What the nose knows – early synthesis of olfactory perception
Scientists have proven for the first time experimentally by means of high-resolution optical methods that the release of transmitter from reciprocal spines takes place by means of an action potential limited to the spine.
Even in a year lacking Christmas markets we associate the Christmas season with smells like cinnamon or candle smoke. While the formation of visual and auditory perceptions has been largely elucidated, we have so far only partially understood how and on which level of the brain such odor signals are mapped and how odor percepts arise from this mapping. The first processing level beyond the nose is the so-called olfactory bulb, whose neurons are innervated by the olfactory sensory cells.
Most neurons of the mammalian brain carry small protusions (termed “spines”) on their dendrites, the cell processes that receive input signals. The synaptic contacts to the upstream neurons are mainly located on these spines. The granule cells of the olfactory bulb – a kind of local neuron and the most common type of nerve cell in this area of the brain – have spines that not only receive excitatory input signals but can also release transmitter substances themselves – so-called reciprocal synapses.
A team of scientists around Veronica Egger, professor of neurophysiology at the University of Regensburg, has now proven for the first time experimentally by means of high-resolution optical methods that the release of transmitter from these reciprocal spines takes place by means of an action potential limited to the spine. Surprisingly, however, the simultaneous activation of so-called NMDA receptors, which are otherwise relevant for the generation of input signals, is required to trigger the release. In collaboration with colleagues from Tehran and Turin, a possible mechanism for such a novel cooperation could be described. The results of the work have now been published in the journal eLife.
Beyond these unusual subcellular processes, the findings also have a wider significance for the coding of odor stimuli. This is done combinatorially, since each odorant usually activates a large number of olfactory receptors types. All receptors of the same type now stimulate a downstream module of neurons of the olfactory bulb, which in turn is connected to the olfactory cortex. From this activation of several modules, an olfactory perception is synthesized (just like a musical chord activates several groups of frequency-selective neurons of the auditory pathway, which finally results in a perception of the overall sound). The receptor modules do not interact directly wihin the bulb, but are connected to each other via granule cells and others. The observed cooperation now suggests that the synthesis of olfactory perception already starts at the level of the olfactory bulb in a very efficient way, in that the granule cells exclusively interconnect simultaneously active modules and synchronize the activity of these modules (eLife 2020).
The project was funded by the German Federal Ministry of Education and Research (funding codes 01GQ1104 and 01GQ1410A).
Prof. Dr. Veronica Egger
Professur für Neurophysiologie
Telefon +49 (0)941 943-3118
Lage-Rupprecht, V., Zhou, L., Bianchini, G., Aghvami, S. S., Mueller, M., Rózsa, B., Sassoè-Pognetto, Egger, V. (2020). Presynaptic NMDARs cooperate with local spikes toward GABA release from the reciprocal olfactory bulb granule cell spine. eLife 2020;9:e63737. DOI:10.7554/eLife.63737