Response noise affects the graphical evaluation of response versus concentration curves

Several theories of chemoreceptor stimulation predict the samesimple relationship between response R and stimulus concentrationC: (1 + K/C),(in which R_max is maximum response and K an equilibriumconstant), also known as Beidler's taste equation. To test whetherdata points fit this equation and estimate R_max and K, severaltransformations are in use which convert the theoretical curveinto a straight line (Lineweaver-Burk, Scatchard, Eadie-Hofsteeand Beidler plots). However, even modest amounts of responsevariability may interfere badly with the evaluation of theseplots. This is not always appreciated; therefore this paperpresents an illustration of the extent of this effect, usinga realistic example. In addition, this effect may also obscurethe presence of theoretically relevant deviations from the aboveequation, caused by multiple receptor sites, or convergenceof receptors, which are expressed in a Hill coefficient n_H 1. These effects are also exemplified. The illustrations areshown graphically and by a Montre-Carlo computer simulation.The conclusion is drawn that the linearizing plots should notbe used at all for the quantitative evaluation of data. Direct,numerical iterative curve-fitting methods seem to give morereliable results.     

Decline of IXth nerve taste responses following nerve transection

Summated impulse discharges to taste solutions were recordedfrom intact and transected IXth nerves in the Mongolian gerbil(Meriones unguiculatus). Five taste stimuli were used: 0.3 MNH₄Cl, 0.3 M NaCl, 0.01 M HCl, 0.01 M quinine hydrochloride,and 0.5 M sucrose. 0.3 M NH₄Cl was the most effective stimulus.Taste responses from intact nerves were stable for more than10 hours. Following IXth nerve transection, the peak summatedresponse to 0.3 M NH₄Cl declined by 50% in a mean of 119 min.(Some animals failed to show this taste response decline inthe winter months.) The transected IXth nerve's spontaneousactivity and responses to other taste solutions also typicallydeclined. The continued presence of normal compound action potentialsindicated that the transection-induced decline in taste responsesdid not result from a failure of impulse propagation mechanismsin the nerve trunk. The results are consistent with the propositionthat transection interferes with axonal transport of materialsvital to the short-term maintenance of taste responses.     

Learned aversions and taste qualities in hamsters

Interralations among taste perceptions in gloden hamsters (Mesocricetusauratus) were examined using generalizaions of learned tasteaversions. If stimulus A is avoided given a taste aversion hasbeen established to stimulus B, and vice versa, A and B ‘cross-generalize’.Stimuli within five groups cross-generalized. The groups ofcompounds were (i) sweeteners (fructose, saccharin, sucrose);(ii) sodium salts (NaCl, NaNO₃, Na₂SO₄): (iii) non-sodium salts(KCl, MgSO₄ NH₄Cl) plus quinine HCl; (iv) acids (acetic, hydrochloric,citric); and (v) urea. Only two pairs of stimuli from differentgroups cross-generalized (HCl—NH₄Cl. quinine HCl—urea).Neural patterns of response recorded form chorda tympam nervefibers in hamsters suggest that taste receptors on the anteriortongue distinguish among three groups of taste stimuli: sweeteners,sodium salts, and a group including non-sodium salts, acids,quinine HCl and urea. Neurons innervating other taste fieldsare likely to provide the information that hamsters use to discriminateamong the tastes of non-sodium-salt and non-sweetener stimuli.     

