Molecular properties and signalling pathways of the histamine H1 receptor

With cloning of the gene encoding the histamine H₁ receptor, a new area of histamine research has become reality. Finally, it seems feasible to study the target of the thera-peutically important clans of antihistamine. Expression of the genes in mammalian cells allows detailed investigations of the various signal transduction routes of the histamine H₁ receptor. Moreover, using molecular biological techniques, it is now possible to investigate ligand receptor interaction at the molecular level. Studies with mutant H₁ receptors have shown that H₁ antagonists bind to a specific amino acid residues in TM3 and 5. It is expected that these new developments will provide much fundamental knowledge on the ligand interaction with the H₁ receptor.     

Cell type-specific dependence of muscarinic signalling in mouse hippocampal stratum oriens interneurones

Cholinergic signalling is critically involved in learning and memory processes in the hippocampus, but the postsynaptic impact of cholinergic modulation on morphologically defined subtypes of hippocampal interneurones remains unclear. We investigated the influence of muscarinic receptor (mAChR) activation on stratum oriens interneurones using whole-cell patch clamp recordings from hippocampal slices in vitro . Upon somatic depolarization, mAChR activation consistently enhanced firing frequency and produced large, sustained afterdepolarizations (ADPs) of stratum oriens–lacunosum moleculare (O-LM) interneurones. In contrast, stratum oriens cell types with axon arborization patterns different from O-LM cells not only lacked large muscarinic ADPs but also appeared to exhibit distinct responses to mAChR activation. The ADP in O-LM cells, mediated by M₁/M₃ receptors, was associated with inhibition of an M current, inhibition of a slow calcium-activated potassium current, and activation of a calcium-dependent non-selective cationic current ( I _CAT). An examination of ionic conductances generated by firing revealed that calcium entry through I _CAT controls the emergence of the mAChR-mediated ADP. Our results indicate that cholinergic specializations are present within anatomically distinct subpopulations of hippocampal interneurones, suggesting that there may be organizing principles to cholinergic control of GABA release in the hippocampus.     

Intraseptal muscarinic ligands and galanin: influence on hippocampal acetylcholine and cognition

The cholinergic neurons in the septohippocampal projection are implicated in hippocampal functions such as spatial learning and memory. The aim of this study was to examine how septohippocampal cholinergic transmission is modulated by muscarinic inputs and by the neuropeptide galanin, co-localized with acetylcholine (ACh) in septohippocampal cholinergic neurons, and how spatial learning assessed by the Morris water maze test is affected. Muscarinic inputs to the septal area are assumed to be excitatory, whereas galanin is hypothesized to inhibit septohippocampal cholinergic function. To test these hypotheses, compounds were microinjected into the medial septum and hippocampal ACh release was assessed by microdialysis probes in the ventral hippocampus of the rat. Blockade of septal muscarinic transmission by intraseptal scopolamine increased hippocampal ACh release suggesting that septal cholinergic neurons are under tonic inhibition. Stimulation of septal muscarinic receptors by carbachol also increased hippocampal ACh release. Despite this increase, both scopolamine and carbachol tended to impair hippocampus-dependent spatial learning. This finding also suggests a revision of the simplistic notion that an increase in hippocampal ACh may be facilitatory for learning and memory. Galanin infused into the medial septum enhanced hippocampal ACh release and facilitated spatial learning, suggesting that septal galanin, contrary to earlier claims, does not inhibit but excites septohippocampal cholinergic neurons. Galanin receptor stimulation combined with muscarinic blockade in the septal area resulted in an excessive increase of hippocampal ACh release combined with an impairment of spatial learning. This finding suggests that the level of muscarinic activity within the septal area may determine the effects of galanin on hippocampal cognitive functions. In summary, a limited range of cholinergic muscarinic transmission may contribute to optimal hippocampal function, a finding that has important implications for therapeutic approaches in the treatment of disorders of memory function.     

