Peptide-driven control of somersaulting in Hydra vulgaris

The cnidarian Hydra vulgaris has a simple nervous system with a few
hundred neurons in distributed networks. Yet Hydra can perform
somersaults, a complex acrobatic locomotion. To understand the neural
mechanisms of somersaulting we used calcium imaging and found that
Rhythmical Potential 1 (RP1) neurons activate before somersaulting.
Decreasing RP1 activity or ablating RP1 neurons reduced somersaulting,
while two-photon activation of RP1 neurons induced somersaulting. Hym-248,
a peptide synthesized by RP1 cells, selectively generated somersaulting.
We conclude that RP1 activity, via release of Hym-248, is necessary and
sufficient for somersaulting. We propose a circuit model to explain the
sequential unfolding of this locomotion, using integrate-to-threshold
decision-making and cross-inhibition. Our work demonstrates that
peptide-based signaling is used by simple nervous systems to generate
behavioral fixed action patterns.