Plasticity of spontaneous excitatory and inhibitory synaptic activity in morphologically defined vestibular nuclei neurons during early vestibular compensation.

After unilateral peripheral vestibular lesions, the brain plasticity underlying early recovery from the static symptoms is not fully understood. Principal cells of the chick tangential nucleus offer a subset of morphologically defined vestibular nuclei neurons to study functional changes after vestibular lesions. Chickens show posture and balance deficits immediately after unilateral vestibular ganglionectomy (UVG), but by 3 days most subjects begin to recover, although some remain uncompensated. With the use of whole cell voltage-clamp, spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) and miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were recorded from principal cells in brain slices 1 and 3 days after UVG. One day after UVG, sEPSC frequency increased on the lesion side and remained elevated at 3 days in uncompensated chickens only. Also by 3 days, sIPSC frequency increased on the lesion side in all operated chickens due to major increases in GABAergic events. Significant change also occurred in decay time of the events. To determine whether fluctuations in frequency and kinetics influenced overall excitatory or inhibitory synaptic drive, synaptic charge transfer was calculated. Principal cells showed significant increase in excitatory synaptic charge transfer only on the lesion side of uncompensated chickens. Thus compensation continues when synaptic charge transfer is in balance bilaterally. Furthermore, excessive excitatory drive in principal cells on the lesion side may prevent vestibular compensation. Altogether, this work is important for it defines the time course and excitatory and inhibitory nature of changing spontaneous synaptic inputs to a morphologically defined subset of vestibular nuclei neurons during critical early stages of recovery after UVG.