4.2 ORN population measurements
4.2.3 Role of glomeruli
In M/T cells of the olfactory bulb, odor identities and odor concentrations are well-predicted by patterns of their first-spike latencies, which are supe-rior to patterns of their firing rate [23]. How are these latencies generated?
For the odorant stimulation with methionine at 50 µM, consistent latency differences between M/T cells up to 500 ms were detected, which cannot be easily explained by individual ORN latencies - the cell-specific contribution to their latency variation (compare standard deviations oft0−tm andt0−tr
at40µM) is lower by a factor of about 5.
Additional measurements at ORN axon terminals in olfactory glomeruli showed that the sum of ORN responses already exhibit the latency pattern observed in M/T cell populations (with slightly less significance). The la-tencies in the olfactory bulb are thus generated by summation over ORN
4.3. Calcium channel microdomains and function of BK channels
ensemble responses, which could be an additional feature of glomeruli in olfactory coding. For the observed concentrations, it is likely that M/T la-tencies are mainly determined by the combined AP firing rates of individual ORNs, essentially computing a transformation of olfactory information from frequency to latency space.
4.3 Calcium channel microdomains and func-tion of BK channels
Using a fast sequential line scanning mode at 1250 f/s, it was possible to detect Ca2+ influx in single ORNs. As the fluorescence signal during depo-larization steps was mostly dynamic (in contrast to steady-state situations described in [43]), influx regions were characterized not by the absolute fluo-rescent amplitude, but by the rate of fluofluo-rescent increase, together with the time constant describing saturation and decay after a pixel-wise fit. Through this method it was possible to increase image contrast in the detection of hotspots of Ca2+ influx.
Ca2+ microdomains were further investigated in our lab by Dr. Guobin Bao (Dep. of Neurophysiology and Cellular Biophysics, University of Göt-tingen) [2]. Here, areas of Ca2+ influx imaged at slower frame rates could be colocalized with BK channels that were imaged using Alexa-488 labeled iberiotoxin. As the main role of BK channels, fast repolarization and thus facilitation of high ORN firing rates could be identified, as the maximum AP frequency was reduced by blocking BK channels with iberiotoxin.
The reduction in response amplitudes measured in Section 3.3, where several ORNs were exposed to the same concentration of iberiotoxin, support these findings in the sense that the BK channel can positively modulate the response to odors. Only a subset of ORNs showed amplitude reductions upon BK inhibition, possibly meaning that BK channels are not expressed in all ORNs at the same level, or that the unaffected ORNs responded at too low AP frequencies to be modulated.
Chapter 5 Summary
Odor space is large. This is one of the main problems the olfactory system has to solve, and it solves it well. Its strategy in detecting and discriminating a multitude of odors with a limited number of neurons lies in the so-called combinatorial coding. By having a set of olfactory receptor neurons (ORN), each with a broad but specific sensitivity to different odors, their response pattern can be used to efficiently recognize many odors. Recent findings sug-gest that response patterns are not necessarily static, but carry information in the time domain. In the olfactory bulb (OB), odor identity was found to be better described by latency patterns than by firing rates.
Motivated by these discoveries, this thesis aimed at the investigation of ORN population responses in how they are able to generate the latency code observable in the OB. For the measured odor concentration range, it was shown that latency coding is not dominant on the ORN level, and the main information is carried by ORN firing rates. Olfactory glomeruli thus seem to play a role in converting the set of ORN firing rates to a latency.
Supplementing the population measurements, Ca2+ influx through voltage-gated Ca2+channels (VGCC) in single ORNs was imaged at high frame rates, discovering clustering of VGCCs.
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Acknowledgements
First of all, I would like to thank my supervisor Prof. Dr. Dr. Schild for giving me the opportunity to work in his lab, his helpful advice and discussion, and the great freedom offered in following my ideas.
Many thanks to Prof. Dr. Salditt and Prof. Dr. Schmidt for being in my thesis committee and giving me advice.
I am greatful to Dr. Stephan Junek for the nice time constructing the LIM, the discussions, and the friendship.
I want to thank Alexander Brinkmann and Sebastian Gliem for proof-reading of the manuscript in the last minute.
And thanks to all friends and collegues from the lab for the creative and friendly atmoshpere, ranging from coffee sessions, scientific debates, to strange philosophy.
Personal
Name Mihai Alevra Date of birth April 29th 1978 Place of birth Bukarest, Romania
E-mail malevra@gwdg.de
Education
since 2/2009 PhD in the laboratory of Prof. Dr. Dr. Schild, Dep. of Neurophysiology and Cellular Biophysics
Enrolled at the International Max Planck Research School “Physics of Biological and Complex Systems“, Göttingen Graduate School for Neurosciences and Molecular Biosciences
3/2008 Diploma in physics, ”Biophysikalische Charakterisierung schneller Signalprozesse in olfaktorischen Rezeptorneu-ronen mit einem konfokalen Linienrastermikroskop“