Regeneration of the neurohemal terminals for identified cerebral neurosecretory cells in an insect

Abstract

The axons of specific neurosecretory cells, L-NSC III, in the brain of the tobacco hornworm, Manduca sexta, were transected during larval-pupal development to study the effects of this type of lesion on these peptidergic neurons and to begin to identify factors that may regulate their regeneration and growth. The two somata of these bilaterally paired neurons produce the prothoracicotropic hormone and are located in the pars intercerebralis. Their axons exit from the contralateral brain lobe via a retrocerebral nerve and pass through the corpus cardiacum before terminating at the glandular corpus allatum. At the corpus allatum, the L-NSC III axons arborize to form the terminal neurohemal organ for prothoracicotropic hormone release. The retrocerebral nerve was severed either in vitro followed by brain transplantation or in situ; in either protocol, the distal axon segments and corpus allatum were removed. The ability of the injured L-NSC III axons to regenerate was assessed immunocytologically by using a monoclonal antibody against the prothoracicotropic hormone. In both treatments, the proximal axon stumps exhibited regenerative growth as early as 1 day after axotomy, and, by the third day, neurites had extended. By the fifth day, the regenerating axons had branched to form terminal varicosities similar to those of a normal neurohemal organ. The regenerated neurohemal structure appeared to be functional, because larvae that had been bilaterally axotomized were able to metamorphose to pupae, a process requiring temporally precise periods of prothoracicotropic hormone release. In addition to the regeneration of the terminal axon structures, several other responses to axotomy and retrocerebral organ excision occurred. These included an apparent accumulation of prothoracicotropic hormone in the axons and regenerating neurohemal-like structure, sprouting of ectopic neurites from the axotomized somata, and a change in shape of the cell bodies from spherical to avoid.