Corneal acetylcholine regulates sensory nerve activity via nicotinic receptors

PURPOSE

Sensory nerve terminals are highly distributed in the cornea, and regulate ocular surface sensation and homeostasis in response to various endogenous and exogenous stimuli. However, little is known about mediators regulating the physiological and pathophysiological activities of corneal sensory nerves. The aim of this study was to investigate the presence of cholinergic regulation in sensory nerves in the cornea.

METHODS

Localization of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (vAChT) was evaluated using western blotting and immunohistochemical analysis. The synthesis and liberation of acetylcholine from the cornea were assessed using corneal segments pre-incubated with [3H]choline. The responsiveness of corneal neurons and nerves to cholinergic drugs was explored using calcium imaging with primary cultures of trigeminal ganglion neurons and extracellular recording from corneal preparations in guinea pigs.

RESULTS

ChAT, but not vAChT, was highly distributed in the corneal epithelium. In corneal segments, [3H] acetylcholine was synthesized from [3H]choline, and was also released in response to electrical stimuli. In cultured corneal neurons, the population sensitive to a transient receptor potential melastatin 8 (TRPM8) agonist exhibited high probability of responding to nicotine in a calcium imaging experiment. The firing frequency of cold-sensitive corneal nerves was increased by the application of nicotine, but diminished by an α4 nicotinic acetylcholine receptor antagonist.

CONCLUSIONS

The corneal epithelium can synthesize and release acetylcholine. Corneal acetylcholine can excite sensory nerves via nicotinic receptors containing the α4 subunit. Therefore, corneal acetylcholine may be one of the important regulators of corneal nerve activity arranging ocular surface condition and sensation.

Acetylcholine release from striatal cholinergic interneurons is controlled differently depending on the firing pattern

How is the quantal size in neurotransmitter release adjusted for various firing levels? We explored the possible mechanisms that regulate acetylcholine (ACh) release from cholinergic interneurons using an ultra-mini superfusion system. After preloading [3 H]ACh in rat striatal cholinergic interneurons, the release was elicited by electrical stimulation under a condition in which presynaptic cholinergic and dopaminergic feedback was inhibited. [3 H]ACh release was reproducible at intervals of more than 10 min; shorter intervals resulted in reduced levels of ACh release. Upon persistent stimulation for 10 min, ACh release transiently increased, before gradually decreasing. Vesamicol, an inhibitor of the vesicular ACh transporter (VAChT), had no effect on the release induced by the first single pulse, but it reduced the release caused by subsequent pulses. Vesamicol also reduced the [3 H]ACh release evoked by multiple pulses, and the inhibition was enhanced by repetitive stimulation. The decreasing phase of [3 H]ACh release during persistent stimulation was accelerated by vesamicol treatment. Thus, it is likely that releasable ACh was slowly compensated for via VAChT during and after stimulation, changing the vesicular ACh content. In addition, ACh release per pulse decreased under high-frequency stimulation. The present results suggest that ACh release from striatal cholinergic interneurons may be adjusted by changes in the quantal size due to slow replenishment via VAChT, and by a reduction in release probability upon high-frequency stimulation. These two distinct processes likely enable the fine tuning of neurotransmission and neuroprotection/limitation against excessive output and have important physiological roles in the brain.

Role of Muscarinic Acetylcholine Receptors in Intestinal Epithelial Homeostasis: Insights for the Treatment of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is an intestinal disorder that causes prolonged inflammation of the gastrointestinal tract. Currently, the etiology of IBD is not fully understood and treatments are insufficient to completely cure the disease. In addition to absorbing essential nutrients, intestinal epithelial cells prevent the entry of foreign antigens (micro-organisms and undigested food) through mucus secretion and epithelial barrier formation. Disruption of the intestinal epithelial homeostasis exacerbates inflammation. Thus, the maintenance and reinforcement of epithelial function may have therapeutic benefits in the treatment of IBD. Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors for acetylcholine that are expressed in intestinal epithelial cells. Recent studies have revealed the role of mAChRs in the maintenance of intestinal epithelial homeostasis. The importance of non-neuronal acetylcholine in mAChR activation in epithelial cells has also been recognized. This review aimed to summarize recent advances in research on mAChRs for intestinal epithelial homeostasis and the involvement of non-neuronal acetylcholine systems, and highlight their potential as targets for IBD therapy.

Evaluation of radiolabeled acetylcholine synthesis and release in rat striatum

Cholinergic transmission underlies higher brain functions such as cognition and movement. To elucidate the process whereby acetylcholine (ACh) release is maintained and regulated in the central nervous system, uptake of [³ H]choline and subsequent synthesis and release of [³ H]ACh were investigated in rat striatal segments. Incubation with [³ H]choline elicited efficient uptake via high-affinity choline transporter-1, resulting in accumulation of [³ H]choline and [³ H]ACh. However, following inhibition of ACh esterase (AChE), incubation with [³ H]choline led predominantly to the accumulation of [³ H]ACh. Electrical stimulation and KCl depolarization selectively released [³ H]ACh but not [³ H]choline. [³ H]ACh release gradually declined upon repetitive stimulation, whereas the release was reproducible under inhibition of AChE. [³ H]ACh release was abolished after treatment with vesamicol, an inhibitor of vesicular ACh transporter. These results suggest that releasable ACh is continually replenished from the cytosol to releasable pools of cholinergic vesicles to maintain cholinergic transmission. [³ H]ACh release evoked by electrical stimulation was abolished by tetrodotoxin, but that induced by KCl was largely resistant. ACh release was Ca²⁺ dependent and exhibited slightly different sensitivities to N- and P-type Ca²⁺ channel toxins (ω-conotoxin GVIA and ω-agatoxin IVA, respectively) between both stimuli. [³ H]ACh release was negatively regulated by M2 muscarinic and D2 dopaminergic receptors. The present results suggest that inhibition of AChE within cholinergic neurons and of presynaptic negative regulation of ACh release contributes to maintenance and facilitation of cholinergic transmission, providing a potentially useful clue for the development of therapies for cholinergic dysfunction-associated disorders, in addition to inhibition of synaptic cleft AChE.

β1-Adrenergic cardiac contractility is increased during early endotoxemic shock: Involvement of cyclooxygenases

AIMS

Endothelial dysfunction is one of the earliest symptoms in septic patients and plays an important role in the cardiovascular alterations. However, the endothelial mechanisms involved in the impaired sympathetic regulation of the cardiovascular system are not clear. This study aimed to determine the role of the endocardial endothelium (EE) in the cardiac β-adrenergic (β-AR) remodeling at the early phase of endotoxemic shock.

MAIN METHODS

Rats received either lipopolysaccharide (LPS) or saline (control) intravenously. Three hours later, β-AR cardiac contractility was evaluated on papillary muscles with or without a functional EE.

KEY FINDINGS

Isoproterenol-induced contractility was strongly increased in papillary muscles from LPS rats. A similar increase was observed with a β₁-AR stimulation, whereas β₂-AR and β₃-AR produced similar contractility in control and LPS treatments. The removal of the EE did not modify β₁-AR-induced contractility in controls, whereas it abolished the increased β₁-AR response in LPS-treated muscles. In LPS-treated papillary muscle, the increased β₁-AR-induced contractility was not modified by pretreatment with a NOS inhibitor or an endothelin receptor antagonist. Conversely, the increased β₁-AR-induced contractility was abolished by indomethacin, a non-selective cyclooxygenase (COX) inhibitor, as well as by selective inhibitors of COX1 and COX2. An early treatment with indomethacin improved the survival of LPS rat.

