Distribution of histaminergic neuronal cluster in the rat and mouse hypothalamus

Histaminergic System Activity in the Central Nervous System: The Role in Neurodevelopmental and Neurodegenerative Disorders

Histamine (HA), a biogenic monoamine, exerts its pleiotropic effects through four H1R-H4R histamine receptors, which are also expressed in brain tissue. Together with the projections of HA-producing neurons located within the tuberomammillary nucleus (TMN), which innervate most areas of the brain, they constitute the histaminergic system. Thus, while remaining a mediator of the inflammatory reaction and immune system function, HA also acts as a neurotransmitter and a modulator of other neurotransmitter systems in the central nervous system (CNS). Although the detailed causes are still not fully understood, neuroinflammation seems to play a crucial role in the etiopathogenesis of both neurodevelopmental and neurodegenerative (neuropsychiatric) diseases, such as autism spectrum disorders (ASDs), attention-deficit/hyperactivity disorder (ADHD), Alzheimer's disease (AD) and Parkinson's disease (PD). Given the increasing prevalence/diagnosis of these disorders and their socioeconomic impact, the need to develop effective forms of therapy has focused researchers' attention on the brain's histaminergic activity and other related signaling pathways. This review presents the current state of knowledge concerning the involvement of HA and the histaminergic system within the CNS in the development of neurodevelopmental and neurodegenerative disorders. To this end, the roles of HA in neurotransmission, neuroinflammation, and neurodevelopment are also discussed.

Targeting Histamine and Histamine Receptors for Memory Regulation: An Emotional Perspective

Histamine has long been accepted as a pro-cognitive agent. However, lines of evidence have suggested that the roles of histamine in learning and memory processes are much more complex than previously thought. When explained by the spatial perspectives, there are many contradictory results. However, using emotional memory perspectives, we suspect that the histaminergic system may interplay with stress, reward inhibition, and attention to modulate emotional memory formation. The functional diversity of histamine makes it a viable target for clinical management of neuropsychiatric disorders. Here, we update the current knowledge about the functions of histamine in emotional memory and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on memory and discuss insights into future research on the roles of histamine in emotional memory. Despite the recent progress in histamine research, the histaminergic emotional memory circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

Histamine H3 and H4 receptors modulate Parkinson's disease induced brain pathology. Neuroprotective effects of nanowired BF-2649 and clobenpropit with anti-histamine-antibody therapy

Military personnel deployed in combat operations are highly prone to develop Parkinson's disease (PD) in later lives. PD largely involves dopaminergic pathways with hallmarks of increased alpha synuclein (ASNC), and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) precipitating brain pathology. However, increased histaminergic nerve fibers in substantia nigra pars Compacta (SNpc), striatum (STr) and caudate putamen (CP) associated with upregulation of Histamine H3 receptors and downregulation of H4 receptors in human cases of PD is observed in postmortem cases. These findings indicate that modulation of histamine H3 and H4 receptors and/or histaminergic transmission may induce neuroprotection in PD induced brain pathology. In this review effects of a potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist, in association with monoclonal anti-histamine antibodies (AHmAb) in PD brain pathology is discussed based on our own observations. Our investigation shows that chronic administration of conventional or TiO₂ nanowired BF 2649 (1mg/kg, i.p.) or CLBPT (1mg/kg, i.p.) once daily for 1 week together with nanowired delivery of HAmAb (25μL) significantly thwarted ASNC and p-tau levels in the SNpC and STr and reduced PD induced brain pathology. These observations are the first to show the involvement of histamine receptors in PD and opens new avenues for the development of novel drug strategies in clinical strategies for PD, not reported earlier.

Histamine: A Key Neuromodulator of Memory Consolidation and Retrieval

In pharmacological studies conducted on animals over the last four decades, histamine was determined to be a strong modulator of learning and memory. Activation of histamine signaling enhances memory consolidation and retrieval. Even long after learning and forgetting, it can still restore the retrieval of forgotten memories. These findings based on animal studies led to human clinical trials with histamine H₃ receptor antagonists/inverse agonists, which revealed their positive effects on learning and memory. Therefore, histamine signaling is a promising therapeutic target for improving cognitive impairments in patients with various neuropsychiatric disorders, including Alzheimer's disease. While the memory-modulatory effects of histamine receptor agonists and antagonists have been confirmed by several research groups, the underlying mechanisms remain to be elucidated. This review summarizes how the activation and inhibition of histamine signaling influence memory processes, introduces the cellular and circuit mechanisms, and discusses the relationship between the human histaminergic system and learning and memory.

Complexin I knockout rats exhibit a complex neurobehavioral phenotype including profound ataxia and marked deficits in lifespan

Complexin I (CPLX1), a presynaptic small molecule protein, forms SNARE complex in the central nervous system involved in the anchoring, pre-excitation, and fusion of axonal end vesicles. Abnormal expression of CPLX1 occurs in several neurodegenerative and psychiatric disorders that exhibit disrupted neurobehaviors. CPLX1 gene knockout induces severe ataxia and social behavioral deficits in mice, which has been poorly demonstrated. Here, to address the limitations of single-species models and to provide translational insights relevant to human diseases, we used CPLX1 knockout rats to further explore the function of the CPLX1 gene. The CRISPR/Cas9 gene editing system was adopted to generate CPLX1 knockout rats (CPLX1^-/-). Then, we characterized the survival rate and behavioral phenotype of CPLX1^-/- rats using behavioral analysis. To further explain this phenomenon, we performed blood glucose testing, Nissl staining, hematoxylin-eosin staining, and Golgi staining. We found that CPLX1^-/- rats showed profound ataxia, dystonia, movement and exploratory deficits, and increased anxiety and sensory deficits but had normal cognitive function. Nevertheless, CPLX1^-/- rats could swim without training. The abnormal histomorphology of the stomach and intestine were related to decreased weight and early death in these rats. Decreased dendritic branching was also found in spinal motor neurons in CPLX1^-/- rats. In conclusion, CPLX1 gene knockout induced the abnormal histomorphology of the stomach and intestine and decreased dendritic branching in spinal motor neurons, causing different phenotypes between CPLX1^-/- rats and mice, even though both of these phenotypes showed profound ataxia. These findings provide a new perspective for understanding the role of CPLX1.

