Introduction
Epilepsy is a serious and deleterious neurodegenerative disease characterized by persistent unprovoked seizures that distress more than 70 million people globally [
1]. Seizure episodes are caused by anomalous electrical activity in the central nervous system (CNS) [
2]. Despite the availability of more than 30 types of antiepileptic drugs (AEDs), 30–40% of patients develop drug resistance affecting their cognitive function and impairing their psychological behavior with prolonged use [
3,
4]. These facts motivated the search for replacement therapy with greater therapeutic potential and fewer side effects.
Various studies have postulated that several etiologies are involved in the pathophysiology of seizures. A disturbed balance between neuronal excitation and inhibition (glutamate/γ-aminobutyric acid (GABA) equilibrium) plays a major role in seizure development [
5‐
7]. Glutamate and GABA transmission can curb the release and/or uptake of other neurotransmitters, affecting neuronal network activity and maintaining normal balance at the cellular level [
8].
Additionally, increased calcium influx through glutamate receptors alters excitability [
9]. In fact, calcium is a key second messenger in several signaling pathways and has a significant role in the pathogenesis of numerous neurological disorders [
10,
11]. Calcineurin is a neuronally abundant, calcium-dependent serine/threonine protein phosphatase that plays an essential role in neuronal excitability as well as apoptosis [
12]. Disturbances in cellular calcium disrupt several crucial cellular cascades through calcineurin, including the regulation of neuronal plasticity [
13] and the induction of neuronal apoptosis [
14]. Accordingly, alterations in cellular calcium levels stimulate calcineurin-mediated signaling, which is intricately involved in acute seizures and status epilepticus [
15].
Furthermore, neuroinflammation, the generation of reactive oxygen species (ROS), and oxidative status contribute significantly to epilepsy progression [
16]. Interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) are the main epileptogenic cytokines, in addition to the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κΒ). Collectively, these factors all have an impact on increased glutamate levels, leading to abnormal neurotransmission and epileptic seizures [
17].
Recently, numerous studies have focused on the impact of neuroinflammation on the promotion of seizure development by neuronal excitability [
18‐
20]. Inflammasome (Nod)-like receptor protein-3 (NLRP3) is a cytosolic protein that combines with an adaptor protein known as apoptosis-associated speck-like protein containing a CARD (ASC) stimulating caspase-1 [
21]. The NLRP3/ASC/Caspase-1 cascade consequently leads to the activation of pro-IL-1β and pro-IL-18 [
22,
23]. NLRP3 activation has been shown to influence the pathophysiology of Parkinson’s disease [
24], multiple sclerosis [
25]
, and epilepsy [
26,
27]. Additionally, an increasing number of researchers have reported that inhibition of the NLRP3 pathway can ameliorate epileptic seizure episodes and cognitive dysfunction [
28‐
30]; therefore, the NLRP3 pathway could be considered a potential treatment target for epilepsy [
31‐
33].
Epileptic seizures affect the balance between Bcl-2-associated death protein (Bad) and Bcl-2-associated X protein (Bax), which are proapoptotic factors, and the antiapoptotic factor B-cell lymphoma 2 (Bcl-2), which in turn activates caspase-3, inducing proteolytic damage and cell death [
34].
Sinapic acid (SA) (4-hydroxy-3,5-dimethoxy cinnamic acid or sinapinic acid) is a recognized carboxylic acid that is a member of the phenylpropanoid family and is naturally present in citrus fruits, vegetables, and cereals [
35,
36]. Examples of citrus fruits include lemon and Murcott orange [
37]. Berries such as strawberries and cranberries also possess a good quantity of SA [
38]. The SA content in rye, wheat, rice, oats and other cereal grains is found to be 8% to 10% of all phenolic acids [
39]. Spices are also found to contain larger amounts of SA [
40,
41]. Additionally, Brassica vegetables such as broccoli, cabbage, turnip and radish have been shown to contain SA and its derivatives [
42].
SA has been shown to have potent antioxidant, antiproliferative and antiapoptotic effects [
35,
43‐
46]. SA exerts its antiapoptotic effects by decreasing Bax and caspase-3 protein expression and increasing Bcl-2 protein expression levels [
43]. Furthermore, the neuroprotective effects of SA have been previously reported to be attributed to its antagonistic effects on the GABA-A receptor [
47,
48].
The machinery through which SA exerts its anti-inflammatory effects comprises
in vivo and
in vitro inhibition of the NLRP3 inflammasome, caspase-1 activation and IL-1β production [
49]. SA effectively reduces inflammation by inhibiting malondialdehyde (MDA), TNF-α and myeloperoxidase expressions in an inflammatory colitis model [
50] and by decreasing NF-κB expression, inhibiting its downstream inflammatory cascade in an acute doxorubicin-induced cardiotoxicity model [
43]. The current study pointed at discovering the potential effects of SA administration at two therapeutic doses on seizure vulnerability, oxidative stress, neuroinflammation and apoptotic events complicated by the pathophysiology of pentylenetetrazol (PTZ)-evoked acute seizures in mice. PTZ is an antagonist of GABA-A receptors. Sequential repeated injections of a subconvulsive dose of PTZ have been used for triggering chemical kindling seizures, whereas a single intraperitoneal (IP) injection at a high dose induces acute, severe seizures. This animal model of acute seizures has been broadly used to study neuronal, behavioral and biochemical alterations after epileptic seizures [
51].
