Kv4.2 mediates histamine modulation of preoptic neuron activity and body temperature

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.

TMEM16C is involved in thermoregulation and protects rodent pups from febrile seizures

As the most common convulsive disorder in infancy and childhood, affecting 2 to 5% of American children in their first 5 y of life, febrile seizures (FSs) are associated with genetic risk factors, including the Tmem16c (Ano3) gene. Whereas central neuronal hyperexcitability has been implicated in FSs, whether FSs may result from compromised body temperature regulation is unknown. To approach this question, we developed rodent models of FSs associated with deficient thermoregulation, including conditional knockout mice with TMEM16C eliminated from a hypothalamic neuronal population important for maintaining body temperature but not from most of the cortical and hippocampal neurons and sensory neurons. Our findings raise the possibility that impaired homeostatic thermoregulation could contribute to the risk of FSs.

Febrile seizures (FSs) are the most common convulsion in infancy and childhood. Considering the limitations of current treatments, it is important to examine the mechanistic cause of FSs. Prompted by a genome-wide association study identifying TMEM16C (also known as ANO3) as a risk factor of FSs, we showed previously that loss of TMEM16C function causes hippocampal neuronal hyperexcitability [Feenstra et al., Nat. Genet. 46, 1274–1282 (2014)]. Our previous study further revealed a reduction in the number of warm-sensitive neurons that increase their action potential firing rate with rising temperature of the brain region harboring these hypothalamic neurons. Whereas central neuronal hyperexcitability has been implicated in FSs, it is unclear whether the maximal temperature reached during fever or the rate of body temperature rise affects FSs. Here we report that mutant rodent pups with TMEM16C eliminated from all or a subset of their central neurons serve as FS models with deficient thermoregulation. Tmem16c knockout (KO) rat pups at postnatal day 10 (P10) are more susceptible to hyperthermia-induced seizures. Moreover, they display a more rapid rise of body temperature upon heat exposure. In addition, conditional knockout (cKO) mouse pups (P11) with TMEM16C deletion from the brain display greater susceptibility of hyperthermia-induced seizures as well as deficiency in thermoregulation. We also found similar phenotypes in P11 cKO mouse pups with TMEM16C deletion from Ptgds-expressing cells, including temperature-sensitive neurons in the preoptic area (POA) of the anterior hypothalamus, the brain region that controls body temperature. These findings suggest that homeostatic thermoregulation plays an important role in FSs.

TRPCing around the hypothalamus

All of the canonical transient receptor potential channels (TRPC) with the exception of TRPC 2 are expressed in hypothalamic neurons and are involved in multiple homeostatic functions. Although the metabotropic glutamate receptors have been shown to be coupled to TRPC channel activation in cortical and sub-cortical brain regions, in the hypothalamus multiple amine and peptidergic G protein-coupled receptors (GPCRs) and growth factor/cytokine receptors are linked to activation of TRPC channels that are vital for reproduction, temperature regulation, arousal and energy homeostasis. In addition to the neurotransmitters, circulating hormones like insulin and leptin through their cognate receptors activate TRPC channels in POMC neurons. Many of the post-synaptic effects of the neurotransmitters and hormones are regulated in different physiological states by expression of TRPC channels in the post-synaptic neurons. Therefore, TRPC channels are key targets not only for neurotransmitters but circulating hormones in their vital role to control multiple hypothalamic functions, which is the focus of this review.

Central thermoreceptors

Homeotherms maintain their core body temperature within a narrow range by employing multiple redundant mechanisms to control heat production and dissipation. Preoptic area/anterior hypothalamic (PO/AH) neurons receive thermal signals from peripheral and deep-body thermoreceptors as well as hormonal and metabolic signals. A population of PO/AH neurons termed warm-sensitive increase their firing temperature with warming and are considered central thermoreceptors. Electrophysiologic and pharmacologic experiments have provided descriptions of their characteristics and signaling mechanisms. These studies have also allowed insights into the mechanisms by which neurochemicals important in thermoregulation exert their influence. Finally, the cellular mechanism involved in the interactions between thermoregulation and other aspects of homeostasis, such as energy metabolism and osmoregulation, have started to be unraveled.

