Histamine H(3) receptor-mediated inhibition of depolarization-induced, dopamine D(1) receptor-dependent release of [(3)H]-gamma-aminobutryic acid from rat striatal slices

The role of heterodimers formed by histamine H3 receptors and dopamine D1 receptors on the methamphetamine-induced conditioned place preference

RATIONALE

The rewarding effect of Methamphetamine (METH) is commonly believed to play an important role in METH use disorder. The altered expression of dopamine D1 receptor (D1R) has been suggested to be essential to the rewarding effect of METH. Notably, D1R could interact with histamine H3 receptors (H3R) by forming a H3R-D1R heteromer (H3R-D1R).

OBJECTIVES

This study was designed to specifically investigate the involvement of H3R-D1R in the rewarding effect of METH.

METHODS

C57BL/6 mice were treated with intraperitoneal injections of a selective H3R antagonist (Thioperamide, THIO; 20 mg/kg), an H1R antagonist (Pyrilamine, PYRI; 10 mg/kg), or microinjections of cytomegalovirus (CMV)-transmembrane domain 5 (TM5) into the nucleus accumbens (NAc). The animal model of Conditioned Place Preference (CPP) was applied to determine the impact of H3R-D1R on the rewarding effect of METH.

RESULTS

METH resulted in a significant preference for the drug-associated chamber, in conjunction with increased H3R and decreased D1R expression in both NAc and the ventral tegmental area (VTA). THIO significantly attenuated the rewarding effect of METH, accompanied by decreased H3R and increased D1R expression. In contrast, pyrilamine failed to produce the similar effects. Moreover, the inhibitory effect of THIO on METH-induced CPP was reversed by SKF38393, a D1R agonist. Furthermore, SCH23390, a D1R antagonist, counteracted the ameliorative effect of SKF38393 on THIO. Co-immunoprecipitation (CO-IP) experiments further demonstrated the specific interaction between H3R and D1R in METH CPP mice. The rewarding effect of METH was also significantly blocked by the interruption of CMV-transmembrane domain 5 (TM5), but not CMV-transmembrane domain 7 (TM7) in NAc.

CONCLUSION

These results suggest that modulating the activity of H3R-D1R complex holds promise for regulating METH use disorder and serves as a potential drug target for its treatment.

Histamine H3 Receptor Isoforms: Insights from Alternative Splicing to Functional Complexity

Alternative splicing significantly enhances the diversity of the G protein-coupled receptor (GPCR) family, including the histamine H3 receptor (H3R). This post-transcriptional modification generates multiple H3R isoforms with potentially distinct pharmacological and physiological profiles. H3R is primarily involved in the presynaptic inhibition of neurotransmitter release in the central nervous system. Despite the approval of pitolisant for narcolepsy (Wakix®) and daytime sleepiness in adults with obstructive sleep apnea (Ozawade®) and ongoing clinical trials for other H3R antagonists/inverse agonists, the functional significance of the numerous H3R isoforms remains largely enigmatic. Recent publicly available RNA sequencing data have confirmed the expression of multiple H3R isoforms in the brain, with some isoforms exhibiting unique tissue-specific distribution patterns hinting at isoform-specific functions and interactions within neural circuits. In this review, we discuss the complexity of H3R isoforms with a focus on their potential roles in central nervous system (CNS) function. Comparative analysis across species highlights evolutionary conservation and divergence in H3R splicing, suggesting species-specific regulatory mechanisms. Understanding the functionality of H3R isoforms is crucial for the development of targeted therapeutics. This knowledge will inform the design of more precise pharmacological interventions, potentially enhancing therapeutic efficacy and reducing adverse effects in the treatment of neurological and psychiatric disorders.

Ligand-directed biased agonism at human histamine H3 receptor isoforms across Gαi/o- and β-arrestin2-mediated pathways

The histamine H3 receptor (H3R) is a neurotransmitter receptor that is primarily found in the brain, where it controls the release and synthesis of histamine, as well as the release of other neurotransmitters (e.g. dopamine, serotonin). Notably, 20 H3R isoforms are differentially expressed in the human brain as a consequence of alternative gene splicing. The hH3R-445, -415, -365 and -329 isoforms contain the prototypical GPCR (7TM) structure, yet exhibit deletions in the third intracellular loop, a structural domain that is pivotal for G protein-coupling, signaling and regulation. To date, the physiological relevance underlying the individual and combinatorial function of hH3R isoforms remains poorly understood. Nevertheless, given their significant implication in physiological processes (e.g. cognition, homeostasis) and neurological disorders (e.g. Alzheimer's and Parkinson's disease, schizophrenia), widespread targeting of hH3R isoforms by drugs may lead to on-target side effects in brain regions that are unaffected by disease. To this end, isoform- and/or pathway-selective targeting of hH3R isoforms by biased agonists could be of therapeutic relevance for the development of region- and disease-specific drugs. Hence, we have evaluated ligand biased signaling at the hH3R-445, -415, -365 and -329 isoforms across various Gαi/o-mediated (i.e. [35S]GTPγS accumulation, cAMP inhibition, pERK1/2 activation, pAKT T308/S473 activation) and non Gαi/o-mediated (i.e. β-arrestin2 recruitment) endpoints that are relevant to neurological diseases. Our findings indicate that H3R agonists display significantly altered patterns in their degree of ligand bias, in a pathway- and isoform-dependent manner, underlining the significance to investigate GPCRs with multiple isoforms to improve development of selective drugs. SUBJECT CATEGORY: Neuropharmacology.

