Effect of conditioning lesions on regeneration of goldfish optic axons: time course of the cell body reaction to axotomy
The time course of the cell body reaction to axotomy was determined in goldfish retinal ganglion cells by measuring cell body size and the amount of labelled protein conveyed by fast axonal transport to the optic tectum, both of which increase during regeneration of the optic axons. Following a single testing lesion of the optic nerve, the regenerating axons began to innervate the tectum at about 14 days after the lesion and the cell body reaction began to decline 2-3 weeks thereafter. If the testing lesion had been preceded by a conditioning lesion 2 weeks earlier, the time for the regenerating axons to arrive in the tectum was reduced by a week, because of the faster rate of axonal outgrowth, but the interval between their arrival and the beginning of the decline of the cell body reaction was unchanged. Electrophysiological measurements showed that synaptic transmission was initiated earlier when the axons reached the tectum faster. These results indicate that the mechanisms initiating the recovery of cell body metabolism are independent of those governing the rate of axonal outgrowth. The recovery of the cell body may begin shortly after synapses are established, regardless of whether they are correctly or incorrectly targetted. The correctness of the target may be a separate factor in determining how rapidly and completely the cell body recovers.