DEVELOPMENT OF GLYCINERGIC AND GLUTAMATERGIC SYNAPTIC TRANSMISSION INTHE AUDITORY BRAIN-STEM OF PERINATAL RATS

In contrast to our knowledge about the anatomical development of the m ammalian central auditory system, the development of its physiological properties is still poorly understood. In order to better understand the physiological properties of the developing mammalian auditory brai nstem, we made intracellular recordings in brainstem slices from perin atal rats to examine synaptic transmission in the superior olivary com plex, the first binaural station in the ascending auditory pathway. We concentrated on neurons in the lateral superior olive (LSO), which in adults, are excited from the ipsilateral side and inhibited from the contralateral side. Already at embryonic day (E) 18, when axon collate rals begin to invade the LSO anlage, synaptic potentials could be evok ed from ipsilateral, as well as from contralateral inputs. Ipsilateral ly elicited PSPs were always depolarizing, regardless of age. They had a positive reversal potential and could be completely blocked by the non-NMDA glutamate receptor antagonist CNQX. In contrast, contralatera lly elicited PSPs were depolarizing from E18-P4, yet they turned into ''adult-like,'' hyperpolarizing PSPs after P8. Their reversal potentia l shifted dramatically from -21.6 +/- 17.7 mV (E18-P0) to -73.0 +/- 7. 1 mV (P10). Regardless of their polarity, contralaterally elicited PSP s were reversibly blocked by the glycine receptor antagonist strychnin e. Bath application of glycine and its agonist beta-alanine further co nfirmed the transitory depolarizing action of glycine in the auditory brainstem. Since the transient excitatory behavior of glycine occurs d uring a period during which glycinergic synaptic connections in the LS O are refined by activity-dependent mechanisms, glycinergic excitation might be a mechanism by which synaptic rearrangement in the contralat eral inhibitory pathway is accomplished.