Dose-related effects of ciproxifan on brain tissue in rats with cerebral ischemia-reperfusion

PURPOSE

Cerebral ischemia is the result of decreased or interrupted blood flow to the brain. It is the third leading cause of death after cardiovascular disease and cancer. Cerebral ischemia is reversible or irreversible in neurons in the affected area, and subsequent free radical damage can be exacerbated if reperfusion occurs. Ciproxifan is used to study the involvement of histaminergic neurons in different phases such as wakefulness and cognition. We wanted to find out whether ciproxifan has a protective effect on the brain of rats with cerebral ischemia-reperfusion injury.

MATERIALS AND METHODS

A total of 64 adult rats (32 male and 32 female) were used for the experiment. Eight cages were formed with randomly selected rats. No substance was administered to the rats in Group 1 and no surgical procedure was performed. The cerebral ischemia-reperfusion model (clamping of the left common carotid artery for 15 min followed by reperfusion for 24 h) was applied to rats in Group 2, Group 3, and Group 4 after 7 days/single dose of saline and ciproxifan (10 mg/kg, 30 mg/kg). After that, the activitymeter, forced swim test (FST), and Morris water maze (MWM) were performed on all animals.

RESULTS

Rats treated with ciproxifan exhibit neurons and glial cells with histologic structures similar to those of the control group, and interestingly, these differences became more pronounced with increasing dose. Rats administered ciproxifan improved motor coordination, decreased total distance behavior, and improved learning ability. However, when the groups were compared by sex, no significant difference was found in the parameters.

CONCLUSION

Thus, we could conclude that ciproxifan has a protective effect on the brain to a certain extent, regardless of the dose.

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.

Therapeutic Potential of Histamine H3 Receptors in Substance Use Disorders

Substance use disorders are a leading cause of morbidity and mortality, and available pharmacological treatments are of modest efficacy. Histamine is a biogenic amine with four types of receptors. The histamine H₃ receptor (H₃R) is an autoreceptor and also an heteroreceptor. H₃Rs are highly expressed in the basal ganglia, hippocampus and cortex, and regulate a number of neurotransmitters including acetylcholine, norepinephrine, GABA and dopamine. Its function and localization suggest that the H₃R may be relevant to a number of psychiatric disorders and could represent a potential therapeutic target for substance use disorders. The purpose of the present review is to summarize preclinical studies investigating the effects of H₃R agonists and antagonists on animal models of alcohol, nicotine and psychostimulant use. At present, the effects of H₃R antagonists such as thioperamide, pitolisant or ciproxifan have been investigated in drug-induced locomotion, conditioned place preference, drug self-administration, reinstatement, sensitization and drug discrimination. For alcohol and nicotine, the effects of H₃R ligands on two-bottle choice and memory tasks, respectively, have also been investigated. The results of these studies are inconsistent. For alcohol, H₃R antagonists generally decreased the reward-related properties of ethanol, which suggests that H₃R antagonists may be effective as a treatment option for alcohol use disorder. However, the effects of H₃R antagonists on nicotine and psychostimulant motivation and reward are less clear. H₃R antagonists potentiated the abuse-related properties of nicotine, but only a handful of studies have been conducted. For psychostimulants, evidence is mixed and suggests that more research is needed to establish whether H₃R antagonists are a viable therapeutic option. The fact that different drugs of abuse have different brain targets may explain the differential effects of H₃R ligands.

Are Histamine H3 Antagonists the Definitive Treatment for Acute Methamphetamine Intoxication?

BACKGROUND

Methamphetamine (METH) is classified as a Schedule II stimulant drug under the United Nations Convention on Psychotropic Substances of 1971. METH and other amphetamine analogues (AMPHs) are powerful addictive drugs. Treatments are needed to treat the symptoms of METH addiction, chronic METH use, and acute METH overdose. No effective treatment for METH abuse has been established because alterations of brain functions under the excessive intake of abused drug intake are largely irreversible due in part to brain damage that occurs in the course of chronic METH use.

OBJECTIVE

Modulation of brain histamine neurotransmission is involved in several neuropsychiatric disorders, including substance use disorders. This review discusses the possible mechanisms underlying the therapeutic effects of histamine H₃ receptor antagonists on symptoms of methamphetamine abuse.

CONCLUSION

Treatment of mice with centrally acting histamine H₃ receptor antagonists increases hypothalamic histamine contents and reduces high-dose METH effects while potentiating lowdose effects via histamine H₃ receptors that bind released histamine. On the basis of experimental evidence, it is hypothesized that histamine H₃ receptors may be an effective target for the treatment METH use disorder or other adverse effects of chronic METH use.

Integrating the monoamine and cytokine hypotheses of depression: Is histamine the missing link?