SIGNIFICANCE

Our results suggest that the EE is involved in the increased cardiac β₁-AR contractility in the early phase of endotoxemic shock. This effect is mediated through the activation of COX1 and COX2 and suggests these may be novel putative therapeutic targets during endotoxemic shock.

Store-operated calcium entry (SOCE) contributes to phosphorylation of p38 MAPK and suppression of TNF-α signalling in the intestinal epithelial cells

Calcium influx via store-operated calcium entry (SOCE) has an important role for regulation of vast majority of cellular physiological events. MAPK signalling is also another pivotal modulator of many cellular functions. However, the relationship between SOCE and MAPK is not well understood. In this study, we elucidated the involvement of SOCE in Gα_q/11 protein-mediated activation of p38 MAPK in an intestinal epithelial cell line HT-29/B6. In this cell line, we previously showed that the stimulation of M3 muscarinic acetylcholine receptor (M3-mAChR) but not histamine H1 receptor (H1R) led to phosphorylation of p38 MAPK which suppressed tumor necrosis factor-α (TNF-α)-induced NF-κB signalling through ADAM17 protease-mediated shedding of TNF receptor-1 (TNFR1). First, we found that stimulation of M3-mAChR and protease-activated receptor-2 (PAR-2) but not H1R induced persistent upregulation of cytosolic Ca²⁺ concentration through SOCE. Activation of M3-mAChR or PAR-2 also suppressed TNF-α-induced NF-κB phosphorylation, which was dependent on the p38 MAPK activity. Time course experiments revealed that M3-mAChR stimulation evoked intracellular Ca²⁺-dependent early phase p38 MAPK phosphorylation and extracellular Ca²⁺-dependent later phase p38 MAPK phosphorylation. This later phase p38 MAPK phosphorylation, evoked by M3-mAChRs or PAR-2, was abolished by inhibition of SOCE. Thapsigargin or ionomycin also phosphorylate p38 MAPK by Ca²⁺ influx through SOCE, leading to suppression of TNF-α-induced NF-κB phosphorylation. Finally, we showed that p38 MAPK was essential for thapsigargin-induced cleavage of TNFR1 and suppression of TNF-α-induced NF-κB phosphorylation. In conclusion, SOCE is important for p38 MAPK phosphorylation and is involved in TNF-α signalling suppression.

Augmentation of Endogenous Acetylcholine Uptake and Cholinergic Facilitation of Hippocampal Long-Term Potentiation by Acetylcholinesterase Inhibition

Hippocampal cholinergic activity enhances long-term potentiation (LTP) of synaptic transmission in intrahippocampal circuits and regulates cognitive function. We recently demonstrated intracellular distribution of functional M1-muscarinic acetylcholine receptors (mAChRs) and neuronal uptake of acetylcholine (ACh) in the central nervous system. Here we examined whether endogenous ACh acts on intracellular M1-mAChRs following its uptake and causes cholinergic facilitation of hippocampal LTP. ACh esterase (AChE) activities and [³H]ACh uptake was measured in rat hippocampal segments. LTP of evoked field excitatory postsynaptic potentials at CA1 synapses was induced by high frequency stimulation in hippocampal slices. Pretreatment with diisopropylfluorophosphate (DFP) irreversibly inhibited AChE, augmented ACh uptake, and significantly enhanced the LTP. This cholinergic facilitation was inhibited by pirenzepine, a membrane-permeable M1 antagonist, while only the early stage of cholinergic facilitation was inhibited by a membrane-impermeable M1 antagonist, muscarinic toxin 7. Tetraethylammonium (TEA) inhibited ACh uptake in hippocampal segments and selectively suppressed late stage cholinergic facilitation without changing the early stage. In contrast, LTP in DFP-untreated slices was not affected by the muscarinic antagonists and TEA. Carbachol (CCh; an AChE-resistant muscarinic agonist) competed with ACh for its uptake and produced cholinergic facilitation of LTP in DFP-untreated slices. The late stage of CCh-induced facilitation was also selectively inhibited by TEA. Our results suggest that when AChE is inactivated by inhibitors, LTP in hippocampal slices is significantly enhanced by endogenous ACh and that cholinergic facilitation is caused by direct activation of cell-surface M1-mAChRs and subsequent activation of intracellular M1-mAChRs after ACh uptake.

Regulation of synaptic acetylcholine concentrations by acetylcholine transport in rat striatal cholinergic transmission

In addition to hydrolysis by acetylcholine esterase (AChE), acetylcholine (ACh) is also directly taken up into brain tissues. In this study, we examined whether the uptake of ACh is involved in the regulation of synaptic ACh concentrations. Superfusion experiments with rat striatal segments pre-incubated with [³ H]choline were performed using an ultra-mini superfusion vessel, which was developed to minimize superfusate retention within the vessel. Hemicholinium-3 (HC-3) at concentrations less than 1 μM, selectively inhibited the uptake of [³ H]choline by the high affinity-choline transporter 1 and had no effect on basal and electrically evoked [³ H]efflux in superfusion experiments. In contrast, HC-3 at higher concentrations, as well as tetraethylammonium (>10 μM), which inhibited the uptake of both [³ H]choline and [³ H]ACh, increased basal [³ H]overflow and potentiated electrically evoked [³ H]efflux. These effects of HC-3 and tetraethylammonium were also observed under conditions where tissue AChE was irreversibly inactivated by diisopropylfluorophosphate. Specifically, the potentiation of evoked [³ H]efflux was significantly higher in AChE-inactivated preparations and was attenuated by atropine. On the other hand, striatal segments pre-incubated with [³ H]ACh failed to increase [³ H]overflow in response to electrical stimulation. These results show that synaptic ACh concentrations are significantly regulated by the postsynaptic uptake of ACh, as well as by AChE hydrolysis and modulation of ACh release mediated through presynaptic muscarinic ACh receptors. In addition, these data suggest that the recycling of ACh-derived choline may be minor in cholinergic terminals. This study reveals a new mechanism of cholinergic transmission in the central nervous system.

TRPA1 Channels Modify TRPV1-Mediated Current Responses in Dorsal Root Ganglion Neurons

The transient receptor potential vanilloid 1 (TRPV1) channel is highly expressed in a subset of sensory neurons in the dorsal root ganglia (DRG) and trigeminal ganglia of experimental animals, responsible for nociception. Many researches have revealed that some TRPV1-positive neurons co-express the transient receptor potential ankyrin 1 (TRPA1) channel whose activities are closely modulated by TRPV1 channel. However, it is less investigated whether the activities of TRPV1 channel are modulated by the presence of TRPA1 channel in primary sensory neurons. This study clarified the difference in electrophysiological responses induced by TRPV1 channel activation between TRPA1-positive and TRPA1-negative DRG. TRPV1 and TRPA1 channel activations were evoked by capsaicin (1 μM), a TRPV1 agonist, and allyl isothiocyanate (AITC; 500 μM), a TRPA1 agonist, respectively. Capsaicin perfusion for 15 s caused a large inward current without a desensitization phase at a membrane potential of −70 mV in AITC-insensitive DRG (current density; 29.6 ± 5.6 pA/pF, time constant of decay; 12.8 ± 1.8 s). The capsaicin-induced currents in AITC-sensitive DRG had a small current density (12.7 ± 2.9 pA/pF) with a large time constant of decay (24.3 ± 5.4 s). In calcium imaging with Fura-2, the peak response by capsaicin was small and duration reaching the peak response was long in AITC-sensitive neurons. These electrophysiological differences were completely eliminated by HC-030031, a TRPA1 antagonist, in an extracellular solution or 10 mM EGTA, a Ca²⁺ chelator, in an internal solution. Capsaicin perfusion for 120 s desensitized the inward currents after a transient peak. The decay during capsaicin perfusion was notably slow in AITC-sensitive DRG; ratio of capsaicin-induced current 60 s after the treatment per the peak current in AITC-sensitive neurons (78 ± 9%) was larger than that in AITC-insensitive neurons (48 ± 5%). The capsaicin-induced current in the desensitization phase was attenuated by HC-030031 in AITC-insensitive DRG. These results indicate that (1) TRPV1-mediated currents in TRPA1-positive neurons characterize small current densities with slow decay, which is caused by TRPA1 channel activities and intracellular Ca²⁺ mobilization and (2) desensitization of TRPV1-mediated current in TRPA1-positive neurons is apparently slow, due to appending TRPA1-mediated current.

