Ecological determinants of reinforcement in the guinea pig

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Testosterone Rapidly Augments Retrograde Endocannabinoid Signaling in Proopiomelanocortin Neurons to Suppress Glutamatergic Input from Steroidogenic Factor 1 Neurons via Upregulation of Diacylglycerol Lipase-α

Testosterone exerts profound effects on reproduction and energy homeostasis. Like other orexigenic hormones, it increases endocannabinoid tone within the hypothalamic feeding circuitry. Therefore, we tested the hypothesis that testosterone upregulates the expression of diacylglycerol lipase (DAGL)α in the hypothalamic arcuate nucleus (ARC) to increase energy intake via enhanced endocannabinoid-mediated retrograde inhibition of anorexigenic proopiomelanocortin (POMC) neurons. Energy intake, meal patterns, and energy expenditure were evaluated in orchidectomized, male guinea pigs treated subcutaneously with testosterone propionate (TP; 400 μg) or its sesame oil vehicle (0.1 mL). TP rapidly increased energy intake, meal size, O2 consumption, CO2 production, and metabolic heat production, all of which were antagonized by prior administration of the DAGL inhibitor orlistat (3 μg) into the third ventricle. These orlistat-sensitive, TP-induced increases in energy intake and expenditure were temporally associated with a significant elevation in ARC DAGLα expression. Electrophysiological recordings in hypothalamic slices revealed that TP potentiated depolarization-induced suppression of excitatory glutamatergic input onto identified ARC POMC neurons, which was also abolished by orlistat (3 μM), the CB1 receptor antagonist AM251 (1 μM), and the AMP-activated protein kinase inhibitor compound C (30 μM) and simulated by transient bath application of the dihydrotestosterone mimetic Cl-4AS-1 (100 nM) and testosterone-conjugated bovine serum albumin (100 nM). Thus, testosterone boosts DAGLα expression to augment retrograde, presynaptic inhibition of glutamate release onto ARC POMC neurons that, in turn, increases energy intake and expenditure. These studies advance our understanding of how androgens work within the hypothalamic feeding circuitry to affect changes in energy balance.

The role of AMP-activated protein kinase in the androgenic potentiation of cannabinoid-induced changes in energy homeostasis

Orexigenic mediators can impact the hypothalamic feeding circuitry via the activation of AMP-dependent protein kinase (AMPK). Given that testosterone is an orexigenic hormone, we hypothesized that androgenic changes in energy balance are due to enhanced cannabinoid-induced inhibition of anorexigenic proopiomelanocortin (POMC) neurons via activation of AMPK. To this end, whole animal experiments were carried out in gonadectomized male guinea pigs treated subcutaneously with either testosterone propionate (TP; 400 μg) or its sesame oil vehicle (0.1 ml). TP-treated animals displayed increases in energy intake associated with increases in meal size. TP also increased several indices of energy expenditure as well as the p-AMPK/AMPK ratio in the arcuate nucleus (ARC) measured 2 and 24 h posttreatment. Subcutaneous administration of the CB1 receptor antagonist AM251 (3 mg/kg) rapidly blocked the hyperphagic effect of TP. This was mimicked largely upon third ventricular administration of AM251 (10 μg). Electrophysiological studies revealed that TP potentiated the ability of the cannabinoid receptor agonist WIN 55,212-2 to decrease the frequency of miniature excitatory postsynaptic currents in ARC neurons. TP also increased the basal frequency of miniature inhibitory postsynaptic currents. In addition, depolarization-induced suppression (DSE) is potentiated in cells from TP-treated animals and blocked by AM251. The AMPK inhibitor compound C attenuated DSE from TP-treated animals, whereas the AMPK activator metformin enhanced DSE from vehicle-treated animals. These effects occurred in a sizable number of identified POMC neurons. Collectively, these results indicate that the androgen-induced increases in energy intake are mediated via an AMPK-dependent augmentation in endocannabinoid tone onto POMC neurons.

