Title: Diverse in- and output polarities and high complexity of local synaptic and nonsynaptic signalling within a chemically defined class of peptidergic Drosophila neurons Authors
Submitted to: Frontiers in Neural Circuits
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 12, 2013
Publication Date: August 1, 2013
Citation: Karsai, G., Pollak, E., Wacker, M., Voemel, M., Selcho, M., Berta, G., Nachman, R.J., Isaac, E., Molnar, L., Wegener, C. 2013. Diverse in- and output polarities and high complexity of local synaptic and nonsynaptic signalling within a chemically defined class of peptidergic Drosophila neurons. Frontiers in Neural Circuits. 7:1-22. Doi:10.3389/fncir.2013.00127. Interpretive Summary: Insect pests have developed resistance to several conventional pesticides, and new approaches are needed for pest management. Although neuropeptides (short chains of amino acids) serve as potent messengers in insects to regulate vital functions, the neuropeptides hold little promise as pest control agents because they can be degraded in the target pest. New, selective control agents may be developed by designing mimics of these neuropeptides that resist degradation and either inhibit or over-stimulate critical neuropeptide-regulated life functions. And, we must investigate and understand the mechanisms employed by these neuropeptide signals within the complex web of circuits within the nervous system of insects. We report on evidence that the action of neuropeptides of the ETH class is dependent on inhibition of certain nerve connections known as synapses, and independent of the action of steroid hormones. Neuropeptides of the ETH class regulate aspects of the transformation of juvenile insects into the adult form, critical for insect survival and propagation. The work brings us one step closer to the development of practical neuropeptide-like substances that will be effective in the management of pest insects in an environmentally-friendly fashion.
Technical Abstract: Peptidergic neurons are not easily integrated into current connectomics concepts, since their peptide messages can be distributed via non-synaptic paracrine signaling or even via volume transmission. Moreover, and especially in insects, the polarity of peptidergic interneurons in terms of in- and output sites is hard to predict and very little explored. We here describe in detail the single cell morphology and the subcellular distribution of fluorescent vesicle and dendrite markers in CCAP-GAL4 neurons (NCCAP), a small but well defined set of peptidergic neurons in the Drosophila larva. NCCAP can be divided into five different subsets depending on the arborisation pattern. In contrast to most other subsets, serial homologous interneurons in the ventral ganglion show a mixed localization of in- and output markers along ventral neuritis that are difficult to assign to dendritic or axonal compartments. Immuno-electron microscopy revealed that these neuritis both contain pre- and postsynaptic sites preferably at their varicosities. While many of the synaptic partners are unidentified, a significant portion of the synaptic events are due to reciprocal synapses. Aldehyde-tannic acid fixation revealed that peptides are mostly non-synaptically or parasynaptically released, and dense-core vesicles and synaptic vesicle pools are typically well separated. Calcium imaging with the genetically encoded reporter GCaMP1.6 suggest that the responsiveness of the NCCAP to ecdysis-triggering hormone is at least partly dependent on a tonic synaptic inhibition, and independent of ecdysteroids. Our results reveal a remarkable variety and complexity of local synaptic circuitry within a chemically similar group of peptidergic neurons, and stress that synaptic transmitter signaling as well as peptidergic paracrine signaling and volume transmission from varicosities can be the main signaling mode of peptidergic interneurons. These features should be taken into account in connectomic studies that aim to dissect the circuitry underlying insect behavior and physiology in which peptidergic neurons such as the NCCAP act as important regulators.