Spectral analysis of extrinsic fluorescence of the nerve membrane labeled with aminonaphthalene derivatives

Shock and detonation waves at an interface and the collision of action potentials

Action potentials in neurons are known to annihilate each other upon collision, while there are cases where they might penetrate each other. The fate of two waves upon collision is critically dependent on the underlying mechanism of propagation and therefore an understanding of possible outcomes of collision under different conditions is important. Previously, compression waves that travel within the plasma membrane of a neuron have been proposed as a thermodynamic basis for the propagation of action potentials. In this context, it was recently shown that two-dimensional compressive shock waves in the model system of lipid monolayers behave strikingly similar to action potentials in neurons and can even annihilate each other upon head-on collision. However, even a qualitative mechanism remained unclear. To this end, we summarise the fundamentals of shock physics as applied to an interface and recap how it explained the observation of threshold and saturation of shockwaves in the lipid monolayer (all - or - none). We then compare the theory with the soliton model that has the same fundamental premise, i.e. the conservation laws and thermodynamics, and was previously proposed as a model for the nerve pulse propagation. We elaborate on how the two approaches make different predictions with regards to collisions and the detailed structure of the wave-front. As a case study and a new qualitative result, we finally show that previously unexplained annihilation of shock waves in the lipid monolayer is a direct consequence of the nature of state changes, i.e. jump conditions, within these shockwaves.

On measuring the acoustic state changes in lipid membranes using fluorescent probes

Ultrasound is increasingly being used to modulate the properties of biological membranes for applications in drug delivery and neuromodulation. While various studies have investigated the mechanical aspects of the interaction such as acoustic absorption and membrane deformation, it is not clear how these effects transduce into biological functions, for example, changes in the permeability or the enzymatic activity of the membrane. A critical aspect of the activity of an enzyme is the thermal fluctuations of its solvation or hydration shell. Thermal fluctuations are also known to be directly related to membrane permeability. Here solvation shell changes of lipid membranes subject to an acoustic impulse were investigated using a fluorescence probe, Laurdan. Laurdan was embedded in multi-lamellar lipid vesicles in water, which were exposed to broadband pressure impulses of the order of 1 MPa peak amplitude and 10 µs pulse duration. An instrument was developed to monitor changes in the emission spectrum of the dye at two wavelengths with sub-microsecond temporal resolution. The experiments show that changes in the emission spectrum, and hence the fluctuations of the solvation shell, are related to the changes in the thermodynamic state of the membrane and correlated with the compression and rarefaction of the incident sound wave. The results suggest that acoustic fields affect the state of a lipid membrane and therefore can potentially modulate the kinetics of channels and enzymes embedded in the membrane.

Spectral analysis of absorption changes associated with nerve excitation in dye-stained crab nerve

Spectral characteristics of absorption changes associated with nerve excitation were studied with crab nerves stained with a homologous series of dyes, merocyanine-rhodanines and rhodanine oxonols. In these classes of dyes, the absorption changes which followed approximately the same time course as that of the action potential (fast responses) depended in a similar fashion on the wavelength and polarization of the incident light. In order to interpret those commonly observed dependencies, a mode of reorientation of the absorption oscillators of the dye molecules in the membrane matrix during nerve excitation was proposed. In addition to the fast changes mentioned above, slow responses which developed during and after the action potential were commonly observed with oxonols. The spectra of the slow changes differed from those of the fast ones, indicating a distinct mechanism on the response production. A possible mechanism of the production of fast responses was also discussed based on the proposed mode of reorientation of the absorption oscillators.

Protein release from the internal surface of the squid giant axon membrane during excitation and potassium depolarization

The proteins in the perfusate collected from intracellularly perfused squid giant axons were analyzed after being labeled with radioactive 125-I-labeled Bolton-Hunter reagent. The rate of protein release into the perfusate was found to be increased by the following electrophysiological manipulations of the axons: (1) repetitive electrical stimulation at 60 Hz in axons perfused with normal potassium fluoride-containing solution or at 0.125 Hz in axons perfused with tetraethylammonium containing solution, (2) perfusion with 4-aminopyridine solution which induces spontaneous electrical activity in the axon, and (3) depolarization of the axon induced by raising the external potassium concentration. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the proteins released under these conditions yielded molecular weight profiles different from those of the extruded axoplasmic proteins. These observations indicate that there exists, in close association with the axonal membrane, aparticular group of proteins, the solubility of which is readily affected by changes in the state of the membrane.

Internal and external application of photodynamic sensitizers on squid giant axons

Squid giant axons were photosensitized by dyes applied internally or externally in air saturated solutions and photochemically modified by visible light. For most dyes the modifications included an irreversible block of sodium channels, a destruction of inactivation in some of the unblocked channels, and a slowing of inactivation. Internal application was up to 100-fold more effective in blocking sodium channels than external application, suggesting a site of block nearer the internal surface. Rose Bengal sensitized channel block and destruction of inactivation when applied internally, but sensitized only channel block when applied externally. In contrast, externally applied Eosin Y sensitized a clear slowing of inactivation plus channel block. Beta-carotene, an effective agent for quenching photochemically generated excited singlet oxygen, inhibited most of the modification sensitized by internally applied Methylene blue but not by Rose Bengal or Merocyanine 540.

Studies of light emission, absorption and energy transfer in nerve membranes labelled with fluorescent probes

Physicochemical properties of fluorescent membrane probes, 2-p-Cl-anilinonaphthalene-6-sulfonate (p-Cl-ANS), 2-p-Br-anilinonaphthalene-6-sulfonate (p-Br-ANS), merocyanine-540, methyl violet, etc., were examined because of the possibility of demonstrating resonance energy-transfer between probes. The emission psectra of p-Cl-ANS and p-Br-ANS and the absorption (or fluorescence excitation) spectra of Eastman Kodak merocyanine-540 (M-540) and other probes were found to be very sensitive to changes in the solvent polarity. The spectra of these probes incorporated in the nerve membrane were determined and compared with the corresponding spectra in various organic solvents and macromolecules. This comparison suggests that the polarity of the binding sites for p-Cl-ANS (and p-Br-ANS) in the membrane is high and that of M-540 is very low. The spectrum of the portion of p-Cl-ANS fluorescence contributing to production of optical responses (i.e., transient changes during action potentials) was determined. Both absorption responses and fluorescence responses were detected from M-540 in the nerve membrane. It was possible to demonstrate resonance transfer of electronic energy from p-Cl-ANS to M-540 incorporated in lysolecithin micelles and in crab nerves. During action potentials, the intensity of M-540 fluorescence excited by energy transfer was found to undergo transient changes. Based on these and other experimental findings, properties of the binding sites for these probes in the nerve membranes are discussed.

The application of fluorescent probes in membrane studies

Fluorescence has been used in biochemical studies for many years but only recently has the information content and the practical applicability of the fluorescence method been fully realized.

Following the early studies of Newton (1954) and Weber (11954) and after the initial utilization of fluorescent probes by Chance and coworkers (Azziet al.1969) and Tasakiet al.(1968), in the study of membranes, the use of fluorescence to provide structural information at microscopic or molecular levels in biological membranes has become widespread. widespread. The application of the fluorescence technique to biological systems has progressed parallel to the development of a theoretical basis for fluorescence data interpretation and the synthesis of a large number of fluorescent probes, organic molecules having fluorescence characteristics that are dependent on their environment.