New sensory or internal information will be encoded initially through primary, short-term processing (Cowan, 2008). Different brain circuits and regions convey various short-term memories. Prefrontal cortex maintains short-term memory by selecting relevant sensory information and ignoring irrelevant information to aid performance in tasks such as object recognition (Courtney, 2010). Additionally, other brain structures including basal ganglia, perceptual, and motor cortices are involved in STM (Courtney, 2010). Several studies indicate that distinct neuronal circuits are recruited in the processing of visuospatial (occipitoparietal cortex), auditory (temporal cortex), or phenological (inferior parietal cortex) STM systems (Vallar, 2017). Furthermore, temporary changes in synapses through neurotransmitters, including HA, and their downstream signaling pathways were found to be involved in STM (Mallakin, 2020). Interestingly, i.c.v. administration of HA and its precursor, l-histidine, improved social recognition memory assessed in experimental rodents (Prast et al., 1996). In the same study, H3R antagonist thioperamide provided the same behavioral results. However, a combination of genetic and pharmacological approaches comprehended the involvement and the requirement of the brain histaminergic system in social recognition learning using the social discrimination paradigm in mice. Hereafter, pharmacological or genetic disruption of the histaminergic system, through α-FMH or HDC^-/-, respectively, failed to disturb short-term recognition memory or fear memory in the inhibitory avoidance paradigm in mice (Rani et al., 2021). Accordingly, VUF16839, an agonist targeting H3Rs, was found to impair performance of tested animals in short-term social recognition in normal and, surprisingly, in HA-depleted mice (Rani et al., 2021), and this impairment was reversed by donepezil, suggesting that VUF16839 activated H3-heteroreceptors that control the release of ACh rather than HA (Rani et al., 2021). Furthermore, H3R antagonists/inverse agonists thioperamide and ciproxifan were described to ameliorate spatial memory deficits (Komater et al., 2005). In addition, the H3R antagonists/inverse agonists E159 and E177 were found to enhance short-term object recognition comparable to the reference drug donepezil, clinically used acetylcholinesterase inhibitor. Moreover, and in the same study, systemic co-administration of brain penetrant H2R antagonist zolantidine nullified the beneficial effects provided by E159 on memory deficits (Alachkar et al., 2017; Alachkar et al., 2019). Contrarily, DL77, another H3R antagonist/inverse agonist, failed to improve short-term memory but enhanced long-term memory in the same behavioral tasks (Eissa et al., 2018). Also, and in another preclinical study, H3R antagonist/inverse agonist CEP-26401 was found to improve social recognition memory in tested animals (Raddatz et al., 2012).

Although many theories and classifications exist, working memory can be viewed as short-term memory combined with goal-directed attention (Cowan, 2008). Attention among other executive brain functions is regulated by the prefrontal cortex, which interacts with other brain regions in maintaining working memory (D'Esposito, 2007). In regard to the role of brain histaminergic neurotransmission in working memory, depletion of brain HA following i.c.v. administration of the HDC inhibitor α-FMH or blockade of H1Rs but not H2Rs was found to disturb cognitive functions in experimental animals in eight-arm radial maze. Interestingly, and in agreement with the latter observations, H1R antagonist but not H2R antagonist was found to impair working memory, while H3R antagonist/inverse agonist clobenpropit was reported to improve cognitive functions in preclinical studies in rodents (Nakazato et al., 2000; Huang et al., 2004). In addition, and in a clinical study, the density of H3Rs in the dorsolateral prefrontal cortex was inversely correlated with performance in the working memory paradigm (Ito et al., 2018).

