Selective pressure on a saccharin intake phenotype and its correlates: a replication study.

The Occidental High- and Low-Saccharin rats (respectively, HiS and LoS lines) were selectively bred for decades to examine mechanisms and correlates of a saccharin intake phenotype. Observed line differences ranged from taste and eating to drug self-administration and defensive behavior, paralleling human research on relationships between gustation, personality, and psychopathology. The original lines were terminated in 2019, and replicate lines (HiS-R and LoS-R) were selectively bred for 5 generations to test for reproducible, rapid selection for the phenotype and its correlates. The line differences chosen for replication included intake of tastants (saccharin, sugars, quinine-adulterated sucrose, sodium chloride, and ethanol) and foods (cheese, peas, Spam, and chocolate) and several noningestive behaviors (deprivation-induced hyperactivity, acoustic startle, and open field behavior). The HiS-R and LoS-R lines diverged on intake of saccharin, disaccharides, quinine-adulterated sucrose, sodium chloride, and complex foods, and open field behavior. Differences from the original lines also were observed. Reasons for and implications of the pattern of replication and lack thereof in 5 generations are discussed.

Parametric Characterization of a Taste Phenotype in Rats Selectively Bred for High Versus Low Saccharin Intake.

Taste signals food quality and reflects energy status and associated processes. Occidental high- and low-saccharin consuming rats (HiS, LoS) have been selectively bred for nearly 60 generations on intake of 0.1% saccharin in a 23-h two-bottle test, as a tool for studying individual differences in taste and its correlates in the domains of feeding, defensive, and social behavior. The saccharin phenotype itself has not been well characterized until now. The present series of parametric studies examined suprathreshold saccharin concentration-intake functions (Experiment 1), saccharin preference threshold (Experiments 2A and 2B), and intra- and inter-sweetener carryforward effects (Experiments 2B, 3A-3D). Results indicate high stability in line differences in behavior toward saccharin and also line-specific mutability of intake of saccharin and certain other sweeteners. Methodological and conceptual implications are discussed.

Sweetener Intake by Rats Selectively Bred for Differential Saccharin Intake: Sucralose, Stevia, and Acesulfame Potassium.

Behavioral responses to sweeteners have been used to study the evolution, mechanisms, and functions of taste. Occidental low and high saccharin consuming rats (respectively, LoS and HiS) have been selectively outbred on the basis of saccharin intake and are a valuable tool for studying variation among individuals in sweetener intake and its correlates. Relative to HiS rats, LoS rats consume smaller amounts of all nutritive and nonnutritive sweeteners tested to date, except aspartame. The lines also differ in intake of the commercial product Splenda; the roles of sucralose and saccharides in the difference are unclear. The present study extends prior work by examining intake of custom mixtures of sucralose, maltodextrin, and sugars and Splenda by LoS and HiS rats (Experiment 1A-1D), stevia and a constituent compound (rebaudioside A; Experiment 2A-2E), and acesulfame potassium tested at several concentrations or with 4 other sweeteners at one concentration each (Experiment 3A-3B). Results indicate that aversive side tastes limit intake of Splenda, stevia, and acesulfame potassium, more so among LoS rats than among HiS rats. In addition, regression analyses involving 5 sweeteners support the idea that both sweetness and bitterness are needed to account for intake of nonnutritive sweeteners, more so among LoS rats. These findings contribute to well developed and emerging literatures on sweetness and domain-general processes related to gustation.

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.

Stardust and feminism: A creatureliness agenda.

People are living, breathing creatures. Dominant feminist discourses are situated within hegemonic human exceptionalism (HHE) which, by framing the body in terms of human forms of meaning-making and social life, eschews first-order embodiment (or creatureliness) as worthy of inquiry. Here, well-known reasons for avoidance of "the biological" are briefly summarized and an argument is advanced for meta-theoretical centering of creatureliness. A three-pronged agenda is proposed that embraces the creaturely body without the "-isms" (e.g., essentialism) and "-izings" (e.g., so-called "naturalizing") that subvert feminist commitments. By unsettling HHE, executing the agenda would promote broader feminist coalitions and new scholarly collaborations aimed at fleshing out gender.

Food dependence in rats selectively bred for low versus high saccharin intake. Implications for "food addiction".

The "food addiction" concept implies that proneness to drug dependence and to food dependence should covary. The latter was studied in low- (LoS) and high- (HiS) saccharin-consuming rats, who differ in drug self-administration (HiS>LoS) and withdrawal (LoS>HiS). Sugary food intake in the first 1-2 h was higher in HiS than LoS rats. Sugar intake predicted startle during abstinence only among LoS rats. These results may suggest bingeing-proneness in HiS rats and withdrawal-proneness among LoS rats. However, intake escalation and somatic withdrawal did not differ between lines. Further study with selectively bred rats, with attention to definitions and measures, is warranted.

