Existence of nicotinic receptors in a subset of type I vestibular hair cells of guinea pigs.

In mammals, vestibular hair cells (VHCs) are classified as type I and II according to morphological criteria. Acetylcholine (ACh) is identified as the primary efferent neurotransmitter. To date, cholinergic activities have been reported in mammalian type II VHCs, but similar activities in type I VHCs have not been pursued presumably because the body of type I VHCs were suggested to be totally surrounded by afferent nerve calyces. A few reports showed that part of type I VHCs were incompletely surrounded by calyces and received contact from the efferent nerve endings in the mammals studied. The possibility of the expression of cholinergic receptors, their subunit composition, and their function in mammals' type I VHCs are still unclear. In this study, nicotinic responses were investigated by the whole-cell patch clamp technique in isolated type I VHCs of guinea pigs. Of the cells, 7.3% were sensitive to cholinergic agonists and showed an excitatory current at -40 mV which was not sensitive to nifedipine, iberiotoxin (IBTX), and apamin. The main carriers of this current were Na+ and K+. The rank order of activation potency was nicotine > 1,1-dimethyl-4-phenyl-piperazinium (DMPP) > ACh. These nicotinic ACh receptors (nAChRs) were not blocked by strychnine and α-bungarotoxin (α-BTX), but sensitive to d-tubocurarine (dTC) and mecamylamine (Mec). The findings provide physiological evidence that some subtypes of nAChRs may be located in a subset of type I VHCs, which were different from α9α10 nAChRs.

Gentamicin blocks the ACh-induced BK current in guinea pig type II vestibular hair cells by competing with Ca²âº at the L-type calcium channel.

Type II vestibular hair cells (VHCs II) contain big-conductance Ca²âº-dependent K⁺ channels (BK) and L-type calcium channels. Our previous studies in guinea pig VHCs II indicated that acetylcholine (ACh) evoked the BK current by triggering the influx of Ca²âº ions through L-type Ca²âº channels, which was mediated by M2 muscarinic ACh receptor (mAChRs). Aminoglycoside antibiotics, such as gentamicin (GM), are known to have vestibulotoxicity, including damaging effects on the efferent nerve endings on VHCs II. This study used the whole-cell patch clamp technique to determine whether GM affects the vestibular efferent system at postsynaptic M2-mAChRs or the membrane ion channels. We found that GM could block the ACh-induced BK current and that inhibition was reversible, voltage-independent, and dose-dependent with an IC50 value of 36.3 ± 7.8 µM. Increasing the ACh concentration had little influence on GM blocking effect, but increasing the extracellular Ca²âº concentration ([Ca²âº]0) could antagonize it. Moreover, 50 µM GM potently blocked Ca²âº currents activated by (-)-Bay-K8644, but did not block BK currents induced by NS1619. These observations indicate that GM most likely blocks the M2 mAChR-mediated response by competing with Ca²âº at the L-type calcium channel. These results provide insights into the vestibulotoxicity of aminoglycoside antibiotics on mammalian VHCs II.

Two distinct channels mediated by m2mAChR and α9nAChR co-exist in type II vestibular hair cells of guinea pig.

Acetylcholine (ACh) is the principal vestibular efferent neurotransmitter among mammalians. Pharmacologic studies prove that ACh activates a small conductance Ca2+-activated K+ channels (KCa) current (SK2), mediated by α9-containing nicotinic ACh receptor (α9nAChR) in mammalian type II vestibular hair cells (VHCs II). However, our studies demonstrate that the m2 muscarinic ACh receptor (m2mAChR) mediates a big conductance KCa current (BK) in VHCs II. To better elucidate the correlation between these two distinct channels in VHCs II of guinea pig, this study was designed to verify whether these two channels and their corresponding AChR subtypes co-exist in the same VHCs II by whole-cell patch clamp recordings. We found that m2mAChR sensitive BK currents were activated in VHCs II isolated by collagenase IA, while α9nAChR sensitive SK2 currents were activated in VHCs II isolated by trypsin. Interestingly, after exposing the patched cells isolated by trypsin to collagenase IA for 3 min, the α9nAChR sensitive SK2 current was abolished, while m2mAChR-sensitive BK current was activated. Therefore, our findings provide evidence that the two distinct channels and their corresponding AChR subtypes may co-exist in the same VHCs II, and the alternative presence of these two ACh receptors-sensitive currents depended on isolating preparation with different enzymes.

