1. Introduction
Epilepsy is a brain disease clinically manifested by epileptic seizures in both humans and animals [
1,
2]. The International League against Epilepsy (ILAE) has introduced a system to classify epileptic seizures according to their etiology in metabolic epileptic seizures, structural epilepsy, idiopathic epilepsy and genetic epilepsy [
3]. Idiopathic epilepsy is most commonly diagnosed in young purebred or mixed-breed dogs and can be compared to human temporal lobe epilepsy or human idiopathic generalized epilepsy [
4,
5,
6,
7,
8]. Structural epilepsy is mainly diagnosed in adult and aged dogs; in these age groups, inflammatory encephalopathy is being diagnosed most frequently in small-breed dogs and extra-axial neoplasia in large-breed dogs [
9,
10]. Diagnosis of the different types of epilepsy is based on advanced diagnostic imaging and/or cerebrospinal fluid (CSF) analysis [
11]. The application of magnetic resonance imaging (MRI) in veterinary medicine has revealed many brain structural and functional abnormalities; however, it cannot identify and explain the molecular basis of the spontaneous abnormal electrical discharge of neurons that causes epileptic seizures in idiopathic epilepsy [
9,
10,
12,
13,
14,
15,
16]. Electroencephalography (EEG), a routine diagnostic test in human patients with epilepsy, can identify the locus of the abnormal electrical discharge in the specific lobe of the brain; however, its use is limited in veterinary medicine [
17].
Regarding epileptogenesis, while significant progress has been recorded in recent years, many aspects of the underlying mechanisms remain unclear [
18,
19]. Oxidative stress occurs when there is an imbalance between the reactive oxygen species (ROS) or reactive nitrogen species (RNS) and the brain’s antioxidant defenses [
20,
21]. In the brain, which has high oxygen consumption and relatively low antioxidant capacity, oxidative stress is especially damaging. Therefore, oxidative stress has a significant role in the development and progression of epilepsy, particularly in the process of epileptogenesis and seizure-induced brain damage [
22]. Oxidative stress has been assessed with specific biomarkers in many studies in blood or brain tissue from patients with epilepsy or from animal models exhibiting
status epilepticus [
22,
23,
24,
25,
26,
27,
28]. However, the bibliography is limited regarding the assessment of oxidative stress markers in cerebrospinal fluid (CSF), which could be the ideal material to study through its direct contact with the brain [
29]. Oxidative stress is strongly connected with neuroinflammation during epileptic seizures and epilepsy. It forms a vicious cycle, where each process amplifies the other and contributes to neuronal damage, epileptogenesis and seizure recurrence [
22,
30,
31]. Many studies have evaluated inflammatory markers and particularly C-reactive protein (CRP) in blood and CSF samples of humans and animals with neurological disorders including epilepsy [
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46]. Blood CRP is found elevated in patients with epilepsy compared with controls, and antiepileptic medication can reduce CSF and blood CRP levels [
24,
32,
33,
35,
36,
38,
40,
41,
42,
43]. Elevated CRP levels are found in CSF of dogs affected with distemper and in serum samples of dogs with
status epilepticus due to idiopathic epilepsy [
44,
45,
46].
Cholinesterase does not play a primary active role in epilepsy; it can have an indirect/modulatory role through its impact on acetylcholine (AChE) levels, which affect neuronal excitability. Therefore, if cholinesterase activity is inhibited, AChE accumulates, leading to neuronal overexcitation, triggering epileptic seizures or
status epilepticus [
47,
48]. The bibliography is limited regarding the assessment of cholinesterase in human patients with epilepsy, probably because of its indirect association with epilepsy. There is a paper indicating elevated cholinesterase activity in patients with epilepsy and decreased cholinesterase levels in the blood and CSF of patients with epilepsy after surgical treatment [
47].
Oxytocin has been evaluated for its antiepileptic properties, mostly in experimental studies of patients with epilepsy, as well as in patients with other mental co-morbidities [
49,
50,
51,
52,
53]. In veterinary medicine, research regarding oxytocin has been performed in mice and no published data in dogs have been identified [
51,
53]. Canine epilepsy shares many clinical and pathophysiological similarities with human epilepsy, therefore a canine model should be considered ideal to study the therapeutic potential of oxytocin in canine epileptic patients.
The current study aimed to assess oxidative stress and inflammatory markers in serum and cerebrospinal fluid (CSF) samples of dogs naturally affected by idiopathic epilepsy. In addition, oxytocin was measured and a new assay for its quantification in CSF was validated.
4. Discussion
Epilepsy is a complex disease entity that involves inflammatory and oxidative stress processes in addition to abnormal electrical activity [
31]. In the current study, inflammatory markers (CRP), oxidative stress markers (PON1, CUPRAC, FRAP), cholinesterase and oxytocin were assessed in serum and CSF samples of epileptic dogs with different types of epilepsy.
