Ionotropic histamine receptors and H2 receptors modulate supraoptic oxytocin neuronal excitability and dye coupling

Angiotensin AT1A receptor signal switching in Agouti-related peptide neurons mediates metabolic rate adaptation during obesity

Resting metabolic rate (RMR) adaptation occurs during obesity and is hypothesized to contribute to failed weight management. Angiotensin II (Ang-II) type 1 (AT1A) receptors in Agouti-related peptide (AgRP) neurons contribute to the integrative control of RMR, and deletion of AT1A from AgRP neurons causes RMR adaptation. Extracellular patch-clamp recordings identify distinct cellular responses of individual AgRP neurons from lean mice to Ang-II: no response, inhibition via AT1A and Gαi, or stimulation via Ang-II type 2 (AT2) receptors and Gαq. Following diet-induced obesity, a subset of Ang-II/AT1A-inhibited AgRP neurons undergo a spontaneous G-protein "signal switch," whereby AT1A stop inhibiting the cell via Gαi and instead begin stimulating the cell via Gαq. DREADD-mediated activation of Gαi, but not Gαq, in AT1A-expressing AgRP cells stimulates RMR in lean and obese mice. Thus, loss of AT1A-Gαi coupling within the AT1A-expressing AgRP neuron subtype represents a molecular mechanism contributing to RMR adaptation.

β subunits of GABAA receptors form proton-gated chloride channels: Insights into the molecular basis

Gamma-aminobutyric acid type A receptors (GABAARs) are ligand gated channels mediating inhibition in the central nervous system. Here, we identify a so far undescribed function of β-subunit homomers as proton-gated anion channels. Mutation of a single H267A in β3 subunits completely abolishes channel activation by protons. In molecular dynamic simulations of the β3 crystal structure protonation of H267 increased the formation of hydrogen bonds between H267 and E270 of the adjacent subunit leading to a pore stabilising ring formation and accumulation of Cl- within the transmembrane pore. Conversion of these residues in proton insensitive ρ1 subunits transfers proton-dependent gating, thus highlighting the role of this interaction in proton sensitivity. Activation of chloride and bicarbonate currents at physiological pH changes (pH50 is in the range 6- 6.3) and kinetic studies suggest a physiological role in neuronal and non-neuronal tissues that express beta subunits, and thus as potential novel drug target.

Anatomical Markers of Activity in Hypothalamic Neurons

The scientific community has searched for years for ways of examining neuronal tissue to track neural activity with reliable anatomical markers for stimulated neuronal activity. Existing studies that focused on hypothalamic systems offer a few options but do not always compare approaches or validate them for dependence on cell firing, leaving the reader uncertain of the benefits and limitations of each method. Thus, in this article, potential markers will be presented and, where possible, placed into perspective in terms of when and how these methods pertain to hypothalamic function. An example of each approach is included. In reviewing the approaches, one is guided through how neurons work, the consequences of their stimulation, and then the potential markers that could be applied to hypothalamic systems are discussed. Approaches will use features of neuronal glucose utilization, water/oxygen movement, changes in neuron-glial interactions, receptor translocation, cytoskeletal changes, stimulus-synthesis coupling that includes expression of the heteronuclear or mature mRNA for transmitters or the enzymes that make them, and changes in transcription factors (immediate early gene products, precursor buildup, use of promoter-driven surrogate proteins, and induced expression of added transmitters. This article includes discussion of methodological limitations and the power of combining approaches to understand neuronal function. © 2020 American Physiological Society. Compr Physiol 10:549-575, 2020.

The role of co-neurotransmitters in sleep and wake regulation

Sleep and wakefulness control in the mammalian brain requires the coordination of various discrete interconnected neurons. According to the most conventional sleep model, wake-promoting neurons (WPNs) and sleep-promoting neurons (SPNs) compete for network dominance, creating a systematic "switch" that results in either the sleep or awake state. WPNs and SPNs are ubiquitous in the brainstem and diencephalon, areas that together contain <1% of the neurons in the human brain. Interestingly, many of these WPNs and SPNs co-express and co-release various types of the neurotransmitters that often have opposing modulatory effects on the network. Co-transmission is often beneficial to structures with limited numbers of neurons because it provides increasing computational capability and flexibility. Moreover, co-transmission allows subcortical structures to bi-directionally control postsynaptic neurons, thus helping to orchestrate several complex physiological functions such as sleep. Here, we present an in-depth review of co-transmission in hypothalamic WPNs and SPNs and discuss its functional significance in the sleep-wake network.