Pharmacological evidence of specific acetylcholine transport in rat cerebral cortex and other brain regions

Functional acetylcholine receptors (AChRs) were recently demonstrated to exist not only in the plasma membrane but also intracellularly in brain tissues. In order to activate intracellular AChRs, endogenous hydrophilic ACh must cross the plasma membrane. Here, we examined the pharmacological characteristics of this process, including whether it is mediated by active ACh uptake. When ACh esterase (AChE) was suppressed by diisopropylfluorophosphate, [³ H]ACh was effectively taken up into segments of rat cerebral cortex and other brain regions, in contrast to peripheral tissues such as liver and kidney. The uptake of [³ H]ACh in rat cerebral cortex was temperature-dependent, and the uptake capacity was comparable to that of [³ H]choline. However, [³ H]ACh uptake was inhibited by lower concentrations of ACh, carbachol, tetraethylammonium (TEA), compared with uptake of [³ H]choline. Uptake of [³ H]ACh was also inhibited by several organic cations, including choline, hemicholinium-3 (HC-3), quinidine, decynium 22, clonidine, diphenhydramine, but was little affected by some amino acids and biogenic amines, corticosterone, spermine, atropine, and tetrodotoxin. Unlike diisopropylfluorophosphate, several ACh esterase inhibitors, including drugs for Alzheimer's disease, such as donepezil, galantamine, and rivastigmine, also suppressed the uptake of [³ H]ACh, but not [³ H]choline. These results indicate that in the brain, ACh is specifically taken up through a unique transport system with different pharmacological properties from known organic cation transporters (OCTs), and suggest that this mechanism may be involved in intracellular cholinergic transmission in the brain.

Long-Term Activation of Group I Metabotropic Glutamate Receptors Increases Functional TRPV1-Expressing Neurons in Mouse Dorsal Root Ganglia

Damaged tissues release glutamate and other chemical mediators for several hours. These chemical mediators contribute to modulation of pruritus and pain. Herein, we investigated the effects of long-term activation of excitatory glutamate receptors on functional expression of transient receptor potential vaniloid type 1 (TRPV1) in dorsal root ganglion (DRG) neurons and then on thermal pain behavior. In order to detect the TRPV1-mediated responses in cultured DRG neurons, we monitored intracellular calcium responses to capsaicin, a TRPV1 agonist, with Fura-2. Long-term (4 h) treatment with glutamate receptor agonists (glutamate, quisqualate or DHPG) increased the proportion of neurons responding to capsaicin through activation of metabotropic glutamate receptor mGluR1, and only partially through the activation of mGluR5; engagement of these receptors was evident in neurons responding to allylisothiocyanate (AITC), a transient receptor potential ankyrin type 1 (TRPA1) agonist. Increase in the proportion was suppressed by phospholipase C (PLC), protein kinase C, mitogen/extracellular signal-regulated kinase, p38 mitogen-activated protein kinase or transcription inhibitors. Whole-cell recording was performed to record TRPV1-mediated membrane current; TRPV1 current density significantly increased in the AITC-sensitive neurons after the quisqualate treatment. To elucidate the physiological significance of this phenomenon, a hot plate test was performed. Intraplantar injection of quisqualate or DHPG induced heat hyperalgesia that lasted for 4 h post injection. This chronic hyperalgesia was attenuated by treatment with either mGluR1 or mGluR5 antagonists. These results suggest that long-term activation of mGluR1/5 by peripherally released glutamate may increase the number of neurons expressing functional TRPV1 in DRG, which may be strongly associated with chronic hyperalgesia.

Evidence for Conservation of the Calcitonin Superfamily and Activity-regulating Mechanisms in the Basal Chordate Branchiostoma floridae: INSIGHTS INTO THE MOLECULAR AND FUNCTIONAL EVOLUTION IN CHORDATES

The calcitonin (CT)/CT gene-related peptide (CGRP) family is conserved in vertebrates. The activities of this peptide family are regulated by a combination of two receptors, namely the calcitonin receptor (CTR) and the CTR-like receptor (CLR), and three receptor activity-modifying proteins (RAMPs). Furthermore, RAMPs act as escort proteins by translocating CLR to the cell membrane. Recently, CT/CGRP family peptides have been identified or inferred in several invertebrates. However, the molecular characteristics and relevant functions of the CTR/CLR and RAMPs in invertebrates remain unclear. In this study, we identified three CT/CGRP family peptides (Bf-CTFPs), one CTR/CLR-like receptor (Bf-CTFP-R), and three RAMP-like proteins (Bf-RAMP-LPs) in the basal chordate amphioxus (Branchiostoma floridae). The Bf-CTFPs were shown to possess an N-terminal circular region typical of the CT/CGRP family and a C-terminal Pro-NH2. The Bf-CTFP genes were expressed in the central nervous system and in endocrine cells of the midgut, indicating that Bf-CTFPs serve as brain and/or gut peptides. Cell surface expression of the Bf-CTFP-R was enhanced by co-expression with each Bf-RAMP-LP. Furthermore, Bf-CTFPs activated Bf-CTFP-R·Bf-RAMP-LP complexes, resulting in cAMP accumulation. These results confirmed that Bf-RAMP-LPs, like vertebrate RAMPs, are prerequisites for the function and translocation of the Bf-CTFP-R. The relative potencies of the three peptides at each receptor were similar. Bf-CTFP2 was a potent ligand at all receptors in cAMP assays. Bf-RAMP-LP effects on ligand potency order were distinct to vertebrate CGRP/adrenomedullin/amylin receptors. To the best of our knowledge, this is the first molecular and functional characterization of an authentic invertebrate CT/CGRP family receptor and RAMPs.

Effects of the 5α-reductase inhibitor dutasteride on rat prostate α1A-adrenergic receptor and its mediated contractility

OBJECTIVE

To clarify the possible interference of the 5α-reductase inhibitor dutasteride with α-adrenergic blockers, whose action is mainly mediated by α1A-adrenergic receptor.

METHODS

Male rats were divided into dutasteride and vehicle-treated groups. The drug treatment group was treated with oral dutasteride 0.5 mg/kg/d, and the control group received vehicle only for 2 months. After the 2-month treatment, the rats' ventral prostate weight changes and the testosterone and dihydrotestosterone levels in the serum were measured. In vitro organ-bath studies, real-time polymerase chain reaction, and tissue-segment binding were performed to determine the expression of α1A-adrenergic receptors and its mediated contractility.

RESULTS

Dutasteride treatment significantly decreased the rats' ventral prostate weight, increased their testosterone levels, and decreased the dihydrotestosterone levels in their serum. There were no marked changes in the α1A-adrenergic receptor messenger ribonucleic acid expression, relative phenylephrine-induced contractility, or nerve-mediated contractility between the groups. Dutasteride treatment caused no marked changes in the relative binding capacity of α1A-adrenergic receptor, whereas it greatly decreased the total protein expression of this subtype and its mediated maximal contraction in the whole ventral prostate.