Inhibition of hamster chorda tympani neural response by copper chloride

Copper chloride was evaluated as a specific inhibitor of neuralresponses to sweet taste stimuli in the goldern hamster (Mesocricetusauratus). The chorda tympani whole-nerve response to taste stimuliwas recorded before and after the tongue was treated for 30s with 0.01, 0.1 and 1 mM CuCl₂. Sweet stimuli [sucrose, fructose,saccharin (calcium salt), D-phenylalanine], which primarilystimulate chorda tympani S fibers, and non-sweet stimuli (NaCl,NH₄Cl) were used. At 0.01 mM, copper chloride had little effect.At 0.10 mM it partially inhibited responses to sucrose and saccharin,but had little effect on responses to D-Phe, fructose, NaCl,NH₄Cl, or a mixture of sucrose plus L-Phe. L-Phe, which hasthe same chelating properties as D-Phe, is not an S-fiber stimulusand likely reduced sucrose inhibiton by chelating the cupricion.Analysis of concentration–response functions revealedthat 0.1 mM copper chloride inhibited the neural response tolow concentrations of sucrose by about 25%, but did not significantlyinhibit high concentrations of surcrose, suggesting competitiveinhibition. In contrast, 0.1 mM CuCl₂ reduced saccharin responsesby 25% throughtout the effective range, suggesting non-competitiveinhibition. Occupation of a saccharide receptor site by coppermay interfere with dimer but not monomer reception and distortthe saccharin receptor site. At 1 mM, CuCl₂ non-competitivelyinhibited responses to sucrose, fructose, saccharin and thenon-sweet NaCl (an N-fiber stimulus), but not NH₄Cl (an H-fiberstimulus). The mechanisms of copper chloride inhibition aredifficult to establish because its effects are weak at concentrationswhere they are specific.     

Transport pathways in canine lingual epithelium involved in sweet taste

The responses of canine lingual epithelium to D-glucose weremeasured in an Ussing chamber to determine the possible contributionof the osmotic changes of taste cells to the response of saccharides.With the mucosal solution containing 50 mM NaCl, 2 mM HEPES,pH 7.4 (solution A) and the serosal solution containing Krebs—Henseleit(KH) buffer the addition of up to 0.5 M D-glucose in the mucosalsolution increased the short circuit current (I_sc) in a sigmoidalmanner. The D-glucose-stimulated I_sc was inhibited by 0.1 mMamiloride or 1 mM ouabain added to either the mucosal or theserosal solution, and partially inhibited by 5 mM BaCl₂ addedto the serosal solution. The inhibition by these three compoundswas also observed in the presence of 0.5 M NaCl. Ouabain alsoinhibited transport when added to solution A. These experimentssuggest that in canine lingual epithelium the paracellular pathwaypermits molecules as large as ouabain (mol. wt 586) to diffusefrom the mucosal to the serosal solution and vice versa underall osmotic conditions. These results may explain the phenomenonof intravascular taste. Such is not the case in rat tongue whereouabain only inhibited transport when added to the serosal solution.Increasing the osmolality of the serosal KH buffer by additionof relatively membrane-impermeable saccharides such as sucroseor L-glucose did not significantly alter the I_sc, whereas makingthe serosal KH solution hypo-osmotic resulted in a transientdecrease in I_sc. These data suggest that the increase in I_scinduced by saccharides, such as D-glucose, is not simply anosmotic response of the epithelium but more likely the consequenceof saccharides binding weakly to receptors. That the responseto both salts by themselves and in the presence of saccharidesexhibits the same cation selectivity, and that both are inhibitedby amiloride, ouabain, BaCl₂ and LaCl₃ suggest that in caninelingual epithelia, in contrast to rat epithelium, the responsesto hyperosmotic concentrations of salts and saccharides mightoccur via the same transcellular pathways.     

Effects of the bitter additives naringin and sucrose octa-acetate on sweet persistence and sweet quality of neohesperidin dihydrochalcone

The maximal perceived sweet intensity (Ip_max) and the sweetpersistence constant (T) of neohesperidin dihydrochalcone (NHD),were significantly reduced in a mixture containing naringin(NAR), a bitter flavone analog of NHD Sucrose octa-acetate (SOA),another bitter stimulus, reduced the Ip_max of NHD in mixtures,but no appreciable decrease in T values was found. Linear regressionanalyses performed on the IP_max data of either NHD + NAR orNHD + SOA (logIp_max versus log concentration) produced slopevalues lower than those of NHD alone. Moreover, taste similarityexperiments revealed that the mixture of NHD + NAR was locatedfurther than NHD from the sugar area in the multi-dimensionalscaling (MDS) map. It is concluded that the reduction in Tvaluesof NHD by NAR was apparently related to the reduced Ip_max levelsand that such a mixture produces a sweet quality inferior tothat of NHD.     