Eating and wheel running across the estrous cycle in rat lines selectively bred on a taste phenotype.

The Occidental Low- and High-Saccharin-Consuming rats (respectively, LoS and HiS) have been selectively bred for decades to study the relationship between taste and behaviors in and beyond the ingestive domain. Whether the saccharin phenotype is associated with behavioral periodicities tied to reproductive status is not known. Here we describe for the first time variation across the estrous cycle in chow intake and wheel running by LoS and HiS rats. This study also shed light on why rats, humans, and some other mammals eat less and become more active as fertility increases. Wheel running increases when eating is reduced through restricted chow access, more so in LoS rats than in HiS rats (Dess et al., 2000). If the decrease in food intake from diestrus through estrus causes the increase in running (Eat Less â†’ Run More hypothesis, ELRM), then the running peak should follow the eating nadir and be greater in LoS rats. Bayesian cyclic regression showed that estrous cycles were shorter in LoS rats than in HiS rats; implications are discussed. Contrary to ELRM, the running peak did not follow the eating nadir, and cycle amplitude did not distinguish LoS rats from HiS rats. These results indirectly support the No Time To Eat hypothesis (Fessler, 2003), according to which the periovulatory eating nadir and running peak reflect fitness-enhancing consequences of shifts away from eating and toward mating as fertility increases.

"Us vs. Them" Pair Housing: Effects on Body Weight, Open Field Behavior, and Gut Microbiota in Rats Selectively Bred on a Taste Phenotype.

Taste is increasingly recognized as being related to reward, risk, and social processes beyond the ingestive domain. Occidental High (HiS) and Low (LoS) Saccharin Consuming rats have been selectively bred for more than 25 years to study those relationships. The present study examined LoS and HiS rats' sensitivity to a social partner's lineage. The role of gut microbiome transfer between lines was also explored as a possible mediating mechanism. Rats were pair-housed with a rat from either their own line (same-line condition) or the other line (other-line condition); weight gain, saccharin intake, acoustic startle, and open field behavior were measured. Results show for the first time that the lines express different behavioral strategies in a novel open field. In addition, weight gain and open field measures indicate that other-line housing was stressful. Saccharin intake, however, was unaffected by housing condition. A previous finding that the lines possess different gut microbiota was replicated. Although microbial transfer occurred between social partners, no clear evidence was obtained that housing-condition effects on weight gain or behavior were mediated by microbial transfer. Overall, these findings add to the characterization of non-gustatory correlates of a taste phenotype and suggest that rats differing strikingly on the taste phenotype and/or its correlates may be socially incompatible.

Consumption of SC45647 and sucralose by rats selectively bred for high and low saccharin intake.

Mammals' affinity for sweet tastes exists alongside dramatic variation among species and individuals in responses to sweeteners. The present paper focused on consumption by Occidental High- (HiS) and Low-Saccharin (LoS)-consuming rats in 23-h 2-bottle tests of 2 sweeteners for which few data from rats are available: SC45647 and sucralose. Every HiS and LoS rat preferred SC45647 to water at every concentration, with HiS rats consuming it more avidly. Most HiS rats preferred sucralose to water at one or more concentrations; some HiS rats and most LoS rats avoided sucralose at every concentration. However, both HiS and LoS rats preferred a sucralose-maltodextrin mixture (Splenda) to water; thus, Splenda's "bulking" ingredient maltodextrin transforms highly variable responses to sucralose into a relatively homogeneous preference for the product. Implications for the study of variation in sweet taste are discussed.

Taste as a marker for behavioral energy regulation:Replication and extension of meal pattern evidence from selectively bred rats.

A key feature of energy regulation among species that eat discrete meals is meal patterning - meal frequency, size, and duration. Such animals can adjust to internal states and external circumstances with changes in one or more of those meal parameters, with or without altering total food intake. Relatively little is known about individual differences in meal patterning. We previously reported meal patterning differences between rat lines selectively bred for differential saccharin solution intake, lines that also differ in sensitivity to metabolic challenges: Relative to high-saccharin-consuming counterparts (HiS), male low-saccharin-consuming rats (LoS) ate smaller, more frequent meals. Those findings provided evidence of an association between taste and short term satiety. Twenty generations later, we describe systematic replication of the line difference in meal patterns in males and females using two different kinds of reinforcer pellet. The previous study was further extended by examining meal parameters (1) with bi- and multivariate analyses and (2) after acute food restriction and a moderate stressor. Results are discussed within a behavior-systems framework incorporating taste as a marker for behavioral energy regulation.

