The segmented reticulospinal system (zebrafish, hindbrain)

1. To determine the extent of cell proliferation, respecification of cell identities and cell death during development, reticulospinal (RS) neurons of the adult zebrafish were retrogradely labelled with horseradish peroxidase (HRP). Structural analysis of the RS system showed that cell death is virtually absent and there is little remodelling of major morphological features among neurons. Despite cell proliferation, the segmental arrangement of neurons is not obscured. Since the pattern which is established during embryogenesis is conserved ontogenetically, perhaps the morphogenetic mechanisms involved in early pattern formation are used to maintain structural integrity of the adult hindbrain. 2. The hindbrain of the zebrafish is small and not conducive for functional studies of segmentally homologous neurons. The goldfish, a related cyprinid, may be used for this purpose if the segmental organization of RS neurons observed in zebrafish is evolutionarily conserved in related species. To examine this possibility, HRP was used to label RS neurons of the goldfish. All major neurons types identified in zebrafish, including their segmental arrangement, are present in the goldfish hindbrain. These neurons are identifiable either as individuals or as isomorphic clusters. Hence, the RS system of the goldfish provides a unique preparation for neurobiological investigations of vertebrate neurons in much the same way as the identifiable nerve nets of invertebrates. 3. The functional properties of 3 pairs of segmentally homologous neurons, the Mauthner series, were studied with immunocytochemical methods. Gamma-amino butyric acid (GABA), a major inhibitory transmitter, is important for regulating membrane threshold of the largest RS neuron, the Mauthner cell. Do the Mauthner homologues receive GABAergic input as well? An antibody directed against glutamic acid decarboxylase (GAD), the synthesizing enzyme for GABA, was used to label presumptive GABAergic terminals. Immunoreactive puncta were found on all members of the Mauthner series suggesting their functioning is regulated by the same inhibitory transmitter. Together, these studies show that the segmental arrangement of the reticulospinal system and its homologous neurons are conserved ontogenetically and across taxa. Within a species, the functioning of homologous neurons may be regulated by similar synaptic mechanisms. Whether homologous RS neurons share similar afferent and efferent projections remains to be determined. (Abstract shortened with permission of author.)