Histamine H₃ receptor antagonists in relation to epilepsy and neurodegeneration: a systemic consideration of recent progress and perspectives

Abstract

The central histaminergic actions are mediated by H₁, H₂, H₃ and H₄ receptors. The histamine H₃ receptor regulates the release of histamine and a number of other neurotransmitters and thereby plays a role in cognitive and homeostatic processes. Elevated histamine levels suppress seizure activities and appear to confer neuroprotection. The H₃ receptors have a number of enigmatic features like constitutive activity, interspecies variation, distinct ligand binding affinities and differential distribution of prototypic splice variants in the CNS. Furthermore, this Gi/Go-protein-coupled receptor modulates several intracellular signalling pathways whose involvement in epilepsy and neurotoxicity are yet to be ascertained and hence represent an attractive target in the search for new anti-epileptogenic drugs. So far, H₃ receptor antagonists/inverse agonists have garnered a great deal of interest in view of their promising therapeutic properties in various CNS disorders including epilepsy and related neurotoxicity. However, a number of experiments have yielded opposing effects. This article reviews recent works that have provided evidence for diverse mechanisms of antiepileptic and neuroprotective effects that were observed in various experimental models both in vitro and in vivo. The likely reasons for the apparent disparities arising from the literature are also discussed with the aim of establishing a more reliable basis for the future use of H₃ receptor antagonists, thus improving their utility in epilepsy and associated neurotoxicity.

Histamine has been one of the most studied substances in medicine for a century, regulating a wide spectrum of activities, including its function in neurotransmission (Brown et al., 2001). The association between the histaminergic system with the pathogenesis of epilepsy, which is currently the subject of extensive evaluation, is still in its infancy, owing to the complex brain neurophysiology of histamine (Haas et al., 2008) and pleiotropic receptor ligand pharmacology (Leurs et al., 2005; Esbenshade et al., 2008; Tiligada et al., 2009). Our understanding of the pathophysiology of epilepsy is mostly confined to the conventional theory of deranged inhibitory GABAergic and protracted excitatory glutamatergic neurotransmission in excitotoxic neuronal death (reviewed by Naylor, 2010; Werner and Coveñas, 2011; Rowley et al., 2012). The imbalance could be modulated by various other neurotransmitter systems including the histaminergic system. The latter, through H₃ heteroreceptors, modulates the release of a wide spectrum of vital neurotransmitters, for example, GABA, glutamate, dopamine, 5-HT, noradrenaline and acetylcholine, in a pathway-dependent manner (reviewed by Passani et al., 2007; Haas et al., 2008). Again, histamine release is not only regulated by its own H₃ autoreceptor system but also by GABA via GABA_A and GABA_B receptors and by glutamate via NMDA receptors (Okakura et al., 1992; Okakura-Mochizuki et al., 1996; Sherin et al., 1998).

Excitotoxicity is considered to be the key mechanism underlying neurodegenerative processes including epileptic seizures (Choi, 1992). Neurodegeneration followed by neuronal cell death resulting from excessive activation of excitatory ionotropic glutamate receptors [NMDA, AMPA and kainic acid (KA)] or malfunctioning of the inhibitory mechanism due to altered functioning of GABA_A receptor, usually underlies the dissemination of epileptic activity (reviewed by Acharya et al., 2008). The compensatory yet abnormal neuronal activities that follow, viz. mossy fibre sprouting, significantly modify the neuronal organization and alter the propagation of neurotransmission processes that is attributable to the epileptogenic mechanisms (Dichter, 2009). Epileptogenesis refers to the progressive transformation of normal brain to a hyperexcitable epileptic brain, after an initial precipitating injury (IPI), for example status epilepticus (SE), and continues even after the withdrawal of the inciting stimulus. This is a characteristic of temporal lobe epilepsy (TLE), the most intractable form of epilepsy (Acharya, 2002; Loscher and Brandt, 2010). The conventional and new antiepileptic drugs (AEDs) affect seizure expression, merely providing symptomatic treatment without having any influence on the course of the disease (epileptogenesis). The current consensus is to employ anti-epileptogenic strategies for treating TLE (Walker, 2007; Acharya et al., 2008; Pitkanen, 2010). Hence, there is a growing interest that the AEDs, besides controlling seizure activity, must also prevent concurrent injury responses that result in epileptogenesis in the hippocampal structures, a phenomenon linked to the transformation of a normal non-epileptic brain to a hyperexcitable epileptic brain in which recurrent, spontaneous seizures occur. In fact, epilepsy treatment has always been associated with the protection of neural tissue, since it intends to diminish the duration or totally suppress seizures. Even though the debate on the capacity of simple seizures to induce neuronal injury is still in progress, there is compelling evidence for the disastrous effects of prolonged episodes of status epilepticus (Willmore, 2005; Acharya et al., 2008). The role of the brain histaminergic system in neuroprotection remains a challenging area of research that is currently under consideration. Based on recent findings, which include changes in H₁ and H₃ receptor expression in a KA-induced epileptic model, it has been proposed that the brain histaminergic system is involved in experimental SE and subsequent neurodegeneration (Jin et al., 2005; Lintunen et al., 2005).

Histamine is considered to be an anticonvulsive neurotransmitter as its low levels are associated with convulsions and seizures (Kiviranta et al., 1995; Chen et al., 2003; Hirai et al., 2004). Until now, research on the anticonvulsive role of histamine has largely focused on electrically- or PTZ-induced seizures (Vohora et al., 2000; 2001; Yawata et al., 2004). However, an understanding of the possible role of histamine in temporal lobe epilepsy and the brain regions affected has begun to evolve (Jin et al., 2007; Kukko-Lukjanov et al., 2012). Prolonged release of histamine in response to brain ischaemic insults and alleviation of neuronal damage upon post-ischaemic histamine administration suggests it has a neuroprotective role (reviewed by Adachi, 2005). Histaminergic drugs, most importantly H₃ receptor antagonists or inverse agonists, which are considered to be a class of perspective drugs for the treatment of diverse neurological and neuropsychiatric disorders (reviewed by Leurs et al., 2011; Passani and Blandina, 2011), in view of the findings from various basic experimental models of epilepsy in rodents, are envisaged to possess promising anticonvulsive properties (Vohora et al., 2000; 2001; 2010; Harada et al., 2004a,b).

In this review, an attempt has been made to scrutinize the recent experimental evidence that has evoked the possibility that the histaminergic system, via modulation of H₃ receptor function, can be engaged to mediate a neuroprotective effect in epilepsy-related neurotoxicity and also to address the possible mechanisms involved, which would explain the above effects. Histamine and various agents, which act by modulating the histaminergic system, especially H₃ receptor antagonists, and their impact on epileptogenesis are then examined with particular reference to the neuroprotective action of some of these molecules. The core of the review deals with the current status of the literature on the use of some of these inhibitors as antiepileptic and neuroprotective agents as well as on their possible role in intervening in disease modifications of the brain (epileptogenesis). Probable explanations for many of the conflicting results arising from the literature are discussed in view of establishing a more reliable platform for future use of histaminergic agents to treat epilepsy and associated neurotoxicity.

