Early detection of optic nerve-evoked response in the superior colliculus of the neonatal rat.

Optic nerve-evoked responses were measured in the superior colliculus (SC) of neonatal rats in vivo from postnatal day (P) 0 to P11. At P1, a biphasic response was recorded in superficial layers and the amplitude diminished as the electrode penetrated into the deeper layers of the SC. By P2, a similar response, with a fast positive-going potential followed by a more prolonged negative potential was observed at the surface. The polarity of the response reversed as the electrode was moved into the deeper laminae of the SC. Such a reversal in the polarity of optic nerve-evoked responses resembled those observed in more mature preparations. Using current source density analysis, a single pair of source-sink could be identified following optic nerve stimulation at P2, and this changed to a more complex pattern by P11. Our results suggest that synaptic transmission in the retinocollicular pathway of the rat is functional as early as P2.