Essential Role of the Mushroom Body in Context-Dependent CO sub(2) Avoidance in Drosophila

• Published in: Current biology Vol. 23; no. 13; pp. 1228 - 1234
• Main Authors: Braecker, Lasse B, Siju, K P, Varela, Nelia, Aso, Yoshinori, Zhang, Mo, Hein, Irina, Vasconcelos, Maria Luisa, Kadow, Ilona CGrunwald
• Format: Journal Article
• Language: English
• Published: 01.07.2013
• Subjects: Apis mellifera; Drosophila melanogaster
• ISSN: 0960-9822
• DOI: 10.1016/j.cub.2013.05.029

Internal state as well as environmental conditions influence choice behavior. The neural circuits underpinning state-dependent behavior remain largely unknown. Carbon dioxide (CO sub(2)) is an important olfactory cue for many insects, including mosquitoes, flies, moths, and honeybees [1]. Concentrations of CO sub(2) higher than 0.02% above atmospheric level trigger a strong innate avoidance in the fly Drosophila melanogaster [ 2 and 3]. Here, we show that the mushroom body (MB), a brain center essential for olfactory associative memories [ 4, 5 and 6] but thought to be dispensable for innate odor processing [ 7], is essential for CO sub(2) avoidance behavior only in the context of starvation or in the context of a food-related odor. Consistent with this, CO sub(2) stimulation elicits Ca super(2+) influx into the MB intrinsic cells (Kenyon cells: KCs) in vivo. We identify an atypical projection neuron (bilateral ventral projection neuron, biVPN) that connects CO sub(2) sensory input bilaterally to the MB calyx. Blocking synaptic output of the biVPN completely abolishes CO sub(2) avoidance in food-deprived flies, but not in fed flies. These findings show that two alternative neural pathways control innate choice behavior, and they are dependent on the animal's internal state. In addition, they suggest that, during innate choice behavior, the MB serves as an integration site for internal state and olfactory input.