Automaintenance in guinea pigs: effects of feeding regimen and omission training

Behavior maintained by stimulus-reinforcer pairings was examined. Guinea pigs maintained at 85 per cent of free-feeding weights reliably contacted a retractable lever presented before delivery of a single piece of guinea-pig chow or a 45-milligram guinea-pig pellet. When animals were given free access to one food and received the second food preceded by the lever, contact responses persisted. Such responses seldom occurred when a single food was freely available and was also delivered after lever presentation. Introduction of an omission training (negative automaintenance) procedure, in which lever contacts resulted in lever retraction and prevented food delivery, strongly reduced lever contacts. Observation indicated that mouthing the food cup, instead of the lever, became the prominent behavior during the prefood stimulus under the omission training procedure.

Quantification of rats' behavior during reinforcement periods

What is treated as a single unit of reinforcement often involves what could be called a reinforcement period during which two or more acts of ingestion may occur, and each of these may have associated with it a series of responses, some reflexive, some learned, that lead up to ingestion. Food-tray presentation to a pigeon is an example of such a "reinforcement period." In order to quantify this behavior, a continuous-reinforcement schedule was used as the reinforcement period and was chained to a fixed-ratio schedule. Both fixed-ratio size and reinforcement-period duration were manipulated. Rats were used as subjects, food as reinforcement, and a lever press as the operant. Major findings included (a) a rapid decline in response rates during the first 15 to 20 seconds of the reinforcement periods, and (b) a strong positive relationship between these response rates and the size of the fixed ratio. Also revealed was a short scallop not normally found in fixed-ratio response patterns, whose length was a function of fixed-ratio size and reinforcement-period duration. It is suggested that rapidly fluctuating excitatory processes can account for many of these findings and that such processes are functionally significant in terms of behavioral compensation.

Contrast, induction, facilitation, suppression, and conservation

Ten rats received all of their water in daily 1-hr sessions. Following a baseline phase in which lever and water spout were freely available throughout each session, subjects were trained to press the lever for water on mixed schedules composed of two alternating components. Each component gave access to water for a fixed cumulation of drinking time every time the rat cumulated a fixed amount of lever-pressing time. Changes in one component produced contrast and induction effects, both positive and negative, with respect to both lever pressing and drinking in the unchanged component. All schedules facilitated lever pressing relative to baseline. All schedules suppressed drinking relative to baseline, even though contingency sessions allowed ample time to perform the baseline amount of drinking. The entire pattern of results was predicted in quantitative detail by assuming that the total amount of a dimension apportioned to lever pressing and drinking is conserved between baseline and contingency sessions. Conservation theory was shown to predict several effects produced by simple fixed-ratio schedules, and was compared favorably with probability-differential (Premack, 1971) and response-deprivation (Timberlake and Allison, 1974) theory.

The matching law in and within groups of rats

In each of the two experiments, a group of five rats lived in a complex maze containing four small single-lever operant chambers. In two of these chambers, food was available on variable-interval schedules of reinforcement. In Experiment I, nine combinations of variable intervals were used, and the aggregate lever-pressing rates (by the five rats together) were studied. The log ratio of the rates in the two chambers was linearly related to the log ratio of the reinforcement rates in them; this is an instance of Herrnstein's matching law, as generalized by Baum. Summing over the two food chambers, food consumption decreased, and response output increased, as the time required to earn each pellet increased. In Experiment II, the behavior of individual rats was observed by time-sampling on selected days, while different variable-interval schedules were arranged in the two chambers where food was available. Individual lever-pressing rates for the rats were obtained, and their median bore the same "matching" relationship to the reinforcement rates as the group aggregate in Experiment I. There were differences between the rats in their distribution of time and responses between the two food chambers; these differences were correlated with differences in the proportions of reinforcements the rats obtained from each chamber.

The economics of daily consumption controlling food- and water-reinforced responding

In the first experiment, two rhesus monkeys earned their entire ration of food and water during daily sessions with no provisions to ensure constant daily intakes. Two variable-interval schedules of food presentations were concurrent with one variable-interval schedule of water presentations; the maximum rate of food presentations arranged by one food schedule was varied. As the rate of food presentations was increased, the absolute level of responding on the two food schedules combined decreased, while responding on the water schedule increased. The preference for the variable food schedule compared to the other food schedule approximately matched the proportion of reinforcers obtained from it. The preference for the variable food schedule compared to the water schedule did not match, but greatly decreased, as the proportion of reinforcers from the food schedule increased. When Experiment I was replicated, with provisions to ensure constant daily intakes of food and water (Experiment II), the absolute response rates under the two food schedules combined and under the water schedule no longer changed with increases in the rate of food during the sessions. On the other hand, choice between the two food schedules remained proportional to the distribution of obtained food pellets. These results were interpreted as indicating that behavior to obtain nonsubstitutable commodities, such as food and water, is strongly controlled by the economic conditions of daily consumption, while choice between substitutable commodities is independent of these factors.

