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 fo rmed much earlier and are present before birth, indicating that a subs tantial amount of maturation occurs without acoustic input. Electrical activity is thought to influence neuronal development, but the physio logical properties of auditory brainstem neurons during perinatal matu ration are barely known. The present study focuses on the development of electrophysiological membrane properties of neurons in the rat's su perior olivary complex (SOC), the first binaural station in the mammal ian 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 inj ection of biocytin or neurobiotin. By E18, i.e. 4 days before birth, S OC 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 d evelopment. in contrast, input resistance, time constant and spike dur ation decreased significantly, and maximal spike frequency increased s ignificantly during the age period sampled. Our results show that rat SOC neurons display mature as well as immature electrical membrane pro perties during the same developmental period when anatomical connectio ns are refined and when the soma-dendritic morphology develops. We con clude, therefore, that their membrane properties represent adequate ph ysiological adaptations to the immature auditory brainstem microcircui ts and that they form a basis upon which the development of these micr ocircuits is shaped.