Dynamic characteristics of vestibular nuclear neurons responses to vestibular and optokinetic stimulation during vestibular compensation in the rat.

In albino rats and pigmented rats, neurons were recorded extracellularly in the vestibular nuclei during the first 2 weeks after unilateral labyrinthectomy in order to study the neuronal events occurring during vestibular compensation and the effect of unilateral vestibular lesion on the optokinetic responses of neurons in the vestibular nuclei. It was found in albino rats that a re-equilibration took place in the gains of type I neurons between both lesioned and intact sides. The gain of the rare type I neurons on the deafferented side, which was low just after the lesion (t less than 48 h) subsequently increased (48 h less than t less than 14 d) while on the intact side the gain was greater just after the lesion, and then decreased. This re-equilibration of the gains of type I neurons is considered to be the neuronal equivalent of behavioural effects which occur after hemilabyrinthectomy. In pigmented rats, most type I and II VN neurons recorded on both sides 24 to 96 h after the lesion did not respond to pure horizontal OKS, while in controls almost all of them responded. It seems evident that the tonic activation of VN neurons by vestibular afferences is necessary for their responsiveness to pure OKS. However, when comparing the gain/phase of their responses to pure, vestibular and combined optokinetic-vestibular stimulations, it was found that optokinetic inputs improved the performance of type I and II VN neurons on both lesioned and intact sides. Finally, the time course of vestibular compensation is shorter in pigmented rats than in albino rats, since the re-equilibration in gains between the two sides was already reached 4 days after the lesion in the former.