The Histamine H3 Receptor Differentially Modulates Mitogen-activated Protein Kinase (MAPK) and Akt Signaling in Striatonigral and Striatopallidal Neurons

Sexual dimorphism in histamine regulation of striatal dopamine

Background

Many neuropsychiatric disorders show sex differences in prevalence and presentation. For example, Tourette's Syndrome (TS) is diagnosed 3-5 times more often in males. Dopamine modulation of the basal ganglia is implicated in numerous neuropsychiatric conditions, including TS. Motivated by an unexpected genetic finding in a family with TS, we previously characterized the modulation of striatal dopamine by histamine.

Methods

We used microdialysis to analyze striatal dopamine response to the targeted infusion of histamine and histamine agonists. siRNA knockdown of histamine receptors was used to identify the cellular mediators of observed effects.

Results

Intracerebroventricular histamine reduced striatal dopamine in male mice, replicating previous work. Unexpectedly, histamine increased striatal dopamine in females. Targeted infusion of selected agonists revealed that the effect in males depends on H2R receptors in the substantia nigra pars compacta (SNc). Knockdown of H2R in SNc GABAergic neurons abrogated the effect, identifying these cells as a key locus of histamine's regulation of dopamine in males. In females, in contrast, H2R had no role; instead, H3R agonists in the striatum increased striatal dopamine. Strikingly, the effect of histamine on dopamine in females was modulated by the estrous cycle, appearing in estrus/proestrus but not in metestrus/diestrus.

Conclusions

These findings confirm the regulation of striatal dopamine by histamine but identify marked sexual dimorphism in and estrous modulation of this effect. These findings may shed light on the mechanistic underpinnings of other sex differences in the striatal circuitry, perhaps including the marked sex differences seen in TS and related neuropsychiatric conditions.

Postsynaptic histamine H3 receptors in ventral basal forebrain cholinergic neurons modulate contextual fear memory

Overly strong fear memories can cause pathological conditions. Histamine H3 receptor (H3R) has been viewed as an optimal drug target for CNS disorders, but its role in fear memory remains elusive. We find that a selective deficit of H3R in cholinergic neurons, but not in glutamatergic neurons, enhances freezing level during contextual fear memory retrieval without affecting cued memory. Consistently, genetically knocking down H3R or chemogenetically activating cholinergic neurons in the ventral basal forebrain (vBF) mimics this enhanced fear memory, whereas the freezing augmentation is rescued by re-expressing H3R or chemogenetic inhibition of vBF cholinergic neurons. Spatiotemporal regulation of H3R by a light-sensitive rhodopsin-H3R fusion protein suggests that postsynaptic H3Rs in vBF cholinergic neurons, but not presynaptic H3Rs of cholinergic projections in the dorsal hippocampus, are responsible for modulating contextual fear memory. Therefore, precise modulation of H3R in a cell-type- and subcellular-location-specific manner should be explored for pathological fear memory.

Chronic H3R activation reduces L-Dopa-induced dyskinesia, normalizes cortical GABA and glutamate levels, and increases striatal dopamine D1R mRNA expression in 6-hydroxydopamine-lesioned male rats

RATIONALE

Dyskinesias induced by L-3,4-dihydroxyphenylalanine, L-Dopa (LIDs), are the major complication in the pharmacological treatment of Parkinson's disease. LIDs induce overactivity of the glutamatergic cortico-striatal projections, and drugs that reduce glutamatergic overactivity exert antidyskinetic actions. Chronic administration of immepip, agonist at histamine H₃ receptors (H₃R), reduces LIDs and diminishes GABA and glutamate content in striatal dialysates (Avila-Luna et al., Psychopharmacology 236: 1937-1948, 2019).

OBJECTIVES AND METHODS

In rats unilaterally lesioned with 6-hydroxydopamine in the substantia nigra pars compacta (SNc), we examined whether the chronic administration of immepip and their withdrawal modify LIDs, the effect of L-Dopa on glutamate and GABA content, and mRNA levels of dopamine D₁ receptors (D₁Rs) and H₃Rs in the cerebral cortex and striatum.

RESULTS

The administration of L-Dopa for 21 days induced LIDs. This effect was accompanied by increased GABA and glutamate levels in the cerebral cortex ipsi and contralateral to the lesioned SNc, and immepip administration prevented (GABA) or reduced (glutamate) these actions. In the striatum, GABA content increased in the ipsilateral nucleus, an effect prevented by immepip. L-Dopa administration had no significant effects on striatal glutamate levels. In lesioned and L-Dopa-treated animals, D₁R mRNA decreased in the ipsilateral striatum, an effect prevented by immepip administration.

CONCLUSIONS

Our results indicate that chronic H₃R activation reduces LIDs and the overactivity of glutamatergic cortico-striatal projections, providing further evidence for an interaction between D₁Rs and H₃Rs in the cortex and striatum under normal and pathological conditions.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

White matter abnormalities in the Hdc knockout mouse, a model of tic and OCD pathophysiology

Introduction

An inactivating mutation in the histidine decarboxylase gene (Hdc) has been identified as a rare but high-penetrance genetic cause of Tourette syndrome (TS). TS is a neurodevelopmental syndrome characterized by recurrent motor and vocal tics; it is accompanied by structural and functional abnormalities in the cortico-basal ganglia circuitry. Hdc, which is expressed both in the posterior hypothalamus and peripherally, encodes an enzyme required for the biosynthesis of histamine. Hdc knockout mice (Hdc-KO) functionally recapitulate this mutation and exhibit behavioral and neurochemical abnormalities that parallel those seen in patients with TS.

Materials and methods

We performed exploratory RNA-seq to identify pathological alterations in several brain regions in Hdc-KO mice. Findings were corroborated with RNA and protein quantification, immunohistochemistry, and ex vivo brain imaging using MRI.

Results

Exploratory RNA-Seq analysis revealed, unexpectedly, that genes associated with oligodendrocytes and with myelin production are upregulated in the dorsal striatum of these mice. This was confirmed by qPCR, immunostaining, and immunoblotting. These results suggest an abnormality in myelination in the striatum. To test this in an intact mouse brain, we performed whole-brain ex vivo diffusion tensor imaging (DTI), which revealed reduced fractional anisotropy (FA) in the dorsal striatum.

Discussion

While the DTI literature in individuals with TS is sparse, these results are consistent with findings of disrupted descending cortical projections in patients with tics. The Hdc-KO model may represent a powerful system in which to examine the developmental mechanisms underlying this abnormality.

