Dopaminergic Control of Serotonergic Neuron Development in the Grasshopper Central Nervous System

Abstract

The central nervous system (CNS) of the grasshopper consists of a brain and a set of segmental ganglia that together make up the ventral nerve cord. Each ventral nerve cord ganglion develops very similarly during early embryogenesis. The availability of cells that make stereotypical decisions in the developing CNS and that can be identified in the living embryo makes the grasshopper an attractive system for researching specific questions about neural development. Serotonin is a very important neuromodulator in invertebrates. Application of this compound to small neuronal circuits or whole nervous systems profoundly affects their physiological behavior. Invertebrates have small numbers of dopaminergic and serotonergic neurons that send extensive processes that innervate most of the central neuropil. Dopamine and serotonin receptors have been characterized in insects and mollusks, both pharmacologically and molecularly. These are remarkably conserved between vertebrates and invertebrates. Thus, these experimental observations made in insects may also be applicable to vertebrates. The experimental access afforded by insects makes such studies an attractive approach to understanding the much more complex vertebrate system. Serotonergic innervation of the neuropil of each ventral nerve cord hemiganglion derives from one local serotonergic interneuron, s1. It expresses serotonin uptake activity before making serotonin. Serotonin synthesis begins when the early branches reach the target neuropil. Dopamine seems to be required both for the serotonin synthesis and for controlling the rate of neurite extension. The receptors controlling these processes have been pharmacologically characterized.