Functional development of the neonatal rat retinotectal pathway.

Electrophysiological activity in the neonatal rat superior colliculus was recorded to measure neuronal and synaptic activity, and, therefore, functional development. Neonatal rat pups were studied from five days to two weeks of age. The earliest activity in the superior colliculus were spontaneous discharges at a frequency of one unit per animal on postnatal day 6 (P6). Spontaneously discharging units were more numerous at P8, and the number peaked on P10. The first clear response to optic nerve stimulation was seen on P10, with relatively long and variable latencies. By P14, electrically evoked responses had much shorter latencies. The results are in line with the first response to light flash in the superior colliculus at P12/13. The evidence suggests that functional development of the rat retinotectal pathway begins at the end of the first week after birth, and that much of the functional maturation occurs mainly during the second week after birth.