In PWE suffering from TLE, hippocampal 5-HT depletion (da Fonseca et al., 2015) and reduced 5-HT_1AR availability have been observed. The latter somatodendritic 5-HT_1A autoreceptor is one of the best characterized subtypes of the 14 known 5-HTRs and is clearly implicated in seizure modulation. A large body of evidence on this receptor subtype in epilepsy has arisen from many positron emission tomography (PET) studies in PWE, and reduced 5-HT_1AR binding in the epileptic focus has been repeatedly and consistently found in these temporal lobe epilepsy (TLE) patients (Hasler et al., 2007; Lothe et al., 2008; Giovacchini et al., 2009; Assem-Hilger et al., 2010). All of these studies point to the fact that diminished 5-HT_1AR expression and subsequent less activation by endogenous 5-HT may lead to the epileptic phenotype. PET imaging of brain 5-HT_1ARs has also helped in the correct identification of the epileptogenic zone during the preoperative evaluation of temporal lobe of PWE subjected to epilepsy surgery (Didelot et al., 2008; Theodore et al., 2012).

More recently, it has been shown that both 5-HT₆Rs (Wang et al., 2015) and 5-HT₇Rs (Yang et al., 2012) were upregulated in the human neocortex of PWE with refractory TLE. These interesting findings call for more studies with 5-HT₆R and 5-HT₇R ligands.

Acute seizure evocation with KA led to increases in hippocampal 5-HT tissue content and extracellular 5-HT levels (Alfaro-Rodriguez et al., 2011; Tchekalarova et al., 2015a) while during the spontaneous recurrent limbic seizures in the KA model decreases in 5-HT content were found (Tchekalarova et al., 2011). In another well-established post-SE rat model for focal epilepsy using pilocarpine (i.e., a muscarinergic receptor agonist) as the chemoconvulsant, 5-HT hippocampal content (Cavalheiro et al., 1994) and hippocampal 5-HT levels (Meurs et al., 2008) were increased during the acute seizure phase but not during the following spontaneous recurrent seizure phase (Cavalheiro et al., 1994; Szyndler et al., 2005). Comparing three acute limbic seizure models, which differed only in the chemoconvulsant used to evoke the seizures in rats, no straightforward correlation between the seizure activity and increased hippocampal extracellular 5-HT concentrations could be found (Meurs et al., 2008).

Concerning the role of the somatodendritic 5-HT_1A autoreceptor in focal epilepsy, the majority of pharmacological studies clearly highlight the anticonvulsant effects of 5-HT_1AR agonists against limbic seizures evoked in various rat models, e.g., against pilocarpine-induced seizures (Clinckers et al., 2004; Lopez-Meraz et al., 2005; Pericic et al., 2005; Orban et al., 2013), as well as against status epilepticus evoked by lithium pilocarpine (Yang et al., 2014).

Activation of the 5-HT_2CRs do not appear to play a pivotal role in focal epilepsy or on the contrary is proepileptic (Di Giovanni and De Deurwaerdere, 2016). Indeed, 5-HT_2CR agonists with different pharmacological profiles such as meta-chlorophenylpiperazine (mCPP) and lorcaserin, but not RO60-0175, were able to stop the elongation of the electrically triggered hippocampal maximal dentate gyrus activation in a limbic seizure model in anesthetized rats, an effect that was not blocked but rather potentiated by pre-treatment of SB 242084 (Orban et al., 2014), a selective 5-HT_2CR antagonist. In addition, 5-HT₃Rs display also no importance in focal hippocampal seizures (Watanabe et al., 1998).

A selective 5-HT₆R antagonist was able to attenuate spontaneous recurrent seizures in the post-SE pilocarpine rat model, and diminished hippocampal mechanistic target of rapamycin (mTOR) activity, suggesting that 5-HT₆Rs may mediate limbic seizures via mTOR signaling (Wang et al., 2015). Moreover, 5-HT₆R expression was upregulated in the hippocampus and neocortex of the pilocarpine-treated rats (Wang et al., 2015), confirming the finding in PWE as described above. There is one study showing that 5-HT₇R antagonism also diminished the number of limbic seizures in pilocarpine-treated rats (Yang et al., 2012). More confirmatory results with 5-HT₆R and 5-HT₇R antagonists in focal epilepsy models might be interesting to obtain.