Contrarily, and in another preclinical study, histaminergic interactions with both H1- and H2Rs were found to be involved in the process of enhancing spatial memory and following i.c.v. administration of HA (Xu et al., 2005). In this study, bilateral ventral intrahippocampal infusion of NMDA receptor antagonist MK-801 impaired the retrieval process in both working memory and reference memory. However, intrahippocampal injection of HA or intraperitoneal injection of histidine markedly ameliorated the spatial memory deficits induced by MK-801. Moreover, both the H1R antagonist pyrilamine and the H2R antagonist cimetidine abolished the ameliorating effect of histidine on reference memory deficits, but not that on working memory deficits induced by MK-801. Accordingly, the latter observations demonstrate that in addition to H1Rs, H2Rs could be involved in the memory processes in experimental animals when memory deficits are induced (Xu et al., 2005). Moreover, physostigmine, an acetylcholinesterase inhibitor, was found to reverse the impairment induced following the administration of an H1R antagonist in working memory, demonstrating the interrelationship of brain histaminergic and cholinergic neurotransmissions in the observed effects on cognitive functions of tested animals (Nakazato et al., 2000). Furthermore, H3-heteroreceptors were extensively reported, as discussed above, to directly influence the release of other neurotransmitters including ACh, glutamate, and NE, all of which are capable of facilitating cognition and attention. Interestingly, atomoxetine and methylphenidate, clinically established drugs in the treatment of ADHD and working memory deficits, were found to increase the release of brain HA in the prefrontal cortex among their pharmacological roles (Horner et al., 2007). Therefore, brain HA may participate in the improvement of cognition and attention necessary for working memory performance.

Information intended for long-term learning is further processed. Such processing involves further changes in signaling pathways and consequently altered gene expression for prolonged and larger storage (Mallakin, 2020). STM and LTM are viewed in many studies as separate processes; therefore, it is not uncommon for a particular disease or treatment to affect one of these memory types while sparing the other. The hippocampus is well reported to be the encoding structure for LTM (Passani et al., 2017; Santangelo et al., 2017). Other structures are additionally involved like the basolateral amygdala, septum, cerebellum, and prefrontal cortex (Benetti et al., 2015; Passani et al., 2017; Rani et al., 2021). LTM can be divided into two categories, namely, explicit and implicit memories, which will be discussed in the following in association with the brain HA-provided effects in preclinical studies.

Numerous H3 antagonists/inverse agonists attracted attention by their capability to alleviate several neurological symptoms in experimental models in rodents. The relationship between HA and memory has been well established in the literature (de Almeida and Izquierdo, 1986; Alvarez, 2009), since H1-, H2-, and H3Rs are expressed in high densities in areas of the brain known to be involved in learning and cognition like the cortex, thalamus, hypothalamus, hippocampus, and amygdala (Martinez-Mir et al., 1990; Pollard et al., 1993). The evidence regarding the involvement of HA receptor subtypes in learning and memory are contradicting. For example, and in an active avoidance task during acquisition, stimulating H1Rs while blocking H2Rs was found to lead to complete inhibition of learning (Alvarez, 2009). Contrarily, stimulating H2Rs while blocking H1Rs yielded a normal learning curve, and the same applies to the retrieval memory phase (Alvarez, 2009). The latter contradictive observations may indicate that H2Rs are solely involved in learning and retrieval processes. However, in a study performed on mice lacking either H1- or H2Rs, both genotypes showed similar learning ability enhancement in the auditory and contextual fear conditioning test, and impairment in the object recognition and Barnes maze task (Dai et al., 2007). In addition, memory enhancement in the passive avoidance paradigm and following systemic administration of several H3R antagonists/inverse agonists has been reported to be moderately reversed by H2R antagonists but not H1R antagonists (Alachkar et al., 2017; Panayi et al., 2017; Eissa et al., 2018). This indicates that H3R antagonist-mediated memory improvement occurs through muscarinic cholinergic receptors and to a lesser extent H2Rs (Leentjens, 2015; Alachkar et al., 2017; Eissa et al., 2018). Overall, the interaction of HA with its receptor subtypes and in the context of memory assessments appears to be task-dependent, complex, and far from being fully understood (Figure 2A). More recently, the role of brain-derived neurotrophic factor (BDNF) has also been suggested (Miranda et al., 2019). Mainly expressed in the hippocampus, cortex, amygdala, and striatum and also in the hypothalamus, BDNF is a protein known to be involved in brain plasticity and depression (Dwivedi, 2009; Miranda et al., 2019). Neuroplasticity is stimulated through neurogenesis, dendritogenesis, and synaptogenesis, and promoting plasticity in dopaminergic, serotoninergic, cholinergic, and noradrenergic neurons. It is also involved in potentiating of signal transmission and induction (Je et al., 2012; Gibon et al., 2016; Liu and Nusslock, 2018). Interestingly, several H3R antagonists/inverse agonists were found to increase brain levels of BDNF in the chronic cerebral hypoperfusion model and age-dependent effect (Guilloux et al., 2017; Wang et al., 2020).

