Huoviala, P. | Search bioRxiv

bioRxiv2018

Neurogenetic dissection of the Drosophila innate olfactory processing center

Animals exhibit innate behaviours in response to a variety of sensory stimuli such as olfactory cues. In Drosophila, a higher olfactory centre called the lateral horn (LH) is implicated in innate behaviour. However, our knowledge of the structure and function of the LH is scant, due to the lack of sparse neurogenetic tools for this brain region. Here we generate a collection of split-GAL4 driver lines providing genetic access to 82 LH cell-types. We identify the neurotransmitter and axo-dendritic polarity for each cell-type. Using these lines were create an anatomical map of the LH. We found that [~]30% of LH projections converge with outputs from the mushroom body, the site of olfactory learning and memory. Finally, using optogenetic activation of small groups of LH neurons. We identify cell-types that drive changes in either valence or specific motor programs, such as turning and locomotion. In summary we have generated a resource for manipulating and mapping LH neurons in both light and electron microscopy and generated insights into the anatomy and function of the LH.

neuroscience↗

bioRxiv2018

Neural circuit basis of aversive odour processing in Drosophila from sensory input to descending output.

Evolution has shaped nervous systems to produce stereotyped behavioural responses to ethologically relevant stimuli. For example when laying eggs, female Drosophila avoid geosmin, an odorant produced by toxic moulds. Here we identify second, third, and fourth order neurons required for this innate olfactory aversion. Connectomics data place these neurons in a complete synaptic circuit from sensory input to descending output. We find multiple levels of valence-specific convergence, including a novel form of axo-axonic input onto second order neurons conveying another danger signal, the pheromone of parasitoid wasps. However, we also observe extensive divergence: second order geosmin neurons connect with a diverse array of 80 third order cell types. We find a pattern of convergence of aversive odour channels at this level. Crossing one more synaptic layer, we identified descending neurons critical for egg-laying aversion. Our data suggest a transition from a labelled line organisation in the periphery to a highly distributed central brain representation that is then coupled to distinct descending pathways.

neuroscience↗

bioRxiv2017

An olfactogenetic approach identifies olfactory neurons and brain centers directing negative oviposition decisions in Drosophila.

The sense of smell influences behaviors in animals, yet how odors are represented in the brain remains unclear. The nose contains different types of olfactory sensory neurons (OSNs), each expressing a particular odorant receptor, and OSNs expressing the same receptors converge their axons on a brain region called a glomerulus. In Drosophila, second order neurons (projection neurons) typically innervate a single glomerulus and send stereotyped axonal projections to the lateral horn. One of the greatest challenges to studying olfaction is the lack of methods allowing activation of specific types of olfactory neurons in an ethologically relevant setting. Most odorants activate many olfactory neurons, and many olfactory neurons are activated by a variety of odorants. As such, it is difficult to identify if individual types of olfactory neurons directly influence a behavior. To address this, we developed a genetic method in Drosophila called olfactogenetics in which a narrowly tuned odorant receptor, Or56a, is ectopically expressed in different olfactory neuron types. Stimulation with geosmin (the only known Or56a ligand), in an Or56a mutant background leads to specific activation of only the target olfactory neuron type. We used this approach to identify which types of olfactory neurons can directly guide oviposition decisions. We identified 5 OSN-types (Or71a, Or47b, Or49a, Or67b, and Or7a) that, when activated alone, suppress oviposition. Projection neurons partnering with these OSNs share a region of innervation in the lateral horn, suggesting that oviposition site-selection might be encoded in this brain region.\n\nSignificance StatementThe sense of smell begins by activation of olfactory neurons in the nose. These neurons express an olfactory receptor that binds odorants (volatile chemicals). How the sense of smell is encoded in the brain remains unclear. A key challenge is due to the nature of olfactory receptors themselves - most respond to a wide range of odorants - so it is often impossible to activate just a single olfactory neuron type. We describe here a novel approach in Drosophila called olfactogenetics which allows the specific experimental activation of any desired olfactory neuron. We use olfactogenetics to identify olfactory neurons and brain regions that guide egg-laying site selection. Olfactogenetics could be a valuable method to link olfactory neuron activities with circuits and behaviors.

neuroscience↗