Serotonin inhibited bicuculline (i.e., a GABA_A receptor antagonist)- and KA-evoked epileptiform activity in brain slices via membrane hyperpolarization (Salgado and Alkadhi, 1995). The use of a 5-HT₃R agonist showed no effect on cortical epileptiform activity (Bobula et al., 2001). In rat hippocampal brain slices, 5-HT₄R agonism aggravated population spikes, evoked by electrical stimulation and spontaneous epileptiform activity (Tokarski et al., 2002). The influence of many other 5-HT receptor subtypes on epileptiform activity remains elusive.

In PWE suffering from TLE, alterations in the neocortical DA content, D1-like and D2-like receptor expression, and DA transporter (DAT) binding have been reported (Rocha et al., 2012). Reduced D₂R/D₃R binding, sustained impairment of the DAergic system, was demonstrated in extrastriatal and/or striatal brain regions of PWE, specifically with TLE (Bernedo Paredes et al., 2015).

Similarly as described above for 5-HT, acute kainic acid-induced seizures increased hippocampal DA tissue content and extracellular DA levels, (Alfaro-Rodriguez et al., 2011; Tchekalarova et al., 2015b) while during the spontaneous recurrent limbic seizures in the kainic acid model, decreases in DA content were found (Tchekalarova et al., 2011). In the pilocarpine rat model, DA hippocampal content (Cavalheiro et al., 1994) and hippocampal DA levels (Meurs et al., 2008) were increased during the acute seizure phase. During the chronic recurrent seizure phase in the pilocarpine model, hippocampal DA content was elevated in one study (Cavalheiro et al., 1994) while no alterations were described in another study (Szyndler et al., 2005). Interestingly, when comparing three acute limbic seizure models, a direct relationship between the seizure activity and increased hippocampal extracellular concentrations of DA were established (Meurs et al., 2008).

In line with the majority of data described for generalized seizures, D₁-like receptor-activation results in seizure enhancement in the limbic pilocarpine model (Barone et al., 1990). Activation of hippocampal D₂-like receptors, leading to inhibition of adenylyl cyclase (AC) via Gi coupling, consistently protected rodents against limbic motor seizures, supporting seizure facilitation via D₁R-mediated increases in cyclic adenosine monophosphate (cAMP; Bozzi and Borrelli, 2013). Moreover, the D₂-like receptors seem to play a pivotal role in the overall anticonvulsant effect of hippocampal DA since a selective D₂R antagonist abolished DA-mediated anticonvulsant effects in the acute pilocarpine limbic seizure model (Clinckers et al., 2004). Most interestingly, D₂-like receptor signaling in the hippocampus also leads to glycogen synthase kinase 3β inhibition and hippocampal cell survival following kainic acid administration (Dunleavy et al., 2013).

Electrophysiological data demonstrated that DA can affect hippocampal excitability in a biphasic fashion but the predominant DA action was an D₂-like receptor-mediated inhibitory effect via hyperpolarization of the resting membrane potential and a long-lasting increase in after-hypolarization (Benardo and Prince, 1982). D₄R KO mice displayed cortical hyperexcitability, as measured with electrophysiological current and voltage-clamp recordings, suggesting that D₄R activation can negatively modulate glutamate (GLU) activity in the frontal cortex (Rubinstein et al., 2001). This is not unexpected from a receptor from the D₂-like receptor family that mainly exhibits decreased excitation upon activation.

Patients with juvenile myoclonic epilepsy showed a reduction in D₂R/D₃R binding restricted to the bilateral posterior putamen, suggesting an alteration of the DAergic system within this region (Landvogt et al., 2010).

