Research reportReactive nitrogen species contribute to blood–labyrinth barrier disruption in suppurative labyrinthitis complicating experimental pneumococcal meningitis in the rat
Introduction
The mortality of meningitis due to Streptococcus (S.) pneumoniae is 20–30% [11], [42], [43], and up to one-third of survivors are left with significant neurologic sequelae, most commonly hearing impairment, mental retardation, and focal neurologic deficits [7], [39], [44]. The frequency of hearing impairment varies with the causative pathogen: in a prospective study of 185 children with acute bacterial meningitis, the incidence of hearing loss was 31.5% with S. pneumoniae, 10.5% with Neisseria meningitidis, and 6% with Haemophilus (H.) influenzae infections [10]. The high incidence of hearing impairment after pneumococcal meningitis was confirmed by a more recent prospective study of 180 children with pneumococcal meningitis, which showed moderate to severe hearing loss in 32% of the cases [3]. Hearing loss during bacterial meningitis is sensorineural, unilateral or bilateral, often severe, and permanent [3], [10], [36]. The site of the lesion probably is the cochlea [35], [37]. Electron microscopic histopathology showed damage to the organ of Corti (involving the hair cells, supporting cells, and nerve terminals) and the stria vascularis during experimental meningogenic bacterial labyrinthitis in the rabbit [37], [40]. Pathophysiological studies have shown that NO contributes to hearing damage during cochlear inflammation induced by microperfusion with pneumolysin [1], [2]. However, the exact pathophysiological mechanisms of cochlear damage in suppurative labyrinthitis are still largely unknown. In a well-established adult rat model of pneumococcal meningitis, we recently identified peroxynitrite as a mediator of brain damage [23]. Nitrotyrosine residues on proteins (a marker of peroxynitrite) were detected immunohistochemically in the brain of rats with pneumococcal meningitis, and pretreatment with the peroxynitrite scavenger uric acid (UA) attenuated meningeal inflammation, blood–brain barrier (BBB) disruption, and increased intracranial pressure. In this study, we investigated the integrity of the blood–labyrinth barrier (BLB) during cochlear inflammation in our rat model of pneumococcal meningitis and conducted an immunohistochemical study of the presence of eNOS, iNOS, and tyrosine nitration in cochlear inflammation. In order to determine if RNS contribute to BLB disruption, we evaluated the effect of two different peroxynitrite scavengers (UA and MnTBAP) on cochlear Evans Blue extravasation.
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Rat model of pneumococcal meningitis
Adult male Wistar rats (270–350 g) were anesthetized with halothane (Hoechst AG, Frankfurt, Germany), and meningitis was induced by transcutaneous intracisternal injection of 150 μl 106 colony forming units ml−1 of Streptococcus pneumoniae type 3 [29]. Controls were injected with 150 μl sterile phosphate-buffered saline (PBS, pH 7.4, n=4). Each rat was put into an individual cage, allowed to wake up, and fed with a standard diet and water ad libitum. Twenty h after infection, clinical signs of
Histological characterization of labyrinthitis
Cochlear inflammation was present in all infected rats, but in none of the uninfected control animals. All ears of the infected animals showed stonger inflammation in the tympanic scale than in the vestibular scale. Within the tympanic scale, inflammation was strongest in the first basal turn of the cochlea. In all ears from infected animals, in which the cochlear aqueduct was discernible, it was infiltrated by inflammatory cells. Inflammatory cells were not observed in the scala media in any
Discussion
The main findings of this study are: (a) labyrinthitis is a regular complication in our rat model of pneumococcal meningitis, (b) labyrinthitis is accompanied by a disruption of the BLB, (c) eNOS and iNOS are possible sources of NO during cochlear inflammation and possibly contribute to oxidative cochlear damage through the formation of RNS, and (d) BLB disruption is attenuated by the peroxynitrite scavengers UA and MnTBAP.
The route of infection of the inner ear during bacterial meningitis is
Acknowledgements
We thank Dr. V. Arbusow and Dr. P. Schulz (Department of Neurology) for their advice on dissection and handling of rat cochleae. We also thank Ms. S. Walter for technical assistance and Ms. J. Benson for copyediting the manuscript. The financial support of the Volkswagenstiftung (AZ I/75 119) and of the Graduiertenkolleg (Infektion und Immunität) of the Deutsche Forschungsgemeinschaft to HWP is appreciated gratefully.
References (11)
Inhibition of nitric oxide synthase attenuates blood–brain barrier disruption during experimental meningitis
Brain Res.
(1996)- et al.
N-methyl-l-arginine protects the guinea pig cochlea from the cytotoxic effects of pneumolysin
Acta Otolaryngol. Stockh.
(1995) - et al.
Possible involvement of nitric oxide in the sensorineural hearing loss of bacterial meningitis
Acta Otolaryngol.
(1997) - et al.
Three-year multicenter surveillance of pneumococcal meningitis in children: clinical characteristics, and outcome related to penicillin susceptibility and dexamethasone use
Pediatrics
(1998) - et al.
Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly
Am. J. Physiol.
(1996)