Prenatal development of functional connections in the cat's retinogeniculate pathway

The development of functional connections between the axons of retinal ganglion cells and the neurons of the dorsal lateral geniculate nucleus (LGNd) of fetal and neonatal cats was studied using an in vitro assay. Extracellular microelectrode recordings of single units were made from histologically identified sites in the LGNd of isolated diencephalon preparations between embryonic day 39 ( E39 ) and postnatal day 2 (P2). (Gestation is 65 days in the cat.) Postsynaptic units activated by electrical stimulation of one or both optic nerves were found at all ages tested from E39 onwards. Over 90% of the units studied in the fetal preparations received convergent excitation from both optic nerves, compared with roughly half of the units studied in the neonatal optic nerves, compared with roughly half of the units studied in the neonatal preparations. Inhibition was detected in the LGNd of the neonatal preparations, but in only the oldest of the fetal preparations ( E59 ). This physiological change from predominantly convergent excitation to an adult-like mixture of excitation and inhibition seen at birth coincides with the change from mixed to segregated afferent input from the two eyes seen anatomically ( Shatz , C. J. (1983) J. Neurosci . 3: 482-499). These results indicate that attainment of the adult pattern of retinogeniculate connectivity involves the elimination of already functional synapses.