Drivers of changing urban flood risk: A framework for action

This study focuses on drivers for changing urban flood risk. We suggest a framework for guiding climate change adaptation action concerning flood risk and manageability in cities. The identified key drivers of changing flood hazard and vulnerability are used to provide an overview of each driver's impact on flood risk and manageability at the city level. We find that identified drivers for urban flood risk can be grouped in three different priority areas with different time horizon. The first group has high impact but is manageable at city level. Typical drivers in this group are related to the physical environment such as decreasing permeability and unresponsive engineering. The second group of drivers is represented by public awareness and individual willingness to participate and urbanization and urban sprawl. These drivers may be important and are manageable for the cities and they involve both short-term and long-term measures. The third group of drivers is related to policy and long-term changes. This group is represented by economic growth and increasing values at risk, climate change, and increasing complexity of society. They have all high impact but low manageability. Managing these drivers needs to be done in a longer time perspective, e.g., by developing long-term policies and exchange of ideas.

Accuracy of the FAMACHA system for on-farm use by sheep and goat producers in the southeastern United States

FAMACHA is a practical on-farm system designed to provide small ruminant producers a tool for improving their management of Haemonchus contortus infections. Although this system has become very popular and widely accepted by small ruminant producers in many regions of the southern United States, there is very limited data reported on the effectiveness of the FAMACHA system when performed by farmers. The objective of this study was to evaluate the accuracy of the FAMACHA system for on-farm use by small ruminant producers during the summer season. Small ruminant producers from Georgia, Louisiana, Florida, and Puerto Rico were trained to use the FAMACHA system by veterinarians and scientists experienced with this method. FAMACHA scores were assigned at least every 2 weeks by producers to weaned and mature sheep (n=552) and goats (n=676) of various breeds and ages between April and September 2004. At intervals that varied among farms from 2 to 8 weeks, researchers determined body condition scores (BCS; 1=thin and 5=fat) and collected blood and feces from a group of animals selected randomly to determine packed cell volume (PCV) and fecal egg counts (FEC). Two separate anemia thresholds were evaluated; these were defined by either FAMACHA score (>or=3 versus >or=4) or PCV (<or=19 versus <or=15%). The correlation between FAMACHA scores and PCV or FEC was high for both sheep and goats (P<0.001). Specificity was maximized when FAMACHA scores of 4 and 5 were considered anemic, but sensitivity was low. Sensitivity for detecting anemic animals was 50% for sheep and 89% for goats when eye score values of >or=3 were considered anemic and PCV cutoff was <or=15%. The percentage of false negatives (anemic animals not identified by FAMACHA evaluation) was less than 5% in sheep and less than 1% in goats when FAMACHA scores>or=3 were considered anemic and PCV cutoff was <or=15%. In both sheep and goats, predictive value of a negative was greater than 90% for all anemia and eye score categories. These data indicate that the FAMACHA method used by producers is a valuable tool for identifying anemic sheep and goats in the southern United States and Puerto Rico.

Validation of the FAMACHA© eye color chart for detecting clinical anemia in sheep and goats on farms in the southern United States

