Role of histamine as a putative inhibitory transmitter in the honeybee antennal lobe

BACKGROUND: Odors are represented by specific spatio-temporal activity patterns in the olfactory bulb of vertebrates and its insect analogue, the antennal lobe. In honeybees inhibitory circuits in the AL are involved in the processing of odors to shape afferent odor responses. GABA is known as an inhibitory transmitter in the antennal lobe, but not all interneurons are GABAergic. Therefore we sought to analyze the functional role of the inhibitory transmitter histamine for the processing of odors in the honeybee AL. RESULTS: We optically recorded the representation of odors before, during and after histamine application at the input level (estimated from a compound signal), and at the output level (by selectively measuring the projection neurons). For both, histamine led to a strong and reversible reduction of odor-evoked responses. CONCLUSION: We propose that histamine, in addition to GABA, acts as an inhibitory transmitter in the honeybee AL and is therefore likely to play a role in odor processing

Stabile neuronale Repräsentation von Duftinformation in uniglomerulären Projektionsneuronen während olfaktorischen Lernens in der Honigbiene Apis mellifera

Titelblatt und Inhaltsverzeichnis Einleitung Material und Methoden Ergebnisse Diskussion Ausblick Englische Zusammenfassung Deutsche Zusammenfassung Danksagung Literaturangaben LebenslaufOdors elicit specific spatio-temporal combinatorial patterns of activity in the olfactory bulb (OB) of vertebrates and the antennal lobe (AL) of insects. In recent years, there have been several reports of changes in these patterns following olfactory learning. These studies pose a conundrum: How can an animal learn to efficiently respond to a particular odor with an adequate response, if its representation is changing during this process? A plastic odor-code cannot ensure reliable odor recognition. In this study, we offer a possible solution for this problem. Using the honey bee as an experimental animal, I measured odor evoked calcium responses in a subpopulation of AL output neurons, the uniglomerular projection neurons (PNs). I demonstrate that neural representations of odors in these neurons are remarkably resistant to plasticity following a variety of appetitive olfactory learning paradigms. There was no significant difference in the changes of odor-evoked activity between single and multiple trial forward or backward conditioning, differential conditioning, or unrewarded successive odor stimulation. In a behavioural learning experiment we show that uniglomerular projection neurons are necessary for conditioned odor responses. As a consequence, I discuss the possibility of parallel odor processing in the antennal lobe: lACT uniglomerular projection neurons are necessary for reliable odor coding and are not modified by learning. Parallel processing of olfactory information might ensures the separate encoding of the neural representations of a variety of aspects of olfactory stimuli, for example their temporal structure, their intensity fluctuations and the sequence, their evaluation and meaning The role that other neurons (such as multiglomerular projection neurons) play in olfactory memory remains to be investigated.Düfte werden durch ein spezifisches räumlich-zeitliches kombinatorisches Aktivitätsmuster im olfaktorischen Bulbus der Vertebraten und im Antennallobus der Insekten neuronal repräsentiert. In den letzten Jahren haben einige Studien gezeigt, daß sich diese neuronalen Repräsentationen des Dufts durch olfaktorisches Lernen verändert. Dies stellt ein Rätsel dar, denn wie kann ein Tier lernen, auf einen Duft mit einem adäquaten Verhalten zu antworten, wenn sich die neuronale Repräsentation während des Lernvorganges ändert. Diese Arbeit nimmt sich dieses Problems an, in dem die Geruchskodierung im Antennallobus der Honigbiene Apis mellifera untersucht wird. Mit Hilfe von kalziumsensitiven Farbstoffen, werden Duftantworten in einer bestimmten Neuronenpopulation, den uniglomerulären lACT Projektionsneuronen, im Antennallobus mittels optischer Imagingmethoden gemessen. Ich konnte zeigen, daß diese Neurone auch nach einer Vielzahl an verschiedenen Lernparadigmen keine Veränderungen in ihren Duftantworten zeigen. Einfache und multiple Wiederholung von Vorwärtspaarungen des Duftes mit einer Belohnung, ebenso Rückwärtspaarung, sowie differentielle Konditionierung haben keinen modulatorischen Einfluß auf die neuronale Repräsentation der Duftantwort in den uniglomerulären lACT Projektionsneuronen auf der Ebene des Antennallobus. In einem Verhaltensversuch ließ sich zeigen, daß Tiere deren Projektionsneurone durchtrennt sind, nicht auf einen erlernten Duft reagieren. Diese Ergebnisse zeigen, daß es eine Subpopulation von Projektionsneuronen im Antennallobus gibt, die sich aufgrund von Lernvorgängen nicht verändern und somit komplexe und stabile Duftkodierung sicherstellen

Appetitive odor learning does not change olfactory coding in a subpopulation of honeybee antennal lobe neurons

Odors elicit spatio-temporal patterns of activity in the olfactory bulb of vertebrates and the antennal lobe of insects. There have been several reports of changes in these patterns following olfactory learning. These studies pose a conundrum: how can an animal learn to efficiently respond to a particular odor with an adequate response, if its primary representation already changes during this process? In this study, we offer a possible solution for this problem. We measured odor-evoked calcium responses in a subpopulation of uniglomerular AL output neurons in honeybees. We show that their responses to odors are remarkably resistant to plasticity following a variety of appetitive olfactory learning paradigms. There was no significant difference in the changes of odor-evoked activity between single and multiple trial forward or backward conditioning, differential conditioning, or unrewarded successive odor stimulation. In a behavioral learning experiment we show that these neurons are necessary for conditioned odor responses. We conclude that these uniglomerular projection neurons are necessary for reliable odor coding and are not modified by learning in this paradigm. The role that other projection neurons play in olfactory learning remains to be investigated