Gastrin-releasing peptide receptor mediates the excitation of preoptic GABAergic neurons by bombesin

Bombesin, a pan agonist of the bombesin-like peptide receptor family, elicits potent hypothermia when applied centrally. The signaling mechanisms involved are not known. Here we report that GABAergic preoptic neurons express gastrin-releasing peptide (GRP) receptors and are directly excited by GRP or bombesin. This effect was abolished by a GRP receptor antagonist. A partially overlapping group of preoptic GABAergic neurons express bombesin-like receptor 3 (BRS3), however their activation results in a decrease in firing rate. The excitatory effects of bombesin or GRP were not affected by BRS3 antagonist. GRP activated a Ca²⁺-dependent inward nonselective cationic current and Ca²⁺ release from intracellular stores. Our data indicate that GRP receptors mediate the excitatory effects of bombesin in preoptic neurons.

Identification of histaminergic neurons through histamine 3 receptor-mediated autoinhibition

Using a reporter mouse model with expression of the tomato fluorescent protein under the dopamine transporter promoter (Tmt-DAT) we discovered a new group of neurons in the histaminergic tuberomamillary nucleus (TMN), which, in contrast to tuberoinfundibular dopaminergic neurons of the dorsomedial arcuate nucleus, do not express tyrosine hydroxylase but can synthesize and store dopamine. Tmt-DAT neurons located within TMN share electrophysiological properties with histaminergic neurons: spontaneous firing at a membrane potential around -50 mV and presence of hyperpolarization-activated cyclic nucleotide-gated ion channels. Histamine (30 μM) depolarizes and excites Tmt-DAT neurons through H1R activation but inhibits histaminergic neurons through H3R activation thus allowing a pharmacological identification of the different neurons. Single-cell RT-PCR revealed that all histaminergic neurons expressing histidine decarboxylase (HDC) also express H3R. This includes neurons retrogradely traced from the striatum whose inhibition by a selective H3R agonist was indistinguishable from the whole population. Prolonged depolarization reduces the autoinhibition. The potency of histamine at H3R depends on membrane potential and on extracellular and intracellular calcium. Autoinhibition can be impaired by preincubation with capsaicin, a ligand of the calcium-permeable TRPV1 channel or by blockade of Ca(2+)-ATPase with thapsigargin. The pharmacology of autoinhibition is revisited and physiological conditions for its functionality are determined. Usage of reporter mouse models for the safe identification of aminergic neurons under pathophysiological conditions is recommended. This article is part of the Special Issue entitled 'Histamine Receptors'.

Central nervous system regulation of brown adipose tissue

Thermogenesis, the production of heat energy, in brown adipose tissue is a significant component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature in many species from mouse to man and plays a key role in elevating body temperature during the febrile response to infection. The sympathetic neural outflow determining brown adipose tissue (BAT) thermogenesis is regulated by neural networks in the CNS which increase BAT sympathetic nerve activity in response to cutaneous and deep body thermoreceptor signals. Many behavioral states, including wakefulness, immunologic responses, and stress, are characterized by elevations in core body temperature to which central command-driven BAT activation makes a significant contribution. Since energy consumption during BAT thermogenesis involves oxidation of lipid and glucose fuel molecules, the CNS network driving cold-defensive and behavioral state-related BAT activation is strongly influenced by signals reflecting the short- and long-term availability of the fuel molecules essential for BAT metabolism and, in turn, the regulation of BAT thermogenesis in response to metabolic signals can contribute to energy balance, regulation of body adipose stores and glucose utilization. This review summarizes our understanding of the functional organization and neurochemical influences within the CNS networks that modulate the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolic alterations in BAT thermogenesis and BAT energy expenditure that contribute to overall energy homeostasis and the autonomic support of behavior.