CONCLUSION

These results suggest that dutasteride does not interfere with α-adrenergic blockers but otherwise has beneficial effects on their actions. Therefore, the long-term administration of the combination of dutasteride with an α-adrenergic blocker might be a better choice for the treatment of lower urinary tract symptoms due to benign prostatic hyperplasia.

Agonist pharmacology at recombinant α1A - and α1L -adrenoceptors and in lower urinary tract α1 -adrenoceptors

BACKGROUND AND PURPOSE

Two distinct α1 -adrenoceptor phenotypes (α1A and α1L ) have recently been demonstrated to originate from a single α1A -adrenoceptor gene. Here, we examined the agonist profiles of recombinant α1A and α1L phenotypes and of lower urinary tract (LUT) α1 -adrenoceptors.

EXPERIMENTAL APPROACH

A series of drugs (A61603, Ro 115-1240, NS-49 , MK017 and ESR1150) originally developed for stress urinary incontinence (SUI) therapy were used to stimulate recombinant α1A - and α1L -adrenoceptor phenotypes, and their potencies and intrinsic activity estimated from Ca(2+) responses. Agonist-induced contractions were also examined in LUT tissues of rats and humans and in human mesenteric artery and rat tail artery.

KEY RESULTS

All the drugs were potent agonists of the α1A -adrenoceptor compared with the α1L -adrenoceptor phenotype. Among them, Ro 115-1240 was shown to be an α1A -specific partial agonist that produced partial contractions through α1A -adrenoceptors in rat prostate and tail artery, but not in the other LUT tissues and human mesenteric artery. In contrast, P-come 102 showed full agonist activity at α1A - and α1L -adrenoceptors, but was less selective than noradrenaline for α1A -adrenoceptors. Like noradrenaline, P-come 102 was highly potent at inducing contractions in all of the LUT tissues tested. However, the potency and intrinsic activity of P-come 102 were significantly lower than those of noradrenaline in human mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

The α1A - and α1L -adrenoceptor phenotypes and LUT α1 -adrenoceptors were demonstrated to have distinct agonist profiles. As adrenergic contractions in LUT are predominantly mediated through α1L -adrenoceptors, the development of α1L -selective agonists may provide clinically useful drugs for SUI therapy.

Pharmacologically distinct phenotypes of α1B -adrenoceptors: variation in binding and functional affinities for antagonists

BACKGROUND AND PURPOSE

The pharmacological properties of particular receptors have recently been suggested to vary under different conditions. We compared the pharmacological properties of the α1B -adrenoceptor subtype in various tissue preparations and under various conditions.

EXPERIMENTAL APPROACH

[(3) H]-prazosin binding to α1B -adrenoceptors in rat liver (segments, dispersed hepatocytes and homogenates) was assessed and the pharmacological profiles were compared with the functional and binding profiles in rat carotid artery and recombinant α1B -adrenoceptors.

KEY RESULTS

In association and saturation-binding experiments with rat liver, binding affinity for [(3) H]-prazosin varied significantly between preparations (KD value approximately ten times higher in segments than in homogenates). The binding profile for various drugs in liver segments also deviated from the representative α1B -adrenoceptor profile observed in liver homogenates and recombinant receptors. L-765,314 and ALS-77, selective antagonists of α1B -adrenoceptors, showed high binding and antagonist affinities in liver homogenates and recombinant α1B -adrenoceptors. However, binding affinities for both ligands in the segments of rat liver and carotid artery were 10 times lower, and the antagonist potencies in α1B -adrenoceptor-mediated contractions of carotid artery were more than 100 times lower than the representative α1B -adrenoceptor profile.

CONCLUSIONS AND IMPLICATIONS

In contrast to the consistent profile of recombinant α1B -adrenoceptors, the pharmacological profile of native α1B -adrenoceptors of rat liver and carotid artery varied markedly under various receptor environments, showing significantly different binding properties between intact tissues and homogenates, and dissociation between functional and binding affinities. In addition to conventional 'subtype' characterization, 'phenotype' pharmacology must be considered in native receptor evaluations in vivo and in future pharmacotherapy.

Intracellular localization of M1 muscarinic acetylcholine receptor through clathrin-dependent constitutive internalization via a C-terminal tryptophan-based motif

The M1 muscarinic acetylcholine receptor (M1-mAChR) is a G-protein-coupled membrane receptor that is activated by extracellular cholinergic stimuli. Recent investigations have revealed the intracellular localization of M1-mAChRs. In this study, we observed constitutive internalization of M1-mAChRs in mouse neuroblastoma N1E-115 cells without agonist stimulation. Constitutive internalization depends on dynamin, clathrin, and the adaptor protein-2 (AP-2) complex. A WXXI motif in the M1-mAChR C-terminus is essential for its constitutive internalization, since alanine mutations of W442 or I445 abolished constitutive internalization. This WXXI motif resembles YXXΦ, which is the canonical binding motif for the μ2 subunit of the AP-2 complex. The M1-mAChR C-terminal WXXI motif interacted with AP-2 μ2. W442A and I445A mutants of the M1-mAChR C-terminal sequence lost AP-2 μ2 binding activity, while the W442Y mutant bound more effectively than wild-type. Consistent with these results, W442A and I445A M1-mAChR mutants selectively localized to the cell surface. Conversely, the W442Y receptor mutant was found only at intracellular sites. Our data indicate that the cellular distribution of M1-mAChRs is governed by the C-terminal tryptophan-based motif that mediates constitutive internalization.

Activation of focal adhesion kinase via M1 muscarinic acetylcholine receptor is required in restitution of intestinal barrier function after epithelial injury

Impairment of epithelial barrier is observed in various intestinal disorders including inflammatory bowel diseases (IBD). Numerous factors may cause temporary damage of the intestinal epithelium. A complex network of highly divergent factors regulates healing of the epithelium to prevent inflammatory response. However, the exact repair mechanisms involved in maintaining homeostatic intestinal barrier integrity remain to be clarified. In this study, we demonstrate that activation of M1 muscarinic acetylcholine receptor (mAChR) augments the restitution of epithelial barrier function in T84 cell monolayers after ethanol-induced epithelial injury, via ERK-dependent phosphorylation of focal adhesion kinase (FAK). We have shown that ethanol injury decreased the transepithelial electrical resistance (TER) along with the reduction of ERK and FAK phosphorylation. Carbachol (CCh) increased ERK and FAK phosphorylation with enhanced TER recovery, which was completely blocked by either MT-7 (M1 antagonist) or atropine. The CCh-induced enhancement of TER recovery was also blocked by either U0126 (ERK pathway inhibitor) or PF-228 (FAK inhibitor). Treatment of T84 cell monolayers with interferon-γ (IFN-γ) impaired the barrier function with the reduction of FAK phosphorylation. The CCh-induced ERK and FAK phosphorylation were also attenuated by the IFN-γ treatment. Immunological and binding experiments exhibited a significant reduction of M1 mAChR after IFN-γ treatment. The reduction of M1 mAChR in inflammatory area was also observed in surgical specimens from IBD patients, using immunohistochemical analysis. These findings provide important clues regarding mechanisms by which M1 mAChR participates in the maintenance of intestinal barrier function under not only physiological but also pathological conditions.

M1 is a major subtype of muscarinic acetylcholine receptors on mouse colonic epithelial cells

BACKGROUND

Muscarinic acetylcholine receptors (mAChRs) are major regulators of gut epithelial functions. However, the precise subtype composition has not been clarified.

METHODS

We characterized the pharmacological profile of mAChRs on mouse colonic crypts, employing [(3)H]-N-methyl scopolamine chloride as a radioligand and several subtype-selective chemicals, and the functional aspect by measuring short-circuit current (I sc) in Ussing chambers and by evaluating MAP kinase phosphorylation in mouse colonic mucosal sheets.

