TRANSPLANTATION OF NEURONS REVEALS PROCESSING AREAS AND RULES FOR SYNAPTIC CONNECTIVITY IN THE CRICKET NERVOUS-SYSTEM

In order to assess the nature of spatial cues in determining the chara cteristic projection sites of sensory neurons in the CNS, we have tran splanted sensory neurons of the cricket Acheta domesticus to ectopic l ocations. Thoracic campaniform sensilla (CS) function as proprioceptor s and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a vent ral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific p rojection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neur ons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when tran splanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each s egmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encount er after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the h ost ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of ne uropil in each segmental ganglion, or with novel targets, which lack d endrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molec ular markers are utilized for cell-cell recognition in each segmentall y homologous ganglion. Regenerating sensory neurons can recognize nove l postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segm entation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity. (C) 1993 John Wil ey & Sons, Inc.