Effects of antihistamines on the function of human α7-nicotinic acetylcholine receptors

Effects of the histamine H₁ receptor (H1R) antagonists (antihistamines), promethazine (PMZ), orphenadrine (ORP), chlorpheniramine (CLP), pyrilamine (PYR), diphenhydramine (DPH), citerizine (CTZ), and triprolidine (TRP) on the functional properties of the cloned α7 subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were investigated. Antihistamines inhibited the α7-nicotinic acetylcholine receptor in the order PYR>CLP>TRP>PMZ>ORP≥DPH≥CTZ. Among the antihistamines, PYR showed the highest reversible inhibition of acetylcholine (100 µM)-induced responses with IC₅₀ of 6.2 µM. PYR-induced inhibition was independent of the membrane potential and could not be reversed by increasing the concentration of acetylcholine. Specific binding of [¹²⁵I] α-bungarotoxin, a selective antagonist for α7-nicotinic acetylcholine receptor, was not changed in the presence of PYR suggesting a non-competitive inhibition of nicotinic receptors. In line with functional experiments, docking studies indicated that PYR can potentially bind allosterically with the α7 transmembrane domain. Our results indicate that the H₂-H₄ receptor antagonists tested in this study (10 µM) showed negligible inhibition of α7-nicotinic acetylcholine receptors. On the other hand, H₁ receptor antagonists inhibited the function of human α7-nicotinic acetylcholine receptor, with varying potencies. These results emphasize the importance of α7-nicotinic acetylcholine receptor for future pharmacological/toxicological profiling.

Effects of cannabidiol on the function of α7-nicotinic acetylcholine receptors

The effects of cannabidiol (CBD), a non-psychoactive ingredient of cannabis plant, on the function of the cloned α7 subunit of the human nicotinic acetylcholine (α7 nACh) receptor expressed in Xenopus oocytes were tested using the two-electrode voltage-clamp technique. CBD reversibly inhibited ACh (100 μM)-induced currents with an IC50 value of 11.3 µM. Other phytocannabinoids such as cannabinol and Δ(9)-tetrahydrocannabinol did not affect ACh-induced currents. CBD inhibition was not altered by pertussis toxin treatment. In addition, CBD did not change GTP-γ-S binding to the membranes of oocytes injected with α7 nACh receptor cRNA. The effect of CBD was not dependent on the membrane potential. CBD (10 µM) did not affect the activity of endogenous Ca(2+)-dependent Cl(-) channels, since the extent of inhibition by CBD was unaltered by intracellular injection of the Ca(2+) chelator BAPTA and perfusion with Ca(2+)-free bathing solution containing 2mM Ba(2+). Inhibition by CBD was not reversed by increasing ACh concentrations. Furthermore, specific binding of [(125)I] α-bungarotoxin was not inhibited by CBD (10 µM) in oocytes membranes. Using whole cell patch clamp technique in CA1 stratum radiatum interneurons of rat hippocampal slices, currents induced by choline, a selective-agonist of α7-receptor induced currents were also recoded. Bath application of CBD (10 µM) for 10 min caused a significant inhibition of choline induced currents. Finally, in hippocampal slices, [(3)H] norepinephrine release evoked by nicotine (30 µM) was also inhibited by 10 µM CBD. Our results indicate that CBD inhibits the function of the α7-nACh receptor.