Reversible inhibition of GABAA receptors by α7-containing nicotinic receptors on the vertebrate postsynaptic neurons

Nicotinic acetylcholine receptors (nAChRs) are expressed throughout the central nervous system and influence a variety of higher order functions including learning and memory. While the effects of presynaptic nAChRs on transmitter release have been well documented, little is known about possible postsynaptic actions. A major species of neuronal nAChRs contains the α7 gene product and has a high relative permeability to calcium. Both on rodent hippocampal interneurons and on chick ciliary ganglion neurons these α7-nAChRs are often closely juxtaposed to GABA_A receptors. We show here that in both cases activation of α7-nAChRs on the postsynaptic neuron acutely down-regulates GABA-induced currents. Nicotine application to dissociated ciliary ganglion neurons diminished subsequent GABA_A receptor responses to GABA. The effect was blocked by α7-nAChR antagonists, by chelation of intracellular Ca²⁺ with BAPTA, and by inhibition of both Ca²⁺–calmodulin-dependent protein kinase II and mitogen-activated protein kinase. A similar outcome was obtained in the hippocampus where electrical stimulation to activate cholinergic fibres reduced the amplitude of subsequent GABA_A receptor-mediated inhibitory postsynaptic currents. The reduction showed the same calcium and kinase dependence seen in ciliary ganglion neurons and was absent in hippocampal slices from α7-nAChR knockout mice. Moreover, α7-nAChR blockade in hippocampal slices reduced rundown of GABA_A receptor-mediated whole-cell responses, indicating ongoing endogenous modulation. The results demonstrate regulation of GABA_A receptors by α7-nAChRs on the postsynaptic neuron and identify a new mechanism by which nicotinic cholinergic signalling influences nervous system function.     

New findings in pharmacological effects induced by antihistamines: from PET studies to knock-out mice

Antihistamines are efficacious drugs to be used for the symptomatic relief of allergic diseases. The safety issue of antihistamines is of central importance because of their widespread use in current medical practice. To better understand the pharmacological effects of antihistamines on the central nervous system (CNS), we used two kinds of new methods, positron emission tomography (PET) and gene targeting regarding on histamine H₁ receptors. The histamine H₁ receptor occupancy was examined in young male volunteers with[¹¹C]-doxepin (a potent H₁ antagonist) after the oral or intravenous administration of antihistamines. In other studies, the cognitive performance was also measured tachistoscopically before and after taking antihistamines. The mutant mice lacking H₁ receptors were used in the behavioural and neurochemical experiments to re-evaluate the role of H₁ receptors. The H₁-receptor occupancy in the human frontal cortex caused by antihistamines is significantly correlated with the reported values of incidence of sleepiness in clinical trials, and the occupancy is well proportional to the impaired cognitive performance. The behavioural studies of the H₁-receptor knock-out mice confirmed the role of H₁ receptors in arousal, the sleep–wake cycle, locomotion, nociception and aggressive behaviour. The pharmacological effects induced by H₁ antagonism were re-evaluated by the PET and gene-targetting. Although any serious effects could not be observed in mice by the destruction of the H₁-receptor gene, the cognitive performance was impaired in humans after taking first generation antihistamines in recommended doses.     

Induction by kainate of theta frequency rhythmic activity in the rat medial septum–diagonal band complex in vitro

The medial septum–diagonal band (MSDB) complex, via the septohippocampal pathway, is thought to be critical for the generation and/or maintenance of the hippocampal theta rhythm in vivo . The aim was to determine whether the MSDB is capable of generating and maintaining its own rhythmic firing activity, a mechanism by which it could impose a theta frequency oscillatory activity on the hippocampus. Bath application of 50–300 n m kainate to an in vitro preparation of 20- to 25-day-old rat MSDB elicited rhythmic extracellular field activity primarily within the theta frequency band (4–12 Hz). This activity was observed both at 33°C and at 37°C, and was localized to the midline part of the MSDB that is rich in parvalbumin-containing neurones. The application of neurotransmitter receptor antagonists and putative gap junction blockers showed that the oscillatory field activity was dependent upon the activation of GABA_A receptors and possibly gap junctions, but not on the activation of NMDA, GABA_B, muscarinic or nicotinic receptors. The frequency of the oscillatory activity was reduced by the application of diazepam or low doses of baclofen. Intracellular recording showed that concomitant action potential firing activity in putative GABAergic and cholinergic neurone populations was of a single spiking rather than a bursting firing nature, and was coherent with extracellularly recorded oscillatory field activity. We conclude that kainate activation of neuronal circuitry in the MSDB is capable of synchronization of rhythmic activity in the MSDB, and that this may underlie the mechanism for phase-locking rhythmic burst activity in the MSDB in vivo .     