A computational analysis of central CO2 chemosensitivity in Helix aspersa

We created a single-compartment computer model of a CO₂ chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in CO₂ chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current (I_KA), a delayed rectifier current (I_KDR), a calcium-activated potassium current (I_KCa), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation. pH-dependent inhibition of I_KA initiated the response to CO₂, but hypercapnic inhibition of I_KDR and I_KCa affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH (pH_i) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to CO₂ chemosensitivity, but the primary sensor is probably I_KA. pH_i may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either pH_i or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of pH_i regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of pH_i regulation during hypercapnia probably depends on intracellular activation of pH_i regulation but extracellular inhibition of pH_i regulation. These general principles are applicable to all CO₂ chemosensory cells in vertebrate and invertebrate neurons. hypercapnia; potassium channels; computer modeling; central chemoreceptors     

Selective interactions of lectins with amino acid taste receptor sites in the channel catfish

Five lectins of varying carbohydrate specificities, Dolichosbiflorus (DBA), jacalin, Phaseolus vulgaris (PHA), Pisum sativum(PSA) and Ricinus communis (RCA I), were used to extend characterizationof the glycoprotein nature of taste plasma membranes and todifferentially affect the binding of two taste stimuli, L-alanineand L-arginine, to their respective taste receptor sites inthe cutaneous taste system of the channel catfish (Ictaluruspunctatus). The binding of the taste stimulus L-arginine toa partial membrane fraction (P_–) from taste epitheliumwas inhibited by 68 and 74% by preincubation in the presenceof the unconjugated lectins PHA and RCA I respectively. A correspondinglevel of inhibition of L-alanine binding was seen in the presenceof RCA I (76%); however, PHA had little effect upon L-alaninebinding. DBA appeared to selectively inhibit L-alanine but notL-arginine binding (60 versus 8% respectively) while jacalinmoderately inhibited the binding of both stimuli to fractionP2. PSA had little effect upon the binding of either L-alanineor L-arginine (4 and 5% inhibition respectively). Inhibitionof taste receptor binding by all lectins was time- and dose-dependent,and was fully abolished by incubation in the presence of theappropriate hapten sugar. The biotinylated lectins DBA, jacalin,PHA, RCA I and concanavalin A (Con A) were used to identifythe glycoprotein components of the chemosensory plasma membranesafter polyacrylamide gel electrophoresis. As previously shown,numerous protein components were labeled by Con A. In contrast,only a few minor protein components were labeled by PHA, DBAand RCA I. This differential labeling of the taste membranesand the differential inhibition of receptor binding by lectinssuggest that they may prove useful as tools in the isolationand purification of taste receptor proteins.     

Activation of calcium release assessed by calcium release-induced inactivation of calcium current in rat cardiac myocytes

In mammalian cardiac myocytes, calcium released into the dyadic space rapidly inactivates calcium current (I_Ca). We used this Ca²⁺ release-dependent inactivation (RDI) of I_Ca as a local probe of sarcoplasmic reticulum Ca²⁺ release activation. In whole cell patch-clamped rat ventricular myocytes, Ca²⁺ entry induced by short prepulses from —50 mV to positive voltages caused suppression of peak I_Ca during a test pulse. The negative correlation between peak I_Ca suppression and I_Ca inactivation during the test pulse indicated that RDI evoked by the prepulse affected only calcium channels in those dyads in which calcium release was activated. Ca²⁺ ions injected during the prepulse and during the subsequent tail current suppressed peak I_Ca in the test pulse to a different extent. Quantitative analysis indicated that equal Ca²⁺ charge was 3.5 times less effective in inducing release when entering during the prepulse than when entering during the tail. Tail Ca²⁺ charge injected by the first voltage-dependent calcium channel (DHPR) openings was three times less effective than that injected by DHPR reopenings. These findings suggest that calcium release activation can be profoundly influenced by the recent history of L-type Ca²⁺ channel activity due to potentiation of ryanodine receptors (RyRs) by previous calcium influx. This conclusion was confirmed at the level of single RyRs in planar lipid bilayers: using flash photolysis of the calcium cage NP-EGTA to generate two sequential calcium stimuli, we showed that RyR activation in response to the second stimulus was four times higher than that in response to the first stimulus. excitation-contraction coupling     

