Early onset of glutamatergic and GABAergic synaptic activity in the visuallayers of the rodent superior colliculus

During postnatal development. the retinocollicular pathway undergoes activi ty-dependent refinement, resulting in the precise retinotopic map seen in a dults. Previous studies established that retinal efferents reach the mouse Superior colliculus (SC) by embryonic day 16. Morphologically, synapses wer e found in the rat SC before birth. As part of an extended project aimed at understanding the development of synaptic transmission in the visual layer s of the SC, we report here the presence of functionally active synapses im mediately after birth. Circuit activity in mouse SC neurons was detected in horizontal slices of the visual layers using cell-attached voltage clamp. The spontaneous discharge of action potentials was abolished by glutamaterg ic blockers and facilitated by bicuculline, showing that circuit activity i s based on synaptic transmission and that the action of gamma -aminobutyric acid is inhibitory. Using whole-cell voltage clamp, spontaneous glutamater gic postsynaptic currents as well as miniature GABAergic postsynaptic curre nts were recorded on postnatal day 1. Excitatory and inhibitory postsynapti c currents could also be evoked by electrical stimulation. Glutamatergic po stsynaptic currents comprised both (S)-alpha -amino-3-hydroxy-5-methyl-4-is oxazolepropionic acid and N-methyl-D-aspartate receptor-mediated components . The early function of glutamatergic and GABAergic synaptic transmission i n the visual layers of SC suggests that SC neurons are able to process info rmation originating From retinal axons immediately after birth. (C) 2001 IS DN. Published by Elsevier Science Ltd. All rights reserved.