Orexin-saporin lesions of the medial septum impair spatial memory

The medial septum and diagonal band of Broca (MSDB) provide a major input to the hippocampus and are important for spatial learning and memory. Although electrolytic MSDB lesions have prominent memory impairing effects, selective lesions of either cholinergic or GABAergic MSDB neurons do not or only mildly impair spatial memory. MSDB neurons are targets of orexin-containing neurons from the hypothalamus. At present, the functional significance of orexin afferents to MSDB is unclear, and the present study investigated a possible involvement of orexin innervation of the MSDB in spatial memory. Orexin-saporin, a toxin that damages neurons containing the hypocretin-2 receptor, was administered into the MSDB of rats. Rats were subsequently tested on a water maze to assess spatial reference memory and a plus maze to assess spatial working memory. At 100 ng/microl, orexin-saporin destroyed primarily GABAergic septohippocampal neurons, sparing the majority of cholinergic neurons. At 200 ng/microl, orexin-saporin almost totally eliminated GABAergic septohippocampal neurons and destroyed many cholinergic neurons. Spatial reference memory was impaired at both concentrations of orexin-saporin with a dramatic impairment observed for 24-h retention. Short-term reference memory was also impaired at both concentrations. Rats treated with 200 ng/microl, but not 100 ng/microl, of orexin-saporin were also impaired on a spontaneous alternation task, showing a deficit in spatial working memory. Our results, together with previous studies, suggest that orexin innervation of the MSDB may modulate spatial memory by acting on both GABAergic and cholinergic septohippocampal neurons.     

Group I metabotropic glutamate receptor activation produces a direct excitation of identified septohippocampal cholinergic neurons

Septohippocampal cholinergic neurons innervate the hippocampus and provide it with almost its entire acetylcholine. Axon collaterals of these neurons also release acetylcholine within the septum and thereby maintain the firing activity of septohippocampal GABAergic neurons. A loss of septohippocampal cholinergic neurons occurs in various neurodegenerative disorders associated with cognitive dysfunctions. group I metabotropic glutamate receptors have been implicated in septohippocampal-dependent learning and memory tasks. In the present study, we examined the physiological and pharmacological effects of a potent and selective group I metabotropic glutamate receptor (mGluR) agonist S-3,5-dihydroxyphenylglycine (DHPG) on rat septohippocampal cholinergic neurons that were identified in brain slices using a selective fluorescent marker. In whole cell recordings, DHPG produced a reversible, reproducible and a direct postsynaptic and concentration-dependent excitation in 100% of septohippocampal cholinergic neurons tested with an EC(50) of 2.1 microM. Pharmacologically, the effects of DHPG were partially/completely reduced by the mGluR1 antagonists, 7-hydrox-iminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester and (+)-2-methyl-4-carboxyphenylglycine. Addition of the mGluR5 antagonist, 2-methyl-6-(phenylethnyl)pyridine hydrochloride, reduced the remaining response to DHPG, suggesting involvement of both receptor subtypes in a subpopulation of septohippocampal cholinergic neurons. In double-immunolabeling studies, 74% of septohippocampal cholinergic neurons co-localized mGluR1alpha-immunoreactivity and 35% co-localized mGluR5-immunoreactivity. Double-immunolabeling studies at the light and electron-microscopic levels showed that vesicular glutamate transporter 2 terminals make asymmetric synaptic contacts with septohippocampal cholinergic neurons. These findings may be of significance in treatment of cognitive deficits associated with neurodegenerative disorders as a group I mGluR-mediated activation of septohippocampal cholinergic neurons would enhance the release of acetylcholine both in the hippocampus and in the septum.     

