Inescapable shock increases finickiness about drinking quinine-adulterated water in rats

Taste aversion learning during successive negative contrast

Previous experiments found that acceptance of saccharin by rats was reduced if they had prior experience of sucrose or some other highly palatable solution. This study tested whether such successive negative contrast (SNC) effects involve acquisition of an aversion to the new taste. In three experiments, rats were switched from sucrose exposure in Stage 1 to a less palatable solution containing a new taste in Stage 2. In Experiments 1 and 2, a novel flavor was added to a saccharin solution at the start of Stage 2. In Experiment 1, preference tests revealed a weak aversion to the added vanilla flavor in the Suc-Sacch group, while in Experiment 2 an aversion was found in the Suc-Sacch group to the salty flavor that was used, compared with controls given access either saccharin or water in Stage 1. In Experiment 3, the Suc-Quin group, given quinine solution in Stage 2, displayed a greater aversion to quinine than a Water-Quin control group. These results support the suggestion that taste aversion learning plays a role in the initial suppression of intakes in a qualitative consummatory SNC effect. However, in the light of other evidence, it seems that the unusual persistence of successive negative contrast when rats are switched from sucrose to saccharin is not due to a long-lasting reduction in the value of saccharin.

Uncontrollable chronic stress affects eating behavior in rats

The issue of whether the decrease in food intake induced by inescapable shock is due to the uncontrollability of the stressor or the shock per se has not yet been settled. Besides, whether food intake is differentially affected by an uncontrollable chronic stressor has been explored only by a few studies. Thus, we evaluated the effects of chronic escapable or inescapable electric shocks on eating behavior. Rats were exposed to shock sessions for 20 days in two occasions separated by baseline sessions with no shock in an ABAB design. Results showed a reduction in food and water intake and body weight gain during stress periods, especially with inescapable shocks. The findings support a close link between learned helplessness, chronic stress, and eating behavior.

Gut Microbiota and a Selectively Bred Taste Phenotype: A Novel Model of Microbiome-Behavior Relationships

OBJECTIVES

The microbiota-gut-brain axis is increasingly implicated in obesity, anxiety, stress, and other health-related processes. Researchers have proposed that gut microbiota may influence dietary habits, and pathways through the microbiota-gut-brain axis make such a relationship feasible; however, few data bear on the hypothesis. As a first step in the development of a model system, the gut microbiome was examined in rat lines selectively outbred on a taste phenotype with biobehavioral profiles that have diverged with respect to energy regulation, anxiety, and stress.

METHODS

Occidental low and high-saccharin-consuming rats were assessed for body mass and chow, water, and saccharin intake; littermate controls had shared cages with rats in the experimental group but were not assessed. Cecum and colon microbial communities were profiled using Illumina 16S rRNA sequencing and multivariate analysis of microbial diversity and composition.

RESULTS

The saccharin phenotype was confirmed (low-saccharin-consuming rats, 0.7Δ% [0.9Δ%]; high-saccharin-consuming rats, 28.1Δ% [3.6Δ%]). Regardless of saccharin exposure, gut microbiota differed between lines in terms of overall community similarity and taxa at lower phylogenetic levels. Specifically, 16 genera in three phyla distinguished the lines at a 10% false discovery rate.

DISCUSSION

The study demonstrates for the first time that rodent lines created through selective pressure on taste and differing on functionally related correlates host different microbial communities. Whether the microbiota are causally related to the taste phenotype or its correlates remains to be determined. These findings encourage further inquiry on the relationship of the microbiome to taste, dietary habits, emotion, and health.

Stress increases voluntary alcohol intake, but does not alter established drinking habits in a rat model of posttraumatic stress disorder

BACKGROUND

Life-altering anxiety disorders, such as posttraumatic stress disorder (PTSD), can co-occur at high rates with substance use disorders. Alcoholism, compared with other substance use disorders, is particularly common. Rodent studies of acute stress effects on alcohol consumption show that stress can alter ethanol (EtOH) consumption. This study examined voluntary EtOH consumption in male Long-Evans rats that had undergone a stress-enhanced fear learning (SEFL) procedure.