Psychiatric diseases, like depression, largely affect the central nervous system (CNS). While the underlying neuropathology of depressive illness remains to be elucidated, several hypotheses have been proposed as molecular underpinnings for major depressive disorder, including the monoamine hypothesis and the cytokine hypothesis. The monoamine hypothesis has been largely supported by the pharmaceuticals that target monoamine neurotransmitters as a treatment for depression. However, these antidepressants have come under scrutiny due to their limited clinical efficacy, side effects, and delayed onset of action. The more recent, cytokine hypothesis of depression is supported by the ability of immune-active agents to induce "sickness behaviour" akin to that seen with depression. However, treatments that more selectively target inflammation have yielded inconsistent antidepressive results. As such, neither of these hypotheses can fully explain depressive illness pathology, implying that the underlying neuropathological mechanisms may encompass aspects of both theories. The goal of the current review is to integrate these two well-studied hypotheses and to propose a role for histamine as a potential unifying factor that links monoamines to cytokines. Additionally, we will focus on stress-induced depression, to provide an updated perspective of depressive illness research and thereby identify new potential targets for the treatment of major depressive disorder.

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.

Mechanism of Action of Atypical Antipsychotic Drugs in Mood Disorders

Atypical antipsychotic drugs were introduced in the early 1990s. Unlike typical antipsychotics, which are effective only against positive symptoms of schizophrenia, atypical antipsychotics are effective against negative and cognitive symptoms as well. Furthermore, they are effective not only in psychotic but also in affective disorders, on their own or as adjuncts to antidepressant drugs. This review presents the neural mechanisms of currently existing atypical antipsychotics and putative antipsychotics currently being investigated in preclinical and clinical studies and how these relate to their effectiveness in mood disorders such as depression, anxiety, and post-traumatic stress disorder (PTSD). Typical antipsychotics act almost exclusively on the dopamine system. Atypical drugs, however, modulate serotonin (5-HT), norepinephrine, and/or histamine neurotransmission as well. This multimodal mechanism of action putatively underlies the beneficial effect of atypical antipsychotics in mood and anxiety disorders. Interestingly, novel experimental drugs having dual antipsychotic and antidepressant therapeutic potential, such as histamine, adenosine, and trace amine-associated receptors (TAAR) ligand, are also characterized by a multimodal stimulatory effect on central 5-HT, norepinephrine, and/or histamine transmission. The multimodal stimulatory effect on central monoamine neurotransmission may be thus primarily responsible for the combined antidepressant and antipsychotic therapeutic potential of certain central nervous system (CNS) drugs.

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.

Chronic administration of the histamine H3 receptor agonist immepip decreases L-Dopa-induced dyskinesias in 6-hydroxydopamine-lesioned rats

RATIONALE

Histamine H₃ receptors (H₃Rs) are co-expressed with dopamine D₁ receptors (D₁Rs) by striato-nigral medium spiny GABAergic neurons, where they functionally antagonize D₁R-mediated responses.

OBJECTIVES AND METHODS

We examined whether the chronic administration of the H₃R agonist immepip modifies dyskinesias induced by L-3,4-dihydroxyphenylalanine, L-Dopa (LIDs), in rats lesioned with 6-hydroxydopamine in the substantia nigra pars compacta, and the effect of D₁R and H₃R co-activation on glutamate and GABA content in dialysates from the dorsal striatum of naïve rats.

RESULTS

The systemic administration (i.p.) of L-Dopa for 14 days significantly increased axial, limb, and orolingual abnormal involuntary movements (AIMs) compared with the vehicle group. The chronic administration of the H₃R agonist immepip alongside L-Dopa significantly decreased axial, limb, and orolingual AIMs compared with L-Dopa alone, but AIMs returned to previous values on immepip withdrawal. Chronic immepip was ineffective when administered prior to L-Dopa. The chronic administration of immepip significantly decreased GABA and glutamate content in striatal dialysates, whereas the administration of L-Dopa alone increased GABA and glutamate content.

CONCLUSIONS

These results indicate that chronic H₃R activation reduces LIDs, and the effects on striatal GABA and glutamate release provide evidence for a functional interaction between D₁Rs and H₃Rs.

Adenosine A2A and histamine H3 receptors interact at the cAMP/PKA pathway to modulate depolarization-evoked [3H]-GABA release from rat striato-pallidal terminals