Demand for food on fixed-ratio schedules as a function of the quality of concurrently available reinforcement

Six rats lever pressed for food on concurrent fixed-ratio schedules, in a two-compartment chamber. In one compartment, mixed diet pellets were delivered on fixed-ratio schedules of 1, 6, 11, and 16; in the other, either no food was delivered, or sucrose or mixed diet pellets were delivered on fixed-ratio 8. The number of pellets obtained in the first compartment declined as a function of fixed-ratio size in that compartment in all three conditions, but the decline was greatest overall with mixed diet pellets concurrently available in the other compartment, and least with no food concurrently available. The result is discussed in terms of economic demand theory, and is consistent with the prediction that elasticity of demand for a commodity (defined in operant terms as the ratio of the proportionate change in number of reinforcements per session to the proportionate change in fixed-ratio size) is greater the more substitutable for that commodity are any concurrently available commodities.

Sex differences in the cannabinoid modulation of appetite, body temperature and neurotransmission at POMC synapses

We sought to determine whether sex differences exist for the cannabinoid modulation of appetite, body temperature and neurotransmission at pro-opiomelanocortin (POMC) synapses. Gonadectomized male and female guinea pigs were outfitted to monitor core body temperature and injected with either the CB1 receptor agonist WIN 55,212-2 (1 mg/kg s.c.), antagonist AM251 (3 mg/kg s.c.) or vehicle (1 ml/kg s.c.) and evaluated for changes in six indices of feeding behavior under ad libitum conditions for 7 days. WIN 55,212-2 elicited an overt, sexually differentiated hyperphagia in which males displayed larger increases in hourly and daily intake, consumption/gram body weight, meal size and meal duration. The agonist also produced a more robust acute hypothermia in males than in females. In addition, males were more sensitive to the hypophagic effect of AM251, manifested by comparatively sizeable decreases in hourly intake, consumption/gram body weight, meal frequency and hyperthermia. To gain additional insight into the cellular mechanism underlying cannabinoid regulation of energy homeostasis, we performed whole-cell patch clamp recordings in hypothalamic slices prepared from gonadectomized male and female guinea pigs, and monitored miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) in arcuate (ARC) neurons. ARC neurons from females exhibited a higher basal mEPSC frequency. WIN 55,212-2 dose-dependently reduced mEPSC and mIPSC frequency; however, cells from males were far less sensitive to the CB1 receptor-mediated decrease in mIPSC frequency. These effects were observed in neurons subsequently identified as POMC neurons. These data reveal pronounced sex differences in how cannabinoids influence the hypothalamic control of homeostasis.

"An ecological perspective": the value of species comparisons

George Collier is known for promoting the incorporation of an ecological framework into the study of feeding behavior. He has tirelessly reminded psychologists and physiologists that the animals they study are members of species designed by evolution to solve problems of acquiring, handling and storing energy in particular ways. A less-appreciated contribution is the comparative approach within his laboratory. George and his students used laboratory simulations of foraging, but ranged far beyond the laboratory rat to examine the responses of a variety of species representing different dietary needs and foraging strategies. Species comparisons enhance our ability to test the generality of responses to challenges, apply ecological theory to the explanation of behavior, and incorporate the feeding strategy of a species in the study of its ingestive behavior.

The evolution of an operant ecologist

George Collier has maintained a research program concerned with the controls of eating for forty-three years. In the first ten years, Collier approached the problem within the operant conditioning framework proposed by Skinner. This consisted of the intrameal analysis of the reflex strength of a food-rewarded operant in short sessions in food-deprived rats. At the end of the first decade, Collier shifted his attack on the problem by studying the ecological control of meal patterns by procurement and consummatory costs in free-feeding rats. This paper analyzes the reasons for this apparently abrupt evolution from operant conditioning to operant ecology and concludes that they accumulated over about ten years and were intellectual and personal, not technical or professional. This paper also summarizes the major achievements of the research program of operant ecology, notes some emerging problems with economic explanations of ecological controls, and argues that Collier's robust and quantitative behavioral results are ripe for physiological analysis.