M2 muscarinic ACh receptors sensitive BK channels mediate cholinergic inhibition of type II vestibular hair cells.

There are two types of hair cells in the sensory epithelium of vestibular end organ. Type II vestibular hair cell (VHC II) is innervated by the efferent nerve endings, which employ a cholinergic inhibition mediated by SK channels through the activation of α9-containing nAChR. Our previous studies demonstrated that a BK-type cholinergic inhibition was present in guinea pig VHCs II, which may be mediated by an unknown mAChR. In this study, BK channel activities triggered by ACh were studied to determine the mAChR subtype and function. We found the BK channel was insensitive to α9-containing nAChR antagonists and m1, m3, m4 muscarinic antagonists, but potently inhibited by the m2 muscarinic antagonist. Muscarinic agonists could mimic the effect of ACh and be blocked by m2 antagonist. cAMP analog activated the BK current and adenyl cyclase (AC) inhibitor inhibited the ACh response. Inhibitor of Giα subunit failed to affect the BK current, but inhibitor of Giα and Gißγ subunits showed a potent inhibition to these currents. Our findings provide the physiological evidence that mAChRs may locate in guinea pig VHCs II, and m2 mAChRs may play a dominant role in BK-type cholinergic inhibition. The activation of m2 mAChRs may stimulate Gißγ-mediated excitation of AC/cAMP activities and lead to the phosphorylation of Ca(2+) channels, resulting in the influx of Ca(2+) and opening of the BK channel.

[Co-location of ACh-sensitive BK channels and L-type calcium channels in type II vestibular hair cells of guinea pig].

OBJECTIVE: To explore the mechanisms of the influx of calcium ions during the activation of ACh-sensitive BK channel (big conductance, calcium-dependent potassium channel) in type II vestibular hair cells of guinea pigs. METHODS: Type II vestibular hair cells were isolated by collagenase type IA. Under the whole-cell patch mode, the sensitivity of ACh-sensitive BK current to the calcium channels blockers was investigated, the pharmacological property of L-type calcium channel activator-sensitive current and ACh-sensitive BK current was compared. RESULTS: Following application of ACh, type II vestibular hair cells displayed a sustained outward potassium current, with a reversal potential of (-70.5 +/- 10.6) mV (x +/- s, n = 10). At the holding potential of -50 mV, the current amplitude of ACh-sensitive potassium current activated by 100 micromol/L ACh was (267 +/- 106) pA (n = 11). ACh-sensitive potassium current was potently sensitive to the BK current blocker, IBTX (iberiotoxin, 200 nmol/L). Apamin, the well-known small conductance, calcium-dependent potassium current blocker, failed to inhibit the amplitude of ACh-sensitive potassium current at a dose of 1 micromol/L. ACh-sensitive BK current was sensitive to NiCl2 and potently inhibited by CdCl2. NiCl2 and CdCl2 showed a dose-dependent blocking effect with a half inhibition-maximal response of (135.5 +/- 18.5) micromol/L (n = 7) and (23.4 +/- 2.6) micromol/L (n = 7). The L-type calcium channel activator, (-) -Bay-K 8644 (10 micromol /L), mimicked the role of ACh and activated the IBTX-sensitive outward current. CONCLUSION: ACh-sensitive BK and L-type calcium channels are co-located in type II vestibular hair cells of guinea pigs.

[Properties of cholinergic receptor-mediated ion channels on type I vestibular hair cells of guinea pigs].

To confirm the existence of cholinergic receptors on type I vestibular hair cells (VHCs I) of guinea pigs and to study the properties of the cholinergic receptor-mediated ion channels on VHCs I, electrophysiological responses of isolated VHCs I to external ACh were examined by means of whole-cell patch-clamp recordings. The results showed that 7.5% (21/279) VHCs I were found to be sensitive to ACh (10-1000 µmol/L). ACh generated an outward current in a steady, slow, dose-dependent [EC(50) was (63.78±2.31) µmol/L] and voltage-independent manner. In standard extracellular solution, ACh at the concentration of 100 µmol/L triggered a calcium-dependent current of (170±15) pA at holding potential of -50 mV, and the current amplitude could be depressed by extracellularly added calcium-dependent potassium channel antagonist TEA. The time interval for the next complete activation of ACh-sensitive current was no less than 1 min. The ion channels did not shut off even when they were exposed to ACh for an extended period of time (8 min). The results suggest that dose-dependent, calcium-dependent and voltage-independent cholinergic receptors were located on a few of the VHCs I investibular epithelium of guinea pigs. The cholinergic receptors did not show desensitization to ACh. This work reveals the existence of efferent neurotransmitter receptors on VHCs I and helps in understanding the function of vestibular efferent nervous system, and may provide some useful information on guiding the clinical rehabilitative treatment of vertigo.