A conjugate of decyltriphenylphosphonium with plastoquinone can carry cyclic adenosine monophosphate, but not cyclic guanosine monophosphate, across artificial and natural membranes

The present study demonstrated for the first time the interaction between adenosine 3',5'-cyclic monophosphate (cAMP), one of the most important signaling compounds in living organisms, and the mitochondria-targeted antioxidant plastoquinonyl-decyltriphenylphosphonium (SkQ1). The data obtained on model liquid membranes and human platelets revealed the ability of SkQ1 to selectively transport cAMP, but not guanosine 3',5'-cyclic monophosphate (cGMP), across both artificial and natural membranes. In particular, SkQ1 elicited translocation of cAMP from the source to the receiving phase of a Pressman-type cell, while showing low activity with cGMP. Importantly, only conjugate with plastoquinone, but not dodecyl-triphenylphosphonium, was effective in carrying cAMP. In human platelets, SkQ1 also appeared to serve as a carrier of cAMP, but not cGMP, from outside to inside the cell, as measured by phosphorylation of the vasodilator stimulated phosphoprotein. The SkQ1-induced transfer of cAMP across the plasma membrane found here can be tentatively suggested to interfere with cAMP signaling pathways in living cells.

Design principles of electrical synaptic plasticity

Essentially all animals with nervous systems utilize electrical synapses as a core element of communication. Electrical synapses, formed by gap junctions between neurons, provide rapid, bidirectional communication that accomplishes tasks distinct from and complementary to chemical synapses. These include coordination of neuron activity, suppression of voltage noise, establishment of electrical pathways that define circuits, and modulation of high order network behavior. In keeping with the omnipresent demand to alter neural network function in order to respond to environmental cues and perform tasks, electrical synapses exhibit extensive plasticity. In some networks, this plasticity can have dramatic effects that completely remodel circuits or remove the influence of certain cell types from networks. Electrical synaptic plasticity occurs on three distinct time scales, ranging from milliseconds to days, with different mechanisms accounting for each. This essay highlights principles that dictate the properties of electrical coupling within networks and the plasticity of the electrical synapses, drawing examples extensively from retinal networks.

Characteristics and plasticity of electrical synaptic transmission

Electrical synapses are an omnipresent feature of nervous systems, from the simple nerve nets of cnidarians to complex brains of mammals. Formed by gap junction channels between neurons, electrical synapses allow direct transmission of voltage signals between coupled cells. The relative simplicity of this arrangement belies the sophistication of these synapses. Coupling via electrical synapses can be regulated by a variety of mechanisms on times scales ranging from milliseconds to days, and active properties of the coupled neurons can impart emergent properties such as signal amplification, phase shifts and frequency-selective transmission. This article reviews the biophysical characteristics of electrical synapses and some of the core mechanisms that control their plasticity in the vertebrate central nervous system.

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.

Mechanisms and functions of GABA co-release

The 'one neuron, one neurotransmitter' doctrine states that synaptic communication between two neurons occurs through the release of a single chemical transmitter. However, recent findings suggest that neurons that communicate using more than one classical neurotransmitter are prevalent throughout the adult mammalian CNS. In particular, several populations of neurons previously thought to release only glutamate, acetylcholine, dopamine or histamine also release the major inhibitory neurotransmitter GABA. Here, we review these findings and discuss the implications of GABA co-release for synaptic transmission and plasticity.

Oxytocin-secreting system: A major part of the neuroendocrine center regulating immunologic activity

Interactions between the nervous system and immune system have been studied extensively. However, the mechanisms underlying the neural regulation of immune activity, particularly the neuroendocrine regulation of immunologic functions, remain elusive. In this review, we provide a comprehensive examination of current evidence on interactions between the immune system and hypothalamic oxytocin-secreting system. We highlight the fact that oxytocin may have significant effects in the body, beyond its classical functions in lactation and parturition. Similar to the hypothalamo-pituitary-adrenal axis, the oxytocin-secreting system closely interacts with classical immune system, integrating both neurochemical and immunologic signals in the central nervous system and in turn affects immunologic defense, homeostasis, and surveillance. Lastly, this review explores therapeutic potentials of oxytocin in treating immunologic disorders.

Connexins in neurons and glia: targets for intervention in disease and injury

Both neurons and glia throughout the central nervous system are organized into networks by gap junctions. Among glia, gap junctions facilitate metabolic homeostasis and intercellular communication. Among neurons, gap junctions form electrical synapses that function primarily for communication. However, in neurodegenerative states due to disease or injury gap junctions may be detrimental to survival. Electrical synapses may facilitate hyperactivity and bystander killing among neurons, while gap junction hemichannels in glia may facilitate inflammatory signaling and scar formation. Advances in understanding mechanisms of plasticity of electrical synapses and development of molecular therapeutics to target glial gap junctions and hemichannels offer new hope to pharmacologically limit neuronal degeneration and enhance recovery.