METHODS

Adult Long-Evans rats were exposed to a stress that consisted of 15 inescapable foot-shocks (1 mA, 1 second) known to cause a long-lasting nonassociative enhancement of subsequent fear learning. Control animals received no shock. One day later, animals were placed in a novel and very different context and received a single foot-shock. On day 3, animals were returned to the single shock context and freezing was used as a measure of learned fear. The intermittent access 2-bottle choice (2BC) regimen consisted of 1 bottle of water and 1 bottle of experimental solution, either 19% EtOH or 28.4% sucrose-0.08% quinine, for a 24-hour period, 3 days a week, and all other times 2 water bottles. This regimen lasted until stable levels of experimental solution drinking were reached, at which point the experimental solution was removed for 40 days and then returned to measure the resumption of consumption.

RESULTS

Rats that received stress prior to EtOH consumed significantly more EtOH than control rats before and after reinstatement. Rats that received stress after drinking was established did not consume significantly more EtOH when the drug was returned. Stress had no significant effect on sucrose-quinine drinking, our calorie and taste control for EtOH.

CONCLUSIONS

A single traumatic event sufficient to produce long-lasting enhancement of fear learning increases voluntary EtOH consumption, but does not alter previously acquired EtOH drinking habits or alter the consumption of a calorically equivalent sweet-bitter-tasting solution.

The taste of sickness: Lipopolysaccharide-induced finickiness in rats

Decrease in food intake is one of the most documented non-specific symptoms of inflammatory processes. However, attention has been mainly focused on quantitative analysis. The present paper reports studies undertaken to test the possible contribution of changes in taste processes in inflammatory-induced alteration of feeding behavior. In a first experiment, the effects of lipopolysaccharide-induced sickness were assessed on preference for saccharin and aversion for quinine in rats using the two-bottle test paradigm. In a second experiment, effect of lipopolysaccharide (LPS) on the behavioral reactivity to palatable, unpalatable and mixed solutions was analyzed using the taste-reactivity paradigm. Our results show that LPS decreased total fluid intake but did not change taste responses to unpalatable or palatable substances. However, LPS increased aversive reactions and decreased hedonic responses to mixed taste. These LPS-induced changes are interpreted as an increase in finickiness and are discussed in regard to their potential role in the adaptation of individuals to sickness.

Behavioral and Neurochemical Mechanisms of the Action of Mild Stress in the Enhancement of Feeding

Rats were trained to respond under a cyclic-ratio schedule of reinforcement composed of an ascending, followed by a descending, sequence of ratio values. Subjects were trained while exposed to 70 dB white noise, then tested while exposed to 70 or 90 dB white noise. Exposure to 90 dB white noise elevated the response function (p<.02). Naloxone was then administered intraperitoneally at 0.3. 1.0. and 3.0 mg/kg under 70 dB and 90 dB white noise. Naloxone administration (1.0 and 3.0 mg/kg) significantly depressed the response function obtained under 90 dB white noise (ps<.01) but did not affect the function obtained under 70 dB white noise. These findings suggest that mild stress increases food intake through a mechanism affecting palatability enhanced by the release of endogenous opioids.

Poststress glucose mitigates behavioral impairment in rats in the "learned helplessness" model of psychopathology

Three experiments examined the effects of poststress glucose treatment in the learned helplessness model of psychopathology in rats. In experiment 1, rats were given access to water or 40% aqueous glucose immediately following exposure to inescapable tailshocks or simple restraint in a 2 x 2 factorial design. Inescapably shocked rats failed to drink the glucose solution during the poststress interval and failed to show any improvement 24 hours after stress induction in shuttle-escape performance. Consequently, all rats received preexposure to a sweetened glucose cocktail in an attempt to increase poststress ingestion following inescapable shock treatment in experiment 2. Under these conditions, poststress intake of the glucose cocktail eliminated behavioral impairment in inescapably shocked rats relative to water-treated shocked rats and water- and glucose-treated restrained controls. Experiment 3 demonstrated that glucose prophylaxis occurs in the absence of sucrose when rats are preexposed to a 40% glucose solution prior to stress induction.

