Synaptic connections between identified neuron types in the antennal lobe glomeruli of the cockroach, Periplaneta...

Synapses between three types of antennal lobe neurons, namely, local multiglomerular interneurons, antennal receptor neurons, and gamma-aminobutyric acid (GABA)-immunoreactive neurons, were studied by means of a combination of three different markers. The interneurons were labeled by intracellular injection of horseradish peroxidase (HRP) into a single soma or a small group of neurons. Antennal receptor cells were marked by experimentally induced anterograde degeneration, and GABA-containing neurons were identified by postembedding immunogold staining. The following types of connections were found: Local interneurons receive input synapses from 1) degenerated receptor neuron axons, 2) GABA-immunogold-labeled neurites, and 3) non-GABA-immunoreactive neurons. The interneurons form output synapses onto the same three neuron groups. Contacts were also found between HRP-labeled interneurons themselves. The majority of synapses were dyadic. In most cases, only one postsynaptic neuronal process of the dyads was labeled and, thus, was identified. Polysynaptic connections were found between GABA-immunoreactive neurites, HRP-labeled interneuron processes, and nonlabeled neurites or between HRP-labeled interneuron processes and two interconnected GABA-immunoreactive processes. The present findings provide anatomical evidence for an earlier suggested monosynaptic connection between afferent receptor fibers and local, at that time putative, GABAergic interneurons. They further reveal that local multiglomerular interneurons are synaptically interconnected. The interneurons, in addition, form serial connections via more than one GABA-immunoreactive neuron with non-GABA-immunoreactive and putative projection neurons. Such polysynaptic connections would be a substrate for a feed-forward "disinhibition" of projection neurons, which has been suggested on the basis of electrophysiological findings.