Discussion
In the present study, pretreatment with oral SA significantly alleviated oxidative stress, restored the GABA/glutamate neurotransmitter balance, restored calcium/calcineurin signaling and downregulated NLRP3 activation, which collectively ameliorated PTZ-induced seizures and improved spatial working memory in PTZ-treated mice. The biochemical and neurobehavioral findings presented herein reveal, for the first time, a multifaceted neuroprotective mechanism for SA in a PTZ model of acute seizures and highlight a framework for the involvement of three dominant and intermingled axes in seizure pathogenesis related to calcium/calcineurin signaling, NLRP3 activation and the GABA/glutamate balance.
Several compounds, including PTZ, tetanus toxin, penicillin, N-methyl-D, L-aspartate, and strychnine, are utilized to induce acute seizures in animal models. In fact, acute seizure models, especially the PTZ model, have been broadly employed for the evaluation of AEDs [
68,
69]. The PTZ-induced convulsive seizure model is the most commonly used model because it is considered a clinical epilepsy model since it mimics epilepsy in people with head trauma [
70].
Seizure severity in different experimental seizure models was assessed via behavioral scoring. Racine's scale is an intensity measure frequently applied in acute seizure modeling [
10,
71]. According to our behavioral observations, PTZ-treated mice were less sensitive to PTZ after pretreatment with both doses of SA, indicating that SA succeeded in preventing PTZ-induced seizures in mice. As a bioactive phenolic acid, SA and other phenolic acids are widely known for their neuroprotective effects through complex cellular mechanisms [
72]. Neuroprotective effects of SA were previously reported in a model of Alzheimer's disease induced by the amyloid β1–42 protein in mice [
47] and in a 6-hydroxydopamine-induced hemi-parkinsonian model in rats [
48], in addition to oxidative stress-evoked disorders in addition to aging [
35,
54,
73].
Changes in brain connectivity and cognitive impairments, in addition to impaired memory and learning functions, are linked to epilepsy in adult patients [
74], as are a number of epileptogenesis and acute seizure animal models [
75]. The Y-maze test has been used to assess short-term recognition capacity in animals. Spontaneous alternation is a measurement of spatial working memory that can be evaluated by permitting animals to freely discover all arms of the Y-maze depending solely on the innate curiosity of rodents, which directs it to explore formerly unvisited parts. A mouse with integral spatial working memory will remember the previously visited arms and express less drive to visit them again [
76].
In our study, PTZ significantly impaired short-term recognition capacity, as indicated by decreased SAP and total arm entries in PTZ-treated mice. PTZ-induced seizures are known to affect short-term memory in rodents [
77]. In fact, several earlier studies have associated seizure-induced deterioration in cognitive performance with the extent of neuronal damage, mainly in the hippocampus [
78‐
80]. Herein, SA pretreatment successfully reversed PTZ-induced short-term memory impairments. SA has cerebral protective effects and cognitive-improving effects, as reported previously in a mouse model of scopolamine-induced memory deficit [
81]. Additionally, SA was found to be effective in preventing memory loss in intracerebroventricular streptozotocin-induced cognitive dysfunction [
82] and in toluene-induced dementia [
83].
An ideal working brain requires regulated and well-balanced excitatory and inhibitory input. At the cellular level, glutamate and GABA are the master excitatory and inhibitory neurotransmitters, respectively [
84]. Perturbation of the excitatory/inhibitory balance leads to defective signaling, which causes impaired cognitive and motor handling and eventually neuronal damage [
8]. In fact, epileptic seizures are known to be associated with an imbalance in GABA/glutamate neurotransmission [
84]. PTZ administration causes enhanced glutamatergic transmission with concurrent decreases in GABAergic transmission in the mouse brain [
85].
In our study, we established that pretreatment with SA significantly improved GABA levels and decreased glutamate levels in the brain. Interestingly, the high dose of SA (40 mg/kg) succeeded in normalizing the glutamate content in the mice. The previously reported neuroprotective effects of SA have been attributed to the fact that SA is a GABA
A receptor agonist. Such GABA
A receptor activation is believed to hinder the neurotoxicity induced by agonists of glutamate receptors, such as kainic acid, which is a potent epileptogenic drug [
73,
86]. Previously, SA was shown to significantly attenuate kainic acid-induced neuronal apoptosis by activating GABA
A receptors and scavenging free radicals [
86].
The production of ROS is natural and inevitable, even under normal physiological conditions. In the CNS, oxidative stress underlies the pathogenesis of numerous neurological illnesses, including stroke, neurodegenerative diseases, and epilepsy. In fact, oxidative stress is extensively involved in the initiation and progression of epileptic seizures [
87,
88]. PTZ-induced acute seizure modeling has been fundamentally linked to an unbalanced oxidative status [
70]. Our results are in accordance with several previous reports in which PTZ-induced oxidative stress was reflected by significantly increased MDA content in brain tissues with concurrent depletion of the antioxidant GSH in the brain [
70,
89,
90].
