Mechanisms of N-oleoyldopamine activation of central histaminergic neurons

Impact of TRPV1 on Pathogenesis and Therapy of Neurodegenerative Diseases

Transient receptor potential vanilloid 1 (TRPV1) is a transmembrane and non-selective cation channel protein, which can be activated by various physical and chemical stimuli. Recent studies have shown the strong pathogenetic associations of TRPV1 with neurodegenerative diseases (NDs), in particular Alzheimer's disease (AD), Parkinson's disease (PD) and multiple sclerosis (MS) via regulating neuroinflammation. Therapeutic effects of TRPV1 agonists and antagonists on the treatment of AD and PD in animal models also are emerging. We here summarize the current understanding of TRPV1's effects and its agonists and antagonists as a therapeutic means in neurodegenerative diseases, and highlight future treatment strategies using natural TRPV1 agonists. Developing new targets and applying natural products are becoming a promising direction in the treatment of chronic disorders, especially neurodegenerative diseases.

Orexin neurons inhibit sleep to promote arousal

Humans and animals lacking orexin neurons exhibit daytime sleepiness, sleep attacks, and state instability. While the circuit basis by which orexin neurons contribute to consolidated wakefulness remains unclear, existing models posit that orexin neurons provide their wake-stabilizing influence by exerting excitatory tone on other brain arousal nodes. Here we show using in vivo optogenetics, in vitro optogenetic-based circuit mapping, and single-cell transcriptomics that orexin neurons also contribute to arousal maintenance through indirect inhibition of sleep-promoting neurons of the ventrolateral preoptic nucleus. Activation of this subcortical circuit rapidly drives wakefulness from sleep by differentially modulating the activity of ventrolateral preoptic neurons. We further identify and characterize a feedforward circuit through which orexin (and co-released glutamate) acts to indirectly target and inhibit sleep-promoting ventrolateral preoptic neurons to produce arousal. This revealed circuitry provides an alternate framework for understanding how orexin neurons contribute to the maintenance of consolidated wakefulness and stabilize behavioral state.

OLHA (Nα-oleoylhistamine) modulates activity of mouse brain histaminergic neurons

Histaminergic (HA) neurons are located in the tuberomamillary nucleus (TMN) of the posterior hypothalamus, from where they project throughout the whole brain to control wakefulness. We examined the effects of N^α-oleoylhistamine (OLHA), a non-enzymatic condensation product of oleic acid (OLA) and histamine, on activity of mouse HA neurons in brain slices. OLHA bidirectionally modulated the firing of HA neurons. At 10 nM OLHA inhibited or had no action, whereas at 1 μM it evoked excitatory and inhibitory responses. Inhibition was not seen in presence of the histamine receptor H3 (H₃R) antagonist clobenpropit and in calcium-free medium. Pre-incubation with a histamine-reuptake blocker prevented the decrease in firing by OLHA. OLHA-evoked increase in firing (EC₅₀ ∼44 nM) was insensitive to blockers of cannabinoid 1 and 2 receptors and of the capsaicin receptor, but was significantly impaired by the peroxisome proliferator-activated receptor-alpha (PPAR-alpha) antagonist MK886, which suppressed also the rise in intracellular calcium level caused by OLHA. The OLHA-evoked excitation was mimicked by synthetic PPAR-alpha agonists (gemfibrozil and GW7647) and was abolished by the PKA inhibitor H-89. The H₃R affinity (Ki) for histamine, measured in HEK293 cells with stable expression of human H₃R, was higher than for OLHA (Ki: 42 vs 310 nM, respectively). Expression of PPAR-alpha was not different between TMN regions of males and females, responses to OLHA did not differ. Molecular modelling of PPAR-alpha bound to either OLHA or OEA showed similar binding energies. These findings shed light on a novel biotransformation product of histamine which may play a role in health and disease.

Tachykinins amplify the action of capsaicin on central histaminergic neurons

Substance P (SP), a product of the tachykinin 1 (Tac1) gene, is expressed in many hypothalamic neurons. Its wake-promoting potential could be mediated through histaminergic (HA) neurons of the tuberomamillary nucleus (TMN), where functional expression of neurokinin receptors (NKRs) waits to be characterized. As in the process of nociception in the peripheral nervous system (PNS) capsaicin-receptor (transient potential vanilloid 1: TRPV1) signalling is amplified by local release of histamine and SP, we tested the involvement of tachykinins in the capsaicin-induced long-lasting enhancement (LLEcaps) of HA neurons firing by investigating selective neurokinin receptor ligands in the hypothalamic mouse brain slice preparation using patch-clamp recordings in cell-attached mode combined with single-cell RT-PCR. We report that the majority of HA neurons respond to SP (EC50 3 nM), express the SP precursor tachykinin 1 (Tac1) gene and at least one of the neurokinin receptors. Responses to selective agonists of three known neurokinin receptors were sensitive to corresponding antagonists. LLEcaps was significantly impaired by the neurokinin receptor antagonists, indicating that in hypothalamus, as in the PNS, release of tachykinins downstream to TRPV1 activation is able to boost the release of histamine. The excitatory action of SP on histaminergic neurons adds another pathway to the noradrenergic and orexinergic ones to synergistically enhance cortical arousal. We show NK1R to play a prominent role on HA neurons and thus the control of wakefulness.

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.

A Duet Between Histamine and Oleoylethanolamide in the Control of Homeostatic and Cognitive Processes

In ballet, a pas de deux (in French it means "step of two") is a duet in which the two dancers perform ballet steps together. The suite of dances shares a common theme of partnership. How could we better describe the fine interplay between oleoylethanolamide (OEA) and histamine, two phylogenetically ancient molecules controlling metabolic, homeostatic and cognitive processes? Contrary to the pas de deux though, the two dancers presumably never embrace each other as a dancing pair but execute their "virtuoso solo" constantly exchanging interoceptive messages presumably via vagal afferents, the blood stream, the neuroenteric system. With one exception, which is in the control of liver ketogenesis, as in hepatocytes, OEA biosynthesis strictly depends on the activation of histaminergic H₁ receptors. In this review, we recapitulate our main findings that evidence the interplay of histamine and OEA in the control of food consumption and eating behaviour, in the consolidation of emotional memory and mood, and finally, in the synthesis of ketone bodies. We will also summarise some of the putative underlying mechanisms for each scenario.

An Inhibitory Lateral Hypothalamic-Preoptic Circuit Mediates Rapid Arousals from Sleep

Among the neuronal populations implicated in sleep-wake control, the ventrolateral preoptic (VLPO) nucleus has emerged as a key sleep-promoting center. However, the synaptic drives that regulate the VLPO to control arousal levels in vivo have not to date been identified. Here, we show that sleep-promoting galaninergic neurons within the VLPO nucleus, defined pharmacologically and by single-cell transcript analysis, are postsynaptic targets of lateral hypothalamic GABAergic (LH^GABA) neurons and that activation of this pathway in vivo rapidly drives wakefulness. Ca²⁺ imaging from LH^GABA neurons indicate that they are both wake and rapid eye movement (REM)-sleep active. Consistent with the potent arousal-promoting property of the LH^GABA → VLPO pathway, presynaptic inputs to LH^GABA neurons originate from several canonical stress- and arousal-related network nodes. This work represents the first demonstration that direct synaptic inhibition of the VLPO area can suppress sleep-promoting neurons to rapidly promote arousal.