Local interneuron diversity in the primary olfactory center of the moth Manduca sexta

Local interneurons (LNs) play important roles in shaping and modulating the activity of output neurons in primary olfactory centers. Here, we studied the morphological characteristics, odor responses, and neurotransmitter content of LNs in the antennal lobe (AL, the insect primary olfactory center) of the moth Manduca sexta. We found that most LNs are broadly tuned, with all LNs responding to at least one odorant. 70% of the odorants evoked a response, and 22% of the neurons responded to all the odorants tested. Some LNs showed excitatory (35%) or inhibitory (33%) responses only, while 33% of the neurons showed both excitatory and inhibitory responses, depending on the odorant. LNs that only showed inhibitory responses were the most responsive, with 78% of the odorants evoking a response. Neurons were morphologically diverse, with most LNs innervating almost all glomeruli and others innervating restricted portions of the AL. 61 and 39% of LNs were identified as GABA-immunoreactive (GABA-ir) and non-GABA-ir, respectively. We found no correlations between odor responses and GABA-ir, neither between morphology and GABA-ir. These results show that, as observed in other insects, LNs are diverse, which likely determines the complexity of the inhibitory network that regulates AL output.