We previously reported that the activation of histamine H₃ receptors (H₃Rs) selectively counteracts the facilitatory action of adenosine A_2A receptors (A_2ARs) on GABA release from rat globus pallidus (GP) isolated nerve terminals (synaptosomes). In this work, we examined the mechanisms likely to underlie this functional interaction. Three possibilities were explored: (a) changes in receptor affinity for agonists induced by physical A_2AR/H₃R interaction, (b) opposite actions of A_2ARs and H₃Rs on depolarization-induced Ca²⁺ entry, and (c) an A_2AR/H₃R interaction at the level of adenosine 3',5'-cyclic monophosphate (cAMP) formation. In GP synaptosomal membranes, H₃R activation with immepip reduced A_2AR affinity for the agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680) (K_i control 4.53 nM; + immepip 9.32 nM), whereas A_2AR activation increased H₃R affinity for immepip (K_i control 0.63 nM; + CGS-21680 0.26 nM). Neither A_2AR activation nor H₃R stimulation modified calcium entry through voltage-gated calcium channels in GP synaptosomes, as evaluated by microfluorometry. A_2AR-mediated facilitation of depolarization-evoked [2,3-³H]-γ-aminobutyric acid ([³H]-GABA) release from GP synaptosomes (130.4 ± 3.6% of control values) was prevented by the PKA inhibitor H-89 and mimicked by the adenylyl cyclase activator forskolin or by 8-Bromo-cAMP, a membrane permeant cAMP analogue (169.5 ± 17.3 and 149.5 ± 14.5% of controls). H₃R activation failed to reduce the facilitation of [³H]-GABA release induced by 8-Bromo-cAMP. In GP slices, A_2AR activation stimulated cAMP accumulation (290% of basal) and this effect was reduced (- 75%) by H₃R activation. These results indicate that in striato-pallidal nerve terminals, A_2ARs and H₃Rs interact at the level of cAMP formation to modulate PKA activity and thus GABA release.

Molecular Imaging in Huntington's Disease

Huntington's disease (HD) is a rare monogenic neurodegenerative disorder caused by a trinucleotide CAG repeat expansion in the huntingtin gene resulting in the formation of intranuclear inclusions of mutated huntingtin. The accumulation of mutated huntingtin leads to loss of GABAergic medium spiny neurons (MSNs); subsequently resulting in the development of chorea, cognitive dysfunction and psychiatric symptoms. Premanifest HD gene expansion carriers, provide a unique cohort to examine very early molecular changes, occurring before the development of overt symptoms, to elucidate disease pathophysiology and identify reliable biomarkers of HD progression. Positron emission tomography (PET) is a non-invasive molecular imaging technique allowing the evaluation of specific molecular targets in vivo. Selective PET radioligands provide invaluable tools to investigate the role of the dopaminergic system, brain metabolism, microglial activation, phosphodiesterase 10A, and cannabinoid, GABA, adenosine and opioid receptors in HD. PET has been employed to monitor disease progression aiming to identify a reliable biomarker to predict phenoconversion from premanifest to manifest HD.

Functional histamine H3 and adenosine A2A receptor heteromers in recombinant cells and rat striatum

In the striatum, histamine H3 receptors (H3Rs) are co-expressed with adenosine A2A receptors (A2ARs) in the cortico-striatal glutamatergic afferents and the GABAergic medium-sized spiny neurons that originate the indirect pathway of the basal ganglia. This location allows H3Rs and A2ARs to regulate the striatal GABAergic and glutamatergic transmission. However, whether these receptors can physically interact has not yet been assessed. To test this hypothesis, a heteromer-selective in vitro assay was used to detect functional complementation between a chimeric A2AR302-Gαqi4 and wild-type H3Rs in transfected HEK-293T cells. H3R activation with the agonist RAMH resulted in Ca2+ mobilization (pEC50 7.31 ± 0.23; maximal stimulation, Emax 449 ± 25% of basal) indicative of receptor heterodimerization. Functional H3R-A2AR heteromers were confirmed by co-immunoprecipitation and observations of differential cAMP signaling when both receptors were co-expressed in the same cells. In membranes from rat striatal synaptosomes, H3R activation decreased A2AR affinity for the agonist CGS-21680 (pKi values 8.10 ± 0.04 and 7.70 ± 0.04). Moreover, H3Rs and A2ARs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a H3R-A2AR heteromer with possible physiological implications for the modulation of the intra-striatal transmission.

Histamine H3 receptor antagonists display antischizophrenic activities in rats treated with MK-801

BACKGROUND

Animal models based on N-methyl-d-aspartate receptor blockade have been extensively used for schizophrenia. Ketamine and MK-801 produce behaviors related to schizophrenia and exacerbated symptoms in patients with schizophrenia, which led to the use of PCP (phencyclidine)- and MK-801 (dizocilpine)-treated animals as models for schizophrenia.

METHODS

The study investigated the effect of subchronic dosing (once daily, 7 days) of histamine H3 receptor (H3R) antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p.) on MK-801 (0.2 mg/kg, i.p.)-induced locomotor activity and also measured dopamine and histamine levels in rat's brain homogenates. The study also included clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively.

RESULTS

Atypical and typical antipsychotic was used to serve as clinically relevant reference agents to compare the effects of the H3R antagonists. MK-801 significantly increased horizontal locomotor activity, which was reduced with CPX and CBP. MK-801-induced locomotor hyperactivity attenuated by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised striatal dopamine level, which was reduced in rats pretreated with CPX and CBP. CPZ also significantly lowered striatal dopamine levels, although the decrease was less robust compared to CLZ, CPX, and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increased histamine levels in the hypothalamus compared to MK-801 treatment alone. Histamine H3 receptor agonist, R-α methylhistamine (10 mg/kg, i.p.), counteracted the effect of CPX and CBP.

CONCLUSIONS

The present study shows the positive effects of CPX and CBP on MK-801-induced schizophrenia-like behaviors in rodents.

