Envelope gating and noise shaping in populations of noisy neurons.

Narrowband signals have fast and slow time scales. The transmission of narrowband signal features on both times cales, by spiking neurons, is demonstrated experimentally and theoretically. The interaction of the narrowband input and the threshold nonlinearity may create out-of-band interference, hindering the transmission of signals in a low-frequency range. The resultant out-of-band signal is the "envelope," or time-varying modulation of the narrowband signal. The levels of noise and nonlinearity intrinsic to the neuron gate transmission on the slow "envelope" time scale. When a narrowband and a distinct slow signal drive the neuron, the slow signal may be poorly transmitted. Increasing intrinsic noise in an averaging network removes the envelope in favor of the slow signal, paradoxically increasing the signal-to-noise ratio. These gating effects are generic for threshold and excitable systems.

Reversible inhibition of proton release activity and the anesthetic-induced acid-base equilibrium between the 480 and 570 nm forms of bacteriorhodopsin.

In purple membrane added with general anesthetics, there exists an acid-base equilibrium between two spectral forms of the pigment: bR570 and bR480 (apparent pKa = 7.3). As the purple 570 nm bacteriorhodopsin is reversibly transformed into its red 480 nm form, the proton pumping capability of the pigment reversibly decreases, as indicated by transient proton release measurements and proton translocation action spectra of mixture of both spectral forms. It happens in spite of a complete photochemical activity in bR480 that is mostly characterized by fast deprotonation and slow reprotonation steps and which, under continuous illumination, bleaches with a yield comparable to that of bR570. This modified photochemical activity has a correlated specific photoelectrical counterpart: a faster proton extrusion current and a slower reprotonation current. The relative areas of all photocurrent phases are reduced in bR480, most likely because its photochemistry is accompanied by charge movements for shorter distances than in the native pigment, reflecting a reversible inhibition of the pumping activity.

Lipid-protein interactions in the purple membrane: structural specificity within the hydrophobic domain.

In the absence of native-like interactions between bacteriorhodopsin and its neighbouring lipids, the pigment chromophore is reversibly titrated from its purple 570 nm form to a blue-shifted 480 nm form in the moderately alkaline pH range. Quantitation of this acid-base chromophore equilibrium in vesicles prepared from modified lipid mixtures shows that it is absent under conditions where bacteriorhodopsin is allowed to interact with methyl-substituted alkyl chains. The peculiar homogeneous structure of purple membrane alkyl chain lipids is thus likely to be an essential requirement for maintenance of the native bacteriorhodopsin structure over a wide pH range.