Dendritic amputation redistributes sprouting evoked by axotomy in lamprey central neurons

In the previous paper (Hall and Cohen, 1988), we showed that axotomy of anterior bulbar cells (ABCs) in the hindbrain of the larval lamprey results in the sprouting of axonlike neurites from either the end of the proximal axon stump, the dendritic tips, or both, depending on the site of axotomy. Here we show that, unlike axotomy, dendritic amputation (dendrotomy) does not by itself induce sprouting from ABCs. However, dendrotomy does induce sprouting from dendrites in the immediate vicinity of the dendritic lesion in cells that have been previously axotomized. We found that dendrotomy acts primarily to rearrange the distribution of sprouts induced by axotomy rather than serving as an additional stimulus to neurite outgrowth. We propose that (1) dendritic sprouting in ABCs occurs because the dendritic tips become attractive sites for sprout initiation when they are either directly injured (as with dendrotomy) or are situated relatively close to the site of injury (as with axotomy close to the soma), and (2) the axon stump, dendritic stumps, and uninjured dendritic tips of the cell compete to initiate a limited total amount of sprouting induced by axotomy. The probability that a given locus will support sprouting is determined both by its proximity to the nearest lesion site and by whether there are other attractive potential sprouting sites in the cell.