Sexual Dimorphism in the Effect of Neonatal Inflammatory Pain on Stress Reactivity of Hormonal Response and Cognitive Functions in Adult Rats

The effect of moderate neonatal stress induced by inflammatory pain in rat pups of both sexes on the hormonal response and cognitive processes in adult animals was studied in the Morris water maze. No significant differences in spatial learning and memory were found in experimental rats exposed to neonatal inflammatory pain vs. control animals. However, experimental rats exhibited sex differences in long-term spatial memory whose efficiency was higher in males vs. females. After long-term memory testing, stress responsiveness of the hypothalamic-pituitary-adrenocortical axis, as assessed by the plasma corticosterone level in the formalin test, was higher in experimental males vs. females. Only experimental females exhibited differences between short-term and long-term memory, with the efficiency being higher in the former. Thus, sexual dimorphism was found in the effect of neonatal nociceptive stress on long-term spatial memory in adult rats: experimental males vs. females demonstrated more effective long-term memory combined with a higher stress reactivity of the hormonal response.

Role of histidine decarboxylase gene in the pathogenesis of Tourette syndrome

Tourette syndrome (TS) is caused by complex genetic and environmental factors and is characterized by tics. Histidine decarboxylase (HDC) mutation is a rare genetic cause with high penetrance in patients with TS. HDC-knockout (KO) mice have similar behavioral and neurochemical abnormalities as patients with TS. Therefore, HDC-KO mice are considered a valuable TS pathophysiological model as it reveals the underlying pathological mechanisms that cannot be obtained from patients with TS, thus advancing the development of treatment strategies for TS and other tic disorders. This review summarizes some of the recent research hotspots and progress in HDC-KO mice, aiming to deepen our understanding of brain mechanisms relevant to TS. Furthermore, we encapsulate the possible brain nerve cell changes in HDC-KO mice and their potential roles in TS to provide multiple directions for the future research on tics.

Histamine H3 receptor antagonism modulates autism-like hyperactivity but not repetitive behaviors in BTBR T+Itpr3tf/J inbred mice

BACKGROUND

Autism spectrum disorders (ASDs) are a group of neurodevelopmental conditions defined by behavioral deficits in social communication and interactions, mental inflexibility and repetitive behaviors. Converging evidence from observational and preclinical studies suggest that excessive repetitive behaviors in people with ASD may be due to elevated histaminergic H3 receptor signaling in the striatum. We hypothesized that systemic administration of pharmacological histamine H3 receptor antagonists would attenuate the expression of repetitive behaviors in the BTBR T+Itpr3tf/J (BTBR) mouse inbred strain, an established mouse model presenting autism-like repetitive behaviors and novelty-induced hyperactivity. We further aimed to investigate whether agonism of the histamine H3 receptor would be sufficient to induce repetitive behaviors in the C57BL/6J control mouse strain.

METHODS

Different doses of H3 receptor agonists (i.e., (R)-α-methylhistamine and immethridine) and H3 receptor antagonists/inverse agonists (i.e., ciproxifan and pitolisant) were administered via intraperitoneal (i.p.) injection in male mice to characterize the acute effects of these compounds on ASD-related behavioral readouts.

RESULTS

The highly selective H3 receptor agonist immethridine significantly increased the time spent in stereotypic patterns in C57BL/6J mice, but this effect appeared to be driven by general sedative properties of the compound. High doses of pitolisant significantly decreased locomotor hyperactivity in novel environments in BTBR mice, without significant effects on repetitive behaviors.

CONCLUSIONS

Based on our findings, we conclude that acute H3 receptor manipulation mainly affected general motor activity levels in novel environments. Small changes in stereotyped behaviors were observed but appeared to be driven by altered general activity levels.

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

Histamine, Neuroinflammation and Neurodevelopment: A Review

The biogenic amine, histamine, has been shown to critically modulate inflammatory processes as well as the properties of neurons and synapses in the brain, and is also implicated in the emergence of neurodevelopmental disorders. Indeed, a reduction in the synthesis of this neuromodulator has been associated with the disorders Tourette's syndrome and obsessive-compulsive disorder, with evidence that this may be through the disruption of the corticostriatal circuitry during development. Furthermore, neuroinflammation has been associated with alterations in brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there are suggestions that histamine deficiency may leave the developing brain more vulnerable to proinflammatory insults. While most studies have focused on neuronal sources of histamine it remains unclear to what extent other (non-neuronal) sources of histamine, e.g., from mast cells and other sources, can impact brain development. The few studies that have started exploring this in vitro, and more limited in vivo, would indicate that non-neuronal released histamine and other preformed mediators can influence microglial-mediated neuroinflammation which can impact brain development. In this Review we will summarize the state of the field with regard to non-neuronal sources of histamine and its impact on both neuroinflammation and brain development in key neural circuits that underpin neurodevelopmental disorders. We will also discuss whether histamine receptor modulators have been efficacious in the treatment of neurodevelopmental disorders in both preclinical and clinical studies. This could represent an important area of future research as early modulation of histamine from neuronal as well as non-neuronal sources may provide novel therapeutic targets in these disorders.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Histamine H3 receptor and cholinesterases as synergistic targets for cognitive decline: Strategies to the rational design of multitarget ligands

The role of histamine and acetylcholine in cognitive functions suggests that compounds able to increase both histaminergic and cholinergic neurotransmissions in the brain should be considered as promising therapeutic options. For this purpose, dual inhibitors of histamine H₃ receptors (H₃ R) and cholinesterases (ChEs) have been designed and assessed. In this context, this paper reviews the strategies used to obtain dual H₃ R/ChEs ligands using multitarget design approaches. Hybrid compounds designed by linking tacrine or flavonoid motifs to H₃ R antagonists were obtained with high affinity for both targets, and compounds designed by merging the H₃ R antagonist pharmacophore with known anticholinesterase molecules were also reported. These reports strongly suggest that key modifications in the lipophilic region (including a second basic group) seem to be a strategy to reach novel compounds, allied with longer linker groups to a basic region. Some compounds have already demonstrated efficacy in memory models, although the pharmacokinetic and toxicity profile should be considered when designing further compounds. In conclusion, the key features to be considered when designing novel H₃ R/ChEs inhibitors with improved pharmacological profile were herein summarized.