Serum CRP can be temporarily increased in patients exhibiting generalized tonic–clonic seizures,
status epilepticus, or prolonged seizures. This increase is modest unless there is another underlying condition [
33,
38]. Most patients with epilepsy have normal CRP, especially between seizures [
38]. In the current study, CRP was assessed in serum samples of epileptic dogs. Median values were 5.55 μg/mL for Group A (control group), 4.1 μg/mL for Group B, 3.4 μg/mL for Group C, and 3.8 μg/mL for Group D. None of the median values exceeded the reference range for CRP in serum samples (<10 μg/mL). All comparisons among the four groups did not reveal any significant differences. Multiple human studies have indicated increased serum CRP values in patients with epilepsy compared with controls [
41,
42]. In particular, despite the increased serum CRP concentration in refractory epilepsy cases, CRP values were decreased when patients received antiepileptic medication but still remained increased compared with controls [
33,
35,
37]. Levetiracetam antiepileptic treatment decreased serum CRP concentration compared with other antiepileptics [
40,
43]. In an experimental rat model assessing CRP at different time points after electrically induced
status epilepticus, there were no concentration changes identified [
34]. In contrast to this study, other studies involving epileptic dogs indicated increased serum CRP levels in dogs diagnosed with structural epilepsy compared with idiopathic epilepsy dogs and in dogs exhibiting
status epilepticus [
44,
45]. In the current study, there was no significant difference in CRP levels among the three groups of epileptic dogs compared with controls. The time elapsing from the last seizure till serum sampling and the different antiepileptic medications administered (Group B and Group D dogs) could have influenced the results. In particular, concerning the time interval between the last seizure and serum sampling, it was not standardized for the study population; therefore, sampling was performed regardless of the time the last epileptic seizure occurred. Furthermore, no inflammatory encephalopathy cases were included in the structural epilepsy Group D. In a previously published study, including dogs diagnosed with distemper encephalitis, serum CRP levels were elevated compared with controls [
46]. The results of the current study support evidence from human patients; CRP had been within reference ranges in patients with epilepsy suffering from tonic–clonic epileptic seizures [
38]. Results from the current study indicate that CRP is not a reliable inflammatory marker for either idiopathic or structural epilepsy in dogs.
Oxidative stress has been associated with epilepsy in both human and canine patients [
24,
25,
27,
46]. Although there are multiple studies assessing oxidative stress in human neurological diseases, including epilepsy, the bibliography is limited in canine epilepsy [
26,
28,
64,
65]. In the current study, selective oxidative stress markers were evaluated in both serum (PON1 and CUPRAC) and CSF (PON1, CUPRAC, FRAP) samples of three groups of dogs diagnosed with different types of epilepsy and a control group (Group A). Paraoxonase 1 (PON1) has an important anti-inflammatory and antioxidant role; it protects lipids and lipoproteins from oxidative damage by preventing lipid peroxidation in cell membranes and lipoproteins [
66,
67]. In general, PON1 concentration was decreased in oxidative stress [
66,
67]. The overall assessment of median values of PON1 of the current study indicated that serum concentrations were much lower compared with CSF concentrations. To the authors’ knowledge, there is no available literature indicating the reference range of PON1 in serum or CSF in dogs with epilepsy. In the study of [
65], where antioxidant markers, including PON1, in dogs with idiopathic epilepsy were assessed, it was concluded that serum PON1 was lower compared with healthy controls, but no reference ranges were provided. Contrary to the results of comparisons of the serum PON1 values among the four study groups, there was a statistically significant difference in CSF PON1 when healthy controls (Group A) and dogs with idiopathic epilepsy that did not receive antiepileptic medication (Group C) were compared with structural epilepsy (Group D). A possible explanation for this finding could be the severity of brain damage in Group D cases (structural epilepsy) and the demand for further antioxidant protection of the nervous tissue from further damage. Since PON1 cannot cross the blood–brain barrier (BBB), even if it is impaired [
68], the results of the current study are an important finding that requires further investigation. The same research group mentioned that, despite the fact that there is no documented gene expression in mouse or human brain tissue, a hypothesis of transport of PON1 via “discoidal HDL” with unspecified mechanisms could not be excluded [
68]. The BBB not only limits the passive diffusion of these markers but also influences the dynamics of redox homeostasis in the brain, often resulting in systemic markers that do not reflect the actual oxidative condition within the CNS [
69]. There were additional studies of PON1 identification in CSF samples of patients suffering from neurodegenerative diseases and they speculate that CSF PON1 originated from the periphery [
70,
71]. Therefore, CSF PON1 identification, origin and mechanism of action in epilepsy need further investigation.