Activity-dependent bidirectional regulation of GABAA receptor channels by the 5-HT4 receptor-mediated signalling in rat prefrontal cortical pyramidal neurons

Emerging evidence has implicated a potential role for 5-HT₄ receptors in cognition and anxiolysis. One of the main target structures of 5-HT₄ receptors on 'cognitive and emotional' pathways is the prefrontal cortex (PFC). As GABAergic signalling plays a key role in regulating PFC functions, we examined the effect of 5-HT₄ receptors on GABA_A receptor channels in PFC pyramidal neurons. Application of 5-HT₄ receptor agonists produced either an enhancement or a reduction of GABA-evoked currents in PFC neurons, which are both mediated by anchored protein kinase A (PKA). Although PKA phosphorylation of GABA_A receptor β3 or β1 subunits leads to current enhancement or reduction respectively in heterologous expression systems, we found that β3 and β1 subunits are co-expressed in PFC pyramidal neurons. Interestingly, altering PKA activation levels can change the direction of the dual effect, switching enhancement to reduction and vice versa. In addition, increased neuronal activity in PFC slices elevated the PKA activation level, changing the enhancing effect of 5-HT₄ receptors on the amplitude of GABAergic inhibitory postsynaptic currents (IPSCs) to a reduction. These results suggest that 5-HT₄ receptors can modulate GABAergic signalling bidirectionally, depending on the basal PKA activation levels that are determined by neuronal activity. This modulation provides a unique and flexible mechanism for 5-HT₄ receptors to dynamically regulate synaptic transmission and neuronal excitability in the PFC network.     

Disinhibition of perifornical hypothalamic neurones activates noradrenergic neurones and blocks pontine carbachol-induced REM sleep-like episodes in rats

Studies in behaving animals suggest that neurones located in the perifornical (PF) region of the posterior hypothalamus promote wakefulness and suppress sleep. Among such cells are those that synthesize the excitatory peptides, orexins (ORX). Lack of ORX, or their receptors, is associated with narcolepsy/cataplexy, a disorder characterized by an increased pressure for rapid eye movement (REM) sleep. We used anaesthetized rats in which pontine microinjections of a cholinergic agonist, carbachol, can repeatedly elicit REM sleep-like episodes to test whether activation of PF cells induced by antagonism of endogenous, GABA_A receptor-mediated, inhibition suppresses the ability of the brainstem to generate REM sleep-like state. Microinjections of the GABA_A receptor antagonist, bicuculline (20 nl, 1 m m ), into the PF region elicited cortical and hippocampal activation, increased the respiratory rate and hypoglossal nerve activity, induced c-fos expression in ORX and other PF neurones, and increased c-fos expression in pontine A7 and other noradrenergic neurones. The ability of pontine carbachol to elicit any cortical, hippocampal or brainstem component of the REM sleep-like response was abolished during the period of bicuculline-induced activation. The activating and REM sleep-suppressing effect of PF bicuculline was not attenuated by systemic administration of the ORX type 1 receptor antagonist, SB334867. Thus, activation of PF neurones that are endogenously inhibited by GABA_A receptors is sufficient to turn off the brainstem REM sleep-generating network; the effect is, at least in part, due to activation of pontine noradrenergic neurones, but is not mediated by ORX type 1 receptors. A malfunction of the pathway that originates in GABA_A receptor-expressing PF neurones may cause narcolepsy/cataplexy.     