Immunomodulatory Effects of Dopamine in Inflammatory Diseases

Dopamine (DA) receptor, a significant G protein-coupled receptor, is classified into two families: D1-like (D1 and D5) and D2-like (D2, D3, and D4) receptor families, with further formation of homodimers, heteromers, and receptor mosaic. Increasing evidence suggests that the immune system can be affected by the nervous system and neurotransmitters, such as dopamine. Recently, the role of the DA receptor in inflammation has been widely studied, mainly focusing on NLRP3 inflammasome, NF-κB pathway, and immune cells. This article provides a brief review of the structures, functions, and signaling pathways of DA receptors and their relationships with inflammation. With detailed descriptions of their roles in Parkinson disease, inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis, this article provides a theoretical basis for drug development targeting DA receptors in inflammatory diseases.

Histamine H3 Receptor Function Biases Excitatory Gain in the Nucleus Accumbens

BACKGROUND

Histamine (HA), a wake-promoting monoamine implicated in stress-related arousal states, is synthesized in histidine decarboxylase-expressing hypothalamic neurons of the tuberomammillary nucleus. Histidine decarboxylase-containing varicosities diffusely innervate striatal and mesolimbic networks, including the nucleus accumbens (NAc). The NAc integrates diverse monoaminergic inputs to coordinate motivated behavior. While the NAc expresses various HA receptor subtypes, mechanisms by which HA modulates NAc circuit dynamics are undefined.

METHODS

Using male D1tdTomato transgenic reporter mice, whole-cell patch-clamp electrophysiology, and input-specific optogenetics, we employed a targeted pharmacological approach to interrogate synaptic mechanisms recruited by HA signaling at glutamatergic synapses in the NAc. We incorporated an immobilization stress protocol to assess whether acute stress engages these mechanisms at glutamatergic synapses onto D₁ receptor-expressing [D1(+)] medium spiny neurons (MSNs) in the NAc core.

RESULTS

HA negatively regulates excitatory gain onto D1(+)-MSNs via presynaptic H₃ receptor-dependent long-term depression that requires G_βγ-directed Akt-GSK3β signaling. Furthermore, HA asymmetrically regulates glutamatergic transmission from the prefrontal cortex and mediodorsal thalamus, with inputs from the prefrontal cortex undergoing robust HA-induced long-term depression. Finally, we report that acute immobilization stress attenuates this long-term depression by recruiting endogenous H₃ receptor signaling in the NAc at glutamatergic synapses onto D1(+)-MSNs.

CONCLUSIONS

Stress-evoked HA signaling in the NAc recruits H₃ heteroreceptor signaling to shift thalamocortical input onto D1(+)-MSNs in the NAc. Our findings provide novel insight into an understudied neuromodulatory system within the NAc and implicate HA in stress-associated physiological states.

The Multi-Targeting Ligand ST-2223 with Histamine H3 Receptor and Dopamine D2/D3 Receptor Antagonist Properties Mitigates Autism-Like Repetitive Behaviors and Brain Oxidative Stress in Mice

Autism spectrum disorder (ASD) is a complex heterogeneous neurodevelopmental disorder characterized by social and communicative impairments, as well as repetitive and restricted behaviors (RRBs). With the limited effectiveness of current pharmacotherapies in treating repetitive behaviors, the present study determined the effects of acute systemic treatment of the novel multi-targeting ligand ST-2223, with incorporated histamine H₃ receptor (H₃R) and dopamine D₂/D₃ receptor affinity properties, on ASD-related RRBs in a male Black and Tan BRachyury (BTBR) mouse model of ASD. ST-2223 (2.5, 5, and 10 mg/kg, i.p.) significantly mitigated the increase in marble burying and self-grooming, and improved reduced spontaneous alternation in BTBR mice (all p < 0.05). Similarly, reference drugs memantine (MEM, 5 mg/kg, i.p.) and aripiprazole (ARP, 1 mg/kg, i.p.), reversed abnormally high levels of several RRBs in BTBR (p < 0.05). Moreover, ST-2223 palliated the disturbed anxiety levels observed in an open field test (all p < 0.05), but did not restore the hyperactivity parameters, whereas MEM failed to restore mouse anxiety and hyperactivity. In addition, ST-2223 (5 mg/kg, i.p.) mitigated oxidative stress status by decreasing the elevated levels of malondialdehyde (MDA), and increasing the levels of decreased glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT) in different brain parts of treated BTBR mice (all p < 0.05). These preliminary in vivo findings demonstrate the ameliorative effects of ST-2223 on RRBs in a mouse model of ASD, suggesting its pharmacological prospective to rescue core ASD-related behaviors. Further confirmatory investigations on its effects on various brain neurotransmitters, e.g., dopamine and histamine, in different brain regions are still warranted to corroborate and expand these initial data.

Antibodies From Children With PANDAS Bind Specifically to Striatal Cholinergic Interneurons and Alter Their Activity

OBJECTIVE

Pediatric obsessive-compulsive disorder (OCD) sometimes appears rapidly, even overnight, often after an infection. Pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, or PANDAS, describes such a situation after infection with Streptococcus pyogenes. PANDAS may result from induced autoimmunity against brain antigens, although this remains unproven. Pilot work suggests that IgG antibodies from children with PANDAS bind to cholinergic interneurons (CINs) in the striatum. CIN deficiency has been independently associated with tics in humans and with repetitive behavioral pathology in mice, making it a plausible locus of pathology. The authors sought to replicate and extend earlier work and to investigate the cellular effects of PANDAS antibodies on cholinergic interneurons.

METHODS

Binding of IgG to specific neurons in human and mouse brain slices was evaluated ex vivo after incubation with serum from 27 children with rigorously characterized PANDAS, both at baseline and after intravenous immunoglobulin (IVIG) treatment, and 23 matched control subjects. Binding was correlated with symptom measures. Neural activity after serum incubation was assessed in mouse slices using molecular markers and electrophysiological recording.

RESULTS

IgG from children with PANDAS bound to CINs, but not to several other neuron types, more than IgG from control subjects, in three independent cohorts of patients. Post-IVIG serum had reduced IgG binding to CINs, and this reduction correlated with symptom improvement. Baseline PANDAS sera decreased activity of striatal CINs, but not of parvalbumin-expressing GABAergic interneurons, and altered their electrophysiological responses, in acute mouse brain slices. Post-IVIG PANDAS sera and IgG-depleted baseline sera did not alter the activity of striatal CINs.

CONCLUSIONS

These findings provide strong evidence for striatal CINs as a critical cellular target that may contribute to pathophysiology in children with rapid-onset OCD symptoms, and perhaps in other conditions.