Repeated D₁-like receptor activation results in generalized seizures, disrupted hippocampal plasticity, and impaired long-term recognition memory (Gangarossa et al., 2014). Initially, the majority of results on D₁-like receptor agonist effects on behavioral seizure thresholds clearly indicated proconvulsant effects (Starr, 1996). Nevertheless, recent reports showed that D₁-like receptor agonists, linked to stimulation of adenylate cyclase (AC; but not phospholipase C, PLC), led to prominent behavioral seizures in rodents, whereas D₁-like receptor agonists, linked to stimulation of phospholipase C (PLC, but not AC), did not (O'Sullivan et al., 2008). The D₅R belongs to the D₁-like receptor family and upon its activation mainly increased excitation is observed, although less prominent in comparison with D₁R activation and subsequent increases in cAMP (O'Sullivan et al., 2008).

The role of the D₃R in seizure modulation appears to be more complex. D₃R KOs were less sensitive to picrotoxin-induced clonic seizures and mortality suggesting proconvulsant D₃R-mediated signaling (Micale et al., 2009). On the other hand, D₃R-mediated agonist actions protected against acute and cocaine-kindled seizures in mice and reduced lethal effects of acute cocaine toxicity (Witkin et al., 2008). Most probably, different D₃R downstream signaling cascades in different implicated brain regions may explain the contrasting results (Bozzi and Borrelli, 2013); however more investigations are required.

The DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA) improved the clinical outcome of a male patient suffering from intractable epileptic encephalopathy (Horvath et al., 2016), again sustaining an overall anticonvulsant DA-mediated action. Nevertheless, DA concentrations in media collected from neural cultures, derived from induced pluripotent stem cells from a patient with the Dravet syndrome, were higher than those from wild-type neural cultures (Maeda et al., 2016).

Activation of both D₁-like and D₂-like receptors showed anti-absence effects (Deransart et al., 2000), probably by decreasing GABA_A receptor-mediated tonic inhibition (Yague et al., 2013) that is altered in animal models of absence seizures (Cope et al., 2009). Up-regulation of D₃ (but not D₁, D₂, or D₅) receptor mRNA seems part of the epileptic phenotype in absence-epilepsy prone rats (Deransart et al., 2001). The role of D₁-like and D₂-like receptors in non-convulsive epilepsy is thus less clear-cut in comparison with the data obtained in focal and generalized seizure and epilepsy models.

In this context, the receptor-binding assays with prazosin as a ligand, performed on isolated cortical cell membranes from 10 PWE subjected to temporal lobectomy due to intractable partial epilepsy, showed a reduced density of α₁ adrenoceptor (AR) in the sites of the epileptic foci with no change in affinity. It was concluded that the lower receptor density may result in noradrenergic hyposensitivity that could contribute to a localized diminution in inhibitory mechanisms in epileptic foci (Briere et al., 1986).

Likewise, VNS is an effective adjunctive treatment for medically refractory epilepsy, and was found to produce its anticonvulsive effect by increasing NA levels in the hippocampus that is critically involved in the generation of limbic seizures (Raedt et al., 2011). The anticonvulsant action of VNS on pilocarpine-induced seizures in rats can be abolished by the blockade of hippocampal α₂-AR, indicating a strong causal link between the seizure-suppressing effect of VNS and hippocampal noradrenergic signaling (Raedt et al., 2011). Interestingly, combined but not separate α₂- and β₂-AR stimulation inhibited limbic seizures induced by pilocarpine infusion in the hippocampus of rats. On the other hand, α_1A-AR stimulation and α_1D-AR antagonism alone also inhibited seizures associated with respectively significant hippocampal GABA increases and GLU decreases (Clinckers et al., 2010).

NA has demonstrated both proconvulsant and antiepileptic properties; however, the specific pharmacology of these actions has not been clearly established. For instance, under conditions of impaired GABAergic inhibition, the excitatory and inhibitory effects of NA on hippocampal CA3 epileptiform activity are mediated primarily via β- and α₂-ARs respectively. Moreover, the NA antiepileptic effect in CA3 epileptiform in vitro is not dependent on the increase in GABAergic function (Jurgens et al., 2005) but is due to the activation of the α₂-AR on presynaptic glutamatergic terminals of the recurrent axon collaterals of the CA3 pyramidal neurons (Jurgens et al., 2007). While the α₁-AR antagonists prazosin and terazosin had no effect on hippocampal CA3 epileptiform activity in slice with GABA system blocked, (Jurgens et al., 2005) there is in vitro evidence showing that α₁-AR subtype activation was able to release GABA and somatostatin at the single cell level. This suggests that α₁-AR activation may also represent one mechanism by which NA exerts anti-epileptic effects within the hippocampus.

