The ontogeny of inhibition and excitation in the gerbil lateral superior olive.

While the development of excitatory responses has been the focus of considerable research, the ontogeny of inhibitory connections has received relatively little attention. The lateral superior olive (LSO), an auditory nucleus in the ventral brain stem, is a favorable system in which to compare the maturation of an inhibitory and an excitatory input. Neurons in the LSO are excited by stimuli delivered to the ipsilateral ear and inhibited by similar stimuli to the contralateral ear. Single-neuron recordings were made to characterize tone-evoked responses at the onset of hearing and in adult Mongolian gerbils. The results indicated that frequency selectivity was significantly poorer in young than adult animals. In several cases, neurons within the same animal were found to have disparate tuning properties, such that one of the units had "adult-like" tuning, while the other was much more broadly tuned. No difference existed between excitatory and inhibitory tuning within any age group. The degree to which the excitatory and inhibitory characteristic frequencies of an LSO neuron were correlated was used as a measure of tonotopic map alignment. A significant improvement of matching was seen with increasing age. A comparison of excitatory and inhibitory thresholds indicated that the inhibitory system was relatively more efficacious in young than adult animals. The ability of LSO neurons to respond to interaural intensity differences, the binaural parameter to which they are sensitive, indicated 3 differences between adult and young animals: the dynamic range was smaller, the slope was shallower, and the sample of neurons encoded a constrained range of interaural intensity difference values. We conclude that the maturation of the inhibitory and excitatory systems are nearly identical.