Development of electrical membrane properties and discharge characteristics of superior olivary complex neurons in fetal and postnatal rats.

Although hearing onset occurs relatively late during ontogeny of rats [around postnatal day (P) 12], anatomical brainstem connections are formed much earlier and are present before birth, indicating that a substantial amount of maturation occurs without acoustic input. Electrical activity is thought to influence neuronal development, but the physiological properties of auditory brainstem neurons during perinatal maturation are barely known. The present study focuses on the development of electrophysiological membrane properties of neurons in the rat's superior olivary complex (SOC), the first binaural station in the mammalian auditory brainstem. In in vitro slice preparations, intracellular recordings were obtained from 115 SOC cells from embryonic day (E) 18 to P17, and cells were morphologically identified by intracellular injection of biocytin or neurobiotin. By E18, i.e. 4 days before birth, SOC neurons were capable of generating Na(+)-dependent action potentials. Several passive and active membrane properties, including the resting potential, spike threshold and spike amplitude, did not change with development. In contrast, input resistance, time constant and spike duration decreased significantly, and maximal spike frequency increased significantly during the age period sampled. Our results show that rat SOC neurons display mature as well as immature electrical membrane properties during the same developmental period when anatomical connections are refined and when the soma-dendritic morphology develops. We conclude, therefore, that their membrane properties represent adequate physiological adaptations to the immature auditory brainstem microcircuits and that they form a basis upon which the development of these microcircuits is shaped.