Postnatal development of spike generation in rat medial vestibular nucleusneurons

Image stability during self motion depends on the combined actions of the v estibuloocular and optokinetic reflexes (VOR and OKR, respectively). Neuron s in the medial vestibular nucleus (MVN) participate in the VOR and OKR by firing in response to both head and image motion. Their intrinsic spike-gen erating properties enable MVN neurons to modulate firing rates linearly ove r a broad range of input amplitudes and frequencies such as those that occu r during natural head and image motion. This study examines the postnatal d evelopment of the intrinsic spike-generating properties of rat MVN neurons with respect to maturation of peripheral vestibular and visual function. Sp ike generation was studied in a brain stem slice preparation by recording f iring responses to current injected intracellularly through whole cell patc h electrodes. MVN neurons fired spontaneously and modulated their firing ra te in response to injected current at all postnatal ages. However, the inpu t-output properties of the spike generator changed dramatically during the first two postnatal weeks. Neurons younger than postnatal day 10 could not fire faster than 80 spikes/s, modulated their firing rates over a limited r ange of input amplitudes, and tended to exhibit a nonlinear relationship be tween input current and mean evoked firing rate. In response to sustained d epolarization, firing rates declined significantly in young neurons. Respon se gains tended to be highest in the first few postnatal days but varied wi dely across neurons and were not correlated with age. By about the beginnin g of the third postnatal week, MVN neurons could fire faster than 100 spike s/s in response to a broad range of input amplitudes, exhibited predominant ly linear current-firing rate relationships, and adapted little in response to sustained depolarization. Concomitant decreases in action potential wid th and the time course of the afterhyperpolarization suggest that changes i n potassium currents contribute to the maturation of the MVN neuronal spike generator. The results demonstrate that developmental changes in intrinsic membrane properties enable MVN neurons to fire linearly in response to a b road range of stimuli in time for the onset of visual function at the begin ning of the third postnatal week.