RESULTS

The mAChRs were detected on the crypts (K d = 163.2 ± 32.3 pM, B max = 47.3 ± 2.6 fmol/mg of total cell protein). Muscarinic toxin 7 (MT-7, M1 subtype selective) gave a displacement curve with high affinity, but there was a part insensitive to MT-7 (18.8 ± 0.4 % of the total specific binding). The MT-7-insensitive component was displaced completely by darifenacin (M3 selective) with high affinity. ACh induced an increase in I sc, which was significantly enhanced by MT-7 but was completely inhibited by darifenacin or atropine. Colitis induction resulted in a significant decrease in the density of mAChRs, which occurred mainly in the MT-7-sensitive component (M1 subtype). Immunological experiments exhibited a reduction of M1 but not of M3 signal after colitis induction. Muscarinic stimulation induced an increase in MAP kinase phosphorylation, which was completely suppressed by MT-7 and was attenuated by inflammation, in mouse colonic epithelium.

CONCLUSIONS

These results suggest that mAChRs in mouse colonic epithelial cells consist of two subtypes, M1 (80 %) and M3 (20 %). The major M1 subtype was likely to regulate epithelial chloride secretion negatively and was susceptible to inflammation and may be relevant to inflammatory gut dysfunction.

Regional quantification of muscarinic acetylcholine receptors and β-adrenoceptors in human airways

BACKGROUND AND PURPOSE Muscarinic acetylcholine receptors (mAChRs) and β-adrenoceptors in the airways and lungs are clinically important in chronic obstructive pulmonary disease (COPD) and asthma. However, the quantitative and qualitative estimation of these receptors by radioligand binding approaches in human airways has not yet been reported because of tissue limitations. EXPERIMENTAL APPROACH The regional distribution and relative proportion of mAChR and β-adrenoceptor subtypes were evaluated in human bronchus and lung parenchyma by a tissue segment binding method with [(3)H]-N-methylscopolamine ([(3)H]-NMS) for mAChRs and [(3)H]-CGP-12,177 for β-adrenoceptors. Functional responses to carbachol and isoprenaline were also analysed in the bronchus. KEY RESULTS The M(3) subtype predominantly occurred in the bronchus, but the density decreased from the segmental to subsegmental bronchus, and was absent in lung parenchyma. On the other hand, the M(1) subtype occurred in the lung only, and the M(2) subtype was distributed ubiquitously in the bronchus and lungs. β(2)-adrenoceptors were increased along the airways, and their densities in the subsegmental bronchus and lung parenchyma were approximately twofold higher than those of mAChRs in the same region. β(1)-adrenoceptors were also detected in lung parenchyma but not in the bronchus. The muscarinic contractions and adrenoceptor relaxations in both bronchial regions were mediated through M(3)-mAChRs and β(2)-adrenoceptors, respectively. CONCLUSIONS AND IMPLICATIONS From the present radioligand binding approach with intact tissue segments, we constructed a distribution map of mAChRs and β-adrenoceptors in human bronchus and lung parenchyma for the first time, providing important evidence for future pharmacotherapy and new drug development for respiratory disorders.

Phenotype pharmacology of lower urinary tract α(1)-adrenoceptors

α(1)-Adrenoceptors are involved in numerous physiological functions, including micturition. However, the pharmacological profile of the α(1)-adrenoceptor subtypes remains controversial. Here, we review the literature regarding α(1)-adrenoceptors in the lower urinary tract from the standpoint of α(1L) phenotype pharmacology. Among three α(1)-adrenoceptor subtypes (α(1A), α(1B) and α(1D)), α(1a)-adrenoceptor mRNA is the most abundantly transcribed in the prostate, urethra and bladder neck of many species, including humans. In prostate homogenates or membrane preparations, α(1A)-adrenoceptors with high affinity for prazosin have been detected as radioligand binding sites. Functional α(1)-adrenoceptors in the prostate, urethra and bladder neck have low affinity for prazosin, suggesting the presence of an atypical α(1)-adrenoceptor phenotype (designated as α(1L)). The α(1L)-adrenoceptor occurs as a distinct binding entity from the α(1A)-adrenoceptor in intact segments of variety of tissues including prostate. Both the α(1L)- and α(1A)-adrenoceptors are specifically absent from Adra1A (α(1a)) gene-knockout mice. Transfection of α(1a)-adrenoceptor cDNA predominantly expresses α(1A)-phenotype in several cultured cell lines. However, in CHO cells, such transfection expresses α(1L)- and α(1A)-phenotypes. Under intact cell conditions, the α(1L)-phenotype is predominant when co-expressed with the receptor interacting protein, CRELD1α. In summary, recent pharmacological studies reveal that two distinct α(1)-adrenoceptor phenotypes (α(1A) and α(1L)) originate from a single Adra1A (α(1a)-adrenoceptor) gene, but adrenergic contractions in the lower urinary tract are predominantly mediated via the α(1L)-adrenoceptor. From the standpoint of phenotype pharmacology, it is likely that phenotype-based subtypes such as the α(1L)-adrenoceptor will become new targets for drug development and pharmacotherapy.

Re-evaluation of nicotinic acetylcholine receptors in rat brain by a tissue-segment binding assay

Nicotinic acetylcholine receptors (nAChRs) of the cerebral cortex and cerebellum of rats were evaluated by a radioligand binding assay, employing tissue segments, or homogenates as materials. [³H]-epibatidine specifically bound to nAChRs in rat cortex or cerebellum, but the dissociation constants for [³H]-epibatidine differed between segments and homogenates (187 pM for segments and 42 pM for homogenates in the cortex and 160 pM for segments and 84 pM for homogenates in the cerebellum). The abundance of total nAChRs was approximately 310 fmol/mg protein in the segments of cortex and 170 fmol/mg protein in the segments of cerebellum, which were significantly higher than those estimated in the homogenates (115 fmol/mg protein in the homogenates of the cortex and 76 fmol/mg protein in the homogenates of the cerebellum). Most of the [³H]-epibatidine binding sites in the cortex segments (approximately 70% of the population) showed high affinity for nicotine (pK_i = 7.9), dihydro-β-erythroidine, and cytisine, but the binding sites in the cerebellum segments had slightly lower affinity for nicotine (pK_i = 7.1). An upregulation of nAChRs by chronic administration of nicotine was observed in the cortex segments but not in the cerebellum segments with [³H]-epibatidine as a ligand. The upregulation in the cortex was caused by a specific increase in the high-affinity sites for nicotine (probably α4β2). The present study shows that the native environment of nAChRs is important for a precise quantitative as well as qualitative estimation of nAChRs in rat brain.