Histaminergic system in CNS

In the CNS, the synthesis of histamine [2-(4-imidazolyl)-ethylamine] from 1-histidine by the catalytic activity of the rate-limiting enzyme histidine decarboxylase (HDC, EC 4.1.1.22; Moya-Garcia et al., 2005) takes place in a restricted population of neurons located in the tuberomammillary nucleus (TMN) of the posterior hypothalamus (Panula et al., 1984; Watanabe et al., 1984; Wouterlood et al., 1986). They give rise to widespread and diffuse projections extending through the basal forebrain virtually to the entire brain including the cortex, striatum, thalamus, hippocampus, hypothalamus, locus coeruleus and spinal cord (Watanabe et al., 1984; Panula et al., 1990; Zimatkin et al., 2006). This morphology renders histamine to be able to act as a neurotransmitter and neuromodulator of a wide spectrum of physiological functions and behaviours of the CNS, such as the circadian rhythms, catalepsy, energy homeostasis, thermoregulation, neuroendocrine and cardiovascular control, drinking and feeding, learning and memory, locomotion, sexual behaviour, analgesia and emotion (reviewed by Brown et al., 2001; Passani et al., 2007; Yanai and Tashiro, 2007; Haas et al., 2008).

Histamine H₃ receptor

The histamine neuroreceptor system is one of the major aminergic systems exerting key neurological functions via pharmacologically distinct subtypes of histamine receptors, which belong to a large superfamily of GPCRs that are characterized by the presence of seven putative transmembrane spanning domains (Leurs et al., 2005; Parsons and Ganellin, 2006). To date, histamine has been recognized to be an endogenous ligand for four subtypes of metabotropic histamine receptor (H_1–4; Parsons and Ganellin, 2006; Nuutinen and Panula, 2010), of which H_1–3 receptors are very widely expressed throughout the mammalian brain (Garbarg and Schwartz, 1985; Hill et al., 1997; Tashiro and Yanai, 2007). For the H₃ receptor, so far, six isoforms have been reported with varying distribution (H_3A-F; Lovenberg et al., 1999; Drutel et al., 2001; Bakker et al., 2006) and pharmacological properties (reviewed in Hancock et al., 2003). Both the H_3A and H_3D isoforms contain the full-length third intracellular loop but have different C-termini. All the other isoforms have deletions in that loop (H_3B and H_3E: 32 amino acids; H_3C and H_3F: 48 amino acids). Isoforms H_3A-C have the same C-terminus, while isoforms H_3E-F have an alternative C-terminus (reviewed in Leurs et al., 2005; Arrang et al., 2007).

Numerous studies have established that all subtypes (H₁ and H₂) are located postsynaptically and are found in the CNS and periphery (reviewed in Parsons and Ganellin, 2006), whereas the recently discovered H₄ receptor is found predominantly in mast cells and leucocytes (reviewed by Oda and Matsumoto, 2001; Leurs et al., 2009), although their presence in the CNS has also been detected (Connelly et al., 2009); with the exception of the H₃ receptor, which was originally characterized as a presynaptic autoreceptor that is located on histaminergic and other cell somata, dendrites and axonal varicosities (Arrang et al., 1983; Lovenberg et al., 1999). Although, later on, a detailed mapping of the H₃ receptors in rat brain revealed their presence post-synaptically on the perikarya, dendrites and projections of many neuronal populations (Pillot et al., 2002), but their postsynaptic physiological role is yet to be elucidated (reviewed in Arrang et al., 2007). Both rat and human H₃ receptors (hH₃ receptors) again have the striking feature of being constitutively (or spontaneously) active, by virtue of their existence in their active conformation even in the absence of histamine (agonist), thereby negatively regulating the synthesis and release of histamine (Schwartz et al., 1990). Inverse agonists by attenuating this constitutive inhibitory effect enhance histamine release (Jansen et al., 1998; Morisset et al., 2000; Rouleau et al., 2002; Arrang et al., 2007).

The relative expression of histamine H₃ receptors is very high in the CNS, where they participate in the modulation of arousal, learning and memory and food intake by means of their autoreceptor and heteroreceptor functions. So far, the H₃ receptor has been postulated to be a good target for drug discovery for a variety of CNS conditions (Gemkow et al., 2009; Schwartz, 2011). The H₃ receptor, being an auto- as well as hetero-receptor, was subsequently found to restrict the release of histamine and other neurotransmitters, including acetylcholine, dopamine, glutamate, noradrenaline and 5-HT (Schwartz et al., 1990; Schlicker et al., 1994). Consequently, specific ligands for the H₃ receptor show potential therapeutic effects in models of obesity, depression, mood disorders, neuropathic pain, epilepsy, sleep–wake disorders including narcolepsy and cognitive and sensorimotor deficits that are considered to be the more common deleterious features of CNS disorders such as Parkinson's disease (PD), attention deficit hyperactivity disorder (ADHD), Alzheimer's disease (AD) and schizophrenia (Fox et al., 2005; Leurs et al., 2005; Nuutinen and Panula, 2010; Passani and Blandina, 2011; Schwartz, 2011).

H₃ receptor signalling pathways

The seminal work of Arrang et al. (1983) was the first to identify the third histamine receptor and this has opened a new chapter in the pharmacology of histamine. With the cloning of the hH₃ receptor gene pioneered by Tim Lovenberg in 1999, H₃ receptor research has received a tremendous boost and enormous progress has been made in the field ever since. At present, we are buoyed by the detailed knowledge of its modulation of several signalling pathways based on the findings in the last 10 years. The notion of G-protein (especially G_i/G_o) involvement in the H₃ receptor was confirmed by the heterologous expression of the H₃ receptor (Lovenberg et al., 1999) and this was further substantiated by the reported inhibitory effects of G-protein toxins, such as cholera and Pertussis toxin, on H₃ receptor function (Rouleau et al., 2002) and [³⁵S]-GTPγS binding studies in rat brain (Clark and Hill, 1996).

Studies using recombinant cell systems have revealed that brain H₃ receptors are coupled to G_i/o-proteins modulating protein kinases such as cAMP-dependent protein kinase (PKA), ERK2 (Giovannini et al., 2003) and GSK-3β (Bongers et al., 2007b). Thus, a number of signal transduction pathways have been identified as being modulated by H₃ receptors (Figure 1), including inhibition of AC and activation of MAPK (reviewed by Leurs et al., 2005; Bongers et al., 2007a). H₃ receptor activation of G_i/o-proteins might also lead to the activation of PLA₂ to induce the release of arachidonic acid (AA; Rouleau et al., 2002), inhibition of the Na⁺/H⁺ exchanger (NHE; Silver et al., 2001) as well as modulation of intracellular Ca²⁺ ([Ca²⁺]_i) levels (Silver et al., 2002). In view of the interest in the H₃ receptor as a potential therapeutic target, each of these signalling pathways is discussed below and any evidence germane to epilepsy (if any) and neurodegeneration is cited.