Furthermore, systemic i.c.v. administration of HA in rats was reported to facilitate memory in multiple behavioral models (de Almeida and Izquierdo, 1986; Izquierdo et al., 1992; Prast et al., 1996; Haas and Panula, 2003; Izquierdo et al., 2006; Izquierdo et al., 2016; Sadek et al., 2016a; Sadek and Stark, 2016). The impact of H3R antagonists/inverse agonists on memory can also be mediated by the brain neurotransmitter ACh. As discussed earlier, blocking H3Rs may lead to an increase in brain ACh release, given that ACh depletion in the brain was linked to cognitive impairments in both normal aging and AD patients (Perry et al., 1978; Decker, 1987). Collectively, the evidence showing the effects of anticholinergic agents, e.g., scopolamine and atropine, on memory may indicate another mechanism of enhancing memory impairment by H3R antagonists/inverse agonists (Drachman, 1977; Gallagher and Colombo, 1995). In a recent study evaluating the involvement of the central histaminergic system in social recognition memory on both short and long terms, the H3R antagonist/inverse agonist ciproxifan showed a procognitive effect on LTM in a test of social discrimination. In the same study, the CNS-penetrant H3R agonist was found to impair both short- and long-term social recognition memory (Rani et al., 2021). Interestingly, the genetic or pharmacological blockade of HA release impaired LTM but not STM, and the latter observation may be explained with the actions of H3R agonists on H3-heteroreceptors and the subsequent effect on ACh release. Moreover, Fox et al. reported enhancement of social recognition memory by H3R antagonists/inverse agonists in both adult and aged rats (Fox et al., 2005). Another study explored the effect of H3R antagonist/inverse agonist E159 on memory impairment induced by MK801 using step-through passive avoidance and novel object recognition tasks (Alachkar et al., 2017). The results obtained showed that the H3R antagonist/inverse agonist was able to enhance STM but not LTM through mechanisms related to cholinergic muscarinic neurotransmission and partially by the activation of H2Rs (Alachkar et al., 2017). Accordingly, the results observed for E159 showed that the E159-provided memory-enhancing effects on MK801-induced amnesia were moderately abrogated following acute systemic co-administration of scopolamine, H2R antagonist zolantidine (ZOL), but not with H1R antagonist pyrilamine to the animals (Alachkar et al., 2017). Contrary to the effects observed for E159, and using the same test battery, H3R antagonist/inverse agonist DL77 was found to alleviate deficits of LTM, without any appreciable enhancing effects on STM impairments induced by MK801 in rats (Eissa et al., 2018). The contradictory observations for H3R antagonists/inverse agonists might be explained with the different pharmacokinetic profiles of both compounds, and also probably with the difference in their antagonist affinity to H3Rs with pK _i of 6.1 and 8.03 for E159 and Dl77, respectively. Also, different species, namely mice and rats, were used in the latter preclinical studies to assess the memory-enhancing effects of both H3R antagonists/inverse agonists E159 and DL77. Furthermore, and in a study that used the 6-OHDA bilateral brain lesion as a model for PD, administration of H3R antagonist/inverse agonist thioperamide ameliorated memory deficits in the novel object recognition task (Masini et al., 2017). Spatial memory is another domain where H3R antagonists/inverse agonists have shown a promising potential therapeutic use. Using Morris water maze—a known behavioral test to assess spatial-working memory, a study showed improvement in scopolamine-induced amnesia following the administration of the H3R antagonist/inverse agonist S 38,093 (Panayi et al., 2017). Likewise, another research group reported improvement in escape latencies and task recall following the systemic administration of the H3R antagonist/inverse agonist GSK189254 in a Morris water maze task and using scopolamine-induced amnesia (Medhurst et al., 2007). Also, the H3R antagonist/inverse agonist ABT-239 enhanced spatial working memory in ketamine-induced memory deficits assessed in a cross maze task (Brown et al., 2013). In this study, ABT-239 and A-431404, but not the reference drugs risperidone and olanzapine, attenuated ketamine-induced deficits on spontaneous alternation in cross-maze. However, both H3R antagonists/inverse agonists failed to affect alternation performance on their own. Moreover, ABT-239 and A-431404 were found to also attenuate MK801-induced impairments in inhibitory avoidance, demonstrating