NA may have proconvulsant and anticonvulsant properties under certain conditions; activated noradrenergic transmission could be an etiological factor in some epilepsies. The available clinical data on the subject (NA boosting antidepressants, α₂ AR agonists, pheophromocytoma, etc.) are discussed in detail by Fitzgerald (Fitzgerald, 2010). It has been shown that co-administration of β-ARs ligands with conventional AEDs, i.e., diazepam, phenobarbital, lamotrigine, valproate, enhance the anticonvulsive efficacy of the latter ones (De Sarro et al., 2002; Fischer, 2002; Luchowska et al., 2002).

It has been shown that the animals treated with the monoaminergic toxin 6-hydroxydopamine (6-OHDA) or the noradrenergic toxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), as well as DA beta-hydroxylase (DBH) KO mice that lack NA, expressed increased susceptibility to convulsing stimuli (Bortolotto and Cavalheiro, 1986; McIntyre and Edson, 1989) while, on the contrary, the stimulation of the locus coeruleus in the same animals consistently reduced it (Libet et al., 1977; Weiss et al., 1990). Further confirmation of inhibitory effects of NA on epileptogenesis was obtained by studying the genetic epilepsy-prone rat (GEPR) model (Yan et al., 1993, 1998). With regard to a question related to the receptors involved, the studies yielded conflicting results; the same ligands—agonists or antagonists, could have proconvulsant or anticonvulsant effects, depending on the animal species, the strain, the model of epilepsy employed (for refs see Fitzgerald, 2010) and also receptor location, as in case of α₂-AR (Szot et al., 2004). Specifically, in flurothyl model of generalized convulsive seizures, it has been suggested that presynaptic α₂-AR is responsible for the proconvulsant effect of α₂-AR agonists, while the postsynaptic α₂-AR is responsible for the anticonvulsant effect of α₂-AR agonists (Szot et al., 2004). That α₂-ARs mediate anticonvulsive effects is supported also by the observation that D79N mice which carry a point mutation in the locus of α₂-AR develop amygdala kindling very easily (Janumpalli et al., 1998). Fewer studies concerned with α₁-AR have been conducted. In DBH KO mice, pre-treatment with α₁-AR agonist protected these mice against PTZ induced seizures while α₁-AR antagonist exacerbated PTZ induced seizures in the control mice (Weinshenker et al., 2001b). As to β-ARs, the anticonvulsive activity of propranolol, a non-selective antagonist, has been demonstrated in a variety of animal models of generalized tonic-clonic seizures. Propranolol reduced seizures induced in mice by lidocaine, PTZ, strychnine, low frequency, and maximal electroshock (Saelens et al., 1977; Akkan et al., 1989; Fischer, 2002) as well as by sound in DBA/2 mice (Anlezark et al., 1979; De Sarro et al., 2002), and increased the threshold for lidocaine-induced convulsions in awake animals (Nakamura et al., 2008). Other beta blockers that showed some protective effects include metoprolol, for instance against audio seizures (De Sarro et al., 2002). On the other hand, the anticonvulsant effects of higher doses of clenbuterol against generalized tonic-clonic seizures has also been demonstrated in a couple of used tests (Fischer et al., 2001). Pre-treatment with α₁-AR or β₂-AR, but not α₂-AR or β₁-AR agonist significantly protected against PTZ-induced seizures in DBH^−/− mice. Therefore, activation of the α₁-AR is primarily responsible for the anticonvulsant activity of endogenous NA in the murine PTZ model of epilepsy. Endogenous NA probably does not activate the β₂-AR under these conditions, but exogenous activation of the β₂-AR produces an anticonvulsant effect (Weinshenker et al., 2001a).

