Sidney A. Simon

Food Reward in the Absence of Taste Receptor Signaling

Food palatability and hedonic value play central roles in nutrient intake. However, postingestive effects can influence food preferences independently of palatability, although the neurobiological bases of such mechanisms remain poorly understood. Of central interest is whether the same brain reward circuitry that is responsive to palatable rewards also encodes metabolic value independently of taste signaling. Here we show that trpm5-/- mice, which lack the cellular machinery required for sweet taste transduction, can develop a robust preference for sucrose solutions based solely on caloric content. Sucrose intake induced dopamine release in the ventral striatum of these sweet-blind mice, a pattern usually associated with receipt of palatable rewards. Furthermore, single neurons in this same ventral striatal region showed increased sensitivity to caloric intake even in the absence of gustatory inputs. Our findings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction.

Nonelectrolyte substitution for water in phosphatidylcholine bilayers

Abstract Glycerol substitutes for water in multilamellar phosphatidylcholine liposomes in that the fluid spaces between bilayers, as well as their main transition temperatures, heat capacities, and ethalpies are very similar in water and in pure glycerol. One major difference is that the gel state phase of dipalmitoylphosphatidylcholine (DPPC) in glycerol consists of bilayers with fully interdigitated hydrocarbon chains. Interdigitated DPPC phases are also formed in ethylene glycol or in methanol (at low methanol content). In solutions of glycerol and water, the fluid spacing between bilayers is a function of mole fraction of glycerol Xg, reaching maximum values at X g ≌ 0.1 for lipid in the liquid crystalline phase and at X g ≌ 0.3 for the gel phase. These changes are explained in terms of a modification of the long-range Van der Waals attractive forces by glycerol.

Induction of an interdigitated gel phase in fully hydrated phosphatidylcholine bilayers

Abstract Several surface active small molecules induce an unusual phase in dipalmitoylphosphatidylcholine (DPPC) suspensions. In this phase, the lipid hydrocarbon chains from apposing monolayers interpenetrate or interdigitate. A structural analysis by X-ray diffraction shows that with incorporation of the drug chlorpromazine, the bilayer thickness, or lipid headgroup separation, in DPPC liposomes is only 30 A, which is about 20 A smaller than two fully-extended DPPC molecules. This interdigitated phase may be a more general phenomenon than previously believed, as several other molecules, both charged and uncharged, such as tetracaine and benzyl alcohol, can cause the lipid hydrocarbon chains to interpenetrate.

Structure, composition, and peptide binding properties of detergent soluble bilayers and detergent resistant rafts.

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70 degrees C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 A, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 A for DRMs, which was approximately 5 A greater than that of 1:1:1 PC:SM:cholesterol and 10 A greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.

The organization of n-alkanes in lipid bilayers

The interaction of n-alkanes (C6--C16) with phosphatidylcholine has been studied by the combined use of differential scanning calorimetry, X-ray diffraction and monolayer techniques. It has been found that the thermal properties and ultrastructure of lipid-alkane vesicles are strongly dependent on the length of the n-alkanes. Long alkanes, such as tetradecane and hexadecane, increase the transition temperature of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine, while the X-ray data indicate that these long alkanes align parallel to the lipid acyl chains. In contrast, shorter alkanes, such as hexane and octane, decrease and broaden the thermal transition and electron density profiles show that these alkanes increase bilayer width by partitioning between the apposing monolayers of the bilayer. For lipids in the gel and liquid crystalline states, the short alkanes form an alkane region in the geometric center of the bilayer.

The neural mechanisms of gustation: a distributed processing code

Whenever food is placed in the mouth, taste receptors are stimulated. Simultaneously, different types of sensory fibre that monitor several food attributes such as texture, temperature and odour are activated. Here, we evaluate taste and oral somatosensory peripheral transduction mechanisms as well as the multi-sensory integrative functions of the central pathways that support the complex sensations that we usually associate with gustation. On the basis of recent experimental data, we argue that these brain circuits make use of distributed ensemble codes that represent the sensory and post-ingestive properties of tastants.

Capsazepine, a vanilloid receptor antagonist, inhibits nicotinic acetylcholine receptors in rat trigeminal ganglia

Vanilloid receptors are activated by capsaicin, the pungent ingredient in hot pepper. They are also specifically and competitively inhibited by capsazepine (CPZ). To determine whether CPZ is specific to vanilloid receptors, its effects were tested on the currents evoked by nicotine in rat trigeminal ganglia. We found that 10 microM CPZ, a concentration frequently used to inhibit capsaicins physiological responses attributed to capsaicin, reversibly inhibits (40%) the magnitude of the currents activated by 100 microM nicotine. We conclude that 10 microM capsazepine can alter the effects of channels other than those activated by capsaicin, and thus caution must be used in attributing all the CPZ-sensitive physiological effects to those only produced by blocking of vanilloid receptors.