Systemic lack of canonical histamine receptor signaling results in increased resistance to autoimmune encephalomyelitis

Histamine (HA) is a key regulator of experimental allergic encephalomyelitis (EAE), the autoimmune model of multiple sclerosis. HA exerts its effects through four known G-protein-coupled receptors: H1, H2, H3, and H4 (histamine receptors; H(1-4)R). Using HR-deficient mice, our laboratory has demonstrated that H1R, H2R, H3R, and H4R play important roles in EAE pathogenesis, by regulating encephalitogenic T cell responses, cytokine production by APCs, blood-brain barrier permeability, and T regulatory cell activity, respectively. Histidine decarboxylase-deficient mice (HDCKO), which lack systemic HA, exhibit more severe EAE and increased Th1 effector cytokine production by splenocytes in response to myelin oligodendrocyte gp35-55. In an inverse approach, we tested the effect of depleting systemic canonical HA signaling on susceptibility to EAE by generating mice lacking all four known G-protein-coupled-HRs (H(1-4)RKO mice). In this article, we report that in contrast to HDCKO mice, H(1-4)RKO mice develop less severe EAE compared with wild-type animals. Furthermore, splenocytes from immunized H(1-4)RKO mice, compared with wild-type mice, produce a lower amount of Th1/Th17 effector cytokines. The opposing results seen between HDCKO and H1-4RKO mice suggest that HA may signal independently of H1-4R and support the existence of an alternative HAergic pathway in regulating EAE resistance. Understanding and exploiting this pathway has the potential to lead to new disease-modifying therapies in multiple sclerosis and other autoimmune and allergic diseases.

Under construction: building the macromolecular superstructure and signaling components of an electrical synapse

A great deal is now known about the protein components of tight junctions and adherens junctions, as well as how these are assembled. Less is known about the molecular framework of gap junctions, but these also have membrane specializations and are subject to regulation of their assembly and turnover. Thus, it is reasonable to consider that these three types of junctions may share macromolecular commonalities. Indeed, the tight junction scaffolding protein zonula occluden-1 (ZO-1) is also present at adherens and gap junctions, including neuronal gap junctions. On the basis of these earlier observations, we more recently found that two additional proteins, AF6 and MUPP1, known to be associated with ZO-1 at tight and adherens junctions, are also components of neuronal gap junctions in rodent brain and directly interact with connexin36 (Cx36) that forms these junctions. Here, we show by immunofluorescence labeling that the cytoskeletal-associated protein cingulin, commonly found at tight junctions, is also localized at neuronal gap junctions throughout the central nervous system. In consideration of known functions related to ZO-1, AF6, MUPP1, and cingulin, our results provide a context in which to examine functional relationships between these proteins at Cx36-containing electrical synapses in brain--specifically, how they may contribute to regulation of transmission at these synapses, and how they may govern gap junction channel assembly and/or disassembly.

The effector and scaffolding proteins AF6 and MUPP1 interact with connexin36 and localize at gap junctions that form electrical synapses in rodent brain

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) occur in most major structures in the mammalian central nervous system. These synapses link ensembles of neurons and influence their network properties. Little is known about the macromolecular constituents of neuronal gap junctions or how transmission through electrical synapses is regulated at the level of channel conductance or gap junction assembly/disassembly. Such knowledge is a prerequisite to understanding the roles of gap junctions in neuronal circuitry. Gap junctions share similarities with tight and adhesion junctions in that all three reside at close plasma membrane appositions, and therefore may associate with similar structural and regulatory proteins. Previously, we reported that the tight junction-associated protein zonula occludens-1 (ZO-1) interacts with Cx36 and is localized at gap junctions. Here, we demonstrate that two proteins known to be associated with tight and adherens junctions, namely AF6 and MUPP1, are components of neuronal gap junctions in rodent brain. By immunofluorescence, AF6 and MUPP1 were co-localized with Cx36 in many brain areas. Co-immunoprecipitation and pull-down approaches revealed an association of Cx36 with AF6 and MUPP1, which required the C-terminus PDZ domain interaction motif of Cx36 for interaction with the single PDZ domain of AF6 and with the 10th PDZ domain of MUPP1. As AF6 is a target of the cAMP/Epac/Rap1 signalling pathway and MUPP1 is a scaffolding protein that interacts with CaMKII, the present results suggest that AF6 may be a target for cAMP/Epac/Rap1 signalling at electrical synapses, and that MUPP1 may contribute to anchoring CaMKII at these synapses.

The wake-promoting transmitter histamine preferentially enhances α-4 subunit-containing GABAA receptors

Histamine is an important wake-promoting neurotransmitter that activates seven-transmembrane G-protein coupled histamine receptors. However, histamine demonstrates target promiscuity, including direct interaction with the structurally unrelated glutamate (NMDA) and GABA(A) receptor channels. Previous work showed that histamine enhances the activity of recombinant GABA(A) receptor isoforms typically found in synaptic locations, although co-release of histamine and GABA is not known to occur in vivo. Here we used patch clamp recordings of various recombinant GABA(A) receptor isoforms (α1-6, β1-3, γ1-3, δ) to test the hypothesis that histamine might show subunit preference under low GABA concentration (extrasynaptic) conditions. We found that histamine potentiated the whole-cell responses to GABA for all tested subunit combinations. However, the magnitude of enhancement was largest (∼400% of EC(10) GABA-evoked currents) with α4β3 and α4β3X isoforms, where X could be γ or δ. In contrast, histamine (1 mM) had small effects on prolonging deactivation of α4β3γ2 receptors following brief (5 ms) pulses of 1 mM GABA. These findings suggest GABA-histamine cross-talk may occur preferentially at low GABA concentrations, which could theoretically be inhibitory (via enhancing tonic inhibition), directly excitatory (via enhancing presynaptic GABAergic signaling), or indirectly excitatory (via inhibiting GABAergic interneurons).