Recent studies on sheep and goat farms in the southern United States indicate that multiple-anthelmintic resistance in Haemonchus contortus is becoming a severe problem. Though many factors are involved in the evolution of resistance, the proportion of the parasite population under drug selection is believed to be the single most important factor influencing how rapidly resistance develops. Therefore, where prevention of resistance is an important parallel goal of worm control, it is recommended to leave a portion of the animals untreated. Recently, a novel system called FAMACHA was developed in South Africa, which enables clinical identification of anemic sheep and goats. When H. contortus is the primary parasitic pathogen, this system can be applied on the farm level to reduce the number of treatments administered, thereby increasing the proportion of the worm population in refugia. Since most studies validating the FAMACHA method have been performed in South Africa, it is important that the method be tested in other regions before its use is broadly recommended. We performed a validation study of FAMACHA by testing the system in sheep (n = 847) and goats (n = 537) of various breeds and ages from 39 farms located in Arkansas, Georgia, Louisiana, Florida, and the US Virgin Islands. The color of the ocular conjunctiva of all animals were scored on a 1-5 scale using the FAMACHA card, and blood samples were collected from each animal for determination of packed cell volume (PCV). Fecal samples were also collected from a majority of the animals tested for performance of fecal egg counts (FEC). Correlations between PCV and eye scores, PCV and FEC, and FEC and eye scores were all highly significant for both sheep and goats (P < 0.001). Data for both FAMACHA scores and PCV were evaluated using two separate criteria for anemia: eye score values of 3, 4 and 5 or 4 and 5, and PCV values of < or =19 or < or =15 were considered anemic. Specificity was maximized when eye score values of 4 and 5 were considered anemic and PCV cut off for anemia was < or =19, but sensitivity was low. In contrast, sensitivity was 100% for both sheep and goats when eye score values of 3, 4 and 5 were considered anemic and PCV cut off was < or =15, but specificity was low. In both sheep and goats, predictive value of a negative was greater than 92% for all anemia and eye score categories, and was greater than 99% for both eye score categories when an anemia cutoff of < or =15 was used. Predictive value of a positive test was low under all criteria indicating that many non-anemic animals would be treated using this system. However, compared to conventional dosing practices where all animals are treated, a large proportion of animals would still be left untreated. These data indicate that the FAMACHA method is an extremely useful tool for identifying anemic sheep and goats in the southern US and US Virgin Islands. However, further studies are required to determine optimal strategies for incorporating FAMACHA-based selective treatment protocols into integrated nematode control programs.

Perceptual channels for the texture of a food

The present study makes a start on characterising the cognitive processes by which physical effects of eating on the senses are transformed into quantitative judgments about perceived characteristics of a food. It was hypothesised that there is a discrete perceptual channel for an aspect of texture sensed during each of the three initial movements of eating a piece of a cookie. The results showed that the force required for initial compression of the surface of the biscuit related to how 'crisp' it was. Scores on 'hard' were sensitive to higher forces being required to bite off a piece. Ratings of crunchiness responded to both amplitude and frequency of the cracks opened up in this heterogeneously structured material during the first crushing of the bitten piece. These findings are being pursued to identify the stimulation patterns more precisely and to measure how the percepts are integrated into judgments of overall texture.

Antagonism by WAY-100635 of the effects of 8-OH-DPAT on performance on a free-operant timing schedule in intact and 5-HT-depleted rats

In this experiment we examined the effect of a serotonin receptor (5-HT1A) agonist and antagonist WAY-100635 (N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide) on temporal differentiation, in intact rats and rats whose serotonergic (5-HTergic) pathways had been destroyed by 5,7-dihydroxytryptamine (5,7-DHT). Thirteen rats received 5,7-DHT-induced lesions of the median and dorsal raphe nuclei; 14 rats received sham lesions. They were trained to press two levers (A and B) in 50-s trials, in which reinforcement was contingent upon responding on A in the first half, and B in the second half, of the trial. Logistic psychophysical curves were fitted to the relative response rate data (percent responding on B, %B), for derivation of timing indices [T50 (time corresponding to %B=50%), slope, Weber fraction] following WAY-100635, 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin], combinations of WAY-100635+8-OH-DPAT, and vehicle alone. WAY-100635 (30, 100 and 300 microg/kg, s.c.) did not affect the timing indices. 8-OH-DPAT (100, 200 microg/kg, s.c.) reduced T50 without affecting the Weber fraction. WAY-100635 (300 microg/kg) abolished the effect of 8-OH-DPAT on T50 in both the lesioned and sham-lesioned groups. 5-HT levels in the neocortex, hippocampus, amygdala, nucleus accumbens and hypothalamus of the lesioned group were <20% of those in the sham-lesioned group; catecholamine levels were unaffected. The results confirm that 8-OH-DPAT disrupts temporal differentiation in a free-operant psychophysical schedule, reducing T50, and indicate that this effect of 8-OH-DPAT is mediated by postsynaptic 5-HT1A receptors.

Effects of quinolinic acid-induced lesions of the orbital prefrontal cortex on inter-temporal choice: a quantitative analysis

RATIONALE

Lesions of the orbital prefrontal cortex (OPFC) can cause pathologically impulsive behaviour in humans. Inter-temporal choice behaviour (choice between reinforcers differing in size and delay) has been proposed as a model of "impulsive choice" in animals.

OBJECTIVE

A quantitative method was used to analyse inter-temporal choice in rats with lesions of the OPFC and sham-lesioned control rats.