Further confirmation of inhibitory effects of NA on epileptogenesis was obtained in GAERS (Micheletti et al., 1987). It has also been shown that various antiepileptic drugs, i.e., carbamazepine, have a modulatory, activating effect on NA system (Olpe and Jones, 1983; Post, 1988).

In an early electrophysiological study, epileptiform field potentials were elicited by omission of Mg²⁺ from the superfusate and recorded from layers II–V of human temporal neocortical slices cut from tissue resected for surgical treatment of epilepsy. The frequency of occurrence of epileptiform field potentials was halved with application of MT (Fauteck et al., 1995).

Clinical studies on a group of 54 children have demonstrated that during a convulsive crisis, the MT concentration in blood, as measured in patient's serum, significantly peaked but normalized within 1 h. The MT production stimulated by the convulsive crisis may be part of the response of the organism counteracting the seizures effects (Molina-Carballo et al., 2007). Nevertheless, in other studies no changes in serum or salivary MT concentrations were found after epileptic seizures (Rao et al., 1989; Motta et al., 2014). Clinical reports have shown the beneficial effect of MT treatment on seizure activity during the day and night (Goldberg and Spealman, 1983; Peled et al., 2001) in patients with intractable epilepsy. Systematic review of all so far published clinical data on MT in relation to epilepsy, including therapeutic use of MT, lead to the conclusion that there is no marked improvement or worsening of seizures with MT (Jain and Besag, 2013) or its use as add-on treatment (Brigo and Igwe, 2016). Only large randomized double blind placebo-controlled trials could give the final answer. In addition, it has been recently suggested that melatonergic drugs may be effective in treating comorbid depression in PWE (Tchekalarova et al., 2015b)

The anticonvulsant activity exerted by MT seen in animal models of epilepsy (Golombek et al., 1992, 1996; Cardinali et al., 2008; Solmaz et al., 2009) has been suggested to be executed via increasing the activity of GABAergic system (Golombek et al., 1996). A selective MT₁/MT₂R agonist mimicked the MT effects in rat rapid kindling model and in the spontaneously epileptic mice lacking voltage-gated Kv1.1 channels (Kcna1-null mice; Fenoglio-Simeone et al., 2009). Beneficial actions of MT in epilepsy have also been attributed to its free radical scavenging properties (Mohanan and Yamamoto, 2002). MT has been reported to have an anticonvulsant action in many models of acute seizures, such as those produced by the administration of PTZ, picrotoxin, bicuculline, pilocarpine, l-cysteine, kainate, 3-mercaptopropionic acid, quinolinate, GLU, strychnine, N-methyl-d-aspartate (NMDA), or penicillin, as well as in the maximal electroshock seizure (MES) test in rats, mice, gerbils, and hamsters (see Banach et al., 2011). MT treatment during epileptogenesis can have beneficial effects against the deleterious consequences of SE in the KA model of TLE. Melatonin chronic treatment increased the seizure-latent period, decreased the frequency of spontaneous recurrent seizures (SRSs), and attenuated the circadian rhythm of seizure activity (Petkova et al., 2014; Tchekalarova et al., 2015b). These findings are in agreement with the earlier evidence that pinealectomy facilitates the epileptogenic process that follows the long-lasting SE. This facilitation can be partially reverted by the simultaneous administration of MT (De Lima et al., 2005). Agomelatine is a novel antidepressant agent, which is structurally homologous to MT. It is a potent MT1 and MT2 MTR agonist as well as a 5-HT_2CR receptor antagonist (Millan et al., 2003). It was recently approved as an antidepressant medication with comparable efficacy to classical antidepressant drugs (Sansone and Sansone, 2011). Agomelatine has anticonvulsant activity shown in PTZ- or pilocarpine-induced seizure models due to its combined action at MT1/2 and 5-HT_2CRs (Aguiar et al., 2012).

It has been shown that subchronic and systemic administration of agomelatine and MT displayed considerable antiepileptic effects on absence seizures in WAG/Rij rats (Hatice et al., 2015).

Agomelatine seems to be recommendable as a potential drug for absence epilepsy and many other complications such as depression and sleep disorders associated with epilepsy.