Escalation of i.v. cocaine self-administration and reinstatement of cocaine-seeking behavior in rats bred for high and low saccharin intake.

RATIONALE: Rats selectively bred for high saccharin (HiS) intake consume more alcohol, acquire intravenous (i.v.) cocaine self-administration more rapidly, and show more dysregulated patterns of cocaine self-administration than their low saccharin-consuming (LoS) counterparts. OBJECTIVES: The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, extinction, and reinstatement of i.v. cocaine self-administration. MATERIALS AND METHODS: Two experiments were conducted in separate groups of rats. In the first experiment, HiS and LoS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h), and FR1-maintained cocaine self-administration was examined. In the second experiment, additional groups of HiS and LoS female rats were given access to cocaine (0.4 mg/kg; FR 1) self-administration during 2-h sessions for 10 days. Subsequently, saline was substituted for cocaine, and responding was extinguished. After a 14-day extinction period, saline- and cocaine-[5, 10, and 15 mg/kg, intraperitoneal (i.p.)] induced reinstatement of drug-seeking behavior was measured. RESULTS: HiS LgA rats escalated their cocaine intake more rapidly than LoS rats, and during the 2 h sessions after escalation cocaine self-administration was significantly higher in HiS LgA rats, compared to LoS LgA rats. HiS rats responded on the cocaine-paired lever more than LoS rats during maintenance, extinction, and cocaine-(15 mg/kg) induced reinstatement. CONCLUSIONS: These results suggest that HiS and LoS rats have distinct drug-seeking and drug-taking profiles. The HiS and LoS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of excessive intake of dietary substances and vulnerability to drug abuse.

Escalation of intravenous cocaine self-administration, progressive-ratio performance, and reinstatement in rats selectively bred for high (HiS) and low (LoS) saccharin intake.

RATIONALE: Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (IV) cocaine self-administration more rapidly than their low saccharin (LoS) consuming counterparts. OBJECTIVES: The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, and reinstatement of IV cocaine self-administration. METHODS: LoS and HiS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h) and (1) FR1-maintained cocaine (0.4 mg/kg) self-administration, (2) progressive ratio (PR)-maintained cocaine (0.2-1.6 mg/kg) self-administration, and (3) saline-induced and cocaine (10 mg/kg, IP)-induced reinstatement of drug-seeking behavior were examined. RESULTS: HiS LgA rats escalated their cocaine intake more rapidly and self-administered more cocaine (mg/kg) than LoS LgA rats; however, there was no LoS versus HiS phenotype difference in the number of infusions self-administered by Day 21. Post-escalation cocaine self-administration under an FR1 schedule did not differ as a function of phenotype (LoS versus HiS) or access condition (ShA versus LgA); however, LoS rats responded more for cocaine under the PR schedule than HiS rats, and they showed a greater reinstatement of cocaine-seeking behavior than HiS rats. In contrast, ShA versus LgA did not affect PR or reinstatement performance in the LoS and HiS groups. CONCLUSIONS: These results suggest that LoS and HiS rats have distinct drug-seeking and drug-taking profiles that differ as a function of the experimental phase and access condition. The LoS and HiS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of feeding, emotionality, and other factors related to vulnerability to drug abuse.

Ethanol withdrawal and proclivity are inversely related in rats selectively bred for differential saccharin intake.

Withdrawal severity and voluntary alcohol consumption are inversely related in rats and mice. The present study demonstrated this empirical relation and extended it in two ways. First, the rats were selectively bred for low (LoS) and high (HiS) saccharin intake, a phenotype that correlates positively with ethanol intake and inversely with emotional reactivity. Withdrawal has not yet been studied in these rats. Second, proclivity to consume ethanol was measured as conditioned preference for an ethanol-paired flavor. After 2 weeks of forced exposure to ethanol and a period of abstinence, LoS rats showed elevated acoustic startle; HiS rats did not (Exp. 1). When ethanol- and no-ethanol solutions were available freely during conditioning, both LoS and HiS rats preferred a flavor paired with 4% ethanol, but only HiS rats preferred a flavor paired with 10% ethanol (Exp. 2A); when exposure to the two solutions was controlled, all groups except LoS males preferred flavors paired with 4% or 10% ethanol (Exp. 2B). Thus, as predicted, withdrawal was more severe in the line with less ethanol proclivity (LoS). These results implicate basic associative and affective processes in individual differences in patterns of alcohol use.