Intracellular distribution of functional M(1) -muscarinic acetylcholine receptors in N1E-115 neuroblastoma cells

Signaling by muscarinic agonists is thought to result from the activation of cell surface acetylcholine receptors (mAChRs) that transmit extracellular signals to intracellular systems. In N1E-115 neuroblastoma cells, we detected both plasma membrane and intracellular M(1) -mAChRs using both biochemical and pharmacological methods. In intact cells, both plasma membrane and intracellular M(1) -mAChRs were detected by the hydrophobic ligand probe, 1-quinuclidinyl-[phenyl-4-(3) H]-benzilate ([(3) H]-QNB) whereas the hydrophilic probe, 1-[N-methyl-(3) H] scopolamine ([(3) H]-NMS), detected only cell surface receptors. These probes detected comparable numbers of receptors in isolated membrane preparations. Immunohistochemical studies with M(1) -mAChR antibody also detected both cell-surface and intracellular M(1) -mAChRs. Carbachol-stimulated phosphatidylinositol hydrolysis and Ca(2+) mobilization were completely inhibited by a cell-impermeable M(1) antagonist, muscarinic toxin -7 and the G(q/11) inhibitor YM-254890. However, carbachol-stimulated extracellular-regulated kinase 1/2 activation was unaffected by muscarinic toxin-7, but was blocked by the cell-permeable antagonist, pirenzepine. extracellular regulated kinase 1/2 phosphorylation was resistant to blockade of G(q/11) (YM-254890) and protein kinase C (bisindolylmaleimide I). Our data suggest that the geographically distinct M(1) -mAChRs (cell surface versus intracellular) can signal via unique signaling pathways that are differentially sensitive to cell-impermeable versus cell-permeable antagonists. Our data are of potential physiological relevance to signaling that affects both cognitive and neurodegenerative processes.

Influence of tissue integrity on pharmacological phenotypes of muscarinic acetylcholine receptors in the rat cerebral cortex

Distinct pharmacological phenotypes of muscarinic acetylcholine receptors (mAChRs) have been proposed. We compared the pharmacological profiles of mAChRs in intact segments and homogenates of rat cerebral cortex and other tissues by using radioligand binding assays with [(3)H]N-methylscopolamine ([(3)H]NMS). Recombinant M(1) and M(3) mAChRs were also examined. The density of mAChRs detected by [(3)H]NMS binding to rat cerebral cortex segments and homogenates was the same (approximately 1400 fmol/mg tissue protein), but the dissociation constant of [(3)H]NMS was significantly different (1400-1700 pM in segments and 260 pM in homogenates). A wide variation in [(3)H]NMS binding affinity was also observed among the segments of other tissues (ranging from 139 pM in urinary bladder muscle to 1130 pM in the hippocampus). The mAChRs of cerebral cortex were composed of M(1), M(2), M(3), and M(4) subtypes, which showed typical subtype pharmacology in the homogenates. However, in the cortex segments the M(3) subtype showed a low selectivity for M(3) antagonists (darifenacin, solifenacin) and was not distinguished by the M(3) antagonists from the other subtypes. Recombinant M(1) and M(3) mAChRs showed high affinity for [(3)H]NMS and subtype-specific pharmacology for each tested ligand. The present binding study under conditions where tissue integrity was kept demonstrates a wide variation in [(3)H]NMS binding affinity among mAChRs of many rat tissues and the presence of an atypical M(3) phenotype in the cerebral cortex, suggesting that the pharmacological properties of mAChRs are not necessarily constant, rather they may be significantly modified by tissue integrity and tissue type.

A cytotoxic peptide from a marine sponge exhibits ion channel activity through vectorial-insertion into the membrane

A cytotoxic peptide, polytheonamide B (pTB), from marine sponge was examined for cytotoxic spectrum and specific activity to mammalian cells was demonstrated. pTB is composed of alternative D- and L-amino acid residues throughout the 48-mer peptide. This suggests the formation of a β-helix similar to gramicidin channels. Planar bilayer experiments revealed that pTB forms monovalent cation-selective channels, being compatible with the inner pore diameter of ∼4Å for a β-helical structure. pTB penetrated vectorially into the membrane, formed a channel by means of a single molecule, and remained in the membrane. These functional properties may account for specific cytotoxic activity.

Endothelin type B receptor-induced sustained Ca2+ influx involves G(q/11)/phospholipase C-independent, p38 mitogen-activated protein kinase-dependent activation of Na+/H+ exchanger

The mechanism for sustained Ca2+ influx activated by G protein-coupled receptors was examined. In Chinese hamster ovary cells expressing recombinant human endothelin type B receptor (ET(B)R) and endogenous P2Y receptor (P2Y-R), endothelin-1 elicited a sustained Ca2+ influx depending on G(q/11 )protein, phospholipase C (PLC), Na+/H+ exchanger (NHE), and p38 mitogen-activated protein kinase (p38MAPK), whereas P2Y-R-induced sustained Ca2+ influx was negligible. Functional studies showed that NHE activation by ET(B)R was mediated via p38MAPK but not G(q/11)/PLC, while that by P2Y-R involves only G(q/11)/PLC/p38MAPK. These results suggest that G(q/11)/PLC-independent NHE activation via p38MAPK plays an important role in ET(B)R- mediated sustained Ca2+ influx.

Visualization and tissue distribution of alpha1L-adrenoceptor in human prostate by the fluorescently labeled ligand Alexa-488-silodosin

PURPOSE

Although alpha(1L)-adrenoceptor is recognized as a target of alpha(1) antagonist therapy for benign prostatic hyperplasia, the most common techniques, such as immunohistochemistry and in situ hybridization, are not applicable to examine alpha(1L)-AR vs alpha(1A)-AR tissue distribution because alpha(1L)-AR is now considered another phenotype sharing the alpha(1A)-AR gene and protein molecule. We labeled the alpha(1A) and alpha(1L)-adrenoceptor selective antagonist silodosin (Kissei Pharmaceutical, Matsumoto, Japan) with the fluorophore Alexa Fluor(R) 488 (Alexa-488-silodosin) to visualize alpha(1L)-AR expression.

MATERIALS AND METHODS

Radioligand binding and functional bioassay experiments were done to assess alpha(1)-AR expression in Chinese hamster ovary cells and human prostate tissues. Confocal imaging was subsequently performed.

RESULTS

Although Alexa-488-silodosin had about 10 times lower affinity for all alpha(1)-AR subtypes than silodosin in binding and functional studies, it had high selectivity to alpha(1A) and alpha(1L)-ARs. Confocal imaging revealed clear localization of fluorescence on the membrane of Chinese hamster ovary cells expressing alpha(1A)-AR but not alpha(1B)-and alpha(1D)-ARs, and in the muscle layer of the human prostate. The fluorescent signal in Chinese hamster ovary cells disappeared in the presence of 3 nM prazosin but fluorescence was observed in the human prostate even in the presence of 100 nM prazosin.

CONCLUSIONS

Alexa-488-silodosin is a powerful fluorescent probe with high selectivity to alpha(1A) and alpha(1L)-ARs. Thus, Alexa-488-silodosin successfully visualizes the site of alpha(1L)-ARs in the muscle layer of the human prostate without losing its distinct pharmacological profile.

Differential coupling of human endothelin type A receptor to G(q/11) and G(12) proteins: the functional significance of receptor expression level in generating multiple receptor signaling

This study examines the influence of receptor expression level on signaling pathways activated via endothelin type A receptor (ET(A)R) expressed in Chinese hamster ovary cells at 32,100 (ET(A)R-high-CHO) and 893 (ET(A)R-low-CHO) fmolmg protein(-1). Endothelin-1 (ET-1) elicited a sustained increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which was dependent on G(q/11) protein, phospholipase C (PLC), Na(+)/H(+) exchanger (NHE), and p38 mitogen-activated protein kinase (p38MAPK) in ET(A)R-high-CHO, whereas the sustained [Ca(2+)](i) increase was negligible in ET(A)R-low-CHO. Functional study with Cytosensor(TM) microphysiometer showed that ET-1 evoked an NHE1-mediated increase in extracellular acidification rate (ECAR) in ET(A)R-high-CHO and ET(A)R-low-CHO. In ET(A)R-high-CHO, the ECAR response at 30 min after ET-1 stimulation was insensitive to G(q/11) and PLC inhibitors, but sensitive to the p38MAPK inhibitor. In ET(A)R-low-CHO, the ECAR response at 30 min was sensitive to these inhibitors. Western blot analysis demonstrated that ET-1-induced p38MAPK phosphorylation in ET(A)R-low-CHO but not in ET(A)R-high-CHO was mediated via G(q/11) and PLC. The G(q/11)/PLC-independent p38MAPK phosphorylation in ET(A)R-high-CHO was suppressed by expression of the C terminus of G(alpha12) protein to disrupt receptor-G(12) protein coupling. These results provide evidence for multiple signaling pathways of ET(A)R that were activated via at least the G(q/11)/PLC/NHE, G(12)/p38MAPK/NHE, and G(q/11)/PLC/p38MAPK/NHE cascades in an expression level-dependent manner.

