Distinctive Neuronal Networks and Biochemical Pathways for Appetitive and Aversive Memory inDrosophilaLarvae

Abstract

Associative strength between conditioned stimulus (CS) and unconditioned stimulus (US) is thought to determine learning efficacy in classical conditioning. Elucidation of the neuronal mechanism that underlies the association between CS and US in the brain is thus critical to understand the principle of memory formation. With a simple brain organization, the Drosophila larva provides an attractive model system to investigate learning at the neurocircuitry level. Previously, we described a single-odor paradigm for larval associative learning using sucrose as a reward, and showed that larval appetitive memory lasts longer than 2 h. In this work, we describe behavioral and genetic characterization of larval aversive olfactory memory formed in our paradigm, and compare its stability and neurocircuitry with those of appetitive memory. Despite identical training paradigms, larval olfactory memory formed with quinine or NaCl is shortlived to be lost in 20 min. As with appetitive memory, larval aversive memory produced in this paradigm depends on intact cAMP signaling, but neither mutation of amnesiac nor suppression of CREB activity affects its kinetics. Neurocircuitry analyses suggest that aversive memory is stored before the presynaptic termini of the larval mushroom body neurons as is the case with appetitive memory. However, synaptic output of octopaminergic and dopaminergic neurons, which exhibit distinctive innervation patterns on the larval mushroom body and antennal lobe, is differentially required for the acquisition of appetitive and aversive memory, respectively. These results as a whole suggest that the genetically programmed memory circuitries might provide predisposition in the efficacy of inducing longer-lived memory components in associative learning.

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

“…(3) Blocking synaptic output via shibire ts using the MB-specific driver lines 201y and OK301 (Honjo and Furukubo-Tokunaga, 2005) or 201y and NP1131 (this study, Fig. 3) blocks larval appetitive and aversive learning (Honjo and Furukubo-Tokunaga, 2009). Specifically, the latter authors showed that output from chemical synapses of the MBs is required at test, but is dispensable during training to support normal appetitive and aversive memory.…”

“…1, 3) imply the necessity and sufficiency of the larval MB for olfactory learning and memory: (1) Classical learning mutants like dunce and rutabaga are impaired in larval appetitive Furukubo-Tokunaga, 2005, 2009) and aversive olfactory learning (Aceves-Piña and Quinn, 1979;Honjo and Furukubo-Tokunaga, 2009). As their gene products are highly enriched in the MBs, cAMP-pathwaydependent molecular coincidence detection of odor and reinforcement signaling in this brain structure was suggested (Crittenden et al, 1998).…”

“…Interestingly, the learning phenotype in sitters can be rescued to rover levels by boosting expression of the protein kinase G in the MBs via the driver strains 201Y, H24, and c739 (Kaun et al, 2007). (5) Reinforcement signaling by dopaminergic and octopaminergic neurons onto the MBs is likely to be necessary during training for aversive and appetitive olfactory learning (Honjo and Furukubo-Tokunaga, 2009;Selcho et al, 2009). Together these data strongly suggest that the larval MBs harbor an olfactory memory trace, similar to the role proposed for its adult counterpart (Heisenberg, 2003;Gerber et al, 2004;McGuire et al, 2005;Keene and Waddell, 2007).…”

“…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.…”

Classical conditioning through auditory stimuli in Drosophila: methods and models

“…For example, the learning mutants dunce and rutabaga fail to learn odors in a shock-avoidance protocol but learn normally in an appetitive task (Tempel et al, 1983), while aversive and appetitive learning of odors take place through different neural and biochemical pathways in Drosophila larvae (Honjo and Furukubo-Tokunaga, 2009). Learning mutants may act differently not only in different protocols but also with different CS modalities.…”