Figure 1.

The schematic diagram is a hypothetical depiction of the H₃ receptor-mediated signalling pathways in the CNS and their probable implications. Activation of H₃ receptor, constitutively or in presence of an agonist, leads to the activation of many G_i/o-associated intracellular pathways. Active G_i/o-proteins negatively couple AC, thereby inhibiting the cAMP/PKA cascade and subsequent lowering of cAMP responsive element binding protein (CREB), a pro-survival transcription factor. Reduction in PKA activity may decrease release of neurotransmitters, for example histamine (HA), GABA, etc. Additionally, H₃ receptor activation also activates PI3K and MAPK pathways. Activated PI3K then activates Akt, a serine threonine kinase, which phosphorylates and hence inactivates pro-apoptotic GSK-3β. In the MAPK pathway, especially p44 and p42 MAPK (ERK1/2) are coupled to H₃ receptors; activated ERK has also been linked to epilepsy. The other three pathways known so far that are modulated upon H₃ receptor activation include increased activity of the enzyme PLA₂, whose inhibition confers neuroprotection in epileptic models; mobilization of intracellular Ca²⁺[Ca²⁺]_i; and finally the inhibition of Na⁺/H⁺ exchanger (NHE), a protein that buffers intra-neuronal pH and it's defective functioning is also implicated in epilepsy. Also, H₃ receptors possibly influence the function of other GPCRs and vice versa (e.g. dopamine D₁ receptor) by virtue of cross talk in heteromer containing cells.

PKA/cAMP/CREB pathway

AC catalyses the formation of cAMP, which in turn, activates PKA and, subsequently, induces cAMP-responsive-element-binding protein (CREB) to modulate gene transcription. The H₃ receptor negatively couples to AC; hence, H₃ receptor activation lowers cAMP levels and reduces downstream events, such as CREB-dependent gene transcription (Lovenberg et al., 1999). The lowering of intracellular levels of cAMP through the activation of H₃ autoreceptors (by an agonist or via a constitutive property) results in the modulation of histamine synthesis in histaminergic nerve terminals through the AC/PKA pathway (Gomez-Ramirez et al., 2002). Many classical H₃ receptor antagonists, for example thioperamide, clobenpropit, ciproxifan, were found to reverse this process in various transfected cell lines (Morisset et al., 2000; Wieland et al., 2001). Similarly, ABT-239 (H₃ receptor antagonist) restored cortical CREB level and increased its phosphorylation in mice (Bitner et al., 2011).

The transcriptional activation of CREB is induced by an array of kinases including PKA (Gonzalez and Montminy, 1989), Ca²⁺-activated calmodulin kinases (CaMKII) and MAPK-activated protein kinase 2 (MAPKAPK2; Davis et al., 2000) and was found to regulate many facets of neuronal functioning, including neuronal excitation, growth and survival, development and regeneration, circadian entrainment and long-term synaptic plasticity (reviewed by Lonze and Ginty, 2002; Borlikova and Endo, 2009). In rat cortical cells, an H₃ receptor antagonist was found to alleviate NMDA-induced neurotoxicity by activating the cAMP/PKA pathway, which subsequently facilitated GABA release (Dai et al., 2007). It is well known that the signalling pathway of PKA/CREB has an anti-apoptotic effect (Lonze et al., 2002; Ao et al., 2006), and there is experimental evidence suggesting that CREB is actively involved in supporting neuroprotection (Lee et al., 2005, 2009; Kim et al., 2011; reviewed by Sakamoto et al., 2011) and that its disruption in the brain leads to neurodegeneration (Lee et al., 2002); thus, supporting a pivotal role for CREB in preventing neuronal death (reviewed by Ferrer, 2002). In transgenic mice, the gene profiling of degenerating hippocampal tissue with electron microscopy reveals that the sustained inhibition of CREB function is associated with neuron degeneration, whereas its strong chronic activation primarily causes excitotoxic hippocampal cell death and inflammation, indicating that a fine tuned control of CREB expression is critical for the viability of pyramidal neurons (Valor et al., 2010). Altered expression of CREB and ICER (inducible cAMP early repressor), an endogenous CREB antagonist, and their possible alteration of GABA receptor subunits in various epileptic models suggest a definitive role for the PKA/CREB pathway in epileptogenesis (reviewed by Borlikova and Endo, 2009; Brooks-Kayal et al., 2009). On the other hand, a number of experimental (Lee et al., 2007; Zhu et al., 2012) and clinical (Park et al., 2003; Rakhade et al., 2005) findings appear, in some respects, to contradict the established neuroprotective role of this pathway as they indicate that an increased expression of CREB is associated with seizure-induced pathological alterations in the CNS. Therefore, further studies with H₃ receptor ligands are needed to elucidate the possible role of this particular pathway in epilepsy and the subsequent pathological remodelling of the hippocampus.

Activation of the MAPK pathway

G-protein-coupled activation of the MAPK pathway involves the Ras/Raf/MEK (MAP/ERK kinase) pathway that results in the phosphorylation of ERK1/2s. Activated ERKs phosphorylate several substrates, for example MAPK-activated protein kinases (MAPKAPKs; Wada and Penninger, 2004), of which MAPKAPK2 is associated with Akt phosphorylation (Rane et al., 2001). However, the occurrence of such cross-talk in the H₃ receptor pathways remains to be further examined, as H₃ receptor-mediated phosphorylation of Akt was observed to be independent of MAPK pathway in transfected SK-N-MC cells (Bongers et al., 2007b).

Notably, different components of the MAPK family are known to have opposing effects; the ERK/MAPK pathway is deemed to be pro-survival (Karmarkar et al., 2011), whereas the p38/MAPK and SAPK/JNK pathways are often implicated in cell death (de Lemos et al., 2010). More importantly, inhibition of ERK1/2 activation increases epileptiform activity and animal mortality in the pilocarpine model of status epilepticus (Berkeley et al., 2002) and ERK1/2 activation is thought to play a pivotal role in neuronal survival following hypoxic injury (Jin et al., 2002). Although the results of these studies indicated that ERK1/2 has a neuroprotective role, other studies have suggested ERK1/2 signalling has a detrimental role in neurotoxic responses (reviewed by Subramaniam and Unsicker, 2010). The role of ERK1/2 in status epilepticus is further complicated by evidence indicating that ERK1/2 activation is epileptogenic (Merlo et al., 2004) and triggers epilepsy in mice by augmenting NMDA receptor (GluN2B) function (Nateri et al., 2007).