A Focused Update on Tardive Dyskinesia

Tardive dyskinesia (TD) is a delayed and potentially irreversible motor complication following chronic exposure to centrally acting dopamine receptor antagonists, mainly of the class of antipsychotics drugs. New generations of antipsychotic drugs reduced its mean prevalence to 20%, but it continues to mar the drug experience and social integration in a significant fraction of patients. The underlying molecular cascade remains elusive, explaining in part why TD management is so often difficult. Protocol variations between experimental laboratories and inter-species differences in the biological response to antipsychotic drugs have added layers of complexity. The traditional dopamine D2 receptor supersensitivity hypothesis was revisited in an experimental nonhuman primate model. Findings in the striatum revealed a strong upregulation of D3, not D2, receptors specific to dyskinetic animals, and indirect evidence suggestive of a link between overactivation of glycogen synthase kinase-3β signaling and TD. New effective vesicular monoamine transporter type 2 inhibitors alleviating TD have been approved in the USA. They were integrated to an emerging stepwise treatment algorithm for troublesome TD, which also includes consideration for changes in the current antipsychotic drug regimen and recognition of potentially aggravating factors such as anticholinergic co-medications. These advances may benefit TD.

Region-specific regulation of central histaminergic H3 receptor expression in a mouse model of cow's milk allergy

Central histaminergic H3 receptor (H3R) has been extensively investigated as a potential therapeutic target for various neurological and neurodegenerative disorders. Despite promising results in preclinical rodent models, clinical trials have not provided conclusive evidence for the benefit of H3R antagonists to alleviate cognitive and behavioral symptoms of these disorders. Inconsistent pharmacological efficacies may arise from aberrant changes in H3R over time during disease development. Because H3R is involved in feedback inhibition of histamine synthesis and secretion, the expression of the autoreceptor may also be reciprocally regulated by altered histamine levels in a pathological condition. Thus, we investigated H3R expression in a mouse model of cow's milk allergy, a condition associated with increased histamine levels. Mice were sensitized to bovine whey proteins (WP) over 5 weeks and H3R protein and transcript levels were examined in the brain. Substantially increased H3R immunoreactivity was observed in various brain regions of WP-sensitized mice compared to sham mice. Elevated H3R expression was also found in the thalamic/hypothalamic region. The expression of histaminergic H1, but not H2, receptor subtype was also increased in this and the midbrain regions. Unlike the brain, all three histaminergic receptors were increased in the small intestine. These results indicated that the central histaminergic receptors were altered in WP-sensitized mice in a subtype- and region-specific manner, which likely contributed to behavioral changes we observed in these mice. Our study also suggests that altered levels of H3R could be considered during a pharmacological intervention of a neurological disease.

Histaminergic Control of Corticostriatal Synaptic Plasticity during Early Postnatal Development

A reduction in the synthesis of the neuromodulator histamine has been associated with Tourette's syndrome and obsessive-compulsive disorder. Symptoms of these disorders are thought to arise from a dysfunction or aberrant development ofcorticostriatal circuits. Here, we investigated how histamine affects developing corticostriatal circuits, both acutely and longer-term, during the first postnatal weeks, using patch-clamp and field recordings in mouse brain slices (C57Bl/6, male and female). Immunohistochemistry for histamine-containing axons reveals striatal histaminergic innervation by the second postnatal week, and qRT-PCR shows transcripts for H1, H2, and H3 histamine receptors in striatum from the first postnatal week onwards, with pronounced developmental increases in H3 receptor expression. Whole-cell patch-clamp recordings of striatal spiny projection neurons and histamine superfusion demonstrates expression of functional histamine receptors from the first postnatal week onwards, with histamine having diverse effects on their electrical properties, including depolarization of the membrane potential while simultaneously decreasing action potential output. Striatal field recordings and electrical stimulation of corticostriatal afferents revealed that histamine, acting at H3 receptors, negatively modulates corticostriatal synaptic transmission from the first postnatal week onwards. Last, we investigated effects of histamine on longer-term changes at developing corticostriatal synapses and show that histamine facilitates NMDA receptor-dependent LTP via H3 receptors during the second postnatal week, but inhibits synaptic plasticity at later developmental stages. Together, these results show that histamine acutely modulates developing striatal neurons and synapses and controls longer-term changes in developing corticostriatal circuits, thus providing insight into the possible etiology underlying neurodevelopmental disorders resulting from histamine dysregulation.SIGNIFICANCE STATEMENT Monogenic causes of neurologic disorders, although rare, can provide opportunities to both study and understand the brain. For example, a nonsense mutation in the coding gene for the histamine-synthesizing enzyme has been associated with Tourette's syndrome and obsessive-compulsive disorder, and dysfunction of corticostriatal circuits. Nevertheless, the etiology of these neurodevelopmental disorders and histamine's role in the development of corticostriatal circuits have remained understudied. Here we show that histamine is an active neuromodulator during the earliest periods of postnatal life and acts at developing striatal neurons and synapses. Crucially, we show that histamine permits NMDA receptor-dependent corticostriatal synaptic plasticity during an early critical period of postnatal development, which suggests that genetic or environmental perturbations of histamine levels can impact striatal development.

Modulation of dopamine D1 receptors via histamine H3 receptors is a novel therapeutic target for Huntington's disease

Early Huntington's disease (HD) include over-activation of dopamine D₁ receptors (D₁R), producing an imbalance in dopaminergic neurotransmission and cell death. To reduce D₁R over-activation, we present a strategy based on targeting complexes of D₁R and histamine H₃ receptors (H₃R). Using an HD mouse striatal cell model and HD mouse organotypic brain slices we found that D₁R-induced cell death signaling and neuronal degeneration, are mitigated by an H₃R antagonist. We demonstrate that the D₁R-H₃R heteromer is expressed in HD mice at early but not late stages of HD, correlating with HD progression. In accordance, we found this target expressed in human control subjects and low-grade HD patients. Finally, treatment of HD mice with an H₃R antagonist prevented cognitive and motor learning deficits and the loss of heteromer expression. Taken together, our results indicate that D₁R - H₃R heteromers play a pivotal role in dopamine signaling and represent novel targets for treating HD.

