Neuronal competition determines the spatial pattern of neuropeptide expression by identified neurons of the leech

Staining adult and embryonic leech ventral nerve cords with antibodies raised against the molluscan neuropeptides small cardioactive peptide B (SCP) and FMRFamide results in segment-specific and bilaterally asymmetric patterns of cell staining. One immunoreactive neuron, the RAS interneuron, is present in only four rostral segmental ganglia, while another, the CAS interneuron, is restricted to the four most caudal abdominal ganglia and tail. In addition to their segment-specific distributions, only one RAS or CAS cell is found in each segmental ganglion, and they alternate sides between adjacent ganglia (either L-R-L-R or R-L-R-L) with a fidelity of about 95%. This paper utilizes cell deletion techniques to investigate the determination of the asymmetric and alternating pattern of RAS and CAS neurons. We show that developmentally equivalent RAS and CAS homologs are present on both sides of the appropriate ganglia, and that within each ganglion one of the initially paired homologs loses the ability to assume the immunoreactive RAS or CAS fate 2-3 days after axonogenesis has begun. These experiments suggest that there is a competitive interaction between bilateral homologs which ensures that only one mature RAS/CAS neuron is formed per ganglion, and that contralateral RAS/CAS neurons are not required in the same or adjacent ganglia for the determination of the RAS or CAS developmental pathways. Nerve cord transections between ganglia in the CAS domain can alter the spatial pattern of CAS neuron determination, confirming that both bilateral homologs retain the ability to express neuropeptide until late embryonic stages, and suggesting that the alternating pattern of RAS/CAS cells requires communication between adjacent ganglia through the longitudinal connectives.