METHODS

Under halothane anaesthesia, rats received injections of the excitotoxin quinolinate into the OPFC (0.1 M, 0.5 micro l; two injections in each hemisphere), or sham lesions (injections of the vehicle). They were trained to press two levers (A and B) for sucrose reinforcement (0.6 M) in discrete-trials schedules. In free-choice trials, a press on A resulted in delivery of 50 micro l of the sucrose solution after a delay d (A); a press on B resulted in delivery of 100 micro l of the same solution after a delay d (B). d (B) was increased progressively across successive blocks of six trials in each session, while d (A) was manipulated systematically across phases of the experiment. The indifference delay, d (B(50)) (value of d (B) corresponding to 50% choice of B) was estimated for each rat in each phase. Linear functions of d (B(50)) versus d (A) were derived, and the parameters of the function compared between the groups. The locations of the lesions were verified histologically at the end of the experiment.

RESULTS

In both groups, d (B(50)) increased linearly with d (A) ( r(2)>0.98 in each case). The slope of the function was significantly steeper in the lesioned group than the sham-lesioned group, whereas the intercept did not differ significantly between the groups. The brains of the lesioned rats showed extensive atrophy/gliosis of the OPFC, with sparing of the dorsolateral prefrontal cortex.

CONCLUSIONS

The results indicate that lesions of the OPFC can alter inter-temporal choice, either promoting or suppressing "impulsive choice", depending upon the relative sizes and delays of the two choice alternatives. Theoretical analysis based on a quantitative model of inter-temporal choice indicates that the pattern of effect of the OPFC lesion is likely to reflect two actions: (i) an increase in the rate of time discounting; (ii) an increase in sensitivity to the ratio of the sizes of two reinforcers.

Effect of 8-OH-DPAT on temporal discrimination following central 5-hydroxytryptamine depletion

The 5-hydroxytryptamine (5-HT)(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) alters performance in discrete-trials timing schedules. 5-HT(1A) receptors occur both presynaptically and postsynaptically, but it is not known which receptor population mediates the effects of 8-OH-DPAT on timing. Rats received intra-raphe injections of 5,7-dihydroxytryptamine (n=16) or sham lesions (n=14). They were trained in a discrete-trials psychophysical procedure in which levers were presented at a predetermined time after the onset of each trial (2.5, 7.5,., 47.5 s). A response on lever A was reinforced if lever presentation occurred < 25 s after trial onset; a response on lever B was reinforced if lever presentation occurred >25 s after trial onset. After 70 preliminary sessions, the rats received 8-OH-DPAT (25, 50, 100, 200 microg kg(-1) sc) and saline vehicle. The percentage of responses on lever B (%B) increased as a function of time from trial onset. Under the baseline (vehicle-treatment) condition, performance did not differ between the two groups. 8-OH-DPAT did not alter the indifference point (time corresponding to %B=50%), but dose-dependently increased the Weber fraction in both groups. Forebrain concentrations of 5-HT and 5-HIAA in the lesioned group were approximately 10% of control levels. The results suggest that the effect of 8-OH-DPAT on performance on discrete-trials timing schedules is mediated by postsynaptic 5-HT(1A) receptors.

Effects of lesions of the orbitofrontal cortex on sensitivity to delayed and probabilistic reinforcement

RATIONALE

Lesions of the orbital prefrontal cortex (OPFC) can cause pathologically impulsive behaviour in humans. Inter-temporal choice behaviour (choice between reinforcers differing in size, delay and/or probability) has been proposed as a model of "impulsive choice" in animals.

OBJECTIVE

The effect of lesions of the OPFC on rats' inter-temporal choice behaviour was examined in two experiments: (1) rats chose between a smaller immediate reinforcer and a larger delayed reinforcer; (2) rats chose between a smaller certain reinforcer and a larger probabilistic reinforcer.

METHODS

Under halothane anaesthesia, rats received injections of the excitotoxin quinolinate into the OPFC (0.1 M, 0.5 microl, two injections in each hemisphere), or sham lesions (injections of vehicle). They were trained to press two levers (A and B) for food-pellet reinforcers in discrete-trials schedules. In free-choice trials, a press on A resulted in immediate delivery of one food pellet; a press on B resulted in delivery of two pellets, either following a delay ( d) (experiment 1), or with a probability ( p) <1 (experiment 2). The values of d and p were manipulated across phases of the experiments. The locations of the lesions were verified histologically at the end of the experiment.