Iontophoretic application of bitter taste stimuli in hamsters

Recent electrophysiological studies on the iontophoretic applicationof taste stimuli by weak electric currents using rodents andfrogs have produced stimuli which appear to mimic the actionof salty, sour and sweet solutions. However, there has beenno report of an ionic stimulus which might serve as a bitteriontophoretic probe. Many common bitter stimuli are either uncharged(e.g. quinine, urea) or have mixed quality sensations (e.g.the bitter salts KCl, MgCl₂) and therefore are unsuitable. Thisreport investigates the use of four organic anions, all of whichare bitter to humans, which may serve as potential bitter stimulifor iontophoretic application to the tongue of the hamster whilerecording electrophysiologically from its chorda tympani nerve.These anions are m-nitrobenzene sulfonate (NBSA), picrate, cholateand m-nitrobenzoate (NBA). The electrophysiological responsesto cathodal polarization via these four anions plus saccharin,an effective cathodal stimulus in the hamster, form the sameefficacy series as chemical (i.e. normal sapid) presentationsof sodium salts of these anions, i.e. saccharin > NBSA >picrate > NBA > cholate. Behavioral evidence suggeststhat NBSA is sweet to hamsters while the latter three anions,picrate, NBA and cholate, are bitter. Electrophyiological observations,based on magnitude of response, appear to support these behavioralfindings. It was concluded that picrate, NBA and cholate mayserve as useful bitter stimulus probes for ionto-phoretic applicationin the hamster.     

The control of nest climate in bumblebee (Bombus terrestris) colonies: interindividual variability and self reinforcement in fanning response

Interindividual variability in response to environmental stimuliis believed to have a major impact on collective behaviors insocial insects. The present study presents a detailed investigationof the variability in individual fanning behavior underlyingthe collective control of nest climate in bumblebee (Bombusterrestris) colonies. Four colonies were repeatedly exposedto increasing temperature and CO₂ levels. The response thresholdof each worker (defined as the mean stimulus intensity at whicha worker responded by fanning) was determined. Temperature responsethresholds of 118 workers and CO₂ response thresholds of 88workers were analyzed. Workers differed in their response thresholds.Some consistently responded to low stimulus intensities, othersconsistently responded to high stimulus intensities. No consistentcorrelation between temperature and CO₂ thresholds was foundwithin individuals. Response thresholds of fanning bees decreasedover successive trials, providing empirical support for theidea of specialization through individual threshold reinforcement.In addition to variability in individual response thresholds,workers of a colony differed in two other parameters of responsiveness:response probability (the probability of responding to a stimulusonce it exceeded an individual's response threshold) and responseduration (the persistency with which fanning was performed oncean individual responded). The results of the present study suggestthat response threshold, response probability and response durationare important independent parameters of individual responsivenessin the collective control of nest climate in bumblebee colonies.     

