Postnatal development of auditory central evoked responses and thalamic cellular properties.

During development, the sense of hearing changes rapidly with age, especially around hearing onset. During this period, auditory structures are highly sensitive to alterations of the acoustic environment, such as hearing loss or background noise. This sensitivity includes auditory temporal processing, which is important for processing complex sounds, and for acquiring reading and language skills. Developmental changes can be observed at multiple levels of brain organization-from behavioral responses to cellular responses, and at every auditory nucleus. Neuronal properties and sound processing change dramatically in auditory cortex neurons after hearing onset. However, development of its primary source, the auditory thalamus, or medial geniculate body (MGB), has not been well studied over this critical time window. Furthermore, to understand how temporal processing develops, it is important to determine the relative maturation of temporal processing not only in the MGB, but also in its inputs. Cellular properties of rat MGB neurons were studied using in vitro whole-cell patch-clamp recordings, at ages postnatal day (P) 7-9; P15-17, and P22-32. Auditory evoked potentials were measured in P14-17 and P22-32 rats. MGB action potentials became about five times faster, and the ability to generate spike trains increased with age, particularly at frequencies of 50 Hz and higher. Evoked potential responses, including auditory brainstem responses (ABR), middle latency responses (MLR), and amplitude modulation following responses, showed increased amplitudes with age, and ABRs and MLRs additionally showed decreased latencies with age. Overall, temporal processing at subthalamic nuclei is concurrently maturing with MGB cellular properties.