Histamine Receptors Regulate the Activity, Surface Expression, and Phosphorylation of Serotonin Transporters

Reuptake and clearance of released serotonin (5-HT) are critical in serotonergic neurotransmission. Serotonin transporter (SERT) is mainly responsible for clearing the extracellular 5-HT. Controlled trafficking, phosphorylation, and protein stability have been attributed to robust SERT activity. H₃ histamine receptors (H₃Rs) act in conjunction and regulate 5-HT release. H₃Rs are expressed in the nervous system and located at the serotonergic terminals, where they act as heteroreceptors. Although histaminergic and serotonergic neurotransmissions are thought to be two separate events, whether H₃Rs influence SERT in the CNS to control 5-HT reuptake has never been addressed. With a priori knowledge gained from our studies, we explored the possibility of using rat hippocampal synaptosomal preparations. We found that treatment with H₃R/H₄R-agonists immepip and (R)-(-)-α-methyl-histamine indeed resulted in a time- and concentration-dependent decrease in 5-HT transport. On the other hand, treatment with H₃R/H₄R-inverse agonist thioperamide caused a moderate increase in 5-HT uptake while blocking the inhibitory effect of H₃R/H₄R agonists. When investigated further, immepip treatment reduced the level of SERT on the plasma membrane and its phosphorylation. Likewise, CaMKII inhibitor KN93 or calcineurin inhibitor cyclosporine A also inhibited SERT function; however, an additive effect with immepip was not seen. High-speed in vivo chronoamperometry demonstrated that immepip delayed 5-HT clearance while thioperamide accelerated 5-HT clearance from the extracellular space. Immepip selectively inhibited SERT activity in the hippocampus and cortex but not in the striatum, midbrain, and brain stem. Thus, we report here a novel mechanism of regulating SERT activity by H₃R-mediated CaMKII/calcineurin pathway in a brain-region-specific manner and perhaps synaptic 5-HT in the CNS that controls 5-HT clearance.

Identification of new antiviral agents against Kaposi's sarcoma-associated herpesvirus (KSHV) by high-throughput drug screening reveals the role of histamine-related signaling in promoting viral lytic reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) causes several human cancers, such as Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). Current treatment options for KSHV infection and virus associated diseases are sometimes ineffective, therefore, more effectively antiviral agents are urgently needed. As a herpesvirus, lytic replication is critical for KSHV pathogenesis and oncogenesis. In this study, we have established a high-throughput screening assay by using an inducible KSHV+ cell-line, iSLK.219. After screening a compound library that consisted of 1280 Food and Drug Administration (FDA)-approved drugs, 15 hit compounds that effectively inhibited KSHV virion production were identified, most of which have never been reported with anti-KSHV activities. Interestingly, 3 of these drugs target histamine receptors or signaling. Our data further confirmed that antagonists targeting different histamine receptors (HxRs) displayed excellent inhibitory effects on KSHV lytic replication from induced iSLK.219 or BCBL-1 cells. In contrast, histamine and specific agonists of HxRs promoted viral lytic replication from induced iSLK.219 or KSHV-infected primary cells. Mechanistic studies indicated that downstream MAPK and PI3K/Akt signaling pathways were required for histamine/receptors mediated promotion of KSHV lytic replication. Direct knockdown of HxRs in iSLK.219 cells effectively blocked viral lytic gene expression during induction. Using samples from a cohort of HIV+ patients, we found that the KSHV+ group has much higher levels of histamine in their plasma and saliva than the KSHV- group. Taken together, our data have identified new anti-KSHV agents and provided novel insights into the molecular bases of host factors that contribute to lytic replication and reactivation of this oncogenic herpesvirus.

Histamine H3 receptor activation reduces the impairment in prepulse inhibition (PPI) of the acoustic startle response and Akt phosphorylation induced by MK-801 (dizocilpine), antagonist at N-Methyl-d-Aspartate (NMDA) receptors

We have investigated the effect of the local activation of histamine H₃ receptors (H₃Rs) in the rat prefrontal cortex (PFCx) on the impairment of pre-pulse inhibition (PPI) of the startle response induced by the systemic administration of MK-801, antagonist at glutamate N-Methyl-d-Aspartate (NMDA) receptors, and the possible functional interaction between H₃Rs and MK-801 on PFCx dopaminergic transmission. Infusion of the H₃R agonist RAMH (19.8 ng/1 μl) into the PFCx reduced or prevented the inhibition by MK-801 (0.15 mg/kg, ip) of PPI evoked by different auditory stimulus intensities (5, 10 and 15 dB), and the RAMH effect was blocked by the H₃R antagonist/inverse agonist ciproxifan (30.6 ng/1 μl). MK-801 inhibited [³H]-dopamine uptake (-45.4 ± 2.1%) and release (-32.8 ± 2.6%) in PFCx synaptosomes or slices, respectively, and molecular modeling indicated that MK-801 binds to and blocks the rat and human dopamine transporters. However, H₃R activation had no effect on the inhibitory action of MK-801 on dopamine uptake and release. In PFCx slices, MK-801 and the activation of H₃Rs or dopamine D₁ receptors (D₁Rs) stimulated ERK-1/2 and Akt phosphorylation. The co-activation of D₁Rs and H₃Rs prevented ERK-1/2 and Akt phosphorylation, and H₃R activation or D₁R blockade prevented the effect of MK-801. In ex vivo experiments, the intracortical infusion of the D₁R agonist SKF-81297 (37 ng/1 μl) or the H₃R agonist RAMH increased Akt phosphorylation, prevented by D₁R/H₃R co-activation. These results indicate that MK-801 enhances dopaminergic transmission in the PFCx, and that H₃R activation counteracts the post-synaptic actions of dopamine.

7-Pyrrolidinethoxy-4'-Methoxyisoflavone Prevents Amyloid β-Induced Injury by Regulating Histamine H3 Receptor-Mediated cAMP/CREB and AKT/GSK3β Pathways

In studies on the treatment of Alzheimer’s disease (AD), in which cognition is enhanced even modestly or selectively, it has been considered that the histamine H3 receptor (H3R) may be a potential target. In this study, we aimed at evaluating the ability of 7-pyrrolidinethoxy-4′-methoxyisoflavone (indicated as LC1405), a novel potential H3R antagonist identified from our H3R antagonist screening system, to ameliorate amyloid β (Aβ)-induced cognitive deficits, and to explore the underlying mechanisms that are related to H3R-modulated signaling. Our results demonstrated that LC1405 effectively reduced the progression of Aβ-associated disorders, such as improved learning and memory capabilities, preserved tissues from suffering neurodegeneration and ultrastructural abnormalities, and ameliorated cholinergic dysfunction in an APP/PS1 double transgenic mouse model of AD. In an in vitro model, LC1405 protected neuronal cells against copper-induced Aβ toxicity, as demonstrated by the improvement in cell viability and decrease in neuronal apoptotic ratio. In addition, treatment with LC1405 resulted in the up-regulation of acetylcholine (ACh) or histamine release and provided neuroprotection through cellular signaling cascades involving H3R-mediated cAMP/CREB and AKT/GSK3β pathways. Furthermore, the beneficial effects of LC1405 on Aβ-mediated toxicity and H3R-mediated cAMP/CREB and AKT/GSK3β axes were reversed after pharmacological activation of H3R. In conclusion, our results demonstrated that LC1405 blocked Aβ-induced toxicity through H3R-modulated signaling transduction both in vitro and in vivo. The results also suggested that LC1405 might have translational potential as a complementary therapy to control disease progression in AD patients who developed cognitive deficits with H3R-related ACh neurotransmission abnormality.