Histaminergic responses by hypothalamic neurons that regulate lordosis and their modulation by estradiol

How do fluctuations in the level of generalized arousal of the brain affect the performance of specific motivated behaviors, such as sexual behaviors that depend on sexual arousal? A great deal of previous work has provided us with two important starting points in answering this question: (i) that histamine (HA) serves generalized CNS arousal and (ii) that heightened electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMN) is necessary and sufficient for facilitating the primary female sex behavior in laboratory animals, lordosis behavior. Here we used patch clamp recording technology to analyze HA effects on VMN neuronal activity. The results show that HA acting through H1 receptors (H1R) depolarizes these neurons. Further, acute administration of estradiol, an estrogen necessary for lordosis behavior to occur, heightens this effect. Hyperpolarization, which tends to decrease excitability and enhance inhibition, was not affected by acute estradiol or mediated by H1R but was mediated by other HA receptor subtypes, H2 and H3. Sampling of mRNA from individual VMN neurons showed colocalization of expression of H1 receptor mRNA with estrogen receptor (ER)-alpha mRNA but also revealed ER colocalization with the other HA receptor subtypes and colocalization of different subtypes with each other. The latter finding provides the molecular basis for complex "push-pull" regulation of VMN neuronal excitability by HA. Thus, in the simplest causal route, HA, acting on VMN neurons through H1R provides a mechanism by which elevated states of generalized CNS arousal can foster a specific estrogen-dependent, aroused behavior, sexual behavior.

Ligand-gated chloride channels are receptors for biogenic amines in C. elegans

Biogenic amines such as serotonin and dopamine are intercellular signaling molecules that function widely as neurotransmitters and neuromodulators. We have identified in the nematode Caenorhabditis elegans three ligand-gated chloride channels that are receptors for biogenic amines: LGC-53 is a high-affinity dopamine receptor, LGC-55 is a high-affinity tyramine receptor, and LGC-40 is a low-affinity serotonin receptor that is also gated by choline and acetylcholine. lgc-55 mutants are defective in a behavior that requires endogenous tyramine, which indicates that this ionotropic tyramine receptor functions in tyramine signaling in vivo. Our studies suggest that direct activation of membrane chloride conductances is a general mechanism of action for biogenic amines in the modulation of C. elegans behavior.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Histamine depolarizes neurons in the dorsal vagal complex

We sought to determine whether histamine has effects on single neurons in the dorsal vagal complex of the brainstem since previous studies have suggested a role for histamine receptors in this region. Using whole-cell patch clamp recordings from neurons within the nucleus of the tractus solitarius (NTS) and the dorsal vagal nucleus (DVN), histamine (20 microM) depolarized a small proportion of neurons in these regions accompanied by a decrease in input resistance. Although few neurons were depolarized (21% of NTS neurons and 15% of DVN neurons), those that were affected showed robust depolarizations of 13 mV. These depolarizations were antagonized by the histamine H1 receptor antagonist triprolidine (2 microM) and were subject to a level of desensitization. Neither histamine nor the H3 receptor agonist imetit caused any change in the amplitudes of excitatory or inhibitory postsynaptic potentials elicited in NTS neurons by stimulation of the solitary tract. These data indicate that histamine has a restricted but profound effect on neurons in the dorsal vagal complex.

Indirect mechanism of histamine-induced nociception in temporomandibular joint of rats

A considerable amount of evidence suggests that temporomandibular joint (TMJ) pain associated with temporomandibular disorder results, at least in part, from an inflammatory episode. Although histamine can cause pain, it is not clear whether this mediator induces nociception in the TMJ. In this study, we investigated the contribution of endogenous histamine to formalin-induced nociception in the TMJ of rats. We also investigated whether the administration of histamine induces nociception in the TMJ and, if so, whether this effect is mediated by an indirect action on primary afferent nociceptors. Local administration of the H1-receptor antagonist pyrilamine prevented formalin-induced nociception in the TMJ in a dose-dependent manner. Local administration of histamine (250 microg) in the TMJ induced nociceptive behavior that was inhibited by co-administration of the lidocaine N-ethyl bromide quaternary salt QX-314 (2%) or the selective H1-receptor antagonist pyrilamine (400 microg). Nociception induced by histamine was also inhibited by pre-treatment with sodium cromoglycate (800 microg) and by co-administration of the 5-HT(3) receptor antagonist tropisetron (400 mug), while pyrilamine (400 mug) did not inhibit nociception induced by 5-hydroxytryptamine (5-HT, 250 microg) in the TMJ. Furthermore, histamine, in a dose that did not induce nociception by itself, strongly enhanced 5-HT-induced nociception. Finally, the administration of a sub-threshold dose of 5-HT (100 microg), but not of histamine (100 microg), elicited nociception in the TMJ previously challenged with the inflammatory agent carrageenan (100 microg). In conclusion, these data suggest that histamine induces TMJ nociception by an indirect mechanism involving endogenous release of 5-HT and activation of 5-HT(3) receptors on sensory afferents. It is proposed that histamine activates the H1 receptor to induce the release of 5-HT which depolarizes the nociceptor by activating 5-HT(3) receptor.