Convergent control of synaptic GABA release from rat dorsal horn neurones by adenosine and GABA autoreceptors

Perforated patch clamp recordings were performed on cultured superficial neonatal rat dorsal horn (DH) spinal cord neurones in order to study the presynaptic modulation of GABA release at unitary synaptic connections. Since ATP can be coreleased with GABA at about two-thirds of GABAergic synapses between DH neurones, and can be rapidly metabolized to adenosine in the extracellular space, we investigated the potential role of A1 adenosine receptors and GABA_B receptors which might function as inhibitory autoreceptors. Adenosine and GABA_B receptor agonists reduced the amplitude of electrically evoked GABAergic inhibitory postsynaptic currents (eIPSCs) as well as the frequency of GABAergic miniature IPSCs, suggesting a presynaptic action of these substances. The actions of adenosine were blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The effects of adenosine and GABA_B agonists were occlusive, indicating a functional convergence of the signalling pathways engaged by A1 and GABA_B receptors. A1 and GABA_B antagonists increased the amplitude of eIPSCs in a supra-additive manner, suggesting a tonic activation of these receptors by ambient adenosine and GABA. Moreover, using trains of electrical stimulations, we were able to unravel a phasic (activity-dependent) activation of presynaptic A1 and GABA_B autoreceptors only in the case of neurones coreleasing ATP and GABA, despite the presence of functional presynaptic A1 and GABA_B receptors on all GABAergic DH neurones. This selective, convergent and activity-dependent inhibition of GABA release by A1 and GABA_B autoreceptors might modulate the integrative properties of postsynaptic DH neurones under physiological conditions and/or during the development of pathological pain states.     

Is antihistamine (H1-receptor antagonist) therapy useful in clinical asthma?

Airway hyperresponsiveness to histamine is a hallmark of asthma, and histamine inhalation reproduces asthma symptoms. Plasma histamine concentrations are elevated during the early and late responses to inhaled allergens, and may also increase during spontaneous acute asthma episodes. Ordinary doses of currently available antihistamines (H₁-receptor antagonists) have minimal bronchodilator and bronchoprotective activity. In severe persistent asthma, H₁ antagonists have no significant clinical effect. In moderate persistent asthma, clinical benefits of H₁ antagonists are apparent, but may not be worth the potential risks of the higher-than-usual doses required. When mild seasonal asthma and allergic rhinitis coexist, however, relief of rhinitis symptoms with H₁ antagonists administered in ordinary dose is associated with significant improvement in asthma symptoms.     

Possible Role of Histamine in Rheumatoid Arthritis

Basophilocytes from patients with rheumatoid arthritis (RA) responded to leukocyte nuclei from normal persons with histamine release; a similar histamine release induced by the nuclear components RNA and DNA has been demonstrated previously. A role of histamine in RA is also supported by the findings of clinical improvement during treatment with H₁ and H₂ antihistamines in six of 12 patients with RA in active phase, whereas four showed definite deterioration.     

Profound regulation of neonatal CA1 rat hippocampal GABAergic transmission by functionally distinct kainate receptor populations

Neonatal hippocampus exhibits distinct patterns of network activity that are dependent on the interaction between inhibitory and excitatory transmission. Kainate receptors are ideally positioned to regulate this activity by virtue of their ability to regulate presynaptic function in GABAergic interneurones. Indeed, kainate receptors are highly expressed in neonatal hippocampal interneurones, yet the role and mechanisms by which they might regulate neonatal circuitry are unexplored. To address this we investigated the kainate receptor-dependent regulation of GABAergic transmission onto neonatal CA1 pyramidal neurones. Kainate receptor activation produced two distinct opposing effects, a very large increase in the frequency of spontaneous IPSCs, and a robust depression of evoked GABAergic transmission. The up-regulation of spontaneous transmission was due to activation of somatodendritic and axonal receptors while the depression of evoked transmission could be fully accounted for by a direct regulation of GABA release by kainate receptors located at the terminals. None of the effects of kainate receptor agonists were sensitive to GABA_B receptor antagonists, nor was there any postsynaptic kainate receptor-dependent effects observed in CA1 pyramidal cells that could account for our findings. Our data demonstrate that kainate receptors profoundly regulate neonatal CA1 GABAergic circuitry by two distinct opposing mechanisms, and indicate that these two effects are mediated by functionally distinct populations of receptors. Thus kainate receptors are strategically located to play a critical role in shaping early hippocampal network activity and by virtue of this have a key role in hippocampal development.     