Expression, regulation, and function of P2X4 purinergic receptor in human cervical epithelial cells

Micromolarconcentrations of ATP stimulate biphasic change in transepithelialconductance across CaSki cultures on filters, an acute transientincrease (phase I response; triggered by P2Y₂ receptor and mediated by calcium mobilization-dependent cell volume decrease) followed by a slower decrease in permeability (phase II response). Phase II response is mediated byaugmented calcium influx and protein kinase C-dependent increase intight junctional resistance. The objective of the study was todetermine the role of P2X₄ receptor as a mediator ofphase II response. Human cervical epithelial cells expressP2X₄ receptor mRNA (1.4-, 2.2-, and 4.4-kb isoforms byNorthern blot analysis) and P2X₄ protein. Depletion ofvitamin A reversibly downregulated P2X₄ receptor mRNA andprotein and ATP-induced calcium influx. Depletion of vitamin Aabrogated phase II response, and the effect could bepartially reversed only with retinoic acid receptor (RAR)-selectiveretinoids but not retinoid X receptor (RXR) agonists. Depletion ofvitamin A also abrogated protein kinase C increase in tight junctionalresistance, and the effect could not be reversed with retinoids.Depletion of vitamin A also abrogated phase I increase inpermeability and reversibly downregulated P2Y₂ receptormRNA and ATP-induced calcium mobilization. However, in contrast tophase II response, both RAR and RXR agonists could fullyreverse those effects. These results suggest that phase IIresponse is mediated by a P2X₄ receptor mechanism.

     

Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction

Taste receptor cells (TRCs)respond to acid stimulation, initiating perception of sour taste.Paradoxically, the pH of weak acidic stimuli correlates poorly with theperception of their sourness. A fundamental issue surrounding sourtaste reception is the identity of the sour stimulus. We tested thehypothesis that acids induce sour taste perception by penetratingplasma membranes as H⁺ ions or as undissociated moleculesand decreasing the intracellular pH (pH_i) of TRCs. Our datasuggest that taste nerve responses to weak acids (acetic acid andCO₂) are independent of stimulus pH but strongly correlatewith the intracellular acidification of polarized TRCs. Taste nerveresponses to CO₂ were voltage sensitive and were blockedwith MK-417, a specific blocker of carbonic anhydrase. Strong acids(HCl) decrease pH_i in a subset of TRCs that contain apathway for H⁺ entry. Both the apical membrane and theparacellular shunt pathway restrict H⁺ entry such that alarge decrease in apical pH is translated into a relatively smallchange in TRC pH_i within the physiological range. Weconclude that a decrease in TRC pH_i is the proximate stimulus in rat sour taste transduction.

     

MAGNITUDE ESTIMATES OF GUSTATORY QUALITY CHANGES AS A FUNCTION OF SOLUTION CONCENTRATION OF SIMPLE SALTS

Concentration-dependent quality changes in inorganic salt solutionswere investigated by obtaining quality reports, and magnitudeestimates, from four subjects, of the taste of LiCl, KCl, andNaCl solutions in the concentration range from 0.004 M to 0.050M; and Li₂SO₄ and K₂SO₄ solutions in the concentration rangefrom 0.002 F to 0.025 F. Plots of the geometric mean of themagnitude estimates for each quality as a function of concentrationconfirmed the general findings of earlier investigators (Höberand Kiesow, 1898; Renqvist, 1919; Dzendolet and Meiselman, 1967).In addition, differences found among the various studies werelinked to differences in procedure which established variablelevels of subject adaptation in the studies and permitted uncontrolledwater tastes to affect the data. It was concluded that a concentration-dependentphysicochemical change in the salt solutions, as proposed byDzendolet (1968), combines with a water taste to produce thequality of low concentration inorganic salt solutions. The relativecontribution of these two mechanisms to the overall qualityof the solution at any concentration will depend on the proceduresemployed. ^* Reprint requests should be sent to present address: John B.Pierce Foundation Laboratory, 290 Congress Ave., New Haven,CT 06519.     

Phytoplanktonic carbon isotope fractionation: equations accounting for CO2-concentrating mechanisms

A model of carbon isotope discrimination by phytoplankton wasdeveloped which took into account the occurrence of a carbon-concentratingmechanism (CCM). A simple equation was obtained for the modelinvolving CO₂ active transport. In the case of HCO₃^– activetransport, another equation was developed based on a seriesof approximations. The former equation was used to analyse reportedand newly obtained data from culture experiments and field observationsin both freshwater and marine environments. In most cases, alinear relationship between a combined parameter, (1–f)Ci,which was made up of the relative contribution of active CO₂uptake to total carbon uptake (f) and the intracellular CO₂concentration (Ci), and CO₂ concentration in bulk solution (Ce)was obtained as (1–f)Ci = ace–b, with a high correlationcmfficient (r²>0.9). The slope a is suggested as a measureof the ratio of diffusive to total (diffusive+active) CO₂ transport,while bla represents CO₂ demand.     

