Dissecting the Translation of Olfactory Information Processing into Approach Behavior using Orco and Octopamine Signaling in Drosophila melanogaster

Environmental stimuli present animals with input that needs to be processed and judged according to relevance. The animal is challenged with the decision to act and if, how to act in response. How components of the olfactory machinery function in odor processing with hindsight to behavioral output is not well understood. Neurotransmitters such as octopamine might be modulating behavioral output and decision making but how and what behaviors are affected is not known in detail. This thesis attempts to further the understanding of these processes. The general organization of the olfactory system in insect olfaction is similar to that of vertebrates. Both express one olfactory receptor (OR) gene per olfactory sensory neuron (OSN). In contrast to vertebrates, insects have an additional almost ubiquitously present olfactory co-receptor (Orco) in OSNs encoded by the orco gene. Orco has been proposed to be essential for insect olfaction. Still how Orco contributes to the relevant content of the olfactory information for the live animal is not known in detail. In this thesis the function of the Orco is dissected using Orco loss-of-function mutants (Orco1) in olfactory preference experiments. In general complex odor mixtures are more attractive for adult Drosophila melanogaster than single odors. Previously Orco1 mutants were shown to be unable to distinguish between similar complex odor mixtures. It is shown here, that Orco1 mutants can distinguish between two similar complex odor mixtures but in an odor concentration dependent manner. Furthermore, Orco is required for olfactory preference at low attractive odorant concentrations, while aversive behavior is mostly Orco independent at high odorant concentrations. In addition, olfactory preference for ethyl acetate and acetic acid is shifted to higher concentrations. These findings suggest that Orco increases odor sensitivity for olfactory preference behavior. Odor specific receptors are thought to give odors identities and have been proposed to be mis-localized in the absence of Orco. Here it is shown that Orco1 mutants are not anosmic and can sense and prefer single odors. To address whether Orco plays a functional role in the perception of odor identity two similar attractive single odors were tested against each other as single odors and in a complex odor mixture background. Indeed Orco mutants fail to distinguish between the two odors and odor containing mixtures. This provides evidence that Orco is not essential for odor perception and plays a role in odor identity assignment. Taken together the findings here indicate that Orco is not essential but probably aids odor sensing for olfactory preference and odor identity assignment by increasing odor sensitivity. OA has been associated as a reward reinforcer and DA with punishment and negative reinforcement in behavior in Drosophila melanogaster. Furthermore, OA has been associated with response selection and decision making. Schneider et al. (2012) previously showed that activating OA/TA neurons with a sequence of 40Hz and 8Hz frequency in an optogenetic approach could induce site-preference in Drosophila melanogaster. However, whether this specific OA/TA neuron activation frequency is needed to induce site-preference and if activation of DA neurons can induce site-aversion is unclear. Here the blue light sensitive cation channel Channelrhodopsin-2 was expressed in DA and OA neurons to allow optogenetic activation in the site-preference assay. It is shown here that activation of DA neurons can promote site-aversion, probably due to the negative reinforcement properties of DA. In addition, OA/TA neuron activation with different activation frequencies failed to induce site-preference indicating that site-preference promotion requires a specific OA/TA neuron activation frequency. OA/TA mediated response selection for site-preference and possible other behaviors might be activation frequency dependent. Site-preference and aversion involves targeted movement, a form of locomotion. Therefore it could be that the same set of OA/TA neurons also modulates locomotion. In addition, OA has been proposed to play a role in locomotion behavior. To investigate locomotion behavior an optogenetic locomotion setup was developed. Direct activation of DA neurons has been shown to promote increases in locomotion. To validate the setup it is shown that activation of DA neurons increases the locomotor output. To address whether the same set of OA/TA neurons that regulate site-preference also influence locomotion behavior OA/TA neurons were activated with different activation frequencies. Depending on the OA/TA neuron activation frequency, increased or prolonged locomotion as well as no change in behavior could be observed. These findings indicate that locomotor output is modulated by OA/TA neurons and changes in locomotion can be induced by specific OA/TA neuron activation frequencies. Finally, employing the GRASP system, cellular contact between OA/TA neurons and OSNs can be detected in the AL providing a possible site for modulation of the olfactory information processing. This study provides insights into the function of Orco from a behavioral output point of view. Furthermore, it is shown that behavioral output can differ and be regulated by activating the same population of OA/TA neurons with different frequencies. This indicates that response selection might be regulated by neuronal activity/spiking patterns and not simple on and off of neurons.