Although cuts and scrapes are painful, most injuries eventually heal and recover. The only exception to this rule was thought to be brain tissue, but mounting evidence had suggested that this is not the case and in 1998, P. S. Erikkson and colleagues reported in Nature Medicine that new neurones can be born and grow even in adult humans. However, getting to grips with the nuts and bolts of the systems regulating neurone birth is a complex problem;which is why Barbara Beltz and her team at Wellesley College, USA, turned to a relatively simple creature, the American lobster, to satisfy their curiosity about neurogenesis. Beltz explains that lobsters tend to become active around dusk, and when she investigated neurogenesis activity in the crustacean's brain, found that it also coincided with dusk. Neurogenesis seemed to be under circadian control. Beltz also knew that a key neurotransmitter, serotonin,regulates neurogenesis. Could serotonin be a key component of circadian controlled neurogenesis (p. 3765)?
Intrigued, Beltz and Miriam Wildt, a graduate student from the University of Ulm in Germany, began manipulating the crustacean's daylight exposure to see how it affected brain serotonin levels. But even after 18 months of struggling against enormous technical challenges, only a weak circadian pattern in serotonin levels began to emerge. Beltz explains that they had expected significant increases in serotonin over the course of the day, but Wildt found relatively weak peaks in her serotonin HPLC traces, and these were hard to reproduce. Beltz suspected that something wasn't quite right, but she didn't know what.
Around the same time, Beltz's team was also investigating the effects of nerve stimulation on serotonin production in the brain's olfactory and accessory lobes. They suspected that if they looked for circadian patterns in these regions `there was a chance we could see something really interesting'says Beltz; and they did! Suddenly the reason for the weak pattern in the whole brain samples became clear. The serotonin fluctuations in both lobes were strong and rhythmic, but each had a different pattern. They had found the strong rhythms they were seeking. Each lobe's serotonin levels were under circadian control, although the olfactory and accessory lobe's patterns differed dramatically; serotonin levels in the olfactory lobe peaked just before dusk, falling significantly at night, while the accessory lobe's serotonin levels also peaked at dusk, but remained high throughout the night.
But Beltz is still puzzled. She believes that the dorsal giant neurone provides the primary serotonergic input to both brain regions, so how does a single nerve cell produce two such different circadian rhythms? Beltz suspects that the branches of the dorsal giant neurone that innervate the olfactory and accessory lobes could have independent mechanisms for regulating serotonin levels in their respective lobes. She explains that these brain regions are likely to have different neuronal activity levels; the olfactory lobe processes information for one sense, while the accessory lobe handles many different inputs. It is possible that these differences in activity levels could in turn regulate serotonin levels in both the olfactory and accessory lobe branches of the dorsal giant neuron.