Alpha 1-adrenoceptor pharmacome: alpha 1L-adrenoceptor and alpha 1A-adrenoceptor in the lower urinary tract

Alpha(1)-adrenoceptors are involved in physiological functions such as urinary excretion and ejaculation in the lower urinary tract (LUT). Several alpha(1) antagonists are clinically used for the treatment of urinary obstruction in patients with benign prostatic hyperplasia. At present, three classical alpha(1)-adrenoceptor subtypes (alpha(1A), alpha(1B), and alpha(1D)) have been identified, among which the alpha(1A) and alpha(1D)-adrenoceptor subtypes have been regarded as the main targets of alpha(1) antagonist therapy for LUT symptoms. Prazosin has been used as a prototypic, classical antagonist, to characterize alpha(1)-adrenoceptors pharmacologically, (i.e. all classical alpha(1)-adrenoceptor subtypes show high-affinity for the drug). However, we found that alpha(1)-adrenoceptors in the LUT show atypical low-affinity for prazosin. Therefore, the concept alpha(1L)-receptor, which indicates alpha(1)-adrenoceptor(s) showing low-affinity for prazosin has been introduced. A recent study demonstrated that the alpha(1L)-adrenoceptor is a specific phenotype present in the many intact tissues including human LUT, and that it originates from the ADRA1A gene. Therefore, the alpha(1L)-adrenoceptor in the LUT is now re-defined as alpha(1A(L))-adrenoceptor. The physiological and pharmacological difference between classical alpha(1A(H),) and alpha(1A(L)) which is the native receptor expressed in the LUT is of special interest as it provides fundamental bases for urological alpha(1A)-adrenoceptor blocking pharmacotherapy. Here, we briefly review the alpha(1)-adrenoceptors in the LUT with special reference to phenotype-based (pharmacome) analysis.

Evaluation of beta1L-adrenoceptors in rabbit heart by tissue segment binding assay

[(3)H]-CGP12177 biphasically bound to beta-adrenoceptors with high and low affinities in the segments and crude membranes of rabbit left ventricle. The low-affinity sites for [(3)H]-CGP12177 in the segments was double in density, compared to the density of high-affinity sites. Total abundance of the beta-adrenoceptors decreased to approximately 10% upon tissue homogenization, and the proportion of low-affinity sites was the same as that of the high-affinity sites in the membranes. The majority of the high-affinity binding sites of [(3)H]-CGP12177 in the segments and the membranes were beta(1H)-adrenoceptor, being highly sensitive to propranolol and beta(1)-antagonists (atenolol and ICI-89,406), whereas the low-affinity binding sites showed a beta(1L)-profile (less sensitive to propranolol and beta(1)-, beta(2)-, and beta(3)-antagonists). Furthermore, a part of the beta(1L)-adrenoceptors was insensitive to atenolol, ICI-89,406, and/or isoproterenol. The present binding study clearly shows that beta(1L)-adrenoceptors occur as a distinct phenotype from beta(1H)-adrenoceptors in rabbit ventricle. However, quantitative imbalance between beta(1H)- and beta(1L)-adrenoceptors and divergent ligand-beta(1L)-adrenoceptor interactions suggest a possibility that the beta(1L)-adrenoceptor may not reflect a simple conformational change or allosteric state in the beta(1)-adrenoceptor molecule.

Expression of distinct alpha 1-adrenoceptor phenotypes in the iris of pigmented and albino rabbits

BACKGROUND AND PURPOSE

The expression of multiple pharmacological phenotypes including alpha(1L)-adrenoceptor has recently been reported for alpha(1)-adrenoceptors. The purpose of the present study was to identify alpha(1)-adrenoceptor phenotypes in the irises of pigmented and albino rabbits.

EXPERIMENTAL APPROACH

Radioligand binding and functional bioassay experiments were performed in segments or strips of iris of pigmented and albino rabbits, and their pharmacological profiles were compared.

KEY RESULTS

[(3)H]-silodosin at subnanomolar concentrations bound to intact segments of iris of pigmented and albino rabbits at similar densities (approximately 240 fmol x mg(-1) protein). The binding sites in the iris of a pigmented rabbit were composed of a single component showing extremely low affinities for prazosin, hydrochloride [N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride (RS-17053)] and 5-methylurapidil, while two components with high and low affinities for prazosin, RS-17053 and 5-methylurapidil were identified in irises from albino rabbits. In contrast, specific binding sites for [(3)H]-prazosin were not clearly detected because a high proportion of non-specific binding and/or low affinity for prazosin occurred. Contractile responses of iris dilator muscle to noradrenaline were antagonized by the above ligands, and their antagonist affinities were consistent with the binding estimates at low-affinity sites identified in both strains of rabbits.

CONCLUSIONS AND IMPLICATIONS

A typical alpha(1L) phenotype with extremely low affinity for prazosin is exclusively expressed in the iris of pigmented rabbits, while two distinct phenotypes (alpha(1A) and alpha(1L)) with high and moderate affinities for prazosin are co-expressed in the iris of albino rabbits. This suggests that a significant difference in the expression of phenotypes of the alpha(1)-adrenoceptor occurs in the irises between the two strains of rabbits.

Identification of alpha 1L-adrenoceptor in mice and its abolition by alpha 1A-adrenoceptor gene knockout

BACKGROUND AND PURPOSE

The alpha(1L)-adrenoceptor has pharmacological properties that distinguish it from three classical alpha(1)-adrenoceptors (alpha(1A), alpha(1B) and alpha(1D)). The purpose of this was to identify alpha(1L)-adrenoceptors in mice and to examine their relationship to classical alpha(1)-adrenoceptors.

EXPERIMENTAL APPROACH

Radioligand binding and functional bioassay experiments were performed on the cerebral cortex, vas deferens and prostate of wild-type (WT) and alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptor gene knockout (AKO, BKO and DKO) mice.

KEY RESULTS

The radioligand [(3)H]-silodosin bound to intact segments of the cerebral cortex, vas deferens and prostate of WT, BKO and DKO but not of AKO mice. The binding sites were composed of two components with high and low affinities for prazosin or RS-17053, indicating the pharmacological profiles of alpha(1A)-adrenoceptors and alpha(1L)-adrenoceptors. In membrane preparations of WT mouse cortex, however, [(3)H]-silodosin bound to a single population of prazosin high-affinity sites, suggesting the presence of alpha(1A)-adrenoceptors alone. In contrast, [(3)H]-prazosin bound to two components having alpha(1A)-adrenoceptor and alpha(1B)-adrenoceptor profiles in intact segments of WT and DKO mouse cortices, but AKO mice lacked alpha(1A)-adrenoceptor profiles and BKO mice lacked alpha(1B)-adrenoceptor profiles. Noradrenaline produced contractions through alpha(1L)-adrenoceptors with low affinity for prazosin in the vas deferens and prostate of WT, BKO and DKO mice. However, the contractions were abolished or markedly attenuated in AKO mice.

