Modification of the pacemaker activity of vestibular neurons in brainstem slices during vestibular compensation in the guinea pig.

In the guinea pig, unilateral labyrinthectomy causes an immediate and severe depression of the spontaneous activity of the ipsilateral central vestibular neurons, which subsequently recovers completely within one week. A possible underlying mechanism could be an increase in the endogenous activity of the neurons deprived of their labyrinthine input. Here, we addressed this hypothesis. The endogenous activity of the neurons was assessed by their spontaneous activity recorded extracellularly in brainstem slices in the presence of a cocktail of neurotransmitter blockers (CNQX, D-APV, bicuculline and strychnine) which freed them from their main synaptic influences. The left medial vestibular nucleus (MVN) was explored in a very systematic way and strict methodological precautions were taken in order to validate comparisons between the numbers of spontaneously active neurons recorded in the MVN of distinct slices. In the presence of neurotransmitter antagonists, the mean number of spontaneously active neurons detected in a single MVN increased dramatically from 9.5 in slices from control guinea pigs to 26.3 in slices from animals labyrinthectomized on the left side one week beforehand. The mean firing rate of the recorded neurons also increased from 7.5 +/- 5.6 spikes/s in slices from control animals to 12.3 +/- 7.6 spikes/s in slices from guinea pigs labyrinthectomized one week beforehand. These results show that deprivation of the vestibular neurons of their labyrinthine input caused a change in the deprived neurons themselves. They suggest that an increase in pacemaker activity might be a factor responsible for the restoration of spontaneous activity in the vestibular neurons after labyrinthectomy.