Auditory behaviour and brainstem histochemistry in adult rats with characterized ear damage after neonatal ossicle ablation or cochlear disruption.

Binaural and monaural ossicle ablation in neonate rats before the time of onset of auditory input resulted in hearing deficits as detected by behavioural responses to sound stimuli in these rats as young adults. Cochlear disruption at the same neonatal age similarly resulted in the absence of startle reflexes in many of the rats. When the middle and inner ears of the rats were analysed postmortem in serial sections, it was observed that most ears after neonatal ossicle ablation contained only small remnants of the malleus-incus unit, separated from the stapes; in other ears an apparent continuity of ossicles had been restored. The rats with blind-ending ear canals and ossicle atrophy were those that had shown little response to sound stimuli. In the cochlear-disrupted rats, those with modiolar damage and loss of most spiral ganglion cells had shown substantial impairment of sound perception, even in some rats with only monaural modiolar loss. The chronic conduction deficit caused by neonatal ossicle removal did not result in detectable differences in relative cytochrome oxidase activity in the dorsal cochlear nuclei and central nucleus of the inferior colliculus. For monaurally ossicle-ablated rats, quantitation of the average intensity of enzyme reaction product in sections of dorsal or ventral cochlear nuclei, or central nucleus, did not reveal a difference between operated and non-operated sides. However, in binaurally ossicle-ablated rats, the relative enzyme activity in the anteroventral cochlear nuclei was reduced in comparison to this nucleus in control rats. The volume of the anteroventral cochlear nucleus in rats that had had neonatal binaural cochlear disruption was reduced relative to the volume in control rats or in rats that had had binaural ossicle ablation (P < 0.001); the latter procedure did not result in a statistically significant difference from controls in AVCN volume. In cochlear-operated rats with monaural modiolar damage, the AVCN contralateral to the damaged cochlea had a lower mean level of cytochrome oxidase activity in its neurons measured individually than that for neurons in the ipsilateral AVCN. These results suggested the importance during development of input from contralateral cochlear neurons.