On the kinetics of acetylcholine at the synapse

Collision and annihilation of nonlinear sound waves and action potentials in interfaces

Nerve impulses, previously proposed as manifestations of nonlinear acoustic pulses localized at the plasma membrane, can annihilate upon collision. However, whether annihilation of acoustic waves at interfaces takes place is unclear. We previously showed the propagation of nonlinear sound waves that propagate as solitary waves above a threshold (super-threshold) excitation in a lipid monolayer near a phase transition. Here we investigate the interaction of these waves. Sound waves were excited mechanically via a piezo cantilever in a lipid monolayer at the air-water interface and their amplitude is reported before and after a collision. The compression amplitude was observed via Förster resonance energy transfer between donor and acceptor dyes, measured at fixed points along the propagation path in the lipid monolayer. We provide direct experimental evidence for the annihilation of two super-threshold interfacial pulses upon head-on collision in a lipid monolayer and conclude that sound waves propagating in a lipid interface can interact linearly, nonlinearly, or annihilate upon collision depending on the state of the system. Thus we show that the main characteristics of nerve impulses, i.e. solitary character, velocity, couplings, all-or-none behaviour, threshold and even annihilation are also demonstrated by nonlinear sound waves in a lipid monolayer, where they follow directly from the thermodynamic principles applied to an interface. As these principles are equally unavoidable in a nerve membrane, our observations strongly suggest that the underlying physical basis of action potentials and the observed nonlinear-pules is identical.

On the Physical Basis of Biological Signaling by Interface Pulses

Currently, biological signaling is envisaged as a combination of activation and movement, triggered by local molecular interactions and molecular diffusion, respectively. However, here, we suggest that other fundamental physical mechanisms might play an at least equally important role. We have recently shown that lipid interfaces permit the excitation and propagation of sound pulses. Here, we demonstrate that these reversible perturbations can control the activity of membrane-embedded enzymes without a requirement for molecular transport. They can thus facilitate rapid communication between distant biological entities at the speed of sound, which is here on the order of 1 m/s within the membrane. The mechanism described provides a new physical framework for biological signaling that is fundamentally different from the molecular approach that currently dominates the textbooks.

Protons at the speed of sound: Predicting specific biological signaling from physics

Local changes in pH are known to significantly alter the state and activity of proteins and enzymes. pH variations induced by pulses propagating along soft interfaces (e.g. membranes) would therefore constitute an important pillar towards a physical mechanism of biological signaling. Here we investigate the pH-induced physical perturbation of a lipid interface and the physicochemical nature of the subsequent acoustic propagation. Pulses are stimulated by local acidification and propagate - in analogy to sound - at velocities controlled by the interface's compressibility. With transient local pH changes of 0.6 directly observed at the interface and velocities up to 1.4 m/s this represents hitherto the fastest protonic communication observed. Furthermore simultaneously propagating mechanical and electrical changes in the lipid interface are detected, exposing the thermodynamic nature of these pulses. Finally, these pulses are excitable only beyond a threshold for protonation, determined by the pKa of the lipid head groups. This protonation-transition plus the existence of an enzymatic pH-optimum offer a physical basis for intra- and intercellular signaling via sound waves at interfaces, where not molecular structure and mechano-enyzmatic couplings, but interface thermodynamics and thermodynamic transitions are the origin of the observations.

On the excitation of action potentials by protons and its potential implications for cholinergic transmission

One of the most conserved mechanisms for transmission of a nerve pulse across a synapse relies on acetylcholine (ACh). Ever since the Nobel Prize-winning works of Dale and Loewi, it has been assumed that ACh-subsequent to its action on a postsynaptic cell-is split into inactive by-products by acetylcholinesterase (AChE). Herein, the widespread assumption of inactivity of ACh's hydrolysis products is falsified. Excitable cells (Chara braunii internodes), which had previously been unresponsive to ACh, became ACh-sensitive in the presence of AChE. The latter was evidenced by a striking difference in cell membrane depolarization upon exposure to 10 mM intact ACh (∆V = -2 ± 5 mV) and its hydrolysate (∆V = 81 ± 19 mV), respectively, for 60 s. This pronounced depolarization, which also triggered action potentials, was clearly attributed to one of the hydrolysis products: acetic acid (∆V = 87 ± 9 mV at pH 4.0; choline ineffective in the range 1-10 mM). In agreement with our findings, numerous studies in the literature have reported that acids excite gels, lipid membranes, plant cells, erythrocytes, as well as neurons. Whether excitation of the postsynaptic cell in a cholinergic synapse is due to protons or due to intact ACh is a most fundamental question that has not been addressed so far.