Neuronal Processing of Complex Mixtures Establishes a Unique Odor Representation in the Moth Antennal Lobe

Abstract

Animals typically perceive natural odor cues in their olfactory environment as a complex mixture of chemically diverse components. In insects, the initial representation of an odor mixture occurs in the first olfactory center of the brain, the antennal lobe (AL). The contribution of single neurons to the processing of complex mixtures in insects, and in particular moths, is still largely unknown. Using a novel multicomponent stimulus system to equilibrate component and mixture concentrations according to vapor pressure, we performed intracellular recordings of projection and interneurons in an attempt to quantitatively characterize mixture representation and integration properties of single AL neurons in the moth. We found that the fine spatiotemporal representation of 2–7 component mixtures among single neurons in the AL revealed a highly combinatorial, non-linear process for coding host mixtures presumably shaped by the AL network: 82% of mixture responding projection neurons and local interneurons showed non-linear spike frequencies in response to a defined host odor mixture, exhibiting an array of interactions including suppression, hypoadditivity, and synergism. Our results indicate that odor mixtures are represented by each cell as a unique combinatorial representation, and there is no general rule by which the network computes the mixture in comparison to single components. On the single neuron level, we show that those differences manifest in a variety of parameters, including the spatial location, frequency, latency, and temporal pattern of the response kinetics.

Antennal Lobe Processing Correlates to Moth Olfactory Behavior

“…However, the strong nonlinear interactions observed at the output of six of seven animals support conclusions concerning the important role of the modulatory interneuron network in insects (Hildebrand et al, 1992;Chou et al, 2010;Lei et al, 2011;Reisenman et al, 2011;Kuebler et al, 2011a,b), and thus strengthen the idea of a novel feature percept generated by the AL. Moreover, these results are consistent with our previous electrophysiological study at the single neuron level (Kuebler et al, 2011b) and indicate that the mechanisms of mixture processing revealed at the single neuron level are maintained across the glomerular array in the AL.…”

“…In contrast to honeybees (Fonta et al, 1993), most local neurons in Manduca show a typical broad "symmetrical" arborization pattern innervating the majority of AL glomeruli (Matsumoto and Hildebrand, 1981;Kuebler et al, 2011b;Reisenman et al, 2011). Consequently, an inhibitory global network incorporating a population of neurons with glomerulus-specific connectivity for general odor mixtures seems unlikely in Manduca (but compare the Manduca MGC: Christensen and Hildebrand, 1997;Lei et al, 2002;Heinbockel et al, 2004).…”

“…We differentiated between two main network interaction types, synergism and suppression (Duchamp-Viret et al, 2003;Silbering and Galizia, 2007;Kuebler et al, 2011b). Our criteria were highly restrictive: to be considered as a suppressive interaction, at least one of the activity regions in the mixture pattern (black bars) had to completely disappear compared with the projected mixture (white bars).…”

“…All odors were acquired from Sigma at highest purity available and diluted in mineral oil at 1 ϫ 10 Ϫ4 , a concentration previously used for an electrophysiological study of the Manduca AL (see Kuebler et al, 2011b for rationale). Odors were selected because of their ecological and physiological relevance as shown in previous studies of M. sexta (King et al, 2000;Shields and Hildebrand, 2000;Fraser et al, 2003;Hansson et al, 2003;Reisenman et al, 2004;Kuebler et al, 2011b;Olsson et al, 2011). For optical recordings, stimuli were presented in a similar repetitive sequence across animals: We performed three experimental runs, first presenting each animal with a random stimulation sequence of each of the five single components and the control (mineral oil), followed by the complete quintuple mixture.…”

“…The double staining of input and output AL neurons in a single preparation allows direct assessment of input-output modulation without requiring topographical orientation to specific landmarks (i.e., glomeruli). The mixture compounds used were previously assessed in a single unit study of the Manduca AL (Kuebler et al, 2011b) and provide a chemically diverse excerpt of the odor bouquet of flowers and plants on which Manduca feeds and oviposits (Kessler and Baldwin, 2001;Allmann and Baldwin, 2010). In an attempt to correlate our physiological results with olfactory behavior, we subsequently tested components shown to undergo different levels of input-output spatiotemporal modulation in our optophysiological study with a known behaviorally attractive synthetic flower mixture (Riffell et al, 2009a).…”