Abnormal auditory brainstem responses for mice treated with mercurial compounds: involvement of excessive nitric oxide
Introduction
Mercury is a notorious, toxic heavy metal, its different chemical forms accounting for various differing degrees of toxicity (Sundberg et al., 1999). For example, methyl mercury (MeHg) is more toxic and more easily transferred across the blood-brain-barrier than mercuric chloride (HgCl2) (Yoshida et al., 1994). Recognizing that mercuric sulfide (HgS) is virtually insoluble, it is not surprising that pharmacological details regarding its absorption, tissue distribution and toxic effects still remain to be clarified. In Asia, cinnabar (a naturally occurring HgS compound) has been used in combination with Chinese herbal medicine as a sedative for more than 2000 years. Additionally, the abuse of cinnabar as a sedative for infants has resulted in cases of Hg intoxication (Schoof and Nielsen, 1997). We therefore consider it an urgent priority to investigate how well HgS is absorbed via the gastrointestinal tract, and what are the specific toxic effects of HgS for animals and humans.
Audiological tests may be selected to help determine the hearing dysfunction in the sensorineural and/or central auditory system, this being monitored by assessing any abnormality in the auditory brainstem response (ABR) (Musiek and Hanlon, 1999, Zheng et al., 1999). Impaired hearing and deafness have been reported to result from developmental and adult exposure to MeHg (Rice, 1998). The waveform of ABR demonstrated a significant correlation between the prolonged interwave latency I–III and blood-Hg level following increased exposure to MeHg during development. Thus, a delay of the latency of wave III of ABR has been reported to serve as a biomarker of prenatal MeHg toxicity from contaminated seafood (Murata et al., 1999). Other data indicate that the subchronic (4, 8, and 12 weeks) administration of a relatively low-level of HgCl2 (0.4, 0.8, and 1.6 mg/kg, per os) produced a dose- and time-dependent effect upon the examined auditory brainstem response parameters of test individuals (Desi et al., 1996).
The oto-neurotoxicity of MeHg and HgCl2 have been associated with toxic effects to the peripheral or central auditory system, including eliciting auditory deficits and damage to the sensory organ of the cochlea (Igarashi et al., 1992, Counter et al., 1998, Whitworth et al., 1999). Therefore, in this paper, we have attempted to investigate the extent of the gastric/intestinal absorption of HgS and the subsequent tissue distribution of HgS in association with its oto-neurotoxicity as compared with the same parameters for MeHg. The potential toxic effects of mercurial compounds on ABR thresholds were measured for adult mice as also the corresponding Hg content in blood, liver, kidney, cerebral cortex and brainstem, these being determined by continuous flow and cold-vapor atomic absorption spectrometry (AAS).
The characteristics of chronic mercury intoxication have previously been reported as alterations of the activity of Na+/K+-ATPase in certain brain regions of experimental animals, with the associated stimulation of nitric oxide (NO) production and the subsequent alteration of neuronal excitability (Numagami et al., 1997, Pontiggia et al., 1998). Nitric oxide is a messenger molecule that mediates several physiological functions and pathological processes. It has been previously reported that sodium nitroprusside (SNP), a NO donor, may cause a loss of auditory nerve compound action potential (CAP) following topical application of SNP to round-window membrane (RWM) of guinea pig (Kong et al., 1996). It has been claimed that the existence of the NO/cGMP pathway is involved in both the ascending and descending pathways of the auditory brainstem. Based upon these findings, we considered whether NO is involved in the regulation of Na+/K+-ATPase activity in the central auditory system. By measuring the Na+/K+-ATPase activity coupled with NO analysis in the brainstem of mice, we hope to shed some light on the oto-neurotoxicity of mercurial compounds.
Section snippets
Animal preparations
In the present experiment, male mice weighing 18–20 g were administered HgS suspended in saline (0.1 and 1.0 g/kg per day) or MeHg (0.2, 2.0 and 10 mg/kg per day) orally, for 7 consecutive days. The study was conducted in accordance with the guidelines for the care and use of laboratory animals by the Animal Research Committee in the National Taiwan University, College of Medicine. These animals were sacrificed after deep anesthesia with an intraperitoneal injection of pentobarbital (60 mg/kg)
Abnormal ABR induced by mercurial compounds
The purpose of this study was directed to investigating whether the oral administration of HgS as well as MeHg can produce auditory brainstem dysfunction for test mice. Following HgS or MeHg administration once daily for a period of 7 consecutive days, the ototoxicity in terms of elevations of ABR threshold is detailed in Fig. 1. According to our data, MeHg- (2.0 or 10 mg/kg per day) treatment for 7 consecutive days significantly elevated the hearing threshold and revealed a more severe degree
Discussion
Methylmercury (MeHg) is recognized as a significant environmental hazard, particularly to the development of the nervous system. On the other hand, HgS (inorganic form) is almost insoluble in water, and is generally considered to be less toxic in vivo (Sin et al., 1990a, Sin et al., 1990b, Sin and The, 1992). Recently, some reports have suggested that HgS can be absorbed and accumulated in the kidney and liver following the oral administration of cinnabar or HgS (Yeoh et al., 1986). In order to
Acknowledgements
This investigation was supported by a research grant (CCMP88-RD-045) from the Committee on Chinese pharmacy, Department of Health, Executive Yuan, Taipei, Taiwan.
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