The relation between meal size and the time between meals: effects of cage complexity and food cost

Certain popular models of the regulation of food intake predict a positive correlation between the size of a meal and the preceding and/or following intermeal interval. However, the reported strength of these prandial correlations has varied widely in the literature. To determine what factors may influence the strength of these relationships, we measured the timing of and amount consumed in meals of laboratory rats as a function of 1) whether they were housed in isolation or with partial access to peers or a running wheel, 2) whether they were disturbed daily or weekly for maintenance procedures, and 3) whether food was free or contingent on operant responding. We also compared two definitions of "meal" and "intermeal interval." Strong prandial correlations were found only occasionally. Caging, food cost, and maintenance frequency did not affect the size of the correlations, although these factors did influence the rats' meal patterns. We conclude the regulation of food intake cannot be explained by models relying on a regular relationship between meal size and intermeal interval.

Who is in charge? Animal vs experimenter control

In the 1920s Curt Richter (1927) stated that the central problem for psychology was to discover the determinants of the initiation and termination of bouts of behavior. Ignoring this challenge, experimentation in animal psychology has been dominated by the session paradigm in which animals work in brief sessions for a resource of which they have been deprived. In this open economy, no behavioral strategy of the animal can meet its demand, and the beginnings and ends of bouts are controlled by the experimenter; thus, Richter's problem cannot be addressed. In contrast, in a free-feeding, closed economy, the animal controls the initiation and termination of feeding and can regulate its intake, and bout patterns can be observed. If the paradigm is modified to simulate a habitat where resources are distributed discontinuously and the animal must work to discover and procure access to a commodity before it can be used, behavioral strategies allowing the animal to regulate its intake while tending to maximize the ratio of benefits to costs are revealed. We offer an answer to Richter's question based on a cost/benefit analysis of feeding behavior in this foraging paradigm. We show that the time and energy costs of resource acquisition and resource consumption are powerful determinants of the pattern of resource use, and that they have different and independent effects. The former costs are reduced by reducing the frequency of initiating bouts, and the latter costs, by altering the rate and amount of consumption. Further, the time window of these relations is much longer than expected from analyses in the session paradigm. We conclude that the recurrent nature of behavior is due to the discontinuous distribution of resources rather than to cycles of physiological depletion and repletion, and that the determinants of bout initiation and termination lie in the economics of the allocation of time and effort to different resources and activities.

Responses of fowls to an operant feeding procedure and its potential use for reducing randomness in meal occurrence

Adult laying hens were tested with increasing and decreasing procurement costs (pecks at a disc) which allowed access to food for unlimited periods. This was done to observe changes in feeding behaviour, to see whether apparent randomness in meal occurrence can be reduced, and, if so, to identify an appropriate fixed ratio (FR) for incorporating in investigations of physiological correlates of hunger and satiety. Daily food intake stayed the same at all but the highest (160 pecks) FR. FR sizes were related negatively to total time feeding and meal frequency, and positively to intermeal interval length, meal size and rate of eating within meals. They were also related positively to meal length, but only weakly so. Frequency distributions of meal and interval lengths, and postprandial correlation coefficients, indicated that randomness in feeding did decline at higher FRs, and FR20 seems the most appropriate schedule for potential use in physiological experiments.

The effects of varying procurement costs on food intake in baboons

Food intake of four adult male baboons (Papio anubis), ranging in weight from 18 to 33 kg, was monitored during daily experimental sessions lasting 22 hours. Food was available under a two-component operant schedule designed to mimic food availability in the natural ecology. The first component was a "procurement" component which consisted of pulling the manipulandum a set number of times under a fixed-ratio (FR) operant schedule. Following completion of the procurement response requirement, access to food, i.e., a meal, became available under the second "consumption" component during which each response produced a food pellet (one-gram banana-flavored pellets, 3.1 kcal/g). After a 10-minute interval in which no response occurred, the consumption component was terminated. In order to gain access to another meal, the baboon had to complete the ratio requirement of the procurement component. Increasing the ratio requirement of the procurement component from 10 to 200 had no significant effect on mean total daily intake, but significantly reduced the number of meals from 7.4 under the FR10 requirement to 3.6 under the FR200 procurement component. There were no differences among baboons in mean meal size, or intermeal-interval under each procurement requirement. Similar patterns of cumulative daily intake were observed within baboons under all ratio requirements of the procurement component, and there were few significant correlations between meal size and postmeal-interval, or premeal-interval and meal size in any baboon.