CUPRAC measurement is a reliable method for assessing the antioxidant capacity of a sample by reducing Cu
2+to Cu
1+ [
58]. Therefore, decreased CUPRAC values may indicate reduced antioxidant defense in multiple diseases [
58]. Limited data are available regarding the assessment of CUPRAC in human and canine epilepsy. Overall assessment of median CUPRAC values between the two different sample types (serum and CSF) indicates a tendency for higher CUPRAC values in serum compared with CSF (except for Group D). There was no significance identified in either serum or CSF CUPRAC among the four groups. To the authors’ knowledge, there are no other previously published papers assessing CUPRAC in patients with epilepsy.
FRAP (ferric reducing ability) is a method that assesses the antioxidant capacity of a sample by reducing ferric ion (Fe
3+) to ferrous ion (Fe
2+) [
59]. In the current study, FRAP was evaluated in CSF. Statistical analysis did not reveal any significance of FRAP among the four groups. Previous studies reported increased serum and CSF FRAP values in canine patients with distemper encephalitis and decreased values in human patients diagnosed with Fabry disease [
46,
64]. Since the published literature is limited and involves different species (human vs. canine) and/or different disease entities, secure conclusions could not be extrapolated regarding FRAP in canine epilepsy.
Cholinesterase activity (acetylcholinesterase and butyrylcholinesterase) is correlated with epilepsy through cholinergic neurotransmission, which is closely linked to neuronal excitability and seizure activity [
47,
65]. In this study cholinesterase was assessed in both serum and CSF samples of epileptic dogs and healthy controls. Serum cholinesterase activity was not significant among the four study groups. On the contrary, CSF cholinesterase activity was significant when Group D dogs (structural epilepsy) were compared with the other two groups of idiopathic epilepsy (Groups B and C) and the control group (Group A). CSF cholinesterase activity is altered (increased) probably through a localized release in the brain, as a compensatory mechanism [
72]. In this study, both serum and CSF median cholinesterase values are increased, but the increase in CSF is more prominent. Interestingly, an increase was also recorded in Group A (control group). A possible explanation could be that stress may be responsible since these dogs were thoroughly investigated and no abnormalities were identified during routine physical examination or clinicopathological testing. The bibliography supports the influence of acute stress episode on cholinesterase by increasing its activity in the brain and peripheral nervous system [
73].
In this report an AlphaLISA assay for the measurement of oxytocin in CSF of dogs was analytically validated given the adequate values of precision and accuracy, indicating that this assay can be applied for oxytocin CSF quantification. In humans and rats, exogenous oxytocin administration (intranasally, intra-hippocampal microinjection) may reduce seizure severity and frequency on a long-term basis [
49,
50,
51,
52,
53]. In this study, CSF endogenous oxytocin levels were evaluated in the four groups of dogs. There was a statistically significant increase in CSF oxytocin between Group D dogs compared with the other two groups of idiopathic epilepsy dogs (Groups B and C). This increase in Group D could be due to the presence of more severe brain lesions when structural epilepsy is suspected and could increase to compensate for the damage since it produces neuroprotection [
53]. However, the small sample size of Group D dogs (eight dogs) necessitates further investigation in a larger animal population.
The limitations of the current study include missing data. Notably, the CSF sample size was modest (26 samples), and some biomarkers were unavailable in certain groups because of limited CSF volume, resulting in missing data. This reduces statistical power—particularly for subgroup comparisons—and constrains generalizability. Therefore, these results require confirmation in larger, adequately powered cohorts with more complete CSF profiling and independent validation. In addition, some limitations are related to the quantification and analytical interpretation of oxidative stress and inflammatory biomarkers. Single time-point measurements may not fully capture the dynamic fluctuations associated with seizure activity. Importantly, due to the clinical nature of this study and the inclusion of naturally occurring epilepsy cases, a strict control of sampling in relation to seizure timing was not feasible, reflecting real-world clinical conditions. Despite these inherent constrains, the study design was strengthened by standardized sample processing, batch analysis of samples, and use of validated analytical assays, supporting the internal consistency and reliability of the findings. The heterogeneity of the study population with respect to antiepileptic medications, which constitutes one of this study’s limitations, reflects the differing needs of each individual case and is manifested in the variable response to treatment, as demonstrated by the seizure frequency and severity. Additional research is required to evaluate cholinesterase, oxytocin and oxidative stress, and inflammatory markers in larger groups of epileptic dogs. Homogeneity is quite difficult to obtain in naturally occurring animal studies since each individual requires specific antiepileptic medication and seizure frequency is unique and unpredictable for every case.