NEUROSYSTEMS Environment-specific modulation of odorant representations in the honeybee brain Neloy Kumar Chakroborty, 1,2 Randolf Menzel 1 and Marco Schubert 1 1 Department of Biology, Chemistry and Pharmacy, Institute of Biology/Neurobiology, Free University Berlin, K€ onigin-Luise-Strasse 28/30, 14195 Berlin, Germany 2 Indian Statistical Institute, Computer Vision and Pattern Recognition (CVPR) Unit, Kolkata, West Bengal, India Keywords: antennal lobe, calcium imaging, in vivo, olfaction, projection neurons Edited by Giovanni Galizia Received 3 November 2015, revised 5 October 2016, accepted 10 October 2016 Abstract Ca 2+ imaging techniques were applied to investigate the neuronal behavior of projection neurons in the honeybee antennal lobe (AL) to examine the effects of long-lasting adaptation on odorant coding. Responses to eight test odorants were measured before, during, and after an odor adaptation phase. Bees were exposed to the adapting odor for 30 min. Test odorant responses were only recorded from a sub-population of accessible glomeruli on the AL surface. Projection neurons, the output neurons of the antennal lobes, are projecting through the lateral, mediolateral, and medial AL tract to higher centers of the olfactory pathway. Due to our staining techniques, we primarily focused our study on projection neurons going through the lateral and medial tract. Test odorants comprised compounds with different functional groups (alcohol, aldehyde, ketone, and ester) representing floral and/or pheromone odorants. Strength and discriminability between combinatorial activity patterns induced by the test odorants were quantified. In two independent experiments, we investigated one group of animals adapted to a colony odor and another adapted to a synthetic odor. Within the experimental groups, we found test odorant responses either decreased or increased in AL projection neurons. Additionally, the discriminability between test odorant patterns became less distinct in the colony odor experiment and more distinct during adaptation in the synthetic mixture experiment. These results are interpreted as odor depen- dent adaptation effects, increasing or decreasing response strength and discriminability by altered neural coding mechanisms in the AL neuropile. Introduction Adaptation is a form of neuronal plasticity that adjusts the sensitiv- ity of sensory systems to varying stimulus intensities. This plasticity can be used as a filter for static stimuli after prolonged exposure. Such a filter could for example function as a gating mechanism for relevant sensory information suppressing invariable background information such as monotonic noise (e.g., sound, light, or odors). Among the different sensory modalities, the olfactory system has a remarkable capacity to adapt to environmental conditions. Adapta- tion may last for short (milliseconds, seconds) or long (hours, days) durations modulating the responses of the olfactory system to test stimuli (Dethier, 1976; Chaput & Panhuber, 1982; Wang et al., 1993; Colbert & Bargmann, 1995). Such reformatting processes are reported for the peripheral receptor neurons and for central neurons in both vertebrates and invertebrates (Baylin & Moulton, 1979; Cha- put & Panhuber, 1982; Mair, 1982; Kaissling et al., 1987; Colbert & Bargmann, 1995; Zufall & Leinders-Zufall, 1997; Galizia et al., 1999a; St€ ortkuhl et al., 1999; Kelling et al., 2002; Best & Wilson, 2004). These studies show increased and/or decreased responses to test stimuli after adaptation that do not follow the simple scheme of adaptation only leading to a reduced response. In previous work, we found that background odors can weaken the odor learning performance in honeybees using the proboscis extension responses (PER) paradigm for quantification (Chakroborty et al., 2015). In the present study we were looking for a neuronal correlate of these findings and addressed the question whether and how long-term adaptation induces changes in neural representations of test odorants at the level of output neurons of the honeybee (Apis mellifera) antennal lobe (AL), the primary olfactory information pro- cessing center. Odors are detected by olfactory sensory neurons (OSN) in the antenna and the palps. Together they serve as input neurons to the AL. Projection neurons (PN) are the output neurons of the AL and receive input from OSNs either directly or indirectly via local neu- rons (LN). Most of the LNs are inhibitory implementing a global normalization network (Olsen et al., 2010; Galizia, 2014) that flat- tens the dose–response functions of PNs (Sachse & Galizia, 2003). Additionally, negative feedback loop connections between PNs and LNs are presumed to create a push/pull mechanism keeping the Correspondence: Dr Marco Schubert, as above. E-mail: m.schubee@gmx.de © 2016 Federation of European Neuroscience Societies and John Wiley & Sons Ltd European Journal of Neuroscience, Vol. 44, pp. 3080–3093, 2016 doi:10.1111/ejn.13438