CONDUCTION AND SYNAPTIC TRANSMISSION IN THE OPTIC-NERVE AND THE SUPERIOR COLLICULUS DURING DEVELOPMENT OF THE RETINOCOLLICULAR PROJECTION IN THE WALLABY (MACROPUS-EUGENII)

When do the developing connections between mammalian retinal ganglion cells and the superior colliculus become functional? Evoked potentials elicited by optic nerve stimulation in the pouch young of the wallaby were used to answer the question. Up to 42 days after birth, the evok ed potentials in the colliculus appeared to be generated by axon condu ction. Synaptic activity was first recorded from the rostral colliculu s at 45 days, and was found to be progressively more caudal, spreading to cover the colliculus, by 65 days. From the earliest indication of synaptic activity until eye opening at 140 days, current source densit y (CSD) analysis consistently showed the same basic pattern: an initia l deep sink from synaptic activity of fast (Y type) fibres, and a more superficial longer-latency sink from slower (W type) fibres. All feat ures became more clearly delineated with age. The indirect retinocorti cocollicular connection appeared between 134 days and 146 days. The ab ility of optic nel-ve fibres to sustain action potentials precedes the ir formation of functional synapses with collicular neurons, which hap pens abruptly at three months before eye opening. CSD analysis showed that the relationship between the conduction velocity of optic nerve f ibres and their depth of termination is evident from the first signs o f synapse formation. (C) 1997 Wiley-Liss, Inc.