The origins of the human hypothalamus go to the worm stage

Aug 6, 2007 08:31 GMT  ·  By

Next time when you call somebody "a worm", you must know you're talking about a long lost cousin. Scientists have found strong proof that the hypothalamus and other hormone-secreting brain centers are much older than previously thought and probably have their origins in the multifunctional neurons of the last common ancestor of vertebrates, insects and worms.

Hormones are chemicals controlling growth, metabolism, reproduction and other biological processes in all vertebrates (including humans). They are synthesized by specialized brain centers like the hypothalamus, reaching all the cells of the body through the blood stream.

A team at the European Molecular Biology Laboratory [EMBL] has now discovered that the evolutionary basis of these advanced hormonal brain centers were multifunctional neurons that sensed directly the environment and released hormones accordingly. So, next time you step on a cockroach ...

The slow, long-lasting and wide effects of the hormones complete with the fast and precise nervous commands. Insects and worms too have hormones, but their formula is very different from that of the vertebrate counterparts.

"This suggested that hormone-secreting brain centers have arisen after the evolution of vertebrates and invertebrates had split. But then vertebrate-type hormones were found in annelid worms and mollusks, indicating that these centers might be much older than expected." said co-author Detlev Arendt.

The team compared two types of hormone-secreting neurons of zebrafish, a vertebrate and the annelid worm Platynereis dumerilii and encountered some amazing similarities. Both cell types were situated at the same positions in the developing brains of the two species, but also had the same shape and molecular makeup.

One of these neurons releases vasotocin, a hormone controlling reproduction and water balance of the organism, the other synthesizes a hormone called RF-amide. Each cell type possesses a unique molecular fingerprint, a pattern of regulatory genes that give the cell's identity.

The level of similarity between the fingerprints of vasotocin and RF-amide-secreting neurons in zebrafish and Platynereis cannot be explained by convergent evolution, but by a common evolutionary origin of these cells. "It is likely that they existed already in Urbilateria, the last common ancestors of vertebrates, insects and worms" explained Arendt.

Both of the neurons from Platynereis and fish are multifunctional: they synthesize hormones and at the same time possess sensory properties. The vasotocin-releasing cells contain a light-sensitive pigment, while RF-amide is synthesized as a response to certain chemicals.

The researchers now believe that such multifunctional sensory neurons are among the oldest neuron types, which directly processed sensory information from the ancient marine environment to shifts in the animal's body. In time, these cells clustered and specialized forming complex brain centers like the hypothalamus in vertebrates.

"These findings revolutionize the way we see the brain. So far we have always understood it as a processing unit, a bit like a computer that integrates and interprets incoming sensory information. Now we know that the brain is itself a sensory organ and has been so since very ancient times." said co-researcher Kristin Tessmar-Raible.