The histidine decarboxylase model of tic pathophysiology: a new focus on the histamine H3 receptor

Histamine dysregulation was implicated as a rare cause of Tourette syndrome and other tic disorders a decade ago by a landmark genetic study in a high density family pedigree, which implicated a hypomorphic mutation in the histidine decarboxylase (Hdc) gene as a rare but high penetrance genetic cause. Studies in Hdc knockout (KO) mice have confirmed that this mutation causes tic-relevant behavioural and neurochemical abnormalities that parallel what is seen in patients and thus validate the KO as a potentially informative model of tic pathophysiology. Recent studies have focused on the potential role of the histamine H₃ receptor in this model, and by association in tic disorders and related neuropsychiatric conditions. The H₃ receptor is up-regulated in the striatum in Hdc KO mice. As the H₃ receptor has constitutive activity in the absence of ligand, this receptor up-regulation may have significant cellular effects despite the absence of neurotransmitter histamine in these mice. Activation in vivo of H₃ receptors in wild type mice regulates signalling in striatal medium spiny neurons (MSNs) that interacts non-linearly with dopamine receptor signalling. Baseline signalling alterations in MSNs in Hdc KO mice resemble those seen after H₃ receptor agonist treatment in wild type animals. H₃ receptor agonist treatment in the KOs further accentuates most of these signalling abnormalities and produces behavioural stereotypy. Together, these data suggest the intriguing hypothesis that constitutive signalling by up-regulated H₃ receptors explains many of the molecular and behavioural abnormalities seen in these animals. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Parkinson's Disease is Associated with Dysregulations of a Dopamine-Modulated Gene Network Relevant to Sleep and Affective Neurobehaviors in the Striatum

In addition to the characteristic motor symptoms, Parkinson’s disease (PD) often involves a constellation of sleep and mood symptoms. However, the mechanisms underlying these comorbidities are largely unknown. We have previously reconstructed gene networks in the striatum of a population of (C57BL/6J x A/J) F2 mice and associated the networks to sleep and affective phenotypes, providing a resource for integrated analyses to investigate perturbed sleep and affective functions at the gene network level. Combining this resource with PD-relevant transcriptomic datasets from humans and mice, we identified four networks that showed elevated gene expression in PD patients, including a circadian clock and mitotic network that was altered similarly in mouse models of PD. We then utilized multiple types of omics data from public databases and linked this gene network to postsynaptic dopamine signaling in the striatum, CDK1-modulated transcriptional regulation, and the genetic susceptibility of PD. These findings suggest that dopamine deficiency, a key aspect of PD pathology, perturbs a circadian/mitotic gene network in striatal neurons. Since the normal functions of this network were relevant to sleep and affective behaviors, these findings implicate that dysregulation of functional gene networks may be involved in the emergence of non-motor symptoms in PD. Our analyses present a framework for integrating multi-omics data from diverse sources in mice and humans to reveal insights into comorbid symptoms of complex diseases.

Dopamine signaling in the striatum

The striatum integrates dopamine-mediated reward signals to generate appropriate behavior in response to glutamate-mediated sensory cues. Such associative learning relies on enduring neural plasticity in striatal GABAergic spiny projection neurons which, when altered, can lead to the development of a wide variety of pathological states. Considerable progress has been made in our understanding of the intracellular signaling mechanisms in dopamine-related behaviors and pathologies. Through the prism of the regulation of histone H3 and ribosomal protein S6 phosphorylation, we review how dopamine-mediated signaling events regulate gene transcription and mRNA translation. Particularly, we focus on the intracellular cascades controlling these phosphorylations downstream of the modulation of dopamine receptors by psychostimulants, antipsychotics and l-DOPA. Finally, we highlight the importance to precisely determine in which neuronal populations these signaling events occur in order to understand how they participate in remodeling neural circuits and altering dopamine-related behaviors.

Striatal Signaling Regulated by the H3R Histamine Receptor in a Mouse Model of tic Pathophysiology

Histamine dysregulation has been identified as a rare genetic cause of tic disorders; mice with a knockout of the histidine decarboxylase (Hdc) gene represent a promising model of this pathophysiology. How alterations in the histamine system lead to neuropsychiatric disease, however, remains unclear. The H3R histamine receptor is elevated in the striatum of Hdc KO mice, and H3R agonists, acting in the dorsal striatum, trigger tic-like movements in the model. In wild-type mice, H3R in the dorsal striatum differentially regulates mitogen-activated protein kinase (MAPK) and protein kinase B (Akt) signaling in D1R dopamine receptor-expressing striatonigral medium spiny neurons (dMSNs) and D2R dopamine receptor-expressing striatopallidal MSNs (iMSNs), respectively. We examined the effects of H3R agonist treatment on MSN signaling in the Hdc-KO model. In dMSNs, MAPK signaling was elevated at baseline in the Hdc-KO model, resembling what is seen after H3R activation in WT animals. Similarly, in iMSNs, Akt phosphorylation was reduced at baseline in the KO model, resembling what is seen after H3R activation in WT animals. H3R activation in Hdc-KO mice further enhanced the baseline effect on Akt phosphorylation in iMSNs but attenuated the abnormality in MAPK signaling in dMSNs. These observations support the hypothesis that constitutive activity of upregulated H3R receptors in the Hdc-KO model mediates the observed alterations in baseline MSN signaling; but further activation of H3R, which produces tic-like repetitive movements in the model, has more complex effects.