that ABT-239 and A-431404 may have the potential to ameliorate cognitive deficits associated with SCH (Brown et al., 2013). In addition, the H3R antagonist/inverse agonist pitolisant improved episodic-like memory in scopolamine-induced amnesia and natural forgetting situation in male C57BL/6J mice assessed in the two-trial object recognition test (Ligneau et al., 2007). In this study, and on the two-trial object recognition test in mice, a promnesiant effect was observed regarding either scopolamine-induced or natural forgetting, signifying that the H3R antagonist/inverse agonist pitolisant was a valuable drug candidate for further development in wakefulness or memory deficits and other cognitive disorders (Ligneau et al., 2007). Furthermore, the potent H3R antagonist/inverse agonist samelisant, and when combined with sub-effective doses of donepezil, was able to counteract the amnestic effect of scopolamine in the Morris water maze task (Nirogi et al., 2021b). Another study compared the ability of ciproxifan and thioperamide to counteract the amnesic effect of scopolamine in both Morris water maze and Barnes maze; while both were able to diminish the memory impairment in the water maze, only ciproxifan had a less robust effect in the Branes maze (Komater et al., 2005). Also and in another preclinical study, thioperamide was also able to enhance spatial working memory in a cross maze task, although the mice used were not challenged (Vohora et al., 2005). In this study, thioperamide and tacrine enhanced performance on a spontaneous alternation task in mice, and a combination of sub-effective doses of the two drugs had a synergistic effect on the alternation scores in a cross maze test (Vohora et al., 2005). Moreover, sleep-deprived mice developed working memory impairment that was reversed by ciproxifan administration in a T-maze spontaneous alternation task (Chauveau et al., 2014). In addition, sleep restriction failed in the latter study to significantly modify immunopositive cells in control animals; however, ciproxifan administration prevented working memory deficits in sleep-restricted mice through significant increases in Fos labeling in several brain areas, including the prelimbic, infralimbic, and two cingulate cortex regions (Chauveau et al., 2014). Likewise, and in a Y-maze task, the H3R antagonist/inverse agonist SAR110894 reversed cognitive impairment associated with schizophrenia and attention-deficit/hyperactivity disorder, and improved memory performances in several variants of the object recognition task in mice or rats (0.3–1 mg/kg, p.o.) (Griebel et al., 2012). In another behavioral test model, the standard H3R antagonist/inverse agonist pitolisant enhanced consolidation of contextual fear memory and mitigated amnesia induced by dizocilpine (Brabant et al., 2013) (Table 1). In addition, the H3R antagonist/inverse agonist GSK189254 was found to significantly improve performance of rats in diverse cognition paradigms, including inhibitory passive avoidance, water maze, object recognition, and attentional set-shifting, signifying the potential role of targeting H3Rs for the symptomatic treatment of dementia in AD and other cognitive disorders (Medhurst et al., 2007). Furthermore, the H3R antagonists/inverse agonists GT-2331 (Fox et al., 2002) and S38093 (Panayi et al., 2017) were reported to significantly and dose-dependently improve spatial working memory, reverse scopolamine-induced memory deficits, and promote episodic memory of the spontaneously hypertensive rat (SHR) pups in different behavioral paradigms (Table 1). Despite the numerous aforementioned preclinical observations for the effectiveness of several H3R antagonists/inverse agonists to enhance cognitive functions in rodents, a clinical trial revealed that the highly potent and brain penetrant H3R antagonist/inverse agonist GSK239512 used as monotherapy was capable of improving episodic memory in patients with mild-to-moderate AD. However, no improvements were detected for GSK239512 on executive function/working memory or other domains of cognitive functions, indicating that GSK239512 failed to show benefit in this population (Grove et al., 2014). These clinical observations suggested that H3R antagonists/inverse agonists may, at most, have modest and selective effects on cognitive function in patients with mild-to-moderate AD. In addition, another clinical study was carried out with ABT-288, a highly selective H3R antagonist/inverse agonist, demonstrating no efficacy of ABT-288 in the symptomatic treatment of subjects with mild-to-moderate AD (Haig et al., 2014b).

