An improved model of the synaptic organization of insect olfactory glomeruli

In order to characterize the neuronal pathways underlying central nervous processing of odor signals, the synaptic circuitry of identified neuron individuals or types within cockroach antennal lobe glomeruli has been studied in the electron microscope. The anatomical findings are consistent with physiological data from other investigations. For the characterization of the synaptic connections, different combinations of neuron labelings have been applied including (a) horseradish peroxidase injection of local interneurons (INs) or uniglomerular projection neurons (uPNs), (b) gamma-aminobutyric acid (GABA)-immunogold labeling, and (c) experimentally induced degeneration of antennal receptor neuron projections (RNs). The following pathways between glomerular input and output neurons are proposed: (1) A monosynaptic excitatory route, and (2) at least two polysynaptic routes, one of them probably inhibitory and the other disinhibitory. (3) Inhibitory feedback connections exist from these two paths via GABA-positive INs onto both RNs as well as uPNs. (4) Since a given GABA-positive IN innervates a large number of antennal lobe glomeruli, inhibitory signals could also be passed to neighbored glomeruli.