[Intrinsic membrane properties of rat medial vestibular nucleus neurons and their responses to simulated vestibular input signals].

OBJECTIVE: To study the membrane properties of rat medial vestibular nucleus (MVN) neurons and their firing responses to simulated input signals of peripheral vestibular system, and to discuss how the intrinsic membrane properties contribute to physiologic functions in central vestibular system. METHODS: By using infrared differential interference contrast technique, whole-cell recordings were made from rat MVN neurons under direct observation. On the basis of their averaged action potential shapes, the MVN neurons were classified. Linear and non-linear currents were put into the neurons to simulate the input signals of peripheral vestibular system. The differences of intrinsic membrane properties and firing response dynamics were observed between two types. RESULTS: The discharge activities were recorded in MVN neurons, which remained in low Ca2+-high Mg2+ artificial cerebrospinal fluid (ACSF). Neurons are classified as type A (33%) characterized by a single deep after-hyperpolarization (AHP) and A-like rectification, or type B (63%) characterized by double AHP, and another two neurons with all or none of the characters. The passive membrane properties were not significantly different between type A and type B neurons, while part of active membrane properties was significantly different. Both type A and B neurons well responded to simulated current inputs, but disparities existed in response range and firing dynamics. CONCLUSIONS: The discharge activities of MVN neurons were initiated by their intrinsic membrane properties. Most MVN neurons were classified as type A and B, while several showed unrepresentative firing properties. Linear and nonlinear inputs evoked a heterogeneous range of firing responses. The differences of response range and firing dynamics between neurons may determine their different physiological functions.

Association of body position with sleep architecture and respiratory disturbances in children with obstructive sleep apnea.

CONCLUSIONS: Sleep position in children with obstructive sleep apnea (OSA) did not affect the sleep architecture. Supine position had a significant influence on respiratory parameters and gas exchange. Lateral position did not affect respiratory parameters in children aged 3-5 years and little affected gas exchange except right lateral decubitus (RLD) position in children aged 11-13 years. Children with OSA breathe best when in the left lateral decubitus (LLD) position. OBJECTIVE: To analyze the relationship between OSA and body position during sleep in children. SUBJECTS AND METHODS: A total of 45 consecutive children (age class A: 3-5 years, n = 15; B: 6-10 years, n = 14; C: 11-13 years, n = 16) with OSA were studied. The following variables were evaluated: apnea hypopnea index (AHI) in supine, LLD, and RLD positions, sleep architecture, respiratory parameters, and oxygen saturation. RESULTS: There were no no significant differences in AHI between LLD AHI, RLD AHI, and supine position in age class A, LLD AHI was significantly lower than supine AHI in age classes B and C. There was no correlation between position AHI and sleep architecture. There was no correlation between lateral position AHI and the parameters of respiratory disturbance in age class A. The parameters of respiratory disturbance were related to LLD and RLD position in age class B; supine and RLD position in age class C were similar. Position AHI was related to some of the parameters of gas exchange except for RLD AHI in age classes A and B.

[Feature of ACh-sensitive potassium current in outer hair cells of guinea pig cochlea].