Brain HA plays an important role in anxiety, as there have been numerous studies indicating a functional relationship between anxiety and histaminergic neurotransmission in classical animal models. Accordingly, the H1R antagonist chlorpheniramine improved anxiety of assessed rats in the elevated plus maze test and the open field test (Hasenohrl et al., 1999). Moreover, several research groups reported different effects of brain HA in preclinical models of anxiety in both male and female mice lacking HDC enzyme (HDC−/−). Accordingly, (HDC−/−)phenotypes were found to present behavioral features related to an increased anxiety level, which are mostly confirmed through animal behaviors in elevated plus maze, zero maze, light dark test (Acevedo et al., 2006), open field test (Dere et al., 2004; Acevedo et al., 2006), and graded anxiety test and height-fear task (Dere et al., 2004). Moreover, mice knocked out of H3Rs showed reduced anxiety in both elevated plus maze and zero maze, but not in the acoustic startle test (Rizk et al., 2004). Furthermore, rats that received bilateral HA infusion in the lateral septum showed decreased anxiety response in novelty-induced suppression of feeding (NISF) test and elevated plus maze (Chee et al., 2014). Intriguingly, the anxiolytic effect observed in the NISF test seems to be mediated by the histaminergic activation of postsynaptic H1R and H2Rs, since administration of either antagonist abrogated the HA-provided effects. However, administration of an H3R antagonist/inverse agonist was reported to reverse the effect seen in elevated plus maze, indicating possible signaling mediated by H3Rs (Chee and Menard, 2013). Conversely, the effect of HA injection on the lateral septal was reported to induce anxiety-like behaviors in the open field test, which was possibly mediated by neurotransmission through H1Rs and H2Rs (Mohsen et al., 2014). The reported results for numerous H3R antagonists/inverse agonists on anxiety models also differed momentously, with the most observed effect being the lack of any effects on anxiety-like behaviors assessed for ABT-239, ciproxifan, clobenpropit, DL77, and E159 in both rats and mice (Femenía et al., 2015; Alachkar et al., 2017; Masini et al., 2017; Eissa et al., 2018; Alachkar et al., 2019; Chauveau et al., 2019; Eissa et al., 2019; Soliani et al., 2020; Trofimiuk et al., 2020). However, the existing studies found that some H3R antagonists/inverse agonists, e.g., the H3R antagonist/inverse agonist ST-1283, may have an anxiolytic effect using paradigms such as open field test and marble-burying test (Bahi et al., 2014; Eissa et al., 2019). On the other hand, the contradicting results indicated an anxiogenic effect of the H3R antagonist/inverse agonist JNJ-10181457 observed in the open field test and the elevated zero maze test, and the anxiety parameters were explained with the increase in the locomotor activity witnessed with this test compound. In addition, the JNJ-10181457-induced anxiogenic and locomotor effects were reversed for the most part by co-administration with an H2R antagonist, suggesting a possible role of the H2Rs in exploratory and anxious behaviors of tested animals (Mohsen et al., 2014). The contradictory observations for the involvement of brain histaminergic neurotransmission in anxiety-like behaviors of experimental rodents shed light on the different task models and animal species used to evaluate the effects of different antagonists targeting H1-, H2-, or H3Rs. Consequently, there is no conclusive statement of the proved anxiolytic and/or anxiogenic effectiveness of several H3R antagonists/inverse agonists.

H3R antagonists/inverse agonists were also reported to have an antidepressant-like effect in experimental rodents (Lamberti et al., 1998; Pérez-García et al., 1999; Bahi et al., 2014; Femenía et al., 2015). However, the exact mechanism of the observed antidepressant-like effects is not fully explored in the literature. A study that explored possible mechanisms of the antidepressant actions of H3R antagonists/inverse agonists addressed the possibility of the involvement of BDNF. This neurotrophic factor is known to be involved in brain plasticity and mood, and recently was linked to depression (Dwivedi, 2009; Miranda et al., 2019). Stress and depression were reported to be capable of reducing the concentrations of BDNF in the hippocampus and prefrontal cortex, as several studies reported restoration of the BDNF factor upon administration of several reference antidepressant drugs (Castrén and Rantamäki, 2010; Yu and Chen, 2010). In preclinical models, mice exposed to chronic unpredictable stress were found to exhibit reduced levels of the BDNF factor in the hippocampus and prefrontal cortex, which was reversed by the administration of ciproxifan, a very well-known standard H3R antagonist/inverse agonist used in numerous preclinical studies in rodents (Kumar