Effects of hydroxyurea parallel the effects of radiation in developing olfactory glomeruli in insects

Previous observations (Oland and Tolbert: J. Comp. Neurol. 255:196–207, '87, Soc. Neurosci. Abstr. 13:1144, '87; Oland et al.: J. Neurosci. 8:353–367, '88) have provided evidence that the afferent‐axon‐induced development of synaptic glomeruli in the antennal lobe of the moth Manduca sexta depends upon an interaction between ingrowing sensory axons and the glial cells of the antennal lobe. In order to differentiate between the roles of glial cells and of afferent axons on the partitioning of the lobe into glomeruli, we have used the antimitotic agent hydroxyurea to produce lobes deficient in glial cells but retaining sensory input. The resulting lobes were analyzed in the light and electron microscopes, and the integrity of their antennal input was evaluated by examining the gross and microscopic structure of the antennae, the number of antennal afferent axons, and electroantennogram responses to odors. Our results with hydroxyurea show that in treated animals with adequate antennal input the degree to which the antennallobe neuropil becomes glomerular varies with the number of glial cells remaining in the lobe; when less than approximately one‐quarter of the normal glial complement is present, glomeruli do not develop at all. These experiments complement and extend previous experiments in which the number of glial cells was reduced with radiation (Oland et al.: J. Neurosci. 8:353–367, '88). The fact that the present results mimic the previous results with radiation strongly suggest that glial cells do mediate the afferent‐axon‐induced formation of olfactory glomeruli in the moth.