Local processing of auditory information in the thorax of Ensifera
Final Report Abstract
This project focused on DUM-neurons (dorsal unpaired median neurons) as elements potentially involved in local auditory processing in the duetting bushcricket Ancistrura nigrovittata, which shows a sex difference in song frequency and acoustically guided mate finding. Since DUM-neurons are hard to record in their very thin terminals in the neuropile, they were targeted at their soma, which uniquely is located unpaired in the posterior midline. It turned out that there are many more sound-activated DUM-neurons than anticipated from mostly incomplete stainings in the neuropile – we suspect at least 50 sound activated DUM-neurons to exist. We classified the sound-activated DUM-neurons into 7 morphological classes, 5 local ones and two intersegmental (with ascending or descending axons). Any of these “types” may indeed contain several types (definitely so for the intersegmental neurons and for type “outside”, which likely contains a set of neurons with increasing contact with vibratory sensory neurons and decreasing contact with auditory neurons) – but cell numbers recorded in these types are too low to identify these “subclasses”. It also appears that transitions or “hybrids” between neuron types may occur (e.g. “narrow” and “loops”). Definitely in some types multiples with nearly identical morphology exist (like in “narrow” and “loops”). These facts together make it so far not possible to discriminate intraindividual variation from differences between neuron types (or twins/triples etc.). As for “best frequency” (frequency with strongest response), the majority of neuron types exhibits a broad range, which includes nearly the whole range of tested frequencies between 4 and 50 kHz. At least for certain neuron types this is due to multiples (twins, triples …) of cells existing. This is exemplified for neurons, which receive frequency dependent inhibition visible as IPSPs at certain frequencies and intensities. Such IPSPs are found for the types “narrow, loops and outside” and physiologically clearly differ between individuals. We assume at least 6 or 7 cells to exist showing IPSPs and belonging to “narrow” and “loops” and few types belonging to “outside” and potentially “bushy”. Again, also such cells have best frequencies spanning nearly the whole range of frequencies tested. Neurons of the type “outside” in most cases are more responsive to lower frequencies and are additionally sensitive to vibration. Otherwise, the expectation that there are direct and easy to recognize correlations between neuron types and physiology (especially responses to carrier frequency variation) was not met. The majority of DUM-neurons, when recorded in the soma, produce at least small action potentials, but some do not spike at all. Members of type “outside”, in contrast to most other DUM, have a tonic response (often with many spikes) and not a phasic to phaso-tonic response. While nearly all DUM-neurons tested appear to receive more or less symmetrical input from the left and right ear, they may differ clearly in their responses to different temporal patterns of acoustic stimuli – some showing a stronger response to long stimuli and stimuli with short pauses (in the ms-range), others showing the reverse or no obvious dependence. In an artificial duet between males and females, three neurons from one individual may respond either to the male or to the female part or to the whole duet. Applying a potent blocker of inhibition (picrotoxin = PTX), which has shown to change response properties of other local and ascending neurons in A. nigrovittata, has an effect on many, but not all DUM-neurons tested. The most typical response is an increase in response strength – especially spike production. Visible IPSPs may or may not disappear after PTX- application. The average range of tuning significantly changes (increases) after PTX-application. Preliminary data with antibodies against -aminobutyric acid (GABA) indicate that some sound-activated DUM-neurons may, others do not seem to contain GABA. Relatively few experiments have been performed with filling a DUM-neuron with Lucifer Yellow and killing it by laser illumination while recording extracellularly from potentially postsynaptic neurons. On the first view no clear changes have been seen in the extracellular units, however, these data have not been evaluated yet in detail, since spike sorting needs to be applied as well as comparison to controls to check for potential effects. For being able to draw any conclusions, more experiments of this type are needed and are planned for the near future.
Publications
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(2013). "How are DUM neurons involved in the frequency processing of bush?crickets?" GGNB Science Day, Göttingen, Germany
Lefebvre P, Stumpner A
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(2013). "Local neurons in the auditory system of the bush cricket Ancistrura nigrovittata" The 14th Invertebrate Sound and Vibration international meeting, Glasgow, UK
Lefebvre P, Stumpner A, Molina J
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(2014). "Local neurons in the auditory system of the bush?cricket Ancistrura nigrovittata" 11th International Congress of Neuroethology, Hokkaido, Japan
Lefebvre P, Stumpner A
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(2014). Are DUM?neurons involved in local auditory processing of Ancistrura nigrovittata?" 107th Annual Meeting of the German Zoological Society, Göttingen, Germany
Lefebvre P, Stumpner A
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(2016) Local processing of auditory information in the thorax of Ensifera. Dissertation Georg-August-University Göttingen
Lefebvre, Paule
