (Matos et al., 2021). From a structural point of view, the flavonoids found (2, 3, 4, 5, 6 and 7) are characterized by the presence of two aromatic rings and a heterocyclic benzopyran ring containing oxygen and hydroxyl substituents [
24].
Saponins are widely found in the Rhamanaceae family. In the species
S. joazeiro, tetracyclic triterpenes of the dammarane type are frequently reported [
25]. Masullo et al., (2019) proved the presence of these saponins through structural elucidation by 2D NMR. From a structural point of view, these derivatives are differentiated by their side chains, which correspond to 16.23:16.30 diepoxidammarane (8, 9, 10 e 11). Previous studies of species from the Rhamnaceae family, including
Z. jujuba [
19], [
26], and
Z. mauritiana [
27] report the presence of these saponins.
The antioxidant activity of species of the genus Ziziphus reported in the literature agrees with these data, in which Z. cotinifolia exhibited IC50 values of 7.2 ± 1.5 μg/ml and 6.4 ± 0.3 μg/mL, for the DPPH• and ABTS•⁺ method, while Z. jujuba showed IC50 of 47.37 mmol of trolox, for ABTS•⁺. On the other hand, in the DPPH• elimination test, Brito et al., (2015) found higher IC50 values than those found in this study (735,72 μg/mL). In an HPLC/DAD analysis, the authors confirmed a low content of rutin (0.97 %) and quercetin (0.52 %) in the extract.
The high antioxidant action exerted by EEFSJ in this study can be attributed to the higher content of rutin (2) and quercetin (6) in the composition, which may justify the difference in the results found in the studies, considering that these are the main natural flavonoids responsible for enhancing the antioxidant effects of plant extracts. This variation in composition is due to the influence of extractive methods, organic solvents, as well as seasonal factors, such as location, collection period and type of soil [
28].
The structures of flavonoids found in the present study allow them to act by combating free radicals through the donation of hydrogen atoms from a hydroxyl group of their aromatic structure, which has the ability to support an unpaired electron through its displacement around the entire molecule electron system [
29], [
20]. Furthermore, they can also act as scavengers of pro-inflammatory and neurotoxic species in the CNS [
9].
The intensity of the anxiolytic effect observed in this study was inversely proportional to the concentration of EEFSJ. Silva et al., (2023) recently reported similar behavior of
S. joazeiro extract in the same experimental model of
zebrafish. These data propose that EEFSJ when administered in high doses (> 200 mg/kg) may cause tolerance and adverse effects similar to DZP. The administration of high doses of DZP (> 30 mg/kg) is capable of developing tolerance, causing compensatory changes in GABA receptors, which become less responsive, and as a consequence, reduce their inhibitory actions [
30]. Furthermore, higher doses of DZP can cause CNS depression, whose additional non-classical BZD binding sites on GABA
ARs can cause adverse action [
15].
EEFSJ also altered the locomotion of
zebrafish at all concentrations tested, exhibiting behavior similar to DZP. Recently, studies of
Z. cotinifolia (Rhamnaceae) using the same experimental model showed similar results [
14]. BZDs produce anxiolytic and sedative effects by binding to high-affinity sites located at the α1/2/3/5+/γ2− interface of synaptic GABA
ARs to enhance GABA-mediated post-synaptic membrane hyperpolarization [
15]. Therefore, the alteration of
zebrafish locomotion caused by EEFSJ can be attributed to the high affinity for the GABA
A receptor binding sites, which are capable of reducing central activity, causing an anxiolytic and/or sedative action similar to BDZ.
The involvement of GABAergic neurotransmission was evaluated through pre-treatment with flumazenil (Fmz) [
7]. EEFSJ demonstrated anxiolytic behavior in animals and its effect was reversed by FMZ (
Figure 6), proving that EEFSJ is capable of activating the GABA
A receptor in the same region as BZD [
31].
Clinical and pre-clinical studies prove that flavonoids and saponins act through selective binding to GABA
A receptors, promoting membrane hyperpolarization by allowing the influx of chloride anions, accompanied by inhibition of excitatory transmission, which consequently contribute to the reduction of anxiety disorders [
9], [
32]. This hypothesis is corroborated by previous studies that show the anxiolytic potential of
S. joazeiro, flavonoids and saponins through GABAergic neurotransmission in
zebrafish and mice [
8], [
11], [
33].
Quercetin (6) has been widely investigated due to its antagonistic actions on GABA
Umρ1R, this effect may be mediated by a redox-independent allosteric mechanism, in addition to affecting the expression of GABA
Umα5R, which could be a mechanism to attenuate the severity of crises [
34]. In addition, rutin (2) is involved in GABAergic neurotransmission without involvement of BZD receptors [
35]. Previous research also shows that rutin (2) can act on different anxiolytic mechanisms, for example, combating oxidative stress in the CNS, modulating the release of the neurotransmitters serotonin, norepinephrine, opioids and activating opioid receptors [
8]. Quantitative models prove that the increased affinity of these flavonoids (2, 3, 4, 5, 6, 7) for the BZD receptor binding site is associated with the presence of electronegative substituents at 6 and 3' of the flavone backbone [
13].
This study shows the potential of EEFSJ as an antioxidant and anxiolytic agent, making it a promising candidate for therapy against psychological disorders, especially anxiolytics.