The histamine H3 receptor antagonist thioperamide rescues circadian rhythm and memory function in experimental parkinsonism

Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by motor impairment and a wide range of non-motor symptoms, including sleep disorders and cognitive and affective deficits. In this study, we used a mouse model of PD based on 6-hydroxydopamine (6-OHDA) to examine the effect of thioperamide, a histamine H3 receptor antagonist, on circadian activity, recognition memory and anxiety. A partial, bilateral 6-OHDA lesion of the striatum reduces motor activity during the active phase of the 24 h cycle. In addition, the lesion disrupts the endogenous circadian rhythm observed when mice are maintained in constant darkness. Administration of thioperamide to 6-OHDA-lesion mice rescues the normal rest/activity cycle. Moreover, thioperamide counteracts the deficit of novel object recognition produced by 6-OHDA. Our experiments show that this memory impairment is accompanied by disrupted gamma oscillations in the hippocampus, which are also rescued by thioperamide. In contrast, we do not observe any modification of the anxiogenic effect of 6-OHDA in response to administration of thioperamide. Our results indicate that thioperamide may act as a multifunctional drug, able to counteract disruptions of circadian rhythm and cognitive deficits associated with PD.

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 Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H₁-H₄) identified as mediators of the biologic effects of histamine, the H₃ receptor (H₃R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H₃Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H₃R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H₃R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H₃R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.

Histamine and the striatum

The neuromodulator histamine is released throughout the brain during periods of wakefulness. Combined with an abundant expression of histamine receptors, this suggests potential widespread histaminergic control of neural circuit activity. However, the effect of histamine on many of these circuits is unknown. In this review we will discuss recent evidence for histaminergic modulation of the basal ganglia circuitry, and specifically its main input nucleus; the striatum. Furthermore, we will discuss recent findings of histaminergic dysfunction in several basal ganglia disorders, including in Parkinson's disease and most prominently, in Tourette's syndrome, which has led to a resurgence of interest in this neuromodulator. Combined, these recent observations not only suggest a central role for histamine in modulating basal ganglia activity and behaviour, but also as a possible target in treating basal ganglia disorders. This article is part of the Special Issue entitled 'Histamine Receptors'.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Synthesis and structural elucidation of a novel polymorph of alcaftadine

In this study, we have synthesized and elucidated the structure of the H1 histamine antagonist, 2-(1-methylpiperidin-4-ylidene)-4,7-diazatricyclo[8.4.0.0((3,7))]tetradeca-1(14),3,5,10,12-pentaene-6-carbaldehyde in the solution and solid-state. We have also studied the thermal dilapidation of the compound. Solution structure analysis was achieved by employing NMR spectroscopy including 2D experiments NOESY, HSQC and HMBC, while solid state investigations were undertaken using SXRD, PXRD, TGA, DSC, and IR spectroscopy. For the first time the single crystal structure of alcaftadine has now been solved. Crystallographic data are as follows: monoclinic, Cc, a=11.5694(6)Å, b=14.5864(6)Å, c=10.2688(4)Å, α=90°, β=111.793(3)°, γ=90°, V=1609.07(13)Å(3), Z=4. The Hirshfeld surface analyses also have been performed using the crystal structure.

Histamine H₃ receptors, the complex interaction with dopamine and its implications for addiction

Histamine H₃ receptors are best known as presynaptic receptors inhibiting the release of histamine, as well as other neurotransmitters including acetylcholine and dopamine. However, in the dorsal and ventral striatum, the vast majority of H₃ receptors are actually located postsynaptically on medium sized spiny output neurons. These cells also contain large numbers of dopamine (D₁ and D₂) receptors and it has been shown that H₃ receptors form heterodimers with both D₁ and D₂ receptors. Thus, the anatomical localization of H₃ receptors suggests a complex interaction that could both enhance and inhibit dopaminergic neurotransmission. Dopamine, especially within the striatal complex, plays a crucial role in the development of addiction, both in the initial reinforcing effects of drugs of abuse, as well as in maintenance, relapse and reinstatement of drug taking behaviour. It is, therefore, conceivable that H₃ receptors can moderate the development and maintenance of drug addiction. In the present review, we appraise the current literature on the involvement of H₃ receptors in drug addiction and try to explain these data within a theoretical framework, as well as provide suggestions for further research.

Histamine H3 receptor activation counteracts adenosine A2A receptor-mediated enhancement of depolarization-evoked [3H]-GABA release from rat globus pallidus synaptosomes