Menthol inhibits 5-HT3 receptor-mediated currents

The effects of alcohol monoterpene menthol, a major active ingredient of the peppermint plant, were tested on the function of human 5-hydroxytryptamine type 3 (5-HT3) receptors expressed in Xenopus laevis oocytes. 5-HT (1 μM)-evoked currents recorded by two-electrode voltage-clamp technique were reversibly inhibited by menthol in a concentration-dependent (IC50 = 163 μM) manner. The effects of menthol developed gradually, reaching a steady-state level within 10-15 minutes and did not involve G-proteins, since GTPγS activity remained unaltered and the effect of menthol was not sensitive to pertussis toxin pretreatment. The actions of menthol were not stereoselective as (-), (+), and racemic menthol inhibited 5-HT3 receptor-mediated currents to the same extent. Menthol inhibition was not altered by intracellular 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid injections and transmembrane potential changes. The maximum inhibition observed for menthol was not reversed by increasing concentrations of 5-HT. Furthermore, specific binding of the 5-HT3 antagonist [(3)H]GR65630 was not altered in the presence of menthol (up to 1 mM), indicating that menthol acts as a noncompetitive antagonist of the 5-HT3 receptor. Finally, 5-HT3 receptor-mediated currents in acutely dissociated nodose ganglion neurons were also inhibited by menthol (100 μM). These data demonstrate that menthol, at pharmacologically relevant concentrations, is an allosteric inhibitor of 5-HT3 receptors.

Menthol binding and inhibition of α7-nicotinic acetylcholine receptors

Menthol is a common compound in pharmaceutical and commercial products and a popular additive to cigarettes. The molecular targets of menthol remain poorly defined. In this study we show an effect of menthol on the α₇ subunit of the nicotinic acetylcholine (nACh) receptor function. Using a two-electrode voltage-clamp technique, menthol was found to reversibly inhibit α7-nACh receptors heterologously expressed in Xenopus oocytes. Inhibition by menthol was not dependent on the membrane potential and did not involve endogenous Ca²⁺-dependent Cl⁻ channels, since menthol inhibition remained unchanged by intracellular injection of the Ca²⁺ chelator BAPTA and perfusion with Ca²⁺-free bathing solution containing Ba²⁺. Furthermore, increasing ACh concentrations did not reverse menthol inhibition and the specific binding of [¹²⁵I] α-bungarotoxin was not attenuated by menthol. Studies of α₇- nACh receptors endogenously expressed in neural cells demonstrate that menthol attenuates α₇ mediated Ca²⁺ transients in the cell body and neurite. In conclusion, our results suggest that menthol inhibits α7-nACh receptors in a noncompetitive manner.

Involvement of the α7-nicotinic acetylcholine receptors in the anti-inflammatory action of the thymulin-related peptide (PAT)

BACKGROUND AND PURPOSE

Peptide analog of thymulin (PAT) has been shown to have anti-hyperalgesic and anti-inflammatory properties in animal models of inflammation. Recent reports suggest that the peripheral cholinergic system has an anti-inflammatory role mediated by α7-nicotinic acetylcholine receptor (α7-nAChR). Our aim is to investigate whether the action of PAT is mediated, via the cholinergic pathway.

EXPERIMENTAL APPROACH

The anti-hyperalgesic and anti-inflammatory action of PAT was assessed in rat models of inflammatory nociceptive hyperactivity (carrageenan and endotoxin) and in a mice air-pouch model for localized inflammation, respectively; the possible attenuation of PAT's effects by pretreatment with the α7-nAchR specific antagonist methyllycaconitine citrate (MLA) was also investigated. In another series of experiments, using two electrode recordings, the effect of PAT on the α7-nAChRs, expressed in Xenopus Oocytes, was also determined.

KEY RESULTS

Administration of PAT reversed inflammatory nociceptive hyperactivity and cold and tactile hyperactivity in rats. This effect was partially or totally prevented by MLA, as assessed by different behavioral pain tests. Treatment with PAT also reduced the alteration of cytokines and NGF levels by carrageenan injection in the mouse air pouch model; this effect was partially antagonized by MLA. Electrophysiological recording demonstrated that PAT significantly potentiated the α7-nAchR expressed in Xenopus Oocytes. These effects were not observed when a control peptide, with a reverse sequence (rPAT), was utilized.

CONCLUSIONS AND IMPLICATIONS

The behavioral and electrophysiological observations described in this report demonstrate that PAT mediates, at least partially, its anti-inflammatory action by potentiating the α7-nAChR. These results indicate that PAT has a potential for new therapeutic applications as anti-inflammatory and analgesic agent.

