The Mixture Counts – Odorants in Urine Trigger Complex Behaviour in Mice

_{The MUP´s convey important information about the animal, for example about its aggressive or mating behaviour. (source: Winandy).}

In the animal kingdom, social behaviour is controlled by a wide range of signals. The urine of mice, for instance, contains major urinary proteins (MUPs). These proteins convey important information about the animal, for example about its aggressive or mating behaviour. Together with international cooperation partners, Prof. Marc Spehr, Lichtenberg Professor of Biology and Biotechnology at RWTH Aachen University, has now shown for the first time how these different types of information are produced and recognized.

The research findings have been published in the highly respected journal Cell
With the aid of a special olfactory organ, the vomeronasal organ, mice are able to deduce important information from odorants. "In order to correctly interpret the scent mark of the urine or rather the pheromones contained in it, mice must discriminate between the self-deposited mark and that of other mice," says Spehr. "A self-deposited mark must not under any circumstances initiate aggression or change the hierarchical group structure," adds Spehr, who is head of the Department of Chemosensation at the Institute of Biology II. But how does the nervous system, with a limited number of pheromonal substances, succeed in controlling stereotypical behaviours such as aggression or willingness to mate and at the same time discriminate between the "self" and "other"?

Individual mixture of MUPs represents a personal note
The scientists have taken a decisive step forward in answering this question. "Although the mouse genome encodes 21 different MUPs, each mouse only produces between four and twelve different proteins," explains Spehr. From these basic ingredients, each individual mixes a cocktail of different MUPs and excretes it in its urine. This results in a kind of personal scent mark containing a wide range of relevant information. This includes the important information on whether the urine is the mouse's own or from another mouse. In behavioural experiments and neurophysiological studies, the researchers used manipulated mixtures of MUPs to show that even minor modifications to the MUP cocktail change the mice's identity. For example, a dominant male's "countermarking" of his own territory was observed whenever just one "foreign" MUP was added to his own urine. If the animals were presented with marks consisting of their own urine or their own MUP cocktail then there was no reaction at all.

Some neurons in the vomeronasal organ are decoding experts
In the next step, the Aachen scientists worked out how the animals' behaviour is triggered on the neuronal level. To do so, they investigated individual neurons in the vomeronasal organ and recorded their activity during stimulation with odorants. In the search for the neurons responsible, the PhD students Tobias Ackels and Annika Cichy reached an astonishing conclusion. Of the more than 1,000 neurons studied in the vomeronasal organ, only a fraction, to be precise 2.5 %, reacted sensitively to the MUPs. Whereas some neurons reacted very specifically to a certain MUP, others displayed a broad response pattern to various MUPs. "We had previously assumed that individual sensors of the vomeronasal organ reacted exclusively to quite specific pheromones," explains Spehr. "The new findings now suggest that some neurons are real decoding experts."

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