High levels of histamine H3 receptors (H3Rs) are found in the globus pallidus (GP), a neuronal nucleus in the basal ganglia involved in the control of motor behavior. By using rat GP isolated nerve terminals (synaptosomes), we studied whether H3R activation modified the previously reported enhancing action of adenosine A2A receptor (A2AR) stimulation on depolarization-evoked [(3)H]-GABA release. At 3 and 10 nM, the A2AR agonist CGS-21680 enhanced [(3)H]-GABA release induced by high K(+) (20 mM) and the effect of 3 nM CGS-21680 was prevented by the A2AR antagonist ZM-241385 (100 nM). The presence of presynaptic H3Rs was confirmed by the specific binding of N-α-[methyl-(3)H]-histamine to membranes from GP synaptosomes (maximum binding, Bmax, 1327 ± 79 fmol/mg protein; dissociation constant, Kd, 0.74 nM), which was inhibited by the H3R ligands immepip, clobenpropit, and A-331440 (inhibition constants, Ki, 0.28, 8.53, and 316 nM, respectively). Perfusion of synaptosomes with the H3R agonist immepip (100 nM) had no effect on K(+)-evoked [(3)H]-GABA release, but inhibited the stimulatory action of A2AR activation. In turn, the effect of immepip was blocked by the H3R antagonist clobenpropit, which had no significant effect of its own on K(+)-induced [(3)H]-GABA release. These data indicate that H3R activation selectively counteracts the facilitatory action of A2AR stimulation on GABA release from striato-pallidal projections.

Procognitive properties of drugs with single and multitargeting H3 receptor antagonist activities

The histamine H3 receptor (H3 R) is an important modulator of numerous central control mechanisms. Novel lead optimizations for H3 R antagonists/inverse agonists involved studies of structure-activity relationships, cross-affinities, and pharmacokinetic properties of promising ligands. Blockade of inhibitory histamine H3 autoreceptors reinforces histaminergic transmission, while antagonism of H3 heteroreceptors accelerates the corticolimbic liberation of acetylcholine, norepinephrine, glutamate, dopamine, serotonin and gamma-aminobutyric acid (GABA). The H3 R positioned at numerous neurotransmission crossroads indicates therapeutic applications of small-molecule H3 R modulators in a number of psychiatric and neurodegenerative diseases with various clinical candidates available. Dual target drugs displaying H3 R antagonism/inverse agonism with inhibition of acetylcholine esterase (AChE), histamine N-methyltransferase (HMT), or serotonin transporter (SERT) are novel class of procognitive agents. Main chemical diversities, pharmacophores, and pharmacological profiles of procognitive agents acting as H3 R antagonists/inverse agonists and dual H3 R antagonists/inverse agonists with inhibiting activity on AChE, HMT, or SERT are highlighted here.

Cocaine disrupts histamine H3 receptor modulation of dopamine D1 receptor signaling: σ1-D1-H3 receptor complexes as key targets for reducing cocaine's effects

The general effects of cocaine are not well understood at the molecular level. What is known is that the dopamine D1 receptor plays an important role. Here we show that a key mechanism may be cocaine's blockade of the histamine H3 receptor-mediated inhibition of D1 receptor function. This blockade requires the σ1 receptor and occurs upon cocaine binding to σ1-D1-H3 receptor complexes. The cocaine-mediated disruption leaves an uninhibited D1 receptor that activates Gs, freely recruits β-arrestin, increases p-ERK 1/2 levels, and induces cell death when over activated. Using in vitro assays with transfected cells and in ex vivo experiments using both rats acutely treated or self-administered with cocaine along with mice depleted of σ1 receptor, we show that blockade of σ1 receptor by an antagonist restores the protective H3 receptor-mediated brake on D1 receptor signaling and prevents the cell death from elevated D1 receptor signaling. These findings suggest that a combination therapy of σ1R antagonists with H3 receptor agonists could serve to reduce some effects of cocaine.

Histamine H₃ receptors modulate depolarization-evoked [³H]-noradrenaline release from rat olfactory bulb slices

We have studied the effect of histamine H(3) receptor (H(3)R) activation on the depolarization-evoked release of labeled neurotransmitters from slices of the rat olfactory bulb (rOB). The presence of pre-synaptic H(3)Rs was evidenced by the specific binding of the H(3)R ligand N-α-[methyl-(3)H]histamine to membranes from rOB synaptosomes (maximum binding, B(max), 106 ± 19 fmol/mg protein; dissociation constant, K(d), 0.68 ± 0.11 nM) which was inhibited by selective H(3)R ligands (immepip, (R)(-)-α-methylhistamine (RAMH) and clobenpropit) with affinities similar to those previously reported for H(3)Rs expressed in other rat brain areas. Perfusion of rOB slices with the selective H(3)R agonist RAMH (0.1 and 1 μM) had no effect on the release of [(3)H]-γ-aminobutyric acid ([(3)H]-GABA), [(3)H]-d-aspartate, [(3)H]-dopamine or [(3)H]-5-hydroxytryptamine ([(3)H]-5-HT) evoked by depolarization with high K(+) (20 or 40 mM). [(3)H]-Noradrenaline release induced by 20 mM K(+) was reduced in a modest but significant manner by RAMH (94.9 ± 1.7% and 83.1 ± 2.1% of control release at 0.1 and 1 μM, respectively). The effect of 1 μM RAMH was blocked by the selective H(3)R antagonist/inverse agonist clobenpropit (5 μM). When tested alone clobenpropit and a second H(3)R antagonist/inverse agonist, ciproxifan (both at 1 μM) significantly increased K(+)-evoked [(3)H]-noradrenaline release to 119.4 ± 4.2% and 120.0 ± 3.7% of K(+) alone, respectively. Ciproxifan (1 μM) had no effect on the depolarization-evoked release of the other labeled neurotransmitters. These data indicate that H(3)Rs with constitutive activity modulate noradrenaline release in rOB, presumably through a pre-synaptic action. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Histamine H3 receptor agonists decrease hypothalamic histamine levels and increase stereotypical biting in mice challenged with methamphetamine

