Organization of the olfactory pathway and odor processing in the antennal lobe of the ant Camponotus floridanus

Abstract

Ants rely heavily on olfaction for communication and orientation. Here we provide the first detailed structure-function analyses within an ant's central olfactory system asking whether in the carpenter ant, Camponotus floridanus, the olfactory pathway exhibits adaptations to processing many pheromonal and general odors. Using fluorescent tracing, confocal microscopy, and 3D-analyses we demonstrate that the antennal lobe (AL) contains up to approximately 460 olfactory glomeruli organized in seven distinct clusters innervated via seven antennal sensory tracts. The AL is divided into two hemispheres regarding innervation of glomeruli by either projection neurons (PNs) with axons leaving via the medial (m) or lateral (l) antennocerebral tract (ACT). M- and l-ACT PNs differ in their target areas in the mushroom-body calyx and lateral horn. Three additional ACTs project to the lateral protocerebrum only. We analyzed odor processing in AL glomeruli by retrograde loading of PNs with Fura-2 dextran and fluorimetric calcium imaging. Odor responses were reproducible and comparable across individuals. Calcium responses to pheromonal and nonpheromonal odors were very sensitive (10(-11) dilution) and patterns were partly overlapping, indicating that processing of both odor classes is not spatially segregated within the AL. Response patterns to the main trail-pheromone component nerolic acid remained stable over a wide range of intensities (7-8 log units), while response durations increased indicating that odor quality is maintained by a stable pattern and intensity is mainly encoded in response durations. The structure-function analyses contribute new insights into important aspects of odor processing in a highly advanced insect olfactory system.

Complementary Function and Integrated Wiring of the Evolutionarily DistinctDrosophilaOlfactory Subsystems

“…C, Summed responses of OR (magenta) (Hallem and Carlson, 2006) and IR (green) OSNs to a set of odors common to both studies. Odor names are color coded by functional group as in present in the antennal lobes of honeybees (T4 glomeruli), cockroaches (T10 glomeruli), and ants (T7 glomeruli) (Zube et al, 2008;Nishino et al, 2009;Galizia and Rössler, 2010;Watanabe et al, 2010).…”

Parallel Processing via a Dual Olfactory Pathway in the Honeybee

“…For visualization of electrode tracks in penetrated brain tissue, electrodes were dipped in fluorescent dye (Alexa 568 hydrazide; A-10437, Invitrogen) before the recordings. After successful experiments, brains were rinsed with fresh bee Ringer's solution (37 mM NaCl, 2.7 mM KCl, 8 mM Na 2 HPO 4 , 1.4 mM KH 2 PO 4 , pH 7.2), glands and trachea were removed, and crystals of MicroRuby (tetramethylrhodamin dextran, Invitrogen) were inserted in the recorded AL to trace the m-APT and l-APT (Zube et al, 2008;Rössler and Zube, 2011). After 30 -45 min, brains were dissected and immediately rinsed in 4% formaldehyde in 0.1 M PBS overnight at 4°C.…”

“…The honeybee dual olfactory pathway is anatomically separated in PNs that innervate the MBs and LH in reverse order (Mobbs, 1982;Abel et al, 2001;Kirschner et al, 2006;Zube et al, 2008;Rössler and Zube, 2011). This pattern provoked different hypotheses about the pathway's functional significance: "dual segregated" (different odors in different tracts) or "dual parallel" (similar input, differential feature extraction) (Galizia and Rössler, 2010).…”

“…Anatomically, M/T cells in rodents target different areas in the olfactory cortex, whereas both l-APT and m-APT PNs in honeybees target the MB calyces and LH. However, innervation within the MB and LH shows segregated and overlapping zones (Kirschner et al, 2006;Zube et al, 2008). Previous intracellular recordings revealed latency differences between PNs of both tracts (Müller et al, 2002).…”

“…After AL processing (Martin et al, 2011;Wilson, 2011), odor information is transferred via projection (output) neurons (PNs) as spatiotemporal response patterns (Sachse and Galizia, 2002;Wilson and Laurent, 2005;Krofczik et al, 2008;Arenas et al, 2009) to higher brain centers in the mushroom bodies (MBs) and lateral horn (LH) (Mobbs, 1982). Hymenoptera (ants, bees, wasps) possess a unique dual olfactory pathway (Abel et al, 2001;Kirschner et al, 2006;Zube et al, 2008;Galizia and Rössler, 2010;Rössler and Zube, 2011;Nishikawa et al, 2012) with a medial AL protocerebral tract (m-APT) and a lateral AL protocerebral tract (l-APT). Both tracts comprise axons from ϳ920 uniglomerular PNs (ϳ410 m-APT PNs and ϳ510 l-APT PNs) .…”