Experimental manipulations of eating: advances in animal models for studying anorectic agents

The material set out in this text has been designed to show the wide range of procedures which have the capacity to modify eating behavior--to produce hyper- or hypophagia, to alter the profile of eating patterns, or to adjust dietary preferences and selection. Accordingly, in investigating anorectic drugs it seems necessary to observe the effects of drug actions in a variety of experimental models. This strategy will provide a more complete description of the effect of a drug, will throw light on the mechanism of action, and will provide a more realistic base for predicting the effects of drugs in man.

The economics of the law of effect

A corollary of the law of effect predicts that the larger the reinforcement, the greater the rate of responding. However, an animal must eat more small portions than large portions to obtain the same daily intake, and one would predict, therefore, that when eating smaller portions an efficient animal would eat less (conserving time and energy) and/or respond faster (conserving time). The latter of these predictions was supported by the present experiments with free-feeding rats for which portion size (pellet size or duration of feeder presentation) and portion price within meals were varied. Response rate was a function of the unit price (responses/g) of food: Rats responded faster when portions were smaller or when prices were higher. Meal size and frequency were relatively unaffected by unit price, but were influenced by the price of meal initiation. The results are discussed in relation to the economic differences between traditional operant and free-feeding paradigms and to both traditional and more recent formulations of the law of effect.

Influence of ambient temperature on feeding activity parameters and digestive function in domestic fowls

In order to better understand the way in which domestic fowls regulate their food intake at different ambient temperatures, measurements were made of feeding activity parameters, drinking, rate of food passage, apparent digestibility and gut dimensions of immature female fowls acclimated to cool (8 degrees C), moderate (20 degrees C) and warm (32 degrees C) environments. At 8 degrees C birds had longer (p less than 0.05) intervals between meals, ate larger meals, ate faster within meals and digested their food more efficiently than at 20 degrees C. At 32 degrees C they ate less food per day, drank more water, had a higher water/food intake ratio, ate slower within meals, had a slower rate of food passage, and had smaller proventriculi, gizzards and caeca than at 20 degrees C. It is concluded that differences in daily food intake between temperatures are associated more closely with differences in meal size and rate of eating within meals than with differences in meal frequency. Effects of temperature on behavioural thermoregulation, drinking and rate of food passage all influence patterns of feeding activity shown.

Effect of procurement cost on the drinking of a saccharin-sucrose solution by non-deprived rats

Four non-deprived female rats were required to run in a wheel to obtain 20 min unconstrained access to a saccharin and sucrose solution. Each was run in a series of conditions in which the requirement was a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the amount of running performed in a condition in which the wheel alone was available. A condition in which no running was required to gain access to the solution and one in which the subject was locked in the stationary wheel for the time taken to complete the highest requirement before being allowed access to the solution were also included. The results showed that as the requirement increased the amount of solution consumed also increased, and this relationship did not depend on the time taken to perform the requirement.

Effect of procurement wheel running on the rate of drinking in rats

Four rats, 22.5 hr deprived of water, were tested in 3 conditions, in which they were required to run zero, 5 or 300 one-sixth revolutions in a wheel to gain 30 min access to water in a drinking tube. The number of licks performed in the 30 min increased monotonically with the procurement cost, and was 20% greater following the larger cost than when there was no cost. However, examination of the rates of drinking throughout the 30 min revealed that differences occurred between the conditions only at the beginning of the period. In the first 3 min there was a monotonic relationship between the proportion of time spent drinking and the procurement cost, but no effect of the cost on the rate could be detected after the first 6 min. The results are consistent with the hypothesis that the effect is mediated by a transient elevation of the subject's arousal level.