Intravenous cocaine and heroin self-administration in rats selectively bred for differential saccharin intake: phenotype and sex differences.

RATIONALE: Rats selectively bred for high intake of a sweet saccharin solution (HiS) consume more ethanol than their low-saccharin intake (LoS) counterparts. The HiS phenotype may be a predictor of abuse of other drugs via other routes of administration. OBJECTIVE: HiS and LoS, male and female rats were tested for acquisition of IV cocaine and heroin self-administration under a fixed-ratio 1 (FR1) schedule, and cocaine-reinforced behavior was examined under a progressive-ratio (PR) schedule. METHODS: Four groups of rats (HiS males and females and LoS males and females) were trained to self-administer IV cocaine (0.2 mg/kg), and another four groups were trained to self-administer heroin (0.015 mg/kg) using an automated autoshaping procedure. Rats were allowed 30 days to reach a criterion whereby a mean of 100 (cocaine) or 20 (heroin) infusions were self-administered during 6-h sessions over 5 consecutive days. RESULTS: The HiS female rats acquired cocaine self-administration significantly more rapidly than the LoS rats, and females of both phenotypes met the acquisition criteria more rapidly than males. In both HiS and LoS cocaine groups a greater percentage of females (compared with males) met the acquisition criteria within 30 days. The only cocaine group in which 100% met the criterion was the HiS females. The female (compared with male) heroin groups showed a more rapid rate of acquisition, but there was no difference due to saccharin phenotype. In each of the four heroin groups 100% of all rats met the criteria within 30 days. Results of the PR schedule in the HiS females and males and LoS females indicated significantly higher break points in the HiS females (compared with HiS males), but there were no differences in females due to phenotype. CONCLUSION: Female rats selectively bred for higher saccharin intake show more rapid and successful acquisition of IV self-administration of a low dose of cocaine than those bred for low saccharin intake. Female rats (compared with males) consistently showed accelerated rates of acquisition and maintenance (PR) of cocaine self-administration and acquisition of heroin self-administration.

Ingestional responses to metabolic challenges in rats selectively bred for high and low saccharin intake.

Rats selectively bred on the basis of saccharin intake also differ on some measures of emotional reactivity. The present studies were designed to contribute to our understanding of this association. Rats selectively bred for relatively high (HiS) versus low (LoS) saccharin intake were tested in two paradigms useful in assessing the ability to respond adaptively to internal perturbations of metabolic regulation or to external events that may produce metabolic challenges. The first study concerned slow-onset (regular insulin) and rapid-onset (2-deoxy-D-glucose [2-DG], fast-acting insulin) glucoprivation and resultant feeding behavior. LoS and HiS lines did not differ in response to saline or slow-onset challenges, but LoS rats ate less in the first half hour after rapid-onset challenges; the line differences were eliminated by pretreatment with caffeine. The second study revealed significantly higher plasma corticosterone (CORT) among LoS rats relative to HiS rats, both in the light and in the dark. Preliminary assessments after a single stressor and a single dose of dexamethasone showed, respectively, CORT elevation and suppression that was comparable in the two lines. These results add further support to the ideas that voluntary consumption of saccharin is related to the expression of classically defined emotional behaviors, and that responsiveness to diverse metabolic challenges may share a common basis, such as genetic pleiotropism.

"Jello® shots" and cocktails as ethanol vehicles: parametric studies with high- and low-saccharin-consuming rats.

Naïve humans and rats voluntarily consume little ethanol at concentrations above ~6% due to its aversive flavor. Developing procedures that boost intake of ethanol or ethanol-paired flavors facilitates research on neural mechanisms of ethanol-associated behaviors and helps identify variables that modulate ethanol intake outside of the lab. The present study explored the impact on consumption of ethanol and ethanol-paired flavors of nutritionally significant parametric variations: ethanol vehicle (gelatin or solution, with or without polycose); ethanol concentration (4% or 10%); and feeding status (chow deprived or ad lib.) during flavor conditioning and flavor preference testing. Individual differences were modeled by testing rats of lines selectively bred for high (HiS) or low (LoS) saccharin intake. A previously reported preference for ethanol-paired flavors was replicated when ethanol had been drunk during conditioning. However, indifference or aversion to ethanol-paired flavors generally obtained when ethanol had been eaten in gelatin during conditioning, regardless of ethanol concentration, feeding status, or caloric value of the vehicle. Modest sex and line variations occurred. Engaging different behavioral systems when eating gelatin, rather than drinking solution, may account for these findings. Implications for parameter selection in future neurobiological research and for understanding conditions that influence ethanol intake outside of the lab are discussed.