Earlier, phosphorylation of the MAPK pathway induced by activation of rat H₃ (H_3A isoform) receptors in receptor transfected COS 7 cells (Drutel et al., 2001) led to the discovery that the MAPK pathway is linked to H₃ receptors. However, in rat hippocampal CA3 pyramidal cells H₃ receptor activation was also shown to be coupled indirectly to ERK2 phosphorylation, as thioperamide failed to alter phospho-ERK levels (Giovannini et al., 2003). The relevance of this H₃ receptor signalling pathway in the aetiology of chronic epilepsies has yet to be ascertained.

Activation of the PI3K/Akt/GSK3β axis

H₃ receptor activation in transfected cells leads to the activation of PI3K pathways and the subsequent phosphorylation–activation of another ubiquitous signalling molecule, Akt (PKB, serine/threonine kinase), which phosphorylates at serine residue and thereby inhibits the action of GSK-3β (pGSK-3β_ser9; Bongers et al., 2007b), a constitutively active, τ kinase. Advances in the understanding of signal transduction in the CNS regulated by Akt have paved the way to study the implications of the PI3K/Akt/GSK-3β axis (Brazil et al., 2004; Chuang, 2005). Akt has been shown to play an important role in preventing excitotoxic apoptosis by acting as a regulatory gate, preventing JNK activation (Kim et al., 2002).

In the CNS, the Akt/GSK-3β axis has emerged as a potential target for various neurological disorders. Activation of GSK-3β (pGSK-3β_tyr216) promotes apoptosis in a wide variety of conditions, while its inhibition promotes cell survival (Rickle et al., 2004; Takashima, 2009). The role of H₃ receptor-mediated activation of Akt/GSK-3β in the brain is still under investigation, but dysregulation of GSK-3β is linked to several neurodegenerative conditions including AD and other neurological disorders (Rickle et al., 2004; Takashima, 2009). The pro-apoptotic effect of activated GSK-3β suggests a potential role for its inhibitors in protection against neuronal cell death (Eldar-Finkelman, 2002; Takashima, 2009). Inhibition of GSK-3β was found to be critical in imparting neuroprotection in various models of neurodegeneration (Chin et al., 2005; Rosa et al., 2008), including KA-induced neurotoxicity (Goodenough et al., 2004). Selective GSK-3β inhibitors were found to reduce the development of complications in models of TLE, by limiting the extent of neuronal damage associated with TLE, without affecting the EEG parameters of KA-induced seizures (Busceti et al., 2007; Luna-Medina et al., 2007). Moreover, inhibition of GSK-3β has also been implicated in the mechanism of action of AEDs like valproic acid (Qing et al., 2008), suggesting a possible role for GSK-3β in the pathophysiology of epilepsy.

The novel finding that stimulation of H₃ receptors triggers activation of the Akt/GSK-3β pathway and protects against NMDA-induced neurotoxicity, and that both are blocked by the H₃-antagonist thioperamide (Bongers et al., 2007b; Mariottini et al., 2009), is contradicted by an in vivo study where inhibition of H₃ receptors by the H₃-antagonist ABT-239 conferred neuroprotection by elevating hippocampal GSK-3β_ser9 levels, which resulted in reduction in τ-phosphorylation in an Alzheimer model in mice (Bitner et al., 2011). These experimental anomalies require further elucidation to delineate this H₃ receptor-linked pathway. Reducing neuronal death is an important therapeutic goal in the context of a number of neurodegenerative diseases including refractory epilepsies (Sloviter, 2011). Therefore, the emerging finding that H₃ receptors are coupled to GSK-3β throws up an exciting avenue to explore a new therapeutic opportunity, if any, germane to TLE-induced neurodegeneration.

Activation of PLA₂

H₃ receptor-induced activation of G_i/o-proteins seems to stimulate PLA₂ to induce the release of AA (Rouleau et al., 2002). Usually, activation of mammalian PLA₂ cleaves membrane phospholipids liberating two major fatty acids of brain, AA and docosahexaenoic acid. Under pathological conditions, these metabolites act as precursors for various inflammatory compounds, for example PAF, prostaglandins, leukotrienes and lipid peroxides, which are crucial in mediating the oxidative and inflammatory responses in CNS pathologies such as stroke, AD, PD and multiple sclerosis (Sun et al., 2010). As PLA₂ inhibitors confer neuroprotection in KA-induced neurodegeneration of hippocampal slices (Farooqui et al., 2004) and increased activity of this enzyme in the hippocampus is associated with human TLE (Gattaz et al., 2011), it would be interesting to investigate the significance of this pathway in epileptogenic mechanisms further by employing H₃ receptor ligands.

Inhibition of the Na⁺/H⁺ exchanger

H₃ receptor activation leads to the inhibition of the Na⁺/H⁺ exchanger (NHE; Silver et al., 2001) by an as yet unknown mechanism, although a direct interaction of Gα_i/o-proteins has been proposed (Bongers et al., 2007a). Activation of the H₃ receptor was shown to diminish neuronal NHE activity, and this pathway is proposed as the mechanism by which the H₃ receptor inhibits the excessive release of noradrenaline during protracted myocardial ischaemia (Hashikawa-Hobara et al., 2012).

The NHE is of profound importance as it maintains intracellular physiological pH by the extrusion of one intracellular H⁺ in exchange for one extracellular Na⁺ across the plasma membrane and thereby prevents neural acidosis, as the loss of NHE function is hypothesised to lower seizure threshold (Cox et al., 1997). Earlier, the defective functioning or absence of the NHE1 gene was shown to contribute to the neuronal death in the brains of mutant mice with epilepsy (Cox et al., 1997; Zhao et al., 2005). In line with this, NHE is assumed to favourably regulate neuronal excitability and post-ictal recovery in different transgenic models (reviewed by Obara et al., 2008). However, in contrast, several studies have obtained experimental findings indicating that inhibitors targeting NHE are of significant therapeutic value in epilepsy (reviewed by Ali et al., 2008). In this context, the influence of H₃ receptor ligands on NHE and subsequent implications in epilepsy need to be assessed.