The effects of the histamine H(3) receptor agonists (R)-α-methylhistamine, imetit and immepip on methamphetamine (METH)-induced stereotypical behavior were examined in mice. The administration of METH (10 mg/kg, i.p.) to male ddY mice induced behaviors including persistent locomotion and stereotypical behaviors, which were classified into four categories: stereotypical head-bobbing (1.9%), circling (1.7%), sniffing (14.3%), and biting (82.1%). Pretreatment with (R)-α-methylhistamine (3 and 10 mg/kg, i.p.) significantly decreased stereotypical sniffing, but increased stereotypical biting induced by METH, in a dose-dependent manner. This effect of (R)-α-methylhistamine on behavior was mimicked by imetit or immepip (brain-penetrating selective histamine H(3) receptor agonists; 10 mg/kg, i.p. for each drug). Hypothalamic histamine levels 1 h after METH challenge were significantly increased in mice pretreated with saline. These increases in histamine levels were significantly decreased by pretreatment with histamine H(3) receptor agonists, effects which would appear to underlie the shift from METH-induced stereotypical sniffing to biting.

Pre-synaptic histamine H₃ receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H(3) receptors (H(3)Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H(3)Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H(3)R agonist immepip had no effect on the release of [(3)H]-GABA ([(3)H]-γ-aminobutyric acid) or [(3)H]-dopamine evoked by depolarization with high (20 mM) K(+), but significantly reduced [(3)H]-d-aspartate release (-44.8 ± 2.6% and -63.7 ± 6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H(3)R antagonist A-331440 (4'-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4'-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H(3)Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

Dopamine D1-histamine H3 receptor heteromers provide a selective link to MAPK signaling in GABAergic neurons of the direct striatal pathway

Previously, using artificial cell systems, we identified receptor heteromers between the dopamine D(1) or D(2) receptors and the histamine H(3) receptor. In addition, we demonstrated two biochemical characteristics of the dopamine D(1) receptor-histamine H(3) receptor heteromer. We have now extended this work to show the dopamine D(1) receptor-histamine H(3) receptor heteromer exists in the brain and serves to provide a novel link between the MAPK pathway and the GABAergic neurons in the direct striatal efferent pathway. Using the biochemical characteristics identified previously, we found that the ability of H(3) receptor activation to stimulate p44 and p42 extracellular signal-regulated MAPK (ERK 1/2) phosphorylation was only observed in striatal slices of mice expressing D(1) receptors but not in D(1) receptor-deficient mice. On the other hand, the ability of both D(1) and H(3) receptor antagonists to block MAPK activation induced by either D(1) or H(3) receptor agonists was also found in striatal slices. Taken together, these data indicate the occurrence of D(1)-H(3) receptor complexes in the striatum and, more importantly, that H(3) receptor agonist-induced ERK 1/2 phosphorylation in striatal slices is mediated by D(1)-H(3) receptor heteromers. Moreover, H(3) receptor-mediated phospho-ERK 1/2 labeling co-distributed with D(1) receptor-containing but not with D(2) receptor-containing striatal neurons. These results indicate that D(1)-H(3) receptor heteromers work as processors integrating dopamine- and histamine-related signals involved in controlling the function of striatal neurons of the direct striatal pathway.

An elderly female patient with tardive oromandibular dystonia after prolonged use of the histamine analog betahistine

Tardive oromandibular dystonia (OMD) is iatrogenic in origin and is characterised by orofacial and lingual stereotypes more frequently than the idiopathic form of OMD Tardive OMD is often associated with anti-dopaminergic treatment involving drugs such as anti-psychotics, anti-emetics, and anti-vertigo agents, although the syndrome can also be triggered by anti-epileptic or anti-depressant drugs that do not have anti-dopaminergic properties. We report an elderly female patient with OMD after prolonged, self-administered treatment with betahistine dihydrochloride, a histamine analogue.

Iron-dextran injection into the substantia nigra in rats decreases striatal dopamine content ipsilateral to the injury site and impairs motor function

Iron chloride injections into the rat SNc can cause chronic decreases in striatal dopamine (DA) levels. However, changes in striatal DA content after iron-dextran injection into rat SNc have not been completely elucidated. The aim of this work was to measure striatal DA concentrations after iron-dextran injection into the SNc. We divided 40 male Wistar rats into five groups, including control, saline injected then sacrificed 7 days or 30 days later, and iron-dextran injected then sacrificed 7 days or 30 days later. Striatal DA content was measured in control animals and in all animals sacrificed 7 days or 30 days after injection, and motor performance was assessed in iron-dextran and saline injected groups 30 days after injection. The striatal DA levels were determined using HPLC. There were significant (P < 0.05) decreases in DA concentrations in the striatum ipsilateral to the injection site in the iron-dextran treated rats compared to control and saline-injected rats. There were no significant differences in DA concentration between the sham-operated (i.e., saline-injected) and control rats. We also observed motor deficits in the iron-dextran injected rats. The striatal DA reduction observed after iron-dextran injection may be attributable to iron-induced oxidative injury in the SNc. Motor deficits, in turn, may be explained by subsequent disturbances in striatal and cortical dopaminergic neuromodulation.