Refeeding after acute food restriction: differential reduction in preference for ethanol and ethanol-paired flavors in selectively bred rats.

Rats' voluntary ethanol intake varies with dispositional factors and energy status. The joint influences of these were of interest here. We previously reported that rats selectively bred for high voluntary saccharin intake (HiS) consume more ethanol and express more robust conditioning of preference for flavors paired with voluntarily consumed ethanol than do low-saccharin consuming counterparts (LoS). Three new experiments examined the effect of refeeding after an episode of food restriction on ethanol intake and on preference for ethanol-paired flavors in HiS and LoS rats. A 48-h episode of food restriction with wheel running reduced intake of and preference for 4% ethanol (Exp. 1a) and preference for an ethanol-paired flavor (Exp. 1b) during refeeding. Food restriction alone was sufficient to reduce the flavor preference (Exp. 2). Adding fat to the refeeding diet or extending the food restriction period exacerbated the effect (Exp. 3), yielding a frank aversion to ethanol-paired flavors in LoS rats. These studies indicate that rebound from negative energy balance shifts responses to ethanol-associated cues from preference toward aversion. Analyses of bodyweight changes and caloric intake during refeeding support this conclusion and further suggest that lower metabolic efficiency may be a marker for enhanced preference mutability.

Sweet success, bitter defeat: a taste phenotype predicts social status in selectively bred rats.

For social omnivores such as rats and humans, taste is far more than a chemical sense activated by food. By virtue of evolutionary and epigenetic elaboration, taste is associated with negative affect, stress vulnerability, responses to psychoactive substances, pain, and social judgment. A crucial gap in this literature, which spans behavior genetics, affective and social neuroscience, and embodied cognition, concerns links between taste and social behavior in rats. Here we show that rats selectively bred for low saccharin intake are subordinate to high-saccharin-consuming rats when they compete in weight-matched dyads for food, a task used to model depression. Statistical and experimental controls suggest that differential resource utilization within dyads is not an artifact of individual-level processes such as apparatus habituation or ingestive motivation. Tail skin temperature measurements showed that LoS rats display larger hyperthermic responses to social interaction after status is established, evidence linking taste, social stress, autonomic reactivity, and depression-like symptoms. Based on regression using early- and late-competition predictors to predict dyadic disparity in final competition scores, we tentatively suggest that HiS rats emerge as dominant both because of an "early surge" on their part and because LoS acquiesce later. These findings should invigorate the comparative study of individual differences in social status and its relationship to mental and physical health.

Modulation of methylphenidate effects on wheel running and acoustic startle by acute food deprivation in commercially and selectively bred rats.

Behavioral effects of the same dose of the same drug can vary in degree and direction between and within individuals. The present study examines behavioral base rates, feeding status, and dispositional differences as sources of inter- and intra-individual heterogeneity in drug response. Modulation of the effects of methylphenidate (MPD) on wheel running and acoustic startle by food deprivation was examined in three experiments. Freely fed or food deprived Harlan Sprague-Dawley rats (running study) or rats selectively bred for low (LoS) and high (HiS) saccharin intake (running and startle studies) were given MPD (10 mg/kg) or saline before testing. Overall drug effects and predictors of drug response were assessed. MPD increased running and startle amplitude and disrupted prepulse inhibition; systematic variation among rats of these effects and their modulation by food deprivation was observed. Deprivation-induced running predicted MPD's effect in Harlan SD and LoS rats. Observation of this relationship among commercial rats suggests that acute deprivation sensitivity has utility as a noninvasive marker for drug responses. Its observation in rats selected on a taste phenotype with known correlates points to fruitful avenues of research on stimulant drugs' mechanisms, especially in dopaminergic pathways, and may be relevant to their clinical usage.

Stress-induced attenuation of acoustic startle in low-saccharin-consuming rats.

Exposure to stress can lead to either increased stress vulnerability or enhanced resiliency. Laboratory rats are a key tool in the exploration of basic biobehavioral processes underlying individual differences in the effect of stress on subsequent stressors' impact. The Occidental low (LoS) and high (HiS) saccharin-consuming rats, which differ in emotional reactivity, are useful in this effort. In the present study, footshock affected acoustic startle amplitude 4 h later among LoS but not HiS rats. Surprisingly, shock attenuated startle rather than sensitizing it, a finding not previously reported for male rats exposed to shock. Attenuation was blocked by administering the anxiolytic drug alprazolam prior to stress, implicating anxiety in the effect. Preliminary tests provided no evidence of mediation by adenosine or corticosterone. This novel result encourages further study of the stressor and dispositional variables that modulate the timecourse of effects of stress on startle and identification of its mechanisms.