Modulation of [Ca²⁺]_i

Neuronal H₃ receptor agonism decreases K⁺-induced extracellular calcium inflow, probably by impairing voltage operated ion channel (Silver et al., 2002; Seyedi et al., 2005), and inhibits histamine synthesis through the AC/PKA (Gomez-Ramirez et al., 2002) and CaMKII pathways (Torrent et al., 2005; Moreno-Delgado et al., 2009). Conversely, H₃ receptor agonists, acting on recombinantly expressed H₃ receptors, transiently mobilize calcium from intracellular depots (Bongers et al., 2006). To complicate this further, the H₃ receptor antagonist clobenpropit was found to reverse NMDA-induced Ca²⁺ accumulation in rat cortical neurons (Dai et al., 2007). Also, it has been reported that both PKA and CaMKII tend to regulate phosphorylation and hence activation of the histamine synthesizing enzyme, HDC. Depolarization–stimulation of histamine synthesis is mediated by calcium entry and subsequent calmodulin and CaMKII activation, although it is unlikely that PKA-dependent stimulation of histamine synthesis is linked to depolarization (Gomez-Ramirez et al., 2002; Torrent et al., 2005). Previously, it was reported that, in rat brain cortical slices containing histaminergic nerve endings, the prototypical H₃ receptor antagonist (now considered an inverse agonist) thioperamide stimulated histamine synthesis in a concentration-dependent manner (Gomez-Ramirez et al., 2002). These effects of thioperamide were mimicked by clobenpropit and A331440 (Moreno-Delgado et al., 2006).

Neuronal Ca²⁺ homeostasis and Ca²⁺ signalling via CaMKII regulate neuronal well-being. Ca²⁺ dyshomeostasis due to inhibition of CaMKII results in aberrant calcium and glutamate signalling, which can lead to neurodegenerative diseases including glutamate excitotoxicity and epilepsy (Ashpole et al., 2012). The implications of H₃ receptor-mediated alteration in neuronal [Ca²⁺]_i level pertaining to epileptogenesis still need to be elucidated.

Epilepsy and histamine H₃ receptors

The possibility that the brain histaminergic system is involved in epilepsy is being driven by plethora of experimental and clinical findings suggesting that histamine acts as an antiepileptic neurotransmitter. Activation of the central histaminergic system increases seizure threshold and decreases seizure susceptibility in electrically and chemically induced seizures and kindling mediated by histamine H₁ receptors (Tuomisto and Tacke, 1986; Tuomisto et al., 1987; Yokoyama et al., 1992; Iinuma et al., 1993; Kamei et al., 1998; Chen et al., 2003; Hirai et al., 2004; Ishikawa et al., 2007; Nishida et al., 2007; Zhu et al., 2007). In addition, high doses of several centrally acting H₁ receptor antagonists when used as anti-allergic drugs occasionally induce convulsions in clinical settings (Yokoyama et al., 1993b; Jang et al., 2010; Takano et al., 2010; Miyata et al., 2011) and experimental models (Yokoyama et al., 1993a, 1996; Kamei et al., 2000; Singh and Goel, 2010), further supporting an association between the neuronal histaminergic system and epileptic seizure pathophysiology.

The involvement of histamine H₃ receptors in epilepsy is currently under investigation. The pharmacological properties of H₃ antagonists/inverse agonists with respect to their anticonvulsive potential has begun to receive increased attention, as mounting experimental evidence from both acute and chronic models of epilepsy indicate the effectiveness of H₃ receptor antagonists. They were found to protect against experimental convulsions by increasing the release of histamine in the brain, which in turn interacts with H₁ receptors (Yokoyama et al., 1993c; 1994; Kakinoki et al., 1998; Ishizawa et al., 2000; Vohora et al., 2000; 2001; Chen et al., 2002). In addition, they are assumed to mediate their anticonvulsive action by several other mechanisms (Table 1), such as facilitation of GABA release (Ishizawa et al., 2000; Vohora et al., 2001; Zhang et al., 2003a,b), increasing histidine decarboxylase (HDC) activity (Yokoyama et al., 1994; Hirai et al., 2004) and synergism with AEDs (Ishizawa et al., 2000; Vohora et al., 2001; Uma Devi et al., 2011). Buoyed by encouraging results in experimental epilepsy, many academic and industrial units have begun extensive research to increase the efficacy of possible H₃ antagonists in clinical epilepsy. So far, the only success is in the form of pitolisant, which has progressed to phase II of a clinical trial for patients suffering from photosensitive epilepsy, a rare form of the disease where seizures are evoked by photic stimuli. The compound at a dose range of 20–60 mg, when administered alone as well as in combination with AEDs, dose-dependently improved the photoparoxysomal response. The onset of action was1–2 h post application and appreciably lasted from 8 h up to 36 h. Further progress in this direction has not emerged. (Tiligada et al., 2009; Kuhne et al., 2011).

Table 1.

H₃ recepor ligands and the mechanisms entailed in their antiepileptic effect in diverse models of epilepsy