Analysis and Significance of Gravity-induced Asymmetric Growth in the Grass Leaf-sheath Pulvinus

The negative gravitropic response in the grass leaf-sheath pulvinusis a consequence of cell elongation involving all cells exceptthose of the uppermost region of the upper flank of an horizontallyoriented pulvinus. The lowermost layer of cells elongate maximally,and the regions in between elongate to intermediate extents.The resulting curvatures of a responding pulvinus can be expressedmathematically by relating the angle of curvature () to theoriginal length (L₀) and the maximal length of the lower surface(L₁) and the diameter of the organ (D), using the equation, = (L₁–L₀)/D, where is in radians. The elongation response(S) of any individual cell within the pulvinus can be expressedby the equation, S = 0.5-r cos, where r is the radius of thepulvinus and is in degrees. Microscopic measurement of celllengths in different regions of the pulvinus supports the mathematicalpredictions. Indirect support is also obtained from the useof colchicine, coumarin, dichloro-benzonitrile (DCBN) and isopropylN-chlorophenyl carbamate which exaggerate the inherent asymmetryduring gravitropic response. Coumarin and DCBN also induce thickeningsin the radial walls which appear first in the statenchyma, andlater, in cells located towards the outer periphery of the pulvinus.The distribution patterns of these thickenings suggest thatthe asymmetric growth response of the pulvinus may be due toa differential and radial, centrifugal transport of growth promotorsfrom the central statenchyma region. Gravity perception, grass pulvinus     

The electromotive force of the Chara membrane during the hyperpolarizing response

The hyperpolarizing response of the Chara internodal cell wasstudied by applying the voltage clamp and constant current techniques.By assuming the membrane as an electromotive force (emf) inseries with a resistance r (which is the sum of a series resistancer_s and the membrane resistance r_m), it was shown that the hyperpolarizingresponse was brought about not only by the increase in membraneresistance but by the increase in membrane emf. The time dependentcurrent-voltage (I–V_m) curve obtained under the voltageclamp during the hyperpolarizing response showed a negativeresistance. The hyperpolarizing response is also an excitation,since it is a transition process of the membrane across a negativeresistance region. (Received July 22, 1974; )     

Reanalysis of Vernalization Data of Wheat and Carrot

Vernalization is an important determinant of the growth, development,and yield of biennial and perennial crops. Accurate simulationof its response to temperature is thus an important componentof successful crop systems modelling. Vernalization has a lowoptimum temperature compared to other temperature responsesof plants, and thus may be difficult to treat using expressionsthat are appropriate for other plant processes. This paper examinesthe application of a simple equation that has been used forother processes. It reads as v=V_max(T_max-T_{Tmax- Topt} ) (T_Topt)^{T_optTmax-T_opt}, where v is thedaily rate of vernalization progress at temperature T, T_optandT_maxare the optimum and maximum temperatures for vernalization,respectively, andV_max is the maximum daily rate of vernalization(the inverse of the minimum number of days required to completevernalization), which occurs at T_opt. The model was appliedto published vernalization data for wheat and carrot. The fitsto data were good (adjusted R²for wheat of 0.94, for carrot0.98), with estimatedT_opt and T_maxbeing 5.7±0.5 and 21.3±1.4°C, respectively, for wheat ‘Norin 27’ and 6.6±0.2and 14.1±0.3 °C for carrot ‘ Chantenay RedCored’. The estimated parameters, in particular the highT_maxfor wheat, were close to those reported using differentanalytical approaches. It was suggested that the function wouldbe useful for summarizing vernalization data, and that its usewould avoid the abrupt changes that are inevitable when differentlinear relationships are used for part of the overall response.It was also suggested the high T_maxshould be taken into accountwhen interpreting data obtained with wheat grown under warmconditions. Copyright 1999 Annals of Botany Company Plant, vernalization, temperature response, modelling, wheat (Triticum aestivum L.), carrot (Daucus carota L.).     

