Modification of the macaque's vestibulo-ocular reflex by picrotoxin.

The vestibulo-ocular reflex (VOR) in the macaque was tested by a modified Bárány spinning test before and after subcutaneous administration of 1 mg/kg of picrotoxin. The drug caused a decrease in the nystagmus time constant to a value near that of the cupula and a decrease in mystagmus asymmetry. Picrotoxin prevented diazepam from increasing the time constant of vestibular nystagmus. Furthermore, picrotoxin reduced the nystagmus time constant after vermisectomy, a lesion that increases the nystagmus time constant and prevents some VOR modifications. Because picrotoxin and diazepam interact antagonistically in the gamma-aminobutyric acid (GABA) neurotransmitter system, these results show a role of GABA in the neural network that transforms a short cupula time constant into a longer nystagmus time constant.