H3 receptor ligands	Model used	Effect observed	Mechanism	Reference
Thioperamide (3.75, 7.5 and 15 mg·kg−1 i.p.)	MES seizure in mice	↓ Duration of each phase of convulsion	H3 antagonism/inverse agonism: reversal of auto-inhibitory HA synthesis and release; H1-mediated seizure inhibition	Yokoyama et al., 1993c
Clobenpropit (0.1, 0.3, 1 and 3 mg·kg−1 i.p.)	MES seizure in mice	↓ Duration of each phase of convulsion	As first mechanism; ↑ HDC activity	Yokoyama et al., 1994
AQ-0145	MES seizure in mice	↓ Duration of each phase of convulsion	As first mechanism	Murakami et al., 1995
Thioperamide (7.5 and 15 mg·kg−1 i.p.)	Proconvulsive effect of theophylline on MES seizure in mice	↓ Mortality	As first mechanism	Yokoyama et al., 1997
Clobenpropit (20–40 mg·kg−1 i.p.)	MES- and PTZ-induced seizure in mice	↑ Threshold for tonic seizure in MES test ⊗ PTZ seizure threshold	As first mechanism; pharmacokinetic synergism with AEDs	Fischer and van der Goot, 1998
Thioperamide (5–10 mg·kg−1 i.p.) and Clobenpropit (1, 5, 10 and 50 µg i.c.v.)	Amygdaloid kindled (electrically) rats	↓ Seizure stage; ↓ AD duration	As first mechanism	Kakinoki et al., 1998
Clobenpropit (0.5, 2, 5, and 10 mg·kg−1 i.p.)	Amygdaloid kindled (electrically) rats	↓ Seizure stage; ↓ AD duration	As first mechanism; interaction with GABA actions	Ishizawa et al., 2000
Thioperamide (3.75, 7.5 and 15 mg·kg−1 i.p.) R-α-methylhistamine (10 µg i.c.v.)	PTZ-induced seizure in mice	↑ Latency of myoclonic jerks & clonic generalized seizures; ↓ % incidence; H3 agonism reversed the above	As first mechanism;	Vohora et al., 2000
Thioperamide and AQ 0145 (each 5 and 10 mg·kg−1 i.p. and 10–50 µg i.c.v.)	Amygdaloid kindled (electrically) rats	↓ Seizure stage; ↓ AD duration	As first mechanism	Kamei, 2001
Thioperamide (3.75–7.5 mg·kg−1 i.p.); R-α-methylhistamine (10 µg i.c.v.)	MES- and PTZ-induced seizure in mice	↑ Latency of myoclonic jerks and clonic generalized seizures; ↓ Duration of tonic and clonic phases; H3 agonism reversed the above	As first mechanism; potentiation of AEDs	Vohora et al., 2001
Thioperamide (5, 10 and 20 µg; i.c.v.)	PTZ-induced kindling in rats	↓ Duration of each phase of convulsion; ↑ seizure threshold	As first mechanism	Chen et al., 2002
Clobenpropit (10 and 20 µg i.c.v.) Immepip (5 and 10 µg i.c.v.)	PTZ-induced kindling in rats	Delayed onset of kindling; ↑ latency of seizure stage; H3 agonism reversed the above	As first mechanism	Zhang et al., 2003a,b
Carcinine and Thioperamide (each 2–20 mg·kg−1 i.p.)	PTZ induced kindling in mice	↑ Latency of seizures; ↓ Seizure stages	↑ HDC activity; ↑ HA release	Chen et al., 2004
Iodophenpropit, Clobenpropit, Thioperamide (each 10 mg·kg−1 i.p./i.c.v.), AQ0145 (20 mg·kg−1 i.p.); VUF5514, VUF5515 VUF4929 (i.c.v.)	Amygdaloid kindled (electrically) rats; MES seizure in rats	↓ Seizure stages; ↓ AD duration; ↓ tonic seizure ⊗ Duration of clonic phase	As first mechanism	Harada et al., 2004a,b
Thioperamide (25 mg·kg−1 i.p.)	EL mice (model of human TLE)	↑ Latency; ↓ % of seizure incidence	↑ HDC activity; ↑ Brain HA release	Yawata et al., 2004
Clobenpropit (i.c.v.)	PTZ-induced kindling in mice	↑ Latency of myoclonic jerks and clonic generalized seizures; ↓ Seizure stages	As first mechanism	Zhang et al., 2004
Thioperamide (10, 20 and 50 µg i.c.v.)	Imipramine-induced seizures in amygdaloid kindle (electrically) rats	↓ Behavioural seizure; ↓ EEG seizure	As first mechanism	Ago et al., 2006
Clobenpropit (0.75 and 1.5 mg·kg−1 i.p.)	MES seizure in mice	Inhibition of seizure	Pharmacodynamic synergism with pyridoxine	Uma Devi et al., 2011
Thioperamide (2–10 mg·kg−1 i.p.) and R-α-methylhistamine (10–40 mg·kg−1 i.p.)	MES and PTZ induced seizure in mice	⊗ Seizure threshold in both model	H3 antagonism and agonism	Scherkl et al., 1991
Thioperamide and Burimamide	Picrotoxin induced seizure	↑ Severity of clonic convulsions	H3 antagonism	Sturman et al., 1994
Thiopermide (20 and 30 mg·kg−1 i.p.) Betahistine (200 and 400 mg·kg−1 i.p.)	Amygdaloid kindled rats	No anticonvulsive effect; mild effect only at very high dose	H3 antagonism	Yoshida et al., 2000
Thioperamide (15 mg·kg−1 i.p.)	Methionine-sulfoximine induced convulsion in mice	⊗ Seizure latency and mortality	H3 antagonism	Vohora et al., 2010

AD, after discharge; MES, Maximal electroshock; PTZ, pentylenetetrazole; ↑ increase; ↓ decrease; ⊗ no change.

By contrast, conflicting results have been published on the role of H₃ receptors (either induce seizure or mainly have no effect) in various models of experimental convulsions (Scherkl et al., 1991; Sturman et al., 1994; Wada et al., 1996; Fischer and van der Goot, 1998). These paradoxes could be suggestive of the involvement of non-histaminergic mechanisms for the epileptic disposition in the model studied. All this notwithstanding, it is proposed that the intrinsic histaminergic system exerts a powerful inhibitory function during epileptic seizure episodes, evidently via an H₁- and H₃- dependent mechanism. In line with this, H₃ receptor antagonists can be of therapeutic value in epilepsy, or at least a viable approach that can be combined with AEDs, especially in patients who are refractory or non-responsive to conventional therapies.

Histamine H₃ receptor and neuroprotection

The status of the histaminergic system with regard to its neuroprotective role is still vague and inconclusive. The importance of histamine in neuroprotection is illustrated by the association of brain histamine levels with seizure threshold and duration (Yawata et al., 2004) and the ability of histaminergic neurons to protect the developing hippocampus from KA-induced neuronal damage, which is partly mediated by H₁ and H₃ receptors (Kukko-Lukjanov et al., 2006; 2012). Likewise, KA-induced neurotoxicity was aggravated in mice treated with an H₁-receptor antagonist (Kukko-Lukjanov et al., 2010). At present, there is very little information regarding the effect of H₃ antagonists on neuronal survival and function. A role for histamine H₃ receptors in various neurodegenerative diseases, such as AD and PD, has been suggested (Alguacil and Pérez-García, 2003; Gemkow et al., 2009).

Numerous experimental findings have shown that histamine confers neuroprotection in degenerative changes due to ischaemia. For example, middle cerebral artery occlusion in the rat induces a long-lasting increase in neuronal histamine release in the striatum (Adachi et al., 1992). Moreover, histamine depletion with α-fluoromethylhistidine, an inhibitor of histidine decarboxylase (its synthesizing enzyme), significantly increases the number of necrotic pyramidal cells in the hippocampal CA1 region in rats subjected to cerebral ischaemia (Adachi et al., 1993). Post-infarction loading with histidine, a precursor of histamine, decreases the amount of neuronal damage in the rat brain (Adachi et al., 2005). In cultured cortical neurons, histamine protects against NMDA-induced necrosis via the histamine H₂ receptor (Dai et al., 2006).

Unlike the anticonvulsive role attributed to H₃ receptors in several studies, relatively little attention has been paid to their possible neuroprotective role especially in seizure-associated neurotoxicity. H₃ receptor-dependent modulation of neuroprotective mechanisms has also been demonstrated in diverse studies. A robust up-regulation of H₃ receptor mRNA was observed in certain brain areas that participate in epileptogenic processes after KA-induced seizures, indicative of a modulatory role of H₃ receptors (Lintunen et al., 2005) as well as during experimental ischaemia (Lozada et al., 2005). Increased H₃ receptor expression and subsequent constitutive activation of the downstream Akt/GSK-3β pathway could be the mechanism by which the H₃ receptor exerts its endogenous neuroprotective role (Bongers et al., 2007b). On the flip side and in line with an antiepileptic mechanism, H₃ receptor agonism aggravated delayed neuronal death in a rat model of cerebral ischaemia (Adachi et al., 1993). Moreover, H₃ receptor antagonism conferred neuroprotection by elevating the hippocampal GSK-3β_ser9 level, which resulted in a reduction in τ phosphorylation in an Alzheimer model in mice (Bitner et al., 2011). Physiological chaperons like heat shock proteins, HSPs (HSP 27 and 70) are up-regulated in various neurodegenerative conditions including epilepsy, but their exact role is as yet uncertain as they may have a protective mechanism, but they have also been linked to deleterious effects (reviewed in Turturici et al., 2011). An H₃ receptor antagonist was found to provide neuroprotection arising from their interaction with HSPs in a KA-induced neurotoxic model in teleost (Giusi et al., 2008).