Cortical serotonin receptor subtypes after lesion of ascending cholinergic neurones in rat

Serotonin (5-HT) S₁ and S₂ receptors were studied in rat cortex after lesion of the ascending cholinergic systems by injection of ibotenic acid into the nucleus basalis. The lesions produced a large (56%) decrease in choline acetyltransferase activity with no change in markers of γ-aminobutyric acid, dopamine and 5-HT containing neurones. Lesions of the ascending cholinergic neurones were accompanied by a loss of 5-HT S₁ receptor binding sites with no change in S₂ receptors. These results suggest that a proportion of S₁ receptors may be associated with cholinergic terminals.     

Localization and function of histamine H3 receptor in the nasal mucosa

Background Histamine is an important chemical mediator of allergic rhinitis (AR). Histamine H₃ receptors (H₃R) are located on cholinergic and NANC neurons of the myenteric plexus, and activation of H₃R regulates gastric acid secretion. However, little is known about the localization and function of H₃R in the upper airway.
Objective The objective of this study was to examine the localization and possible function of H₃R in the nasal mucosa.
Methods We extracted total RNA from the inferior turbinate mucosa of patients with AR. H₃R mRNA and β-actin mRNA were amplified by RT-PCR. We used immunohistochemistry to examine localization of H₃R protein in the inferior turbinate mucosa excised during clinically indicated surgery. We used alcian blue/periodic acid-shiff staining to examine the effects of the H₃R agonist (R)-α-methylhistamine and the H₃R antagonist thioperamide on secretion from rat submucosal glands.
Results H₃R protein was expressed around submucosal gland cells. Thioperamide induced degranulation in the submucosal gland in the nasal septum.
Conclusion The present results suggest that H₃R is localized mainly around submucosal glands, and that H₃R plays an important role in the secretion of submucosal glands in the nose.     

Anaphylactic reaction to ranitidine

Ranitidine is a well-tolerated H₂-receptor antagonist with a furan ring a nucleus. Anaphylactic reactions are seldom reported despite the wide use the drug. We report a patient who presented an anaphylactic reaction with ranitidine (Zantac). The positive skin prick test and oral challenge suggest type I hypersensitivity. Specific IgE determination by RAST technique and histamine release test with ranitidine were negative. The patient did not react to other H₂-receptor antagonists or to another furan-derivative (nitrofurantoin).     

NMDA receptors trigger neurosecretion of 5-HT within dorsal raphé nucleus of the rat in the absence of action potential firing

Activity and calcium-dependent release of neurotransmitters from the somatodendritic compartment is an important signalling mechanism between neurones throughout the brain. NMDA receptors and vesicles filled with neurotransmitters occur in close proximity in many brain areas. It is unknown whether calcium influx through these receptors can trigger the release of somatodendritic vesicles directly, or whether postsynaptic action potential firing is necessary for release of these vesicles. Here we addressed this question by studying local release of serotonin (5-HT) from dorsal raphé nucleus (DRN) neurones. We performed capacitance measurements to monitor the secretion of vesicles in giant soma patches, in response to short depolarizations and action potential waveforms. Amperometric measurements confirmed that secreted vesicles contained 5-HT. Surprisingly, two-photon imaging of DRN neurones in slices revealed that dendritic calcium concentration changes in response to somatic firing were restricted to proximal dendritic areas. This implied that alternative calcium entry pathways may dominate the induction of vesicle secretion from distal dendrites. In line with this, transient NMDA receptor activation, in the absence of action potential firing, was sufficient to induce capacitance changes. By monitoring GABAergic transmission onto DRN 5-HT neurones in slices, we show that endogenous NMDA receptor activation, in the absence of postsynaptic firing, induced release of 5-HT, which in turn increased the frequency of GABAergic inputs through activation of 5-HT₂ receptors. We propose here that calcium influx through NMDA receptors can directly induce postsynaptic 5-HT release from DRN neurones, which in turn may facilitate GABAergic input onto these cells.