The histamine H3R antagonist DL77 attenuates autistic behaviors in a prenatal valproic acid-induced mouse model of autism

Autistic spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairment in social communication and restricted/repetitive behavior patterns or interests. Antagonists targeting histamine H3 receptor (H3R) are considered potential therapeutic agents for the therapeutic management of different brain disorders, e.g., cognitive impairments. Therefore, the effects of subchronic treatment with the potent and selective H3R antagonist DL77 (5, 10, or 15 mg/kg, i.p.) on sociability, social novelty, anxiety, and aggressive/repetitive behavior in male Tuck-Ordinary (TO) mice with ASD-like behaviors induced by prenatal exposure to valproic acid (VPA, 500 mg/kg, i.p.) were evaluated using the three-chamber test (TCT), marble burying test (MBT), nestlet shredding test (NST), and elevated plus maze (EPM) test. The results showed that VPA-exposed mice exhibited significantly lower sociability and social novelty preference compared to VPA-exposed mice that were pretreated with DL77 (10 or 15 mg/kg, i.p.). VPA-exposed mice presented a significantly higher percentage of buried marbles in MBT and shredded nestlet significantly more in NST compared to the control groups. However, VPA-exposed animals pretreated with DL77 (10 or 15 mg/kg, i.p.) buried a reduced percentage of marbles in MBT and presented a significantly lower percentage of shredding behavior in NST. On the other hand, pretreatment with DL77 (5, 10, or 15 mg/kg, i.p.) failed to restore the disturbed anxiety levels and hyperactivity observed in VPA-exposed animals in EPM, whereas the reference drug donepezil (DOZ, 1 mg/kg, i.p.) significantly palliated the anxiety and reduced the hyperactivity measures of VPA-exposed mice. Furthermore, pretreatment with DL77 (10 or 15 mg/kg, i.p.) modulated oxidative stress status by increasing GSH and decreasing MDA, and it attenuated the proinflammatory cytokines IL-1β, IL-6 and TNF-α exacerbated by lipopolysaccharide (LPS) challenge, in VPA-exposed mouse brain tissue. Taken together, these results provide evidence that modulation of brain histaminergic neurotransmission, such as by subchronic administration of the H3R antagonist DL77, may serve as an effective pharmacological therapeutic target to rescue ASD-like behaviors in VPA-exposed animals, although further investigations are necessary to corroborate and expand these initial data.

Dopamine D1 receptor activation maintains motor coordination and balance in rats

Dopamine (DA) modulates motor coordination, and its depletion, as in Parkinson's disease, produces motor impairment. The basal ganglia, cerebellum and cerebral cortex are interconnected, have functional roles in motor coordination, and possess dopamine D₁ receptors (D₁Rs), which are expressed at a particularly high density in the basal ganglia. In this study, we examined whether the activation of D₁Rs modulates motor coordination and balance in the rat using a beam-walking test that has previously been used to detect motor coordination deficits. The systemic administration of the D₁R agonist SKF-38393 at 2, 3, or 4 mg/kg did not alter the beam-walking scores, but the subsequent administration of the D₁R antagonist SCH-23390 at 1 mg/kg did produce deficits in motor coordination, which were reversed by the full agonist SKF-82958. The co-administration of SKF-38393 and SCH-23390 did not alter the beam-walking scores compared with the control group, but significantly prevented the increase in beam-walking scores induced by SCH-23390. The effect of the D₁R agonist to prevent and reverse the effect of the D₁R antagonist in beam-walking scores is an indicator that the function of D₁Rs is necessary to maintain motor coordination and balance in rats. Our results support that D₁Rs mediate the SCH-23390-induced deficit in motor coordination.

Behavioral and stereological characterization of Hdc KO mice: Relation to Tourette syndrome

A premature termination codon in the human histidine decarboxylase (Hdc) gene has been identified in a family suffering from Guilles de la Tourette syndrome (GTS). In the current study we investigated if mice lacking the histamine producing enzyme HDC share the morphological and cytological phenotype with GTS patients by using magnetic resonance (MRI) and diffusion tensor imaging (DTI), unbiased stereology and immunohistochemistry. Behavior of Hdc knock-out (Hdc KO) mice was assessed in an open field test. The results of stereological, volumetric and DTI analysis measurements showed no significant differences between control and Hdc KO mice. The numbers and distribution of GABAergic parvalbumin or nitric oxide-expressing and cholinergic interneurons were normal in Hdc KO mice. Cortical morphology and layering in adult Hdc KO mice were also preserved. In open field test Hdc KO mice showed impaired exploratory activity and habituation when introduced to novel environment. Our results indicate that Hdc deficiency in mice does not disturb the development of striatal and cortical interneurons and does not lead to the morphological and cytological phenotypes characterized by humans with GTS. Nevertheless, histamine deficiency leads to behavioral alterations probably due to neurotransmitter dysbalance on the level of the striatum.

Targeted Interneuron Depletion in the Dorsal Striatum Produces Autism-like Behavioral Abnormalities in Male but Not Female Mice

BACKGROUND

Interneuronal pathology is implicated in many neuropsychiatric disorders, including autism spectrum disorder (ASD) and Tourette syndrome (TS). Interneurons of the striatum, including the parvalbumin-expressing fast-spiking interneurons (FSIs) and the large cholinergic interneurons (CINs), are affected in patients with TS and in preclinical models of both ASD and TS.

METHODS

To test the causal importance of these neuronal abnormalities, we recapitulated them in vivo in developmentally normal mice using a combination transgenic-viral strategy for targeted toxin-mediated ablation.

RESULTS

We found that conjoint ~50% depletion of FSIs and CINs in the dorsal striatum of male mice produces spontaneous stereotypy and marked deficits in social interaction. Strikingly, these behavioral effects are not seen in female mice; because ASD and TS have a marked male predominance, this observation reinforces the potential relevance of the finding to human disease. Neither of these effects is seen when only one or the other interneuronal population is depleted; ablation of both is required. Depletion of FSIs, but not of CINs, also produces anxiety-like behavior, as has been described previously. Behavioral pathology in male mice after conjoint FSI and CIN depletion is accompanied by increases in activity-dependent signaling in the dorsal striatum; these alterations were not observed after disruption of only one interneuron type or in doubly depleted female mice.

CONCLUSIONS

These data indicate that disruption of CIN and FSI interneurons in the dorsal striatum is sufficient to produce network and behavioral changes of potential relevance to ASD, in a sexually dimorphic manner.