Brain histamine modulates recognition memory: possible implications in major cognitive disorders

Several behavioural tests have been developed to study and measure emotionally charged or emotionally neutral memories and how these may be affected by pharmacological, dietary or environmental manipulations. In this review, we describe the experimental paradigms used in preclinical studies to unravel the brain circuits involved in the recognition and memorization of environmentally salient stimuli devoid of strong emotional value. In particular, we focus on the modulatory role of the brain histaminergic system in the elaboration of recognition memory that is based on the judgement of the prior occurrence of an event, and it is believed to be a critical component of human declarative memory. The review also addresses questions that may help improve the treatment of impaired declarative memory described in several affective and neuropsychiatric disorders such as ADHD, Alzheimer's disease and major neurocognitive disorder. 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.

Monoamines Inhibit GABAergic Neurons in Ventrolateral Preoptic Area That Make Direct Synaptic Connections to Hypothalamic Arousal Neurons

The hypothalamus plays an important role in the regulation of sleep/wakefulness states. While the ventrolateral preoptic nucleus (VLPO) plays a critical role in the initiation and maintenance of sleep, the lateral posterior part of the hypothalamus contains neuronal populations implicated in maintenance of arousal, including orexin-producing neurons (orexin neurons) in the lateral hypothalamic area (LHA) and histaminergic neurons in the tuberomammillary nucleus (TMN). During a search for neurons that make direct synaptic contact with histidine decarboxylase-positive (HDC+), histaminergic neurons (HDC neurons) in the TMN and orexin neurons in the LHA of male mice, we found that these arousal-related neurons are heavily innervated by GABAergic neurons in the preoptic area including the VLPO. We further characterized GABAergic neurons electrophysiologically in the VLPO (GABA^VLPO neurons) that make direct synaptic contact with these hypothalamic arousal-related neurons. These neurons (GABA^VLPO→HDC or GABA^{VLPO→orexin} neurons) were both potently inhibited by noradrenaline and serotonin, showing typical electrophysiological characteristics of sleep-promoting neurons in the VLPO. This work provides direct evidence of monosynaptic connectivity between GABA^VLPO neurons and hypothalamic arousal neurons and identifies the effects of monoamines on these neuronal pathways.SIGNIFICANCE STATEMENT Rabies-virus-mediated tracing of input neurons of two hypothalamic arousal-related neuron populations, histaminergic and orexinergic neurons, showed that they receive similar distributions of input neurons in a variety of brain areas, with rich innervation by GABAergic neurons in the preoptic area, including the ventrolateral preoptic area (VLPO), a region known to play an important role in the initiation and maintenance of sleep. Electrophysiological experiments found that GABAergic neurons in the VLPO (GABA^VLPO neurons) that make direct input to orexin or histaminergic neurons are potently inhibited by noradrenaline and serotonin, suggesting that these monoamines disinhibit histamine and orexin neurons. This work demonstrated functional and structural interactions between GABA^VLPO neurons and hypothalamic arousal-related neurons.

Hypocretin/orexin loss changes the hypothalamic immune response

Hypocretin, also known as orexin, maintains the vigilance state and regulates various physiological processes, such as arousal, sleep, food intake, energy expenditure, and reward. Previously, we found that when wild-type mice and hypocretin/ataxin-3 littermates (which are depleted of hypothalamic hypocretin-expressing neurons postnatally) were administered lipopolysaccharide (LPS), the two genotypes exhibited significant differences in their sleep/wake cycle, including differences in the degree of increase in sleep periods and in recovery from sickness behaviour. In the present study, we examined changes in the hypothalamic vigilance system and in the hypothalamic expression of inflammatory factors in response to LPS in hypocretin/ataxin-3 mice. Peripheral immune challenge with LPS affected the hypothalamic immune response and vigilance states. This response was altered by the loss of hypocretin. Hypocretin expression was inhibited after LPS injection in both hypocretin/ataxin-3 mice and their wild-type littermates, but expression was completely abolished only in hypocretin/ataxin-3 mice. Increases in the number of histidine decarboxylase (HDC)-positive cells and in Hdc mRNA expression were found in hypocretin/ataxin-3 mice, and this increase was suppressed by LPS. Hypocretin loss did not impact the change in expression of hypothalamic inflammatory factors in response to LPS, except for interferon gamma and colony stimulating factor 3. The number of c-Fos-positive/HDC-positive cells in hypocretin/ataxin-3 mice administered LPS injections was elevated, even during the rest period, in all areas, suggesting that there is an increase in the activity of histaminergic neurons in hypocretin/ataxin-3 mice following LPS injection. Taken together, our results suggest a novel role for hypocretin in the hypothalamic response to peripheral immune challenge. Our findings contribute to the understanding of the pathophysiology of narcolepsy.