Histamine receptor signaling in energy homeostasis

Histamine modulates several aspects of energy homeostasis. By activating histamine receptors in the hypothalamus the bioamine influences thermoregulation, its circadian rhythm, energy expenditure and feeding. These actions are brought about by activation of different histamine receptors and/or the recruitment of distinct neural pathways. In this review we describe the signaling mechanisms activated by histamine in the hypothalamus, the evidence for its role in modulating energy homeostasis as well as recent advances in the understanding of the cellular and neural network mechanisms involved. This article is part of the Special Issue entitled 'Histamine Receptors'.

Electrical remodeling of preoptic GABAergic neurons involves the Kv1.5 subunit

The electrogenic machinery of an excitable cell can adapt in response to changes in input, genetic deficit or in pathological conditions, however the underlying molecular mechanisms are not understood. In cases of genetic deletion it is commonly observed that a channel subunit from the same family replaces the missing one. We have previously reported that Kv4.2−/− preoptic GABAergic neurons display identical firing characteristics to those of wild-type neurons despite having reduced A-type currents, and that, surprisingly, they present a robust upregulation of a delayed rectifier current, the nature of which is unknown. Here, using pharmacology, qPCR and Western blots we report that, although the wild-type neurons express several Kv subunits, the upregulated current is conducted by the Kv1.5 subunit exclusively. Thus, this study reveals the molecular nature of a novel mechanism of electrical remodeling in central neurons.

Persistent histamine excitation of glutamatergic preoptic neurons

Thermoregulatory neurons of the median preoptic nucleus (MnPO) represent a target at which histamine modulates body temperature. The mechanism by which histamine excites a population of MnPO neurons is not known. In this study it was found that histamine activated a cationic inward current and increased the intracellular Ca(2+) concentration, actions that had a transient component as well as a sustained one that lasted for tens of minutes after removal of the agonist. The sustained component was blocked by TRPC channel blockers. Single-cell reverse transcription-PCR analysis revealed expression of TRPC1, TRPC5 and TRPC7 subunits in neurons excited by histamine. These studies also established the presence of transcripts for the glutamatergic marker Vglut2 and for the H1 histamine receptor in neurons excited by histamine. Intracellular application of antibodies directed against cytoplasmic sites of the TRPC1 or TRPC5 channel subunits decreased the histamine-induced inward current. The persistent inward current and elevation in intracellular Ca(2+) concentration could be reversed by activating the PKA pathway. This data reveal a novel mechanism by which histamine induces persistent excitation and sustained intracellular Ca(2+) elevation in glutamatergic MnPO neurons.

Loss of histaminergic modulation of thermoregulation and energy homeostasis in obese mice

Histamine acts centrally to increase energy expenditure and reduce body weight by mechanisms not fully understood. It has been suggested that in the obese state hypothalamic histamine signaling is altered. Previous studies have also shown that histamine acting in the preoptic area controls thermoregulation. We aimed to study the influence of preoptic histamine on body temperature and energy homeostasis in control and obese mice. Activating histamine receptors in the preoptic area by increasing the concentration of endogenous histamine or by local injection of specific agonists induced an elevation of core body temperature and decreased respiratory exchange ratio (RER). In addition, the food intake was significantly decreased. The hyperthermic effect was associated with a rapid increase in mRNA expression of uncoupling proteins in thermogenic tissues, the most pronounced being that of uncoupling protein (UCP) 1 in brown adipose tissue and of UCP2 in white adipose tissue. In diet-induced obese mice histamine had much diminished hyperthermic effects as well as reduced effect on RER. Similarly, the ability of preoptic histamine signaling to increase the expression of uncoupling proteins was abolished. We also found that the expression of mRNA encoding the H1 receptor subtype in the preoptic area was significantly lower in obese animals. These results indicate that histamine signaling in the preoptic area modulates energy homeostasis by regulating body temperature, metabolic parameters and food intake and that the obese state is associated with a decrease in neurotransmitter's influence.