The role of H₃ receptor ligands in neuroprotection is further confounded by a number of discrepancies that have emerged (Table 2). H₃ receptor antagonists have been found to be ineffective (Kukko-Lukjanov et al., 2006; Bongers et al., 2007b; Molina-Hernández and Velasco, 2008; Mariottini et al., 2009), and to reverse H₃ receptor agonists-mediated neuroprotective parameters (Shen et al., 2007; Fu et al., 2008) in various models of neurodegeneration. Furthermore, a short lasting but very high up-regulation of the expression of mRNA for all of the H₃ receptor isoforms (particularly H_3A+D) was observed in the areas most vulnerable to the excitotoxic effect of KA (in CA1 and CA3), which correlated well with neuronal damage (Jin et al., 2005; Lintunen et al., 2005), suggestive of a dual role for this receptor. Neurodegeneration could occur by the activation of H_3A receptor-mediated neuronal death through JNK pathways. However, this upregulation of H_3A receptor could be a compensatory rise to carry out regenerative/cell survival process via H_3A-coupled Ras-Raf-MEK pathways (Jin et al., 2005; Lintunen et al., 2005). Neurotoxic insult by 3-nitropropionic acid (3-NP) resulted in increased transcriptional activities of H₃ receptors, which was comparable with deleterious neuronal effects in rat and hamster (Canonaco et al., 2005). In summary, H₃ receptor antagonism cannot be deemed to be neuroprotective until the confounding factors, such as phenotype and isoforms specific variations among the different experimental paradigms used, are taken into consideration. Hence, it would be interesting to explore the modulatory effect, if any, of H₃ receptor antagonists on the neurodegenerative processes associated with epilepsy.

Table 2.

H₃ receptor ligands and the mechanisms entailed in neuroprotective effect in diverse models of neurotoxicity

H3 receptor ligands	Model used	Effect observed	Mechanism	Reference
Thioperamide (40–400 mg·kg−1 i.p.)	Ibotenate-induced brain lesion in IL-9-treated mice	↓ Brain lesion	H3 antagonism/inverse agonism	Patkai et al., 2001
Thioperamide (10−8–10−5 M)	HA-induced neurotoxicity in cultured rat cerebellar granular neurons	↓ % of cell death	As 1st mechanism	Gepdiremen et al., 2003
Thioperamide (10 mg·kg−1 i.p.)	3-NP-induced neurotoxicity in hamster	↑ H1R mRNA, ↓ Motor behaviour	As 1st mechanism	Canonaco et al., 2005
Thioperamide (10−7–10−5 M)	NMDA-induced neurotoxicity in cultured rat cortical neurons	↓ NMDA-induced necrosis, ↑ Neuronal viability	As 1st mechanism, ↑ GABA release	Dai et al., 2006
Thioperamide (1, 10, 102 & 103 nM) Clobenpropit (10, 102, 103 & 5000 nM)	KA-induced neurotoxicity in cultured rat hippocampal slices	↓ Neuronal degeneration, Clobenpropit had no effect	As 1st mechanism	Kukko-Lukjanov et al., 2006
Clobenpropit (10−11–10−7 M)	NMDA-induced neurotoxicity in cultured rat cortical neurons	↓ NMDA-induced necrosis, ↑ Neuronal viability	As 1st mechanism; Activation of cAMP/PKA pathway; ↑ GABA release, ↓[Ca2+]i	Dai et al., 2007
Thioperamide (10 mg·kg−1 i.p.)	Pb-induced neurotoxicity in teleost (T. pavo)	↓ Behavioural hyperactivity, ↓ Neurodegeneration, ↓ mRNA expression of HSPs (70 and 90)	Cross-talk between H3R with HSPs	Giusi et al., 2008
GSK 189254 (0.3 mg·kg−1 p.o.)	GSK 189254-treated rats	↑ Survival of dentate gyrus neurons	↑ NCAM-PSA expression	Foley et al., 2009
ABT-239 (0.03, 0.1, 0.7, 1 mg·kg−1 i.p.)	Tg2576 and TAPP transgenic AD mice	↑ p-CREB, ↑ p-GSK3βser9, ↓ Tau hyperphosphorylation	As 1st mechanism	Bitner et al., 2011
Thioperamide (10−4& 10−4 M·L−1)	NMDA-induced neurotoxicity in rat PC12 cells	Reversal of carnosine induced neuroprotective effect and inhibition of glutamate release	As 1st mechanism	Shen et al., 2007
Thioperamide (10 µM·L−1) Clobenpropit (100 µM·L−1)	SK-N-MC cells transfected with hH3R; Cultured rat cortical neurons and striatal slices	↑ cAMP accumulation, ↓ pAktSer473, ↓ p-GSK3βSer9	As 1st mechanism	Bongers et al., 2007b
Thioperamide (1 µM)	Cultured rat cortical stem cells	⊗ Cell proliferation and differentiation	As 1st mechanism	Molina-Hernández and Velasco, 2008
Thioperamide & Clobenpropit	A beta-42 induced neurotoxic rat PC12 cells	↓ Cell viability; reversal of inhibition of glutamate release	As 1st mechanism	Fu et al., 2008
Thioperamide (10 µM·L−1) Immepip (10 nM·L−1)	NMDA-induced neurotoxicity in cultured mouse cortical neurons	Blockade of H3 agonist induced- ↑ pAktSer473, ↑ Bcl-2 expression, ↓ LDH, GSK-3β and caspase 3 activity; ⊗per se on the above	As 1st mechanism	Mariottini et al., 2009
–	KA in SD rats	Possible anti-seizure, neuroprotective role	Brief ↓ H3A+D mRNA (24 h); ↑↑ H3A+D mRNA (after 1 week)	Jin et al., 2005
–	KA in SD rats	Variable (site & time dependent)- Both anti- and/or seizurogenicity; from neuronal death to survival/regeneration	Transient expression of- ↑↑ H3A+D mRNA followed by ↓ H3A+D mRNA	Lintunen et al., 2005

LDH, lactate dehydrogenase; pAkt_Ser473, phosphorylation of Akt at serine 473 residue; p-GSK3β_ser9, phosphorylation of GSK-3β at serine 9 residue; NCAM-PSA, neural cell adhesion molecule-polysialylation; PC-12, pheochromocytoma cell line; SK-N-MC, a human neuroblastoma cell line that stably expresses the human H₃R; SD, Sprague–Dawley; ↑ increase; ↑↑ very high increase; ↓ decrease; – no H₃R ligand used; ⊗ no change.

