Extracellular single unit recordings were made from regularly discharging medial vestibular nucleus neurons in brain slices from control rats and from rats surviving 7 days after bilateral transection of the inferior cerebellar peduncle. Decreases in firing rate during perfusion with the Îş-aminobutyric acid (GABA) agonists, muscimol (GABA
) and baclofen (GABA
), were greater in lesioned rats than in control rats. For the grouped data, the half-maximally-effective concentrations of muscimol and baclofen were 3.2 µM, as compared with 19.6 µM for control, and 0.8 µM, as compared with 2.7 µM for control, respectively. The antagonists bicuculline (GABA
) and 2-OH-saclofen (GABA
) only minimally affected the spontaneous firing rates of neurons in lesioned rats, significantly less than in control rats. The data suggest that the decreases of endogenous GABA levels in the medial vestibular nucleus after inferior cerebellar peduncle transection are accompanied by up-regulation of GABA
and, to a lesser extent, GABA
receptors.
