Monolayers of synthetic lecithins as well as phosphatidic acid at different stages of ionization were studied with the film balance technique at pressures above the lateral vapour pressure. Pressure (π) versus area (a) curves (isotherms) and, by application of a special technique, area (a) versus temperature (T) curves (isobars) were recorded. From these data, plots of the lateral compressibility, χ, as a function of lateral lipid density (a-1) and of the thermal expansion coefficient, α, as a function of temperature (T) were obtained. The well known transition (at T = TM) between the expanded (fluid) and the condensed (crystalline) states of the films, called the main transition, is characterized by a non-horizontal deflection of the isotherms (and isobars) in the coexistence region. A pronounced hysteresis shows that it is a first order transition. Two additional phase transitions, one at T > TM (fluid region) and one at T < TM (crystalline region), were revealed by breaks in the slopes of the isotherms and the isobars. Discontinuities in the compressibilities and in the expansion coefficients show that these transitions are of second order. A flow experiment showed : that the fluid → fluid transition is also characterized by a pronounced discontinuity in viscosity. In fatty acid monolayers the transition (at T < TM) between two crystalline states is of weak first order ( — 10 J./Mole). The signs of the discontinuities, in χ and a, were analysed using the Landau theory of phase transitions. This provided information on the symmetry of the polymorphic states of the monolayers. The symmetry is described 1) in terms of the de Gennes stretching vector J, characterizing the orientation within both the hydrophobic and the hydrophilic region of the monolayer and 2) in terms of the density wave, p, characterizing the symmetry of the lateral organization of the lipid molecules. For α-Dipalmitoyllecithin a phase diagram is established. For π > 15 dyn./cm the polymorphic states of phospholipid monolayers observed at decreasing areas are : fluid isotropic (I) (hydrocarbon chains normal to water surface) ; fluid anisotropic (II) (tilted chains) ; tilted crystalline (III) ; non tilted crystalline (IV). At π ≦ 10 dyn./cm phase II and IV are not observable. Heats of transition ( QM) of the main transition were obtained from the Clausius Clapeyron equation. QM is constant (37 kJ./Mole) at low temperatures while it converges towards zero at higher temperatures, according to QM = Q0M(T - Tc), thus defining a critical temperature. It is proposed that this is a tricritical point above which the main transition becomes of second order. This allows for a symmetry break at the fluid-to-crystalline (main) transition above this critical temperature. The behaviour is explained by the Landau theory in terms of the strength in coupling between the lateral order (density wave p) and the chain orientational order (stretching vector J). In analogy to the Rodbell-Bean effect in magnetism a decrease in coupling leads from a first to a second or...

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Neurons in the medial nucleus of the trapezoid body (MNTB) receive prominent excitatory input through the calyx of Held, a giant synapse that produces large and fast excitatory currents. MNTB neurons also receive inhibitory glycinergic inputs that are also large and fast, and match the calyceal excitation in terms of synaptic strength. GABAergic inputs provide additional inhibition to MNTB neurons. Inhibitory inputs to MNTB modify spiking of MNTB neurons both in-vitro and in-vivo, underscoring their importance. Surprisingly, the origin of the inhibitory inputs to MNTB has not been shown conclusively. We performed retrograde tracing, anterograde tracing, immunohistochemical experiments, and electrophysiological recordings to address this question. The results support the ventral nucleus of the trapezoid body (VNTB) as at least one major source of glycinergic input to MNTB. VNTB fibers enter the ipsilateral MNTB, travel along MNTB principal neurons and produce several bouton-like presynaptic terminals. Further, the contribution of GABA to the total inhibition declines during development, resulting in only a very minor fraction of GABAergic inhibition in adulthood, which is matched in time by a reduction in expression of a GABA synthetic enzyme in VNTB principal neurons.

A nanometer scale metal/Langmuir-Blodgett (LB) film/metal structure is realized with an atomic force microscope combined with scanning tunneling microscope (AFM/STM). Even in this nanometer scale configuration, increase in conductance can be induced at any point in the LB film by application of a voltage pulse. The AFM/STM observation shows little surface modification has occurred by the voltage application, which shows that the conductance of the LB film changes without pit formation in the LB film or metal cluster deposition from the tip of the probe.

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Monolayers of rod-shaped and disc-shaped liquid crystalline compounds at the air-water interface Abstract: Calamitic (rod-shaped) and discotic (disc-shaped) thermotropic liquid crystalline (LC) compounds were spread at the air-water interface, and their ability to form monolayers was studied. The calamitic LCs investigated were found to form monolayers which behave analogously to conventional amphiphiles such as fatty acids. The spreading of the discotic LCs produced monolayers as well, but with a behaviour different from classical amphiphiles. The areas occupied per molecule are too small to allow the contact of all hydrophilic groups with the water surface and the packing of all hydrophobic chains. Various molecular arrangements of the discotics at the water surface to fit the spreading data are discussed. U n i v e r s i t ä t P o t s d a m