OBJECTIVE: To explore the feature of the ACh-sensitive potassium current in guinea pig cochlear outer hair cells. METHODS: Cochlear outer hair cells of guinea pigs (n=38) were isolated by collagenase type IV. Under the whole-cell patch mode, the ions nature and the pharmacological properties of the ACh-sensitive potassium current were investigated by applying the inhibitors of calcium-dependent potassium currents and the inhibitors of nicotinic ACh receptor. RESULTS: Following application of ACh, cochlear outer hair cells displayed a rapidly activating outward potassium current with a fast desensitized kinetic and a reversal (x +/- s) potential of (-67.3 +/- 8.2) mV (n=10). At the holding potential of -50 mV, the current amplitude of ACh-sensitive potassium current activated by 100 micronmol/L ACh was (506.6 +/- 186.3) pA (n=9). ACh-sensitive potassium current was sensitive to TEA (tetraethylammonium chloride, 10 mmol/L) and potently inhibited by the small conductance calcium-dependent potassium current (SK) blocker, apamin (1 micromol/L). Iberiotoxin (IBTX), the well-known blocker of big conductance calcium-dependent potassium current (BK), failed to inhibit the amplitude of the ACh-sensitive potassium current at the dose of 200 nmol/L. The dose for half-maximal response (EC50) of the ACh-sensitive potassium current was (33.5 +/- 5.7) micromol/L (n=7). The ACh-sensitive potassium current was sensitive to the GABA (gamma-aminobutyric acid)-A receptor blocker, bicuculline, and strongly inhibited by the selective blocker of the alpha 9-nicotinic ACh receptor, strychnine. Strychnine and bicuculline showed the dose-dependent blocking effect with a half inhibition-maximal response (IC50) of (22.3 +/- 2.6) nmol/L (n=7) and (1.2 +/- 0.4) micromol/L (n=6), respectively. CONCLUSIONS: This work provides direct evidences that the ACh-sensitive SK current was present on guinea pig cochlear outer hair cells. The activation of the ACh-sensitive SK current was most possibly mediated by a alpha 9-nicotinic ACh receptor.

[Electrophysiological characteristics of muscarinic cholinergic receptor in rat medial vestibular nucleus neurons by visual patch clamp technique].

OBJECTIVE: To establish the visual patch clamp whole-cell recording technique and study the properties and functional significance of muscarinic receptor-mediated currents in rat medial vestibular nucleus neurons (MVNn). METHODS: Brain slices containing the MVN were prepared from fifteen Wistar rats. By combining infrared differential interference contrast (IR-DIC) technique and CCD-Camera system with visual patch clamp whole-cell recording technique, twenty healthy neurons were located and muscarinic receptor-mediated currents in rat MVNn were observed and analyzed. RESULTS: Visual patch clamp technique can be used to make direct localization and to make sure of active neuron. In MVNn, a comparison of the current-voltage relationships before and during the application of muscarine, which revealed an increase in the slope of the I-V curve and the reversal potential for this response lay at (-88.4 +/- 4.9) mV (x +/- s), indicates that the activation of muscarinic cholinergic receptors leads to a decrease in potassium current. The test in the voltage sensitivity of the muscarine-induced response, which showed that the effect had a linear current-voltage relationship and reversed at (-86.7 +/- 3.5) mV, indicates that the potassium current blocked by muscarine corresponds to the voltage-insensitive leak potassium current. CONCLUSIONS: Visual patch clamp technique, which was considered better than blind patch clamp technique, can improve the success of sealing process. By the analysis of muscarinic receptor-mediated currents, the data provide support that muscarinic cholinergic mechanisms play a prominent role in the modulation of the excitability of MVNn and also offer a new idea for the efficacy of anticholinergic drugs.

Comparison of polygraphic parameters in children with adenotonsillar hypertrophy with vs without obstructive sleep apnea.

OBJECTIVE: To compare the polygraphic parameters in children with adenotonsillar hypertrophy (ATH) with vs without obstructive sleep apnea (OSA). DESIGN: Prospective controlled study. SETTING: Hospital-based pediatric otolaryngology practice. PATIENTS: Children with ATH. INTERVENTIONS: The children enrolled in the study underwent polysomnography. According to the apnea index (AI) (a patient who has at least 1 episode of apnea per hour of sleep is considered to have apnea), they were classified as having ATH with OSA or ATH without OSA. MAIN OUTCOME MEASURES: We evaluated polysomnography parameters to describe the macrostructure of sleep (sleep efficiency, nonrapid eye movement stages 1-4, and rapid eye movement) and the microstructure of sleep (using electroencephalogram results and movement arousals) and respiratory events. RESULTS: Twenty children were classified as having ATH with OSA and 17 as having ATH without OSA. We found no significant differences in sleep macrostructure and microstructure between the ATH groups with vs without OSA. Apnea-hypopnea indices (AHI), respiratory disturbance events, hypopnea events in rapid eye movement and AHI, AI, respiratory disturbance events, obstructive events, hypopnea events, the duration of obstructive events, and hypopnea events during nonrapid eye movement were more frequent or of longer duration in children with OSA vs those without OSA (P < .05). CONCLUSIONS: Obstructive sleep apnea should be considered a disorder on the continuum of ATH. To our knowledge, our results clearly and for the first time demonstrate that more severe respiratory disturbances seem to be important risk factors for ATH to develop into OSA in children.