CONCLUSIONS AND IMPLICATIONS

alpha(1L)-Adrenoceptors were identified as binding and functional entities in WT, BKO and DKO mice but not in AKO mice, suggesting that the alpha(1L)-adrenoceptor is one phenotype derived from the alpha(1A)-adrenoceptor gene.

Different affinities of native alpha1B-adrenoceptors for ketanserin between intact tissue segments and membrane preparations

The pharmacological profiles of alpha1-adrenoceptors for ketanserin, prazosin, silodosin, and BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride) were examined under different assay conditions. Among the tested antagonists and alpha1-adrenoceptors subtypes, ketanserin showed significantly lower affinity for the alpha1B-adrenoceptor subtype in intact tissue sampled from the rat tail artery, thoracic aorta, and cerebral cortex (functional pKB and binding pKi were approximately 6), than in cerebral cortex membrane preparations or whole cell and membrane preparations of alpha1B-adrenoceptor transfected human embryonic kidney 293T (HEK 293T) cells (pKi was approximately 8). In these tissues and cells, however, ketanserin showed a similar affinity (pKi = approximately 8) for alpha1A- and alpha1D-adrenoceptors even though the assays were conducted under different conditions. In contrast, the affinities of alpha1A-, alpha1B-, and alpha1D-adrenoceptors for prazosin, silodosin, and BMY 7378 did not significantly change under different assay conditions and in different tissues. The present study reveals that the pharmacological profiles of native alpha 1B-adrenoceptors for ketanserin is strongly influenced by the assay conditions and suggest that antagonist affinity is not necessarily constant.

Snapin, a new regulator of receptor signaling, augments alpha1A-adrenoceptor-operated calcium influx through TRPC6

Activation of G(q)-protein-coupled receptors, including the alpha(1A)-adrenoceptor (alpha(1A)-AR), causes a sustained Ca(2+) influx via receptor-operated Ca(2+) (ROC) channels, following the transient release of intracellular Ca(2+). Transient receptor potential canonical (TRPC) channel is one of the candidate proteins constituting the ROC channels, but the precise mechanism linking receptor activation to increased influx of Ca(2+) via TRPCs is not yet fully understood. We identified Snapin as a protein interacting with the C terminus of the alpha(1A)-AR. In receptor-expressing PC12 cells, co-transfection of Snapin augmented alpha(1A)-AR-stimulated sustained increases in intracellular Ca(2+) ([Ca(2+)](i)) via ROC channels. By altering the Snapin binding C-terminal domain of the alpha(1A)-AR or by reducing cellular Snapin with short interfering RNA, the sustained increase in [Ca(2+)](i) in Snapin-alpha(1A)-AR co-expressing PC12 cells was attenuated. Snapin co-immunoprecipitated with TRPC6 and alpha(1A)-AR, and these interactions were augmented upon alpha(1A)-AR activation, increasing the recruitment of TRPC6 to the cell surface. Our data suggest a new receptor-operated signaling mechanism where Snapin links the alpha(1A)-AR to TRPC6, augmenting Ca(2+) influx via ROC channels.

Different changes of plasma membrane beta-adrenoceptors in rat heart after chronic administration of propranolol, atenolol and bevantolol

Recently, tissue segment binding method with a hydrophilic radioligand [(3)H]-CGP12177 was developed to detect plasma membrane beta-adrenoceptors in rat heart (Horinouchi et al., 2006). In the present study, propranolol (40 mg kg(-1) day(-1)), atenolol (40 mg kg(-1) day(-1)) and bevantolol (200 mg kg(-1) day(-1)) were administered to rats for 6 weeks, and the changes of plasma membrane beta-adrenoceptors and their mRNA expression in rat ventricle were examined. Chronic administration of propranolol increased the beta(1)-adrenoceptors but decreased the beta(2)-adrenoceptors without changing total amount of plasma membrane beta-adrenoceptors. Atenolol increased both plasma membrane beta(1)- and beta(2)-adrenoceptors, whereas bevantolol had no effect on the beta-adrenoceptor density and their subtype proportions. In contrast, the density of beta-adrenoceptors detected in conventional homogenate binding study was extremely low (approximately 60% of plasma membrane beta-adrenoceptors detected with the tissue segment binding method) and the effects of chronic administration of beta-adrenoceptor antagonists were not necessarily in accord with those at the plasma membrane beta-adrenoceptors. The mRNA levels of beta(1)- and beta(2)-adrenoceptors were not altered by propranolol treatment, while beta(1)-adrenoceptor mRNA significantly decreased after administration of atenolol or bevantolol without changing the level of beta(2)-adrenoceptor mRNA. The present binding study with intact tissue segments clearly shows that the plasma membrane beta(1)- and beta(2)-adrenoceptors of rat heart, in contrast to the homogenate binding sites and the mRNA levels, are differently affected by chronic treatment with three beta-adrenoceptor antagonists; up- and down-regulations of beta(1)- and beta(2)-adrenoceptors, respectively, by propranolol, and up-regulation of both the subtypes by atenolol, but no significant change in both the subtypes by bevantolol.

Identification of alpha-1L and alpha-1A adrenoceptors in human prostate by tissue segment binding

PURPOSE

Silodosin (KMD-3213 or [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2trifluoroethoxy)phenoxy]ethyl}amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]) (Kissei Pharmaceutical Co., Ltd., Matsumoto, Japan) is a selective antagonist for alpha-1A and alpha-1L adrenoceptors. Using this tritiated ligand the 2 alpha-1 adrenoceptors were examined in binding studies with intact tissue segments and membrane preparations of human prostate, and compared with functionally identified alpha-1 adrenoceptor.

MATERIALS AND METHODS

Binding assays with tissue segments and membrane preparations of human prostate samples were performed using [3H]-silodosin and binding affinities for various drugs were estimated. In functional experiments antagonist affinities were evaluated from the inhibitory potency against the contractile response to noradrenaline.

RESULTS

[3H]-silodosin bound to intact segments and membrane preparations of human prostate with subnanomolar affinity. [3H]-silodosin binding sites in intact segments were divided into 2 distinct components with different affinities for prazosin and RS-17053 (N-[2(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl1H-indole-3-ethanamine hydrochloride) (Research Biochemicals International, Natick, Massachusetts), while binding in membrane preparations showed single high affinity for these drugs. [3H]-silodosin binding sites also showed high affinity for silodosin and tamsulosin but low sensitivity to BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4.5)decane-7,9-dione) (Research Biochemicals International) in intact segments and in membrane preparations. In functional experiments silodosin and tamsulosin potently inhibited the contractile response to noradrenaline but prazosin, RS-17053 and BMY 7378 showed low antagonistic affinity.

CONCLUSIONS

The current binding studies in human prostate samples clearly show that alpha-1L and alpha-1A adrenoceptors coexist as pharmacologically distinct entities in intact tissues but not in crude membrane preparations. Also, alpha-1 adrenoceptors involved in the contractile response to noradrenaline are the alpha-1L subtype.

Beta-blockers show inverse agonism to a novel constitutively active mutant of beta1-adrenoceptor

We obtained a new mutant of the beta(1)-adrenergic receptor (beta(1)-AR) by point mutations that can constitutively activate beta(1)-AR. Aspartate104 of the beta(1)-AR in the 2nd transmembrane was replaced with alanine. The beta(1)-AR mutant expressed in human embryonic kidney (HEK)-293 cells displayed high level of constitutive activity with respect to wild-type (P<0.05), which could be partially inhibited by some beta-blockers. The constitutive activity of the mutant was confirmed by the finding that the enhanced activity is dependent on the level of receptor expression. The results of this study might have interesting implications for future studies aiming at elucidating the activation process of the beta(1)-AR as well as the mechanism of action of beta-blockers.