Synaptic connection between olfactory receptor cells and uniglomerular projection neurons in the antennal lobe of the American cockroach, Periplaneta americana.

Both antennal receptor cell axons and uniglomerular projection neurons of the antennal lobe were specifically labeled, and their synaptic relationship was studied at the fine structural level. The labelings were applied in different combinations: i) Experimentally induced anterograde degeneration of sensory-afferent axons was combined with injection of horseradish peroxidase into uniglomerular projection neurons. ii) Lucifer Yellow was injected into uniglomerular projection neurons, and receptor cell axons were anterogradely labeled with the lipophilic dye DiI. The fluorescent dyes were transformed by immuno- or photochemical treatment into electron-dense markers. In both types of preparations, a considerable number of monosynaptic output synapses from antennal receptor neurons onto processes of uniglomerular projection neurons were identified within the glomeruli of the lobe. In most cases, the receptor axon was connected in a dyadic fashion firstly to a process of a projection neuron and secondly to a nonlabeled process. The results clearly demonstrate a direct connection between receptor cells and output neurons of the cockroach antennal lobe which exists in parallel to the already proposed and demonstrated polysynaptic connection via inhibitory local interneurons.