Methylene blue inhibits the function of α7-nicotinic acetylcholine receptors

Methylene Blue (MB) is being investigated in clinical studies for its beneficial effects in the treatment of Alzheimer disease. However, its exact mechanisms of action have not been fully elucidated. The modulation of nicotinic acetylcholine receptors (nAChRs) has been suggested to play a role in the pathogenesis of various neurodegenerative diseases. Therefore, in the present study, the effect of MB on the function of the cloned α7 subunit of the human nAChR expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp technique. MB reversibly inhibited ACh (100 μM)-induced currents in a concentration-dependent manner with an IC50 value of 3.4±0.3 μM. The effect of MB was not dependent on the membrane potential. MB did not affect the activity of endogenous Ca2+-dependent Cl- channels, since the inhibition by MB was unaltered in oocytes injected with the Ca2+ chelator 1,2-bis (o-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid and perfused with Ca2+-free bathing solution containing 1.8 mM Ba2+. MB decreased the maximal ACh-induced responses without significantly affecting ACh potency. Furthermore, specific binding of [125I] α-bungarotoxin, a radioligand selective for the α7 nAChR, was not altered by MB (10 μM), indicating that MB acts as a noncompetitive antagonist on α7 nAChRs. In hippocampal slices, whole-cell recordings from CA1 pyramidal neurons indicated that the increases in the frequency and amplitudes of the γ-aminobutyric acid-mediated spontaneous postsynaptic currents induced by bath application of 2 mM choline, a specific agonist for α7 nAChRs, were abolished after 10 min application of 3 μM MB. These results demonstrate that MB inhibits the function of human α7 nAChRs expressed in Xenopus oocytes and of α7 nAChR-mediated responses in rat hippocampal neurons.

Occupational noise exposure and hearing loss of workers in two plants in eastern Saudi Arabia

OBJECTIVE

To determine the prevalence of hearing loss associated with occupational noise exposure and other risk factors.

DESIGN

A cross-sectional study involving 269 exposed and 99 non-exposed subjects (non-industrial noise exposed subjects) randomly selected. Current noise exposure was estimated using both sound level meter and noise-dosimeter. Past noise exposure was estimated by interview questionnaire. Otoscopic examination and conventional frequency (0.25-8 kHz) audiometry were used to assess the hearing loss in each subject.

RESULTS

75% (202 subjects) from the exposed group were exposed to a daily Leq above the permissible level of 85 dB(A) and most (61%) of these did not and had never used any form of hearing protection. Hearing loss was found to be bilateral and symmetrical in both groups. Bivariate analysis showed a significant hearing loss in the exposed vs non-exposed subjects with a characteristic dip at 4 kHz. Thirty eight percent of exposed subjects had hearing impairment, which was an 8-fold higher rate than that found for non-exposed subjects. Multivariate analysis indicated exposure to noise was the primary, and age the secondary predictor of hearing loss. Odds of hearing impairment were lower for a small sub-group of exposed workers using hearing protection (N=19) in which logistic regression analysis showed the probability of workers adopting hearing protective devices increased with noise exposure, education, and awareness of noise control. Hearing loss was also greater amongst those who used headphones to listen to recorded cassettes.

CONCLUSION

Gross occupational exposure to noise has been demonstrated to cause hearing loss and the authors believe that occupational hearing loss in Saudi Arabia is a widespread problem. Strategies of noise assessment and control are introduced which may help improve the work environment.

High-frequency (10-18 kHz) hearing thresholds: reliability, and effects of age and occupational noise exposure

