Protons have been found to have a role in transmitting information between neurons, communicating acceleration and the position of the head relative to gravity.
Chemically, an excess of protons (H+) makes a substance acidic. Dr Stephen Highstein of the Marine Biological Laboratory (MBL), Woods Hole Massachusetts, led a team that found that sensory cells in the inner ear of their model organism the toadfish (Opsanus tau) continuously transmit information on the head's orientation relative to gravity. The same sensory mechanism, known as the vestibular system, also detects changes in acceleration.
Highstein twice had toadfish taken into zero gravity on the Space Shuttle in order to investigate the species' vestibular mechanisms. He tragically died of leukemia between the work being completed and publication in the Proceedings of the National Academy of Sciences.
Many of our senses are designed to detect rapid changes, such as the appearance of a threat. Longer lasting signals require a different methodology, and it is this that Highstein's team have identified. "This addresses how we sense gravity and other low-frequency inertial stimuli, like acceleration of an automobile or roll of an airplane," says co-author Professor Richard Rabbitt. "These are very long-lasting signals requiring a synapse that does not fatigue or lose sensitivity over time. Use of protons to acidify the space between cells and transmit information from one cell to another could explain how the inner ear is able to sense tonic signals, such as gravity, in a robust and energy efficient way."
Most neurotransmission uses discrete chemical packets or vesicles, often with electrical augmentation. However, the MBL team found that the type 1 vestibular hair cells of the inner ear send messages to the calyx nerve terminals in a continuous manner. These nerve terminals then send signals to the brain. For low-frequency stimuli, including gravity, continuous transmission is more energy efficient than discrete signaling would be.
Recent studies have found that protons modulate messages between cells in fruit flies and worms, but this is the first time they have been found to act directly as a continuous neurotransmitter. The sensory system of the inner ear is highly preserved across vertebrate species, providing confidence that the mechanism observed in the toadfish also applies to us.
“The inner-ear vestibular system is the only place where this particular type of synapse is present,” Rabbitt says. “But the fact that protons are playing a key role here suggests they are likely to act as important signaling molecules in other synapses as well.”
Conflict between the signals received from the vestibular system's sense of movement and what is perceived by the eyes is the main cause of motion sickness. The research team has not proposed a way in which the new information could be used to treat suffering travelers.