RESULTS

In experiment 1, both groups showed declining choice of lever B as a function of d. The lesioned rats showed significantly shorter indifference delays ( D50: the value of d corresponding to 50% choice of lever B) than the sham-lesioned rats. In experiment 2, both groups showed declining choice of lever B as a function of the odds against delivery of the two-pellet reinforcer, theta ( theta =[1/ p]-1). The lesioned rats showed lower indifference odds ( theta50: the value of theta corresponding to 50% choice of lever B) than the sham-lesioned rats. In both experiments, the lesioned rats showed extensive atrophy of the OPFC, with sparing of the dorsolateral prefrontal cortex.

CONCLUSIONS

The results show that lesions of the OPFC can promote preference for the smaller and more immediate, and the smaller and more certain of two reinforcers. The results are consistent with two interpretations: the lesion may have altered (i) the rates of delay and odds discounting, and/or (ii) sensitivity to the ratio of the sizes of the two reinforcers.

Failure of central 5-hydroxytryptamine depletion to alter the effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on timing performance on the free-operant psychophysical procedure

RATIONALE

The 5-hydroxytryptamine (5-HT)(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) alters temporal differentiation of behaviour on the free-operant psychophysical procedure, displacing the psychophysical curve to the left, thereby reducing the indifference point T(50). However, it is not known whether this effect of 8-OH-DPAT is mediated by an action of the drug at somatodendritic autoreceptors or at postsynaptic receptors.

OBJECTIVE

To compare the effects of 8-OH-DPAT on performance on the free-operant psychophysical procedure in normal (sham-lesioned) rats and in rats whose 5-HTergic pathways had been lesioned by means of intra-raphe injections of the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT).

METHODS

Twelve rats received 5,7-DHT-induced lesions of the median and dorsal raphe nuclei, and twelve received sham lesions. They were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials, during which reinforcement was provided intermittently for responding on A in the first half and B in the second half of the trial. Percentage responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data from each rat for the derivation of timing indices [T(50) (time corresponding to %B=50%) and Weber fraction] following treatment with acute doses of 8-OH-DPAT (25, 50, 100, 200 microg kg(-1), s.c.) and saline (vehicle-alone treatment). Levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), noradrenaline and dopamine were measured in forebrain regions after the completion of the experiment.

RESULTS

Under the vehicle-alone condition, the lesioned group displayed a greater propensity for switching between the levers, but T(50) and the Weber fraction did not differ between the groups. In both groups, 8-OH-DPAT shifted the psychophysical curve to the left, significantly reducing T(50) at the 200-microg kg(-1) dose; the effect of 8-OH-DPAT did not differ significantly between the groups. Levels of 5-HT and 5-HIAA in the lesioned group were about 10% of those in the sham-lesioned group; there was no effect of the lesion on catecholamine levels.

CONCLUSIONS

The results confirm that 8-OH-DPAT disrupts temporal differentiation in the free-operant psychophysical schedule, reducing the indifference time, T(50). The failure of central 5-HT depletion to alter the effect of 8-OH-DPAT suggests that this effect may be mediated by stimulation of postsynaptic 5-HT(1A) (or possibly 5-HT(7)) receptors rather than somatodendritic 5-HT(1A) autoreceptors.

Effects of central 5-hydroxytryptamine depletion on sensitivity to delayed and probabilistic reinforcement

RATIONALE

The ascending 5-hydroxytryptaminergic (5-HTergic) pathways are believed to be involved in "impulse control". Rats whose 5-HTergic pathways have been destroyed are more liable than intact rats to select a smaller, immediate reinforcer rather than a larger, delayed reinforcer (impulsive choice), and recent evidence indicates that this effect of central 5-HT depletion reflects a change in the rate of time discounting (i.e. a change in the rate at which reinforcers become devalued as a function of delay). Delay of reinforcement and uncertainty of reinforcer delivery are believed to have equivalent effects on choice behaviour. However, it is not known whether central 5-HT depletion affects choice between probabilistic reinforcers.

OBJECTIVE

We examined the effects of central 5-HT depletion on choice behaviour in two experiments: In experiment 1, rats chose between a smaller immediate reinforcer and a larger delayed reinforcer; in experiment 2, rats chose between a smaller certain reinforcer and a larger probabilistic reinforcer.