In a comprehensive study on amygdala kindled seizures in rats, convincing evidence for the suppressive role of central histamine in epilepsy was provided (Kamei, 2001). In the amygdala of kindled rats, a significant decrease of histamine concentrations was disclosed. Exogenous histamine administered to kindled animals elicited the seizure inhibiting effect that was mimicked by H₁R agonists but not H₂R agonists. Moreover, when administered repeatedly to rats, L-histidine retarded development of amygdala kindling (Kamei et al., 1998). Histidine and metoprine also inhibited seizures, and both treatments were associated with enhanced histamine levels in cerebral cortex, hippocampus, hypothalamus, and amygdala. H₃R antagonists evoked an antiepileptic effect, which was prevented by pretreatment with an H₃R agonist and was sensitive to H₁R antagonists. As shown by others (Jin et al., 2005) amygdala kindled seizure inhibition could also be achieved by administration of dipeptide carnosine (beta-alanyl-L-histidine). The antiepileptic effect was antagonized by H₁R blockers of the first generation (pyrilamine, diphenhydramine), indicating histamine participation.

Metoprine inhibited seizures evoked by amygdala kindling in rats (Kamei et al., 1998; Kamei, 2001), as well as reduced audiogenic convulsions in genetically audiogenic seizure sensitive rats (Tuomisto et al., 1987). Likewise, L-histidine inhibited amygdala kindled seizures (Kamei et al., 1998) as well as PTZ-induced seizures in rats (Chen et al., 2002), an effect potentiated by H₃R antagonist, thioperamide, and antagonized by α-fluoromethylhistidine (inhibitor of histidine decarboxylase) as well as by pyrilamine, H₁R antagonist (Chen et al., 2002). In audiogenic epilepsy prone Krushinski–Molodkina rats, as opposed to epilepsy resistant Wistar rats, brain histamine concentrations are significantly lower (Onodera et al., 1992). In different animal models of epilepsy, imidazole and non-imidazole H₃R antagonists facilitating the release of histamine have proven to be beneficial (Yokoyama et al., 1993; Kakinoki et al., 1998; Kamei, 2001). Pitolisant, an H₃R antagonist, showed excellent antiepileptic activity in animal models of seizure, predictive for generalized, MES test in mice (Sadek et al., 2014).

Interestingly, children with febrile seizures showed significantly lower histamine concentrations in cerebrospinal fluid than febrile children without seizures. Based on these findings, the suggestion was made that brain histaminergic system may be involved in inhibiting seizures associated with febrile illnesses in childhood (Kiviranta et al., 1995). The clinical data amply documented proconvulsant effects of H₁R antagonists administered in clinically relevant doses (Churchill and Gammon, 1949; Yokoyama et al., 1993; Takano et al., 2010; Miyata et al., 2011; Zolaly, 2012). Therefore, centrally acting H₁R antagonists which may increase seizure susceptibility in patients with febrile seizures are neither recommended to these patients nor to PWE. Also, they should be avoided in young infants, more sensitive to the drugs that could potentially disturb the anticonvulsive central histaminergic system. The potential antiseizure activity of pitolisant, a non-imidazole H₃R inverse agonist, was examined in 14 photosensitive adults using the photosensitivity standard model and employing 20/40/60 mg dose. Significant suppression of generalized epileptiform discharges was observed in the majority of the PWE (Kasteleijn-Nolst Trenite et al., 2013; Bialer et al., 2015). Unfortunately, a recent multicenter, national, pragmatic, noncomparative, open-label, exploratory phase II trial reported there was no clinical effects of pitolisant in human epilepsy, in spite of the existing promising animal data (Collart Dutilleul et al., 2016).

WAG/Rij strain showed an increase in the density of H₁R binding in the frontal motor cortex and interposed nucleus of cerebellum and a decrease in the substantia nigra compacta compared to the non-epileptic control group (Midzyanovskaya et al., 2016). Taking into account the bidirectional effect of the H₁R antagonist pyrilamine on SWDs in WAG/Rij rats (Midzyanovskaya et al., 2005), it may be speculated that histamine modulates different areas involved in opposite absence seizure modulation.