Sense and sensibility
                                                                                 01 May 99
 

                                The brain has a different way of dealing with pheromones

                                A NOSE by any other name does not smell the same. Researchers
                                have found that the vomeronasal system, which detects the sexual
                                signal molecules called pheromones, is wired in a completely different
                                way from the main olfactory system.

                                In both systems, each sensory neuron bears a single type of odour
                                receptor. In the main olfactory system, signals from the neurons
                                bearing a particular type of receptor all converge on a pair of relay
                                stations, called "glomeruli", in the olfactory bulb of the brain, from
                                where they travel on to the cortex. Each receptor type is connected
                                to a different pair of glomeruli. The cortex has to piece together the
                                identity of the odour by noting which of the thousand or so receptor
                                types are active (This Week, 13 March, p 19).

                                The vomeronasal system works differently. Peter Mombaerts and his
                                colleagues at Rockefeller University in New York discovered this when
                                they traced the connections of neurons bearing the vomeronasal
                                odour receptor known as VR i2 using strains of mice genetically
                                modified to produce marker proteins only in those neurons. They found
                                that instead of focusing their output onto a pair of glomeruli, neurons
                                bearing this receptor send signals to an average of 15 different
                                glomeruli in the accessory olfactory bulb of the brain ( Cell, vol 97, p
                                199).

                                A second team, led by Catherine Dulac of the Howard Hughes Medical
                                Institute and Harvard University in Boston and Richard Axel at
                                Columbia University in New York, found a similar pattern for neurons
                                bearing two other vomeronasal receptors. They also found that
                                individual glomeruli sometimes receive input from more than one
                                receptor type (Cell, vol 97, p 209).

                                The results seem to suggest that the vomeronasal system
                                immediately compares and integrates signals from different receptor
                                types instead of passing them on separately. "There is more
                                processing happening within the accessory bulb," says Mombaerts.

                                Dulac speculates that this process suits the vomeronasal system's
                                simple function. Unlike the main olfactory system, which must
                                identify tens of thousands of odours, the vomeronasal system need
                                only recognise a small and predetermined set of pheromones. These
                                pheromones trigger a small set of automatic, involuntary hormonal
                                changes, and mating or territorial behaviours. By contrast, animals
                                have a wide variety of uses for information from the main olfactory
                                system, many of them learnt.

                                But Michael Meredith, a neuroscientist at Florida State University in
                                Tallahassee, warns that it is too early to say whether the unexpected
                                cross-wiring of the vomeronasal system really helps it recognise
                                pheromones. "I don't know whether you could conclude that this is a
                                highly sophisticated system or whether it's just sloppily constructed,"
                                he says.

                                Bob Holmes
                                From New Scientist magazine, vol 162 issue 2184, 01/05/1999, page 7
 
 

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