METHODS

Rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained to press two levers for food-pellet reinforcers in discrete-trials schedules. In free-choice trials, selection of lever A resulted in immediate delivery of one food pellet; selection of lever B resulted in delivery of 2 pellets, either following a delay (dB) (experiment 1) or with a probability (pB) less than 1 (experiment 2).

RESULTS

In experiment 1, both groups showed declining choice of lever B (%B) as a function of dB. The lesioned group showed shorter indifference delays (D50: the value of dB corresponding to %B=50) than the sham-lesioned group. In experiment 2, both groups showed declining choice of lever B as a function of the odds against delivery of the two-pellet reinforcer, thetaB (thetaB=[1/pB]-1). There was no difference between the "indifference odds" (theta50: the value of thetaB corresponding to %B=50) between the two groups. In both experiments, the levels of 5-HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered.

CONCLUSIONS

These results provide additional evidence that central 5-HTergic mechanisms are involved in time discounting, but provide no evidence for a similar role of 5-HT in rats' sensitivity to probabilistic reinforcement.

Effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on performance on two operant timing schedules

RATIONALE

Previous experiments have shown that the disruptive effect of central 5-HT depletion on interval timing behaviour is critically dependent upon the particular timing schedule used. However, it is not known how acute disruption of 5-HTergic function brought about by drugs acting at 5-HT receptors affects timing.

OBJECTIVE

To examine the effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on performance on two quantitative timing schedules, a free-operant schedule in which rats were trained to distribute their responses differentially between two levers during the course of a 50-s trial (free-operant psychophysical procedure) and a discrete-trials schedule in which rats were trained to discriminate the durations of light stimuli (interval bisection task).

METHODS

In experiment 1, rats were trained under the free-operant psychophysical procedure to respond on two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half, of the trial. For one group, repetitive switching between levers was permitted; for another group, it was prevented. In experiment 2, rats were trained to press lever A after a 2-s stimulus and lever B after an 8-s stimulus, and were then tested with stimuli of intermediate durations. For one group, a 'poke response' (depression of a central tray flap) was required after stimulus presentation to effect lever presentation; for the other group this requirement did not operate. In both experiments, quantitative indices of timing were derived from the psychophysical functions (%B responding vs time).

RESULTS

In experiment 1, 8-OH-DPAT (25, 50, 100 and 200 microg kg(-1) s.c.) displaced the psychophysical curve to the left in both versions of the schedule. In experiment 2, 8-OH-DPAT increased the Weber fraction in both versions of the task without displacing the curve.

CONCLUSIONS

These results show that 8-OH-DPAT disrupts timing behaviour. The results of experiment 1 are consistent with the proposal that 5-HTergic mechanisms help to regulate the period of the hypothetical pacemaker. However, the results of experiment 2 do not support this suggestion. Taken together, the results support the notion that different neural mechanisms may be involved in timing tasks involving temporal distribution of responding and discrimination of the durations of exteroceptive stimuli.

Comparison of the effects of clozapine, haloperidol, chlorpromazine and d-amphetamine on performance on a time-constrained progressive ratio schedule and on locomotor behaviour in the rat

Performance on progressive ratio schedules has been proposed as a means of assessing the effects of drugs on "reinforcer efficacy". It has been proposed that the effects of neuroleptic drugs on operant behaviour are mediated by a reduction of "reinforcer efficacy". We examined the effects of two "conventional" neuroleptics (haloperidol and chlorpromazine) and an "atypical" neuroleptic (clozapine) on progressive ratio schedule performance; d-amphetamine was used as a comparison compound. In experiment 1, rats responded for a sucrose reinforcer on a time-constrained progressive ratio schedule (75-min sessions). After 66 preliminary training sessions, the rats received single doses (IP) of haloperidol (0.05, 0.1 mg kg(-1)). chlorpromazine (2, 4 mg kg(-1)), clozapine (0.5, 1, 2, 4, 8 mg kg(-1)), and d-amphetamine (0.2, 0.4, 0.8 mg kg(-1)), and the corresponding vehicle solutions. The highest ratio completed was reduced by haloperidol and chlorpromazine, and increased by clozapine. All three neuroleptics reduced the peak response rate, at least at the highest doses administered. Response rates on the lower and intermediate ratios could be described by a three-parameter equation proposed to account for fixed ratio schedule performance. Haloperidol reduced, and clozapine dose-dependently increased the "motivational" parameter (a); d-amphetamine reduced it at low doses and increased it at high doses. The three neuroleptics increased the "response time" parameter (delta). Un-reinforced locomotor behaviour, measured in experiment 2, was not significantly altered by haloperidol, chlorpromazine or clozapine, but was increased by d-amphetamine. These results are consistent with a reduction of reinforcer efficacy produced by haloperidol and an increase produced by clozapine; clozapine's effect is unlikely to reflect a general increase in locomotion. All three neuroleptics induced some degree of motor debilitation. The quantitative analysis of progressive ratio schedule performance may provide a useful adjunct to existing methods for separating effects of drugs on motivational and motor processes.