et al., 2019). The former researchers also found that the effect of HA on the BDNF factor in primary neurons can be fully blocked using H4R antagonists, indicating a possible role of H4Rs in restoring BDNF levels. This is in correlation with another study that reported depression-like symptoms in H4R-knockout (KO) mice (Sanna et al., 2017). Noteworthy, the HA role in immune response is also integrated in depression pathophysiology. The monoamine hypothesis of major depression relies on the notion that levels of multiple neurotransmitters in the brain are disrupted, specifically 5-HT which is found to be lower in depressive patients (Krishnan and Nestler, 2008; Belmaker and Agam, 2009). Another hypothesis is the possible role of immune response in initiating depression and its progression, and this is mainly through the release of proinflammatory cytokines (Haase and Brown, 2015). HA can be viewed as a link between the two hypotheses by means of its vital role in brain immune responses. Initially, stress can induce HA production either by direct activation of neuronal HA or by increasing the microglia production. Thereafter, HA can lead to decreased production of 5-HT through interaction with H3-heteroreceptors. Furthermore, stress can induce the microglia to produce proinflammatory cytokines that leads to further decrease in 5-HT levels in the synaptic cleft through 5-HT transporters (SERT) (Hersey et al., 2021). In line with the previous findings, a study has found that HA infusion was capable of preventing lipopolysaccharide (LPS)-induced cytokine release and neuronal loss in experimental mice (Saraiva et al., 2019), and H3R antagonism was also found to reduce interleukin (IL)-1β production (Iida et al., 2017). It was also shown that agents such as selective 5-HT reuptake inhibitors and lipid mediator oleoylethanolamide with antidepressant activity required the integrity of the intact brain histaminergic neurotransmission system to exert their effects (Figure 2B) (Munari et al., 2015; Costa et al., 2018). Noteworthy, and in two different studies that tested the effect of the H3R antagonist/inverse agonist ciproxifan on depression induced by chronic unpredicted stress, an elevation of depression-like symptoms such as anhedonia, helplessness, and social deficits was reported in numerous rodent models, including forced swim test, tail suspension test (Je et al., 2012), sucrose preference test, and social behavior test (Kumar et al., 2019), respectively. Furthermore, a study group reported a reversing effect of the H3R antagonist ciproxifan on the BDNF factor (Kumar et al., 2019). The old-generation H3R antagonist/inverse agonist clobenpropit was also found to have a similar effect on depressed rats in a forced swim test, possibly mediated by the actions of released HA on postsynaptically located H1- and H2Rs (Femenía et al., 2015). Moreover, and in the LPS-induced depression model in mice, the H3R antagonist/inverse agonist JNJ-10181457 was found to exhibit antidepressant-like effects by reducing the immobility time of tested mice in the tail suspension test, with evidenced reducing effects on the release of proinflammatory cytokines from microglial cells (Iida et al., 2017). Also, the novel H3R antagonist/inverse agonist ST-1283 was found to reduce depression-like behaviors in forced swim test, tail suspension test, and suppressed feeding test (Bahi et al., 2014) (Table 1).

Noteworthy, a very recent review discussed the association between altered neuroinflammation and brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there were suggestions that HA deficiency may leave the developing brain more vulnerable to proinflammatory insults and neurodevelopmental disorders, including Tourette’s syndrome, autism spectrum disorders, attention-deficit hyperactivity disorder, and SCH, in both preclinical and clinical studies (Carthy and Ellender, 2021)

The original basis behind ritualizing the use of H3R antagonists/inverse agonists to treat sleep-related disorders comes from multiple evidence found in the literature. The role of HA in wakefulness is not initiating but more specifically maintaining the state of alertness needed for higher brain functions (Takahashi et al., 2006). Histaminergic neurons project to different areas of the brain that are involved in the sleep–wake cycle such as the cortex, thalamus, hypothalamus, and brain stem (Vanni-Mercier et al., 2003). Brain histaminergic and orexinergic neurons found in the posterior hypothalamus were also found to play a significant role in the sleep–wake cycle. Histaminergic neurons within the tuberomammillary nucleus (TM) and the posterior hypothalamus are marked as “waking selective,” which means that they only fire during the waking state. In fact, these neurons possess the most selective pattern of discharge to wakening status within the CNS (Figure 2C) (Vanni-Mercier et al., 2003). In mice