Elasticity, Strength, and Water Permeability of Bilayers that Contain Raft Microdomain-Forming Lipids

Bilayers composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (CHOL) are commonly used as systems to model the raft-lipid domain structure believed to compartmentalize particular cell membrane proteins. In this work, micropipette aspiration of giant unilamellar vesicles was used to test the elasticities, water permeabilities, and rupture tensions of single-component PC, binary 1:1 PC/CHOL, and 1:1 SM/CHOL, and ternary 1:1:1 PC/SM/CHOL bilayers, one set of measurements with dioleoyl PC (DOPC; C18:1/C18:1 PC) and the other with stearoyloleoyl PC (SOPC; C18:0/C18:1 PC). Defining the elastic moduli (K(A)), the initial slopes of the increase in tension (sigma) versus stretch in lipid surface area (alpha(e)) were determined for all systems at low (15 degrees C) and high (32-33 degrees C) temperatures. The moduli for the single-component PC and binary phospholipid/CHOL bilayers followed a descending hierarchy of stretch resistance with SM/CHOL > SOPC/CHOL > DOPC/CHOL > PC. Although much more resistant to stretch than the single-component PC bilayers, the elastic response of vesicle bilayers made from the ternary phospholipid/CHOL mixtures showed an abrupt softening (discontinuity in slope), when immediately subjected to a steady ramp of tension at the low temperature (15 degrees C). However, the discontinuities in elastic stretch resistance at low temperature vanished when the bilayers were held at approximately 1 mN/m prestress for long times before a tension ramp and when tested at the higher temperature 32-33 degrees C. The elastic moduli of single-component PC and DOPC/CHOL bilayers changed very little with temperature, whereas the moduli of the binary SOPC/CHOL and SM/CHOL bilayers diminished markedly with increase in temperature, as did the ternary SOPC/SM/CHOL system. For all systems, increasing temperature increased the water permeability but decreased rupture tension. Concomitantly, the measurements of permeability exhibited a prominent correlation with the rupture tension across all the systems. Together, these micromechanical tests of binary and ternary phospholipid/CHOL bilayers demonstrate that PC hydrocarbon chain unsaturation and temperature are major determinants of the mechanical and permeation properties of membranes composed of raft microdomain-forming lipids.

Neuropeptides and capsaicin stimulate the release of inflammatory cytokines in a human bronchial epithelial cell line.

The role of neuropeptides in initiating and modulating airway inflammation was examined in a human bronchial epithelial cell line (i.e. BEAS-2B). At a range of concentrations, exposure of BEAS-2B cells to Substance P (SP) or calcitonin gene related protein resulted in immediate increases in intracellular calcium ([Ca(2+)](i)), the synthesis of the transcripts for the inflammatory cytokines, IL-6, IL-8 and TNFalpha after 2 h exposure, and the release of their proteins after 6 h exposure. Addition of thiorphan (100 nM), an inhibitor of neutral endopeptidase, enhanced the levels of SP-stimulated cytokine release. Stimulation of IL-6 by SP occurred in a conventional receptor-mediated manner as demonstrated by its differential release by fragments SP 4-11 and SP 1-4 and by the blockage of IL-6 release with the non-peptide, NK-1 receptor antagonist, CP-99 994. In addition to the direct stimulation of inflammatory cytokines, SP (0.5 microM), in combination with TNFalpha (25 units/ml), synergistically stimulated IL-6 release. BEAS-2B cells also responded to the botanical irritant, capsaicin (10 microM) with increases in [Ca(2+)](i) and IL-8 cytokine release after 4 h exposure. The IL-8 release was dependent on the presence of extracellular calcium. Capsaicin-stimulated increases of [Ca(2+)](i) and cytokine release could be reduced to control levels by pre-exposure to capsazepine, an antagonist of capsaicin (i.e. vanilloid) receptor(s) or by deletion of extracellular calcium from the exposure media. The present data indicate that the BEAS-2B human epithelial cell line expresses neuropeptide and capsaicin-sensitive pathways, whose activation results in immediate increases of [Ca(2+)](i) stimulation of inflammatory cytokine transcripts and the release of their cytokine proteins.

A calorimetric and monolayer investigation of the influence of ions on the thermodynamic properties of phosphatidylcholine

The effects of various ions and 2H2O on the thermal properties of phosphatidylcholine dispersions were studied using differential scanning calorimetry and the change in the surface potential of monolayers with temperature. The phosphatidylcholine in 2H2O dispersion exhibits a slightly higher transition temperature and lower enthalpy of melting than a phosphatidylcholine in H2O dispersion. Monovalent (H+, Na+, and Li+) and some divalent cations of chloride salts (Ba2+, Mg2+, and Sr2+) have no effect on the thermal properties of phosphatidylcholine, while halide salts of the di-positive ions Cd2+ and Ca2+ have an effect on both the enthalpy of melting and transition temperature. No effect attributable to the metal ion was observed in non-halide salts of cadmium. The chloride salt of La3+ has no effect on lipid thermal properties whereas that of Fe3+ affects the transition temperature. The enthalpy of melting of phosphatidylcholine in one molar solutions of potassium salts increases in the order: CNS minus greater than acetate greater than I minus. Such large, polarizable anions clearly interact with phosphatidylcholine and must therefore also confer a negative charge on the lipid. The potassium salt of SO4-2 minus has no effect. Possible origins of the observed trends are discussed.