Dipsogenic potentiation by sodium chloride but not by sucrose or polyethylene glycol in tuberomammillary-mediated polydipsia

The aim of this study was to examine the dipsogenic mechanisms involved in the recently discovered tuberomammillary (TM)-mediated polydipsia. Rats with bilateral electrolytic lesions of each TM subnucleus underwent several dipsogenic treatments, both osmotic and volemic. Animals with ventral (E2) or medial TM lesions (E3 or E4) showed a potentiated hyperdipsic response to hypertonic sodium chloride administration but not to sucrose or polyethylene glycol treatments. The increase in response to sodium chloride was significantly greater in groups E3/E4 and E2 than in the non-lesioned group and in animals with polydipsia induced by lesion of the median eminence. As previously reported, hyperphagia was induced by lesion to ventral TM nuclei (E1 or E2), confirming a possible role for the TM complex in food intake. However, lesions in medial nuclei (E3 or E4) did not produce this increase in food intake. These results are interpreted in relation to the hypothalamic systems involved in food and water intake.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

Histamine-induced enhancement of vasopressin and oxytocin secretion in rat neurohypophyseal tissue cultures

The effects of histamine (HA) on vasopressin (VP) and oxytocin (OT) secretion were studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP and OT contents of the supernatant were determined by radioimmunoassay (RIA) after a 1 or 2-h incubation. Significantly increased levels of VP and OT production were detected in the tissue culture media following HA administration, depending on the HA dose. The elevation of NH hormone secretion could be partially blocked by previous administration of the HA antagonist mepyramine (MEP, an H1 receptor antagonist) or cimetidine (CIM, an H2 receptor antagonist). Thioperamide (TPE, an H3-H4 receptor antagonist) did not influence the VP or OT secretion increase induced by HA. The application of MEP, CIM or TPE after HA administration proved ineffective. The H1 and H2 receptors are mainly involved in the HA-induced increase of both VP and OT secretion in isolated NH tissue cultures. The results indicate that NH hormone release is influenced directly by the histaminergic system, and the histaminergic control of VP and OT secretion from the NH tissue in rats can occur at the level of the posterior pituitary.

Histamine Excites Neonatal Rat Sympathetic Preganglionic Neurons In Vitro Via Activation of H1 Receptors

The role of histamine in regulating excitability of sympathetic preganglionic neurons (SPNs) and the expression of histamine receptor mRNA in SPNs was investigated using whole-cell patch-clamp electrophysiological recording techniques combined with single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in transverse neonatal rat spinal cord slices. Bath application of histamine (100 μM) or the H1 receptor agonist histamine trifluoromethyl toluidide dimaleate (HTMT; 10 μM) induced membrane depolarization associated with a decrease in membrane conductance in the majority (70%) of SPNs tested, via activation of postsynaptic H1 receptors negatively coupled to one or more unidentified K+ conductances. Histamine and HTMT application also induced or increased the amplitude and/or frequency of membrane potential oscillations in electrotonically coupled SPNs. The H2 receptor agonist dimaprit (10 μM) or the H3 receptor agonist imetit (100 nM) were without significant effect on the membrane properties of SPNs. Histamine responses were sensitive to the H1 receptor antagonist triprolidine (10 μM) and the nonselective potassium channel blocker barium (1 mM) but were unaffected by the H2 receptor antagonist tiotidine (10 μM) and the H3 receptor antagonist, clobenpropit (5 μM). Single cell RT-PCR revealed mRNA expression for H1 receptors in 75% of SPNs tested, with no expression of mRNA for H2, H3, or H4 receptors. These data represent the first demonstration of H1 receptor expression in SPNs and suggest that histamine acts to regulate excitability of these neurons via a direct postsynaptic effect on H1 receptors.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

Histamine modulates thalamocortical activity by activating a chloride conductance in ferret perigeniculate neurons