phenotype lacking HDC, decreased sleep latencies were observed. In new surroundings, EEG readings in wild-type mice show significant changes such as increased paradoxical sleep that tends to be preceded with wakefulness for hours, unlike HDC-/- mice that fall asleep spontaneously in comparison with wild-type mice. Furthermore, injecting wild-type mice with the HDC inhibitor α-FMH elicited similar results, thus indicating the importance of brain HA in maintaining the wake state (Parmentier et al., 2002). Likewise, a recent study carried out in mice with reduced expression of HDC and cKO mice using adeno-associated viruses to exclude the chance of developing compensating mechanisms to maintain the sleep–wake cycle concluded that chronic HA depletion resulted in a significant decrease in wakefulness and an increase in nonrapid eye movement (NREM) sleep (Yamada et al., 2020). Also, H1R KO mice and mice administered with an H1R antagonist showed similar disturbances in sleep patterns compared with wild-type mice, except for increased NREM sleep and decreased latencies to initiate NREM (Scott Bitner, 2012). On the other hand, H3R KO mice showed more awakening during changes in environment and motivation tests. Also, H1R antagonists were reported to be precipitating greater in slow-wave sleep in H3R KO mice, supporting the involvement of histaminergic interaction with H1Rs in sleep regulations (Scott Bitner, 2012). Furthermore, numerous H3R antagonists/inverse agonists were studied for their possible therapeutic potential in regulating sleep, some of which, e.g., pitolisant, were approved for treatment of narcolepsy (excessive day-time sleepiness) (Mohsen et al., 2014). Application of H3R antagonists/inverse agonists in preclinical models of sleep disturbances showed great potentials. For instance, GSK189254 enhanced narcoleptic episodes in orexin KO mice; EEG and EMG showed increased wakefulness (W) and decreased paradoxical and slow-wave sleep (Guo et al., 2009). Using the same mouse model, samelisant decreased NREM sleep and direct REM sleep onset episodes, showing an anti-cataplectic effect (Nirogi et al., 2021a). Another H3R antagonist/inverse agonist, namely, enerisant, was able to promote a waking effect by decreasing slow-wave sleep, however, at high doses (Hino et al., 2020). Moreover, the H3R antagonist/inverse agonist SAR110068 produced wakefulness and decreased slow-wave and REM sleep to similar degrees as ciproxifan but for longer durations as shown by EEG (Gao et al., 2013a). In another study, a bilateral lesion of the striatum in mice using 6-OHDA produced a disruption in the normal endogenous circadian rhythm, which was reversed following systemic administration with the H3R antagonist/inverse agonist thioperamide (Masini et al., 2017) (Table 1). Apart from increasing the levels of brain HA, samelisant was also found to modulate DA and NE levels in the cerebral cortex, while it had no effects on DA levels in the striatum or nucleus accumbens (Nirogi et al., 2021a). In addition, systemic treatment with samelisant was reported to produce a significant increase in wakefulness with a concomitant decrease in NREM sleep and direct REM sleep onset (DREM) episodes in orexin knockout mice subjected to sleep EEG, demonstrating its anticataplectic effects in an animal model relevant to narcolepsy (Nirogi et al., 2021a). Importantly, some positive effects of H3R antagonists/inverse agonists have been reported for the narcoleptic rodent model and for patients with narcolepsy (Romigi et al., 2018; Davis et al., 2021; Fabara et al., 2021). Accordingly, pitolisant (BF2.649/tiprolisant/Wakix), one of the H₃R antagonists/inverse agonists, was reported to enhance HA neuronal activity, promote wakefulness, and decrease abnormal onset of REM sleep from the wakefulness in Hcrt knockout mice (Romigi et al., 2018; Davis et al., 2021). The drug (Wakix®), a first-in-class antagonist/inverse agonist of the H3Rs, was approved in the EU (as of March 2016) for the treatment of narcolepsy with or without cataplexy in adults and in the United States (as of August 2019) for the treatment of excessive daytime sleepiness (EDS) in adults with narcolepsy. Moreover, in patients with narcolepsy, two small trials exhibited the effect of pitolisant on recovery from excessive daytime sleepiness (Romigi et al., 2018; Davis et al., 2021). In addition, pitolisant was reported to ameliorate excessive daytime sleepiness with comparable effectiveness to that of modafinil, an approved medicine for narcolepsy (Romigi et al., 2018; Davis et al., 2021). Therefore, pitolisant is currently assessed in different ongoing or completed clinical trials (clinicaltrials.gov), which may provide more insight into the potential role of brain HA in the excessive sleepiness and cataplexy attacks in narcolepsy (Shan et al., 2015; Carthy and Ellender, 2021; Davis et al., 2021; Fabara et al., 2021).