Histamine modulation of the basal ganglia circuitry in the development of pathological grooming

Aberrant histaminergic function has been proposed as a cause of tic disorders. A rare mutation in the enzyme that produces histamine (HA), histidine decarboxylase (HDC), has been identified in patients with Tourette syndrome (TS). Hdc knockout mice exhibit repetitive behavioral pathology and neurochemical characteristics of TS, establishing them as a plausible model of tic pathophysiology. Where, when, and how HA deficiency produces these effects has remained unclear: whether the contribution of HA deficiency to pathogenesis is acute or developmental, and where in the brain the relevant consequences of HA deficiency occur. Here, we address these key pathophysiological questions, using anatomically and cellularly targeted manipulations in mice. We report that specific ablation or chemogenetic silencing of histaminergic neurons in the tuberomammillary nucleus (TMN) of the hypothalamus leads to markedly elevated grooming, a form of repetitive behavioral pathology, and to elevated markers of neuronal activity in both dorsal striatum and medial prefrontal cortex. Infusion of HA directly into the striatum reverses this behavioral pathology, confirming that acute HA deficiency mediates the effect. Bidirectional chemogenetic regulation reveals that dorsal striatum neurons activated after TMN silencing are both sufficient to produce repetitive behavioral pathology and necessary for the full expression of the effect. Chemogenetic activation of TMN-regulated medial prefrontal cortex neurons, in contrast, increases locomotion and not grooming. These data confirm the centrality of striatal regulation by neurotransmitter HA in the adult in the production of pathological grooming.

The magnificent two: histamine and the H3 receptor as key modulators of striatal circuitry

Histaminergic dysfunction has been recently linked to tic disorders and to aberrant striatal function. There is a particular interest in the histamine 3 receptor (H3R) due to its clinical implications for treating multiple disorders and its high expression in the brain. Striatal histamine (HA) modulates through the H3R in complex ways the release of striatal neurotransmitters into this brain region. The H3R has been classically described to be coupled to G_i, although there is evidence that revealed that striatal H3R forms heteromers with the dopamine receptors 1 and 2 in the medium spiny neurons (MSNs) than changes this signaling. Moreover, new data described for the first time a complete, segregated and time dependent signaling after H3R activation in the two types of MSNs (D1R-MSNs and D2R-MSNs). The aim of this review is to update the role of HA and H3R in striatal function at a molecular and signaling levels.

The naturally occurring mutation Y197C does not affect the expression or signaling of the human histamine H3 receptor

There is evidence for genetic polymorphism within the human histamine H₃ receptor (hH₃R), and a Tyr to Cys exchange at position 197 (Y197C), located in the amino terminus of the fifth transmembrane domain, has been reported. In this work we compared the expression and the pharmacological and signaling properties of wild-type (hH₃R_WT) and mutant (hH₃R_Y197C) receptors transiently expressed in CHO-K1 cells. The hH₃R_Y197C cDNA was created by overlap extension PCR amplification. Receptor expression and affinity were assessed by N-α-[methyl-³H]-histamine binding to cell membranes and intact cells. Receptor function was evaluated by stimulation of [³⁵S]-GTPγS binding to cell membranes and by inhibition of forskolin-induced cAMP accumulation in intact cells. The hH₃R_WT and hH₃R_Y197C were expressed at similar levels (761±68 and 663±66fmol/mg protein for membranes, and 13,434±1533 and 15,894±1884 receptors per cell, respectively). There were no significant differences in the affinities for H₃R agonists or antagonists/inverse agonists between the hH₃R_WT and hH₃R_Y197C, and the H₃R agonist RAMH was similarly efficacious and potent to stimulate [³⁵S]-GTPγS binding and to inhibit forskolin-induced cAMP accumulation. These results indicate that the Y197C mutation does not affect the expression, ligand affinity or signaling of the human H₃ receptor.

Histamine H3R receptor activation in the dorsal striatum triggers stereotypies in a mouse model of tic disorders

Tic disorders affect ~5% of the population and are frequently comorbid with obsessive-compulsive disorder, autism, and attention deficit disorder. Histamine dysregulation has been identified as a rare genetic cause of tic disorders; mice with a knockout of the histidine decarboxylase (Hdc) gene represent a promising pathophysiologically grounded model. How alterations in the histamine system lead to tics and other neuropsychiatric pathology, however, remains unclear. We found elevated expression of the histamine H3 receptor in the striatum of Hdc knockout mice. The H3 receptor has significant basal activity even in the absence of ligand and thus may modulate striatal function in this knockout model. We probed H3R function using specific agonists. The H3 agonists R-aminomethylhistamine (RAMH) and immepip produced behavioral stereotypies in KO mice, but not in controls. H3 agonist treatment elevated intra-striatal dopamine in KO mice, but not in controls. This was associated with elevations in phosphorylation of rpS6, a sensitive marker of neural activity, in the dorsal striatum. We used a novel chemogenetic strategy to demonstrate that this dorsal striatal activity is necessary and sufficient for the development of stereotypy: when RAMH-activated cells in the dorsal striatum were chemogenetically activated (in the absence of RAMH), stereotypy was recapitulated in KO animals, and when they were silenced the ability of RAMH to produce stereotypy was blocked. These results identify the H3 receptor in the dorsal striatum as a contributor to repetitive behavioral pathology.

Histidine Decarboxylase Knockout Mice as a Model of the Pathophysiology of Tourette Syndrome and Related Conditions

While the normal functions of histamine (HA) in the central nervous system have gradually come into focus over the past 30 years, the relationship of abnormalities in neurotransmitter HA to human disease has been slower to emerge. New insight came with the 2010 description of a rare nonsense mutation in the biosynthetic enzyme histidine decarboxylase (Hdc) that was associated with Tourette syndrome (TS) and related conditions in a single family pedigree. Subsequent genetic work has provided further support for abnormalities of HA signaling in sporadic TS. As a result of this genetic work, Hdc knockout mice, which were generated more than 15 years ago, have been reexamined as a model of the pathophysiology of TS and related conditions. Parallel work in these KO mice and in human carriers of the Hdc mutation has revealed abnormalities in the basal ganglia system and its modulation by dopamine (DA) and has confirmed the etiologic, face, and predictive validity of the model. The Hdc-KO model thus serves as a unique platform to probe the pathophysiology of TS and related conditions, and to generate specific hypotheses for subsequent testing in humans. This chapter summarizes the development and validation of this model and recent and ongoing work using it to further investigate pathophysiological changes that may contribute to these disorders.