In our study, pretreatment with SA increased the antioxidant GSH and decreased the increase in MDA to ultimately alleviate the oxidative stress status induced by PTZ administration in mice. These results are consistent with the fact that the biological and therapeutic properties of SA are reported to be antioxidative in nature [
36,
73,
83,
91].
Excessive stimulation of excitatory receptors results in a calcium burden in the cytoplasm with a consequent production of free radicals, which has been implicated in neuronal death in several neurological pathologies, including seizures [
92,
93]. Calcium signaling is a self-motivated second messenger structure that may connect extrinsic signals to complex intracellular activities within neuronal cells. Calcineurin, a heterodimeric calcium-binding serine/threonine phosphatase, comprises a catalytic subunit and a regulatory subunit [
10,
15]. Extensive intracellular calcium accumulation and calcineurin upregulation play pivotal roles in neuronal disorders, which has been attributed to the fact that during neurodegeneration and neuronal apoptosis, calcium homeostasis disruptions are mediated by calcineurin [
10,
94]. Our findings of increased total calcium content and upregulated calcineurin expression associated with PTZ intoxication are consistent with several previous reports in temporal lobe epilepsy patients [
95] and several animal models of seizures [
10,
15,
94].
In this study, pretreatment with both doses of SA decreased total calcium levels and downregulated calcineurin gene expression. These results are in line with previous studies in which SA restored calcium levels after ACR-induced neurotoxicity
in vitro in glioma-derived U87MG cells [
73]. Moreover, SA treatment reportedly has calcium-lowering effects in a model of fulminant hepatitis induced by D-galactosamine/lipopolysaccharide in rats [
91].
Inflammasome activation plays a central role in epileptogenesis [
96]. In animal models of PTZ-induced acute seizures and brain tissues from individuals with pharmacoresistant temporal lobe epilepsy, seizure activity triggers NLRP3 signaling, which can consequently cause apoptosis and neurodegeneration [
97‐
100]. Hence, the use of immunomodulatory drugs that negatively regulate NLRP3 inflammasome activity represents an effective strategy for controlling epileptogenesis and reducing seizures [
98].
In our study, the levels of NLRP3 and ASC were significantly decreased by SA pretreatment, which can explain the neuroprotective effect reported herein since SA succeeded in preventing the occurrence of seizures by inhibiting the NLRP3 activation pathway. This postulation can be further proven by the significant decrease in IL-1β reported in our study in both groups pretreated with SA. The capacity of SA to inhibit the activation of the NLRP3 inflammasome was previously reported in rats with diabetic atherosclerosis [
101] and in a Kunming mouse model of dextran sodium sulfate-induced ulcerative colitis [
102] as well as an intestinal fibrosis model in C57/6BL mice [
23].
Another promising finding was that SA formed stable hydrogen bonds with the active site residues of NLRP3 (Ala228 and Arg351) and ASC (Lys15, Glu111, Glu153 and Ala63). Molecular docking analysis was performed for SA in the NLRP3 inflammasome and ASC-CARD to gain deeper insight into the structural features essential for the binding of our ligand to the relevant proteins. Together, the present findings confirmed the in vivo data generated from our study that SA treatment significantly decreased the levels of NLRP3 and ASC.
Naturally, the NLRP3 inflammasome is implicated in epileptic neuronal apoptosis. In fact, PTZ is known to induce apoptotic neurodegeneration in animal models of seizures and in HCN-2 neuronal cells [
103]. The neuropathologic effects of PTZ can be explained by the ability of PTZ to trigger the neuronal intrinsic apoptotic death program, as indicated by the increased expression of proapoptotic players, including Bad, Bax, caspase-9, caspase-3, and cytochrome-c, and the concurrent decrease in the expression of antiapoptotic Bcl-2 [
104‐
106]. Herein, in our study, the PTZ-treated group exhibited significantly upregulated Bad expression with simultaneous downregulation of Bcl2 expression, which agrees with the findings of the aforementioned studies [
104‐
106].
SA has been found to have antiapoptotic effects on several pathologies, including gentamicin-induced nephrotoxicity in rats [
107], cadmium-induced nephrotoxicity [
108] and acute doxorubicin-induced cardiotoxicity [
43]. Moreover, SA has shown beneficial effects in combating apoptosis in an
in vitro model of Parkinson’s disease, as SA protected SH-SY5Y human neuroblastoma cells from 6-hydroxydopamine-induced neuronal apoptosis [
43].
Additional research is needed to confirm our results and gain a deeper understanding of the role of SA in the regulation of calcium/calcineurin signaling, NLRP3 activation, and other pathways that contribute to epileptic seizures. Moreover, investigating the effect of SA on seizures in female animals could shed light on the potential influence of sex on SA-induced neuroprotection in PTZ-treated animals. Finally, exploring the possible neuroprotective effects of SA in patients with seizures may offer valuable insights for future research.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.