@article {LizbinskiENEURO.0212-18.2018, author = {Kristyn M. Lizbinski and Gary Marsat and Andrew M. Dacks}, title = {Systematic analysis of transmitter co-expression reveals organizing principles of local interneuron heterogeneity}, elocation-id = {ENEURO.0212-18.2018}, year = {2018}, doi = {10.1523/ENEURO.0212-18.2018}, publisher = {Society for Neuroscience}, abstract = {Broad neuronal classes are surprisingly heterogeneous across many parameters and sub-classes often exhibit partially overlapping traits including transmitter co-expression. However, the extent to which transmitter co-expression occurs in predictable, consistent patterns is unknown. Here, we demonstrate that pairwise co-expression of GABA and multiple neuropeptide families by olfactory local interneurons (LNs) of the moth Manduca sexta is highly heterogeneous, with a single LN capable of expressing neuropeptides from at least four peptide families and few instances in which neuropeptides are consistently co-expressed. Using computational modeling, we demonstrate that observed co-expression patterns cannot be explained by independent probabilities of expression of each neuropeptide. Our analyses point to three organizing principles that once taken into consideration allow replication of overall co-expression structure: 1- peptidergic neurons are highly likely to -co-express GABA; 2- expression probability of Allatotropin depends upon Myoinhibitory peptide expression; 3- the all-or-none co-expression patterns of Tachykinin neurons with several other neuropeptides. For other peptide pairs, the presence of one peptide was not predictive of the presence of the other and co-expression probability could be replicated by independent probabilities. The stochastic nature of these co-expression patterns highlights the heterogeneity of transmitter content among LNs and argues against clear-cut definition of subpopulation types based on the presence of single neuropeptides. Furthermore, the receptors for all neuropeptides and GABA were expressed within each population of principal neuron type in the AL. Thus, activation of any given LN results in a dynamic cocktail of modulators that have the potential to influence every level of olfactory processing within the AL.Significance Statement Understanding the functional roles of individual local interneurons (LNs) is complex because traits, like transmitter co-expression, are often partially overlapping across the population. Here, we find that single olfactory LNs co-express neuropeptides from at least four individual peptide families, and that GABA and neuropeptides are partially and heterogeneously co-expressed across the entire population. The stochastic nature of many observed co-expression patterns argues against clear-cut and exclusive definition of subpopulations based on the expression of single neuropeptides. Overall, our results suggest that activation of any given LN causes the release of a variable combination of neuropeptides and GABA that, based on receptor expression, target the input, output and local processing stages of the olfactory coding.}, URL = {https://www.eneuro.org/content/early/2018/09/21/ENEURO.0212-18.2018}, eprint = {https://www.eneuro.org/content/early/2018/09/21/ENEURO.0212-18.2018.full.pdf}, journal = {eNeuro} }