The effect of d-amphetamine on performance on two operant timing schedules

RATIONALE

Previous experiments have shown that d-amphetamine disrupts timing behaviour in rats. It has been proposed that d-amphetamine's effects reflect a reduction in the period of the pacemaker of the hypothetical internal clock. However, some studies have obtained conflicting results.

OBJECTIVE

To examine the effects of d-amphetamine (0.2, 0.4, 0.8 mg kg(-1) i.p.) on performance on two quantitative timing schedules: a free-operant schedule, in which rats were trained to distribute their responses differentially between two levers during the course of a 50-s trial (free-operant psychophysical procedure), and a discrete-trials schedule, in which rats were trained to discriminate the duration of light stimuli (interval bisection task).

METHODS

In experiment 1, rats were trained under the free-operant psychophysical procedure to respond on two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A during the first half and on B during the second half of the trial. For one group, repetitive switching between levers was permitted; for another group, it was prevented. In experiment 2, rats were exposed to press lever A after a 2-s stimulus and lever B after an 8-s stimulus, and were then tested with stimuli of intermediate duration. For one group, a 'poke response' (depression of a central tray flap) was required after stimulus presentation to effect lever presentation; for the other group, this requirement did not operate. In both experiments, quantitative indices of timing were derived from the psychophysical functions (%B responding vs time).

RESULTS

In experiment 1, d-amphetamine increased the Weber fraction and displaced the psychophysical curve to the left in both versions of the schedule, as well as producing rate-dependent suppression of responding. In experiment 2, d-amphetamine increased the Weber fraction in both versions of the task without displacing the curve.

CONCLUSIONS

These results confirm the disruptive effect of d-amphetamine on timing. The results of experiment 1 are consistent with the proposal that the drug reduces the period of the hypothetical pacemaker. However, the results of experiment 2 do not support this suggestion. Taken together, the results support the notion that different neural mechanisms may be involved in timing tasks involving temporal distribution of responding and discrimination of the duration of exteroceptive stimuli.

Effect of central 5-hydroxytryptamine depletion on inter-temporal choice: a quantitative analysis

RATIONALE

It has been proposed that the ascending 5-hydroxytryptaminergic (5-HTergic) pathways are involved in "impulse control". Previous experiments have shown that rats whose 5-HTergic pathways have been destroyed are more liable than intact rats to select a smaller, immediate reinforcer rather than a larger, delayed reinforcer (impulsive choice). However, it remains unclear whether this effect of central 5-HT depletion reflects a change in the rate of time discounting (i.e. a change in the rate at which reinforcers become devalued as a function of delay) or a change in sensitivity to reinforcer size.

OBJECTIVE

We examined the effect of central 5-HT depletion on time discounting using a quantitative model of inter-temporal choice (multiplicative hyperbolic model), which enables effects on time discounting to be differentiated from effects on sensitivity to reinforcer size.

METHODS

Rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained to press two levers for food-pellet reinforcers in a discrete-trials adjusting-delay schedule. In free-choice trials, selection of lever A resulted in a brief fixed delay (dA) followed by delivery of one pellet; selection of lever B resulted in a longer variable delay (dB) followed by delivery of two pellets; dB was adjusted in accordance with the subject's choices. The value of dA was varied (0.5-8.0 s) in successive phases of the experiment, and the indifference value of dB was determined in each case.