Richard L. Solomon and learned helplessness

This article reviews the course of development of research on a currently popular explanatory approach to dysfunctional behavior, the learned helplessness analysis. The early history is prominent in this review as it reflects the inspirations of Richard L. Solomon, a scholar who fostered the resurgence of psychologists' interests in Pavlovian conditioning in the 1950s and 1960s. Current research is characterized as having four separate themes: elaboration of "symptoms," elucidating the role of fear, explicit modeling, and extensions involving attributional constructs.

Divergent responses to saccharin vs. sucrose availability after stress in rats

Rats were exposed to 100 5-s inescapable, unpredictable shocks then had access to water and saccharin or sucrose solution for 5-6 days. Shock reduced daily drinking during saccharin tests (Experiments 1, 2B, 4) and increased daily drinking during sucrose tests (Experiment 2B). In addition, shock reduced body weight when saccharin, but not when sucrose, was available (Experiments 1, 2B). The specificity of the reductions to saccharin tests (Experiments 2B, 4), equal intake of flavors shortly after stress (Experiments 2B to 4), and failure of saccharin-naive and saccharin-familiar groups to differ (Experiment 3) argue against a neophobia interpretation. Normal or above-normal drinking during sucrose (Experiment 2B) or water-only (Experiment 4) tests indicate an absence of general hedonic, motoric, or hydrational deficits. Qualitative differences in the tastes of saccharin and sucrose may underlie their disparate effects on drinking after shock. Whatever the mechanisms, the present results show that even a severe stressor can decrease or increase ingestion, depending on the flavors available.

Individual differences in taste, body weight, and depression in the "helplessness" rat model and in humans

The helplessness paradigm is used extensively in basic stress research and is an experimental model of clinical depression. In Experiment 1, exposure to unsignaled, inescapable shock resulted in finickiness about drinking a weak quinine solution, as previously reported. In contrast, exposure to escapable shock resulted in marked individual differences in finickiness that were predicted by prestress body weight. A more sensitive index of finickiness was used in Experiment 2, and a correlation between body weight and finickiness was observed in nonshocked rats. In Experiment 3, measures of quinine reactivity and body weight predicted depressive symptomatology in a nonclinical human sample. Although research in the helplessness paradigm usually focuses on environmental determinants of distress, the paradigm may help identify and explain individual differences in, or intrinsic modulation of, stress and depression.

Suppression of feeding and body weight by inescapable shock: modulation by quinine adulteration, stress reinstatement, and controllability

Three experiments examined food intake and body weight in rats after exposure to one session of intermittent, inescapable electric shock. Quinine adulteration and shock both suppressed feeding (Experiment 1); recovery of feeding after shock was impeded when quinine adulteration was combined with a mild daily stress reinstatement (Experiment 2). Body weight also was suppressed by shock (Experiments 1 and 2); control over shock provided some protection against this deficit (Experiment 3). These results suggest roles for "finickiness" and vulnerability to mild stressors in the maintenance of eating disorders associated with stress and depression. The findings also may have implications for interpretation of deficits in appetitively motivated behaviors after stress.

Stressors in the learned helplessness paradigm: effects on body weight and conditioned taste aversion in rats

Changes in body weight and taste aversion in the learned helplessness paradigm were examined. In Experiment 1, adult male Sprague-Dawley rats drank saccharin or a control solution, followed by either 100 inescapable shocks or simple restraint. Rats were weighted daily and were tested for saccharin aversion two days after the stress session. Shocked rats gained less weight in the days after stress than restrained controls. Saccharin aversion was apparent only among rats that had consumed saccharin before the stress session. Experiment 2 examined whether control over shock affected body weight or taste aversion. Home-cage controls were included to assess the effects of restraint alone. In addition, the combined effects of shock and a toxin on aversion were studied. Rats drank saccharin solution, followed by escapable or inescapable shock, restraint, or no treatment. Then half of each group was injected with saline; the other half was injected with lithium chloride. As in Experiment 1, shock reduced body weight relative to restraint or no treatment, and shock produced a taste aversion among saline-treated rats. However, shock attenuated the aversion produced by lithium chloride, as did simple restraint. There were no differences in body weight or taste aversion between escapably and inescapably shocked rats. These results suggest a role for stress in the anorexia and weight loss associated with clinical depression and may have implications for theories of learning and learned helplessness.