Histamine and H3 receptor-dependent mechanisms regulate ethanol stimulation and conditioned place preference in mice

RATIONALE

Neuronal histamine has a prominent role in sleep-wake control and body homeostasis, but a number of studies suggest that histamine has also a role in higher brain functions including drug reward.

OBJECTIVE

The present experiments characterized the involvement of histamine and its H3 receptor in ethanol-related behaviors in mice.

MATERIALS AND METHODS

Male histidine decarboxylase knockout (HDC KO) and control mice were used to study the role of histamine in ethanol-induced stimulation of locomotor activity, impairment of motor coordination, and conditioned place preference (CPP). Male C57BL/6Sca mice were used to study the effects of H3 receptor antagonist in the effects of ethanol on locomotor activity.

RESULTS

The HDC KO mice displayed a weaker stimulatory response to acute ethanol than the wild-type (WT) mice. No differences between genotypes were found after ethanol administration on accelerating rotarod. The HDC KO mice showed stronger ethanol-induced CPP than the WT mice. Binding of the GABA(A) receptor ligand [(3)H]Ro15-4513 was not markedly changed in HDC KO mouse brain and thus could not explain altered responses in KO mice. Ethanol increased the activity of C57BL/6Sca mice, and H3 receptor antagonist ciproxifan inhibited this stimulation. In CPP paradigm ciproxifan, an H3 receptor inverse agonist potentiated ethanol reward.

CONCLUSIONS

Histaminergic neurotransmission seems to be necessary for the stimulatory effect of ethanol to occur, whereas lack of histamine leads to changes that enhance the conditioned reward by ethanol. Our findings also suggest a role for histamine H3 receptor in modulation of the ethanol stimulation and reward.

Marked changes in signal transduction upon heteromerization of dopamine D1 and histamine H3 receptors

BACKGROUND AND PURPOSE

Functional interactions between the G protein-coupled dopamine D1 and histamine H3 receptors have been described in the brain. In the present study we investigated the existence of D1-H3 receptor heteromers and their biochemical characteristics.

EXPERIMENTAL APPROACH

D1-H3 receptor heteromerization was studied in mammalian transfected cells with Bioluminescence Resonance Energy Transfer and binding assays. Furthermore, signalling through mitogen-activated protein kinase (MAPK) and adenylyl cyclase pathways was studied in co-transfected cells and compared with cells transfected with either D1 or H3 receptors.

KEY RESULTS

Bioluminescence Resonance Energy Transfer and binding assays confirmed that D1 and H3 receptors can heteromerize. Activation of histamine H3 receptors did not lead to signalling towards the MAPK pathway unless dopamine D1 receptors were co-expressed. Also, dopamine D1 receptors, usually coupled to G(s) proteins and leading to increases in cAMP, did not couple to G(s) but to G(i) in co-transfected cells. Furthermore, signalling via each receptor was blocked not only by a selective antagonist but also by an antagonist of the partner receptor.

CONCLUSIONS AND IMPLICATIONS

D1-H3 receptor heteromers constitute unique devices that can direct dopaminergic and histaminergic signalling towards the MAPK pathway in a G(s)-independent and G(i)-dependent manner. An antagonist of one of the receptor units in the D1-H3 receptor heteromer can induce conformational changes in the other receptor unit and block specific signals originating in the heteromer. This gives rise to unsuspected therapeutic potentials for G protein-coupled receptor antagonists.

Anticonvulsant effect of carnosine on penicillin-induced epileptiform activity in rats

Carnosine is a compound of naturally-occurring dipeptide that synthesized by the carnosine synthetase from beta-alanine and l-histidine. Recent reports claim that carnosine plays an important role in the control of epilepsy but its involvement in anticonvulsant functions remains unknown. In this study, we investigated the effects of carnosine in a rat model of epilepsy using the intracortical penicillin injection method. Thirty minutes after penicillin injection, the doses of 125, 250, 500, 1000 mg/kg carnosine and 90 min before penicillin injection the dose of 500 mg/kg carnosine were administered intraperitoneally. The epileptiform activity was verified by electrocorticographic (ECoG) recordings. The mean spike frequency of penicillin-induced epileptiform activity was significantly decreased in all carnosine-treated rats when compared with those of penicillin-injected. The dose of 500 mg/kg for carnosine treated and pretreated rats was found to be the most effective dose in reducing the frequency of penicillin-induced epileptiform activity. There was no significant difference in the mean onset of epileptiform activity between penicillin and 500 mg/kg carnosine pretreated groups. These findings indicate that carnosine has an anticonvulsant effect on penicillin-induced epilepsy in rats. Thus, our data support the hypothesis that carnosine may be a potential anticonvulsant drug for clinical therapy of epilepsy in the future.