Gene silencing of TKTL1 by RNAi inhibits cell proliferation in human hepatoma cells.

We detected a strong upregulation of the mutated transketolase transcript (TKTL1) in human hepatoma cell line HepG2, whereas transketolase (TKT) and transketolase-like-2 (TKTL2) transcripts were not upregulated. We inhibited the expression of TKTL1 by RNAi in HepG2 cells. It was found that total transketolase activity was dramatically downregulated and the proliferation of cancer cells was significantly inhibited in HepG2 cells. These results indicate that TKTL1 gene influences total transketolase activity and cell proliferation in human hepatoma cells, suggesting that TKTL1 gene plays an important role on glycometabolism in tumors and it might become a novel target for tumor gene therapy.

The coupling of acetylcholine-induced BK channel and calcium channel in guinea pig saccular type II vestibular hair cells.

Molecular biological studies and electrophysiological data have demonstrated that acetylcholine (ACh) is the principal cochlear and vestibular efferent neurotransmitter among mammalians. However, the functional roles of ACh in type II vestibular hair cells (VHCs II) among mammalians are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9-containing nAChR)-activated small conductance, calcium-dependent potassium current (SK) in cochlear hair cells and frog saccular hair cells. The activation of SK current was necessary for the calcium influx through the alpha9-containing nAChR. Recently, we have demonstrated that ACh-induced big conductance, calcium-dependent potassium current (BK) was present in VHCs II of the vestibular end-organ of guinea pig. In this study, the nature of calcium influx for the activation of ACh-induced BK current in saccular VHCs II of guinea pig was investigated. Following extracellular perfusion of ACh, saccular VHCs II displayed a sustained outward current, which was sensitive to iberiotoxin (IBTX). High concentration of apamin failed to inhibit the current amplitude of ACh-induced outward current. Intracellular application of Cs(+) completely abolished the current evoked by ACh. ACh-induced current was potently inhibited by nifedipine, nimodipine, Cd(2+) and Ni(2+), respectively. The inhibition potency of these four calcium channel antagonists was nimodipine>nifedipine>cadmium>nickel. The L-type Ca(2+) channels agonist, (-)-Bay-K 8644 mimicked the effect of ACh and activated an IBTX-sensitive current. In addition, partial VHCs II displayed a biphasic waveform. In conclusion, the present data showed that in the guinea pig saccular VHCs II, ACh-induced BK channel was coupled with the calcium channel, but not the receptor. The perfusion of ACh will drive the opening of calcium channels; the influx of calcium ions will then activate the BK current.

[Dependence of Ca2+ on the acetylcholine-sensitive current in guinea pig type II vestibular hair cells].

OBJECTIVE: To explore the dependence of Ca2+ on the acetylcholine (ACh)-sensitive potassium current in guinea pig type II vestibular hair cells. METHODS: Under the whole-cell patch mode, the current amplitude of the ACh-sensitive potassium current was recorded in response to the concentration change of the extracellular or intracellular Ca2+. RESULTS: Following application of ACh, type II vestibular hair cells displayed the sustained potassium current, which was inhibited by tetraethylammonium chloride (TEA), but not inhibited by 4-aminopyrine (4-AP). The activation of the ACh-sensitive potassium current was strongly affected by the concentration of the extracellular Ca2+. The current amplitude of the ACh-sensitive potassium increased following the increase of Ca2+ concentration from 0 mmol/L to 4 mmol/L At the concentration of 4 mmol/L Ca2+, the current amplitude of the ACh-sensitive potassium current reached the maximal response. Lowering the Ca2 concentration in the external solution from 4 mmol/L to 0. 5 mmol/L, the current amplitude of the ACh-sensitive potassium current was inhibited to (36.5 +/- 6.5)%. However, no difference was found in the presence and in the absence of the intracellular heparin, which was a well-known blocker of the inositol trisphosphate-dependent calcium release channels. In addition, the calcium channel blocker, Cd2+, inhibited the ACh-sensitive potassium current. CONCLUSIONS: The activation of the ACh-sensitive potassium current in guinea pig type II vestibular hair cells was dependent on the extracellular Ca2+ influx through the calcium channel. The application of ACh would stimulate membrane Ca2+ channels; the influx of Ca2+ will then activate the calcium-dependent potassium current in guinea pig type II hair cells to mediate the hyperpolarization effect.

