Ontogeny of modulatory inputs to motor networks: Early established projection and progressive neurotransmitter acquisition

Modulatory information plays a key role in the expression and the ontogeny of motor networks. Many developmental studies suggest that the acquisition of adult properties by immature networks involves their progressive innerva tion by modulatory input neurons. Using the stomatogastric nervous system o f the European lobster Homarus gammarus, we show that contrary to this assu mption, the known population of projection neurons to motor networks, as re vealed by retrograde dye migration, is established early in embryonic devel opment. Moreover, these neurons display a large heterogeneity in the chrono logy of acquisition of their full adult neurotransmitter phenotype. We performed retrograde dye migration to compare the neuronal population pr ojecting to motor networks located in the stomatogastric ganglion in the em bryo and adult. We show that this neuronal population is quantitatively est ablished at developmental stage 65%, and each identified projection neuron displays the same axon projection pattern in the adult and the embryo. We t hen combined retrograde dye migration with FLRFamide-like, histamine, and G ABA immunocytochemistry to characterize the chronology of neurotransmitter expression in individual identified projection neurons. We show that this e arly established population of projection neurons gradually acquires its ne urotransmitter phenotype complement. This study indicates that (1) the basi c architecture of the known population of projection inputs to a target net work is established early in development and (2) ontogenetic plasticity may depend on changes in neurotransmitter phenotype expression within preexist ing neurons rather than in the addition of new projection neurons or fibers .