In the mammalian central nervous system only gamma-aminobutyric acid (GABA) and glycine have been firmly linked to inhibition of neuronal activity through increases in membrane Cl(-) conductance, and these responses are mediated by ionotropic receptors. Iontophoretic application of histamine can also cause inhibitory responses in vivo, although the mechanisms of this inhibition are unknown and may involve pre- or postsynaptic factors. Here, we report that application of histamine to the GABAergic neurons of the thalamic perigeniculate nucleus (PGN), which is innervated by histaminergic fibers from the tuberomammillary nucleus of the hypothalamus, causes a slow membrane hyperpolarization toward a reversal potential of -73 mV through a relatively small increase in membrane conductance to Cl(-). This histaminergic action appears to be mediated by the H(2) subclass of histaminergic receptors and inhibits the single-spike activity of these PGN GABAergic neurons. Application of histamine to the PGN could halt the generation of spindle waves, indicating that increased activity in the tuberomammillary histaminergic system may play a functional role in dampening thalamic oscillations in the transition from sleep to arousal.

AMPA receptor properties and coexpression with sodium-calcium exchangers in rat hypothalamic neurons

The histaminergic tuberomamillary (TM) nucleus, a center for the regulation of wakefulness, is excited by glutamatergic, aminergic and peptidergic inputs. AMPA receptor properties in relation to their expression were investigated in acutely isolated TM neurons with the help of whole-cell patch-clamp recordings combined with single-cell RT-PCR. The mRNAs encoding for the AMPA receptor GluR2 (100% of the neurons) and GluR1 (75%) were the most frequently detected, followed by the mRNA for GluR4 (56%), whereas GluR3 cDNA amplification did not yield a PCR product in any neuron. Flip splice variants prevailed over flop, in keeping with a strong glutamate-response potentiation by cyclothiazide. The expression pattern of AMPA subunits in their two splice variants was correlated with the different subtypes of Na+/Ca2+ (NCX) and Na+/Ca2+/K+ (NCKX) exchangers: glutamate receptor subunits GluR1-4 displayed no coordinated pattern with NCX. However, NCKX2 mRNA occurred only in TM cells with a fast desensitizing glutamate response, where it was coexpressed with the GluR4 subunit in the flop splice variant. NCKX3 mRNA was detected in neurons with fast or slow desensitization of glutamate responses. AMPA receptors in TM neurons were Ca2+-impermeable. As reverse Na+/Ca2+ exchange contributes to the immediate rise in intracellular calcium resulting from glutamate receptor activation, we suggest that the coordinated expression of NCKX2 with the fast desensitizing AMPA receptor-type reflects either a receptor-exchanger coupling or separate mechanisms for maintaining calcium homeostasis in neurons with fast or slow glutamate responses.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

Changes in hippocampal histamine receptors across the hibernation cycle in ground squirrels

Hibernation is a physiological state characterized by a dramatic reduction in various functions, such as body temperature, heart rate, and metabolism. The hippocampus is thought to be important for regulation of the hibernation bout because it remains electrophysiologically active throughout this extremely depressed state. The question arises as to what neuronal influences act within the hippocampus during hibernation to sustain its activity. We hypothesized that histaminergic input might be an important contributor. Brain histamine is involved in functions relevant to hibernation, such as the regulation of diurnal rhythms, body temperature, and energy metabolism. Furthermore, we have previously shown that the histaminergic system appears to be activated during the hibernating state. In this study, we used receptor binding autoradiography, in situ hybridization, and GTP-gamma-S binding autoradiography to study changes in histamine receptors across the hibernation bout. We were able to demonstrate an increase in histamine H1 and H2 receptors in the hippocampus during hibernation, whereas the mRNA expression and receptor density of the inhibitory H3 receptor decreased. Histamine H3 receptors were shown to exhibit both histamine-activated and constitutive GTP-gamma-S-binding activity in the ground squirrel hippocampus, both of which decreased during hibernation, indicating a decrease in H3 receptor G-protein activation. Taken together, our results indicate that histamine may be involved in maintaining hibernation by sustaining hippocampal activity, possibly through H1 and H2 receptor activity and decreased inhibition by H3 receptors. The involvement of brain histamine, which is generally thought of as an arousal molecule, in maintaining a depressed state of the brain suggests a more general role for the amine in controlling arousal state.

Improgan antinociception does not require neuronal histamine or histamine receptors

Improgan, a chemical congener of the H(2) antagonist cimetidine, induces antinociception following intracerebroventricular (i.c.v.) administration in rodents, but the mechanism of action of this compound remains unknown. Because the chemical structure of improgan closely resembles those of histamine and certain histamine blockers, and because neuronal histamine is known to participate in pain-relieving responses, the antinociceptive actions of improgan were evaluated in mice containing null mutations in the genes for three histamine receptors (H(1), H(2), and H(3)) and also in the gene for histidine decarboxylase (the histamine biosynthetic enzyme). Similar to earlier findings in Swiss-Webster mice, improgan induced maximal, reversible, dose-related reductions in thermal nociceptive responses in ICR mice, but neither pre-improgan (baseline) nor post-improgan nociceptive latencies were changed in any of the mutant mice as compared with wild-type controls. Improgan also had weak inhibitory activity in vitro (pK(i)=4.7-4.9) on specific binding to three recently-discovered, recombinant isoforms of the rat H(3) receptor (H(3A), H(3B), and H(3C)). The present findings strongly support the hypothesis that neuronal histamine and its receptors fail to play a role in improgan-induced antinociception.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Intrahippocampal histamine delays arousal from hibernation