Fast cholinergic efferent inhibition in guinea pig outer hair cells.

Hair cells of inner ear are suggested to be inhibited by the activation of the alpha9-containing nicotinic acetylcholine (ACh) receptors (alpha9-containing nAChRs). Several studies have suggested that the native nicotinic-like ACh receptors (nAChRs) in hair cells display a significant permeability of Ca(2+) ions and unusual pharmacological properties. The activation of native nAChRs will initiate the hyperpolarization of hair cells by activation of the small conductance, Ca(2+)-activated K(+) channels (SK). In this work, the properties of the ACh-sensitive potassium current (IK(ACh)) in outer hair cells (OHCs) of guinea pigs were investigated by employing whole-cell patch-clamp. Followed by perfusion of ACh, OHCs displayed a rapid desensitized current with an N-shaped current-voltage curve (I-V) and a reversal potential of - 66 +/- 7 mV. The IK(ACh) was still present during perfusion of either iberiotoxin (IBTX, 200 nM) or TEA (5 mM) but was potently inhibited by apamin (1 muM), TEA (30 mM). The IK(ACh) demonstrated a strong sensitivity to alpha-bungarotoxin (alpha-BgTx), bicuculline and strychnine. These results suggested that OHCs display the well-known SK current, which might be gated by the alpha9-containing nAChRs. Two important changes were present after lowering the Ca(2+) concentration in the external conditions from 2 mM to 0.2 mM: one was a flattened N-shape I-V relationship with a maximum shifted toward hyperpolarized potentials from -20 approximately -30 mV approximately -40 to -50 mV, the other was a significant reduction in the agonist maximal response (percentage of maximal response 10.5 +/- 5.4). These results indicated that native nAChRs are both permeable to and modulated by extracellular Ca(2+) ions. Taken together, this work provides direct evidences that SK channels in OHCs of guinea pigs are gated by alpha9-containing nAChRs, which play an important role in the fast cholinergic efferent inhibition. This fast inhibition is both potently dependent on the permeability of Ca(2+) ions through the native nAChRs and modulated by Ca(2+) ions.

[Cation ions modulate the ACh-sensitive current in type II vestibular hair cells of guinea pigs].

Molecular biological studies and electrophysiological data have demonstrated that acetylcholine (ACh) is the principal cochlear and vestibular efferent neurotransmitter among mammalians. However, the functional roles of ACh in type II vestibular hair cells among mammalians are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9-nAChR) in cochlear hair cells and frog saccular hair cells. In this study, the properties of the ACh-sensitive current were investigated by whole-cell patch clamp technique in isolated type II vestibular hair cells of guinea pigs. The direct effect of extracellular ACh was to induce a hyperpolarization effect in type II vestibular hair cells. Type II vestibular hair cells displayed a sustained outward current in response to the perfusion of ACh. It took about 60 s for the ACh-sensitive current to get a complete re-activation. The reversal potential of the ACh-sensitive current was (-66 +/- 8) mV, which indicated that potassium ion was the main carrier of this current. The blocking effect by the submillimolar concentration of tetraethylammonium (TEA) further indicated that extracellular ACh stimulated the calcium-dependent potassium current. Following replacement of the compartment of NaCl in the normal external solution with TrisCl, LiCl or saccharose respectively, the amplitude of the ACh-sensitive current was not affected. Blocking of the release of intracellular Ca(2+) stores by intracellular application of heparin failed to inhibit the ACh-sensitive current. Therefore, extracellular Na(+)and the inositol 1,4,5-trisphosphate (IP(3))-dependent intracellular Ca(2+)release were not involved in the activation of the ACh-sensitive current. However, the ACh-sensitive current was strongly affected by the concentration of the extracellular K(+), extracellular Ca(2+) and intracellular Mg(2+). The amplitude of the ACh- sensitive current was strongly inhibited by high concentration of extracellular K(+). In the Ca(2+)-free external solution, ACh only activated a very small current; however, the ACh-sensitive current demonstrated a Ca(2+)-dependent inhibition effect in high concentration of Ca(2+)solution. In addition, the ACh-sensitive current was inhibited by increasing of the concentration of intracellular Mg(2+). In conclusion, the present results demonstrate that ACh plays an important role in the vestibular efferent system. The fact that Na(+) is not involved in the ACh-sensitive current will not favor the well-known profile of alpha9-nAChR, which is reported to display a small but important permeability to Na(+). It is also suggested that, in vivo, the amplitude of the ACh-induced hyperpolarization may strongly depend on the concentration of extracellular Ca(2+)and intracellular Mg(2+).