RESULTS

In both groups, the indifference value of dB was linearly related to the value of dA, in accordance with the multiplicative hyperbolic model. The lesioned group showed shorter indifference delays than the sham-lesioned group, this being reflected in a parallel displacement of the linear indifference function. In both experiments, the levels of 5-HT and 5-hydroxyindole-acetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered.

CONCLUSIONS

According to the multiplicative hyperbolic model, parallel displacement of the linear indifference function uniquely specifies a change in time discounting. Thus these results indicate that central 5-HT depletion results in an increase in the rate of time discounting for food reinforcers.

Theory and method in the quantitative analysis of "impulsive choice" behaviour: implications for psychopharmacology

Impulsive choice refers to the selection of small immediate gains in preference to larger delayed gains, or the selection of large delayed penalties in preference to smaller immediate penalties. Current theoretical interpretations of impulsive choice are reviewed, and a synthesis of these ideas, the "multiplicative hyperbolic model of choice", is presented. The model assumes that the value of a positive reinforcer increases as a hyperbolic function of its size, and decreases as a hyperbolic function of its delay and the odds against its occurrence. Each hyperbolic function contains a single discounting parameter which quantifies the organism's sensitivity to the variable in question. The hyperbolic discounting functions combine multiplicatively to determine the overall value of the reinforcer. Equivalent functions are postulated to govern the (negative) value of aversive events, the net value of an outcome reflecting the algebraic sum of the positive and negative values. The model gives rise to a quantitative methodology for studying impulsive choice, based on a family of linear indifference (null) equations, which describe performance under conditions of indifference, when the values of the reinforcers are assumed to be equal. This methodology may be used to identify individual differences in sensitivity to the magnitude, delay and probability of reinforcement. The methodology is also suitable for the quantitative evaluation of the effects of some pharmacological interventions on discounting parameters. Recent psychopharmacological studies of impulsive choice are reviewed, and the utility of indifference equations for extending this work, and developing a quantitative psychopharmacology of impulsive choice is discussed.

Molecular investigation of a listeriosis outbreak in goats caused by an unusual strain of Listeria monocytogenes

During a 16-month period, 10 goats with listeriosis were identified in 2 herds that shared 3 bucks, including 1 that died of listeriosis. Using DNA fingerprinting, we determined that a single genetically unique Listeria monocytogenes strain had infected all goats from which isolates were available. All isolates were unable to metabolize rhamnose (rhamnose-negative), whereas as a species, L monocytogenes is considered to have a rhamnose-positive phenotype. Therefore, these isolates would have been characterized as a species other than L monocytogenes if any of a variety of commercial bacterial identification kits had been used for speciation. Silage was not fed to either herd, and L monocytogenes was not isolated from vaginal or rectal swab specimens obtained from healthy goats or from samples of feed. Because the 3 bucks were the only common elements between the 2 herds, our results suggest a venereal route of transmission for listeriosis.

Splenic lymphosarcoma in a horse

A 10-year-old Tennessee Walker gelding, with a history of progressive weight loss, intermittent colic and lethargy, had a slight fever, tachycardia, tachypnea, pallor, ascites and marked ventral edema. Blood analyses revealed anemia, leukocytosis, neutrophilia with a left shift, lymphopenia, monocytosis, hypoproteinemia and a slightly increased SDH level. Abdominocentesis produced red-orange fluid with many RBC and an increased fibrinogen content. Rectal palpation revealed a large mass in the left caudal abdominal quadrant. The animal died shortly after resection of the mass. The histopathologic diagnosis was lymphosarcoma, involving the spleen, liver and lung.

An experimental evaluation of the response of the bull penis to carbon fiber implants

Flexible filamentous carbon fiber was surgically implanted between the dorsal apical ligament and tunica albuginea of the penis in 6 experimental bulls by both an open and a closed technique. The bulls were evaluated grossly and histologically at 30, 60 and 90 days after implantation. Grossly, carbon fiber produced a strong adhesion of the apical ligament to the tunica albuginea.. Histologically, the fibroblasts and collagen fibers were organized and aligned along the individual carbon filament. The implants were well tolerated and produced minimal reaction and appeared to be biologically safe. The implants produced a strong adhesion of the apical ligament to the tunica albuginea by either surgical technique. It should therefore be a satisfactory implant when used for correction of spiral and ventral deviation of the bull penis.