[Immunohistochemical study for monoamine neurons in the brain of unilateral inner ear impaired rats]

Patients with inner ear impairment have complaints of vertigo and also occasionally depression. The present study was undertaken in order to evaluate changes in monoamines which have reportedly been closely related to depression, using cisplatin-induced unilateral inner-ear impaired rats. A dose of 0.5 mg/kg of cisplatin was injected into the right tympanic cavity under pentobarbital Na+ anesthesia. One or two weeks later, animals were fixed with paraformaldehyde, and thereafter immunohistochemical stainings for monoamine-containing cells in the brain were carried out. To visualize 5-hydroxytryptamine (5-HT), noradrenaline (NA) and dopamine (DA) neurons, we used mouse antibodies against 5-HT, NA, and DA syntheses, i.e., tryptophan hydroxylase (TRH), tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH). The number of TRH immunoreactive neurons significantly decreased in the lateral dorsal raphe nucleus of the ipsilateral side when compared with the contralateral side. The number of DA neurons, which were immunoreactive to TH, but not to DBH, significantly decreased in the hypothalamus of the ipsilateral side. The number of NA neurons which were immunoreactive to both TH and DBH significantly decreased in the locus coeruleus and ventral lateral pons of the ipsilateral side. An additional control study with saline-injected rats showed a lack of differences in monoamine syntheses between the injected and contralateral sides, the expressions of the synthesis on both sides being similar to that obtained in the contralateral side in cisplatin-injected rats. These results indicated the decreases in monoamine syntheses at the ipsilateral side only in the cisplatin-administered rats. We conclude that inner ear impairment may diminish the ipsilateral amount of monoamines in the brain but not the cotralateral, possibly inducing a vestibular compensation such as an upregulation of monoamine receptors.