Effect of methylation-associated silencing of the death-associated protein kinase gene on nasopharyngeal carcinoma.

Death-associated protein kinase (DAPK) is a Ca/calmodulin-regulated serine/threonine kinase and a positive mediator of apoptosis. Loss of expression of the DAPK gene by aberrant promoter methylation may play an important role in cancer development and progression. The aim of this study was to investigate the frequency of gene promoter methylation of DAPK in nasopharyngeal carcinoma (NPC) and the effect of 5-Aza-2'-deoxycytidine (5-Aza-CdR), a demethylating agent, on CNE cells, a human nasopharyngeal carcinoma cell line, and on xenografts of CNE cells. Methylation-specific PCR and RT-PCR were used to determine the promoter methylation status and mRNA expression of the DAPK gene in NPC. Furthermore, CNE cells were treated in vitro and in vivo with 5-Aza-CdR to explore the effect of demethylating agents on DAPK mRNA expression and tumor growth. Hypermethylation of the DAPK gene promoter was found in 35 (76.1%) of 46 NPC samples. There was no significant difference in the promoter hypermethylation rate among samples from patients with different TNM stages. No promoter hypermethylation of the DAPK gene was found in all six chronic inflammatory nasopharyngeal tissue specimens. DAPK mRNA expression was not detected in NPC tumor specimens with promoter hypermethylation. However, DAPK mRNA expression was observed in unmethylated NPC tumors and in the chronic inflammatory nasopharyngeal tissue specimens. Promoter hypermethylation of the DAPK gene was found and no DAPK mRNA expression was detected in CNE cells. DAPK mRNA expression in CNE cells and xenografts could be restored by treatment with 5-Aza-CdR. The CNE cell xenografts of nude mice treated with 5-Aza-CdR were obviously smaller in tumor volume than those of nude mice treated with PBS. These results demonstrate that loss of DAPK expression could be associated with promoter region methylation in NPC. 5-Aza-CdR may slow the growth of CNE cells in vitro and in vivo by reactivating the DAPK gene silenced by de novo methylation.

The properties of ACh-induced BK currents in guinea pig type II vestibular hair cells.

Molecular biological studies have demonstrated that both muscarinic receptor subtypes and nicotinic receptor subunits were located in mammalian vestibular sensorineural epithelium. However, the functional roles are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9nAChR). In this study, the properties of acetylcholine (ACh)-induced currents were investigated by whole-cell patch clamp technique in isolated type II vestibular hair cells (VHCs II) of guinea pig. VHCs II displayed a sustained, non-inactivating current when extracellular application of ACh. ACh-induced currents restored gradually and it took about 60 s to get a complete recovery. ACh-induced current was not affected by extracellular Na(+), but strongly affected by extracellular K(+) and Ca(2+). Depletion of the intracellular Ca(2+) stores by intracellular application of inositol 1,4,5-trisphosphate (IP3) or blocking of the release of intracellular Ca(2+) stores by intracellular application of heparin failed to inhibit this current. ACh-induced currents were inhibited by nifedipine, Cd(2+), tetraethylammonium (TEA), charybdotoxin (CTX), iberiotoxin (IBTX), atropine and d-tubocurarine (DTC), respectively, but not by apamin. In conclusion, ACh stimulates a large conductance, Ca(2+)-activated K(+) current (BK) in guinea pig VHCs II by activation of the influx of Ca(2+) ions, which is mediated by an ACh receptor that could not be defined to be the odd-number muscarinic receptor.