Way ahead

The rapid increase in our understanding of the range and mode of operation of different neurotransmitter systems and of synaptic neurophysiology is leading to landmark advancement in the treatment of epilepsy with the advent of new AEDs. However, improvement in terms of clinical outcome has fallen short of expectations, with up to one-third of patients continuing to experience seizures or unacceptable medication-related side effects in spite of efforts to identify optimal treatment regimes with one or more drugs. A plethora of experimental findings lend credence to the anticonvulsive and neuroprotective effects of H₃ receptor ligands and they are currently in advanced stages of clinical development for a broad spectrum of central diseases (e.g. narcolepsy, AD, epilepsy and schizophrenia). As per the literature in hand, in spite of several experimental findings explicitly indicating an important role for central H₃ receptors in the inhibition of seizures, the development of hH₃ receptor ligands has been disappointingly slow, at least partly because of problems with selectivity. The only success met so far is in the form of pitolisant, which has reached phase II trial for photosensitive epilepsy. Nonetheless, the experimental findings accumulated in support of the involvement of H₃ receptors in epilepsy and concomitant neurodegeneration should not be discounted.

On the basis of the present data, the following key points can be taken into consideration to absolve the discrepancies observed and hence to bridge the gap between experimental findings and success in clinical settings.

• Averting the precipitation of status epilepticus is considered the most effective way to minimize neurodegeneration (Walker, 2007) and employment of immediate neuroprotective strategies could prevent epileptogenic changes (Acharya et al., 2008). Intriguingly, H₃ receptor antagonists appear to possess both anticonvulsive and neuroprotective and/or disease modifying activity as suggested in numerous experimental findings. Strategically, a highly localized CNS expression of H₃ receptors (Drutel et al., 2001) is likely to minimize peripheral side effects, making them a promising target for epilepsy and neurotoxicity.

• The intrinsic activity of H₃ receptor inverse agonists that reverse constitutive suppression of histamine and/or GABA release (Morisset et al., 2000; Dai et al., 2007) could contribute to but is unlikely to fully account for receptor-dependent seizure suppression and neuroprotection. The beneficial effects of some H₃ antagonists observed could also be mediated through a non-histaminergic mechanism (Uma Devi et al., 2011). Hence, further investigations are warranted to reveal cross-talk with other cellular factors (Giusi et al., 2008; Moreno et al., 2011).

• An explanation for H₃ receptors being associated with the dual role of both cell death and survival could be the coupling of this receptor to MAPK pathways, different components of which can play an opposing role (Wada and Penninger, 2004). Moreover, ERK1/2 activation may initially defy oxidative damage, but prolonged oxidative stress leads to exhaustion of cellular defences and it serves as a signal to trigger cell death (Luo and DeFranco, 2006). Again, there are findings that indicate that H₃ receptor agonism in vitro (Bongers et al., 2007b) leads to signalling changes in the Akt/GSK-3β pathway analogous to those observed with H₃ receptor antagonism in vivo (Bitner et al., 2011). This paradox could be attributed to the possible phenotypic variation in the signalling pathways between the two experimental systems used. Hence, improved understanding of the signal transduction cascades that are activated by the H₃ receptor is essential in order to elucidate the molecular mechanisms underlying the potential H₃ receptor-mediated modulation of brain function. The neurotoxicity observed in the presence of high levels of histamine may be due to the interaction of the convulsant with other neurotransmitter receptors and/or other pathways (Díaz-Trelles et al., 1999).

• Both H₃ agonism and antagonism have been found to attenuate Ca²⁺ influx and [Ca²⁺]_i levels. The possible explanation for these inconsistencies in H₃ receptor-mediated Ca²⁺ signalling is speculated to be due to the presence of different ligand-bound active state conformations that couple differentially to the signalling system (Seyedi et al., 2005; Dai et al., 2007). A change in cell membrane composition, as in Apoe^−/− mice, could also affect the structural conformation of H₃ receptors and hence alter its ligand binding characteristics (van Meer et al., 2007).

• Due to the advent of powerful tools like genetic engineering, the existence of more than 20 different species-distinct splice variants of H₃ receptor have been identified, generating large molecular heterogeneity (reviewed in Hancock et al., 2003; Leurs et al., 2011). The resultant functional consequences with respect to their different CNS distribution patterns, biased ligand binding and their clinical relevance have yet to be determined. The dichotomous role of H₃ receptors as demonstrated by the transient and variable expression of various H₃ receptor isoforms in different brain regions that correlated with both neuronal survival and death in KA-treated rats (Jin et al., 2005; Lintunen et al., 2005) could be attributed to the existence of its pharmacological variants.

• In addition, it is anticipated that the interaction of histamine receptors with the intracellular transduction pathways is evidently not straightforward. The complexity arisen from the emergence of various functional isoforms differentially coupled to various G-proteins, the possible intertwining of signalling pathways (Drutel et al., 2001) and interaction with other neurotransmitter receptors and/or pathways (Díaz-Trelles et al., 1999) must be taken into account by selective pharmacological targeting, possibly by devising isoform-specific ligands.

• On the basis of the substantial experimental findings generated so far, H₃ receptor antagonists can be envisaged as having a therapeutic effect on epileptic and associated neurodegenerative disorders. An enhanced understanding of the mechanisms of seizure control and neuroprotective effects mediated via H₃ receptors in the light of the emerging signalling pathways coupled to it must be sought; as such mechanisms are likely to be the most valuable candidates for novel therapeutic interventions for improved seizure control. With that in mind, efforts should be taken to develop selective H₃ receptor agonists, antagonists and inverse agonists, which could provide the lead for the potential exploitation of the histaminergic system in the treatment of epilepsy and epileptogenesis.

Glossary

[³⁵S]-GTPγS

[³⁵S]-guanosine 5′-γ-thiotriphosphate

[Ca²⁺]_i

intracellular calcium

3-NP

3-nitropropionic acid

AA

arachidonic acid

AD

Alzheimer's disease

ADHD

attention deficit hyperactivity disorder

AEDs

antiepileptic drugs

CaMKII

Ca²⁺-activated calmodulin kinases

CREB

cAMP-responsive-element-binding protein

HDC

histidine decarboxylase

hH₃

receptor, human H₃ receptor

HSPs

heat shock proteins

ICER

inducible cAMP early repressor

IPI

initial precipitating injury

KA

kainic acid

MAPKAPK2

MAPK-activated protein kinase 2

MEK

MAP/ERK kinase

NHE

Na⁺/H⁺ exchanger

GluN2B

NMDA receptor subunit 2B

PD

Parkinson's disease

SE

status epilepticus

TLE

temporal lobe epilepsy

TMN

tuberomammillary nucleus

Statement of conflict of interest

The authors declare no conflict of interest.