ACTIVITY BLOCKADE DOES NOT PREVENT THE CONSTRUCTION OF OLFACTORY GLOMERULI IN THE MOTH MANDUCA-SEXTA

During metamorphic development, the arrival at the olfactory (antennal ) lobe of olfactory receptor axons initiates the process of glomerulus formation. The glomeruli are discrete spheroidal regions of neuropil that are the sites of synaptic interactions among receptor neurons and their target antennal-lobe neurons. The process of glomerulus formati on begins as groups of receptor axons form protoglomeruli. These dense clusters of terminal branches mostly are discrete entities from the t ime they can be recognized, although a few branches from neighboring p rotoglomeruli overlap laterally. A previous study by Schweitzer ei al. [Schweitzer E. S., Sanes J. R. and Hildebrand J. G. (1976) Ontogeny o f electroantennogram responses in the moth, Manduca sexta. J. Insect P hysiol. 22, 955-960] has shown that odor-induced activity in the recep tor neurons can be detected first in recordings from the axons in the antennal nerve only in the last few days of metamorphic development an d thus could not influence the process of glomerulus formation. In thi s study, we have tested directly the possibility that an earlier prese nce of spontaneous activity in either the receptor axons or the antenn al-lobe neurons could affect the process. Tetrodotoxin, a Nac-channel blocker, was injected into the hemolymph prior to the onset of glomeru lus formation to block any spontaneous Na+-dependent activity. Subsequ ent intracellular recordings from antennal-lobe neurons revealed no sp ike activity. Comparison with vehicle-injected control animals at stag es during and after glomerulus formation revealed no differences in th e localization of receptor-axon terminal branches in the glomeruli, in the border of glial cells that forms around each glomerulus, or in th e morphology of the tufted glomerular arbors of one of the antennal-lo be neurons. We conclude that: (1) the process of glomerulus formation is largely independent of activity; and (2) glomeruli as modular units of the CNS more closely resemble cortical barrels than cortical colum ns, both in their ontogeny and in the lack of an obvious effect of act ivity on the morphology of the neurons arborizing within them. Copyrig ht (C) 1996 ISDN.