The objective was to investigate the reliability and effects of age and noise on high-frequency hearing thresholds. A cross-sectional study was used involving 187 exposed and 52 non-industrial noise-exposed subjects selected randomly from noise-exposed and non-industrial noise-exposed subjects, respectively. Each subject was tested with both conventional-frequency (0.25-8 kHz) and high-frequency (10-18 kHz) audiometry. Test-retest results showed that high-frequency audiometry (HFA) was as reliable as the conventional procedure. Although the inter-subject variation was large, the intra-subject variation was small, indicating that HFA can be used more reliably than the conventional procedure to monitor individual cases over time. Both the hearing threshold at high frequencies and the upper frequency limit deteriorated as a function of age and frequency. The exposed subjects had significantly higher hearing thresholds than the non-exposed subjects at all the high frequencies tested, the difference between the two groups being greatest at 14 kHz. Multivariate analysis indicated that age was the primary predictor and noise exposure the secondary predictor of hearing thresholds in a high frequency range (10-18 kHz). In contrast, multivariate analysis indicated the reverse order-noise exposure as the primary predictor, then age-for a conventional frequency range (0.25-8 kHz). The results of this study suggest that HFA might be used as an early indicator for noise-induced hearing loss and acoustic trauma rather than audiometry at a conventional frequency (4 kHz), particularly for younger groups.

Quality assurance in an ear, nose and throat department in Saudi Arabia. A surgical audit

Quality assurance (QA) in health care is being introduced in developing countries also. However, QA still meets resistance from some clinicians. A prospective voluntary surgical audit was set up in the ENT department of a university teaching hospital in Saudi Arabia. Roles, responsibilities and objectives were defined and designated facilities were provided. There were 2125 admissions and 2389 operations in a 3-yr period. The stated objectives were achieved concerning resource management, post-graduate training, and, especially, the successful establishment of surgical audit as an on-going and open-forum activity in a developing country. We conclude that other clinicians can feel encouraged by our positive experience and implement QA.

Middle ear effusion in children: review of recent literature

Middle Ear Effusion (O.M.E.) is the commonest ear problem among children, with a peak at the age of 2 years. Incidence varies according to geographical location and race variation, environmental and socio-economic factors. The disease is characterized by accumulation of semi-sterile secretion in the middle ear. It is usually due to either a dysfunction of Eustachian tube or of the mucociliary system or both. The most common presenting symptoms are mild to moderate conductive hearing loss, attacks of earache, and deterioration in school. The diagnosis is based on history, clinical findings and hearing assessments. In general, treatment is classified as conservative and surgical. Early screening and health education are the recent concepts advocated, for early detection.

Hearing screening of infants at King Fahd Hospital of the University

The detection of hearing impairment in neonatal life and prompt intervention prevents the hazards of physical and developmental delay in speech. During a 12-month period between April 1987 and March 1988, the hearing of 151 well babies and 30 high-risk babied aged 6 to 12 months attending the pediatric clinics of King Fahd Hospital of the University (KFHU), Al-Khobar, were screened. The modified Ewing-Stykar test was performed on all babies. Tympanometry, stapedial reflex, and brainstem evoked response (BERA) were carried out in the well babies who failed the screening test and in all high-risk babies. The failure rate was found to be 4.6% and 43.3% and the validity index 71.4% and 92.3% for the well and high-risk babies, respectively. The results of this limited study encourages us to recommend the implementation of this screening test in all the well baby and the high-risk baby clinics in the Kingdom of Saudi Arabia.

Sensorineural hearing loss in sickle cell disease patients in Saudi Arabia

Among the complications of sickle cell disease, sensorineural deafness is well documented in the Western literature; little is known about it in Saudi Arabian patients, in whom the general pattern of the disease has been reported to be "mild," attributed partly to high levels of HbF. The auditory function of 42 patients with sickle cell anemia was evaluated clinically and audiometrically and compared with 27 normal controls. Sensorineural deafness was observed in 10 (23.8%) of our patients.

Hearing levels of school children in Dammam

Eight hundred children were examined clinically and audiometrically at four frequencies in order to evaluate the incidence of hearing loss in school children of Dammam City (Saudi Arabia) and to analyse the aetiological factors and geographical distribution of these hearing-impaired children. This study revealed that 57 (7.12 per cent) children had hearing defects, of whom 53 (92.98 per cent) had conductive, two (3.5 per cent) mixed and two (3.5 per cent) sensorineural deafness. The children had originated from various regions of Saudi Arabia, and the study was able to show a similar distribution of hearing defects throughout the country. Otitis media was found to be the major cause of impaired hearing.