Photosynthesis and Light Activation of Ribulose 1, 5-bisphosphate Carboxylase in the Presence of Starch

Owing to a typographical error three equations were omittedfrom page 1294. The correct paragraphs are set out below. The component K₁ corrected for the difference in temperaturebetween the enzyme assay and the leaf and was calculated accordingto the Arrhemus equation. where v10 and v18 are the reaction velocities of carboxylationat 10?C and 18?C, respectively and A is the activation energy(A = 90 kJ mol^–1, as determined for purified wheat RuBPCOby M?chler, Keys and Cornelius, 1980) The components K2 corrected for the difference in CO₂ partialpressure between enzyme assay and leaf and for competitive inhibitionof carboxylation by O₂ and was calculated according to the modifiedMichaelis Menten equation where v_c, is the carboxylation velocity under leaf conditions,V_c. is the maximum carboxylation velocity as determined in theenzyme assay, K_c, and K_o are the Michaelis constants for carboxylationand oxygenation, respectively (K_o = 159 Pa CO₂. K_o = 35.3 kPaO₂, as interpolated for 18?C from spinach data as determinedby Jordan and Ogren, 1984), O is oxygen partial pressure inair and C₁ is intercellular CO₂ partial pressure in leaves (C₁= 29.1 ? 0.8 Pa (? s c , n = 15)) The component K3 corrected for the decrease in CO₂ fixationin leaves due to photorespiration and was calculated accordingto equation 3 Equation 3 is denved from the equation for the substrate specificityof RuBPCO, S= v_c/v_oC (Laing, Ogren, and Hageman, 1974), andfrom the equation for the stoichiometry of photorespiratoryCO₂ release, F=v_c–1/2 v_o, where v_c, and v_c are reactionvelocities of carboxylation and oxygenation, O and C are partialpressures of 02 and intercellular CO₂, F is net photosynthesisand S is the substrate specificity of RuBPCO (S= 3061 Pa/Pa,as interpolated for 18?C from spinach data as determined byJordan and Ogren, 1984)     

FOOD-RELATED CONDITIONING AND NEURONAL CORRELATES IN THE FRESHWATER SNAIL LYMNAEA STAGNALIS

The feeding response of Lymnaea stagnalis shows robust appetitiveconditioning to a novel chemostim-ulus (amyl acetate), whichis retained for more than 4 days. In scrru-isolated centralnervous system (CNS)-hp preparations taken from conditionedsnails, application of amyl acetate to lip tissue led to theonset of fictive feeding in 5/17 individuals, and excitationof an identified cerebral-buccal feeding intemeuron, CV1_a. ina further 8. Control group snails showed no response to amylacetate. Attempts were made to aversively condition L. stagnalis usinga food stimulus (sucrose) as the conditioned stimulus and eitheran electric shock or mechanical stimulus as the unconditionedstimulus. No conditioned response to sucrose (i.e. withdrawal)was seen following training. However, trained snails exhibiteda reduction in responsiveness which was manifest as inhibitionof the feeding response to sucrose and an increase in the timetaken to emerge from the shell following handling. This reducedresponsiveness was seen at 1 hr but was lost by 24 hr aftertraining, and was not observed in control group snails. Semi-isolatedCNS-hp preparations from experimental snails recorded 1-3 hrafter training exhibited either inhibition of fictive feedingor no response following application of sucrose. Control groupsnails showed the normal excitatory response to sucrose includinginduction of feeding motor output Differences in food-related learning between Lymnaea and othergastropod species are discussed in relation to lifestyle andfeeding strategies (Received 14 January 1996; accepted 3 May 1996)