The Role of Dopamine in Drosophila Larval Classical Olfactory Conditioning

Abstract

Learning and memory is not an attribute of higher animals. Even Drosophila larvae are able to form and recall an association of a given odor with an aversive or appetitive gustatory reinforcer. As the Drosophila larva has turned into a particularly simple model for studying odor processing, a detailed neuronal and functional map of the olfactory pathway is available up to the third order neurons in the mushroom bodies. At this point, a convergence of olfactory processing and gustatory reinforcement is suggested to underlie associative memory formation. The dopaminergic system was shown to be involved in mammalian and insect olfactory conditioning. To analyze the anatomy and function of the larval dopaminergic system, we first characterize dopaminergic neurons immunohistochemically up to the single cell level and subsequent test for the effects of distortions in the dopamine system upon aversive (odor-salt) as well as appetitive (odor-sugar) associative learning. Single cell analysis suggests that dopaminergic neurons do not directly connect gustatory input in the larval suboesophageal ganglion to olfactory information in the mushroom bodies. However, a number of dopaminergic neurons innervate different regions of the brain, including protocerebra, mushroom bodies and suboesophageal ganglion. We found that dopamine receptors are highly enriched in the mushroom bodies and that aversive and appetitive olfactory learning is strongly impaired in dopamine receptor mutants. Genetically interfering with dopaminergic signaling supports this finding, although our data do not exclude on naïve odor and sugar preferences of the larvae. Our data suggest that dopaminergic neurons provide input to different brain regions including protocerebra, suboesophageal ganglion and mushroom bodies by more than one route. We therefore propose that different types of dopaminergic neurons might be involved in different types of signaling necessary for aversive and appetitive olfactory memory formation respectively, or for the retrieval of these memory traces. Future studies of the dopaminergic system need to take into account such cellular dissociations in function in order to be meaningful.

DrosophilaLarvae Establish Appetitive Olfactory Memories via Mushroom Body Neurons of Embryonic Origin

“…Flies were cultured according to standard methods. See the Supplemental Data (available at www.jneurosci.org as supplemental material) or Selcho et al (2009) for fly stock source. For the behavioral experiments, UAS-shi ts1 was crossed to the MB-specific GAL4-driver lines.…”

“…Interestingly, how other sensory modalities are signaled onto the MBs and how output neurons collect the information from the MBs is hardly understood. Recent findings suggest that dopaminergic and octopaminergic neurons signal aversive and/or appetitive reinforcement onto the MBs (Schwaerzel et al, 2003;Schroll et al, 2006;Claridge-Chang et al, 2009;Honjo and Furukubo-Tokunaga, 2009;Mao and Davis, 2009;Selcho et al, 2009). Anatomically Tanaka et al (2008) identified at least 24 different types of MB extrinsic neurons (MBENs) in adult flies.…”

Behavioral and Synaptic Plasticity Are Impaired upon Lack of the Synaptic Protein SAP47

“…2 A, B). Notably, unless mentioned otherwise, we here used only one odorant, n-amyl acetate (AM, CAS: 628 -63-7, purity: 99%, Merck) to simplify the paradigm (Selcho et al, 2009;Saumweber et al, 2011). That is, we train groups of 30 larvae each and compare olfactory choice performance after either of two reciprocal training regimens: one group was exposed to the odorant AM in the presence of a positive reinforcer and to a no-odor situation without the reinforcer (AMϩ/noAM); the second group was trained reciprocally, i.e., by unpaired presentations of odorant and reinforcer (AM/noAMϩ).…”

Odor–Taste Learning Assays in Drosophila Larvae

“…For example, you can measure the behavior toward AM of larvae that have received paired presentations of AM with the reward during one kind of trial and exposure to an unrewarded Petri dish with an odorless container during the other type of trial (AM+/empty); these larvae then must be compared with larvae that have received unpaired presentations of AM and reward (AM/empty+). Clearly, the test involves an AMfilled container on the one side and an empty container on the other (Selcho et al 2009).…”