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 sever e depression of the spontaneous activity of the ipsilateral central vestibu lar neurons, which subsequently recovers completely within one week. A poss ible underlying mechanism could be an increase in the endogenous activity o f the neurons deprived of their labyrinthine input. Here, we addressed this hypothesis. The endogenous activity of the neurons was assessed by their s pontaneous activity recorded extracellularly in brainstem slices in the pre sence of a cocktail of neurotransmitter blockers (CNQX, D-APV, bicuculline and strychnine) which freed them from their main synaptic influences. The l eft medial vestibular nucleus (MVN) was explored in a very systematic way a nd strict methodological precautions were taken in order to validate compar isons between the numbers of spontaneously active neurons recorded in the M VN of distinct slices. In the presence of neurotransmitter antagonists, the mean number of spontaneously active neurons detected in a single MVN incre ased dramatically from 9.5 in slices from control guinea pigs to 26.3 in sl ices 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 slice s from guinea pigs labyrinthectomized one week beforehand. These results sh ow that deprivation of the vestibular neurons of their labyrinthine input c aused a change in the deprived neurons themselves. They suggest that an inc rease in pacemaker activity might be a factor responsible for the restorati on of spontaneous activity in the vestibular neurons after labyrinthectomy.