Afferent axons from the antenna influence the number and placement of intrinsic synapses in the antennal lobes of Manduca sexta

The present study compares the placement and frequency of occurrence of synapses in normally developing antennal lobes of the brain of the moth Manduca sexta and in lobes developing in the absence of afferent axons from the antenna. The antennal lobes develop during the metamorphosis from larva to moth, and require antennal afferent input to develop their characteristic knots of synaptic neuropil, the glomeruli, that are arrayed around a central core of coarse neuropil. Synapses begin to form throughout the neuropil of the antennal lobe before ingrowth of antennal axons into the lobe. During normal development, almost all synapses (those among intrinsic neurons as well as those made by antennal afferent axons) become segregated into glomeruli as the glomeruli are established. Thus, in normal mature lobes, the radial segregation between synaptic (glomerular) and nonsynaptic (central) regions is virtually complete. In lobes that have never been allowed to receive antennal axons, an outer ring of “protoglomerular” neuropil develops in place of the glomeruli. A similar segregation of synaptic and nonsynaptic regions occurs, with two major differences: the frequency of synapses that develops in the outer ring of fine‐textured neuropil is almost 50% higher than in normal glomeruli, and the small number of synapses that occur in the coarse central neuropil now occur on aberrant fine processes in that neuropil. Apparently, in the absence of their normal massive input from the antenna, elements intrinsic to the antennal lobe are induced to form extra synapses with each other, and to send aberrant fine branches to synapse in a region of neuropil normally containing predominantly large‐caliber neurite trunks and few synapses.