IDENTIFIED CENTRAL NEURONS CONVEY A MITOGENIC SIGNAL FROM A PERIPHERAL TARGET TO THE CNS

Regulation of central neurogenesis by a peripheral target has been pre viously demonstrated in the ventral nerve cord of the leech Hirudo med icinalis (Baptista, C. A., Gershon, T. R. and Macagno, E. R. (1990), N ature 346, 855-858) Specifically, innervation of the male genitalia by the fifth and sixth segmental ganglia (the sex ganglia) was shown to trigger the birth of several hundred central neurons (PIG neurons) in these ganglia, As reported here, removal of the target early during in duction shows that PIC neurons can be independently induced in each si de of a ganglion, indicating that the inductive signal is both highly localized and conveyed to each hemiganglion independently, Further, si nce recent observations (Becker, T., Berliner, A. J., Nitabach, M. N., Gan, W.-B. and Macagno, E. R. (1995), Development, 121, 359-369) had indicated that efferent projections are probably involved in this phen omenon, we individually ablated all possible candidates, which led to the identification of two central neurons that appear to play signific ant roles in conveying the inductive signal to the CNS, Ablation of a single ML neuron reduced cell proliferation in its own hemiganglion by nearly 50%, on the average, In contrast, proliferation on the opposit e side of the ganglion increased by about 25%, suggesting the possibil ity of a compensatory response by the remaining contralateral ML neuro n, Simultaneous ablation of both ML neurons in a sex ganglion caused s imilar reductions in cell proliferation in each hemiganglion. Deletion of a single AL neuron produced a weaker (7%) but nonetheless reproduc ible reduction, Ablation of the other nine central neurons that might have been involved in PIC neuron induction had no detectable effect, B oth ML and AL neurons exhibit ipsilateral peripheral projections, and both arborize mostly in the hemiganglion where they reside, Thus, we c onclude that peripheral regulation of central neurogenesis is mediated in the leech by inductive signals conveyed retrogradely to each hemig anglion by specific central neurons that innervate this target and the hemiganglion they affect.