The head activator is a neuropeptide that regulates pattern-forming processes in Hydra magnipapillata. Discovery The head activator was first isolated from the sea anemone Anthopleura elegantissima.An identical peptide was also purified and identified in Hydra magnipapillata as well as in the hypothalamus and intestine of rats and humans.

Structure 

The head activator is an undecapeptide characterized by the N-terminal pyroglutamate residue and the X-Pro-Pro sequence. These characteristics are thought to render the peptide resistant to aminopeptidases.

Synthesis and release 

Distribution of the head activator The head activator has been identified in Hydra as well as in the hypothalamus and intestine of rats and humans, where the peptide appears to have a role in neuronal differentiation.It has been suggested that the head activator acts as a neuropeptide,although direct evidence is lacking. Recently, the Hydra Genome Project was completed; however, the Hydra genome sequence does not include a putative head activator gene, and therefore the nature of this factor is uncertain.

Biological functions

Pattern formation Hydra is one of the simplest multicellular animals, having a simple body plan with a single body axis. Gierer and Meinhardt proposed a reaction-diffusion model that involves two counteracting factors, the head activator andthe head inhibitor.Whena certain threshold ratio of head activator to head inhibitor is attained, a head forms. The head activator increases the rate of head regeneration and bud formation, and increases the number of regenerated tentacles. Thus, this peptide was initially characterized as a morphogen. 

The head activator also functions as a mitogen and an inducer of neuronal differentiation. It has been reported that the morphogenetic function of the head activator can be explained by its mitogenetic activity.Thus, it seems doubtful that the head activator is a morphogen. However, it does have the capacity to activate mitosis and neuronal differentiation in Hydra and cultured mammalian cells. The structure of the head inhibitor remains to be determined