How nerve cells distinguish odours

Whether different odours can be quickly distinguished depends on certain synapses in the brain that inhibit nerve stimulation. The researchers in Professor Dr Thomas Kuner's team at the Institute of Anatomy and Cell Biology at Heidelberg University Medical School and Dr Andreas Schaefer at the Max Planck Institute for Medical Research have shown that mice in which a certain receptor in the olfactory centre is missing can distinguish similar smells more quickly than mice without genetic manipulation. This behaviour was directly attributed to inhibitor loops between adjacent nerve cells.

The discovery of the activation principle of 'lateral inhibition' in the eye 43 years ago by Haldan K. Hartline, George Wald, and Ragnar Granit was honoured with a Nobel Prize. The Heidelberg researchers have for the first time succeeded in confirming the same mechanism for the olfactory system, from the molecular level to behaviour. The results of the studies were published in the prestigious journal 'Neuron.'

Odours attach to receptors of olfactory cells in nasal mucosa, where they trigger nerve signals. These signals are processed in what is known as the olfactory bulb, a part of the brain. In the neuronal network, the incoming signal is converted to a specific electrical pattern that is transmitted to the cerebral cortex and other areas of the brain and is recognised there.

Professor Kuner and his team have now shown for the first time how neuronal processing of olfactory stimuli directly affects the behaviour of test animals. 'We manipulated information processing very specifically in the olfactory bulb and then measured the effect of this genetic manipulation based on reaction time. We were thus able to prove that the test animals, due to localised inhibitor loops, could distinguish very similar odour combinations much faster, yet very reliably,' explained Professor Kuner.

Inhibition via interneurones acts as a kind of filter by amplifying strong stimuli and further weakening weak stimuli. This makes the essential information easier to recognise. In the test animals, reaction time was reduced by about 50 ms. The time needed by test animals to learn various odours and their memory capability remained unaffected. Recognition of simple odours was also unchanged.

The researchers delivered a certain enzyme, cre recombinase, directly into the nerve cells of the olfactory bulb of young mice via a viral gene ferry. In the genome of these mice, a certain gene segment was removed using genetically introduced recognition sites of these enzymes. This led to the deletion of a receptor in the interneurones. This targeted manipulation made the inhibitor loops especially active. Using the usual 'knock-out' models, in which the gene is deactivated in the entire body, the subsequent selective behaviour could not have been observed. In a sophisticated experimental design, the mice then had to learn to recognise simple and complex odours composed of several aromatic substances. Using electrophysiological measurements, imaging processes, and anatomical techniques, a link was then created from the molecule to behaviour.