Hibernation is a state of extremely reduced physiological functions and a deep depression of CNS activity, which is thought to be under hippocampal control. Our previous findings indicate increased histamine turnover during hibernation in several brain regions, including the hippocampus. In this study we showed that histamine infused into the hippocampus significantly delayed arousal from hibernation. These findings indicate that histamine may contribute to maintaining the hibernating state, suggesting a novel role for histamine in controlling arousal state.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Histamine H1-receptor modulation of inter-neuronal coupling among vasopressinergic neurons depends on nitric oxide synthase activation

Inter-neuronal coupling is a relatively recently documented property of a wide variety of cell groups in the mammalian central nervous system. For many of these groups there is evidence that the coupling can be modulated by synaptic inputs. Incidence of dye coupling among vasopressin (VP) neurons of the rat supraoptic nucleus (SON) has been shown to increase in response to either activation of histamine H(1)-receptors or to increased NO production. Both of these effects involve activation of cGMP-dependent pathways. We tested the hypothesis that activation of H(1)-receptors resulted in downstream activation of NO synthase, which then mediated the H(1)-receptor effects. Putative VP neurons were intracellularly recorded and dye-injected in horizontal slices of hypothalamus, in which monosynaptic connections from the tuberomammillary nucleus (TM) were intact and electrically stimulated. Single-pulse TM stimulation evoked EPSPs and repetitive stimulation resulted in a nearly 3-fold increase in coupling incidence over unstimulated slices. TM-stimulated increased coupling was completely blocked by inhibitors of NO synthase (L-NAME) or of soluble guanylyl cyclase (ODQ or methylene blue), or pyrilamine, suggesting that the H(1)-receptor is not directly linked to guanylyl cyclase. Addition of the NO precursor, L-arginine or the NO donor, SNP, in combination with TM stimulation produced increases in coupling that were not significantly larger than those seen with stimulation alone, supporting the idea that a common pathway was used. We conclude that H(1)-receptors activate NO synthase via G-protein-coupled pathways and that NO working though its receptor, induces the downstream cGMP-dependent processes that result in increased inter-neuronal coupling.

Putative histamine-gated chloride channel subunits of the insect visual system and thoracic ganglion

Histamine-gated chloride channels, members of the ligand-gated ion channel superfamily, are thought to be peculiar for arthropods. Their cognate ligand, histamine, is the transmitter of all arthropod photoreceptors and of thoracic mechanoreceptors. To identify putative histamine-gated chloride channel subunits we scanned the Drosophila genome for putative ligand-gated chloride channel subunits and found 12 candidate genes. We found four groups of transcripts based on their expression pattern. Only members of the last group show an expression pattern that is consistent with our knowledge about histamine-gated chloride channels in insects. In the brain these transcripts (Dm HA-Cl I and II) are exclusively present in interneurones postsynaptic to photoreceptors. Within the lamina (the first visual ganglion) only the L1-L3 neurones are labelled. The lack of non-photoreceptor dependent staining in the brain indicates that mechanosensory transmission differs between the head and the thorax/abdomen, and that the receptors responding to brain-intrinsic histaminergic cells use different signalling pathways. The putative histamine-gated chloride channels show the greatest homology mammalian glycine receptors. These ion-channels are the first specific molecular markers for postsynaptic cells in the insect visual system, thus representing ideal tools to study its physiology and development.

Increases in amino-cupric-silver staining of the supraoptic nucleus after sleep deprivation

Sleep deprived rats undergo a predictable sequence of physiological changes, including changes in skin condition, increased energy expenditure, and altered thermoregulation. Amino-cupric-silver staining was used to identify sleep deprivation related changes in the brain. A significant increase in staining was observed in the supraoptic nucleus (SON) of the hypothalamus of rats with high sleep loss (>45 h) vs. their yoked controls. Follow-up experiments showed that staining was not significantly different in rats sleep deprived for less than 45 h, suggesting that injurious sleep deprivation-related processes occur above a threshold quantity of sleep loss. These anatomical changes suggest that the effects of sleep deprivation may be related to protein metabolism in certain brain regions.

Nitric oxide, interleukin and prostaglandin interactions affecting the magnocellular system

Magnocellular neurons are innervated by an excitatory histaminergic pathway. They also express neuronal NO synthase, interleukin-1beta (IL-1beta) and cyclo-oxygenase (COX). In normally hydrated rats when NO synthase activity is inhibited with N(G)-nitro-L-arginine methyl ester (L-NAME), administered intracerebroventricularly (i.c.v.), OT concentration in plasma increases. In the present study, the increase in hormone after L-NAME is attenuated by indomethacin, an inhibitor of COX, as well as by antagonists of histamine receptors at H1 (pyrilamine) and H2 (cimetidine) subtypes injected i.c.v. Moreover, enhanced OT secretion induced by centrally administered IL-1beta, but not naloxone (opiate receptor antagonist), is prevented by indomethacin. PGE2 and PGD2 (i.c.v.) stimulate OT release, but only PGD2 affects circulating vasopressin levels. Thus, NO inhibits release of OT stimulated by: (1) a COX-dependent mechanism, i.e. NO-->-(COX-->+PG-->+OT release); (2) histamine, i.e. NO-->-(histamine-->H1 and H2 receptors-->+OT release); and possibly (3) IL-1beta, i.e. NO-->-(IL-1beta-->+COX-->+PG-->+OT release). These interactions of NO, cytokine and histamine may be important for management of stress-induced activation of neuroendocrine systems.

Peripartum interneuronal coupling in the supraoptic nucleus

In the supraoptic nucleus (SON), the incidence of conducting gap junctions (gjs), as indicated by dye coupling, is low in cycling females, but dramatically elevated in nursing mothers. Functionally, this is consistent with the well-established presence of synchronous milk ejection bursts among oxytocin neurons only in the lactating rat. In situ hybridization data, however, revealed elevated gj mRNA expression on the last day of pregnancy, a time when burst firing by putative oxytocin neurons is absent. Using Lucifer Yellow dye coupling, we determined the incidence of high conductance gjs in SONs of proestrous, immediately prepartum, postpartum non-lactating, lactating day 1, and lactating day 9-10 rats. Results indicate that coupling incidence is high only at times when milk ejection bursts are known to occur, and that the elevated gj mRNA expression seen on the last day of pregnancy does not indicate conducting gjs. It is suggested that gj conductance states, but not gj expression, are modulated by plasma estradiol titers.

The histaminergic system in human thalamus: correlation of innervation to receptor expression

The mRNA expression of three histamine receptors (H1, H2 and H3) and H1 and H3 receptor binding were mapped and quantified in normal human thalamus by in situ hybridization and receptor binding autoradiography, respectively. Immunohistochemistry was applied to study the distribution of histaminergic fibres and terminals in the normal human thalamus. mRNAs for all three histamine receptors were detected mainly in the dorsal thalamus, but the expression intensities were different. Briefly, H1 and H3 receptor mRNAs were relatively enriched in the anterior, medial, and part of the lateral nuclei regions; whereas the expression level was much lower in the ventral and posterior parts of the thalamus, and the reticular nucleus. H2 receptor mRNA displayed in general very low expression intensity with slightly higher expression level in the anterior and lateropolar regions. H1 receptor binding was mainly detected in the mediodorsal, ventroposterolateral nuclei, and the pulvinar. H3 receptor binding was detected mainly in the dorsal thalamus, predominantly the periventricular, mediodorsal, and posterior regions. Very high or high histaminergic fibre densities were observed in the midline nuclear region and other nuclei next to the third ventricle, ventroposterior lateral nucleus and medial geniculate nucleus. In most of the core structures of the thalamus, the fibre density was very low or absent. The results suggest that histamine in human brain regulates tactile and proprioceptory thalamocortical functions through multiple receptors. Also, other, e.g. visual areas and those not making cortical connections expressed histamine receptors and contained histaminergic nerve fibres.

Identification of two novel Drosophila melanogaster histamine-gated chloride channel subunits expressed in the eye

Histamine has been shown to play a role in arthropod vision; it is the major neurotransmitter of arthropod photoreceptors. Histamine-gated chloride channels have been identified in insect optic lobes. We report the first isolation of cDNA clones encoding histamine-gated chloride channel subunits from the fruit fly Drosophila melanogaster. The encoded proteins, HisCl1 and HisCl2, share 60% amino acid identity with each other. The closest structural homologue is the human glycine alpha3 receptor, which shares 45 and 43% amino acid identity respectively. Northern hybridization analysis suggested that hisCl1 and hisCl2 mRNAs are predominantly expressed in the insect eye. Oocytes injected with in vitro transcribed RNA, encoding either HisCl1 or HisCl2, produced substantial chloride currents in response to histamine but not in response to GABA, glycine, and glutamate. The histamine sensitivity was similar to that observed in insect laminar neurons. Histamine-activated currents were not blocked by picrotoxinin, fipronil, strychnine, or the H2 antagonist cimetidine. Co-injection of both hisCl1 and hisCl2 RNAs resulted in expression of a histamine-gated chloride channel with increased sensitivity to histamine, demonstrating coassembly of the subunits. The insecticide ivermectin reversibly activated homomeric HisCl1 channels and, more potently, HisCl1 and HisCl2 heteromeric channels.